Tungsten speciation in sulfidic waters: Determination of thiotungstate formation constants and modeling their distribution in natural waters

Using UV/VIS spectrophotometry the equilibrium constants for the formation of monothiotungstate, WO₃S^2- (WO42-+H2S↔WO3S2-+H2O; log K₀₁=3.08±0.11), dithiotungstate, WO₂S₂^2- (WO3S2-+H2S↔WO2S22-+H2O; log K₁₂=3.22±0.22), trithiotungstate, WOS₃^2- (WO2S22-+H2S↔WO3S2-+H2O; log K₂₃=2.76±0.10), and tetrathiotungstate, WS₄^2- (WO3S2-+H2S↔WS42-+H2O; log K₃₄ ~2.36) were determined. The equilibrium constants describing the formation of thiotungstates are approximately two orders of magnitude less than the equilibrium constants for the formation of the analogous thiomolybdates. These equilibrium constants for thiotungstates were used to model tungstate speciation in sulfidic waters. The model predicts that thiotungstate species are negligible in most natural waters, but are likely to be important in circum-neutral, anoxic waters with ≥0.1mM S(-II) concentrations. Natural waters that are conducive to thiotungstate formation include the Black Sea, Tyro and Bannock Basins, and porewaters with high rates of sulfate reduction such as those common in salt marshes. Preliminary field investigations indicate that thiotungstate formation may lead to increased W solubility.