Effect of pH on Cadmium Sorption in Untreated and Farmyard Manure Treated Soil from a Semi-arid Region

The effect of pH on sorption of Cd in untreated and 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:Cd at 10 mmol L⁻¹ total chloride concentration. The pH_eq, of all soil samples decreased from their initial levels on increasing the proportion of Cd in equilibrating solutions. At comparable FYM, ΔG₀, steeply increased with the decrease in soil pH. In the high pH range, ΔG₀, was slightly larger, in the medium pH range almost equal, and in the low pH range considerably smaller for the FYM treated soil samples as compared to the untreated ones. The negative values of ΔG₀ in the high and medium ranges of experimental pH, irrespective of FYM treatment, suggested thermodynamic preference for Cd over Ca; positive values in the low pH range exhibit thermodynamic preference for Ca over Cd.