Bio-adsorption of dyes from aqueous solution by powdered excess sludge (PES): Kinetic,isotherm, and thermodynamic study

Over 30 million tons of excess sludge is discharged from rural municipal sewage plants annually in China and it is predicted that this figure will keep increasing. However, most of the excess sludge is dumped in landfills except for minor applications. In this study, based on low-cost and recycling waste, the excess sludge was used to adsorb organic dyes from aqueous solution after being directly dewatered. The powdered excess sludge (PES) presents selective adsorption property to cationic dyes. Statics batch adsorption experiments of malachite green (MG) on PES were performed to evaluate the effects of pH, adsorbent dosage, and initial MG concentration. Results revealed that the bio-adsorption equilibrium of MG on the PES can be quickly achieved at 30 min with maximum percentage adsorption of 84% at pH 7, initial dye concentration of 20 mg L^?1, and adsorbent dosage of 1.5 g L^?1. Moreover, the adsorption kinetics follows a pseudo-second-order pathway, and the equilibrium adsorption data could be described well by the Langmuir isotherm equation. Intra-particle diffusion is not the only rate-controlling step in the entire adsorption process. The adsorption process is endothermic, spontaneous, and random. PES can be used as a low-cost adsorbent for refractory cationic organic dye in effluent.