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.

The results of this study suggest that bidentate corner-sharing (C-2)>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 C-2>Fe2O2GeOOH(aq)- complex.  These results are consistent with existing EXAFS experimental evidences (Pokrovsky et al., 2006). The calculations show that Ge isotopes can be distinctly fractionated by such adsorption processes to about 1.7 parts per thousand (25 degrees C, in terms of Ge-74/Ge-70).  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.

Publication name	Chemical Geology, Volume 278, Issues 1-2, 1 November 2010, Pages 15-22
Author(s)	Li XF, Liu Y
Corresponding author	LIU Yun  liuyun@vip.gyig.ac.cn,  State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China

View here for full text from the publisher