Photon Factory Activity Report 2016 #34 (2017)

BL-12C, AR-NW10A/2016G086

Oxidation of antimony (III) in soil using X-ray absorption fine structure
Lei Fu*, Katsumi Shozugawa and Motoyuki Matsuo
Graduate School of Arts and Sciences, The University of Tokyo
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

2 Experiment
We used andosol which are widely distributed
throughout Japan in this research. Sb oxidation reaction
experiment was performed by doping with Sb into andosol
for a concentration of Sb-1000 ppm. Soil samples weresealed immediately in polystyrene bags for XAFS
measurement after adjusting the water content to 30%.
Measurement was carried out once immediately after
doping with Sb (reaction time(RT) = 0) representing thebeginning of the oxidation reaction and measured again
several times (RT = 1d, 3d, 5d, 14d).
Sb K-edge XAFS spectra was collected in fluorescencemode using a Lytle detector at the beamline AR-NW10Aof Photon Factory Advanced Ring. Mn K-edge XAFS
measurements were also carried out in fluorescence modeusing a Lytle detector at the beamline BL-12C of Photon
Factory. Both the XANES region and EXAFS region wererecorded. XAFS analysis was conducted using REX2000
software provided by Rigaku Corp (version 2.5.5). The
scattering parameters were obtained by FEFF 7 package(Ankudinov and Rehr, 1997).
3 Results and Discussion
Sb K-edge XANES spectra of andosol are shown in Fig.
1. The XANES spectra of reference compounds Sb2O3 andK[Sb(OH)6] show that Sb(III) has a lower absorption edgeenergy of 30,493.7 eV and a relatively less pronounced
white line in comparison with Sb(V). Result of the firstmeasurement (RT = 0) is almost the same as the spectra of
Sb2O3. However, spectra get much closer to K[Sb(OH)6]

after 2 weeks of reaction time. Based on the speciation
fitting result estimated by REX2000 of RT = 14d, about
71.2% of Sb are in pentavalent form. On the other hand, Sb
K-edge XANES spectra of reaction time 1d – 5d are all
close to the spectra of trivalent oxide. The speciation fitting
results for Sb(V) of RT = 1d, 3d, 5d are 7.6%, 19.4%,30.6%, respectively. Oorts et al. (2008) pointed out that
Sb2O3 could be almost oxidized in two days after entering
the soil. However, results in our experiment show that thespeed of Sb(III) oxidation reaction in andosol was ratherslow. This may be due to the relatively high Sb(III)concentration in our research (1000ppm) in comparison
with the concentration of 510ppm of theirs, and a silt-loamsoil was used in their report, so the differences in soil types
should also be considered.
Fig. 2 gives the Mn K-edge XANES spectra results (RT= 0, 14d) during the process of the Sb(III) oxidation
reaction. As we can see, absorption edge energy is closer
to quadrivalent compounds at the beginning of theexperiment and then change to a lower value after 2 weeks
Sb(III) Sb(V)RT = 0RT = 1d
RT = 3d

Normalized Absorption

1 Introduction
The chemical state of elements in the sediment and soil
has been used as a good indicator to estimate the redox
state which considered as an important research topic ingeochemistry and environmental chemistry. Antimony(Sb)is one of those elements which responds sensitively toredox state and exists multiple valence states in thesediment and soil. Studies on Sb chemical states show thatthe toxicity of Sb(III) is much more poisonous than Sb(V)(Gurnani et al., 1994). In addition, previous reports about
the chemical behaviour of Sb showed that despite being
emitted mostly as Sb(III) into environment, antimony
exists mainly as Sb(V) in soil samples. However, themechanism of the oxidation of Sb(III) in soil is still
unknown.
The purpose of this study is to observe the oxidation
process of Sb(III) in soil using X-ray absorption finestructure (XAFS) spectra and hopefully determine theelement which cause the oxidation reaction.

RT = 5d
RT = 14d
Sb2O3
K[Sb(OH)6]

30.46

30.48

30.5
30.52
Energy (keV)

30.54

Fig. 1. Sb K-edge XANES spectra change over time inandsol. From top: reaction time(RT) = 0, 1d, 3d, 5d,14d.

Photon Factory Activity Report 2016 #34 (2017)

RT = 0RT = 14d
MnSO4

MnCO3
Mn(H2PO4)2
Mn3(PO4)2
MnO2

6520

6540

6560

6580

Table 1EXAFS results during the oxidation reaction of Sb(III) in andosol.

aRT
RT=0Shell
Sb-O
Sb-Sb

CN
3.2±0.3
4.5±1.1R(Å)
1.988±0.010
3.588±0.012

ΔE0 (eV)16.0±2.9

σ20.003a
0.003a

RT=14dSb-O

5.7±0.5

1.993±0.012

13.1±3.10.003a

Fixed during fit; CN: coordination number.

(b)

(a)

6600

Energy (eV)Fig. 2. Mn K-edge XANES spectra change over timein andsol. From top: RT = 0, 14d.

References[1] N. Gurnani et al., Nucleus 37, 71–96 (1994).
[2] K. Oorts et al., Environ. Sci. Technol. 42, 4378 (2008).
* flay@g.ecc.u-tokyo.ac.jp

MnS

Normalized Absorptionand the spectrum characteristics become more like thedivalent reference compounds. This indicates the chemical
form of Mn has been changed in the redox reaction withSb(III).
EXAFS spectra and fit results of Sb in andosol during
the oxidation reaction are shown in Fig. 3 and Table 1.
Spectra of RT = 0 are clearly different from the RT = 14dby their small peaks of first Sb-O bond in the EXAFS
Fourier transforms at about 1.6 Å before correcting forphase shift and the coordination number is close to three(Table 1). On the other hand, second measurement showslarger peak and the coordination number is close to six.
Both first shell Sb-O bonds and distances of the twomeasurements are almost the same which at around 1.99 Å.
Shell Sb-Sb bond of the fist measurement (RT = 0) showsa distance of 3.6 Å. However, Sb-Sb bond disappeared
after 2 weeks of the oxidation reaction of Sb(III) whichindicates that Sb2O3 compound have been all converted
into other forms and possibly adsorbed in andosol. These
results confirm again that Sb(III) has been oxidized toSb(V) during 2 weeks of the reaction and show that theoxidation states of Sb can also be discriminated based onthe coordination numbers of the first shell Sb-O bond.
By using X-ray absorption fine structure spectra, we
successfully observed the process of the oxidation ofantimony (III) in the soil and can assume that Mn(IV) may
plays an important role in the reaction according to thesignificantly XANES spectra change during the reaction.

Measured
SimulatedSb-O
RT = 0

Sb-Sb
RT = 0k3χ(k)

FT magnitude

Sb-O

RT = 14d
RT = 14d34567k (Å-1)

89100123R (Å)

456Fig. 3. Fourier transformed k3-weighted EXAFS spectra of Sb before and after the oxidation reaction in andosol
in k space (a) and R space (b).