Molecular Geochemistry: systematic understanding of
geochemical behaviors of various elements based on XAFS
Yoshio Takahashi1
1 Department of Earth and Planetary Science, The University of Tokyo, Hongo, Bukyo-ku, Tokyo, Japan
E-mail contact: ytakaha@eps.s.u-tokyo.ac.jp
State-of-the-art spectroscopic methods on the chemical speciation developed in a recent quarter-century
has provided various atomic-scale information even for trace elements in natural samples. The information
includes oxidation state and local structural information of elements of interest, and interactions amongatoms and molecules. This information in turn allows us to compare behaviors and cycles of various
elements from chemical properties. Professor V. M. Goldschmidt defined that geochemistry is the study ofthe distribution and amounts of the chemical elements in minerals, rocks, soils, water, and the atmosphere,
and the study of the circulation of the elements in nature on the basis of the properties of their atoms andions.1 In particular, systematic understanding of various elements based on physico-chemical informationand periodic table is important. To achieve comparison of any elements in periodic table, speciation that canbe applied to trace elements with high sensitivity is essential. For this purpose, X-ray absorption finestructure (XAFS) can be an ideal method, which consists of XANES and EXAFS, good tools to identify theoxidation state and local structure of the particular ion, respectively. If we could establish such systematicunderstanding based on such methods, we can predict behaviors of various elements based on the phsicochemical laws. Thus, I here focus on (i) the systematic understanding of various elements (ions) in naturalsystems and (ii) new XAFS tools in terms of the sensitivity and information on the atomic scale interaction.
(i) Systematic understanding of various elements: One important process, which remains poorly understood,
is adsorption of various ions on various adsorbents in water, which controls dissolved concentrations of ionsand isotopic fractionation in water. For example, adsorption reactions on iron oxides and clay minerals arevery different: the former is chemical interaction with hydroxyl group at the solid surface,2 whereas the latter,in particular for smectites, is characterized by electrostatic interaction (except of the readion to the edgesites). Consequently, the stability of adsorbed species on iron oxides strongly depends on the ionic potentialof each ion, whereas larger ions can be preferentially stabilized within the interlayer of smectites. These
processes can explain retention of various toxic elements by the reaction in soils and sediments, enrichmentof metal ions as resources and optimization of extraction procedure, and isotopic fractionation that can begeochemical tools. As for smectites, for example, caesium, or a largest cation, forms stable inner-spherecomplex within the interlayer which makes it fixed at the surface of soil that contains smectites.3 However,smaller ion that forms stable hydrated complex is adsorbed as outer-sphere complex within the interlayerthat makes the ion readily exchanged with other cations in coexistent water.4 This property makes clay
minerals very effective mineral resources for rare earth elements (REE) called ion-adsorption type ore.5
Here, the materials are same, but its macro-scale application is totally different that depends on theproperties of elements, which can be systematically understood based on their ionic radii, or hard and softacids and bases (HSAB) concept, or more quantitatively by nucleophilic constant (En)6.
(ii) New XAFS tools: One important issue in (i) is the interaction of caesium ion with tetrahedral layer of clay
minerals related to the formation of inner-sphere complex, since only the size (ionic radius) cannot be acause for the formation of the inner-sphere complex. There should be some specific interaction between thecation and the tetrahedral layer, but such information cannot be given only by EXAFS, which only tells static
structural information. For this purpose, XANES can be important, and a new technique using high energy
resolution fluorescence X-ray detection system for XANES better than lifetime broadening effect (HERFDXANES) has made XANES more important than before. For this purpose, the HERFD system using Braggtype crystal analyzer (energy resolution better than 1-2 eV) is useful.7 According to our recent studies using
HERFD-XANES and its interpretation by hard and soft acids and bases (HSAB) concept, caesium has
covalent-type interaction with tetrahedral layer to form stable inner-sphere complex for this soft cation. TheHERFD system is also important to achieve sensitive analysis of XAFS for trace elements which can avoid
interferences by fluorescence X-rays from other elements. For this purpose, a new detector, transition-edge
sensor (TES), can be another powerful tool for fluorescence XAFS.9 Various examples will be given in mypresentation.
[1] V. M. Goldschmidt, J. Chem. Soc. 1937, 655. [2] Y. Takahashi et al., Subseafloor Biosphere Linked to Hydrothermal
Systems: TAIGA Concept, 2015, pp. 39–48. [3] Q. H. Fan, Y. Takahashi et al., Geochim. Cosmochim. Acta 2014, 135,49. [4] Y. Takahashi et al., Radiochim. Acta 1998, 82, 227. [5] A. Yamaguchi, Y. Takahashi et al., Geochem. J. 2018, 52,415. [6] J. O. Edwards, J. Am. Chem. Soc. 1954, 76, 1540. [7] O. Proux et al., J. Environ. Qual. 2017, 46, 1146. [8] S.
Yamada, Y. Takahashi et al., Rev. Sci. Instrum. 2021, 92.