No 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 SO42‐ 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.

Pseudomonas stutzeri isolate rapidly reduced both selenite and selenate ions to elemental selenium at initial concentrations of both anions of up to 48.1 mM. Optimal selenium reduction occurred under aerobic conditions between pH 7.0 and 9.0 and at temperatures of 25 to 35°C. Reduction of both selenite and selenate was unaffected by a number of anions except for sulfite, chromate, and tungstate ions, which inhibited both growth and reduction.

SummaySelenite 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 a t 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,) displaced over selenite adsorbed was more than one a t 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:-) and monovalent (HSeO;) 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:-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.

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, KSeP, being 2.2.