Emerging phytotechnologies for remediation of heavy meal contaminated/ polluted soil and water

M.N.V. Prasad Department of Plant Sciences, School of Life Sciences University of Hyderabad Hyderabad 500046 AP, India Tel: +91-40-23011604, 23134509 (Direct lines) Fax: +91-40-23010120/23010145 E-mail: mnvsl@uohyd.ernet.in

Abstract

    The present world is facing problems with a wide variety of pollutants and contaminants (both inorganic and organic). Healthy soil, clean water and air are the soul of life. Often soil, water and air are no longer clean and pure, but pose human health risks. The supposedly most pristine environment in arctic circle and Antarctica are not even spared due to global transport of anthropogenic pollutants/contaminants of three major groups viz, i) heavy metals ii) acidifying gases (SOx), and iii) a variety of persistent organic pollutants that play a major role in global climate change. This presentation would concentrate only on inorganic pollutants and application of emerging phytotechnologies for environmental cleanup and restoration.

    The contamination of the environment with toxic metals has become a worldwide problem, affecting crop yields, soil biomass and fertility, contributing for the bioaccumulation and biomagnifications in the chain. In the last few decades, research groups have recognised that certain chemical pollutants such as toxic metals may remain in the environment for a long period and can eventually accumulate to levels that could harm humans. Moreover, the numerous classes and types of these chemicals apart from the soil structure complicate the removal of many toxic metals from the environment. As an alternative, an ecological technological approach has been developed involving the use of plants to clean up or remediate soils contaminated with toxic metals. A group of plants, termed "hyper-accumulators" are the best candidates capable of toxic metal uptake, transport and accumulate.

Introduction

    A wide range of phytoechnologies have emerged as a feasible technology for environmental restoration (Glass 1999, McCutcheon and Schnoor 2003). The wide recognitions for this approach is supported by the fact that it is considered to be an environmentally friendly technology, safe and also a cheap way to remove contaminants, in some cases doing the same job as a group of engineers for one tenth of the cost.

    In this presentation, the upcoming phytotechnologies for inorganic pollutants in soil and water such as phytoextraction, phytostabilisation, phytostimulation, phytovolatilisation, hydraulic barriers for containment of groundwater migration, vegetative caps for containment of landfill leachate, constructed wetlands, riparian buffer zones and buffer zones for stormwater detention, environmental restoration for erosion control, halophytic (salt-loving) plants, environmental crops. Phytotechnologies are being commercialised in the developed world (Glass 1999, McCutcheon and Schnoor 2003) (Figure 1 and 2)

    The contamination of the environment with toxic metals has become a worldwide problem, affecting crop yields, soil biomass and fertility, contributing to the bioaccumulation and biomagnifications in the food chain (Table 1). In the last few decades, research groups have recognised that certain chemical pollutants such as toxic metals may remain in the environment for a long period and can eventually accumulate to levels that could harm humans. Biological and engineering strategies designed to improve the use of phytoremediation to reduce the amount of heavy metals in contaminated soils has begun to emerge (Adriano et al 2004).

Table 1: Distinction between contaminant and pollutant

    Phytoremediation is a general term for several ways in which plants are used to clean up, or remediate sites by removing pollutants from soil and water. Plants can break down, or degrade, organic pollutants or contain and stabilise metal contaminants by acting as filters or traps. Some of the methods that are being tested are described in this fact sheet. Treating metal contaminants at sites contaminated with metals, plants are used to either stabilise or remove the metals from the soil and groundwater through three mechanisms: phytoextraction, rhizofiltration, and phytostabilisation. Phytoextraction, also called phytoaccumulation, refers to the uptake and translocation of metal contaminants in the soil by plant roots into the aboveground portions of the plants. Certain plants, called hyperaccumulators, absorb unusually large amounts of metals in comparison to other plants. One or a combination of these plants is selected and planted at a particular site based on the type of metals present and other site conditions. After the plants have been allowed to grow for some time, they are harvested and either incinerated or composted to recycle the metals. This procedure may be repeated as necessary to bring soil contaminant levels down to allowable limits. If plants are incinerated, the ash must be disposed of in a hazardous waste landfill, but the volume of ash will be large.

    Indigenous microorganisms are those microorganisms that are found already living at a given site. To stimulate the growth of these indigenous microorganisms, the proper soil temperature, oxygen, and nutrient content may need to be provided. If the biological activity needed to degrade a particular contaminant is not present in the soil at the site, microorganisms from other locations, whose effectiveness has been tested, can be added to the contaminated soil. These are called exogenous microorganisms. The soil conditions at the new site may need to be adjusted to ensure that the exogenous microorganisms will thrive.

    Essential processes involved in phytoremediation

1. Phytoaccumulation: The uptake and concentration of contaminants (metals or organics) within the roots or aboveground portion of plants.

2. Phytoextraction: Element accumulating plants are established on contaminated soil and later harvested in order to remove the specific elements from the soil.

3. Phytoextraction: The use of plants at waste sites to accumulate metals into the harvestable, above-ground portion of the plant and, thus, to decontaminate soils

4. Phytoextraction coefficient: The ratio of metal concentration in the plant (g metal/g dry weight tissue) to the initial soil concentration of the metal (g metal/g dry weight soil) for phytoextraction of metals.

5. Phytomining: Use of plants to extract inorganic substances from mine ore.

6. Phytoremediation: Use of plants to remediate contaminated soil or water.

7. Phytostabilisation: Plants tolerant to the element in question are used to reduce the mobility of elements, thus, they are stabilised in the substrate or roots.

8. Phytovolatilisation: The uptake and transpiration of a contaminant by a plant, with release of the contaminant or a modified form of the contaminant to the atmosphere from the plant.

9. Rhizofiltration/ phytofiltration: Roots or whole plants of element accumulating plants absorb the element from polluted effluents and are later harvested to diminish the metals in the effluents

10. Rhizosecretion: A subset of molecular farming, designed to produce and secrete valuable natural products and recombinant proteins from roots

Constructed wetlands for water treatment :

    Stormwater is an important water resource. Stormwater treatment and management are important topics in tropical countries. In the wake of 2005 Bangalore and Mumbai urban flash floods, all concerned authorities must examine the feasibility of implementing “Sustainable Urban Drainage Systems (SUDS)” a well established operational in developed counties. This is a neglected area in Indian scenario.

    The use of aquatic plants in water quality assessment has been common for years as in-situ biomonitors and for in situ remediation. The occurrence of aquatic macrophytes is unambiguously related to water chemistry and using these plant species or communities as indicators or biomonitors has been an objective for surveying water quality. Aquatic plants have also been used frequently to remove suspended solids, nutrients, heavy metals, toxic organics and bacteria from acid mine drainage, agricultural landfill and urban storm-water runoff. In addition, considerable research has been focused on determining the usefulness of macrophytes, as biomonitors of polluted environments and as bioremediative agents in wastewater treatments (Fritioff and Greger 2003, Mohan and Hosetti 1999, Kamal et al 2004, Lytle et al 1998). The response of an organism to deficient or excess levels of metal (i.e. bioassays) can be used to estimate metal impact. Such studies done under defined experimental conditions can provide results that can be extrapolated to natural environment. There are multifold advantages in using an aquatic macrophyte as a study material. Macrophytes are cost-effective universally available aquatic plants and with their ability to survive adverse conditions and high colonisation rates are excellent tools for studies of phytoremediation. Rooted macrophytes especially play an important role in metal bioavailability through rhizosphere secretions and exchange processes. This naturally facilitates metal uptake by other floating and emergent forms of macrophytes. Macrophytes readily take up metals in their reduced form from sediments, which exist in anaerobic situations due to the lack of oxygen and oxidise them in the plant tissues making them immobile and bioconcentrate them to a great extent. Metals concentrated within macrophytes are available for grazing by fish. These may also be available for epiphytic phytoplankton, herbivorous and detrivorous invertebrates. This may be a major route for incorporating metals in the aquatic food chain. It is therefore of interest to assess the levels of heavy metals in macrophytes due to their importance in ecological processes. The immobile nature of macrophytes makes them a particularly effective bio-indicator of metal pollution, as they represent real levels present at that site. Data on phytotoxicity studies are considered in the development of water quality criteria to protect aquatic life, the toxicity evaluation of municipal and industrial effluents (APHA-American Public Health Association 1998). In addition aquatic plants have been used to assess the toxicity of contaminated sediment elutriates and hazardous waste leachates.

    In the past research with macrophytes has centred mainly on determining effective eradication techniques for nuisance growth of several species such as Elodea Canadensis, Eichhornia crassipes, Ceratophyllum demersum, etc. Scientific literature exists for the use of wide diversity of macrophytes in toxicity tests designed to evaluate the hazard of potential pollutants, but the test species used is quite scattered. Similarly literature concerning the phytotoxicity tests to be used, test methods and the value of the result data is scattered. Estuarine and marine plant species are being used considerably less than freshwater species in toxicity tests conducted for regulatory reasons. The suitability of a test species is usually based on the specimen bioavailability, sensitivity to toxicant, reported data and the like. The sensitivity of various plants to metals was found to be species and chemical specific, differing in the uptake as well as toxicity of metals. Many submersed plants have been used as test species, but there is no widely used single species. In a literature survey only 7% of 528 reported phytotoxicity tests used macrophytic species (Wang and Freemark 1995). Their use in microcosm and mesocosm studies is even rarer and has been highly recommended. Several plant species like Lemna, Myriophyllum, Potamogeton have been exhaustively used in phytotoxicity assessment, but several others have been given less importance as a bioassay tool. Duckweeds have received the greatest attention for toxicity tests as they are relevant to many aquatic environments, including lakes, streams, effluents. Aquatics encompassing:

    Eichhornia crassipes, Elodea Canadensis, Heteranthera dubia, Myriophyllum spicatum, Potamogeton pectinatus, P.richardsonii, Vallisneria American, V. spiralis, Wolffia globosa, Lemna trisulca, Hydrilla verticillata and Typha latifolia have been almost extensively studied both in lab and field.

Metal concentration Factor :             [Metal] plant (g/g dry wt)
                                                             [ Meta ] water(g/mL)

    Pollutants that originate mainly from non-point sources are difficult to control. Constructed wetlands are designed to intercept and remove a wide range of contaminants from wastewater. These wetlands can save time and money by using natural mechanisms to treat non-point source pollution before it reaches lakes, rivers, and oceans. Conventional wastewater treatment plants can effectively remove non-point source pollution, but are expensive to build and operate on. Therefore, local wastewater treatment plants are desirable to reuse water.

    The most important role of plants in wetlands is that they increase the residence time of water, which means that they reduce the velocity and thereby increase the sedimentation of particles and associated pollutants. Thus, they are indirectly involved in water cleaning. Plants also add oxygen, providing a physical site of microbial attachment to the roots generating positive conditions for microbes and bioremediation. For efficient removal of pollutants, a high biomass per volume of water of the submerged plants is necessary. Uptake of metals in emergent plants only accounts for 5% or less of the total removal capacity in wetlands. Not many studies have been performed on submerged plants, however, higher concentration of metals is submerged than emerged plants has been found and in microcosm wetland, the removal by Elodea canadensis and Potamogeton natans showed up to 69 % removal of Zn. (Kadlec 1995; Kadlec and Knight 1996)

    Constructed and engineered wetlands for water treatment (Figures 3-8; Okurut et al 1999)

Natural wetlands :
        used for wastewater treatment for centuries
        uncontrolled discharge irreversible degradation.

Constructed wetlands :
        effective in treating organic matter, nitrogen, phosphorus, decrease the concentrations of heavy metals,
        organic chemicals, and pathogens

 Figure 6: Constructed wetland for removal waste water                     Figure 7: Constructed wetland for removal waste water treatment
 treatment (Vertical flow system with macrophytes)                              (Horizontal flow with macrophytes)

Potential role of aquatic plants in phytotechnology :

    Phytoremediation is defined as the use of plants for environmental cleanup (Prasad 2007). (Table 2 and 3) Aquatic macrophytes have paramount significance in the monitoring of metals in aquatic ecosystems (eg: Lemna minor, Eichhornia crassipes, Azolla pinnata) Aquatic plants are represented by a variety of macrophytes including algal species that occur in various habitats. They are important in nutrient cycling, control of water quality, sediment stabilisation and provision of habitat for aquatic organisms. The use of aquatic macrophytes in water quality assessment has been a common practice employing in-situ biomonitors (Sobolewski 1999)

Table 2: Aquatic plants for and biomonitoring of toxic trace elements in a wide range of toxicity bioassays
(Prasad 2007, Prasad (2001a,b, 2004 a,b, 2006, 2007, Prasad and Freitas 2003, Prasad et al 2001, 2006 )

Table 3: Aquatic macrophyts a for phytotechnologies to treat inorganic pollutants, acid mine drainage, salt water, regulation of storn water and removal of radionuclides. For details of experiments (field and laboraroty), reference may be made to COST action 837, 2003 and COST action 859, 2005, Hattink et al 2000, McCutcheon and Schnoor 2003, Peles et al 2002, Prasad (2001a,b, 2004 a,b, 2006, 2007, Prasad and Freitas 2003, Prasad et al 2001, 2006 )

    The submerged aquatic macrophytes have very thin cuticle and therefore readily take up metals from water through the entire surface. Hence, the integrated amounts of biologically available metals in water and sediment can be indicated to some extent by using macrophytes. Macrophytes with their ability to survive adverse conditions and high colonisation rate are excellent tools for phytoremediation. Further they redistribute metals from sediments to water and finally take up in the plant tissues and hence maintain circulation. Benthic rooted macrophytes (both submerged and emergent) play an important role in metal bioavailability from sediments through rhizosphere exchanges and other carrier chelates. This naturally facilitates metal uptake by other floating and emergent forms of macrophytes. Macrophytes readily take up metals in their reduced form from sediments, which exist in anaerobic situations due to lack of oxygen and oxidise them in the plant tissues making them immobile and hence bioconcentrate them to a high extent Okurut et al 1999)