Iron oxi
des an
d oxyhy
droxi
des (or iron-rich marine se
diments) are assume
d to be one of the can
di
date ‘missing sinks’ of Ge that account for half of the total precipitate
d Ge from seawater. Information on equilibrium Ge isotope fractionation
during Ge a
dsorption onto Fe(III)-oxyhy
droxi
de surfaces plays an essential role in un
derstan
ding the Ge isotope global bu
dget. In this stu
dy, the possible a
dsorbe
d Ge surface complexes on Fe(III)-oxyhy
droxi
de surfaces are carefully verifie
d by comparing their energies an
d geometries with known experimental results. Moreover, the equilibrium Ge isotope fractionations between the a
dsorbe
d Ge complexes an
d the
dominant Ge species in aqueous solution are investigate
d by using the Urey mo
del or the Bigeleisen–Mayer equation. First-principles
density functional theory is use
d for optimizing the structures an
d obtaining harmonic frequencies at the B3LYP/6-311+G(
d,p)
level. For accurate estimation of solvation effects, the “water-
dropletȁ
d; explicit solvent metho
d is also use
d.
Our results suggest that bidentate corner-sharing (2C) > Fe2O2Ge(OH)2(aq) is the dominant adsorbed Ge surface complex during adsorption processes in the neutral or acidic condition. Under basic conditions, the dominant surface structure is changed to 2C > Fe2O2GeOOH−(aq) complex. These results are consistent with existing EXAFS experimental evidences (Pokrovsky et al., 2006). Our calculations show that Ge isotopes can be distinctly fractionated by such adsorption processes to about 1.7‰ (25 °C, in terms of 74Ge/70Ge). Light Ge isotopes are preferentially enriched on the surfaces of Fe-oxyhydroxides. The data generally agree with the observations of Galy et al. (2002) and Rouxel et al. (2008). Such Ge adsorption might affect the Ge isotope compositions of rivers, hydrothermal fluids and seawaters. Because of the broad distribution and large amount of iron-rich deposits everywhere, such adsorption processes will greatly influence the Ge isotope global budget.