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Adsorption of Selenite and Phosphate on an Allophane Clay¹

S. S. S. Rjan and J. H. Watkinson²

ABSTRACT

Selenite was adsorbed on an allophane clay from solutions of different concentrations at pH 5.0, at 30C, and under a N₂ atmosphere, and the amounts of sulfate, silicate and hydroxyl ions released were measured. The results were compared with those from a similar study with phosphate on the same clay.

The results indicate that at low concentrations both phosphate and selenite exchanged with adsorbed sulfate, adsorbed silicate, and aquo and hydroxo groups. About three times more phosphate than selenite was adsorbed, due mainly to phosphate displacing more aquo groups and thus making the surface less positive.

At high concentrations, whereas the selenite adsorption reached a maximum, phosphate continued to be adsorbed. The latter was due to phosphate displacing structural silicate and probably also to disruption of hydrous oxide polymers. A two-term Langmuir equation distinguished adsorption by surface ligand exchange from these other reactions at high concentration.

A selenite desorption experiment showed that phosphate displaced all of the selenite adsorbed. Phosphate was adsorbed with greater strength, the selectivity coefficient, K_Se^P, being 2.2.

¹ Contribution from the Soil Chemistry Group, Ruakura Agric. Res. Ctr., P.B., Hamilton, New Zealand. Acknowledgement is made to the New Zealand N.R.A.C. for a fellowship to the senior author during the period of this study, to Dr. K. W. Perrott for the clay sample and analysis figures and to members of spectro lab. Ruakura for Si and Se determinations.

² Scientists.

Received for publication May 28, 1975. Accepted for publication October 8, 1975.

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