Selenium oxyanion exchange with Mg(II)-Fe(III) and Fe(II)-Fe(III) layered double hydroxides

Abstract

Immobilization of selenium oxyanions from water resources is a helpful strategy to mitigate exposure to Se at concentrations toxic to human and ecosystem health. Pyroaurite-type minerals are layered double hydroxides with high anion exchange capacity with relevance in subsurface environments and applicable to water treatment technologies. Three types of Mg(II)-Fe(III) pyroaurites and Fe(II)-Fe(III) green rusts were evaluated in kinetic studies to determine the influence of different interlayer anions on the uptake of dissolved selenate (SeO42−) and selenite (HSeO3−, SeO32−). Selenate and selenite uptake extents with pyroaurites were controlled by the identity of the interlayer anion according to the order Cl−> SO42−> CO32−, as predicted by known interlayer anion preferences for LDH. The kinetics of Se uptake typically expressed a rapid initial anion exchange step followed by a slow re-exchange back to solution due to competition with expelled interlayer anions. Excess dissolved interlayer anions and high pH values exacerbated this reverse exchange of Se. The results suggest Se oxyanions taken up by pyroaurites may not be firmly sequestered. Selenate uptake by redox-active green rust appears to be subject to the same interlayer anion preference as non-redox active pyroaurite, suggesting that the ability of green rust to reduce selenium oxyanions may be limited by the anion exchange process.