Abstract :
The binding of toxic metals by soil is usually highly pH dependent. Soil acidification resulting in small changes of pH may lead to the mobilization and increased bioavailability of toxic metals. To predict the binding of toxic metals to soil particles, an understanding of physico-chemical retention mechanisms of soil particles as a function of pH is necessary. This study describes the effects of pH and soluble Al on Cd retention by different soil horizons, using a simple binary surface complexation modelling approach. In this approach the importance of metal binding sites associated with both mineral and soil humic components is included. The metal binding sites associated with the mineral component, are described using a constant-capacitance model and those associated with soil humic substances by a diprotic acid discrete functional group binding site. In order to explain Cd retention in acid soil solutions the model assumes that soluble Al species will compete with Cd and protons for metal binding sites. Results comparing model calculations with experimental data suggest that for Cd the competitive effects of Al will be most marked in the pH range 4–5 due to suppression of Cd binding to the soil humic component.
