Germanium isotope fractionation during Ge adsorption on goethite and its coprecipitation with Fe oxy(hydr)oxides

Abstract

Isotopic fractionation of Ge was studied during Ge adsorption on goethite and its coprecipitation with amorphous Fe oxy(hydr)oxides. Regardless of the pH, surface concentration of adsorbed Ge or exposure time, the solution–solid enrichment factor for adsorption (Δ74/70Gesolution–solid) was 1.7±0.1‰. The value of the Δ74Gesolution–solid in Fe–Ge coprecipitates having molar ratio 0.1<(Ge/Fe)solid<0.5 remained constant at 2.0±0.4‰. For (Ge/Fe)solid ratio<0.1, the Δ74Gesolution–solid increased with the decrease of Ge concentration in the solid phase, with the value as high as 4.4±0.2‰ at (Ge/Fe)solid<0.001, corresponding to the majority of natural settings. These results can be interpreted based on available structural data for adsorbed and coprecipitated Ge. It follows that Ge(OH)4° adsorption occurring as bidentate binuclear complexes at the goethite surface is characterised by an enrichment factor of ∼1.7‰, likely related to the distortion of the GeO4 tetrahedron and the formation of Ge–O–Fe bonds at the goethite surface as compared to aqueous solution. In contrast, coprecipitation yields more distorted edge-sharing GeO4 tetrahedra and, in the case of the most diluted samples, part of the Ge is found in coordination 6, replacing Fe(III) in octahedral positions. This produces a greater enrichment of the solid phase in lighter isotopes, mostly due to the increase in Ge–O bond distances and coordination number compared to aqueous solution, which is in line with the basic principles of isotope fractionation. Discharge of hydrothermal fluids, leading to massive Fe(OH)3 precipitation in the vicinity of the springs should, therefore, represent an isotopically-heavy source of dissolved Ge to the ocean. Similarly, groundwater discharge and Fe(OH)3 precipitation at the Earth’s surface, Fe oxy(hydr)oxide formation in soils and riverine organo-ferric colloids coagulation, leading to iron hydroxide precipitation in estuaries, should produce an isotopically heavy Ge aqueous flux to the ocean compared to bedrock sources and particulate fluxes.