Sorption and desorption kinetics of cadmium from soil: influence of phosphate

The mobility and availability of heavy metals is controlled by sorption-des orption characteristics of the soils. There is much literature available ab out the sorption characteristics of heavy metals by soils. However, the inf luence of ionic environments on the desorption kinetics of heavy metals, pa rticularly Cd, have not been studied in detail. The present study reports t he kinetic data of desorption of Cd from two soils, with contrasting Cd ava ilability characteristics, using 1 M NH4Cl. The soils were preadsorbed with different amounts of Cd in the presence of monoammonium phosphate. The Cd was sorbed onto the soils almost instantaneously in the absence of phosphat e, with >97% of the Cd added to the soils sorbed within the first 15 min of reaction time. The Freundlich parameter a, the sorption coefficient, which is related to the Cd sorption capacity, indicated that the phosphate retar ded Cd sorption by both soils by at least one order of magnitude. The overall diffusion coefficient of Cd release from the Luseland soil by 1 M NH4Cl, obtained using the parabolic diffusion model for the desorption k inetics, was 1.62 to 10.1 times higher than that of the Jedburgh soil, depe nding on the initial amount of Cd preadsorbed. The lower the amount of init ial Cd preadsorbed, the greater the difference in the rate of Cd released b etween the two soils. The presence of phosphate during Cd adsorption by the soils increased the amount of Cd released in the initial 30-min reaction p eriod as well as the overall diffusion coefficients of Cd release. The kine tic data of Cd desorption reflect well the Cd availability index and grain Cd content of the durum wheat crops, Kyle and Arcola, grown in the two soil s.