Molybdenum(VI) speciation in sulfidic waters: Stability and lability of thiomolybdates

Molybdenum in sedimentary rocks and sediments has evoked interest as a paleo-environmental indicator of reducing conditions. This application demands better understanding of Mo deposition mechanisms and in turn, requires improved knowledge of Mo speciation in anaerobic natural waters. In sulfidic solution, molybdate undergoes sulfidation in four steps that conserve Mo-VI and lead to tetrathiomolybdate: MoOxS4-x2- + H2S(aq) reversible arrow MoOx-1S5-x2- + H2O(l), 1 less than or equal to x less than or equal to 4. Equilibrium constants (K(4-x)(5-x)) and rate constants (k((4-x)(5-x))) have been measured by UV-visible spectroscopy. At 25 degrees C, I = 0.8-2.2 M, log K-01 = 5.19 +/- 0.03, log K-12 = 4.80 +/- 0.12, log K-23 = 5.00 +/- 0.13, log K-34 = 4.88 +/- 0.28. The monothio to trithio-intermediates have negligible stability fields. If H2S(aq) increases slowly enough to maintain near-equilibrium conditions, a sharp switch will occur from MoO42- to MoS42- at similar to 11 mu M H2S(aq) (298 K, 1 atm). Sulfide actuation of the switch depends on a(H2S)(4), so a similar to 3-fold change in a(H2S) produces a similar to 100-fold change in MoO42-/MoS42-. Conversion of the predominant anion from a hard to a soft base alters Mo's geochemical behavior, increasing its susceptibility to scavenging. Because each successive sulfidation reaction is similar to 1 order of magnitude slower than the previous one, dithio- reversible arrow trithio- and trithio- reversible arrow tetrathiomolybdate equilibria might not be achieved in seasonally or intermittently sulfidic waters. In such environments, unstable mixtures of intermediate thiomolybdates can predominate, contradicting thermodynamic predictions. The sulfidation reactions are acid catalyzed. Anoxic sediment porewaters, which are enriched relative to overlying waters in Bronsted acids, will promote conversion of molybdate to thiomolybdates. This may account for observations that Mo deposition in euxinic environments occurs often by diagenetic pathways rather than by water column scavenging. Slow hydrolysis and oxidation of MoS42- suggests that it could serve as a reservoir of fixed, but acid-labile sulfide in oxic waters. Copyright (C) 2000 Elsevier Science Ltd.