Physics
Scientific paper
Apr 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000gecoa..64.1149e&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 64, Issue 7, pp.1149-1158
Physics
43
Scientific paper
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: MoO x S 4-x 2- + H 2 S (aq) MoO x-1 S 5-x 2- + H 2 O (l) , 1 x 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°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 H 2 S (aq) increases slowly enough to maintain near-equilibrium conditions, a sharp switch will occur from MoO 4 2- to MoS 4 2- at ~11 M H 2 S (aq) (298 K, 1 atm). Sulfide actuation of the switch depends on a H2S 4 , so a ~3-fold change in a H2S produces a ~100-fold change in MoO 4 2- /MoS 4 2- . 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 ~1 order of magnitude slower than the previous one, dithio- trithio- and trithio- 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 Brønsted 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 MoS 4 2- suggests that it could serve as a reservoir of fixed, but acid-labile sulfide in oxic waters.
Erickson Britt E.
Helz George R.
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