We report on the sorption of selenium oxyanions on zeolite sorbents that were either used as obtained from a mine or modified either by cation exchange or by deposition of an iron coating or both. Zeolites , porous aluminosilicate minerals characterized by high cation exchange capacity, have essentially no anion exchange capacity and therefore limited affinity for oxyanions such as selenite, selenium in the +4 oxidation state, and selenate, selenium in the +6 oxidation state. The objective was to enhance the ability of zeolites to bind selenium oxyanions by partially coating the zeolite surface with an iron coating. In addition, the effect of modifying the composition of zeolites by sodium exchange, with and without the presence of iron coating, was also investigated. The effects of zeolite surface modification through iron coating and cation exchange reactions were studied at different geochemical conditions, including pH (4- 10), ionic strength (0.01 M, 0.10 M, and 1.0 M sodium nitrate), selenium concentration (10-6 M, 10-5 M, and 10-4 M), and selenium oxidation state, namely selenium in the oxidized form of selenite and the reduced form of selenite. Four different types of zeolites were used: Unmodified (natural) zeolite used as received, sodium-exchanged, iron-coated, and sodium-exchanged and iron-coated. The experimental results show that iron coating dramatically increases the ability of zeolites to bind selenium oxyanions, especially selenite. The fractional uptake of selenite by iron-coated zeolites reached 80%, compared to 15%, and 16% sorption on natural and sodium-exchanged zeolites, respectively. The fractional uptake of selenite was not affected substantially by the ionic strength of the solution, a finding consistent with the formation of inner-sphere complexes between the selenite oxyanion and the iron oxide coating. Finally, sorption of selenite was strongly pH-dependent, consistent with sorption of the anion on amphoteric iron surface sites, rather than permanent charge sites. Sorption of selenate was much lower compared to selenite, suggesting that the two selenium oxyanions bind through different mechanisms on the iron oxide surface. These results will enhance our ability to design treatment processes for selenium removal from contaminated sites, including mining sites, and develop better predictive tools for the fate and transport of potentially toxic anions in the environment.

Effect of geochemical conditions and chemical treatment of zeolites on their ability to bind selenium oxyanions

254th National Meeting (2017)

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