Cd, Cu, Pb, and Zn coprecipitates in Fe oxide formed at different pH: aging effects on metal solubility and extractability by citrate.

Coprecipitates of heavy metals with Fe oxides may form in contaminated soil, water, and sediment systems, particularly when oxidation-reduction processes are occurring. Once formed, coprecipitates with ferrihydrite could limit heavy metal mobility, solubility, toxicity, and bioavailability in geochemical systems. In this study, Cd, Cu, Pb, and Zn were coprecipitated with Fe by titration to pHs 6 and 7. Metal solubility was monitored during the coprecipitation process to pH 7 and after aging of the product. In the coprecipitate formed at pH 6, metal solubility was determined after the system reached pH 6 and at subsequent time intervals. We also reacted the coprecipitates with citrate and evaluated potential metal availability at increased aging times. The pH of coprecipitate formation had little effect on the long-term solubility of Cu and Zn, whereas soluble Cd was greater in the coprecipitate formed at pH 6. Soluble percentages of metals were low at both pH 6 and 7 for Cu and Pb but averaged 1.5 to 3% for Zn and greater than 40% for Cd. Hysteresis was observed in the coprecipitation curves for Zn and Cd, revealing reduced solubility after adsorption or coprecipitation. Lead and Cu failed to show hysteresis, with strong retention in the solid phase at pH greater than 6. The citrate-extractable fraction was greatest for Cd and Zn, less for Cu, and least for Pb. Citrate-extractable metal was higher for the coprecipitate formed at pH 7 than at pH 6, suggesting increased potential availability from coprecipitates formed at higher pH.