Synchrotron X-ray spectroscopy for investigating vanadium speciation in marine sediment: limitations and opportunities

Vanadium (V) exists in a number of oxidation states in the environment, potentially making it a useful chemical tracer of both modern and ancient redox conditions in Earth's oceans. However, the use of V as a redox tracer relies on a comprehensive understanding of V geochemistry in marine systems, which in turn requires the ability to accurately measure its solid-phase speciation in marine sediments. Here we report the first evaluation of both bulk and microfocused synchrotron X-ray spectroscopy to investigate the speciation of V in a modern coastal marine sediment. Semi-quantitative spectral analysis of pre-edge peak areas, extracted from bulk X-ray absorption near edge structure (XANES) spectra, determined the average oxidation state of V in the sediment samples to be approximate to 3.5, indicating a mixture of V(iii) and V(iv). Signal-to-noise ratios of XANES spectra were not sufficiently high to allow accurate quantitative analysis by commonly used linear combination fitting approaches. We also demonstrate that high-resolution element mapping by scanning X-ray fluorescence microscopy (SXFM) provides useful complementary geochemical information for resolving possible V host phases in sediments, but is limited in its ability to distinguish V and Ti due to the proximity of their fluorescence emission lines. This study highlights the practical limitations and opportunities of X-ray spectroscopy for investigating V speciation in marine sediments, and reinforces the importance of comprehensively evaluating X-ray spectroscopic techniques prior to applying them to investigating new elements or sample types.