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 (X_Ad) 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.