The effect of soil acidification on Cd and Pb precipitation

A model for estimating the precipitation of Cd-, Pb- and As-sulphide in soils at various pH is presented. pH influences not only the solubility of heavy metal-sulphides but also on the distribution of the S atom among sulphide (S²⁻, SH⁻, H₂S) and sulphate (SO₄²⁻) species by the redox potential of the soil solution. The model, based on the Nerst-equation. considers the equilibrium of protolitic processes and complex ion formation. In order to test the model, a long term field experiment polluted with different levels of toxic heavy metals on Hungarian typical soils was performed. Coarse textured soils were treated with Cd. Pb and As salts at pH = 6.8 and 8 and after 6 months and 2 years the heavy metal precipitation was investigated. The Pb ion precipitated at pH = 8.0 and pH = 6.2 while Cd ion at pH = 8. The As ion did not show precipitation effect in any soil according to the majority of arsenate ions at the actual redox potential. Although the solubility of CdS (pK = 26.1) and PbS (pk = 26.6) is similar, the Cd concentration in the solution was higher than that of Pb. The difference between the two ion concentrations is interpreted by the stronger Pb ion adsorption to organic and inorganic soil constituents. It indicated that the adsorbed and complex ion should be considered for the prediction of precipitation.