Sorption of metals and metalloids from reverse osmosis concentrate on drinking water treatment solids

Abstract

Drinking water treatment solids (DWTS) from coagulation processes could be a low-cost sorbent for selective removal of certain metals and metalloids from brine. In this study, continuous-flow sorption processes and factors affecting sorption of boron (III), copper (II), chromium (VI), lead (II), and selenium (VI) from reverse osmosis (RO) concentrate were investigated using different types of iron-derived DWTS. DWTS was demonstrated to be very effective in removal of copper and lead from RO concentrate, attributed to formation of strong inner-sphere complexes between the metal cations and Fe/Al oxide surface sites in the DWTS, as well as attractive electrostatic interactions with the negatively charged DWTS surface. Sorption of boron and selenium on the DWTS was lower. Boron exists primarily as nonionic boric acid at operating pH of 5.5–7.8, while selenate is a weakly bonding anion typically assumed to form outer-sphere surface complexes with Fe/Al oxides, both resulting in low removal. Sorption of the metals and metalloids was affected by their speciation and electrostatic interactions with DWTS. Higher influent pH increased boron and copper sorption but inhibited removal of chromium. Water temperature, solid moisture content, and loading rate did not have significant impact on sorption to the DWTS.