An assessment of cadmium availability in cadmium-contaminated soils using isotope exchange kinetics

There has been much research conducted to find suitable methods for the determination of available Cd in soils. In recent years, an isotope exchange kinetics (IEK) technique has been tested to describe the kinetic transfer of ions from the soil solution to the solid phase. Although the IEK technique has been successful in describing nutrient availability in soils, it has not been widely applied to study contaminant availability. In this study, experimental conditions to determine exchangeable Cd in soils using IEK were determined along with a measurement of Cd availability using the IEK technique for 20 topsoils. The results indicated that isotopically exchangeable Cd [E(t) values] predicted from short-term isotopic kinetics (≤60 min) were only successful in predicting exchangeable Cd up to 24 h of exchange. After 24 h, E(t) values were significantly overestimated when compared with measured E(t) values. A compartmental analysis revealed that there were differences in the distribution of Cd in exchange pools between soils contaminated with Cd from different sources. The percentage of Cd located in the E(1 min) pool, was on average 21% of total Cd for the P-fertilizer soils compared with 13% for the biosolids-amended soils. In contrast, the biosolids-amended soils had on average 42% of total Cd located in the E(1 min–24 h) pool compared with 25% for the P-fertilizer soil. The E(>24 h) pool averaged 57% for the P-fertilizer soil compared with 46% for the biosolids-amended soil. The IEK technique may be a useful tool to provide information on Cd availability in soils