Chromium (Cr) has received considerable attention for its potential toxicity to human beings, as well as its utilisation as a tracer for redox processes operating in Earth’s critical zones and in ocean sediments. Chromium has two stable oxidation states, hexavalent Cr(VI) and trivalent Cr(III), which are respectively soluble and insoluble. Whilst reduction of Cr(VI) largely shapes the Cr distribution in natural environments (and has thus been adopted as a pollution mitigation strategy), redox transformations alone cannot fully explain the geochemical behaviour of Cr. For example, in surface waters, dissolved Cr concentrations are usually higher than predicted by the solubility of Cr minerals, suggesting additional ‘stabilisation’ mechanisms. We need to appraise the role of small organic molecules called ligands, which have been invoked to control the mobilisation, transport and fate of many heavy metals.
This project combines an interdisciplinary set of field sampling, controlled experiments, isotopic analysis alongside geochemical modelling, to ground-truth the hypotheses that (i) solid Cr can be effectively remobilised by organic ligands in reducing environment despite Cr reduction, and (ii) aqueous Cr can be stabilised in the form of organic complexes with distinct Cr isotopic signature. Addressing these questions is important to the geosciences community as well as the wider public. Firstly, as Cr is a known carcinogen, increased levels of organic ligands, such as in paddy field systems, will potentially cause increased ecological and health risks. Secondly, organic ligands play an overlooked role in modulating the input and removal processes of dissolved Cr to/from various environments (including weathering, benthic, and hydrothermal fluxes), which are poorly constrained to date. Lastly, Cr isotopes are proven as promising tool to fingerprint ligand-bound Cr(III) that has been difficult to characterise/quantify with conventional analytical approaches.
