Tungsten (VI) speciation in hydrothermal solutions up to 400°C as revealed by in-situ Raman spectroscopy

Tungsten (VI) speciation in hydrothermal solutions is investigated through in-situ Raman spectroscopy coupled with the fused silica glass capillary technique at temperatures up to 400 degrees C. The effect of temperature, pH, chlorinity and carbonate speciation are evaluated in systems with highly soluble salts Na2WO4 and Na6W12O39. At all investigated temperatures, the tungstate ion WO42- (927 cm-1) is the only W species in solution at pH > 10. At a given pH, the presence of dissolved carbonates and chloride does not affect the tungsten speciation. Tungsten polymers remain stable up to 400 degrees C under acidic to circum-neutral pH conditions and total tungsten concentration above 0.01 mol_skgH2O mers, namely [W7O24]6- (paratungstate-A, -960 cm-1), [W10O32]4- (tungstate-Y, -970 cm-1), and a-[H2W12O40]6- (ametatungstate, -990 cm-1), only the hepta- and dodeca-tungstate are stable at elevated temperature. Combined with revised literature data, these results allow the thermodynamic stability constants of these W polymers to be constrained, enabling quantitative predictions of their relative abundance at temperatures up to 300 degrees C. These predictions suggest that W polymerization occurs under hydrothermal conditions even at low W concentration (down to 10-3 mol center dot kgH2O-1 ) under acidic conditions. These observations imply that the currently available geochemical models on W transport and deposition in deep and hot geological fluids need to be revised. (c) 2021 Elsevier Ltd. All rights reserved.