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Extraction of Se(IV) and Se(VI) from aqueous HCl
solution by using a diamide-containing tertiaryamine†
Hirokazu Narita, *a Motoki Maeda,b Chiharu Tokoro,b Tomoya Suzuki,
Mikiya Tanaka,c Hideaki Shiwakud and Tsuyoshi Yaitad

aHere, 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. Twotypes of aqueous HCl solutions were prepared by dissolving a SeIV oxide or a SeVI 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.
Received 28th February 2023
Accepted 1st May 2023

Extended X-ray absorption ﬁne structure measurements revealed that the inner-sphere of the Se(IV) andDOI: 10.1039/d3ra01341c

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

rsc.li/rsc-advances

extracted from 0.5 M HCl via an anion-exchange-type reaction.

1. Introduction
Selenium (Se) has been widely used in various industrialelds.1,2 Industrial production of Se is mainly achieved byrecovery as a byproduct from the anode slimes produced during
copper smelting.3,4 Solvent extraction is one of the important
methods for the recovery of Se; however, our understanding of
the extraction of Se ions from acidic solutions and the precise
speciation of Se ions in relatively concentrated HCl solutions
remains largely incomplete. Se is also an essential trace element
for humans and animals; however, its consumption in large
amounts is detrimental to health. Therefore, there have been
many studies conducted to determine the concentration andoxidation states of Se in the natural environment and to
examine how best to facilitate its removal.5–7 Thus, more preciseaGlobal Zero Emission Research Center, National Institute of Advanced Industrial
Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
E-mail: hirokazu-narita@aist.go.jpbSchool of Creative Science and Engineering, Waseda University, 3-4-1 Okubo,
Shinjuku, Tokyo 169-8555, Japanc
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1
Umezono, Tsukuba, Ibaraki 305-8560, JapandMaterials Science Research Center, Japan Atomic Energy Agency (JAEA), 1-1-1 Koto,
Sayo, Hyogo 679-5148, Japan† Electronic supplementary information
https://doi.org/10.1039/d3ra01341c(ESI)

available.

See

DOI:

© 2023 The Author(s). Published by the Royal Society of Chemistry

speciation data will facilitate the development of more eﬃcientsolvent extraction methods for Se.
Many microextraction approaches have been reported forthe solvent extraction of trace amounts of Se from media suchas food and environmental water.7 However, approaches forthe extraction of Se from highly acidic solutions are limited.
Tri-n-butyl phosphate (TBP) has been successfully used forsolvent extraction of Se(IV) from HCl solution.8–10 Interestingly,
Chowdhury and Sanyal9 reported that the dominant complex
extracted changes depending on the concentration of the HClsolution, with SeOCl2(TBP)2 being extracted from 3 to 6 M HCl
and H2SeOCl4(TBP)2 being extracted from 6 to 10 M HCl. A
TBP analog, tri-iso-butyl phosphate, has also been shown to
extract Se(IV) from chloride aqueous solution at around pH
8.5.11 A protonated N-n-octylaniline has been shown to form
an ion-pair complex with [HSeO3]−.12 In contrast, diammoniumcalix[4]arene has been shown to extract Se(VI) via
ion-pair formation with [HSeO4]− or [SeO4]2−.13,14 The reactions of these extractants with Se were estimated mainly byslope analyses of the distribution ratio of Se between theorganic and aqueous phases. Moving forward, direct structural information of the dominant Se species in the organic
and aqueous phases is needed to understand the extraction
behavior in more detail.
In acidic chloride solutions, Se(IV) and Se(VI) form selenousacid (H2SeO3) and selenic acid (H2SeO4), respectively, anddissociate to anions such as HSeO3−, SeO32−, HSeO4−, and

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SeO42− with a decrease in the acid concentration.15 For Se(IV), it
has been suggested that an SeOCl2 complex forms in highly
concentrated HCl solutions (4–12 M), and at even higher
concentrations (∼15 M) SeCl5− is produced.16 Compared with
the number of reports providing structural information forsome precious metals and base metals, which are present in
the anode slimes, there are fewer reports providing structural
information about Se ions in acidic chloride solutions.
Knowledge of the structure of Se ions in the aqueous phase isneeded for the development of new extractants because
extraction type is highly dependent on the dominant Se
complex in solution.
Our group has been examining the possibility of using
amide-containing tertiary amines (ACTAs) as extractants ofmetal ions.17–22 ACTAs eﬃciently extract chlorometalates via ananion-exchange reaction because of their stable protonation
ability.21 They also extract rare earth elements from HNO3 via
a solvation reaction.22 In particular, ACTAs have excellent
extraction eﬃciency for platinum-group metals from HCl solutions. For example, diamide- and triamide-type ACTAs are able
to extract [RhCl5(H2O)]2−, an aqua–chloro complex, despite thiscomplex generally being considered unextractable.18,21 ACTAs
are also much more eﬃcient extractants of metal complex
anions than the typical tertiary amine extractant tri-n-octylamine (TOA). We therefore expected that ACTAs could be used
to eﬃciently extract Se from HCl.
Here, we examined the extraction of Se(IV) and Se(VI) from
aqueous HCl solutions by using a diamide-containing tertiaryamine, N-2-ethylhexyl-bis(N-di-2-ethylhexyl-ethylamide)amine(EHBAA) (Fig. 1). We discuss the underlying extraction mechanism based on a slope analysis of the distribution ratio of Se
ions between the organic and aqueous phases and structural
data for the dominant Se complexes in the solutions obtainedby X-ray absorption ne structure (XAFS) analysis. To allow
comparison, the eﬃcacies of the extractants TOA and TBP werealso examined.

2.

Experimental

2.1. Reagents
SeO2 (FUJIFILM Wako Pure Chemical Co.) and Na2SeO4$10H2O(Alfa Aesar) were used as an Se(IV) oxide and Se(VI) salt, respectively. EHBAA (purity: 96%) was provided by Chemicrea Inc. TBP
and TOA were purchased from the Tokyo Chemical Industry Co.

Paper
The extractants were used without further purication. All theother chemicals used were of reagent grade.
2.2. Solutions of Se in aqueous HCl
Aqueous solutions of Se ions were prepared by dissolving SeO2
or Na2SeO4$10H2O in aqueous HCl. Hereaer, the aqueous
solutions are abbreviated as SolAxH and SolBxH, where ‘A’ =SeO2, ‘B’ = Na2SeO4$10H2O, and ‘x’ = HCl concentration (M).
The Se concentration was 0.001 M for the solvent extraction
experiments and 0.1 M for the XAFS measurements. For thesolvent extraction experiments, the aqueous solutions wereused at 12 h aer oxide or salt dissolution. For the XAFS
measurements, mainly the aqueous solutions were used at 3 w
aer oxide or salt dissolution.
2.3. Solvent extraction
An 80/20 (v/v) n-dodecane/2-ethylhexanol mixture was used as
the diluent for the extractants. Equal volumes of one of thediluted extractants (EHBAA, TOA, or TBP) and of Se-free HClsolution were added to a 10 mL glass tube, and the tube wasshaken vertically for 30 min at an amplitude of 100 mm anda frequency of 200 stroke per min and then centrifuged. Next
1 mL of the HCl-equilibrated organic phase and the same
volume of HCl solution containing 0.001 M Se (SolAxH or
SolBxH) was similarly shaken for 60 min and then centrifuged.
The shaking time was suﬃcient to ensure that the extraction
equilibrium was reached. The concentrations of Se ions in the
aqueous phase were measured by ICP-AES (Horiba ULTIMA2).
The concentrations in the organic phase were calculated fromthe mass balance of the metal ions before and aer extraction.
The data accuracy was within 5%. The extractions were performed at room temperature (23 ± 2 °C). The extraction
percentage (E%) and distribution ratio (D) were calculated byusing eqn (1) and (2), respectively, where [Se]init,aq, [Se]eq,org, and[Se]eq,aq denote the Se concentrations in the initial aqueous
phase, the equilibrated organic phase aer extraction, and theequilibrated aqueous phase aer extraction, respectively.
E% = ([Se]eq,org/[Se]init,aq) × 100= {([Se]init,aq − [Se]eq,aq)/([Se]init,aq)} × 100(1)

D = [Se]eq,org/[Se]eq,aq = ([Se]init,aq − [Se]eq,aq)/[Se]eq,aq(2)2.4. XAFS measurements

Fig. 1

Structure of N-2-ethylhexyl-bis(N-di-2-ethylhexyl-ethylamide)amine.

17002 | RSC Adv., 2023, 13, 17001–17007

Four aqueous solutions, SolA0.5H, SolA8H, SolB0.5H, and
SolB8H ([Se] = 0.1 M), were prepared. For SolB8H, the dependence on standing time was investigated because a change inthe oxidation state was suggested during the solvent extraction
experiment. Organic solutions containing Se complexes
extracted with EHBAA were prepared by the solvent extraction
method detailed in Section 2.3. SolA8H and SolB0.5H ([Se] = 0.1
M) and 0.5 M EHBAA were used for the aqueous and organic
phases, respectively. The concentration of the Se complexes
extracted with EHBAA from SolA8H and SolB05H (hereaer© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

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abbreviated to ExSolA and ExSolB, respectively) was 0.08 M for
ExSolA and 0.05 M for ExSolB. The solutions were transferred to
10 mm diameter polyethylene cells, which were then tightly
sealed. Solid reference samples were prepared by grinding the
Se oxide or salt with boron nitrate and then pelleting theresulting powder.
XAFS measurements were performed at the BL11XU beamline of SPring-8 (8 GeV at ∼99 mA). Synchrotron X-ray radiation
was monochromatized using Si(111) crystal monochromators.
Se K-edge XAFS spectra were collected in transmission modeusing two ion chambers (both lled with N2) at room temperature (∼25 °C). The beam size on the sample was about 1 × 1mm2. The SeO2 was used for energy calibration prior to X-ray
absorption near edge structure (XANES) measurements; thepeak top of the Se K-edge (white line) was aligned to 12 664 eV.23
XAFS data were collected three times and then averaged for eachsample.
Data processing was performed with the EXAFS data analysissoware package WinXAS version 4.0.24 Theoretical phases andamplitudes were calculated by using the soware FEFF 8.25 For
the FEFF calculations, the crystal structures of (SeIVO3)2− in[Na2SeO3],26 (SeVIO4)2− in [(NH4)4H2(SeO4)3],27 and (SeIVOCl2) in[SbCl5$SeOCl2]28 were used as model compounds. Multiple
scattering paths were ignored because their contribution was
not signicant for the studied systems. Thus, we considered
only single scattering paths: Se–O and/or Se–Cl. The k-range for
the Fourier transforms was 4–12 Å−1 for SolA0.5H and 4–13 Å−1for the others, and the curve-tting R-range was 0.5–2.5 Å for
SolA8H and ExSolA, and 0.8–2.0 Å for the others. The coordination geometry of Se in 0.5 M HCl solution (SolA0.5H) is
tetragonal SeO3 because SeO32−, HSeO4−, and/or H2SeO4 arepresent.15 Therefore, the coordination number was kept
constant at 3 and the amplitude reduction factor, S02, wasrened; the obtained S02 value (0.86(3)) was used for all ts. Thisvalue was considered reasonable because the obtained numberof correlations (coordination number), N, for Se–O for the solidreference samples, SeIVO2 and Na2SeVIO4$10H2O, was 3.1(3) and4.0(4), respectively (Fig. S3 and Table S1†). The structuralparameters (N; bond distance, r (Å); Debye–Waller factor
squared, s2 (Å2); and the shi in threshold energy, DE0 (eV))
were allowed to vary in the t; the same DE0 value was used foreach shell. The diﬀerence in bond lengths between Se–O and
Se–Cl was distinguishable because their diﬀerence is more than
the smallest separation,29 given by Dr = p/2Dk (Dk z 9 Å−1, Drz 0.17 Å). The tting quality was checked by using the R-factor:{Sjk3c(k)obs − k3c(k)calcj/Sjk3c(k)obsj} × 100.

3.

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Extraction percentage of Se as a function of HCl concentration.
(A) SolAxH: SeIVO2 dissolved and (B) SolBxH: Na2SeVIO4$10H2O dissolved. Organic phase, 0.5 M EHBAA, TOA, or TBP pre-equilibrated
with HCl; aqueous phase, [Se] = 0.001 M.
Fig. 2

above 5 M for each of the extractants (Fig. 2A). In the SolBxH
system, similar rapid increases were observed at HCl concentrations above 5 M for each of the extractants (Fig. 2B); however,extraction percentages up to 50% were obtained at HCl
concentrations below 5 M, depending on the extractant. The
extraction eﬃciency in this region decreased in the order of
EHBAA [ TOA > TBP, indicating that anion-exchange extraction predominantly occurred at concentrations of 0.5–5 M
because TBP is a weak anion exchanger,30 and EHBAA is a more
eﬃcient extractant of metal complex anions than TOA.17–19,21
According to a previous study examining the reduction of Se(VI),
Se(VI) is rapidly reduced to Se(IV) by chloride in a strongly acidic
medium;31 a similar reduction of Se(VI) to Se(IV) was indicated tohave occurred at HCl concentrations above 5 M in the SolBxH
system.
3.2. Dominant Se species in aqueous HCl solution
Fig. 3 shows the Se K-edge XANES spectra obtained for
SolA0.5H, SolA8H, SolB0.5H, and SolB8H, together with theResults and discussion

3.1. Eﬀect of HCl concentration on Se ion extraction
The extraction percentage of Se as a function of HCl concentration with 0.5 M extractants was determined for SolAxH and
SolBxH. Three extractants were examined: EHBAA, TOA, and
TBP. Third phase formation was observed at 0.5 M and 8 M HCl
in the TOA system. In the SolAxH system, very little Se was
extracted at HCl concentrations below 5 M; however, theextraction percentage rapidly increased at HCl concentrations© 2023 The Author(s). Published by the Royal Society of ChemistryFig. 3 Se K-edge XANES spectra for the aqueous solutions SolA0.5H,
SolA8H, SolB0.5H, and SolB8H and the reference solids SeIVO2 andNa2SeVIO4$10H2O.

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Fig. 4 Se K-edge k3-weighted EXAFS spectra (A) and the corresponding Fourier transforms (FT) (B) for SolA0.5H (top), SolA8H(middle), and SolB0.5H (bottom). The phase shifts are not corrected.
Experimental data (solid line) and theoretical ﬁt (dotted line).

spectra for solid samples of SeIVO2 and Na2SeVIO4$10H2O forreference. The peak positions in the spectra for SolA0.5H and
SolA8H were in good agreement with that of the solid SeIVO2
sample. In contrast, although the peak position in the spectrum
for SolB0.5H was in good agreement with that of the solid
Na2SeVIO4$10H2O sample, the peak position for SolB8H was inagreement with that of SeIVO2, indicating that Se(VI) in 8 M HCl
was reduced to Se(IV) aer dissolution. This is consistent with

Table 1the ndings from the extraction experiment discussed in
Section 3.1.
The time for this reduction to occur was investigated by
XANES measurement using SolB8H solutions that were le tostand before analysis (Fig. S1†). The spectra show that Se(VI) was
reduced to Se(IV) within 12 h aer dissolution.
Fig. 4 shows the Se K-edge k3-weighted EXAFS spectra andcorresponding Fourier transforms for SolA0.5H, SolA8H, and
SolB0.5H. An intense peak seen at ∼1.3 Å in each spectrum(Fig. 4B) was assigned to the Se–O correlation. For SolA8H, in
addition to the intense peak, there was a small peak at ∼2 Å,
which was assigned to the Se–Cl correlation. The curve-tting
results are shown in Table 1. According to the pKa1 value ofselenous acid (∼2.5),15 H2SeO3 is predominant in 0.5 M HCl
solution. In contrast, in >7 M HCl solution both [SeOCl2] and[H2SeO3] are present, and in 15 M HCl solution SeCl5− isdominant.16 For SolA0.5H, both the N value (3) and Se–O bondlength (1.71(1) Å) were almost the same as those for the crystal
structure of [SeO3]−.26 The small peak at ∼2 Å for SolA8H wasconrmed to represent the Se–Cl correlation by the obtained
structural parameters. The obtained N values for Se–O and Se–
Cl were 1.8(1) and 1.1(1), which although not consistent withthe structure of the [SeOCl2] complex, are consistent witha mixture of [SeOCl2] and [H2SeO3]. Although Raman studies
have inferred the existence of a [SeO2Cl]− complex anion,32 thatstudy conducted measurements not in HCl aqueous solution
but in solid and acetonitrile. For SolB0.5H, the curve-tting
results (Se–O: N = 4.4(4), r = 1.633(4) Å) indicated that the
inner-sphere structure is tetrahedral SeO4, which is consistent
with the XANES spectrum. According to the pKa2 of selenic acid
(1.6–2.0),15 both [SeO4]2− and [HSeO4]− would exist in low-pH
solutions.

3.3. Structure of the Se complexes extracted with EHBAAThe Se complexes extracted with EHBAA from SolA8H and
SolB0.5H (abbreviated as ExSolA and ExSolB, respectively) were

Curve-ﬁtting results for selenium EXAFS data

SolA0.5H
SolA8H
SolB0.5H
ExSolAExSolB
Crystal structures[Na2SeO3]g[SbCl5$SeOCl2]i[TiCl3(SeOCl2)2]2(m-O)j[(NH4)4H2(SeO4)3]kNa

rb (Å)

s2 c (Å2)

DEd (eV)R-factore

Se–O
Se–O
Se–Cl
Se–O
Se–O
Se–Cl
Se–O

3f
1.8(1)
1.1(1)
4.4(4)
1.0(2)
2.0(3)
3.8(3)

1.71(1)1.70(1)2.29(1)
1.633(4)1.64(2)2.28(2)
1.632(4)0.0048(9)0.0016(5)0.0065(6)0.0014(6)0.002(1)0.006 (1)0.0018(5)9(1)
5.8(5)

18
7.9

8(1)
5(1)

13
14

5(1)

8.5

Se–O
Se–O
Se–Cl
Se–O
Se–Cl
Se–O

3
1
2
1
2
4

1.71h
1.69
2.12h
1.644
2.145h1.64hCoordination number. b Bond distance (Å). c Debye–Waller factor squared (Å2). d The shi in threshold energy (eV). e Residual: R-factor =Sjk3c(k)obs − k3c(k)calcj/Sjk3c(k)obsj × 100. f Fixed parameter. g Ref. 26. h Averaged bond distances. i Ref. 28. j Ref. 33. k Ref. 27. Estimated errors
are shown in parentheses.
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(EHBAA$HCl) + [SeOCl2] % [SeOCl2](EHBAA$HCl)

Fig. 5 Se K-edge k3-weighted EXAFS spectra (A) and the corresponding Fourier transforms (FT) (B) for ExSolA (top), and ExSolB(bottom). [Se] = 0.08 M in ExSolA and 0.05 M in ExSolB. The phaseshifts are not corrected. Experimental data (solid line) and theoreticalﬁt (dotted line).

examined by XAFS. The XANES spectra obtained for these
extracts indicated that the oxidation state of Se is retained aer
being extracted into the organic phase (Fig. S2†). The Se K-edge
k3-weighted EXAFS spectra and the corresponding Fouriertransforms for ExSolA and ExSolB are shown in Fig. 5. In the
Fourier transform for ExSolA, two intense peaks were seen at∼1.3 and ∼2 Å. The ∼2 Å peak was more intense than thatobserved in the spectrum for the aqueous phase (i.e., in the
spectrum for SolA8H). The ratio of N(Se–O) to N(Se–Cl) for
ExSolA was about 1 : 2, which is diﬀerent from that for SolA8H(2 : 1). In the reported crystal structures of [SeOCl2], the bondlengths of Se–O (1.644–1.69 Å) and Se–Cl (2.12–2.145 Å) varyslightly.28,32 The parameter of Se–Cl obtained in the present
study is longer than these values, suggesting that the slight
structural change of the SeOCl2 moiety in solution. In contrast,
the XANES and EXAFS spectra for ExSolB are very similar to
those for SolB0.5H, which is consistent with the obtained
structural parameters indicating the presence of SeO4, as wasobserved for SolB0.5H.

(4)

The SolA8H solution probably contains neutral [H2SeO4] and[SeOCl2], as mentioned in Section 3.2. The very low extraction
percentage of Se in the HCl concentration range of 0.5–5 M,
indicates that [H2SeO4], which was the dominant species at
those concentrations, was unextractable. Although [SeOCl2] is
not anionic, a higher reactivity can be expected than that of[H2SeO4].1 A previous study using TBP also showed that TBP did
not extract [H2SeO4], but instead it extracted [SeOCl2] and[H2SeOCl4].9
Regarding the protonated structure of EHBAA,21 an H+ ischelated by two amide oxygen atoms and an amine nitrogenatom, and a Cl− weakly bonds to an array of polarized C–Hd+
units on the opposite side of EHBAA from the protonation site.
Considering the stable protonation ability of EHBAA, solvation
with [SeOCl2] possibly occurs at the site opposite the diamide
oxygen atoms. The crystal structure of [TiCl3(SeOCl2)2]2(m-O)$
CH2Cl2 has three long-distance Se/Cl interactions (2.930–
3.465 Å) and two direct Se–Cl bonds (avg. 2145 Å).33 The Cl− in
the EHBAA$HCl complex might have a weak interaction with
[SeOCl2] at the outer-sphere of the Se, although the interaction
was too weak to produce a peak in the EXAFS spectrum.
For the EHBAA–SolB0.5H system, the EXAFS results show that
SeO4 was dominant in both the organic and aqueous phases. In
the aqueous phase, both [HSeO4]− and [SeO4]2− existed, as
mentioned in Section 3.2. Two protonated EHBAA may preferentially extract a [SeO4]2− complex via an anion-exchange reaction, as supported by the formation of a 1 : 2 complex in theorganic phase. In addition, the decreasing extraction percentage

3.4. Extraction mechanism of Se(IV) and Se(VI)Fig. 6 shows the dependence of the distribution ratio of Se on
the EHBAA concentration for SolA8H and SolB0.5H. The slope
values are diﬀerent between SolA8H and SolB0.5H, indicatingthat 1 : 1 Se(IV) : EHBAA and 1 : 2 Se(VI) : EHBAA complexes areformed, respectively. Since EHBAA is readily protonated as
contacting with HCl solution,19,21 ion-pair formation witha complex anion preferentially occurs. Also, neutral complexes
may be extracted by EHBAA via a solvation reaction.22 For the
EHBAA–SolA8H system, the EXAFS results suggested thata SeOCl2 moiety is dominant in the organic phase. Therefore,
the extraction reaction can be presented as
EHBAA + HCl % EHBAA$HCl© 2023 The Author(s). Published by the Royal Society of Chemistry(3)

Fig. 6 Dependence of distribution ratio of Se(IV) and Se(VI) on EHBAA
concentration. Organic phase, EHBAA pre-equilibrated with 0.5 or 8 M
HCl; aqueous phase, SolA8H or SolB0.5H, [Se] = 0.001 M.

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RSC Advancesof Se(VI) in the low HCl concentration range of 0.5–5 M (Fig. 2B)
probably stems from the change in the dominant Se species in
the order of [SeO4]2− / [HSeO4]− / [H2SeO4].15 Similarly, thelow extraction percentage reported for TBP in this low concentration region can be explained by its lower basicity (i.e., its low
protonation ability).30 Therefore, the extraction reaction of Se(VI)from 0.5 M HCl can be represented as

Paper
Use Program of JAEA Facilities (Proposal No. 2013B-E08) supported by JAEA Advanced Characterization Nanotechnology
Platform as a program of “Nanotechnology Platform” of the
Ministry of Education, Culture, Sports, Science and Technology(MEXT), Japan. The synchrotron radiation experiments were
performed at JAEA beamline BL11XU in SPring-8 (Proposal No.
2013B3518).

[SeO4]2− + 2(EHBAA$HCl) % [SeO4]$(EHBAA$H)2 + 2Cl− (5)Notes and references
4. Conclusions
Here, we used XAFS to investigate the extraction of Se by EHBAA
from two types of aqueous HCl solution (SolAxH made by dissolving SeO2 and SolBxH made by dissolving Na2SeO4$10H2O).
SolB8H mostly contained Se(IV) species due to a reduction ofSe(VI) to Se(IV). In the EHBAA–SolAxH system, the extraction
percentage of Se rapidly increased at HCl concentrations above
5 M, but was poor below that limit. In contrast, in the SolBxH
systems, ∼50% of Se was extracted with EHBAA at 0.5 M HCl;the extraction percentage of Se gradually decreased with anincrease in HCl concentration up to 5 M; and then it rapidly
increased at HCl concentrations above 5 M, although this rapid
increase was due to the reduction to Se(IV). The XAFS
measurements indicated that the inner-sphere structures of thedominant Se complexes were SeO3 for SolA0.5H, a mixture of
SeO3 and SeOCl2 for SolA8H, and SeO4 for SolB0.5H. The EXAFS
curve-tting results showed that the Se complexes extractedwith EHBAA have inner-sphere structures of SeOCl2 for SolA8H
and of SeO4 for SolB0.5H. The relationship between the distribution ratio of Se and EHBAA concentration revealed that 1 : 1
and 1 : 2 Se : EHBAA complexes were dominant in the organic
phase of the SolA8H and SolB0.5H systems. The obtained
results suggested that the dominant Se complexes with EHBAA
are [SeOCl2](EHBAA$HCl) for SolA8H and [SeO4](EHBAA$H)2 for
SolB0.5H; the former was a solvation-type extraction, and thelatter was an anion-exchange-type.

Author contributions
Hirokazu Narita: conceptualization, methodology, validation,formal analysis, investigation, writing – original dra, visualization, writing – review & editing; Motoki Maeda: validation,
investigation, Chiharu Tokoro: methodology, visualization;
Tomoya Suzuki: data curation, formal analysis; Mikiya Tanaka:writing – review & editing; Hideaki Shiwaku: methodology,
formal analysis; Tsuyoshi Yaita: methodology, formal analysis.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
The authors wish to thank Ms. Tomoko Saito for technicalassistance. A part of this work was performed under the Shared

17006 | RSC Adv., 2023, 13, 17001–17007

1 W. H. Midura and M. Midura, in Encyclopedia of Inorganic
and Bioinorganic Chemistry, John Wiley & Sons, Ltd., 2014.
2 S. Chaudhary, A. Umar and S. K. Mehta, Prog. Mater. Sci.,
2016, 83, 270–329.
3 G. Liu, Y. Wu, A. Tang, D. Pan and B. Li, Hydrometallurgy,
2020, 197, 105460.
4 Z. Dong, T. Jiang, B. Xu, J. Yang, Y. Chen, Q. Li and Y. Yang,
Chem. Eng. J., 2020, 393, 124762.
5 A. Połatajako, N. Jakubowski and J. Szpunar, J. Anal. At.
Spectrom., 2006, 21, 639–654.
6 Y. liu, M. He, B. Chen and B. Hu, Talanta, 2015, 142, 213–220.
7 E. A. Azooz, M. Tuzen, W. I. Mortaba and N. Ullah, Crit. Rev.
Anal. Chem., 2022, DOI: 10.1080/10408347.2022.2153579.
8 Y.-C. Hou, C.-C. Chang, W.-L. Cheng and I.-S. Shaw,
Hydrometallurgy, 1983, 9, 381–392.
9 M. R. Chowdhury and S. K. Sanyal, Hydrometallurgy, 1993,
32, 189–200.
10 M. R. Chowdhury and S. K. Sanyal, Hydrometallurgy, 1994,
34, 319–330.
11 A. Sattari, M. Kavousi and E. K. Alamdari, Trans. Indian Inst.
Met., 2017, 70, 1103–1109.
12 B. M. Sargar, S. V. Mahamuni and M. A. Anuse, J. Saudi
Chem. Soc., 2011, 15, 177–185.
13 W. Aeungmaitrepirom, A. Hagège, Z. Asfari, L. Bennouna,
J. Vicens and M. Leroy, Tetrahedron Lett., 1999, 40, 6389–
6392.
14 W. Aeungmaitrepirom, A. Hagege, Z. Asfari, J. Vicens andM. Leroy, J. Inclusion Phenom. Macrocyclic Chem., 2001, 40,
225–229.
15 F. Séby, M. P. Gautier, E. Giﬀaut, G. Borge andO. F. X. Donard, Chem. Geol., 2001, 171, 173–194.
16 J. Milne and P. LaHaie, Inorg. Chem., 1979, 18, 3180–3183.
17 H. Narita, K. Morisaku and M. Tanaka, Chem. Commun.,
2008, 45, 5921–5923.
18 H. Narita, K. Morisaku and M. Tanaka, Solvent Extr. Ion
Exch., 2015, 33, 407–441.
19 M. Maeda, H. Narita, C. Tokoro, M. Tanaka, R. Motokawa,
H. Shiwaku and T. Yaita, Sep. Purif. Technol., 2017, 177,
176–181.
20 T. Suzuki, T. Ogata, M. Tanaka, T. Kobayashi, H. Shiwaku,
T. Yaita and H. Narita, Metals, 2018, 8, 558.
21 H. Narita, R. M. Nicolson, R. Motokawa, F. Ito, K. Morisaku,
M. Goto, M. Tanaka, W. T. Heller, H. Shiwaku, T. Yaita,
R. J. Gordon, J. B. Love, P. A. Tasker, E. R. Schoeld,
M. R. Antonio and C. A. Morrison, Inorg. Chem., 2019, 58,
8720–8734.

© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

Open Access Article. Published on 06 June 2023. Downloaded on 9/25/2024 11:08:43 AM.
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Paper
22 S. Uchino, H. Narita, K. Kita, H. Suzuki, T. Matsumura,
H. Naganawa, K. Sakaguchi and K. Ohto, Solvent Extr. Res.
Dev., Jpn., 2023, 30, 39–46.
23 F. E. Huggins, C. L. Senior, P. Chu, K. Ladwig and
G. P. Huﬀman, Environ. Sci. Technol., 2007, 41, 3284–3289.
24 T. Ressler, J. Synchrotron Radiat., 1998, 5, 118–122.
25 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson,
Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 7565–
7576.
26 R. B. Helmholdt, E. J. Sonneveld and H. Schenk, Z. Kristall.,1999, 214, 151–153.

© 2023 The Author(s). Published by the Royal Society of Chemistry

RSC Advances
27 A. Pawłowski and A. Haznar, J. Solid State Chem., 2001, 160,
189–194.
28 Y. Hermodsson, Acta Chem. Scand., 1967, 21, 1313–1327.
29 J. E. Penner-Hahn, Coord. Chem. Rev., 1999, 190–192, 1101–
1123.
30 F. L. Bernardis, R. A. Grant and D. C. Sherrington, React.
Funct. Polym., 2005, 65, 205–217.
31 J. Pettersson and Å. Olin, Talanta, 1991, 38, 413–417.
32 P. Lahaie and J. Milne, Inorg. Chem., 1979, 18, 632–637.
33 F. Calderazzo, M. D'Attoma, F. Marchetti and G. Pampaloni,
J. Chem. Soc., Dalton Trans., 2000, 2497–2498.

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