Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

12C/2004G121

Speciation study of hexavalent chromium reduced by Humic acidAtsuyuki OHTA*1, Hiroshi TSUNO2, Hiroyuki KAGI3, Satoshi FUKURA3, Masaharu, NOMURA4
1

Geological Survey of Japan, AIST, Ibaraki 305-8567, Japan
Faculty of Education and Human Sciences, Yokohama National University, Kanagawa, 240-8501, Japan
3
Geochemical Laboratory, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
4
Photon Factory, Institute of Materials Structure Science, KEK, Ibaraki 305-0801, Japan
2Introduction
A reduction of Cr(VI) by organic materials in soil suchas Humic and Fulvic acid (HA and FA) is a very
interesting phenomenon for environmental problems
because pollutant Cr(VI) disappears in a field systemeasily [1, 2]. Recently, it was also found that the Cr(VI) in
soil is reduced even in dry condition [3]. So far, thereduction study of Cr(VI) by HA and FA have targeted forthe reduction kinetics. However, there is little speciation
examinations of Cr(VI) after a reduction process. The
main purpose of this study is to elucidate the speciation ofCr(VI) reduced by HA and distinguish the Cr(III)originated from pollutant Cr(VI) and the Cr(III) in anatural state.

that bind Cr(III) do not change. The comparison of Cr
XANES spectra of HA reacting with Cr(VI) and Cr(III) tothose of chromium compounds suggests that 40% ofCr(III) just adsorbed on HA electrostatically and the restof Cr(III) binds to a CH3COO--like functional group ofHA. Finally, the Cr XANES spectra of JSO-2 wascompared with the above materials. Tsuno et al. [3]reported that the Cr(VI) added to JSO-2 totally reduced toCr(III). However, the spectra is resemble not to those of
HA reacting Cr(VI) and Cr(III) but to those of Cr(OH)3.
The results suggest 1) that Cr(VI) in JSO-2 was reducedby Fe(II) and precipitated as Cr(OH)3 or 2) that the pH ofexperimental solution was too low in this study.

Methods
The K2Cr2O7 and Cr(NO3)3·9H2O were used as reactant
Cr(VI) and Cr(III), respectively. Two kinds of HA
purchased from Aldrich Chemical Co. and Wako Co.
were selected for a reducing compound. The 2.5 µg, 25µg, and 250 µg of Cr(VI) and 50 µg of Cr(III) werereacted with 1mg HA in a 10ml solution at pH 2.0 for 7days. The reactants were filtrated by a 0.1 µm PTFE
membrane filter and dried in under vacuum. The Cr(VI)remaining unreactive was removed from HA by filtration.
Experiment
The Cr K-edge XANES spectra were recorded in afluoresce mode at the BL-12C of KEK-PF under an
atmosphere and at room temperature. The fluorescence Xray was measured by a 19 element pure-Ge SSD. For
comparison, several kinds of chromium compounds(K2Cr2O7, Cr(OH)3, Cr(NO3)3·9H2O, and Cr(CH3COO)3)and JSO-2 that is a geochemical reference material andcontains K2Cr2O7 were measured in a fluorescence mode.
Results and Discussion
Figure 1 shows the Cr k-edge XANES spectra of HA
reacting with Cr(VI) and Cr(III). There are little
differences of Cr XANES spectra between two kinds ofreactant HA. Types and proportions of functional groups
that bind Cr(III) may be common to each other.
Interestingly, the HA reacting with Cr(VI) has the same
spectra as those with Cr(III). The Cr(III) produced by
reduction process binds with HA like Cr(III). Also the Cr
XANES spectra have no significant change with theincreasing amount of Cr(VI). In the range of experimental
conditions, the types and proportions of functional groups

18

Users' Report

Figure 1. The Cr K-edge XANES spectra. [A], [B], [C],and [D] shows Aldrich HA reacting with 2.5 µg, 25 µg,and 250 µg of Cr(VI) and 50 µg of Cr(III), respectively.
[E], [F], [G], and [H] shows Wako HA reacting with 2.5µg, 25 µg, and 250 µg of Cr(VI) and 50 µg of Cr(III) ,respectively.
References[1] P. R. Wittbrodt and C. D. Palmer, Environ. Sci.
Technol., 30, 2470 (1996).
[2] N. Kožuh et al., Environ. Sci. Technol., 34, 112(2000)[3] Tsuno et al. Geostand Geoanalyt Res 30, 55 (2006).
* a.ohta@aist.go.jp