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Sulfate Adsorbed on Hydrous Alumina, Ligands Displaced, and Changes in Surface Charge¹

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, 0.01M 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 SO₄^2- bridging across two aluminum atoms, forming a six-membered ring. At low concentrations sulfate preferentially displaces aquo groups (Al-OH₂) 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.

¹ Contribution from the Soil Chemistry Group, Ruakura Agric. Res. Ctr., Private Bag, Hamilton, New Zealand.

² Soil Scientist.

Received for publication April 28, 1977. Accepted for publication October 3, 1977.

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