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), situ ated in Castellon (eastern coast of Spain), with an important number o f ceramic industries which generate waste waters containing cadmium an d zinc. In this paper, equilibrium adsorption isotherms have been obta ined to examine the behaviour of cadmium and zinc in three selected so ils of Castellon Plain (Spain) by means of batch experiments. Both met als exhibit non-linear adsorption isotherms with adsorption maximum be yond the dissolved concentration range considered in this study. Stati stical analysis showed that data fit slightly better to Freundlich lin earization than that of Langmuir. Adsorption coefficients obtained fro m Freundlich approximation were calculated to evaluate the relative ca dmium and zinc distribution between solution and studied soils obtaini ng 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 z inc. The effects of salt concentration and calcium competition for ads orption sites were investigated providing different concentrations of CaCl2 in background solutions. Adsorption capacities of cadmium and zi nc in the studied soils decreased when the salt concentration increase d. A tenfold increase in calcium concentration reduced the cadmium ads orption capacity approximately by one third whereas the Ca2+ ion does not seem a significant competitor with Zn2+ for adsorption sites in so ils with high organic matter content, where precipitation of zinc can be expected.