Effect of pH on Cobalt Sorption in Untreated and Farmyard Manure Treated Soil from a Semiarid Region

The effect of pH on sorption of Co in an untreated and the corresponding farmyard manure treated fine loamy, Typic Camborthid from a semi-arid region was investigated. The untreated and FYM treated soils were adjusted to three pH levels (i.e. high, medium and low) and made homoionic with Ca. The Ca-saturated soil samples were equilibrated with solutions having a wide range of Ca:Co at 10 mmol L⁻¹ total chloride concentration. The pH_eq of soil samples at all three initial pH levels decreased by 0.2 to 0.7 units with increasing proportion of Co in the equilibrating solutions. At comparable FYM, there was a substantial increase in ΔG_o with decrease in soil pH. In the high pH range, ΔG_o was substantially larger (i.e. by 2.03 kJ mol⁻¹), in the medium pH range slightly larger (i.e. by 0.39 kJ mol⁻¹), and in the low pH range considerably smaller (i.e. by 3.15 kJ mol⁻¹) for the FYM treated soil sample as compared to the untreated ones. The negative values of ΔG_o in the high and medium ranges of experimental pH, irrespective of FYM treatment, suggested thermodynamic preference for Co over Ca, and positive values in the low pH range thermodynamic preference for Ca over Co.