Investigation of tungstate thiolation reaction kinetics and sedimentary molybdenum/tungsten enrichments: Implication for tungsten speciation in sulfidic waters and possible applications for paleoredox studies

Abstract

The kinetics of tungstate (i.e., WO42−) thiolation were investigated in experimental solutions buffered at different pH values and as a function of varying dissolved sulfide concentrations. Similar to molybdate (MoO42−), tungstate undergoes stepwise thiolation to tetrathiotungstate according to: WOxS4-x2-+H2S(aq)↔WOx-1S5-x2-+H2S; 1 ≤ x ≤ 4. Under equilibrium conditions at standard conditions (298.15 K, 105 Pa) WO42− also converts to tetrathiotungate (WS42−) around 1.0 mM H2S(aq), reminiscent of the chemical switch for the MoO42− → MoS42− transition reported in the literature, but at nearly 100-fold higher H2S(aq) concentrations. The laboratory experiments show that thiotungstate formation is first order with respect to H2S concentration, and that the thiolation reactions are catalyzed by general Brønsted acids. Therefore, the high NH4+ and HCO3− concentrations employed in the experiments both favored WO42− thiolation. The experimental data were used to develop Brønsted relationships for the successive thiolation reactions (i.e., WO3S2− → WO2S22−, WO2S22− → WOS32−, WOS32− → WS42−) that allow the acid-catalyzed thiolation rates of WO42− to be estimated as a function of pH in natural waters. Reaction modeling of tungstate thiolation kinetics indicates that di- to trithiotungstate (WO2S22− → WOS32−) conversion and tri- to tetrathiomolybdate (MoOS32− → MoS42−) conversion may not be achieved in temporally variable sulfidic waters. In such environments, intermediate thioanions of W and Mo dominate with Mo exhibiting a higher degree of thiolation than that of W. As the partition coefficient of W into minerals is affected by its speciation in solution, its enrichment in minerals may change due to the level and changes in dissolved sulfide concentrations. Specifically, the partition coefficient of W decreases from oxic environments to sulfidic environments, suggesting W enrichment in sediment is likely to be a good tracer of redox conditions (i.e., oxic and sulfidic conditions) in the overlying waters. In comparison, the partition coefficient of Mo increases from seasonally sulfidic environments to permanently sulfidic environments, indicating that Mo enrichment in sediments is a good tracer of sulfidic conditions. In addition, the Mo/W concentration ratio in black shales shows the potential for identifying fluctuation of redox conditions from ca. 2500 Ma to ca. 0.1 Ma and is a potential proxy for tracking deep time sulfidic conditions. Thus, even though the geochemical behavior of W is different from Mo, W can nevertheless be a potential proxy for tracking changing redox conditions in the modern and ancient ocean.