Sorption of Phosphate on Pure and Cadmium-enriched Calcium Carbonate and Clay

Laboratory investigation was carried out to study the effect of cadmium pretreatment of pure calcium carbonate and clay isolate of a Typic Haplustept on their phosphate sorbing ability to elucidate the mechanism of phosphorus mobility and availability in the cadmium-contaminated soils. For cadmium enrichment CaCO3 and clay were treated with CdCl2 solution containing 10−4 and 10−3 mol Cd2+ L−1 and incubated for 48 h. The cadmium untreated and treated CaCO3 and clay were used as P sorbents. Distribution coefficient (Kd) and the percentage of added phosphate sorbed (XAd) decreased with increasing solution P concentration and Cd enrichment. The Kd decreased with increasing phosphate sorption (x/m). Phosphate sorption by CaCO3 and clay was satisfactorily described by two-surface Langmuir and Freundlich adsorption isotherm equations. The adsorption maxima and bonding energy constants were calculated in accordance with two surface-Langmuir equation of Syers et al. (1973). The values of P sorption maxima (b1 and b2) decreased and bonding energy (k1 and k2) increased with increasing Cd levels but the effect was more pronounced on b2 and k2 rather than b1 and k1. The enrichment of CaCO3 and clay with 10−4 mol Cd2+ decreased b1 by 20.62 and 22.94 and b2 by 72.65 and 24.80%, while at 10−3 mol Cd2+ level b1 decreased by 43.26 and 33.76% and b2 by 91.97 and 44.02% over corresponding b1 and b2 by original CaCO3 and clay. Freundlich constant K decreased and then increased with increasing Cd levels.