Sulfate Adsorbed on Hydrous Alumina, Ligands Displaced, and Changes in Surface Charge

Abstract

Abstract To obtain a better understanding of the short term reactions between sulfate and hydrous alumina the relationships between the amount of sulfate retained by the alumina and reaction time, concentration, ligands displaced by sulfate, and changes in surface charge were studied. Experimental conditions were 3 hours reaction time, 0.01 M NaCl medium, pH 5.0, 30°C, and nitrogen atmosphere. Sulfate adsorption was very rapid and about 90% of the reaction was over in 10 min. There was little change in adsorption after 3 hours. The adsorption increased with increasing final solution concentration up to about 6 µeq/ml after which it decreased. Sulfate adsorption resulted in the release of OH ‐ from, and the neutralization of positive charge on the surface. From the quantitative relationships between sulfate adsorbed, the ligands displaced, and the changes in surface charge it is proposed that sulfate is adsorbed on hydrous alumina as SO 4 2‐ bridging across two aluminum atoms, forming a six‐membered ring. At low concentrations sulfate preferentially displaces aquo groups (Al‐OH 2 ) from the positive sites. With increasing concentration, increasing proportions of hydroxo groups (Al‐OH) from the neutral sites are displaced. The decrease in adsorption above 6 µeq/ml of sulfate concentration is inexplicable at this stage.