Because V experiences marked enrichment in euxinic sediments, the element can be a useful paleoredox proxy. Unfortunately, knowledge of the chemical mechanisms VVO43-, the passive oxic form of V, undergoes as it enters euxinic waters remains woefully vague. Mildly alkaline and sulfidic test solutions initially containing μM concentrations of VVO43- produce polysulfides (Sx2-; x =2-6) at amounts implying reduction to thiovanadites (VIIIO3-xSx3-) and/or thiovanadyl ions (VIVO2+ or VIVS2+). Thus, V appears to eschew the chemistry exhibited by other transition metals (Mo, Re, W) which form thiometalates (e.g., MoVIS42-) in the metals’ highest oxidation state. Rather, V seems to mimic As which forms thioarsenites (AsIIIO3-xSx3-) in sulfidic solutions. In test solutions with below ~40 μM initial VVO43-, predominantly VIII homogenous and heterogenous products form. Suspended colloidal products are visibly manifest only when exposed to laser light. To promote flocculation, tetrabutylammonium hydroxide is used. Flocculated precipitates yield X-ray diffraction (XRD) spectra that do not match tabulated spectra of known V compounds. Energy dispersive spectroscopy (EDS) spectra imply the precipitates contain at least V and S. Further XRD and EDS analyses on the precipitate are ongoing as well as X-ray absorption spectroscopic (i.e., XAFS and XANES) analyses. LC-MS is used to identify and quantify aqueous-phase reaction products. Ultimately, thermodynamic constants will be calculated and employed in modelling V behaviour in natural euxinic waters.

Speciation of vanadium in euxinic natural waters

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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