Solubility of lead, zinc and copper added to mineral soils

Elevated levels of heavy metals in soils are a result of industrial activit ies, atmospheric deposition, and the land application of sewage sludges and industrial by-products. Their persistence in the soil environment has crea ted interest in the possible changes in solubility. In this study, total di ssolved concentrations of Pb, Zn, and Cu were monitored in seven metal-amen ded soils (a calcareous and six acid mineral soils). Single metal solutions were added to soils and equilibrated (aged) for 40 days. During the 40 day s the soil was allowed to air-dry and was rewetted in cycles of about 5 day s. At the end of this reaction period, metal solubility was measured (by at omic absorption spectrometry and direct current plasma spectrometry) at the initial soil pH and at decreased pH values which were induced by addition of small aliquots of acid. As expected, solubility of added Pb, Zn, and Cu increased with a decrease in pH. Furthermore, the results showed that the s olubility relationship with pH was similar in all non-calcareous soils. Thi s suggests that metal solubility may be controlled by similar soil componen ts, presumably involving soil characteristics such as pH, organic matter co ntent, and soil mineralogy. For each metal, an approximate pH value was fou nd at which solubility deviated from the solubility of metals when they occ ur in soils at typical (natural) values. This pH was about (pH +/- 0.2): 5. 2 for Pb, 6.2 for Zn, and 5.5 for Cu. Thus, pH values below these threshold s may enhance metal mobility, biological availability and toxicity in soils . Metals dissolved at higher pH in the calcareous soil (18.8 g kg(-1) inorg anic carbon, initial pH 8.2). In a calcareous soil, a significant fraction of these metals react with carbonates, and decreased pH results in much hig her metal dissolution. Yet, metal solubility in soils is not determined by the formation and dissolution of single metal compounds. (C) 1999 Elsevier Science Ltd. All rights reserved.