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

The mobility and availability of heavy metals is controlled by sorption-desorption characteristics of the soils. There is much literature available about the sorption characteristics of heavy metals by soils. However, the influence of ionic environments on the desorption kinetics of heavy metals, particularly Cd, have not been studied in detail. The present study reports the kinetic data of desorption of Cd from two soils, with contrasting Cd availability 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 phosphate, 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 retarded 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 kinetics, was 1.62 to 10.1 times higher than that of the Jedburgh soil, depending on the initial amount of Cd preadsorbed. The lower the amount of initial Cd preadsorbed, the greater the difference in the rate of Cd released between 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 period as well as the overall diffusion coefficients of Cd release. The kinetic 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 soils.