The Mechanism of Rhenium Fixation in Reducing Sediments
Dolor, Marvourneen Kimranee
Helz, George R
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There is a wide range of uses of information related to rhenium's geochemical behavior. Field studies indicate that rhenium is highly enriched in reducing sediments. However the mechanisms by which this high degree of enrichment is achieved are unclear. Perrhenate's (ReO4-) sorption onto a clay mineral, kaolinite, was investigated and found to be weak. This behavior is consistent with rhenium's conservative behavior in the oceans. Sorption was enhanced by the presence of a reducing agent, lithium borohydride. The presence of sulfide, a reducing agent found in nature, enhanced sorption slightly. Sulfide apparently does not reduce ReO4- rapidly as shown by the failure of polysulfide species to appear in solution. Instead, sulfide causes the formation of thioperrhenates. The mechanism of thiolation is the successive replacement of oxygen atoms in ReO4- by sulfur atoms, but ReO3S- and ReS4- are the only thioperrhenates observed by UV-Vis spectroscopy. The di- and tri- thioperrhenates, ReO2S2- and ReOS3- are absent due to their relative instability. Thiolation of ReO4- appears to be general-acid catalyzed. Quantitative yield of ReS4- from ReO4- was never achieved. Thioperrhenates undergo polymerization in solution to yield a colloidal Re-S species, probably related to Re2S7, which contains polysulfide ions in its structure. This Re-S polymer is stable under different experimental conditions and thermodynamic calculations confirm that it is a very stable, insoluble species. Sulfide may play an important role in Re enrichment in sediments by leading to the formation of this solid Re-S species.