Risk assessment of cadmium contaminated soils varying in physico-chemical characteristics with reference to aging effect

To estimate the effect of aging on the availability of cadmium (Cd) in soil, relationship between availability of Cd in soil and period after contamination were investigated for a range of soil characteristics. Three levels of Cd viz. (0, 20 and 200 mg Cd kg⁻¹ soil) were added as nitrate solution to soil varying in pH, CaCO₃, organic matter and silt + clay content. The soils were incubated at field capacity for 1 day, 14-days (2 weeks) and 28 days (4 weeks) and available fraction of Cd extracted with 0.005M DTPA was determined by atomic absorption spectrophotometer. There was temporal decrease in the availability of Cd i.e. ageing and it was in the order of 1 day > 2 weeks > 4 weeks which was evident for all soils. However, the magnitude of decrease was different under varying levels of contamination. At 20 mg Cd kg⁻¹ soil contamination level and 2 weeks of ageing lowest extraction of Cd was observed under 40% addition of silt + clay (finer fraction) whereas after 4 weeks of aging lowest extraction was observed under soils treated with 5% CaCO₃. But at 200 mg Cd kg⁻¹ soil contamination level, addition of 40% finer fraction resulted in lowest recoveries both at 2 and 4 weeks of ageing. These results enable us to understand natural attenuation of Cd contamination and also to assess the sink capacity of different soils.