Ca-Zn and Ca-Cd exchange equilibria were studied in the surface [0–0.2m (S)) and associated sub-surface [0.4–0.6m (SS)] soil samples of a sandy loam (surface), mixed hyperthermic Aridic Ustochrept, collected from two adjoining fields; one irrigated wilh sewer water (W1) and the other with canalwaler (WO). The per cent adsorption of added Zn (% Ads.zn) and its selectivity coefficient (KZn) in SW1, SSW1, SWO and SSWO soil samples increased and then decreased when the equivalent fraction of Zn (CZn) in the equilibrating Solulion increased from 0.0005 to 0.008 and from 0.1 to 1, respectively. The %Ads.Cd and KCd in SW1. SSW1. SWO and SSWO samples increased and then decreased when CCd increased from 0.0005 to 0.008 and from 0.008 to 1, respectively. The standard free energy change of Ca-Zn and Ca-Cd exchange reactions showed higher thermodynamic preference of Zn over Ca and of Ca over Cd for soil samples from sewer water and canal water irrigated sites. respectively. The effect of sewer water on the change in preference of soil for metallic cation was more pronounced for Zn than for Cd.
Ca-Zn, Ca-Cd exchange, sewer water, selectivity coefficients, free energy
