Rhenium (Re) is an extremely rare element and, as such, has a very sparse crustal abundance of approximately 0.4 parts per billion (ppb). In seawater, it is distributed quite consistently from one ocean to the next and at most depths as the highly soluble perrhenate ion (ReO4-), which contains Re(VII). It is available in seawater at a mere 8.3 parts per trillion (ppt), but in the anoxic sediments just below the sediment/seawater interface it is found in far greater quantities, as much as 110 ppb. The cause of this, at least in part, is the reduction of the Re(VII) in perrhenate to a solid phase containing Re(IV), which is insoluble. One of the possible mechanisms by which this happens is through interactions with the iron (Fe) contained in Fe(II)-bearing minerals. In our study we reacted KReO4 with magnetite (Fe3O4), siderite (FeCO3), vivianite [Fe3(PO4)2 8H20], green rust (a mixed Fe(II)/Fe(III) layered double hydroxide), FeS, and chemically reduced nontronite (NAu-1) in order to investigate this possibility. Analysis by x-ray near edge absorption spectra (XANES) and extended x-ray absorption fine structure (EXAFS) revealed that green rust alone was able to reduce Re(VII) to Re(IV). This clearly indicates that reduction of soluble Re(VII) in ReO4- to insoluble Re(IV) by green rust is a feasible mechanism by which Re could become enriched in anoxic sediments.

Rhenium (IV) enrichment in sedimentary marine environments and the mechanisms by which rhenium (VII) is reduced: Investigating a paleo-redox indicator

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