Investigation of Rhenium's Biogeochemistry
Helz, George R
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The form of the paleoredox indicator, rhenium, that is sequestered in reducing sediments is not known. To probe this question, I used Laser Ablation ICP-MS to look for correlations between Re signals and those of other elements. My results point to the possibility that Re is incorporated into multiple host phases: sulfides and organic material. Laser Ablation ICP-MS also provided a new way to analyze a large suite of elements in Chesapeake Bay sediments simultaneously. A number of rarely determined elements (Ag, Sn, Sb, Te and Bi) were discovered to exhibit profiles similar to known industrial pollutants such as Cu, Zn, Cd and Pb. This is one of the first demonstrations of LA-ICP-MS as a tool to perform a survey of anthropogenic impacts on modern sediments and suggests that the suite of elements monitored by environmental agencies should be broadened. Rhenium is scavenged between iron and sulfate reduction, in the sequence of microbial terminal electron accepting processes, so I investigated whether Re could be reduced directly or indirectly by iron and/or sulfate reducing bacteria. This hypothesis was also based on the fact that Tc (also Group VIIB), is directly and indirectly reduced by the iron and sulfate reducers used in my experiments: <italic>Geobacter metallireducens<italic> GS-15, <italic>Shewanella oneidensis<italic> €MR-1, <italic>Desulfovibrio desulfuricans<italic> subsp. <italic>desulfuricans<italic> and <italic>Desulfovibrio desulfuricans<italic> ND132. I concluded that neither direct nor short-term indirect microbial processes are likely to explain Re fixation in sediments. In order to test the hypothesis that thioperrhenates play a role in Re fixation, experimentally determined stability constants, (K<sub> (4-x)(5-x) <sub>) for the following types of reactions are needed. ReO <sub>x <sub>S <sub>4-x <sub>2<sub>+ H<sub>2<sub>S = ReO<sub>x-1<sub>S<sub>5-x<sub><super>-<super> + H<sub>2<sub>O where 1 ≤ x ≤ 4 These compounds are analogous to thiomolybdates, which are the particle reactive form of Mo fixed under reducing conditions. The di- and tri- thioperrhenates were never observed, by UV-Vis spectrophotometry. The stability constant for mono-thioperrhenate, K<sub>01<sub> = 10<super>4<super>, while the stability constant for tetra-thioperrhenate, K<sub>04<sub> = 10<super>19<super>. Below pH 8, the formation of a rhenium-sulfur nano-phase competed with the formation of thioperrhenates. The formation of this nano-phase is most likely an important step in rhenium fixation. Future work should involve exploring the sorption characteristics of this material.