Sorption–bioavailability nexus of arsenic and cadmium in variable-charge soils

Highlights: ► Demonstrates the nexus between sorption and bioavailability of As and Cd in variable-charge soils. ► Liming variable-charge soils increase negative charge, thereby decreasing Cd bioavailability. ► Ageing of As and Cd increases their immobilization, thereby decreasing bioavailability. ► Phosphate enhances desorption and phytoavailability of As from sheep dip soil. ► Metal(loid)s transfer to food chain can be managed by controlling sorption reactions. -- Abstract: In this work, the nexus between sorption and bioavailability of arsenic (As) and cadmium (Cd) as affected by soil type, soil pH, ageing, and mobilizing agents were examined. The adsorption of As and Cd was examined using a number of allophanic and non-allophanic soils which vary in their charge components. The effect of pH and ageing on the bioavailability of As and Cd was examined using spiked soils in a plant growth experiment. The effect of phosphate (P)-induced mobility of As on its bioavailability was examined using a naturally contaminated sheep dip soil. The results indicated that the adsorption of both As and Cd varied amongst the soils, and the difference in Cd adsorption is attributed to the difference in surface charge. An increase in soil pH increased net negative charge by an average of 45.7 mmol/kg/pH thereby increasing cation (Cd) adsorption; whereas, the effect of pH on anion (As) adsorption was inconsistent. The bioavailability of As and Cd decreased by 3.31- and 2.30-fold, respectively, with ageing which may be attributed to increased immobilization. Phosphate addition increased the mobility and bioavailability of As by 4.34- and 3.35-fold, respectively, in the sheep dip soil. However, the net effect of P on As phytoavailability depends on the extent of P-induced As mobilization in soils and P-induced competition for As uptake by roots. The results demonstrate the nexus between sorption and bioavailability of As and Cd in soils, indicating that the effects of various factors on bioavailability are mediated through their effects on sorption reactions