Phosphate-induced cadmium adsorption in a tropical savannah soil

Abstract

The influence of phosphate (P) on cadmium (Cd) adsorption was examined in a savanna soil with long history of different fertilizer amendment. The soil was incubated with P at 0, 250 and 500 mg P kg-1 soil and left to equilibrate for 2 weeks. Cd was added to the P-incubated soil at concentrations ranging from 27, 49 and 83 mg Cd kg soil. Addition of phosphate as KH2PO4 increased Cd adsorption by the soil. The amount of Cd adsorbed at 250 mg P kg^-1 treatment was more than the amount of Cd adsorbed at 500 mg P kg^-1 at low equilibrium Cd concentration. The sorption maxima and the distribution coefficient also increased; but the distribution coefficient and binding energy at 250 mg P kg^-1 were higher than those at 500 mg P kg^-1. Fitting the experimental data to adsorption, isotherm indicated a shift from the L-curve to the H-curve isotherm with P-amendment indicating a possible precipitation of Cd-P compounds. The Langmuir sorption maxima increased from 61 to 99.3 with P addition while the distribution coefficient also increased from 13.9 to 29.4. Our results indicated that Cd in the presence of phosphate can act as a bridging element between phosphate and organic matter.

Keywords: Cadmium, adsorption isotherms, phosphate.

African Journal of Biotechnology Vol. 12(24), pp. 3822-3827