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

Hirokazu Narita; Motoki Maeda; Chiharu Tokoro; Tomoya Suzuki; Mikiya Tanaka; Hideaki Shiwaku; Tsuyoshi Yaita

doi:10.1039/d3ra01341c

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

RSC Adv. 2023 Jun 6;13(25):17001-17007. doi: 10.1039/d3ra01341c. eCollection 2023 Jun 5.

Authors

Hirokazu Narita¹, Motoki Maeda², Chiharu Tokoro², Tomoya Suzuki¹, Mikiya Tanaka³, Hideaki Shiwaku⁴, Tsuyoshi Yaita⁴

Affiliations

¹ Global Zero Emission Research Center, National Institute of Advanced Industrial Science and Technology (AIST) 16-1 Onogawa Tsukuba Ibaraki 305-8569 Japan hirokazu-narita@aist.go.jp.
² School of Creative Science and Engineering, Waseda University 3-4-1 Okubo Shinjuku Tokyo 169-8555 Japan.
³ National Institute of Advanced Industrial Science and Technology (AIST) 1-1-1 Umezono Tsukuba Ibaraki 305-8560 Japan.
⁴ Materials Science Research Center, Japan Atomic Energy Agency (JAEA) 1-1-1 Koto Sayo Hyogo 679-5148 Japan.

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.