Effect of conditioning for Pb(II) and Hg(II) biosorption on waste activated sludge

This study focuses on adsorption of Pb(II) and Hg(II) on nonviable activated sludge biomass at both batch and continuous-flow operational conditions. The raw biomass was supplied from a nearby municipal wastewater treatment plant activated sludge aeration unit. First, optimum pHs for Pb(II) and Hg(II) adsorption was investigated on raw biomass. Optimum pH for Pb(II) and Hg(II) were found to be 3.5 and 5.8, respectively. Successively raw biomass was conditioned by 12 different methods and the method yielding highest adsorption capacity, at the predetermined optimum pHs, was investigated for sorption kinetics and isotherm modeling. Among the methods tested, the type of biosolids conditioned by holding the biosolids in NaOH solution for 15 min followed by autoclaving at 18 psi at 121 °C for 30 min (Type 4) yielded the highest biosorption capacity. The sorption kinetics and isotherm model fitting studies, applying pseudo-first and -second order rate models as well as long-familiar Langmuir and Freundlich isotherm models, demonstrated that data fit well to pseudo-second order kinetics and Langmuir isotherm models. The highest adsorption equilibrium, optimum pH and temperature for Pb(II) were 0.387 mmol g−1 at 1.206 mM initial concentration test run at pH of 3.5 and 30 °C while the same parameters for Hg(II) were found out to be 0.097 mmol g−1 at 0.099 mM initial concentration test run at pH of 5.8 and 20 °C, respectively. Continuous-flow tests were conducted in downflow operational mode using immobilized filter-matrix and the resulting data fitted well to Thomas kinetics model.