Phosphate application impacts on cadmium sorption in acidic and calcareous soils

Land treatment of sewage sludge and the use of chemical fertilizers have resulted in cadmium (Cd) contamination of some soils. The effect of a soil's ionic environment on the availability of heavy metals in soils and plants is of concern. In this study, we investigated the environmental availability of Cd as it is affected by the application of different sources of the phosphate anion (monopotassium phosphate (MPP) and dipotassium phosphate (DPP)), and we examined pre-application (PAP) and simultaneous application (SAP) of phosphate sources on Cd sorption in acidic and calcareous soils. The soil-solution system pH in the acidic soil decreased with additions of MPP; however, it increased with DPP applications. The pH decreased with the application of both phosphates in the calcareous soil. Values of ionic strength (1) in the acid and calcareous soils were similar after applying the Cd source (CdCl2) even though they were significantly different in the untreated control soils. The I values with SAP, however, were consistently higher than those with PAP. Equilibrium Cd concentration in the soil-solution systems was also usually higher with SAP than with PAP. The value of Cd sorption capacity was the highest with PAP-DPP in both soils. The Cd sorption capacity and the bonding energy were not correlated in the acidic soil, whereas they were negatively correlated in the calcareous soil. The Cd sorption was decreased with decreasing pH in the soil-solution systems. Cadmium sorption tends to decrease with the presence of MPP and increase with the presence of DPP in both soil systems. The solution Cd concentrations with the SAP-P sources were always higher than those with the PAP-P sources in both soil-solution systems. The observed differences of Cd concentration in soil-solution systems may be attributed to (i) higher pH and lower I with PAP, (ii) less formation of Cd-phosphate complexes with PAP, and (iii) a larger increase in net negative charge on soil colloids with PAP than with SAP.