First-principles study of Ge isotope fractionation during adsorption onto Fe(III)-oxyhydroxide surfaces

Iron oxides and oxyhydroxides (or iron-rich marine sediments) are assumed to be one of the candidate ‘missing sinks’ of Ge that account for half of the total precipitated Ge from seawater. Information on equilibrium Ge isotope fractionation during Ge adsorption onto Fe(III)-oxyhydroxide surfaces plays an essential role in understanding the Ge isotope global budget. In this study, the possible adsorbed Ge surface complexes on Fe(III)-oxyhydroxide surfaces are carefully verified by comparing their energies and geometries with known experimental results. Moreover, the equilibrium Ge isotope fractionations between the adsorbed Ge complexes and the dominant Ge species in aqueous solution are investigated by using the Urey model or the Bigeleisen–Mayer equation. First-principles density functional theory is used for optimizing the structures and obtaining harmonic frequencies at the B3LYP/6-311+G(d,p) level. For accurate estimation of solvation effects, the “water-droplet” explicit solvent method is also used.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.