Competitive sorption of molybdate and phosphate in andisols

We have examined the effects of both pH and phosphate concentrations on molybdate sorption by four Chilean Andisols with varying chemical properties. For both anions adsorption decreased with an increase in soil pH and also with organic matter content in the soils. In general, more phosphate than molybdate was sorbed by the soils. The Freundlich equation adequately described adsorption of both anions. Differences in the extent of sorption of molybdate and phosphate and in isotherm shape between soils were ascribed to variations in soil properties. The competitive sorption data were adequately described by the multicomponent Freundlich equations. Phosphate strongly competed with molybdate for sorption sites on the Andisols, causing molybdate sorption to decrease by 10-27%. The competition coefficients for phosphate and molybdate sorption were variable, indicating that competition between the two anions was not symmetrical. The present results provide further evidence to indicate that the surfaces of Andisols are rich in reactive Al-OH and Fe-OH groups, exposed at colloidal particle surfaces. Molybdate sorption appeared to be mainly associated with free Al- and Fe-oxides, while phosphate sorption was primarily regulated by Al- and Fe-humus complexes.