Solubility of Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ in Aged Coprecipitates with Amorphous Iron Hydroxides

Abstract

Coprecipitates of heavy metals in iron oxides are possible long-term sinks that could limit metal mobility and toxicity in soils. To test the efficacy of this process to maintain low metal solubilities, rapid and slow simultaneous titrations of Cd2+, Cu2+, Pb2+, and Zn2+ were conducted in the presence of Fe3+ by titration with KOH to pH 6 in order to form coprecipitates having concentrations of 100 mg kg-1 Cd2+, 1500 mg kg-1 Cu2+, 500 or 1500 mg kg-1 Pb2+, and 3000 mg kg-1 Zn2+. The final soluble concentrations of Cd2+ (0.28 and 0.16 μM), Zn2+ (8.4 μM), Cu2+ (0.60 μM), and Pb2+ (<5 nM) after aging the coprecipitates for long periods, as measured by voltammetry, corresponded to 28 and 59%, 58%, 94%, and 100% removal from solution by the coprecipitate, respectively. Soluble Cd2+ and Zn2+ were virtually unaffected by the pH changes induced by aging, whereas Cu2+ solubility tended to increase as aging decreased pH. The solubility values (pMetal) of Cu2+ and Zn2+ coprecipitates with Fe3+ indicate solubilities ...