Photon Factory Activity Report 2017 #35 (2018)

BL-13A, BL-15A1/2016S2-002, 2016G632

Radiation damage of iron(III)-organic complexes by hard and soft X-ray microscopy
Kohei Sakata1, Yasuo Takeichi2, Yoshio Takahashi3
1
Center for Global Environmental Research, National Institute for Environmental Studies, 16-2
Onogawa, Tsukuba, Japan
2
Institute of Materials Structure Science, Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan
3
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
1 Introduction
In the field of geochemistry, organic carbons (OC) play
an important role in biogeochemical cycles of transition
metals thorough the organic complexations [1]. Recently,
speciation experiments of organic complexes with trace
metals in the surface environmental samples wereconducted by both hard and soft X-ray microscopy.
However, many studies reported X-ray radiation damageof target elements, but knowledge of X-ray radiation
damage of organic complexes with trace metals are notsufficient [2]. In this study, investigations of radiation
damages of Fe(III)-citrate (Fe-cit), Fe(III)-stearate (Fe-ste)and Fe(III)-deferoxamine (Fe-DFO) were conducted by
micro X-ray absorption fine structure (-XAFS) andscanning transmission X-ray microscope (STXM) because
these species were representatives of Fe organic complexes
in the environment.
2 Experiments
Iron K-edge experiments were conducted by -XAFS at
BL-15A1. Iron K-edge spectra were obtained from 7040
eV to 7840 eV by QXAFS. Iron K-edge XANES and
EXAFS spectra were recorded in 30 times andmeasurement time for each scan is 1 min. Shutter wasautomatically opened and closed, when measurements arestarted and finished, respectively. Carbon and oxygen Kedges and Fe L-edge experiments were conducted by
STXM in BL-13A. Carbon and O K-edge NEXAFS
spectra were obtained by image stack in order tounderstand chemical alteration processes.
3 Results and Discussion
Radiation damages of Fe-ste and Fe-DFO were notfound during repetitive scans around Fe K-edge regions. In
contrast, serious photoreduction of Fe K-edge spectra were
found during repetitive scans by -X-ray (Fig. 1a).
Basically, photoreduction of Fe-cit can be explained byfirst-order reaction, but the slope (reactivity constant) werechanged after X-ray irradiation for 20 min. It is expectedthat the process of photodecomposition of Fe-cit ischanged when the irradiation dose exceeds the thresholdvalue. One of the potential reason is that formation of anintermediate and its photoreductions, but the actual reasonof changing the slope was not be able to clarify in this study.
Carbon and O K-edge NEXAFS spectrum of Fe-cit werealso changed by repetitive scans. As a result of X-ray
irradiation, intensity of the peak at 285 eV from C 1s→ *was slightly increased, whereas the peak position of O 1s.

→ * was shifted to high energy region (Figs. 2a and b).
Iron L-edge NEXAFS spectrum was almost no changed byrepetitive scans around C K-edge region, but the spectrum
changed by radiation around Fe L-edge. In addition, C Kedge NEXAFS spectrum is drastically changed after O Kedge and Fe L-edge scans. From these results, impacts ofX-ray radiation damage can be reduced by conducting
experiments from a region with low energy when chemical
species of unknown organic complexes in environmental
samples were determined. At this stage, we cannot
understand detailed process of X-ray radiation processes ofFe-cit, but it is an important advance that the optimizationof speciation methods organic complexes of trace metals
in environmental samples has progressed.
References[1] M. Gledhill and K.N. Buck, Front. Microbiol., 3,Article 69, (2012).
[2] J. Yang and J. Wang, Can. J. Chem. 95, 1191, (2017).

(a)

(b)

Fig. 1 (a) Fe K-edge XANES spectra and (b) relationshipbetween ln(1/Fe-cit conc.) and radiation time.
(a)

(b)

Fig. 2 (a) Carbon K-edge and (b) O K-edge spectrum changes by
X-ray irradiation by STXM.

* sakata.kohei@nies.go.jp