Experimental study of the complexation of silicon and germanium with aqueous organic species: implications for germanium and silicon transport and Ge/Si ratio in natural waters

Abstract

The stability of aqueous complexes formed by Si and Ge with carboxylic acids (acetic, salicylic, oxalic, citric, tartaric) and phenols (phenol and catechol) has been investigated from 25 to 90°C via solubility and potentiometric measurements. Results show that Ge forms stable complexes with the di- and tricarboxylic acids and catechol, but that Si forms much weaker complexes with these ligands. Analysis of our results and of available literature data on Ge complexes formed with other types of aqueous organic species demonstrates that Ge forms complexes of chelate type with the following functional groups: (1) carboxylic in acid solutions (1 ≤ pH ≤ 6), (2) di-phenolic hydroxyls in neutral and basic solutions (pH ≥ 6), and (3) alcoholic hydroxyls in very basic solutions (pH ≥ 10). Conversely, Si forms very weak complexes with these compounds. Stability constants generated in this study for Ge- and Si-organic species have been used to approximate Ge and Si complexing with humic acids which possess the same organic functional groups as those used in this study. Our calculations show that Si-humic acid complexes are negligible in most natural waters. In contrast, the presence of humic acids can considerably affect Ge speciation in aqueous solution. For example, at pH ≥ 6 in a solution containing and 0.1 μg/L of Ge and 20 mg/L of dissolved organic carbon (DOC), Ge-humic acid complexes account for more than 95% of total aqueous Ge. These results can explain the increase of the Ge/Si ratio in organic-rich surficial waters. Ge-organic matter complexation should be thus taken into account when using Ge/Si ratios measured in surface waters and biogenic opals to estimate chemical-weathering intensity and Ge and Si global fluxes.