Biosorption is a potentially attractive technology for treatment of wastewater for removal of dyes from dilute solutions. Studies carried out through environmental biotechnology have shown that many biosorbents present in the nature have great capacity for removal of dyes. Biosorption of Congo red on Psidium guajava was studied and compared by using batch techniques. The biosorption experiments were carried out under various conditions, such as different contact times, pH, initial concentrations, biosorbent dosage, biosorbent average particle size and different temperatures. About 0.1 g of Psidium guajava was found to be enough to remove 92% of Congo red at the concentration of 20 mg/L from 30 mL aqueous solution in 60 min. The optimum pH was found to be 1.0. The pseudo-first-order and pseudo-second-order kinetic models were used to describe the kinetic data. The dynamic data were fitted with the pseudo-second-order kinetic model for Congo red dye. The experimental equilibrium data were tested by the biosorption isotherms like Langmuir, Freundlich, Redlich-Peterson and Temkin, and their equilibrium parameters were determined. The best fitted model to the experimental equilibrium data for Psidium guajava was Langmuir for Congo red dye.

Many industries, such as dyestuffs, textile, paper and plastics, use dyes in order to color their products and also consume substantial volumes of water. As a result, they generate a considerable amount of colored wastewater. It is recognized that public perception of water quality is greatly influenced by the color.

Color is the first contaminant to be recognized in wastewater (Banat et al., 1996). The presence of very small amounts of dyes, such as methylene blue, malachite green, crystal violet, methyl orange, etc., in water (less than 1 ppm for some dyes) is highly visible and undesirable (Robinson et al., 2001). Many of these dyes are also toxic and even carcinogenic and this poses a serious hazard to aquatic living organisms (O'Neill et al., 1999). The major effect of dyes on humans is that they cause severe mutagenic and carcinogenic effects. Due to increasingly stringent restrictions on the organic content of industrial effluents, it is necessary to eliminate dyes from wastewater before it is discharged (Wang et al., 2008).

In the past three decades, several physical, chemical and biological decolorization methods were reported. A few methods have been accepted by the paper and textile industries (Ghoreishi and Haghighi, 2003). Amongst the numerous techniques of dye removal, adsorption is the procedure of choice and gives the best results, as it can be used to remove different types of coloring materials (Jain et al., 2003). In the present investigation, untreated Psidium guajava L.f. leaf powder was examined in an attempt to remove Congo red from its aqueous solution.

Congo red is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acid. It is a secondary diazo dye. Congo red is water soluble, yielding a red colloidal solution; its solubility is better in organic solvents such as ethanol.

Chemical Engineering Journal, Leaf Powder, Biological Decolorization Methods, Coloring Materials, Biosorption Experiments, Biomass Cell Walls, Electrostatic Repulsion, Biosorption Sites, Equilibrium Biosorption, Biosorption Kinetics, Carcinogenic Effects.