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 Cd 2+, Cu 2+, Pb 2+, and Zn 2+ were conducted in the presence of Fe 3+ by titration with KOH to pH 6 in order to form coprecipitates having concentrations of 100 mg kg - 1 Cd 2+, 1500 mg kg - 1 Cu 2+, 500 or 1500 mg kg - 1 Pb 2+, and 3000 mg kg - 1 Zn 2+ . The final soluble concentrations of Cd 2+ (0.28 and 0.16 μM), Zn 2+ (8.4 μM), Cu 2+ (0.60 μM), and Pb 2+ (<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 Cd 2+ and Zn 2+ were virtually unaffected by the pH changes induced by aging, whereas Cu 2+ solubility tended to increase as aging decreased pH. The solubility values (pMetal) of Cu 2+ and Zn 2+ coprecipitates with Fe 3+ indicate solubilities intermediate between that expected from metal (hydr)oxides precipitation and surface adsorption, whereas Cd 2+ solubility (pCd) indicated solubility lower than that expected from precipitation or adsorption.