Optimization of manganese recovery from groundwater treatment sludge for the production of highly-efficient Cu(II) and Pb(II) adsorbents

Manganese is one of the main components of groundwater treatment sludge. Meanwhile, copper (Cu) and lead (Pb) ions are usually present in various metal plating spent rinse waters and their removal prior to effluent release is of great importance to mitigate the adverse effects of these metals on human health and the environment. In this study, manganese recovery from groundwater treatment sludge by reductive acid leaching was optimized to produce manganese dioxide adsorbents for the competitive removal of Cu(II) and Pb(II) from simulated electroplating wastewaters. Box-Behnken design was used to optimize manganese recovery and analyze the two-parameter interactions of reductive leaching parameters for manganese recovery from the sludge, namely: sulfuric acid concentration, hydrogen peroxide dosage and sludge mass. Results showed that manganese recovery via reductive acid leaching was found to be more dependent on sludge mass than on sulfuric acid concentration and hydrogen peroxide dosage. Analysis of the elemental adsorbent composition revealed a 72.47: 27.53 manganese-to-oxygen ratio, while the evaluation of the adsorbent surface functional groups confirmed the presence of hydroxides and manganese oxides. Adsorbent point-of-zero-charge was determined at pH 6.0. Adsorption capacities for Pb(II) and Cu(II) reached 177 and 118 mg g(-1), respectively. Metal uptake for both adsorbates conformed with the extended Langmuir isotherm, implying monolayer adsorption, while film diffusion was the rate-limiting mechanism. Overall, the findings of this study demonstrate high manganese recovery from groundwater treatment sludge and the production of highly-efficient adsorbents for the treatment of heavy metal-containing industrial effluents.