Extraction of Se(iv) and Se(vi) from aqueous HCl solution by using a diamide-containing tertiary amine

Abstract

Here, we investigated the mechanism underlying the extraction of Se(IV) and Se(VI) from aqueous HCl solutions by N-2-ethylhexyl-bis(N-di-2-ethylhexyl-ethylamide)amine (EHBAA). In addition to examining extraction behavior, we also elucidated structural properties of the dominant Se species in solution. Two types of aqueous HCl solutions were prepared by dissolving a Se^IV oxide or a Se^VI salt. X-ray absorption near edge structure analyses revealed that Se(VI) was reduced to Se(IV) in 8 M HCl. Using 0.5 M EHBAA, ∼50% of Se(VI) was extracted from 0.5 M HCl. In contrast, Se(IV) was hardly extracted from 0.5 to 5 M HCl; however, at molar concentrations above 5 M, the extraction efficiency of Se(IV) increased drastically, reaching ∼85%. Slope analyses for the distribution ratios of Se(IV) in 8 M HCl and Se(VI) in 0.5 M HCl showed that apparent stoichiometries of Se(IV) or Se(VI) to EHBAA were 1 : 1 and 1 : 2, respectively. Extended X-ray absorption fine structure measurements revealed that the inner-sphere of the Se(IV) and Se(VI) complexes extracted with EHBAA was [SeOCl₂] and [SeO₄]²⁻, respectively. Together, these results indicate that Se(IV) is extracted from 8 M HCl with EHBAA via a solvation-type reaction, whereas Se(VI) is extracted from 0.5 M HCl via an anion-exchange-type reaction.