5 research outputs found
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 H<sub>2</sub>S(aq) >
10<sup>–5</sup> M. The first product consists simply of FeMoS<sub>4</sub>, with <i>K</i><sub>sp</sub> = 10<sup>–14.95</sup>. Within hours, FeMoS<sub>4</sub> 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 HNO<sub>3</sub>, 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,
Fe<sub>2</sub>Mo<sub>2</sub>S<sub>4</sub><sup>4+</sup> 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 FeMoS<sub>4</sub> 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