Dissolved and Labile Concentrations of Cd, Cu, Pb, and Zn in Aged Ferrihydrite−Organic Matter Systems

The relative efficiencies of an organic and a mineral component of soils in controlling Zn, Cd, Cu, and Pb solubility and dissolution initially and during aging at pH 5.5 for up to 200 days is investigated. Metal retention by a natural organic matter (leaf compost) and a mineral model system (ferrihydrite) were tested for the organic and hydrous ferric oxide components separately (ORG and HFO) and in a mixed system (HFO−ORG). Total dissolved (by ICP) and labile (by dpasv) concentrations of metals in solution were measured. Initial Cd and Zn solubility in the systems followed the order: HFO > HFO−ORG > ORG. After aging for about 200 days, however, Cd and Zn solubility was HFO = HFO−ORG > ORG. Thus, the organic adsorbate proved to be more efficient in Zn and Cd removal from solution under the conditions used in this study. The HFO system resulted in the highest Cu solubility at intermediate aging times. However, during longer aging, total dissolved Cu increased in the ORG system whereas that in the HFO decreased, so that Cu solubility was lower in HFO after about 200 days. Lead solubility generally remained very low (<0.05 μM) except in the ORG system in which the total dissolved Pb reached 0.25 μM. The ORG system shows that about 75% of total dissolved Cu and 80% of total dissolved Pb exist as nonlabile organo-metal complexes, while soluble nonlabile complexes account for about 40% of dissolved Zn. Cadmium complexation (nonlabile) in the ORG system was minimal, thus Cd exists mostly in the free ionic form or as weak (labile) organic complexes.