The influence of pH on the forms of cadmium in four West Australian soils

Abstract

The forms of cadmium in soils affect its uptake by plants and hence its potential toxicity to animals and humans. We studied the effect of pH on the forms of native and added Cd in four West Australian soils which differed in their clay, hydrous oxide and organic matter content. The forms of Cd were extracted sequentially by KCl, BaCl2, NaOCl, ammonium oxalate and concentrated acids. The majority of Cd applied to a sandy soil was found in the soluble (KCl) and the exchangeable (BaCl2) forms at all pH values. In the siliceous sand, the proportion of the Cd present in the exchangeable form increased as the soil solution pH increased. However, in the peaty sand, the opposite trend was observed; at pH 5, approximately 50% of the Cd was found in the exchangeable form, while at higher pH values, < 60% was bound to organic matter and 20% was in the soluble form. In soils in which the main adsorption surface was dominated by hydrous oxides (mainly goethite), 50-70% of the Cd was extracted as bound to oxides and as the residual fraction at pH < 5. At pH values > 5, the majority (90%) of it was extracted in these forms. Soils, containing clay (mainly kaolinite) as the major adsorbent, retained Cd mainly in exchangeable form at all pH values and at all the rates of Cd application. At pH > 5, however, some of the Cd was also found in the residual form and bound to organic matter. This work has shown that the form of added Cd in a soil cannot be elucidated by considering the major adsorbing component alone. It is also necessary to know the pH, the presence of other adsorbing surfaces and the rates of applied Cd.