Photon Factory Activity Report 2017 #35 (2018)

BL-12C/2016G632

Behavior of Zinc in the sediments on Majuro Atoll
1Lisa Ito1, and Yoshio Takahashi1*
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

1 Introduction
Reef island in atoll is composed of biological remains
such as foraminifera and coral gravels made from calcium
carbonate (CaCO3) [1] Recently, anthropogenic impacts
such as heavy metal contamination have been concernedin atolls due to the urbanization of islands. However, theelement behaviours in sediments have not been clarifiedbecause previous studies have been limited the study areasuch as sea areas or aerosol particles. Therefore, thisstudy aims to reveal subsequent postdeposition migration
behaviour Zinc (Zn), which has negative impact onaquatic species. [2]2 Sample
Samples were collected from inland area of Lauraisland in Majuro Atoll, Marshall Island, which has theoldest resident history in the atoll [3]. Samples were taken
based on sediment profile in the soil trench excavated in
1×1 m2 area in the center of Laura island at various depths
with different colors and grain sizes.
3 Experiment
Zinc species in the sediments were estimated throughX-ray absorption near-edge structure (XANES) methods
at BL-12C in Photon Factory. To investigate the insoluble
spices, we prepare two kinds of samples. One is natural
sample and the other one is the samples extracted withacetic acid to identify insoluble Zn. These samples were
homogenized using an agate mortar before themeasurement. Zn K-edge XANES was obtained using 1.0
× 0.5 mm2 beam. Transmission mode and fluorescencemode were used for measuring standard materials andsamples to obtain Zn k-edge XANES spectra, respectively.
The details of the measurement are similar to those thatwere described by Takahashi et al. (2007). [4] The leastsquares fitting of the XANES spectra with the linear
combination of various reference materials was carriedout using REX2000 software (Rigaku Co., Ltd., Japan).
4 Results and Discussion
The results of bulk Zn K-edge XANES are shown in
Fig. 1. The spectra of original samples are shown in blue
and that of residual samples after extraction are shown inpurple, whereas the spectra of Zn standard materials andresults of LSF results are shown in black and dotted black.
These results suggested that the dominant Zn species inthe bulk sediments were Zn coprecipitated with both
calcite and apatite, which are insoluble in water (Fig.1).
Furthermore, the samples contained water-soluble species,
such as ZnCl2 and Zn(NO3)2·6H2O. As expected, watersoluble chemical species were removed from the threesamples after the extraction. Consequently, the main Zn
species found after the extraction were Zn which wascoprecipitated with or adsorbed on apatite as identified
through XAFS analysis (Fig. 1). These results and anothermeasurements using ICP-MS [5] suggested thatapproximately 50% of Zn existed as water-insoluble Zn inapatite. The coprecipitation of Zn in apatite hinders themigration of Zn into deeper layers and groundwater. This
inhibition of concentrated Zn from the surface sediment ishighly important in terms of their exposure to aquaticorganisms and even to the residents on Majuro Atoll.
In summary, our study suggested that naturalattenuation mechanism by the sediment layer has beenfunctioning in Majuro Atoll for heavy metals such as Zn.
Our study also showed that speciation analysis of suchelements is important to predict their mobility andsubsequent influence on the ecosystem. More details canbe found from Ito et al. (2018). [5]References[1] T. Yasukochi et al., Geomorphology. 222, 59 (2014).
[2] H. Clements and M. Kiffney, Can. J. Fish. Aquat. Sci.
52, 1966 (1995).
[3] T. Yamaguchi et al., Pac. Sci. 63, 4 (2009).
[4] Y. Takahashi et al., Geochim. Cosmochim. Acta. 71,984 (2007).
[5] L. Ito et al., Chemosphere. 202, 65 (2018).
* ytakaha@eps.s.u-tokyo.ac.jp

Before A.A.* extraction
After A.A.* extraction*acetic acid

Fig. 1: Zn-K edge XANES spectra obtained from natural
samples and the samples extracted with acetic acid.