Removal and recovery of metal ions from acidic multi-metal mine water using waste digested activated sludge as biosorbent

Abstract

Highly acidic and toxic metal ion containing acid rock drainage (ARD) can cause severe environmental problems and endanger aquatic life. However, due to the high metal ion containing ARD is an auspicious source of metals for recovery and reuse. The feasibility of using waste digested activated sludge (WDAS) as a biosorbent for selective metal recovery from a highly complex mine water characterized by low pH (2.6), high sulfate concentration (80.8 g L−1) and a multitude of metals (Co, Cr, Cu, Fe, Mn, Ni, Th, U, V, Y, and Zn) at concentrations from few mg L−1 to several g L−1 was investigated. The effect of pH (2.0–10.0) and WDAS concentration (3.8–22.2 g L−1) on metal ion removal and the effect of several recovery solutions and their concentrations on metal recovery were explored in batch experiments. Metal removal was influenced by pH and WDAS concentration. A strong selectivity was observed in the recovery step employing 1 M Na2CO3 solution. The combination of a one-step removal and a two-step recovery approach enabled recovery of U (108.0 ± 6.6%), Cu (39.2 ± 2.5%), and Th (53.7 ± 7.7%) due to formation of strong carbonate complexes increasing the purity of U and Cu up to tenfold compared to the mine water. The results revealed that careful adaption of pH, WDAS concentration, and number of steps of the process according to the combination of metals present in solution is of great importance.