An X-ray absorption fine structure spectroscopy study of germanium-organic ligand complexes in aqueous solution
Résumé
The local environment around Ge(IV) in 0.002 to 0.023 m aqueous solutions at ambient temperature was characterised as a function of pH and in the presence of different organic ligands (citric and oxalic acids, and catechol) using high resolution X-ray absorption fine structure spectroscopy (XAFS). Results show that both in organic-free solutions at acid to basic pH and in carboxylic acid-bearing solutions at neutral to basic pH and catechol-bearing solutions at acid pH, Ge is coordinated with four oxygens with mean Ge–O distances of 1.75±0.02 Å. This is consistent with the formation of the aqueous Ge(OH)4° and GeO(OH)3− complexes and confirms the results of previous studies based on solubility, potentiometry and Raman spectroscopy ([Pokrovski, G.S., Schott, J., 1998a. Thermodynamic properties of aqueous Ge(IV) hydroxide complexes from 25 to 350°C: implications for the behavior of germanium and the Ge/Si ratios in hydrothermal fluids. Geochim. Cosmochim. Acta 62, 1631–1642; Pokrovski, G.S., Schott, J., 1998b. Experimental study of the complexation of silicon and germanium with aqueous organic species: implications for Ge and Si transport and Ge/Si ratio in natural waters. Geochim. Cosmochim. Acta 62, 3413–3428]). By contrast, in carboxylic acid-bearing solutions at acid pH and in catechol-bearing solutions at pH>4 Ge is coordinated by 6 oxygen atoms with Ge–O distances ranging from 1.85 to 1.94 Å. This implies the formation of chelate type complexes with the organic ligands. These results are in agreement with germanium speciation scheme in organic-bearing solutions proposed by Pokrovski and Schott (1998b), and explain a stronger affinity of germanium, compared with silicon, to chelating organic compounds. The formation of Ge–organic acid complexes can lead to a significant increase of Ge/Si ratios in organic-rich fluids and should be taken into account when using these ratios measured in surficial waters and biogenic opals to estimate chemical-weathering intensity and Ge and Si global fluxes.