Biosorption : Techniques and Mechanisms
Ramachandra TV        Ahalya N        Kanamadi RD
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CES TECHNICAL REPORT - 110
ENERGY AND WETLANDS RESEARCH GROUP
CENTRE FOR ECOLOGICAL SCIENCES
INDIAN INSTITUTE OF SCIENCE
BANGALORE 560 012

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SUMMARY

Water resources are of critical importance to both natural ecosystem and human developments. Increasing environmental pollution from industrial wastewater particularly in developing countries is of major concern. Heavy metal contamination exists in aqueous waste streams of many industries, such as metal plating facilities, mining operations, tanneries, etc. Some metals associated with these activities are cadmium, chromium, iron, nickel, lead and mercury. Heavy metals are not biodegradable and tend to accumulate in living organisms causing diseases and disorders.  Many industries like dye industries, textile, paper and plastics use dyes in order to colour their products and also consume substantial volumes of water. As a result they generate a considerable amount of coloured wastewater. The presence of small amount of dyes (less than 1 ppm) is highly visible and undesirable. Many of these dyes are also toxic and even carcinogenic and pose a serious threat to living organisms. Hence, there is a need to treat the wastewaters containing toxic dyes and metals before they are discharged into the waterbodies. Many physico-chemical methods like coagulation, flocculation, ion exchange, membrane separation, oxidation, etc are available for the treatment of heavy metals and dyes. Major drawbacks of these methods are high sludge production, handling and disposal problems, high cost, technical constraints, etc. This necessitates cost effective and environmentally sound techniques for treatment of watsewaters containing dyes and metals. During the 1970s, the increasing awareness and concern about the environment motivated research for new efficient technologies that would be capable of treating inexpensively, waste waters polluted by metals and dyes. This search brought biosorption/adsorption to the foreground of scientific interest as a potential basis for the design of novel wastewater treatment processes. Several adsorbents are currently used which are by-products from agriculture and industries, which include seaweeds, molds, yeast, bacteria, crabshells, agricultural products such as wool, rice, straw, coconut husks, peat moss, exhausted coffee waste tea leaves, walnut skin, coconut fibre, etc. Adsorption/Biosorption using low cost adsorbents could be technically feasible and economically viable sustainable technology for the treatment of wastewater streams. Low cost adsorbents are nothing but materials that require little processing, are abundant in nature or is a byproduct or waste material from another industry.

 

INTRODUCTION

Water has the central role in mediating global-scale ecosystem processes, linking atmosphere, lithosphere and biosphere by moving substances between them and enabling chemical reactions to occur. Natural waters are never pure H2O but a complex and ever-changing mixture of dissolved inorganic and organic molecules and suspended particles.

 

1.1 DISTRIBUTION OF WATER IN THE WORLD

Water is essential to human life and to the health of the environment. As a valuable natural resource, it comprises marine, estuarine, freshwater (river and lakes) and groundwater environments, across coastal and inland areas. Most of the water found on this planet is held within the oceans (~97%). The use of this sink of water by humans is limited because of the dissolved salts it contains. Table 1 below describes the major reservoirs of water found on the Earth. Icecaps and glaciers contain about 2 % of the world's total water, and about 60 % of the freshwater supply. The use of this water by humans is very restricted because of its form and location. Humans primarily use the freshwater found in groundwater, lakes, rivers, etc., which is less than 1% of the earth's supply.

Table 1 : Inventory of water at the Earth's surface.

Reservoir
Volume (cubic km x 10,000,000)
Percent of Total
Oceans
1370
97.25
Ice Caps/Glaciers
29
2.05
Deep Groundwater
5.3
0.38
Shallow Groundwater
4.2
0.30
Lakes
0.125
0.01
Soil Moisture
0.065
0.005
Atmosphere
0.013
0.001
Rivers
0.0017
0.0001
Biosphere
0.0006
0.00004

Figure 1 : World water distribution

Water has two dimensions that are closely linked - quantity and quality. Water quality is commonly defined by its physical, chemical, biological and aesthetic (appearance and smell) characteristics. A healthy environment is one in which the water quality supports a rich and varied community of organisms and protects public health (Ramachandra et al., 2002)

Water quality in a body of water influences the way in which communities use the water for activities such as drinking, swimming or commercial purposes. More specifically, the water may be used by the community for:

 

1.2 TYPES OF AQUATIC ECOSYSTEM

The aquatic ecosystem can be broken down into two basic regions, freshwater (i.e, ponds and rivers) and marine (i.e, oceans and estuaries) (Ramachandra and Ahalya, 2001).

1.2.1 Freshwater Regions

Freshwater is defined as having a low salt concentration—usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e, ocean). There are different types of freshwater regions: ponds and lakes, streams and rivers, and wetlands. The following sections describe the characteristics of these three freshwater zones  (Ramachandra and Ahalya, 2001).

1.2.2 Marine Ecosystems

Marine regions cover about three-fourths of the Earth’s surface and include oceans, coral reefs, and estuaries. Marine algae supply much of the world’s oxygen supply and take in a huge amount of atmospheric carbon dioxide. The evaporation of the seawater provides rainwater for the land.

 

1.3 THREATS TO AQUATIC ECOSYSTEMS

Increasingly, aquatic ecosystems are under increasing stress due to the rapidly growing population, technological development, urbanisation and economic growth.Human activities are causing aquatic species to disappear at an alarming rate. It has been estimated that between 1975 and 2015, species extinction will occur at a rate of 1 to 11 percent per decade. Aquatic species are at a higher risk of extinction than mammals and birds. Losses of this magnitude impact the entire ecosystem, depriving valuable resources used to provide food, medicines, and industrial materials to human beings (Ramachandra et al., 2005). Runoff from agricultural and urban areas, the invasion of exotic species, and the creation of dams and water diversion have been identified as the greatest challenges to freshwater environments. Some of the threats and causes to aquatic ecosystem are presented in Table 2.

Table 2 : Causes and type of threats to aquatic ecosystem

Type of Threat
Causes
Water Regime Flooding; reclamation; water diversion; erosion/siltation; roads; irrigation; water works (floods)
Water Pollution Solid waste refuse; siltation; sewage/fecal; mining; pesticides; fertilizers; salinization of soils
Physical Modification Erosion; flooding; clearance and fire; sedimentation; infrastructure/housing; quarrying and sand winning; hunting; recreation; agriculture
Over-exploitation Fishing; fuel wood; hunting of birds and mammals; grazing

Human impacts on the quality and quantity of fresh water can threaten economic prosperity, social stability, and the resilience of ecological services that aquatic systems provide. Rising demand for fresh water can sever ecological connections in aquatic systems, fragmenting rivers from floodplains, deltas, and coastal marine environments. It also can change the quantity, quality, and timing of freshwater supplies on which terrestrial, aquatic, and estuarine ecosystems depend (Ramachandra et al., 2002). Fresh water is already a limiting resource in many parts of the world. In the next century, it will become even more limiting due to increased population, urbanization, and climate change. This limitation will be caused not just by increased demand for water, but also by pollution in freshwater ecosystems. Pollution decreases the supply of usable water and increases the cost of purifying it. Some pollutants, such as heavy metals or chlorinated organic compounds, contaminate aquatic resources and affect food supplies. This nutrient pollution, combined with human demand for water, affects biodiversity, ecosystem functioning, and the natural services of aquatic systems upon which society depends. Point sources are 'pipeline' discharges of pollutants to receiving waters, e.g. domestic sewage discharges or industrial waste effluents from factories or plants. They are relatively easy to identify and isolate. In contrast, non-point pollution results from storm runoff, which transports polluting materials diffusely and over land.
Major water pollutants include a variety of organic and inorganic chemicals such as heavy metals and industrial compounds. They can affect human health and/or interfere with industrial or agricultural water use. If the level of a pollutant in the water supply exceeds an acceptable level for a given water use (e.g., domestic or industrial water supply), the water is considered unsafe or too degraded for that use. Solutions to such pollution problems, therefore, usually focus on reduction of pollution at the source and/or treatment of the polluted water prior to use (Ahalya and Ramachandra, 2002). It is clear that inland aquatic ecosystems are under increasing threat. As the pervasive and intractable nature of threats makes them difficult to manage, avoidance through protection mechanisms is hugely cost-effective and beneficial. Given that aquatic ecosystem degradation is ubiquitous and increasing, the identification and protection of ecosystems value, is urgent. As most of the industrial processes are located near the water bodies, they are increasingly polluted by a number of organic and inorganic materials. Two of the most hazardous pollutants that are affecting them include heavy metals and dyes.

 

1.4 METALS AND DYES IN THE AQUATIC ECOSYSTEMS

Metals, a major category of globally-distributed pollutants, are natural elements that have been extracted from the earth and harnessed for human industry and products for millenia. Metals are notable for their wide environmental dispersion from such activity; their tendency to accumulate in select tissues of the human body; and their overall potential to be toxic even at relatively minor levels of exposure. Today heavy metals are abundant in our drinking water, air and soil due to our increased use of these compounds. They are present in virtually every area of modern consumerism from construction materials to cosmetics, medicines to processed foods; fuel sources to agents of destruction; appliances to personal care products. It is very difficult for anyone to avoid exposure to any of the many harmful heavy metals that are so prevalent in our environment. The distribution of heavy metals in manufacturing industries is given in Table 3. Some metals, such as copper and iron, are essential to life and play irreplaceable roles in, for example, the functioning of critical enzyme systems. Other metals are xenobiotics, i.e., they have no useful role in human physiology (and most other living organisms) and, even worse, as in the case of lead and mercury, may be toxic even at trace levels of exposure. Even those metals that are essential, however, have the potential to turn harmful at very high levels of exposure, a reflection of a very basic tenet of toxicology-- “the dose makes the poison.”

Table 3 : General Distribution of Heavy metals in Particular Industrial Effluents

Industries
Ag
As
Cd
Cr
Cu
Fe
Hg
Mn
Ni
Pb
Se
Ti
Zn
General Industry and Mining       X X X   X   X     X
Plating     X X X       X X     X
Paint Products       X           X   X  
Fertilizers     X X X X X X X X     X
Insecticides / Pesticides   X     X   X            
Tanning   X   X                  
Paper Products       X X   X   X X   X X
Photographic X     X                  
Fibers         X               X
Printing / Dyeing       X           X      
Electronics X                   X    
Cooling Water       X                  
Pipe Corrosion         X         X      

Note : Ag  - Silver;, As – Arsenic; Cd – Cadmium; Cr – Chromium; Cu –Copper; Fe –Iron, Hg – Mercury; Mn – Manganese; Ni – Nickel; Pb – Lead; Se – Selenium; Zn-Zinc.

Another group of pollutants that are increasingly causing pollution in fresh water bodies are dyes. Dyes are basically chemical compounds that can attach themselves to fabrics or surfaces to impart colour. Most dyes are complex organic molecules and are need to be resistant to many things such as the weather and the action of detergents.  Synthetic dyes are extensively used in many fields of up-to-date technology, e.g., in various branches of the textile industry (Gupta et al., 1992; Shukla and Gupta, 1992 and Sokolowska-Gajda et al., 1996), of the leather tanning industry (Tünay et al., 1999 and Kabadasil et al., 1999) in paper production (Ivanov et al., 1996), in food technology (Bhat and Mathur, 1998 and Slampova et al., 2001), in agricultural research (Cook and Linden, 1997 and Kross et al., 1996), in light-harvesting arrays (Wagner and Lindsey, 1996), in photoelectrochemical cells (Wrobel et al., 2001), and in hair colourings (Scarpi et al., 1998). Moreover, synthetic dyes have been employed for the control of the efficacy of sewage (Morgan-Sagastume et al., 1997) and wastewater treatment (Hsu and Chiang, 1997 and Orhon et al., 1999), for the determination of specific surface area of activated sludge (Sorensen and Wakeman, 1996) for ground water tracing (Field et al., 1995), etc.

Dyes can be classified according to their chemical structure or according to their use. However, classifications vary from country to country though there are some fundamental categories that are common to all.

According to the central pollution control board (CPCB), India there are approximately a million known dyes and dye intermediates out of which 5,000 are produced commercially. Based on their use based classification, the dyes are divided into 15 groups.


Table 4 : Classification of dyes based on their use.

Type of Dye
Application
According to CPCB1
According to World Bank2
Acid dyes Wool, silk, nylon Animal fibres
Azo dyes Cotton Cotton
Basic dyes Acrylic Paper
Direct dyes Cotton, leather, paper and synthetics Cotton wool or cotton silk
Disperse dyes Polyster  
Food dyes Food, cosmetics  
Metal complexes Cotton  
Mordant dyes Wool  
Whitening agent Plastics, paper, soap  
Pigment dyes Paints and plastics Paints and inks
Reactive dyes Wool and cotton  
Solvent dyes Synthetics  
Sulphur dyes Cotton and Synthetics  
Vat dye Cotton and Synthetics  

Source : 1 Anon 2002, Effluent toxicity status in water polluting industries, Part 1 – Dye and dye intermediate, bulk drugs and textile industries, Central Pollution Control Board, Ministry of Environment and Forests, Government of India, p7.
2 Pollution prevention and abatement handbook, World Bank, p 298

Unfortunately, the exact amount of dyes produced in the world is not known. It is estimated to be over 10,000 tonnes per year. Exact data on the quantity of dyes discharged in the environment are also not available. It is assumed that a loss of 1–2% in production and 1–10% loss in use are a fair estimate. For reactive dyes, this figure can be about 4%. Due to large-scale production and extensive application, synthetic dyes can cause considerable environmental pollution and are serious health-risk factors. The growing concern of environmental protection has influenced industrial development promoting the development of ecofriendly technologies (Desphande, 2001), reduced consumption of freshwater and lowers output of wastewater (Knittel and Schollmeyer, 1996 and Petek and Glavic, 1996), etc. However, the release of important amounts of synthetic dyes to the environment has posed challenges to environmental scientists apart from increased public concern and legislation problems.

Due to the commercial importance of dyes, their impact (Guaratini and Zanoni, 2000) and toxicity (Walthall and Stark, 1999 and Tsuda et al., 2001) when released in the environment have been extensively studied during the last decade (Hunger, 1995 and Calin and Miron, 1995). Traditional wastewater treatment technologies have proven to be markedly ineffective for handling wastewater of synthetic textile dyes because of the chemical stability of these pollutants. Thus, it has been verified that, of the 18 azo dyes studied 11 compounds passed through the activated sludge process practically untreated, 4 (Acid Blue 113, Acid Red 151, Direct Violet 9, and Direct Violet 28) were adsorbed on the waste activated sludge and only 3 (Acid Orange 7, Acid Orange 8, and Acid Red 88) were biodegraded (Shaul et al., 1991).

Table 5 : Estimation degree of fixation for different dye-fibre combination and loss to effluent

Dye application class
Fibre
Degree of fixation (%)
Loss of effluent (%)
Acid Polymide 89 – 95 5 - 20
Basic Acrylic 95 – 100 0 - 5
Direct Cellulose 70 –95 5 - 30
Disperse Polyester 90 – 100 0 - 10
Metal - complex Wool 90 – 98 2 - 10
Reactive Cellulose 50 – 90 10 -50
Sulphur Cellulose 60 – 90 10 - 40
Vat Cellulose 80 – 95 5 - 20

 

 

1.5 TOXICOLOGICAL ASPECTS OF METALS AND DYES


1.5.1 Toxicological Aspects of Heavy metals

Due to their mobility in aquatic ecosystems and their toxicity to higher life forms, heavy metals in surface and groundwater supplies have been prioritised as major inorganic contaminants in the environment. Even if they are present in dilute, undetectable quantities, their recalcitrance and consequent persistence in water bodies imply that through natural processes such as biomagnification, concentrations may become elevated to such an extent that they begin exhibiting toxic characteristics. These metals can either be detected in their elemental state, which implies that they are not subject to further biodegradative processes or bound in various salt complexes. In either instance, metal ions cannot be mineralized. Apart from environmental issues, technological aspects of metal recovery from industrial waters must also be considered (Wyatt, 1988).

 

1.5.1.1 EFFECTS OF HEAVY METALS ON HUMAN HEALTH

The heavy metals hazardous to humans include lead, mercury, cadmium, arsenic, copper, zinc, and chromium. Such metals are found naturally in the soil in trace amounts, which pose few problems. When concentrated in particular areas, however, they present a serious danger. Arsenic and cadmium, for instance, can cause cancer. Mercury can cause mutations and genetic damage, while copper, lead, and mercury can cause brain and bone damage. Next section presents the harmful effects to the four heavy metals that are prevalent in the environment.

Chromium : Humans are exposed to chromium through breathing, eating or drinking and through skin contact with chromium or chromium compounds. The level of chromium in air and water is generally low. In drinking water the level of chromium is usually low as well, but contaminated well water may contain the dangerous chromium (VI); hexavalent chromium. For most people eating food that contains chromium (III), it is the main route of chromium uptake, as chromium (III) occurs naturally in many vegetables, fruits, meats, yeasts and grains. Various ways of food preparation and storage may alter the chromium contents of food, as in the case of food stored in steel tanks or cans leading to enhanced chromium concentrations.Chromium (VI) is a danger to human health, mainly for people who work in the steel and textile industry. Chromium (VI) is known to cause various health effects. When it is a compound in leather products, it can cause allergic reactions, such as skin rash. Inhaling chromium (VI) can cause nose irritations and nosebleeds.

Other health problems that are caused by chromium (VI) are skin rashes, respiratory problems, weakened immune systems, kidney and liver damage, alteration of genetic material, lung cancer and death. The health hazards associated with exposure to chromium are dependent on its oxidation state. The metal form (chromium as it exists in this product) is of low toxicity. The hexavalent form is toxic. Adverse effects of the hexavalent form on the skin may include ulcerations, dermatitis, and allergic skin reactions. Inhalation of hexavalent chromium compounds can result in ulceration and perforation of the mucous membranes of the nasal septum, irritation of the pharynx and larynx, asthmatic bronchitis, bronchospasms and edema. Respiratory symptoms may include coughing and wheezing, shortness of breath, and nasal itch.
Carcinogenicity- Chromium and most trivalent chromium compounds have been listed by the National Toxicology Program (NTP) as having inadequate evidence for carcinogenicity in experimental animals. According to NTP, there is sufficient evidence for carcinogenicity in experimental animals for the following hexavalent chromium compounds; calcium chromate, chromium trioxide, lead chromate, strontium chromate, and zinc chromate.

Mercury : Mercury is generally considered to be one of the most toxic metals found in the environment (Serpone et al., 1988). Once mercury enters the food chain, progressively larger accumulation of mercury compounds takes place in humans and animals. The major sources of mercury pollution in environment are industries like chlor-alkali, paints, pulp and paper, oil refining, rubber processing and fertilizer (Namasivayam and Periasamy, 1993), batteries, thermometers, fluorescent light tubes and high intensity street lamps, pesticides, cosmetics and pharmaceuticals (Krishnan and Anirudhan, 2002).

Methyl mercury causes deformities in the offspring, mainly affecting the nervous system (teratogenic effects). Children suffer from mental retardation, cerebral palsy and convulsions. Mercury also brings about genetic defects causing chromosome breaking and interference in cell division, resulting in abnormal distribution of chromosome. Mercury causes impairment of pulmonary function and kidney, chest pain and dyspnoea (Beglund and Bertin, 2002; WHO, 1990). The harmful effect of methyl mercury on aquatic life and humans was amply brought out by the Minamata episode in Japan (WHO, 1991).

Nickel : Electroplating is one important process involved in surface finishing and metal deposition for better life of articles and for decoration. Although several metals can be used for electroplating, nickel, copper, zinc and chromium are the most commonly used metals, the choice depending upon the specific requirement of the articles. During washing of the electroplating tanks, considerable amounts of the metal ions find their way into the effluent. Ni (II) is present in the effluents of silver refineries, electroplating, zinc base casting and storage battery industries (Sitting, 1976).

Higher concentrations of nickel cause cancer of lungs, nose and bone. Dermatitis (Ni itch) is the most frequent effect of exposure to Ni, such as coins and jewellery. Acute poisoning of Ni (II) causes headache, dizziness, nausea and vomiting, chest pain, tightness of the chest, dry cough and shortness of breath, rapid respiration, cyanosis and extreme weakness (Al-Asheh and Duvnjak 1997; Kadirvelu, 1998; Beliles1979).

Iron : Iron exists in two forms, soluble ferrous iron (Fe2+) and insoluble ferric particulate iron (Fe3+). The presence of iron in natural water may be attributed to the dissolution of rocks and minerals, acid mine drainage, landfill leachate sewage or engineering industries. Iron in water is generally present in the ferric state. The concentration of iron in well aerated water is seldom high but under reducing conditions, which may exist in some groundwater, lakes or reservoirs and in the absence of sulphate and carbonate, high concentrations of soluble ferrous iron may be found. The presence of iron at concentrations above 0.1mg/l will damage the gills of the fish. The free radicals are extremely reactive and short lived.  The free radicals formed by the iron on the surface of the gills will cause oxidation of the surrounding tissue and this will lead to massive destruction of gill tissue and anaemia.The presence of iron in drinking water supplies is objectionable for a number of reasons. Under the pH condition existing in drinking water supply, ferrous sulphate is unstable and precipitates as insoluble ferric hydroxide, which settles out as a rust coloured silt. Such water often tastes unpalatable even at low concentration (0.3 mg/L) and stains laundry and plumbing fixtures. Iron is an essential element in human nutrition. It is contained in a number of biologically significant proteins, but ingestion in large quantities results in haemochromatosis where in tissue damage results from iron accumulation.

1.5.1.2 EFFECTS OF HEAVY METALS ON AQUATIC ORGANISMS

Aquatic organisms are adversely affected by heavy metals in the environment. The toxicity is largely a function of the water chemistry and sediment composition in the surface water system (Ahalya, et al., 2005).

The above illustration (Source: Volesky, 2005) shows how metal ions can become bioaccumulated in an aquatic ecosystem. The metals are mineralised by microorganisms, which in turn are taken up by plankton and further by the aquatic organisms. Finally, the metals by now, several times biomagnified is taken up by man when he consumes fish from the contaminated water.

  1. Slightly elevated metal levels in natural waters may cause the following sublethal effects in aquatic organisms: histological or morphological change in tissues;
  2. changes in physiology, such as suppression of growth and development, poor swimming performance, changes in circulation;
  3. change in biochemistry, such as enzyme activity and blood chemistry;
  4. change in behaviour; and
  5. and changes in reproduction (Connell et al., 1984).

Many organisms are able to regulate the metal concentrations in their tissues. Fish and crustacea can excrete essential metals, such as copper, zinc, and iron that are present in excess. Some can also excrete non-essential metals, such as mercury and cadmium, although this is usually met with less success (Connell et al., 1984).

Research has shown that aquatic plants and bivalves are not able to successfully regulate metal uptake (Connell et al., 1984). Thus, bivalves tend to suffer from metal accumulation in polluted environments. In estuarine systems, bivalves often serve as biomonitor organisms in areas of suspected pollution (Kennish, 1992). Shellfishing waters are closed if metal levels make shellfish unfit for human consumption.

In comparison to freshwater fish and invertebrates, aquatic plants are equally or less sensitive to cadmium, copper, lead, mercury, nickel, and zinc. Thus, the water resource should be managed for the protection of fish and invertebrates, in order to ensure aquatic plant survivability (USEPA, 1987). Metal uptake rates will vary according to the organism and the metal in question. Phytoplankton and zooplankton often assimilate available metals quickly because of their high surface area to volume ratio. The ability of fish and invertebrates to adsorb metals is largely dependent on the physical and chemical characteristics of the metal (Kennish, 1992). With the exception of mercury, little metal bioaccumulation has been observed in aquatic organisms (Kennish, 1992). Metals may enter the systems of aquatic organisms via three main pathways:

  1. Free metal ions that are absorbed through respiratory surface (e.g., gills) are readily diffused into the blood stream.
  2. Free metal ions that are adsorbed onto body surfaces are passively diffused into the blood stream.

Metals that are sorbed onto food and particulates may be ingested, as well as free ions ingested with water (Connell et al., 1984). For eg: Chromium is not known to accumulate in the bodies of fish, but high concentrations of chromium, due to the disposal of metal products in surface waters, can damage the gills of fish that swim near the point of disposal.

1.5.1.3 IRRIGATION EFFECTS OF HEAVY METALS

Irrigation water contaminated with sewage or industrial effluents may transport dissolved heavy metals to agricultural fields. Although most heavy metals do not pose a threat to humans through crop consumption, cadmium may be incorporated into plant tissue. Accumulation usually occurs in plant roots, but may also occur throughout the plant (De Voogt et al., 1980). Most irrigation systems are designed to allow for up to 30 percent of the water applied to not be absorbed and to leave the field as return flow. Return flow either joins the groundwater or runs off the field surface (tailwater). Sometimes tailwater are rerouted into streams because of downstream water rights or a necessity to maintain streamflow. However, usually the tailwater is collected and stored until it can be reused or delivered to another field (USEPA 1993a).

Tailwater is often stored in small lakes or reservoirs, where heavy metals can accumulate as return flow is pumped in and out. These metals can adversely impact aquatic communities. An extreme example of this is the Kesterson Reservoir in the San Joaquin Valley, California, which received subsurface agricultural drainwater containing high levels of selenium and salts that had been leached from the soil during irrigation. Studies in the Kesterson Reservoir revealed elevated levels of selenium in water, sediments, terrestrial and aquatic vegetation, and aquatic insects. The elevated levels of selenium were cited as relating to the low reproductive success, high mortality, and developmental abnormalities in embryos and chicks of nesting aquatic birds (Schuler et al. 1990).

 
1.5.2 Toxicological aspects of dyes

Dyeing industry effluents are one of the most problematic wastewaters to be treated not only for their high chemical oxygen demand, but also for high biological oxygen demand, suspended solids, turbidity, toxic constituents but also for colour, which is the first contaminant discernible by the human eye. Dyes may affect the photosynthetic activity in aquatic life due to reduced light penetration and may also be toxic to some aquatic life due to the presence of aromatics, metals, etc. in them (Clarke and Anliker 1980; Zollinger 1987; Mishra and Tripathy 1993; Banat et al1996; Fu and Viraraghvan 2001; Robinson et al2001).

Dyes usually have a synthetic origin and complex aromatic molecular structure, which make them more stable and more difficult to biodegrade. Dyes are classified as follows: anionic – direct, acid and reactive dyes; cationic – basic dyes; non-ionic – disperse dyes (Mishra and Tripathy 1993; Fu and Viraraghvan 2001). The chromophores in anionic and non-ionic dyes are mostly azo groups or anthroquinone types. The reductive cleavage of azo linkages is responsible for the formation of toxic amines in the effluent. Anthraquinone based dyes are more resistant to degradation due to their fused aromatic structures and thus remain coloured in the wastewater. Reactive dyes are typically azo-based chromophore combined with different types of reactive groups e.g, vinyl sulphone, chlorotriazine, trichloropyrimidine, difluorochloropyrimidine. They differ from all other dyes in that they bind to textile fibers like cotton to form covalent bonds. They are used extensively in textile industries regarding favourable characteristics of bright colour, water fast, simple application techniques with low energy consumption.Water soluble reactive and acid dyes are problematic; as they pass through the conventional treatment system unaffected, posing problems. Hence, their removal is also of great importance (Robinson et al 2001; Hu 1992; Juang et al 1997; Karcher et al1999; Sumathi and Manju 2000; Aksu and Tezer et al 2000; O’Mahony et al 2002; Moran et al 1997).

Basic dyes have high brilliance and intensity of colours and are highly visible even in very low concentration (Clarke and Anliker, 1980; Banat et al., 1996; Fu and Viraraghavan, 2001; Mittal and Gupta, 1996; Chu and Chen, 2002; Fu and Viraraghavan, 2002) Metal complex dyes are mostly chromium based, which is carcinogenic (Clarke and Anliker, 1980; Banat et al., 1996; Mishra and Tripathy 1993; Gupta et al., 1990). Disperse dyes do not ionize in an aqueous medium and some disperse dyes have also been shown to have a tendency to bioaccumulate (Banat et al., 1996). Due to the chemical stability and low biodegradability of these, conventional biological wastewater treatment systems are inefficient in treating dye wastewater.

Dyes have generated much concern regarding its use, due to its toxic effects. It has been reported to cause carcinogenesis, mutagenesis, chromosomal fractures, teratogenecity and respiratory toxicity. McGeorge et al. (1985) reported the mutagenic activity of textile wastewater effluents, using the salmonella/microsome assay and contributed the highest percentage (67%) of mutagenic effluents. Costan et al. (1993) found that a textile effluent ranked second in toxicity, among eight industrial sectors represented, by using a series of bioassays assessing the acute, sublethal and chronic toxicity at various trophic levels.    

Estimation of LC50 values of many commercial dyes at different time intervals on fish was done earlier by Clarke and Anliker 1980. Srivastava et al. (1995a) also observed changes in LC50 values of malachite green in a fresh water catfish. Gambusia affinis was used to find the LC50 value for acid red 73 and showed higher toxicity (Muthukumar et al., 2005). Over 90% of some 4000 dyes tested in an ETAD (Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry) Survey had LD50 values greater than 2 X 103 mg/kg. The highest rates of toxicity were found amongst basic and diazo direct dyes (Shore, 1996).

Sub – chronic exposure (13 week) to benzidine – based dyes resulted in hepatocellular carcinomas and hepatic neoplastic nodules in rats (National Cancer Institute 1978) and carcinomas in very short duration (National Institute for Occupational Safety, 1980). Histopathological changes in the testes of textile wastewater exposed rats (sub – chronic) included a reduction in the number of germ and Leydig cells, resulting in impaired spermatogenesis (Mathur, et al.  2003).

Umbuzeiro et al. (2005) analysed the mutagenic activity of dyes in environmental samples of the Cristais River, Sao Paulo, Brazil. A low level mutagencity of textile/dye industries in the underground water of Sanganer, Jaipur (India) were also investigated (Mathur et al2005).  A number of studies have demonstrated mutagenic activity in effluents from textile and dye- related industries (Mcgeorge, et al. 1985; Sanchez, et al., 1988; Wells, et al. 1994).

 

1.6 NEED FOR THE REMOVAL OF DYES AND HEAVY METALS

Continuous discharge of industrial, domestic and agricultural wastes in rivers and lakes causes deposit of pollutants in sediments. Such pollutants include heavy metals, which endanger public health after being incorporated in food chain. Heavy metals cannot be destroyed through biological degradation, as is the case with most organic pollutants. Incidence of heavy metal accumulation in fish, oysters, mussels, sediments and other components of aquatic ecosystems have been reported from all over the world (Naimo, 1995; Sayler et al., 1975, Ahalya et al., 2005).

Excessive amounts of some heavy metals can be toxic through direct action of the metal or through their inorganic salts or via organic compounds from which the metal can become easily detached or introduced into the cell. Exposure to different metals may occur in common circumstances, particularly in industrial setting. Accidents in some environments can result in acute, high level exposure. Some of the heavy metals are toxic to aquatic organisms even at low concentration. The problem of heavy metal pollution in water and aquatic organisms including fish, needs continuous monitoring and surveillance as these elements do not degrade and tend to biomagnify in man through food chain. Hence, there is a need to remove the heavy metals from the aquatic ecosystems.

India produces 64,000 tonnes of dyes, 2 per cent of which - 7,040 tonnes - are directly discharged into the environment. With the Indian dyestuff industry growing by over 50 per cent during the last decade, India is now the second largest producer of dyes and intermediaries in Asia. The CPCB puts their number at 900 units. The production is estimated to be around 60,000 tonnes or about 6.6 per cent of the world production. There are around 700 varieties of dyes and dye intermediaries produced in India. In India only a third of the dyestuff producing industries are in organised sector. The rest come from the unregulated small-scale sector, which produces more than half of India's aggregate volumes. Located mainly in Gujarat and Maharashtra, this sector pays no heed to environmental concerns. The domestic textile industry, which consumes up to 80 per cent of the dyestuffs produced, looks for manageable costs rather than consistent quality. So the bulk of its demand for dyes is met by the small- scale sector. The small-scale sector's substantially lower investment in pollution control measures also makes it more economical.

Dyes and colour pigments also contain metals such as copper, nickel, chromium, mercury and cobalt. Metals are difficult to remove from wastewater and may escape the capacities of the effluent treatment system. Moreover, the unused dyes and colour released in effluent from dyeing vats, interferes with the transmission of light in the water bodies that receives the effluent.

This in turn inhibits the photosynthesis activity of aquatic biota besides direct toxic effects on biota. Several textile and food dyes have been linked to carcinogenicity, such as dye intermediaries like benzidines. Hence the ubiquitous colour needs to be regulated. The new drinking water standards prescribed by the Bureau of Indian Standards (IS 10500) set colour standards at five colour units as the desirable limit and 25 colour units as the permissible limit in the absence of alternate source. But removing the colour from effluents is extremely difficult. There is no universally applicable technique for all conditions.

Research and development, therefore focuses on sector-specific methods and technologies to remove colour and heavy metals from different kinds of waste streams. In view of the above toxicological effects of dyes and heavy metals on environment, animals and human beings, it becomes imperative to treat these toxic compounds in wastewater effluents before they are discharged into freshwater bodies.

 

1.7 CONVENTIONAL METHODS FOR THE TREATMENT OF METALS

Over the last few decades, several methods have been devised for the treatment and removal of heavy metals. Numerous industries (e.g., electroplating, metal finishing operations, electronic –circuit production, steel and non-ferrous processes and fine-chemical and pharmaceutical production) discharge a variety of toxic metals into the environment.
For several years now, it is mandatory that industry is required to remove metal pollutants from liquid discharges. The commonly used procedures for removing metal ions from aqueous streams include chemical precipitation, lime coagulation, ion exchange, reverse osmosis and solvent extraction (Rich and Cherry, 1987, Ahalya et al., 2005, 2006).  The process description of each method is presented below.

1.7.1 Chemical precipitation :

Precipitation of metals is achieved by the addition of coagulants such as alum, lime, iron salts and other organic polymers. The large amount of sludge containing toxic compounds produced during the process is the main disadvantage.

1.7.2 Chemical reduction :

Reduction of hexavalent chromium can also be accomplished with electro-chemical units. The electrochemical chromium reduction process uses consumable iron electrodes and an electric current to generate ferrous ions that react with hexavalent chromium to give trivalent chromium as follows (USEPA, 1979)


Another application of reduction process is the use of sodium borohydride, which has been considered effective for the removal of mercury, cadmium, lead, silver and gold (Kiff, 1987).

1.7.3 Xanthate process :

Insoluble starch xanthate (ISX) is made from commercial cross linked starch by reacting it with sodium hydroxide and carbon disulphide. To give the product stability and to improve the sludge settling rate, magnesium sulphate is also added. ISX works like an ion exchanger, removing the heavy metals from the wastewater and replacing them with sodium and magnesium. Average capacity is 1.1-1.5 meq of metal ion per gram of ISX (Anon, 1978).

ISX is most commonly used by adding to it the wastewater as slurry for continuous flow operations or in the solid form for batch treatments. It should be added to the effluent at pH ≥ 3. Then the pH should be allowed to rise above 7 for optimum metal removal (Wing, 1978). Residual metal ion level below 50 μg/L has been reported (Hanway et al., 1978, Wing et al., 1978). The effectiveness of soluble starch xanthate (SSX) for removal of Cd (II), Cr (VI) and Cu (II) and insoluble starch xanthate (ISX) for Cr (VI) and Cu (II) have been evaluated under different aqueous phase conditions. Insoluble starch xanthate had better binding capacity for metals. The binding capacity of SSX and ISX respectively for different metal ions follows the sequence of Cr (VI)> Cu (II)> Cd(II) and Cr (VI)> Cu (II) (Tare et al., 1988).

1.7.4 Solvent extraction :

Liquid-liquid extraction (also frequently referred as solvent extraction) of metals from solutions on a large scale has experienced a phenomenal growth in recent years due to the introduction of selective complexing agents (Beszedits, 1988). In addition to hydrometallurgical applications, solvent extraction has gained widespread usage for waste reprocessing and effluent treatment.

Solvent extraction involves an organic and an aqueous phase. The aqueous solution containing the metal or metals of interest is mixed with the appropriate organic solvent and the metal passes into the organic phase. In order to recover the extracted metal, the organic solvent is contacted with an aqueous solution whose composition is such that the metal is stripped from the organic phase and is reextracted into the stripping solution. The concentration of the metal in the strip liquor may be increased, often 110 to 100 times over that of the original feed solution. Once the metal of interest has been removed, the organic solvent is recycled either directly or after a fraction of it has been treated to remove the impurities.

1.7.5 Membrane process :

Important examples of membrane process applicable to inorganic wastewater treatment include reverse osmosis and eletrodialysis (EPA, 1980). These processes involve ionic concentration by the use of selective membrane with a specific driving force. For reverse osmosis, pressure difference is employed to initiate the transport of solvent across a semipermeable membrane and electro dialysis relies on ion migration through selective permeable membranes in response to a current applied to electrodes. The application of the membrane process described is limited due to pretreatment requirements, primarily, for the removal of suspended solids. The methods are expensive and sophisticated, requiring a higher level of technical expertise to operate.

A liquid membrane is a thin film that selectively permits the passage of a specific constituent from a mixture (Beszedits, 1988). Unlike solid membranes, however liquid membranes separate by chemistry rather than size, and thus in many ways liquid membrane technology is similar to solvent extraction.

Since liquid membrane technology is a fairly recent development, a number of problems remain to be solved. A major issue with the use of supported membranes is the long term stability of the membranes, whereas the efficient breakup of microspheres for product recovery is one of the difficulties encountered frequently with emulsion membranes.

1.7.6 Evaporators :

In the electroplating industry, evaporators are used chiefly to concentrate and recover valuable plating chemicals. Recovery is accomplished by boiling sufficient water from the collected rinse stream to allow the concentrate to be returned to the plating bath. Many of the evaporators in use also permit the recovery of the condensed steam for recycle as rinse water. Four types of evaporators are used throughout the elctroplating industry (USEPA, 1979a) (I) Rising film evaporators; (ii) Flash evaporators using waste heat; (iii) submerged tube evaporators; (iv) Atmospheric evaporators.

Both capital and operational costs for evaporative recovery systems are high. Chemical and water reuse values must offset these costs for evaporative recovery to become economically feasible.

1.7.7 Cementation :

Cementation is the displacement of a metal from solution by a metal higher in the electromotive series. It offers an attractive possibility for treating any wastewater containing reducible metallic ions. In practice, a considerable spread in the electromotive force between metals is necessary to ensure adequate cementation capability. Due to its low cost and ready availability, scrap iron is the metal used often. Cementation is especially suitable for small wastewater flow because a long contact time is required. Some common examples of cementation in wastewater treatment include the precipitation of copper from printed etching solutions and the reduction of Cr (VI) in chromium plating and chromate-inhibited cooling water discharges (Case, 1974). Removal and recovery of lead ion by cementation on iron sphere packed bed has been reported (Angelidis et al., 1988, 1989). Lead was replaced by a less toxic metal in a harmless and reusable form.

1.7.8 Ion exchange :

Ion exchange resins are available selectively for certain metal ions. The cations are exchanged for H+ or Na+. The cation exchange resins are mostly synthetic polymers containing an active ion group such as SO3H. The natural materials such as zeolites can be used as ion exchange media (Van der Heen, 1977). The modified zeolites like zeocarb and chalcarb have greater affinity for metals like Ni and Pb (Groffman et al., 1992). The limitations on the use of ion exchange for inorganic effluent treatment are primarily high cost and the requirements for appropriate pretreatment systems. Ion exchange is capable of providing metal ion concentrations to parts per million levels. However, in the presence of large quantities of competing mono-and divalent ions such as Na and Ca, ion exchange is almost totally ineffective.

1.7.9 Electrodeposition :

Some metals found in waste solution can be recovered by electrodeposition using insoluble anodes. For example, spent solutions resulting from sulphuric acid cleaning of Cu may be saturated with copper sulphate in the presence of residual acid. These are ideal for electro-winning where the high quality cathode copper can be electrolytically deposited while free sulphuric acid is regenerated.

1.7.10 Adsorption :

Since activated carbon also possesses an affinity for heavy metals, considerable attention has been focussed on the use of carbon for the adsorption of hexavalent chromium, complexed cyanides and metals present in various other forms from wastewaters. Watonabe and Ogawa first presented the use of activated carbon for the adsorption of heavy metals in 1929.

The mechanism of removal of hexavalent and trivalent chromium from synthetic solutions and electroplating effluents has been extensively studied by a number of researchers. According to some investigators, the removal of Cr (VI) occurs through several steps of interfacial reactions (Huang and Bowers, 1979).

Adsorption of Cr (III) and Cr (VI) on activated carbon from aqueous solutions has been studied (Toledo, 1994). Granular activated carbon columns have been used to treat wastewaters containing lead and cadmium (Reed and Arunachalam, 1994, Reed et al., 1994). Granular activated carbon was used for the removal of Pb (II) from aqueous solutions (Cheng et al., 1993). The adsorption process was inhibited by the presence of humic acid, iron (III), aluminum (III) and calcium (II).

 

1.8 DISADVANTAGES OF CONVENTIONAL METHODS FOR TREATMENT OF WASTEWATER CONTAINING HEAVY METALS

Metals are a class of pollutants, often toxic and dangerous, widely present in industrial and household wastewaters. Electroplating and metal finishing operations, electronic circuit production, steel and aluminum processes to name but a few industries, produce large quantities of wastewater containing metals. Although metal precipitation using a cheap alkali such as lime (calcium hydroxide) has been the most favoured option, other separation technologies are now beginning to find favour. Precipitation, by adjusting the pH value is not selective and any iron (ferric ion) present in the liquid effluent will be precipitated initially followed by other metals. Consequently precipitation produces large quantities of solid sludge for disposal, for example precipitation as hydroxides of 100 mg/l of copper (II), cadmium (II) or mercury (II) produces as much as 10-, 9- and 5 fold mg/l of sludges respectively. The metal hydroxide sludge resulting from treatment of electroplating wastewater has been classified as a hazardous waste.

The performance characteristics of heavy metal waste water treatment technologies are identified in Table 6. The versatility, simplicity and other technology characteristics will contribute to the overall process costs, both capital and operational. At present many of these technologies such as ion exchange represent significant capital investments by industry.

Table 6 : Performance characteristics of various heavy metal removal /recovery technologies

Technology
pH change
Metal selectivity
Influence of
Suspended solids
Tolerance of
organic molecules
Working level for
appropriate metal (mg/I)
Adsorption, e.g.
Granulated
Activated carbon		 Limited tolerance Moderate Fouled Can be poisoned <10
Electro
chemical		 Tolerant Moderate Can be engineered
to tolerate		 Can be accommodated >10
Ion exchange Limited tolerance Chelate - resins can
be selective		 Fouled Can be poisoned <100
Membrane Limited tolerance Moderate Fouled Intolerant >10
Precipitation
(a) Hydroxide Tolerant Non-selective Tolerant Tolerant >10
(b) Sulphide Limited tolerance Limited selective
pH dependent		 Tolerant Tolerant >10
Solvent extraction Some systems Metal selective Fouled Intolerant >100
  pH tolerant extractants available      

As seen from the table above, conventional methods are ineffective in the removal of low concentrations of heavy metals and they are non-selective. Moreover, it is not possible to recover the heavy metals by the above mentioned methods.

 

1.9 CONVENTIONAL METHODS FOR TREATMENT OF WASTEWATER CONTAINING DYES

Synthetic dyes often receive considerable attention from researchers interested in textile wastewater effluents treatment processes. As discharge standards are becoming more stringent, the development of technological systems for minimizing concentration of dyes and their break down products in wastewater are nowadays necessary. The following are generally used for the removal of colour from wastewaters.

1.9.1 Physicochemical methods for dye removal :

Adsorptive bubble separation techniques (ion flotation, solvent sublation and adsorbing colloid flotation) resulted in the efficient removal (99%) of Direct Blue from wastewater (Horng and Huang, 1993). The application of coagulation processes for the removal of dyes from wastewater has also been assessed. The efficiencies dependent on the type of flocculant and on the pH of the medium (Koprivanac et al., 1993). Electrocoagulation was used for the effective removal of Acilan Blue from the wastewater of an operating textile plant in a bipolar packed-bed electrochemical reactor (Ogutveren et al., 1992).

1.9.2 Photocatalytic decolourisation and oxidation of synthetic dyes :

Commercial dyes are designed to resist photodegradation, so the selection of optimal photocatalytic conditions for the decolourisation of dyes requires considerable expertise. Due to the significant commercial and environmental interest the efficacy of a large number of catalysts and irradiation conditions has been established for the decolourisation of various synthetic dyes.

1.9.2.1 PHOTOCATALYSIS AND OXIDATION WITH HYDROGEN PEROXIDE :

Hydrogen peroxide has been frequently applied to the decolourisation of synthetic dyes in waters. Hydrogen peroxide can effectively decolourize dye wastewaters in the presence of Fe (II) sulphate, with the higher rates of decolourisation at higher concentrations of the reagents (Kuo, 1992). Iron (III) with hydrogen peroxide was successfully employed for the degradation of the dye intermediate anthraquinone-2-sulphonic acid sodium salt (Kiwi et al., 1993). The results indicated that the method could be successfully used for the decolourisation of acid dyes, direct dyes, basic dyes and reactive dyes but it proved to be inadequate for vat dyes and disperse dyes (Yang et al., 1998).

1.9.2.2 OZONATION :

Ozonation, as an effective oxidation process, has found application in the decolourisation of synthetic dyes. The technique employed in the decolouration of Orange II. Oxalate, formate and benzene sulphonate ions were the most important decomposition products (Tang and An, 1995a and Tang and An, 1995b). It was reported that ozone effectively decomposed azo dyes in textile wastewater. The decomposition rate was considerably higher at acidic pH. However, the influence of temperature and UV irradiation on the decomposition rate was negligible (Koyuncu and Afsar, 1996). The negligible influence of UV irradiation on the decomposition rate of azo dyes by ozone has been supported by other authors. The effect of chemical structure on the decomposition rate has been demonstrated (Davis et al., 1994).

1.9.2.3 OTHER OXIDIZING SYSTEMS :

The photodecomposition of five dyes (Reactive Red 2, Reactive Blue 4, Reactive Black 8, Basic Red 13 and Basic Yellow 2) under UV irradiation in the presence of trivalent iron-oxalato complexes was also reported (Nansheng et al., 1997a). It has been established that the rate of photodegradation is highly dependent on the chemical structure of the dye.

1.9.2.4 MEMBRANE PROCESSES :

Membrane filtration is used by many process industries for product purification. Water entering the membrane is called feed water and the water passing through the membrane is called permeate, treated or product water. Membrane technologies have the potential either to remove the dyestuff or allow reuse of the auxillary chemicals used for dyeing or to concentrate the dyestuffs and auxiliaries and produce purified water.
This method has the ability to clarify, concentrate and most importantly to separate dye continuously from effluent (Mishra and Tripathy, 1993; Xu and Lebrun, 1999). It has some special features unrivalled by other methods; resistance to temperature, adverse chemical environments and microbial attack. The concentrated residue left after separation poses disposal problems, high capital cost, possibility of clogging and membrane replacements are its disadvantages.

(i) Ultrafiltration : Ultrafiltration has many good points such as the recovery of dyes and water or the possibility of reusing them. Membrane transport properties are influenced by casting parameters and membrane thickness. The coefficient of dye separation increases when the time of solvent evaporation increases and the temperature of the casting solution decreases. Membranes prepared from casting solutions of an initial temperature of 318K at a solvent evaporation time of 60 s yield a dye separation between 95 and 100 per cent irrespective of the pressures and dye concentrations applied. Polysulphone membranes 90 to 100 μm thick exhibit the best transport properties as reported in the literature (Pawlowski, 1982).

(ii) Reverse osmosis : In the water treatment industry, reverse osmosis is sometimes referred to as hyperfiltration, is a process in which water is forced through a semipermeable membrane. Reverse osmosis is suitable for removing ions and larger species from dye bath effluents (Marcucci, et al. 2001). Majority of commercial reverse osmosis plants are used for the desalination of seawater and brackish water, while the number of reverse osmosis plants treating municipal and industrial wastewater for reuse is still limited (Mavrov, et al. 2001; Abdel – Jawad, and Al- Sulaimi, 2002; Durham and Walton, 1999).

(iii) Nanofiltration : Nanofiltration is a process of separation with membrane and performance characteristics between reverse osmosis and ultrafiltration. Nanofiltration membranes present an asymmetric structure, which consists of a filtering skin supported by a sub-layer of high porosity with thickness varying from 100 to 300 μm. Studies by Stoyko and Pencho (Stoyko and Pencho, 2003) on the purification of water contaminated with reactive dye, using nanofiltration, considered a dye retention of 85 – 90 % and a permeate flux of 30 – 45 L/h. m2, showed satisfactory for the reuse of the water.  Colour and COD retention present in textile industry were reported and the results showed that the colour retention were around 99 % for the DK 1073 (Lopes, et al. 2005).

1.9.3. Microbiological decomposition of synthetic dyes

The application of microorganisms for the biodegradation of synthetic dyes is an attractive and simple method by operation. However, the biological mechanisms can be complex. Large number of species has been tested for decolouration and mineralisation of various dyes. The use of microorganisms for the removal of synthetic dyes from industrial effluents offers considerable advantages. The process is relatively inexpensive, the running costs are low and the end products of complete mineralisation are not toxic. The various aspects of the microbiological decomposition of synthetic dyes have been reviewed by Stolz (2001). Besides the traditional wastewater cleaning technologies, other methods have been employed in the microbial decolourisation of dyes.
The application of microorganisms for the biodegradation of synthetic dyes is an attractive and simple method. Unfortunately, the majority of dyes are chemically stable and resistant to microbiological attack. The isolation of new strains or the adaptation of existing ones to the decomposition of dyes will probably increase the efficacy of microbiological degradation of dyes in the near future.

1.9.4 Enzymatic decomposition of synthetic dyes

The character of enzymes and enzyme systems in microorganisms that are suitable for the decomposition of dyes has been extensively investigated. Effort has been devoted to the separation, isolation and testing of these enzymes. Exact knowledge of the enzymatic processes governing the decomposition of dyes is important in the environmental protection both from theoretical and practical points of view.

Lignin peroxidase isoenzymes were isolated from P. chrysosporium and purified by chromatofocusing. The activity of isoenzymes towards decolouring triphenylmethane dyes, heterocyclic dyes, azo dyes and polymer dyes was compared with that of a crude enzyme preparation. Optimum pH values for the decolourisation of dyes by various isozymes were markedly different. According to the results, the decomposition capacity of crude enzyme preparation and purified isoenzymes showed marked differences while variations in the structure of dyes exerted slight influence (Ollikka et al., 1993). Horseradish peroxidase has been successfully employed for the decomposition and the precipitation of azo dyes. The degradation rate was dependent on the pH (Bhunia et al., 2001). Another study revealed that the enzymes of white rot fungus degraded Crystal Violet via N-demethylation (Bumpus et al., 1991). Interestingly, lignin peroxidase from B. adusta showed very low degradation capacity towards azo dyes and phthalocyanine dyes. However, veratryl alcohol considerably increased the decomposition rate (Heinfling et al., 1998). Similar investigations proved that pure laccase was also unable to decolourize Remazol Brilliant Blue R but the decolouration rate was facilitated by the presence of a mediator (violuric acid) (Soares et al., 2001).

The employment of enzyme preparations shows considerable benefits over the direct use of microorganisms. Commercial enzyme preparations can be easily standardized, facilitating accurate dosage. The application is simple and can be rapidly modified according to the character of the dye or dyes to be removed. But the cost of such enzyme preparations is quite high.

1.9.5 Adsorption :

Adsorption techniques employing solid sorbents are widely used to remove certain classes of chemical pollutants from waters, especially those that are practically unaffected by conventional biological wastewater treatments. However, amongst all the sorbent materials proposed, activated carbon is the most popular for the removal of pollutants from wastewater (Babel and Kurniawan, 2003, Derbyshire et al., 2001 and Ramakrishna and Viraraghavan, 1997). In particular, the effectiveness of adsorption on commercial activated carbons (CAC) for removal of a wide variety of dyes from wastewaters has made it an ideal alternative to other expensive treatment options (Ramakrishna and Viraraghavan, 1997). Table 7 shows a non-exhaustive list of examples of CAC used in wastewater treatment. Because of their great capacity to adsorb dyes, CAC are the most effective adsorbents. This capacity is mainly due to their structural characteristics and their porous texture, which gives them a large surface area, and their chemical nature which can be easily modified by chemical treatment in order to increase their properties. However, activated carbon presents several disadvantages (Babel and Kurniawan, 2003). It is quite expensive, the higher the quality, the greater the cost, non-selective and ineffective against disperse and vat dyes. The regeneration of saturated carbon is also expensive, not straightforward, and results in loss of the adsorbent. The use of carbons based on relatively expensive starting materials is also unjustified for most pollution control applications (Streat et al., 1995). This has led many workers to search for more economic adsorbents.

Table 7 : Recent reported adsorption capacity qmax (mg/g) for commercial activated carbons

Dye
Qmax (mg/g)
Sources
Acid yellow 1179 Chern and Wu (2001)
Remazol yellow 1111 AI-Degs et al. (2000)
Basic yellow 21 860 Allen et al. (2003)
Basic red 22 720 Allen et al. (2003)
Reactive orange 107 714 Aksu and Tezer (2005)
Reactive red 2 712.3 Chiou et al. (2004)
Basic dye 309.2 Meshko et al. (2001)
Basic blue 9 296.3 Kannan and Sundaram (2001)
Reactive red 5 278 Aksu and Tezer (2005)
Direct red 81 240.7 Chiou et al. (2004)
Acid yellow 117 155.8 Choy et al. (2000)
Acid blue 40 133.3 Ozacar and Sengil (2002)
Acid blue 80 112.3 Choy et al. (2000)
Acid red 88 109 Venkata Mohan et al. (1999)
Basic red 46 106 Martin et al. (2003)
Acid red 114 103.5 Choy et al. (2000)
Acid yellow 17 57.47 Ozacar and Sengil (2002)
Direct red 28 16.81 Fu and Viraraghavan (2002a)
Direct brown 1 7.69 VenkataMohan et al. (2002)


 

1.10 DISADVANTAGES OF USING CONVENTIONAL METHODS FOR DYE REMOVAL


Some of the disadvantages of conventional methods for dye removal are listed in Table 8.

Table 8 : Disadvantages of conventional methods for dye removal

Treatment Process
Technology
Disadvantages
Conventional treatment processes Coagulation High sludge production, handling and disposal problems
Flocculation
Biodegradation Slow process, necessary to create an optimal favorable environment, maintenance and nutrition requirements
  Adsorption on activated carbons Ineffective against disperse and vat dyes, the regeneration is expensive and results in loss of the adsorbent, non-destructive process
Established recovery process Membrane separations High pressures, expensive, incapable of treating large volumes.
Ion-exchange Economic constraints, not effective for disperse dyes
Oxidation High energy cost, chemicals required
Emerging removal process Advanced oxidation process Economically unfeasible, formation of by-products, technical constraints

Although, some of these techniques have been shown to be effective, they have limitations. Among these are: excess amount of chemical usage, or accumulation of concentrated sludge with obvious disposal problems; expensive plant requirements or operational costs; lack of effective colour reduction; and sensitivity to a variable wastewater input.
In view of these disadvantages, biosorption or removal by heavy metals/dyes by biological materials has gained momentum from 1990’s.

 

1.11 BIOSORPTION

During the 1970’s increasing environmental awareness and concern led to a search for new techniques capable of inexpensive treatment of polluted wastewaters with metals. The search for new technologies involving the removal of toxic metals from wastewaters has directed attention to biosorption, based on binding capacities of various biological materials.

Till date, research in the area of biosorption suggests it to be an ideal alternative for decontamination of metal/dye containing effluents. Biosorbents are attractive since naturally occurring biomass/adsorbents or spent biomass can be effectively used. Biosorption is a rapid phenomenon of passive metal/dye sequestration by the non-growing biomass/adsorbents. Results are convincing and binding capacities of certain biomass/adsorbents are comparable with the commercial synthetic cation exchange resins.

The biosorption process involves a solid phase (sorbent or biosorbent; adsorbent; biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (adsorbate, metal/dyes). Due to the higher affinity of the adsorbent for the adsorbate species, the latter is attracted and bound there by different mechanisms. The process continues till equilibrium is established between the amount of solid-bound adsorbate species and its portion remaining in the solution. The degree of adsorbent affinity for the adsorbate determines its distribution between the solid and liquid phases.

There are many types of adsorbents; Earth’s forests and plants, ocean and freshwater plankton, algae and fish, all living creatures, that including animals are all “biomass/ adsorbents”. The renewable character of biomass that grows, fuelled directly or indirectly by sunshine, makes it an inexhaustible pool of chemicals of all kinds.

Biosorption has advantages compared with conventional techniques (Volesky, 1999). Some of these are listed below:

Biosorbents intended for bioremediation environmental applications are waste biomass of crops, algae, fungi, bacteria, etc., which are the naturally abundant. Numerous chemical groups have been suggested to contribute to biosorption.  A review of biosorption of heavy metals by microorganisms is presented below followed by biosorption of dyes by microorganisms. Biosorption by microorganisms have various disadvantages, and hence many low cost adsorbents (industrial/agricultural waste products/byproducts) are increasingly used as biosorbents. This report provides review of the low cost adsorbents used for removal of heavy metals (Ahalya et al., 2004; Ahalya et al., 2006) and dyes (in the later part of the Section).
 

2.1 BIOSORPTION OF HEAVY METALS BY MICROORGANISMS

A large number of microorganisms belonging to various groups, viz. bacteria, fungi, yeasts, cyanobacteria and algae have been reported to bind a variety of heavy metals to different extents. The role of various microorganisms by biosorption in the removal and recovery of heavy metal(s) has been well reviewed and documented (Stratton, 1987; Gadd and Griffiths, 1978; Volesky, 1990; Wase and Foster, 1997; Greene and Darnall, 1990; Gadd 1988). Most of the biosorption studies reported in literatures have been carried out with living microorganisms. However due to certain inherent disadvantages, use of living microorganisms for metal removal and recovery is not generally feasible in all situations. For example, industrial effluents contain high concentrations of toxic metals under widely varying pH conditions. These conditions are not always conducive to the growth and maintenance of an active microbial population. There are several advantages of biosorption of using non living biomass and they are as follows:

  1. Growth independent nonliving biomass is not subject to toxicity limitation by cells.
  2. The biomass from an existing fermentation industry, which essentially is a waste after fermentation, can be a cheap source of biomass. 
  3. The process is not governed by physiological constraints of microbial cells.
  4. Because nonliving biomass behaves as an ion exchanger, the process is very rapid, requiring anywhere between few minutes to few hours. Metal loading is very high on the surface of the biomass leading to very efficient metal uptake.
  5. Because cells are non-living processing conditions are not restricted to those conducive for the growth of the cells. Hence, a wider range of operating conditions such as pH, temperature and metal concentrations are possible. Also aseptic operating conditions are not essential.
  6. Metals can be desorbed readily and then recovered. If the value and the amount of metal recovered are insignificant and if the biomass is plentiful, the metal loaded biomass can be incinerated, eliminating further treatment.

Biosorption essentially involves adsorption processes such as ionic, chemical and physical adsorption. A variety of ligands located on the fungal cell walls are known to be involved in metal chelation. These include carboxyl, amine, hydroxyl, phosphate and sulphydryl groups. Metal ions could be adsorbed by complexing with negatively charged reactions sites on the cell surface. Table 9 presents an exhaustive list of microrganisms used for the uptake of heavy metals.

Table 9 : Biosorbent uptake of metals by Microbial Biomass

Metal
Biomass Type
Biomass class
Metal uptake (mg/g)
Reference
Ag Freshwater alga Biosorbent 86-94 Brierley and Vance, 1988; Brierley et al., 1986
  Fungal biomass Biosorbent 65 Brierley et al., 1986
  Rhizopus arrhizus Fungus 54 Tobin et al., 1984
  Streptomyces noursei Filamentous bacter 38.4 Mattuschka et al., 1993
  Sacchromyces cerevisiae Yeast 4.7 Brady and Duncan, 1993
Au Sargassum natans Brown alga 400 Volesky and Kuyucak, 1988
  Aspergillus niger Fungus 176 Kuyuack and Volesky, 1988
15 Gee and Dudeney, 1988
  Rhizopus arrhizus Fungus 164 Kuyuack and Volesky, 1988
  Palmaria tevera Marine alga 164 Kuyuack and Volesky, 1988
  Palmaria palmata Marine alga 124 Kuyuack and Volesky, 1988
  Chlorella pyrenoidosa Freshwater alga 98 Darnall et al., 1988
  Cyanidium caldarium Alga 84 Darnall et al., 1988
  Chlorella vulgaris Freshwater alga 80 Gee and Dudeney, 1988
  Bacillus subtilis Bacteria Cell wall 79 Beveridge, 1986
  Chondrus crispus Marine alga 76 Kuyuack and Volesky, 1988
  Bacillus subtilis Bacterium 70 Gee and Dudeney, 1988
  Spirulina platensis Freshwater alga 71 Darnall et al., 1988
58 Gee and Dudeney, 1988
  Rhodymenia palmata Marine alga 40 Darnall et al., 1988
  Ascophyllum nodosum Brown marine alga 24 Kuyuack and Volesky, 1988
Cd Ascophyllum nodosum Brown markertman
ine alga		 215 Holan et al., 1993
  Sargassum natans Brown marine alga 135 Holan et al., 1993
  Fucus vesiculosus Brown marine alga 73 Holan et al., 1993
  Candida tropicalis Yeast 60 Mattuschka et al., 1993
  Pencillium chrysogenum Fungus 56 Holan and Volesky, 1995
11 Niu et al., 1993
  Rhizopus arrhizus Fungus 30 Tobin et al., 1984
  Sacchromyces cervisiae Yeast 20-40 Volesky et al., 1993
  Rhizopus arrhizus Fungus 27 Fourest and Roux, 1992
  Rhizopus nigricans Fungus 19 Holan and Volesky, 1995
  Pencillium spinulosum Fungus 0.4 Townsley et al., 1996
  Pantoea sp. TEM 18 Bacteria 204.1 Guven Ozdemir et al., 2004
  Chlamydomonas reinhardtii Alga 42.6 Tuzun et al., 2005
  Spirulina sp. Blue green algae 1.77 meq/g Chojnacka et al., 2005
  Enterobacter cloaceae (Exopolysaccharide) Marine bacterium 16 Anita Iyer et al., 2005
  Padina sp. Brown seaweed 0.75 Sheng et al., 2004
  Sargassum sp. Brown seaweed 0.76 Sheng et al., 2004
  Ulva sp. Green seaweed 0.58 Sheng et al., 2004
  Gracillaria sp. Red seaweed 0.30 Sheng et al., 2004
  Gloeothece magna Cyanobacteria 115–425 μg mg−1 Zakaria A. Mohamed, 2001
Co Ascophyllum nodosum Brown marine algae 100 Kuyucak and Volesky, 1989a
  Sacchromyces cerevisiae Yeast 4.7 Brady and Duncan, 1993
  Ulva reticulata Marine green algae 46.1 Vijayaraghavan et al., 2005
  Enterobacter cloaceae Marine bacterium 4.38 Anita Iyer et al., 2005
Cr Bacillus biomass Bacterium 118 Cr3+
60 Cr 6+		 Brierley and Brierley, 1993
  Rhizopus arrhizus Fungus 31 Tobin et al., 1984
  Candida tropicalis Yeast 4.6 Mattuschka et al., 1993
  Streptomyces nouresei Bacteria 1.8 Mattuschka et al., 1993
  Pantoea sp. TEM 18 Bacteria 204.1 Guven Ozdemir et al., 2004
  Spirulina sp. Cyanobacteria 10.7 meq/g Chojnacka et al., 2005
  Spirogyra sp. Filamentous algae 4.7 Gupta et al., 2001
Cu Bacillus subtilis Biosorbent 152 Beveridge, 1986; Brierley et al., 1986; Brierley and Brierley, 1993
  Candida tropicalis Yeast 80 Mattuschka et al., 1993
  Manganese oxidising bacteria MK-2 50 Stuetz et al., 1993
  Cladosporium resinae Fungus 18 Gadd et al., 1988
  Rhizopus arrhizus Fungus 16 Gadd et al; 1988
  Saccharomyces crevisae Yeast 17-40; 10; 6.3 Volesky and May-Phillips, 1995; Mattuschka et al., 1993; Brady and Duncan, 1993
  Pichia guilliermondii Yeast 11 Mattuschka et al., 1993
  Scenedesmus obliquus Freshwater algae 10 Mattuschka et al., 1993
  Rhizopus arrhizus Fungus 10 Gadd et al; 1988
  Pencillium chrysogenum Fungus 9 Niu et al., 1993
  Streptomyces noursei sp. Filamentous bacteria 5 Mattuschka et al., 1993
  Bacillus sp Bacterium 5 Cotoras et al., 1993
  Pencillium spinulosum Fungus 0.4-2 Townsley et al., 1986
  Aspergillus niger Fungus 1.7 Townsley et al., 1986
  Trichoderma viride Fungus 1.2 Townsley et al., 1986
  Pencillium chrysogenum Fungus 0.75 Paknikar et al., 1993
                 Pantoea sp. TEM 18 Bacteria 31.3 Guven Ozdemir et al., 2004.
  Ulva reticulata Marine green alga 56.3 Vijayaraghavan et al., 2005
  Spirulina sp. Blue green algae 6.17 meq/g Chojnacka et al., 2005
  Enterobacter cloaceae (Exopolysaccharide) Marine bacterium 6.60 Anita Iyer et al., 2005
  Padina sp. Brown seaweed 1.14 Sheng et al., 2004
  Sargassum sp. Brown seaweed 0.99 Sheng et al., 2004
  Ulva sp. Green seaweed 0.75 Sheng et al., 2004
  Gracillaria sp. Red seaweed 0.59 Sheng et al., 2004
  Thiobacillus thiooxidans Bacteria 38.54 Liu et al., 2004
  Ulothrix zonata Algae 176.20 Nuhoglu et al., 2002
Fe Bacillus subtillis Bacterial cell wall preparation 201 Beveridge, 1986
  Bacillus biomass Bacterium 107 Brierley and Brierley, 1993
  Sargassum fluitans Brown alga 60 Figueira et al., 1995
Hg Rhizopus arrhizus Fungus 54 Tobin et al., 1984
  Pencillium chrysogenum (biomass not necessarily in its natural state) Fungus 20 Nemec et al., 1977
  Cystoseira baccata Marine alga 178 Herrero et al., 2005
  Chlamydomonas reinhardtii Algae 72.2 Tuzun et al., 2005
Ni Fucus vesiculosus Brown marine algae 40 Holan and Volesky, 1994
  Ascophylum nodosum Brown marine algae 30 Holan and Volesky, 1994
  Sargassum natans Brown marine algae 24-44 Holan and Volesky, 1994
  Bacillus licheniformis Bacterial cell wall preparation 29 Beveridge, 1986
  Candida tropicalis Yeast 20 Mattuschka et al., 1993
  Rhizopus arrhizus Fungus 18 Fourest and Roux, 1992
  Bacillus subtillis Bacterial cell wall preparation 6 Beveridge, 1986
  Rhizopus nigricans Fungus 5 Holan and Volesky, 1995
  Absidia orchidis Fungus 5 Kuycak and Volesky, 1988
  Ulva reticulata Marine green algae 46.5 Vijayaraghavan et al., 2005
  Padina sp. Brown seaweed 0.63 Sheng et al., 2004
  Sargassum sp. Brown seaweed 0.61 Sheng et al., 2004
  Ulva sp. Green seaweed 0.29 Sheng et al., 2004
  Gracillaria sp. Red seaweed 0.28 Sheng et al., 2004
  Polyporous versicolor White rot fungus 57 Dilek et al., 2002
Pb Bacillus subtilis (biomass not necessarily in its natural state)  Biosorbent 601 Brierley et al., 1986
  Absidia orchidis Fungus 351 Holan and Volesky, 1995
  Fucus vesiculosus Brown marine algae 220-370 Holan and Volesky, 1994
  Ascophyllum nodosum Brown marine algae 270-360 Holan and Volesky, 1994
  Sargassum natans Brown marine algae 220-270 Holan and Volesky, 1994
  Bacillis subtilis (biomass not necessarily in its natural state) Biosorbent 189 Brierley and Brierley, 1993
  Pencillium chrysogenum Fungus 122; 93 Niu et al., 1993; Holan and Volesky, 1995
  Rhizopus nigricans Fungus 166 Holan and Volesky, 1995
  Streptomyces longwoodensis Filamentous bacteria 100 Friis and Myers-Keith, 1986
  Rhizopus arrhizus Fungus 91; 55 Tobin et al., 1984; Fourest and Roux, 1992, Holan and Voleky, 1995.
  Streptomyces noursei Filamentous bacteria 55 Mattuschka et al., 1993
  Chlamydomonas reinhardtii Algae 96.3 Tuzun, et al., 2005
  Padina sp. Brown seaweed 1.25 Sheng et al., 2004
  Sargassum sp. Brown seaweed 1.26 Sheng et al., 2004
  Ulva sp. Green seaweed 1.46 Sheng et al., 2004
  Gracillaria sp. Red seaweed 0.45 Sheng et al., 2004
  Ecklonia radiata Marine alga 282 Matheickal and Yu, 1996
Pd Freshwater alga(biomass not necessarily in its natural state) Biosorbent 436 Brierley and Vance, 1988.
  Fungal biomass Biosorbent 65 Brierley et al., 1988
Pt Freshwater alga (biomass not necessarily in its natural state) Biosorbent 53 Brierley and Vance, 1988; Brierley et al., 1988
U Sargassum fluitans Brown algae 520 Yang and Volesky 1999; Yang and Volesky, 1999
  Streptomyces longwoodensis Filamentous bacteria 440 Friis and Myers-Keith, 1986
  Rhizopus arrhizus Fungus 220; 195 Volesky and Tsezos, 1981; Tobin et al., 1984
  Sacchromyces crevisae Yeast 55-140 Volesky and May Phillips, 1995
  Bacillus sp. Bacterium 38 Cotoras et al., 1993
  Chaetomium distortum Fungus 27 Khalid et al., 1993.
  Trichoderma harzianum Fungus 26 Khalid et al., 1993.
  Pencillium chrysogenum (biomass not necessarily in its natural state) Fungus 25 Nemec et al., 1977
  Alternaria tenulis     Khalid et al., 1993.
Th Rhizopus arrhizus Fungus 160; 93 Tsezos and Volesky, 1981; Gadd et al., 1988
  Sacchromyces cerevisae Yeast 70 Gadd et al., 1988
Zn Bacillus subtilis (biomass not necessarily in its natural state) Biosorbent 137 Brierley et al., 1986
  Sargassa sp. Brown algae 70 Davis et al., 2003; Davis et al., 2000; Figueira et al., 1995; Figueira et al., 1997; Figueira et al., 2000; Figueira et al., 1999; Schiewer et al., 1995; Scheiwer and Volesky, 1996; Scheiwer and Volesky 1997; Scheiwer and Wong, 1999.
  Manganese oxidising bacteria (MK-2) 39 Stuetz et al., 1993
  Sacchromyces cerevisae Yeast 14-40 Volesky and May-Phillips, 1995
  Candida tropicalis Yeast 30 Mattuschka et al., 1993
  Rhizopus arrhizus Fungus 20; 14 Tobin et al., 1984; Gadd et al., 1988
  Pencillium chrysogenum Fungus 6.5 Niu et al., 1993; Paknikar et al., 1993
  Bacillus sp. Bacterium 3.4 Cotoras et al., 1993
  Pencillium spinulosum Fungus 0.2 Townsley et al., 1986
  Padina sp. Brown seaweed 0.81 Sheng et al., 2004
  Sargassum sp. Brown seaweed 0.50 Sheng et al., 2004
  Ulva sp. Green seaweed 0.54 Sheng et al., 2004
  Gracillaria sp. Red seaweed 0.40 Sheng et al., 2004
  Thiobacillus thiooxidans Bacteria 43.29 Liu et al., 2004

Among micro-organisms, fungal biomass offers the advantages of having high percentage of cell wall material, which shows excellent metal binding properties (Gadd, 1990; Rosenberger, 1975; Paknikar, Palnitkar and Puranik, 1993).  Many fungi and yeast have shown an excellent potential of metal biosorption, particularly the genera Rhizopus, Aspergillus, Streptoverticullum and Sacchromyces (Volesky and Tsezos, 1981; Galun et al., 1984; de Rome and Gadd, 1987; Siegel et al., 1986; Luef et al., 1991, Brady and Duncan, 1993 Puranik and Paknikar, 1997).

 

2.2 BIOSORPTION OF DYES BY MICRORGANISMS

A wide variety of microorganisms including bacteria, fungi and yeasts are used for the biosorption of a broad range of dyes. Textile dyes vary greatly in their chemistries, and therefore their interactions with microorganisms depend on the chemical structure of a particular dye, the specific chemistry of the microbial biomass and characteristics of the dye solution or wastewater. Depending on the dye and the species of microorganism used different binding capacities have been observed (Table 10).

Table 10 : Biosorbent Uptake of dyes by microorganisms

Biosorbent
Dye
Biosorption capacity
qeq (mgg-l)
Reference
Activated sludge Basic Red 29 113.2 Chu and Chen, 2002
Basic Yellow 24 105.6
Basic Blue 54 86.6
Basic Red 18 133.9
Basic Violet 3 113.6
Basic Blue 4 157.5
Basic Blue 3 36.5
Activated sludge Reactive Blue 2 102.0 Aksu, 2001
Reactive Yellow 2 119.4
Activated sludge Maxilon Red BL-N (123.2) Basibuyuk and Forster, 2003
Aeromonas sp. Reactive Blue 5 124.8 Hu, 1996
Reactive Red 22 116.5
Reactive Violet 2 114.5
Reactive Yellow 2 124.3
Aspergillus niger Basic Blue 9 18.5 (1.2) Fu and Viraraghvan, 2000
Acid Blue 29 13.8 (6.6) Fu and Viraraghvan, 2001
Congo Red 14.7 Fu and Viraraghvan, 2002
Disperse Red I 5.6
Aspergillus niger Reactive Brilliant Red 14.2 Gallagher et al., 1997
Botrytis cinerea Reactive Blue 19 42 (13.0) Polman and Breckenridge, 1996
Sulphur Black I 360 (49.7)
Candida sp.. Remazol Blue 169 Aksu and Donmez, 2003
Candida lipolytica Remazol Blue 230
Candida membranaefaciens Remazol. Blue 149
Candida quilliermendii Remazol Blue 152
Candida tropicalis Remazol Blue 180
Candida utilis Remazol Blue 113
Candida rugosa Reactive Blue 19 8 (8) Polman and Breckenridge, 1996
Reactive Black 5 31 (31)
Sulphur Black I 407 (308)
Cryptococcuss heveanensis Reactive Blue 19 23 (22) Polman and Breckenridge, 1996
Reactive Black 5' 76 (60)
Sulphur Black I 407 (360)
Dekkera bruxellensis Reactive Blue 19 19 (36) Polman and Breckenridge, 1996
Reactive Black 5 36 (38).
Sulphur Black I 589 (527)
Endothiella aggregata Reactive Black 5 44 Polman and Breckenridge, 1996
Sulphur Black I 307
Escherichia coli Reactive Blue 5 89.4 Hu, 1996
Reactive Red 22 76.6
Reactive Violet 2 65.5
Reactive Yellow 2 52.4
Fomitopsis carnea Orlamar Red BG 503.1 Mittal and Gupta, 1996
Orlamar Blue G 545.2
Orlamar Red GTL 643.9
Geotrichum fici Reactive Blue 19 17 (60) Polman and Breckenridge, 1996
Reactive Black 5 45 (7)
Sulphur Black I 37 (60)
Kluyveromyces marxianus Remazol Black B 37 Bustard et al., 1998
Rem. Turquoise Blue 98
Remazol Red 68
Rem. Golden Yellow 33
Cibacron Orange 8.5
Kluyveromyces marxianus Remazol Blue 161 Aksu and Donmez, 2003
Kluyveromyces waltii Reactive Blue 19 14 (20) Polman and Breckenridge, 1996
Reactive Black 5 72 (60)
Sulphur Black 1 549 (445)
Laminaria digitata Reactive Brilliant Red 20.5 Gallagher et al., 1997
Myrothecum verrucaria Orange II 70% Brahimi-Horn et al., 1992
lOB (Blue) 86%
RS (Red) 95%
Phanerochaete chrysosporium Congo red 90% Tatarko and Bumpus, 1998
Pichia carsonii Reactive Blue 19 5 (3) Polman and Breckenridge, 1996
Reactive Black 5 32 (25)
Sulphur Black I 549 (499)
Pseudomonas luteola Reactive Blue 5 102.5 Hu et al., 1996
Reactive Red 22 105.3
Reactive Violet 2 96.4
Reactive Yellow 2 102.6
Rhizopus arrhizus Humic acid 91.9 Zhou and Banks, 1993
Rhizopus arrhizus Reactive Orange 16 190 O’Mahony et al., 2002
Reactive Blue 19 90
Reactive Red 4 150
   
Rhizopus arrhizus Remazol Black B 500.7 Aksu and Tezer, 2000
Rhizopus oryzae (26668) Reactive Brilliant Red 102.6 Gallagher et al., 1997
Rhizopus oryzae (57412) Reactive Brilliant Red 37.2
Rhizopus oryzae Reactive Black 5 452 (99) Polman and Breckenridge, 1996
Sulphur Black 1 3008 (1107)
Saccharomyces cerevisiae Remazol Blue 162 Aksu and Donmez, 2003
Saccharomyces cerevisiae Reactive Blue 19 69 (52) Polman and Breckenridge, 1996
Saccharomyces pombe Remazol Blue 152 Aksu and Donmez, 2003
Streptomycetes BW130 Anthraquinone Blue 114 27.0% Zhou and Zimmerman, 1993
 
Azo-copper Red
171		 73.0%
 
Azo-reactive Red 147 29.0%
Formazan Blue 209 70.0%
Phytalocyanine Blue 116 39.0%
Tremella uciformis Reactive Blue 19 35 (41) Polman and Breckenridge, 1996
Reactive Black 5 79 (92)
Sulphur Black 1 892 (934)
Xeromyces bisporus Reactive Blue 19 60 (0) Polman and Breckenridge, 1996
Reactive Black 5 1 (11)
Sulphur Black 1 60 (63)


 

2.3 DISADVANTAGES OF BIOSORPTION USING MICRORGANISMS

There are certain inherent disadvantages of using microorganisms for the biosorption of heavy metals/dyes and they are as follows: the protein rich algal and fungal biomass projected as metal/dye biosorbents have limitations as proteinious materials are likely to putrefy under moist conditions. Further, most metal/dye sorption reported in literature is based on algal and fungal biomass, which must be cultured, collected from their natural habitats and pre-processed, if available as discards and transported under special conditions, thus introducing the factor of additional costs.

 

2.4 LOW COST ADSORBENTS

The disadvantages of using microorganisms can be overcome by using low cost adsorbents. In general, a sorbent can be assumed to be “low cost” if it requires little processing and is abundant in nature, or is a by product or waste material from another industry, which has lost its economic or further processing values. There have been several low cost adsorbents that have been used for the removal of heavy metal and dyes. The following Section presents a detailed discussion on the low cost adsorbents that have been used for the removal of heavy metals and dyes.

Cost is an important parameter for comparing the sorbent materials. However, cost information is seldom reported, and the expense of individual sorbents varies depending on the degree of processing required and local availability.

2.4.1 Low cost adsorbents for metal removal

Research pertaining to low cost absorbents is gaining importance these days though most of the work is at laboratory levels. Some of the low-cost sorbents reported so far include: Bark/tannin-rich materials; lignin; chitin/chitosan; seaweed/algae/alginate; xanthate; zeolite; clay; flyash; peat moss; modified wool and modified cotton; tea waste; maize coen cob etc., efficacy of which are discussed next

2.4.1.1 BARK AND OTHER TANNIN –RICH MATERIALS :

Timber industry generates bark a by-product that is effective because of its high tannin content. The polyhydroxy polyphenol groups of tannin are thought the active species in the adsorption process. Ion exchange takes place as metal cations displace adjacent phenolic hydroxyl groups, forming a chelate (Randall et al., 1974a; Vasquez et al., 1994).
Another waste product from the timber industry is sawdust. Bryant et al. (1992) showed adsorption of Cu and hexavalent chromium (Cr (VI) by red fir sawdust to take place primarily on components such as lignin and tanin rather onto cellulose backbone of the sawdust (Table 11). While bark is the most likely choice due to its wide availability, other low cost byproducts containing tannin show promise for economic metal sorption as well.
Table 11 : Reported adsorption capacities (mg/g) for tannin containing materials
Material
Source
Cd
Cr (III)
Cr (VI)
Hg
Pb
Activated carbon Teles de Vasconcelos and Gonzàlez Beća, 1994         2.95
Black oak bark Masri et al., 1974 25.9     400 153.3
Douglas fir bark Masri et al., 1974       100  
Exhausted coffee Orhan and Büyükgüngor, 1993 1.48   1.42    
Formaldehyde –polymerised peanut skins Randall et al., 1978 74       205
Hardwickia binata bark Deshkar et al., 1990 34        
Nut shell Orhan and Büyükgüngor, 1993 1.3   1.47    
Pinus pinaster bark Teles de Vasconcelos and Gonzàlez Beća, 1993, 1994 and Vàzquez et al., 1994 8.00 19.45                               3.33, 1.59
Redwood bark Masri et al 1974, Randall et al 1974a, b 27.6, 32     250 6.8, 182
Sawdust Bryant et al., 1992; Dikshit, 1989; Zarraa, 1995     10.1, 16.05, 4.44    
Turkish coffee Orhan and Buyukgungor, 1993 1.17   1.63    
Treated Pinus sylvestris bark Alves et al., 1993   9.77      
Untreated Pinus sylvestris bark Alves et al., 1993   8.69      
Walnut shell Orhan and Buyukgungor, 1993 1.5   1.33    
Waste tea Orhan and Buyukgungor, 1993 1.63   1.55    


2.4.1.2 CHITOSAN :

Among various biosorbents, chitin is the second most abundant natural biopolymers after cellulose. However, more important than chitin is chitosan, which has a molecular structure similar to cellulose. Presently, chitosan is attracting an increasing amount of research interest, as it is an effective scavenger for heavy metals. Chitosan is produced by alkaline N-deacetylation of chitin, which is widely found in the exoskeleton of shellfish and crustaceans. It was estimated that chitosan could be produced from fish and crustaceans (Rorrer and Way 2002). The growing need for new sources of low-cost adsorbent, the increased problems of waste disposal, the increasing cost of synthetic resins undoubtedly make chitosan one of the most attractive materials for wastewater treatment.
Various researches on chitosan have been done in recent years and it can be concluded that chitosan is a good adsorbent for all heavy metals (Table 12). It is widely known that the excellent adsorption behaviour of chitosan for heavy metal removal is attributed to: (1) high hydrophilicity of chitosan due to large number of hydroxyl groups, (2) large number of primary amino groups with high activity, and (3) flexible structure of polymer chain of chitosan making suitable configuration for adsorption of metal ions.

Table 12 : Reported adsorption capacities (m/g) for chitosan

Material
Source
Cd
Cr (III)
Cr (VI)
Hg
Cu
Pb
Chitin Masri et al., 1974       100    
Chitosan Jha et al., 1988; Masri et al., 1974, McKay et al., 1989; Udhaybhaskar et al., 1990 6.4, 558 92 27.3 1123, 815   796
Chitosan (from lobster shell) Peniche-Covas et al., 1992       430    
Chitosan powder Rorrer et al., 1993 420          
Chitosan beads Rorrer et al., 1993 518          
N-acylated chitosan beads Hsien and Rorrer, 1995 216          
N-acylated cross linked chitosan beads Hsien and Rorrer, 1995 136          
Thiol-grafted chitosan gel Merrifield, et al., 2004       8.0 mmol/g    
Aminated chitosan Jeon and. Höll, 2003       2.23 mmol/g    
Chitosan derived from prawn shells Chu, 2002           0.266 mmol/g
Chitosan Wan Ngah et al., 2002         80.71  
Chitosan beads cross-linked with glutaraldehyde Wan Ngah et al., 2002         59.67  
Chitosan beads cross-linked with epichlorohydrin Wan Ngah et al., 2002         62.47  
Chitosan beads cross-linked with thylene glycol diglycidyl ether Wan Ngah et al., 2002         45.62  


2.4.1.3 ZEOLITES :

Basically zeolites are a naturally occurring crystalline aluminosilicates consisting of a framework of tetrahedral molecules, linked with each other by shared oxygen atoms. During 1970s, natural zeolites gained a significant interest, due to their ion-exchange capability to preferentially remove unwanted heavy metals such as strontium and cesium [Grant et al., 1987]. This unique property makes zeolites favorable for wastewater treatment (Table 13). The price of zeolites depending on the quality is considered very cheap and is about US$ 0.03–0.12/kg, [Virta, 2001].

Table 13 : Reported adsorption capacities (mg/g) for zeolite

Material
Source
Cd
Cr (III)
Cr (VI)
Hg
Pb
Zn
Cu
CETYL-amended zeolite Santiago et al., 1992     0.65        
EHDDMA-amended zeolite Santiago et al., 1992     0.42        
                 
Zeolite Leppert, 1990 84.3 26.0   150.4 155.4    
Clinoptilolite zeolites Erdem et al., 2004           133.85             141.12


2.4.1.4 CLAY :

It is widely known that there are three basic species of clay: smectites (such as montmorillonite), kaolinite, and micas; out of which montmorillonite has the highest cation exchange capacity and its current market price is considered to be 20 times cheaper than that of activated carbon [Virta, 2002]. Therefore, a number of studies have been conducted using clays, mainly montmorillonite, to show their effectiveness for removing metal ions such as Zn2+, Pb2+, and Al3+ from aqueous solutions (Brigatti et al., 1996; Staunton and M. Roubaud, 1997 and Turner et al., 1998) (Table 14). Although the removal efficiency of clays for heavy metals may not be as good as that of zeolites, their easy availability and low cost may compensate for the associated drawbacks.

Fly ash, an industrial solid waste of thermal power plants located in India, is one of the cheapest adsorbents having excellent removal capabilities for heavy metals such as copper ions (Panday et al, 1985). It was reported that an adsorption capacity of 1.39 mg of Cu2+/g was achieved by fly ash at a pH of 8.0. It is also known from various studies that fly ash could be easily solidified after the heavy metals are adsorbed. However, since it also contains heavy metals, the possibility of leaching could be considered and evaluated.

Table 14 : Reported adsorption capacities (mg/g) for clays

Material
Source
Cd
Cr (VI)
Pb
Cu2+
Hg2+
Zn
Bentonite Khan et al., 1995; Cadena et al., 1990; Kaya and Ören, 2005    0.512, 55 6     0.921
Na rich bentonite Kaya and Ören, 2005           8.271
Tailored bentonite Cadena et al., 1990   57, 58        
Acid treated bentonite Pradas et al., 1994 4.11          
Heat treated bentonite Pradas et al., 1994 16.50          
China clay Yadava et al., 1991     0.289      
Wollastonite Yadava et al., 1991     0.217      
Wallastonite-fly ash mixture Panday et al., 1984a   2.92   1.18    
Fly ash Panday et al., 1985; Sen and Arnab       1.39    
Fly ash-China clay Panday et al., 1984a   0.31        
Palygorskite clay Potgieter, et al., 2005   58.5 62.1 30.7    
Fly ash Cho et al, 2005   5.0 10.0 2.8   3.2

 

2.4.1.5 PEAT MOSS :

Peat moss, a complex soil material containing lignin and cellulose as major constituents, is a natural substance widely available and abundant, not only in Europe (British and Ireland), but also in the US. Peat moss has a large surface area (>200 m2/g) and is highly porous so that it can be used to bind heavy metals. Peat moss is a relatively inexpensive material and commercially sold at US$ 0.023/kg in the US [Jasinski, 2001]. Peat moss is a good adsorbent for all metals (Table 15). It is widely known that peat moss exhibited a high CEC and complexities towards metals due to the presence of carboxylic, phenolic, and hydroxylic functional groups.

Table 15 : Reported adsorption capacities (mg/g) for peat moss

Material
Source
Cd
Cr (III)
Cr (VI)
Hg
Cu
Pb
Irish sphagnum moss peat Sharma and Forster, 1993, 1995     119.0, 43.9      
Modified peat Kertman et al., 1993   76       230
Rastunsuo peat Tummavuori and Aho, 1980a, b 5.058 4.63   16.2   20.038
Sphagnum moss peat McLelland and Rock, 1988 5.8 29       40
Sphagnum peat Fattahpour Sedeh et al., 1996         40  
Carex peat Fattahpour Sedeh et al., 1996         24 to  33  


2.4.1.6 INDUSTRIAL WASTE :
Several industrial by-products have been used for the adsorption of heavy metals. Table 16 summarises some of the industrial wastes.

Table 16 : Adsorption capacities of industrial waste (mg/g)

Material
Sources
N2+
Pb2+
Hg2+
Cr6+
Zn2+
Cd2+
Cu2+
Waste slurry Srivastava et al., 1985   1030 560 640      
  Lee and Davis, 2001           15.73 20.97
Iron (III) hydroxide Namasivayam and Rangnathan, 1992       0.47      
Lignin Aloki and Munemori, 1982   1865     95    
Blast furnace slag Srivastava et al., 1997   40   7.5      
Sawdust Ajmal et al., 1998             13.80
Activated red mud Zouboulis and Kydros, 1993 160            
  Pradhan et al., 1999       1.6      
Bagasse fly ash Gupta et al., 1999       260      


2.4.1.7 MISCELLANEOUS ADSORBENTS :

Table 17 lists some of the miscellaneous adsorbents used for the removal of heavy metals.

Table 17 : Reported adsorption capacities (mg/g) for several miscellaneous sorbents

Material
Source
Cd
Cr
Hg
Pb
Ni
Zn
Cu
Dry pine needles Masri et al., 1974     175        
Dry redwood leaves Masri et al., 1974     175        
Dyed bamboo pulp (C.I. Reactive orange 13) Shukla and Sakhardande, 1992     15.6 15      
Undyed bamboo pulp Shukla and Sakhardande, 1992     9.2 8.4      
Dyed jute (C.I. Reactive orange 13 Shukla and Sakhardande, 1992     13.7 14.1      
Undyed jute Shukla and Sakhardande, 1992     7.6 7.9      
Dyed sawdust (C.I. Reactive orange 13) Shukla and Sakhardande, 1992     18.0 24.0      
Undyed sawdust Shukla and Sakhardande, 1992     8.5 7.3      
Milogranite (activated sewage sludge) Masri et al., 1974     460 95.3      
Modified wool Masri and Friedman, 1974 87 17 632 135      
Moss Low and Lee, 1991 46.5            
Orange peel (white inner skin) Masri et al., 1974   125          
Orange peel (outer skin) Masri et al., 1974   275          
PEI wool Freeland et al., 1974   330.97          
Senna leaves Masri et al., 1974   250          
Unmodified jute Shukla and Pai, 2005         3.37 3.55 4.23
Modified jute Shukla and Pai, 2005         5.57 8.02 7.73
Papaya wood Saeed et al., 2005 17.35         14.44 19.99
Activated carbon from apricot stone Kobya et al., 2005 3.08 34.70   6.69 2.50   4.86
Lignocellulosic fibres – unmodified Shukla et al., 2005         7.49 7.88  
Lignocellulosic fibres oxidised with hydrogen peroxide Shukla et al., 2005         2.51 1.83  
Carbon aerogel Meena et al., 2005 400.8   45.62 0.70 12.85 1.84 561.71
Dye loaded groundnut shells Shukla and Pai, 2005         9.87 17.09 8.07
Unloaded sawdust Shukla and Pai, 2005         8.05 10.96 4.94
Siderite Erdem and Özverdi, 2005       14.06      
Diatomite Khraisheh, 2004 16.08     24.94     27.55
Manganese treated diatomite Khraisheh, 2004 27.08     99.00     55.56
Wheat shell Basci et al., 2004             10.84
Wheat bran Farajzadeh et al., 2004 21 93 70 62 12   15
Tea industry waste Cay et al., 2004 11.29           8.64
Sawdust of P. sylvestris Taty-Costodes, et al., 2003 19.08     22.22      
Cork biomass Chubar et al., 2003         0.34 meq./g 0.76 meq/g 0.63 meq/g
Cocoa shells Meunier et al., 2003       6.2      
Vermicompost Matos and Arruda, 2003 33.01     92.94   28.43 32.63
Peanut hulls Johnson et al., 2002             9
Peanut pellets Johnson et al., 2002             12
poly(ethyleneglycol dimethacrylate-co-acrylamide) beads Kesenci et al., 2002 0.370mmol/g   0.270 mmol/g 1.825 mmol/g      
Activated carbon derived from bagasse Dinesh Mohan and Kunwar P. Singh, 2002 49.07         14.0  
Polyacrylamide-grafted iron(III) oxide Manju et al., 2002 151.47   163.21 218.53      
Carboxylated alginic acid Jeon et al., 2002       3.09 mmol/g      
Petiolar felt sheath of palm Iqbal et al., 2002 10.8 5.32   11.4 6.89 5.99 8.09
Sheep manure waste Munther Kandah, 2001           13.8  
Peanut husk carbon Ricordel et al., 2001 0.45     0.55 0.28 0.20  
Kudzu (Pueraria lobata ohwi) Brown et al., 2001 15         35 32
Turkish coal Arpa et al., 2000 0.008 mmol/g   0.039 mmol/g 0.041 mmol/g      
Peanut hulls Brown et al., 2000 6     30   9 8
Peanut hull pellets Brown et al., 2000 6     30   10 10
Commercial grade ion exchange Resin Brown et al., 2000 50         90 85
Carrot residue Nasernejad et al., 2005   45.09       29.61 32.74

 

The results of many biosorption studies vary widely because of the different criteria used by the authors in searching for suitable materials. Some researchers have used easily available biomass types, others specially isolated strains, and some processed the raw biomass to different extents to improve its biosorption properties. In the absence of uniform technology, results have been reported in different units and in many different ways, making quantitative comparison impossible.

 
2.4.2 Low cost adsorbents for dye/s removal

In recent times, attention has been focused on various natural solid supports, which are able to remove pollutants from contaminated water at low cost. Cost is actually an important parameter for comparing the adsorbent materials. Certain waste products from industrial and agricultural operations, natural materials and biosorbents represent potentially economical alternative sorbents. Many of them have been tested and proposed for dye removal.

2.4.2.1 WASTE MATERIALS FROM AGRICULTURE AND INDUSTRY :

The by-products from the agriculture and industries could be assumed to be low-cost adsorbents since they are abundant in nature, inexpensive, require little processing and are effective materials.

2.4.2.2 ACTIVATED CARBONS FROM SOLID WASTES :

Commercially available activated carbons (AC) are usually derived from natural materials such as wood, coconut shell, lignite or coal, but almost any carbonaceous material may be used as precursor for the preparation of carbon adsorbents (Rozada et al., 2003, Rodriguez-Reinoso, 1997 and Pollard et al., 1992). Due to its availability and cheapness, coal is the most commonly used precursor for AC production (Carrasco-Marin et al., 1996 and Illan Gomez et al., 1996). Coal is a mixture of carbonaceous materials and mineral matter, resulting from the degradation of plants. The sorption properties of each individual coal are determined by the nature of the original vegetation and the extent of the physical–chemical changes occurring after deposition (Karaca et al., 2004). Coal adsorption capacities are reported in Table 18. Coal based sorbents have been used by Karaca et al., 2004, Venkata Mohan et al., 1999 and Venkata Mohan et al., 2002 and McKay et al. (1999) with success for dye removal. However, since coal is not a pure material, it has a variety of surface properties and thus different sorption properties.

Table 18 : Activated carbons from solid wastes

    Raw material
Dye
Qmax
Sources
Pinewood Acid blue 264 1176 Tseng et al. (2003)
Pinewood Basic blue 69 1119 Tseng et al. (2003)
Corncob Acid blue 25 1060 Juang et al. (2002a)
Bagasse Basic red 22 942 Juang et al. (2002a)
Cane pith Basic red 22 941.7 Juang et al. (2001)
Corncob Basic red 22 790 Juang et al. (2002a)
Bagasse Acid blue 25 674 Juang et al. (2002a)
Cane pith Acid blue 25 673.6 Juang et al. (2001)
Pinewood Basic blue 9 556 Tseng et al. (2003)
Rice husk Basic green 4 511 Guo et al. (2003)
Bagasse Acid blue 80 391 Valix et al. (2004)
Waste newspaper Basic blue 9 390 Okada et al. (2003)
Coal Basic blue 9 250 McKay et al. (1999)
Waste carbon slurries Acid blue 113 219 Jain et al. (2003)
Waste carbon slurries Acid yellow 36 211 Jain et al. (2003)
Waste carbon slurries Ethyl orange 198 Jain et al. (2003)
Sewage sludge Basic red 46 188 Martin et al. (2003)
Mahogany sawdust Acid yellow 36 183.8 Malik (2003)
Coal Basic red 2 120 McKay et al. (1999)
Sewage sludge Basic blue 9 114.94 Otero et al. (2003a)
Charcoal Acid red 114 101 Choy et al. (1999)
Rice husk Acid yellow 36 86.9 Malik (2003)
Rice husk Acid blue 50 Mohamed (2004)
Charfines Acid red 88 33.3 Venkata Mohan et al. (1999)
Lignite coal Basic blue 9 32 Karaca et al. (2004)
Lignite coal Acid red 88 30.8 Venkata Mohan et al. (1999)
Bituminous coal Acid red 88 26.1 Venkata Mohan et al. (1999)
Rice husk Basic blue 9 19.83 Kannan and Sundaram (2001)
Straw Basic blue 9 19.82 Kannan and Sundaram (2001)
Date pits Basic blue 9 17.3 Banat et al. (2003)
Hazelnut shell Basic blue 9 8.82 Aygün et al. (2003)
Coir pith Acid violet 8.06 Namasivayam et al. (2001a)
Charfines Direct brown 1 6.4 Venkata Mohan et al. (1999)
Coir pith Direct red 28 6.72 Namasivayam and Kavitha (2002)
Sugarcane bagasse Acid orange 10 5.78 Tsai et al. (2001)
Coir pith Basic violet 10 2.56 Namasivayam et al. (2001a)

Plentiful agricultural and wood by-products may also offer an inexpensive and renewable additional source of AC. These waste materials have little or no economic value and often present a disposal problem. Therefore, there is a need to valorize these low-cost by-products. So, their conversion into AC would add economic value, help reduce the cost of waste disposal and most importantly provide a potentially inexpensive alternative to the existing commercial activated carbons.

2.4.2.3 AGRICULTURAL SOLID WASTES :

Raw agricultural solid wastes and waste materials from forest industries such as sawdust and bark have been used as adsorbents. These materials are available in large quantities and may have potential as sorbents due to their physico-chemical characteristics and low-cost. Sawdust is an abundant by-product of the wood industry that is either used as cooking fuel or as packing material. Sawdust is easily available in the countryside at zero or negligible price (Garg et al., 2004a). It contains various organic compounds (lignin, cellulose and hemicellulose) with polyphenolic groups that might be useful for binding dyes through different mechanisms (Table 19). The role of sawdust materials in the removal of pollutants from aqueous solutions has been reviewed recently (Shukla et al., 2002).

Table 19 : Adsorption capacity of agricultural solid wastes for the removal of dyes

   Adsorbent
Dye
Qmax
Sources
Bark Basic red 2 1119 McKay et al. (1999)
Bark Basic blue 9 914 McKay et al. (1999)
Rice husk Basic red 2 838 McKay et al. (1999)
Sugar-industry-mud Basic red 22 519 Magdy and Daifullah (1998)
Tree fern Basic red 13 408 Ho et al. (2005)
Pine sawdust Acid yellow 132 398.8 Özacar and Sengil (2005)
Palm-fruit bunch Basic yellow 327 Nassar and Magdy (1997)
Rice husk Basic blue 9 312 McKay et al. (1999)
Pine sawdust Acid blue 256 280.3 Özacar and Sengil (2005)
Vine Basic red 22 210 Allen et al. (2003)
Rice hull ash Direct red 28 171 Chou et al. (2001)
Egyptian bagasse pith Basic blue 69 168 Ho and McKay (2003)
Vine Basic yellow 21 160 Allen et al. (2003)
Egyptian bagasse pith Basic blue 69 152 Chen et al. (2001)
Coir pith Basic blue 9 120.43 Namasivayam et al. (2001b)
Coir pith Basic violet 10 94.73 Namasivayam et al. (2001b)
Eucalyptus bark Remazol BB 90 Morais et al. (1999)
Raw date pits Basic blue 9 80.3 Banat et al. (2003)
Fly ash Basic blue 9 75.52 Janos et al. (2003)
Egyptian bagasse pith Basic red 22 75 Chen et al. (2001)
Treated sawdust Basic green 4 74.5 Garg et al. (2003)
Wood sawdust Basic blue 69 74.4 Ho and McKay (1998a)
Metal hydroxide sludge Reactive red 2 62.5 Netpradit et al. (2003)
Metal hydroxide sludge Reactive red 141 56.18 Netpradit et al. (2003)
Metal hydroxide sludge Reactive red 120 48.31 Netpradit et al. (2003)
Treated sawdust Basic green 4 26.9 Garg et al. (2003)
Fe(III)/Cr(III) hydroxide Basic blue 9 22.8 Namasivayam and Sumithra (2005)
Banana peel Methyl orange 21 Annadurai et al. (2002)
Banana peel Basic blue 9 20.8 Annadurai et al. (2002, 1997)
Banana peel Basic violet 10 20.6 Annadurai et al. (2002)
Orange peel Methyl orange 20.5 Annadurai et al. (1999, 2002)
Egyptian bagasse pith Acid red 114 20 Chen et al. (2001)
Orange peel Acid violet 19.88 Rajeshwarisivaraj et al. (2001b)
Orange peel Basic blue 9 18.6 Annadurai et al. (2002)
Egyptian bagasse pith Acid blue 25 17.5 Chen et al. (2001)
Egyptian bagasse pith Acid blue 25 14.4 Ho and McKay (2003)
Orange peel Basic violet 10 14.3 Annadurai et al. (2002)
Fly ash Alizarin sulfonic 11.21 Woolard et al. (2002)
Coir pith Acid violet 7.34 Namasivayam et al. (2001a)
Wood sawdust Acid blue 25 5.99 Ho and McKay (1998a)
Sugar cane dust Basic green 4 4.88 Khattri and Singh (1999)
Banana pith Direct red 5.92 Namasivayam et al. (1998)
Red mud Direct red 28 4.05 Namasivayam and Arasi (1997)
Neem sawdust Basic violet 3 3.78 Khattri and Singh (2000)
Neem sawdust Basic green 4 3.42 Khattri and Singh (2000)


2.4.2.4 INDUSTRIAL BY-PRODUCTS :

Industrial solid wastes such as metal hydroxide sludge, fly ash and red mud are available locally at low cost can be used as adsorbents for dye removal (Namasivayam and Sumithra, 2005, Netpradit et al., 2003, Netpradit et al., 2004a, Netpradit et al., 2004b, Acemioglu, 2004, Janos et al., 2003, Mohan et al., 2002, Gupta et al., 2000, Ho and McKay, 1999b, Namasivayam and Arasi, 1997, Namasivayam et al., 1994 and Namasivayam and Chandrasekaran, 1991).

Recently, Netpradit et al., 2003, Netpradit et al., 2004a and Netpradit et al., 2004b studied the capacity and mechanisms of metal hydroxide sludge in removing azo reactive dyes. The sludge is a dried waste from the electroplating industry, which is produced by precipitation of metal ions in wastewater with calcium hydroxide. It contains insoluble metal hydroxides and other salts. The authors demonstrated that metal hydroxide sludge was an effective positively charged adsorbent with a high maximum adsorption capacity (48–62 mg dye/g material) for azo reactive (anionic) dyes. The charge of the dyes is an important factor for the adsorption due to the ion-exchange mechanism.

2.4.2.5 NATURAL MATERIALS :

Clay : Natural clay minerals are well known and familiar to humankind from the earliest days of civilization. Because of their low cost, abundance in most continents of the world, high sorption properties and potential for ion-exchange, clay materials are strong candidates as adsorbents. Clay materials possess a layered structure and are considered as host materials. They are classified by the differences in their layered structures. There are several classes of clays such as smectites (montmorillonite, saponite), mica (illite), kaolinite, serpentine, pylophyllite (talc), vermiculite and sepiolite (Shichi and Takagi, 2000). The adsorption capabilities result from a net negative charge on the structure of minerals. This negative charge gives clay the capability to adsorb positively charged species. Their sorption properties also come from their high surface area and high porosity (Alkan et al., 2004). Montmorillonite clay has the largest surface area and the highest cation exchange capacity.

Clay minerals exhibit a strong affinity for both heteroatomic cationic and anionic dyes (Table 20). However, the sorption capacity for basic dye is much higher than for acid dye because of the ionic charges on the dyes and character of the clay. The adsorption of dyes on clay minerals is mainly dominated by ion-exchange processes. This means that the sorption capacity can vary strongly with pH.

Table 20 : Biosorption of dyes by clays

Adsorbent
Dye
Qmax (mg/g)
Sources
Charred dolomite (Ireland) Reactive dye E-4BA 950 Walker et al. (2003)
Activated bentonite (Turkey) Acid blue 193 740.5 Özcan et al. (2004)
Activated bentonite (Spain) Sella fast brown H 360.5 Espantaleon et al. (2003)
Clay (Tunisia) Basic blue 9 300 Bagane and Guiza (2000)
Calcined alunite (Turkey) Reactive yellow 64 236 Özacar and Sengil (2003)
Calcined alunite (Turkey) Acid blue 40 212.8 Özacar and Sengil (2002)
Diatomite (Jordan) Basic blue 9 198 Al-Ghouti et al. (2003)
Calcined alunite (Turkey) Reactive blue 114 170.7 Özacar and Sengil (2003)
Sepiolite (Turkey) Reactive yellow 176 169.1 Ozdemir et al. (2004)
Activated clay (Singapore) Basic red 18 157 Ho et al. (2001)
Diatomite (Jordan) Basic blue 9 156.6 Shawabkeh and Tutunji (2003)
Calcined alunite (Turkey) Reactive red 124 153 Özacar and Sengil (2003)
Calcined alunite (Turkey) Acid yellow 17 151.5 Özacar and Sengil (2002)
Sepiolite (Turkey) Reactive black 5 120.5 Ozdemir et al. (2004)
Zeolite (Turkey) Reactive red 239 111.1 Ozdemir et al. (2004)
Sepiolite (Turkey) Reactive red 239 108.8 Ozdemir et al. (2004)
Zeolite (Turkey) Reactive yellow 176 88.5 Ozdemir et al. (2004)
Clay/carbons mixture Acid blue 9 64.7 Ho and Chiang (2001)
Zeolite (Turkey) Reactive black 5 60.5 Ozdemir et al. (2004)
Activated clay (Singapore) Acid blue 9 57.8 Ho et al. (2001)
Zeolite (Macedonia) Basic dye 55.86 Meshko et al. (2001)
Hydrotalcite Reactive yellow 208 47.8 Lazaridis et al. (2003)
Modified silica Acid blue 25 45.8 Phan et al. (2000)
Silica (Taiwan) Basic blue 9 11.21 Woolard et al. (2002)
Clay (Turkey) Basic blue 9 6.3 Gürses et al. (2004)
Alunite (Turkey) Reactive yellow 64 5 Özacar and Sengil (2003)
Glass powder Acid red 4 4.03 Atun and Hisarli (2003)
Alunite (Turkey) Reactive blue 114 2.92 Özacar and Sengil (2003)
Alunite (Turkey) Reactive red 124 2.85 Özacar and Sengil (2003)

The results presented above show that clay materials may be promising adsorbents for environmental and purification purposes.

Siliceous materials : The use of natural siliceous sorbents such as silica beads, glasses, alunite, perlite and dolomite for wastewater is increasing because of their abundance, availability and low price. Among inorganic materials, silica beads deserve particular attention (Krysztafkiewicz et al., 2002, Crini and Morcellet, 2002, Woolard et al., 2002, Harris et al., 2001 and Phan et al., 2000), considering chemical reactivity of their hydrophilic surface, resulting from the presence of silanol groups. Their porous texture, high surface area and mechanical stability also make them attractive as sorbents for decontamination applications. However, due to their low resistance toward alkaline solutions their usage is limited to media of pH less than 8 (Ahmed and Ram, 1992).

Zeolites : Zeolites are highly porous aluminosilicates with different cavity structures. Their structures consist of a three dimensional framework, having a negatively charged lattice. The negative charge is balanced by cations which are exchangeable with certain cations in solutions. Zeolites consist of a wide variety of species, more than 40 natural species. However, the most abundant and frequently studied zeolite is clinoptilolite, a mineral of the heulandite group. High ion-exchange capacity and relatively high specific surface areas, and more importantly their relatively cheap prices, make zeolites attractive adsorbents. Another advantage of zeolites over resins is their ion selectivities generated by their rigid porous structures (Ghobarker et al., 1999). Zeolites are becoming widely used as alternative materials in areas where sorptive applications are required. Although the removal efficiency of zeolites for dyes may not be as good as that of clay materials, their easy availability and low cost may compensate for the associated drawbacks.

Chitin and chitosan : The sorption of dyes using biopolymers such as chitin and chitosan is one of the reported emerging biosorption methods for the removal of dyes, even at low concentration (ppm or ppb levels). Chitin and chitosan are abundant, renewable and biodegradable resources. Chitin, a naturally occurring mucopolysaccharide, has been found in a wide range of natural sources such as crustaceans, fungi, insects, annelids and molluscs. However, chitin and chitosan are only commercially extracted from crustaceans (crab, krill, and crayfish) primarily because a large amount of the crustacean’s exoskeleton is available as a by-product of food processing. The annual worldwide crustacean shells production has been estimated to be 1.2 × 106 tonnes, and the recovery of chitin and protein from this waste is an additional source of revenue (Teng et al., 2001). These studies demonstrated that chitosan-based biosorbents are efficient materials and have an extremely high affinity for many classes of dyes (Table 21). They are also versatile materials. This versatility allows the sorbent to be used in different forms, from flake-types to gels, bead-types or fibers.

Table 21 : Chitosan biosorents for removal of dyes

Biosorbent
Dye
qmax
Sources
Crosslinked chitosan bead Reactive blue 2 2498 Chiou et al. (2004)
Crosslinked chitosan bead Reactive red 2 2422 Chiou et al. (2004)
Crosslinked chitosan bead Direct red 81 2383 Chiou et al. (2004)
Crosslinked chitosan bead Reactive red 189 1936 Chiou and Li (2002)
Crosslinked chitosan bead Reactive yellow 86 1911 Chiou et al. (2004)
Chitosan bead Reactive red 189 1189 Chiou and Li (2002)
Chitosan (bead, crab) Reactive red 222 1106 Wu et al. (2000)
Chitosan (bead, lobster) Reactive red 222 1037 Wu et al. (2000)
Chitosan Acid orange 12 973.3 Wong et al. (2004)
Chitosan Acid orange 10 922.9 Wong et al. (2004)
Chitosan Acid red 73 728.2 Wong et al. (2004)
Chitosan Acid red 18 693.2 Wong et al. (2004)
Chitosan Acid green 25 645.1 Wong et al. (2004)
Chitosan (flake, lobster) Reactive red 222 398 Wu et al. (2000)
Chitosan (flake, crab) Reactive red 222 293 Wu et al. (2000)

The traditional and commercial source of chitin is from shells of crab, shrimp and krill that are wastes from the processing of marine food products. However, this traditional method of extraction of chitin creates its own environmental problems as it generates large quantities of waste and the production of chitosan also involves a chemical deacetylation process. These problems can explain why it is difficult to develop chitosan-based materials as adsorbents at an industrial scale.

Peat : Peat is a porous and rather complex soil material with organic matter in various stages of decomposition. Based on the nature of parent materials, peat is classified into four groups, namely moss peat, herbaceous peat, woody peat and sedimentary peat. This natural material is a plentiful, relatively inexpensive and widely available biosorbent, which has adsorption capabilities for a variety of pollutants. Raw peat contains lignin, cellulose, fulvic and humic acid as major constituents. These constituents, especially lignin and humic acid, bear polar functional groups, such as alcohols, aldehydes, ketones, carboxylic acids, phenolic hydroxides and ethers that can be involved in chemical bonding.

Because of its polar character, peat can effectively remove dyes from solution (Allen et al., 2004, Ho and McKay, 1998b, Ho and McKay, 2003, Sun and Yang, 2003, Ramakrishna and Viraraghavan, 1997 and Poots et al., 1976). Peat adsorption capacities are reported in Table 22. For acid and basic dyes, the removal performance was comparable with that of activated carbon, while for disperse dyes, the performance was much better.

Table 22 : Recent reported adsorption capacity qmax (mg/g) for peat

Biosorbent
Dye
qmax
Sources
Treated peat Basic violet 14 400 Sun and Yang (2003)
Treated peat Basic green 4 350 Sun and Yang (2003)
Peat Basic blue 69 195 Ho and McKay (1998b)
Peat Acid blue 25 12.7 Ho and McKay (1998b)

However, when raw peat is used directly as an adsorbent, there are many limitations: natural peat has a low mechanical strength, a high affinity for water, poor chemical stability, a tendency to shrink and/or swell, and to leach fulvic acid (Couillard, 1994 and Smith et al., 1977).

2.4.2.6 MISCELLANEOUS SORBENTS :

Other materials have been studied as low-cost sorbents, such as starch (Delval et al., 2001, Delval et al., 2002 and Delval et al., 2003) and cyclodextrins (Crini, 2003, Crini and Morcellet, 2002, Crini et al., 1999, Crini et al., 2002a, Crini et al., 2002b, Martel et al., 2001 and Shao et al., 1996). Adsorption capacities are reported in Table 23. Next to cellulose, starch is the most abundant carbohydrate in the world and is present in living plants as an energy storage material. Starches are mixtures of two polyglucans, amylopectin and amylose, but they contain only a single type of carbohydrate, glucose. Starches are unique raw materials in that they are very abundant natural polymers, inexpensive and widely available in many countries. They possess several other advantages that make them excellent materials for industrial use. They have biological and chemical properties such as hydrophilicity, biodegradability, polyfunctionality, high chemical reactivity and adsorption capacities. However, the hydrophilic nature of starch is a major constraint that seriously limits the development of starch based-materials. Chemical derivatisation has been proposed as a way to solve this problem and to produce water resistant sorbents.

Table 23 : Miscellaneous sorbents for the removal of dyes.

Adsorbent
Dye
qmax
Sources
Cotton waste Basic red 2 875 McKay et al. (1999)
Treated cotton Acid blue 25 589 Bouzaida and Rammah (2002)
Treated cotton Acid yellow 99 448 Bouzaida and Rammah (2002)
Treated cotton Reactive yellow 23 302 Bouzaida and Rammah (2002)
Cotton waste Basic blue 9 277 McKay et al. (1999)
Starch-based material Acid blue 25 249 Delval et al. (2002)
Crosslinked cyclodextrin Acid blue 25 88 Crini (2003)
Chitosan/cyclodextrin material Acid blue 25 77.4 Martel et al. (2001)

There are several disadvantages of using starch-based materials for dye removal. The efficiency of adsorption depends strongly on the control or particle size and the expansion of the polymer network (Crini, 2003). Performance is also dependent on the type of material used. Another problem with these materials is that they are non-porous and possess low surface area. Adsorption by starch-based materials occurs by physical adsorption, complexation and ion-exchange interactions (Delval et al., 2003 and Crini, 2003).

Other materials used to adsorb dyes are cotton waste (Sawada and Ueda, 2003, Bouzaida and Rammah, 2002 and McKay et al., 1999) and alumina (Harris et al., 2001 and Desai et al., 1997). Adsorption capacities are reported in Table 9. Cotton is the most abundant of all naturally occurring organic substrates and is widely used. This material characteristically exhibits excellent physical and chemical properties in terms of stability, water absorbency and dye removal ability. The performance of treated cotton in a continuous system has been demonstrated by Bouzaida and Rammah (2002). They found that the adsorption capacities of cotton for acid blue 25, acid yellow 99 and reactive yellow 23 were 589, 448 and 302 mg/g, respectively. McKay et al. (1999) also evaluated the performance of cotton waste for dye removal. It was found that this waste had the potential to adsorb 875 and 277 mg of basic red 2 and basic blue 9, respectively.

Generally, a suitable non-conventional low-cost adsorbent for dye adsorption should meet several requirements: (i) efficient for removal of a wide variety of dyes; (ii) high capacity and rate of adsorption; (iii) high selectivity for different concentrations; and (iv) tolerant of a wide range of wastewater parameters.

Certain waste products, natural materials and biosorbents have been tested and proposed for dye/metal removal. It is evident from the discussion so far that each low-cost adsorbent has its specific physical and chemical characteristics such as porosity, surface area and physical strength, as well as inherent advantages and disadvantages in wastewater treatment. In addition, adsorption capacities of sorbents also vary, depending on the experimental conditions. Therefore, comparison of sorption performance is difficult to make. However, it is clear from the present literature survey that non-conventional adsorbents may have potential as readily available, inexpensive and effective sorbents for both heavy metals and dyes. They also possess several other advantages that make them excellent materials for environmental purposes, such as high capacity and rate of adsorption high selectivity for different concentrations, and also rapid kinetics.

 However, despite the number of published laboratory data, there is a need to look for viable non-conventional low-cost adsorbents to meet the growing demand due to the enhanced quantum of dyes and heavy metals in the environment

 

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