Solubility of Cd2+, Cu2+, Pb2+, and Zn2+ in aged coprecipitates with amorphous iron hydroxides

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) pb(2+), and 3000 mg kg(-1) Zn2+. The final soluble concentrations of Cd2+ (0.28 and 0.16 mu M), Zn2+ (8.4 mu M), Cu2+ (0.60 mu M), 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 intermediate between that expected from metal (hydr)oxides precipitation and surface adsorption, whereas Cd2+ solubility (pCd) indicated solubility lower than that expected from precipitation or adsorption.