Effect of calcium on the soil adsorption of cadmium and zinc in some Spanish sandy soils

Castellon Plain is a natural area with an extension of 461 km(2), situated in Castellon (eastern coast of Spain), with an important number of ceramic industries which generate waste waters containing cadmium and zinc. In this paper, equilibrium adsorption isotherms have been obtained to examine the behaviour of cadmium and zinc in three selected soils of Castellon Plain (Spain) by means of batch experiments. Both metals exhibit non-linear adsorption isotherms with adsorption maximum beyond the dissolved concentration range considered in this study. Statistical analysis showed that data fit slightly better to Freundlich linearization than that of Langmuir. Adsorption coefficients obtained from Freundlich approximation were calculated to evaluate the relative cadmium and zinc distribution between solution and studied soils obtaining coefficients ranging from 5200 to 5900 mu g kg(-1) (mu g k(-1))(-n) for cadmium and from 3500 to 43200 mu g kg(-1) (mu g l(-1))(-n) for zinc. The effects of salt concentration and calcium competition for adsorption sites were investigated providing different concentrations of CaCl2 in background solutions. Adsorption capacities of cadmium and zinc in the studied soils decreased when the salt concentration increased. A tenfold increase in calcium concentration reduced the cadmium adsorption capacity approximately by one third whereas the Ca2+ ion does not seem a significant competitor with Zn2+ for adsorption sites in soils with high organic matter content, where precipitation of zinc can be expected.