Evaluation of sorption isotherms for selected azo dyes as predictors of fate in an activated sludge wastewater treatment process

Azo dyes are of concern to environmental regulators because of the widespread production and utilization of these compounds as well as their potential adverse impact to environmental and human health. The typical line of defense between a dye-laden effluent and a receiving stream, river or other body of water is an aerobic biological wastewater treatment plant; hence the need for determining the fate of azo dyes undergoing such treatment. Activated sludge pilot plant studies conducted to determine the fate of several azo dyes have been reported in the literature. However, such experiments are time consuming, costly, and require facilities and expertise outside the realm of the dye industry. Therefore, laboratory batch sorption studies have been evaluated for their efficacy as predictors of fate for selected azo dyes, since sorption is considered to be a key removal mechanism for most dyes in aerobic biological systems. The four sorbents utilized in this study were activated sludge solids, powdered activated carbon, XAD-2 polymeric resin and XAD-7 polymeric resin. The sorbates utilized in this study were water-soluble azo dyes of the acid and direct categories. Over forty compounds were investigated with a little over half of them being acid dyes. Sorption data were appliedmore » to three widely accepted sorption models: Freundlich, Langmuir and the Brunauer-Emmett-Teller (BET) isotherm equations. Potential relationships between the various sorbate-sorbent systems and the three sorption models were investigated.« less