Adsorption of heavy metals by EPS of activated sludge

Extracellular polymeric substances (EPSs) were extracted by high-speed centrifugation at 20,000 G for 30 min from an activated sludge treating municipal wastewater. Each gram of sludge, as measured in volatile suspended solids (VSS), contained 7.3 mg of EPS, including 6.5 mg of protein (EPS_p) and 0.8 mg of carbohydrate (EPS_c). The EPSp had a mean MW of 2.0×105; about 18% of EPS_p had MW over 5×104 and 16% below 5×103. For heavy metal concentrations ranging 10-100 mg/l, EPS on average removed 99% of Zn²⁺, 98% of Cu²⁺, 97% of Cr³⁺, 85% of Cd²⁺, 69% of Co²⁺, 37% of Ni²⁺, and 26% of CrO4^2-. The relative degrees of metal removals were inconsistent with those reported for the activated sludge process. Each mg of ESP had the capacity to remove up to 1.48 mg of Zn²⁺, 1.12 mg of Cu²⁺, 0.83 mg of Cr³⁺, 0.90 mg of Cd²⁺, 1.10 mg of Co²⁺, 0.25 mg each of Ni²⁺ and CrO4^2-. Results suggest the feasibility of recovering ESP from waste sludge for use as adsorbent. Freundlich isotherm correlated satisfactorily with the adsorption data of Ni²⁺,Cu²⁺, Cd²⁺, and CrO4^2-, (R2 ranging 0.89-0.97), whereas Langmuir isotherm correlated satisfactorily with those of Zn²⁺, Cr³⁺ and Ni²⁺ (R2 ranging 0.93-0.96). Both correlated poorly for those of Co²⁺.