Changes in the forms of cadmium with time in some Western Australian soils

Abstract

Changes in the forms of Cd with time could affect its uptake by plants and hence potential toxicity to animals and humans. The effect of time on the forms of native and added Cd was studied in four West Australian soils which differed in their clay, hydrous oxide and organic matter content. Sequential extraction of soluble (KCl), exchangeable (BaCl2), bound to organic matter (NaOCl), bound to oxides/clays (ammonium oxalate) and residual (concentrated acids) forms of Cd was carried out at different time intervals after the addition of Cd. The Cd that was added to the soils transformed with time to less soluble forms; the extent depending upon the type of soil. In addition, the rate of transformation in a particular type of soil was affected by both pH and rate of Cd addition. Soluble cadmium in the sandy soil decreased with time whereas the exchangeable form increased. The extent of the changes increased with increase in pH. In the peaty sand at pH < 5, exchangeable Cd increased apparently at the expense of soluble Cd. At higher pH values, however, Cd bound to organic matter increased with time as exchangeable Cd decreased. In the lateritic podzolic soil (dominated by kaolinite), there was no effect of time on the forms of Cd at pH values <. At pH > 6, the exchangeable form of Cd decreased whereas Cd bound to organic matter and residual Cd increased with time. In the yellow earth (dominated mainly by goethite), soluble Cd decreased with time at pH values < 5 and became a negligible fraction at pH 6. Exchangeable Cd decreased with time at pH values > 5 whereas Cd bound to oxides and residual Cd increased with time at all the pH values.