Speciation and Removal Mechanisms of Rhenium in Anoxic Waters: Roles of Zero-Valent Sulfur, Mineral Oxide Catalysis, and Pyrite Surfaces

Global warming is expected to intensify the effects of coastal anoxia. Knowledge of the causes of geohistorical anoxic episodes may prove critical in developing strategies for the mitigation of or adaptation to the consequences of climate change upon the global aquatic environment. The geochemistry of Re provides the means to acquiring this knowledge. Within oxic waters, Re exists as geochemically-inert ReO4-. Under reducing conditions, Re is removed from the aqueous phase and deposited in sediments. Unfortunately, an incomplete understanding of Re geochemistry hampers exploitation of Re as a paleoredox indicator. Authors purport Re sequestration begins in suboxic environments; others produce evidence supporting removal under sulfidic conditions. Some suggest precipitation as ReS2; others hint at scavenging by pyrite. The proposed research will begin clarifying such issues by positing a plausible pathway to Re fixation. Reductively labile thioperrhenates initiate the pathway: ReVIIO4-(aq) + nH2S(aq) ⇌ ReVIIO4-nSn-(aq) + nH2O(l) (n = 1-4) Mineral oxides promote thioperrhenate formation, explaining why Re uptake occurs within the sediments vs. sulfidic water column of seasonally anoxic basins. S0-donors induce Re(VII) reduction to Re(V)-polysulfido species: ReVIIS4-(aq) + 5S52-(aq) ⇌ 5S42-(aq) + ReV(S4)(S4)S-(aq) Reduction of Re(VII) facilitates uptake by pyrite, yielding surface Fe-Re-S cubane clusters, which require reduced Re to overcome coulombic impediments. Laboratory experiments will (a) define Re speciation in sulfidic and polysulfidic environments, (b) quantify kinetic constants for mineral oxide catalysis of thioperrhenate formation and define the chemistry of the catalytic process, and (c) identify preferentially pyrite-scavenged Re species and elucidate chemical controls on sequestration

Molybdenum Burial Mechanism in Sulfidic Sediments: Iron-Sulfide Pathway

Relative to continental crust, sediments underlying sulfidic marine waters are molybdenum-rich, a property preserved in the rock record and useful for characterizing paleoenvironments. The enrichment mechanism is not agreed upon but is attributed at least partly to deposition of Fe–Mo–S compounds, which are as yet uncharacterized. Here, we determine the composition and stability of colloidal Fe–Mo–S precipitates formed at mildly basic pH and H₂S­(aq) > 10^–5 M. The first product consists simply of FeMoS₄, with <i>K</i>_sp = 10^–14.95. Within hours, FeMoS₄ irreversibly transforms by internal self-reduction to a Mo­(IV) product of similar composition. The reduced product is insoluble in 1 M HCl but soluble in concentrated HNO₃, implying that it would be recovered with pyrite in a common assay of sediments. X-ray absorption fine structure data show that Mo­(IV) in the colloids is coordinated by a split first shell of about five sulfur atoms at average distances of 2.31 and 2.46 Å and in its second shell by an iron atom at about 2.80 Å. These properties resemble those determined for Mo in modern anoxic lake sediments and in Phanerozoic black shales. The atomic environment around Mo suggests that the colloidal products may be inorganic polymers containing cuboid, Fe₂Mo₂S₄⁴⁺ cores. Such materials are so far unreported by mineralogists, although a rare mineral, jordisite, may be a related, but more Mo-rich material. The low solubility of FeMoS₄ makes it a feasible precipitate in euxinic waters like those in the modern Black Sea. We propose that colloids similar to those studied here could account for Mo-enrichment in euxinic basin sediments and black shales