Photon Factory Activity Report 2019 #37 (2020)
BL-4A, BL-12C/2018G089, 2018G575

Enrichment mechanisms of antimony and arsenic in marine ferromanganese oxides:
insights from the structural similarity
Hai-Bo QIN, Soichiro UESUGI, Shitong YANG, Masato TANAKA,
Xiaolan ZHAO, Takaaki ITAI, and Yoshio TAKAHASHI*
The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, JapanThe ferromanganese oxides, aggregates of iron(oxyhydr)oxides and manganese oxides, are mainlypresent in the forms of nodules and crusts in a variety ofenvironments. Marine ferromanganese oxides can begenerally classified into hydrogenetic (HG) nodules andcrusts on deep sea floor, diagenetic (DG) deposits insediments, and hydrothermal (HT) deposits close to
oceanic ridges, based on the Mn/Fe ratio, growth rate, andconstituent Mn minerals. It is generally considered thatmarine ferromanganese oxides are one of potential mineraldeposits as an additional supplement to land-based
resources, because they can act as the efficient scavengers
for many rare and critical elements like tellurium (Te),cobalt (Co), molybdenum (Mo), and REEs from seawater(e.g., Hein et al., 2013). For example, Te can be extremely
enriched in marine ferromanganese oxides with anenrichment factor of ~109 times relative to seawater (Heinet al., 2003; Kashiwabara et al., 2013; Qin et al., 2017).
Over the past decades, marine ferromanganese crusts andnodules as potential deep-ocean resources have received
increasing attention, however, much lesser knowledge isavailable concerning the incorporation and enrichment
mechanisms for each trace element in marine
ferromanganese oxides. However, much lesser knowledge
is available concerning the incorporation and enrichment
mechanisms for antimony (Sb) and arsenic (As) in marine
ferromanganese oxides. The purposes of this work were (i)to reveal the dominant surface complexes for Sb and As
adsorbed on Fe and Mn oxides as examples of traceelementd enriched in marine ferromanganese oxides; (ii) toclarify different enrichment mechanisms for Sb and As in
marine ferromanganese oxides at the molecular scale; and(iii) to provide new insights into geochemical processes
and environment fates of trace elements in marine
environment.
In the present study, the dominant adsorbed structureof Sb(V) and As(V) in synthetic ferrihydrite and Mn oxides
at the molecular scale were investigated by combination ofEXAFS analysis and quantum chemical calculation.
Furthermore, the speciation, distribution, and local
structure of Sb and As in different types of natural marine
ferromanganese oxides were determined by the XAFS
spectroscopy. XAFS spectra. XAFS spectra at As K-edge
and Sb K-edge were measured at bamlines BL-12C andNW10A, respectively. Micro-XRF-XAFS was measured atBL-4A.

The EXAFS results suggested that Sb(V) waspreferentially adsorbed as bidentate-mononuclear complex
on δ-MnO2 due to the symmetry and ionic radius ofoctahedron SbV(OH)6- are very similar with Mn4+, buttetrahedral AsVO43- was adsorbed as bidentate-binuclear
surface complex on δ-MnO2 (Fig. 1). Both Sb(V) andAs(V) were predominantly adsorbed as bidentatebinuclear complex on ferrihydrite and birnessite. In marine
ferromanganese oxides, As can be largely distributed to Fe(oxyhydr)oxide, but Sb can be retained by both Fe(oxyhydr)oxide and Mn oxide. The distribution of Sb toMn oxides could be dependent on the Mn/Fe ratio andconstituting mineral in ferromanganese samples, while As
is selectively distributed to birnessite in addition to Fe(oxyhydr)oxides in marine ferromanganese oxides. The
significant discrepancy of the distribution between Sb andAs in ferromanganese oxides can be attributed to theirdifferent molecule geometry and preferential surface
complexes on Fe and Mn oxides. The findings in this study
suggest that the adsorption site is likely to be different for
the trace elements dissolved as anions in seawater, and thetrace elements with similar structure to the host phases
would show higher enrichment factors in marine
ferromanganese oxides.

Figure 1. As K-edge EXAFS spectra of adsorbed samples:(A) k3-weighted x(k) spectra, and (B) their RSFs (phaseshift not corrected).
References[1] S. Yang et al., ACS Earth Space Chem. 3, 29 (2019).
[2] H. B. Qin et al., Geochim. Cosmochim. Acta 257, 110(2019).
* ytakaha@eps.s.u-tokyo.ac.jp