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. 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 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 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.

The objective of the present study is to explore the feasibility of using four agricultural by-products namely channa dal (Cicer arientinum) husk, tur dal (Cajanus cajan) husk, tamarind pod (Tamarindus indica) shells and coffee (Coffee arabica) husk in the removal of four dyes namely methylene blue, fast green, amaranth and rhodamine B. Batch-mode kinetic and equilibrium studies have been carried out. Regeneration of dyes from the spent adsorbent to recover the adsorbent and adsorbate has been studied. The attractive features of the adsorbents used in the present study are that it is environmentally friendly and of low cost.

The results showed that coffee husk was not efficient in the biosorption of dyes. The maximum adsorption of methylene blue by the four husks was at pH 8.0; fast green 3.0; amaranth at pH 2.0 and rhodamine B at 7.0. The percentage of adsorption increased with increase in time till equilibrium was achieved. The equilibrium time varied with the initial dye concentration and the type of the adsorbent. The amount of adsorbent required to reach equilibrium differed with the type of the dye. In general, an increase in adsorption was seen with increase in adsorbent concentration. The adsorption capacity and intensity was calculated from Langmuir and Freundlich isotherm models and generally varied with the type of the adsorbent and dye under investigation. The Langregren equation was used to calculate the disassociation constant, which varied with type of adsorbent, and dyes. The infrared spectra of the adsorbents before and after binding to the dye revealed the presence of carboxylic, amine and hydroxyl groups which facilitated the biosorption of dyes.

The agricultural byproducts used in the present investigation are attractive alternatives to the existing adsorbents. The adsorption capacity was comparable to activated carbon. The type of adsorption was mainly chemisorption showing that the adsorption was due to ion exchange or chemical bonding or both. The optimal conditions for adsorption varied on the basis of the charge carried by the individual dye; i.e., whether the dye under investigation was a cation or anion. Chemical reactions representing the adsorption mechanism have been worked out.