American Chemical Society
Petroleum Research Fund
Funding Agency Type
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.
Chemistry and Geology
Science, Engineering and Technology
Vorlicek, Trent P., "Speciation and Removal Mechanisms of Rhenium in Anoxic Waters: Roles of Zero-Valent Sulfur, Mineral Oxide Catalysis, and Pyrite Surfaces" (2012). All Private Grants. 1.
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