## 2 Use of Agricultural Residues as Low-Cost Sorbents in Colour Removal  
[PERSON] et al. [6] have investigated in detail a few non-chemical methods for textile-effluent treatment. Researchers used three renewable natural substrates that are widely available globally as agricultural residues, either free of cost or at a very low cost. Corn-cob shreds, wood-chips, and wheat-straw were used in experiments for their ability to adsorb individual dyes and mixture of dyes from solutions consisting of synthetic dyes commonly used in textile companies. Nine individual Ciba Geigy dyes were used: Cibacron Red C-2G-reactive dye, Remazol Navy Blue GG-diazo dye, Remazol Red RB-diazo dye, Cibacron Orange CG-reactive dye, Remazol Golden Yellow RNL-azo dye, Hisperse Navy D2 GR-disperse dye, Remazol Blue B-diazo dye, Remazol Turquoise Blue G 133 -phthalocyanine dye, and Remazol Black B-diazo dye. The textile effluent was simulated as a mixture of all nine dyes. These residual agricultural substrates were used for removing colour from dye-contaminated water instead of using chemicals in order to study them as an economical and eco-friendly method of bio-remediation. Researchers have reported that up to 70-75% colour removal was achieved using corncob shreds and wheat straw from solutions of 500 mg/L dye [6; 7; 8]. Increasing the temperature of the process to above room temperature for the adsorption of dyes had no appreciable effect on the adsorption capacity of both residues. Corncob was the most efficient in dye colour removal, whereas wood chips had no adsorption of dyes due to their lignin-containing complex structure. There are other reports where researchers have used some locally/regionally available, low-cost, agricultural materials as biosorbents, such as residual apple pomace left after the juice extraction and wheat straw, for the removal of colour from a solution of textile dyes [9]. Experiments were performed with a synthetic effluent prepared in a lab, consisting of an equal mixture of five types of textile dyes at total dye concentrations of 10, 20, 30, 40, 50, 100, 150, and 200 mg/L of solution. The factors studied by these workers included the effect of dye concentration in solution, particle size of agricultural residues used as bio-sorbent material, amount of biosorbent added in dye-solution, the rate of adsorbance, percentage of dye removal, and the application of isotherms of [PERSON] and [PERSON]. The results proved apple pomace to be a better material for colour adsorption, adsorbing 81% of dyes from the synthetic effluent when used in the form of \(2\times 4\) mm particles. Even higher colour removal was achieved, up to 91%, when using dried apple pomace in a fine powdered form containing 600-\(\upmu\)m particles. Apple pomace showed adsorption of dyes at a faster rate than the other substrate of wheat straw; the reason is that wheat-straw has less sorption power in its complex lignin-containing structure, and apple pomace has negligible lignin. The use of these cheaply available substrates has been compared with the biosorption capacity of other commonly available agricultural residues, e.g., corncob and barley-husk [10]. The effectiveness of adsorption mechanism, as a method of colour removal, has made it an ideal alternative to other reported methods, which proved to be expensive treatments. Researchers conducted experiments in simulated effluents made of an equal amount of five commonly used textile dyes, setting the total dye concentrations in solution up to 200 mg L\({}^{-1}\). Several parameters were studied for the optimisation of the process, including dye concentration in solution, size of agricultural residue-particles used as sorbent, dose of biosorbent added in solution, effective adsorbance of dyes on particles of solid substrates, and the kinetics of process of dye removal.