Thermodynamics of Aqueous Phase Interactions of Divalent Cadmium (Spiked with ^115mCd) with Fine Soil Particles

The role of fine suspended soil particles (FSP, particle size 0.2–4.1 μm) in scavenging of externally added cadmium ions (Cd²⁺) in soil-water suspension has been investigated under different chemical environments by tracer techinque using ^115mCd radioisotope. The Cd²⁺ immobilisation on FSP has been found to be an extremely fast process and is a function of the concentrations of FSP and Cd²⁺ ions, pH and temperature. Experimental observations and the calculation of thermodynamic parameters suggest that surface adsorption/ion exchange (at acidic or neutral pH) and precipitation/coprecipitation (at alkaline pH) mainly govern the fate of externally added Cd²⁺ ions in aqueous suspensions of FSP. It has been shown thermodynamically that the process of Cd²⁺ immobilisation on FSP at different pH values proceeds spontaneously till the attainment of equilibrium at a ξ corresponding to minimum free energy.