Abstract
This work investigated the removal of selenite and selenate from water by green rust (GR) sulfate. Selenite was immobilized by simple adsorption onto GR at pH 8, and by adsorption–reduction at pH 9. Selenate was immobilized by adsorption–reduction to selenite and zero valent selenium (Se0) at both pH 8 and 9. In the process, GR oxidized to a mixture of goethite (FeOOH) and magnetite (Fe3O4). The kinetics of selenite and selenate sorption at the GR–water interface was described through a pseudo-second-order model. X-ray absorption spectroscopy data enabled to elucidate the concentration profiles of Se and Fe species in the solid phase and allowed to distinguish two removal mechanisms, namely adsorption and reduction. Selenite and selenate were reduced by GR through homogeneous solid-phase reaction upon adsorption and by heterogeneous reaction at the solid–liquid interface. The selenite reduced through heterogeneous reduction with GR was adsorbed onto GR but not reduced further. The redox reaction between GR and selenite/selenate was kinetically described through an irreversible second-order bimolecular reaction model based on XAFS concentration profiles. Although the redox reaction became faster at pH 9, simple adsorption was always the fastest removal mechanism.
Highlights
Selenium is an essential element for humans, but it is associated with acute toxicity above 1.4 mg l21 and with mutagenicity above 12 mM [1,2]
This study addresses the removal of selenite and selenate at pH 8 and 9 by green rust (GR)-sulfate
The immobilization of selenium species by GR occurred through different phenomena depending on pH and Se species
Summary
Selenium is an essential element for humans, but it is associated with acute toxicity above 1.4 mg l21 and with mutagenicity above 12 mM [1,2]. Mining activities contribute to the contamination issue, especially when acid mine drainage dissolves the selenium naturally contained in ores/minerals [6,7]. These anthropogenic activities are carried out worldwide and involve large amounts of water.
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