COMPETITIVE INTERACTIONS OF PHOSPHATE AND MOLYBDATE ON ARSENATE ADSORPTION

Abstract

The adsorption of arsenate by a Cecil clay was found to be suppressed by the competitive effects of phosphate and molybdate. Competitive Langmuir-type equations have been used in other studies to describe the adsorption data, but in this investigation the adsorption of arsenate alone and in binary mixtures followed a Freundlich-type of response. Competitive Freundlich-type isotherm equations have not been used extensively in soil chemistry studies. The usefulness of two competitive Freundlich-type equations that have been used to model the competitive interactions of dilute organic solutes by activated carbon was examined for application to arsenate adsorption by a Cecil clay as influenced by phosphate and molybdate. The equation of DiGiano et al. (1978) had the potential of being able to predict the effects of competition on adsorption based entirely on single-component data. The Sheindorf et al. (1981) equation required the collection of competitive data to derive a competitive coefficient. Both expressions were found to be useful, although there was no clear indication of which, if either, was the more accurate. The ability of both expressions to describe the data appeared to be limited to situations where the ratios of the equilibrium concentrations of As/P and As/Mo were greater than 20, i.e., when the equilibrium concentration of As was much greater than that of P or Mo. This limitation was partly attributed to the regression procedure used to calculate competitive coefficients.