Changes in composition of soil aqueous phase influence chemistry of indigenous heavy metals in alkaline sodic and acidic soils

The effect of major cations and ionic strength (I) on the chemistry of indigenous heavy metals, Cd, Cu and Zn in alkaline sodic and acidic soils was investigated. Both I and the nature of index cations had a marked effect on the concentrations of dissolved organic carbon (DOC), Cd, Cu and Zn in the soil liquid phase. With increasing I, the concentration of DOC decreased in all soils. A similar effect of index cation was also recorded; i.e. divalent cations led to a decrease in DOC relative to the monovalent cations. These changes in DOC were also accompanied by marked differences in the concentrations of Cd, Zn and Cu in the two soil types. While the concentration of Cd increased with I in all soil types, that of Zn increased in acid soils and decreased markedly in alkaline sodic soils. The concentration of Cu also decreased in sodic soils with increasing I. The concentration of Cd and Zn in solution phase increased in the order Ca < Mg < Na in acidic soils. In contrast, there was little obvious effect of index cations on Cu in alkaline sodic soils. MINTEQA2 speciation studies showed that the ratio of free hydrated Zn2+ to the total Zn increased from < 1% and 18% to 5% and 76% with I in two alkaline sodic soils studied, while the ratio of org-Zn to the total Zn decreased from 99% and 78% to 92% and 15% in the same conditions, respectively. This shows that DOC is an important factor controlling the solubility of Zn and Cu in alkaline sodic soils through organo-mineral complexation reactions. (C) 1998 Elsevier Science B.V. All rights reserved.