Selenite retention by nanocrystalline magnetite: Role ofadsorption, reduction and dissolution/co-precipitation processes

摘要

We studied selenite (SeO_3~(2-)) retention by magnetite (Fe~(II) III)_2O_4) using both surface complexation modeling and X-rayabsorption spectroscopy (XAS) to characterize the processes of adsorption, reduction, and dissolution/co-precipitation.The experimental sorption results for magnetite were compared to those of goethite (Fe~(III)OOH) under similar conditions. Sel-enite sorption was investigated under both oxic and anoxic conditions and as a function of pH, ionic strength, solid-to-liquidratio and Se concentration. Sorption onto both oxides was independent of ionic strength and decreased as pH increased, asexpected for anion sorption; however, the shape of the sorption edges was different. The goethite sorption data could be mod-eled assuming the formation of an inner-sphere complex with iron oxide surface sites (SOH). In contrast, the magnetite sorp-tion data at low pH could be modeled only when the dissolution of magnetite, the formation of aqueous iron-selenite species,and the subsequent surface complexation of these species were implemented. The precipitation of ferric selenite was the pre-dominant retention process at higher selenite concentrations (>1 × 10~(-4)M) and pH < 5, which was in agreement with theXAS results. Sorption behavior onto magnetite was similar under oxic and anoxic conditions. Under anoxic conditions,we did not observe the reduction of selenite. Possible reasons for the absence of reduction are discussed. In conclusion, weshow that under acidic reaction conditions, selenite retention by magnetite is largely influenced by dissolution and co-precip-itation processes.