Adsorption of Selenite, Phosphate and Sulphate on Hydrous Alumina

Summary Selenite was adsorbed on a positively charged hydrous alumina at a solution pH of 5.0, and the OH − released and changes in the surface charge were measured. The adsorption isotherm levelled off at high concentrations suggesting a definite adsorption maximum. The OH − released yielded a curvilinear relationship of increasing slope with the selenite adsorbed. The positive charge on the alumina surface was neutralized by the adsorbed selenite and the net charge became close to zero as the adsorption of selenite was near maximum. The molar ratio of the ligands (OH − + OH 2 ) displaced over selenite adsorbed was more than one at surface saturations of <0.5, but it was approaching one with increasing adsorption of selenite. The results are explained in terms of preferential adsorption of divalent (SeO 2 3 − ) and monovalent (HSeO − 3 selenite on a highly positively and less positively charged surface respectively, in agreement with the VSC − VSP model of Bowden et al . (1973; 1977). The supply of SeO 2 3 − for adsorption is considered to be favoured by an expected lower H activity in solution adjacent to the positively charged surface than that in the bulk solution. A general discussion is given on the ligand exchange adsorption of selenite, phosphate and sulphate on hydrous alumina.