Vanadium biogeochemical cycling in a lake (Kinghorn Loch, U.K.) following historical red mud pollution
Item statusRestricted Access
Embargo end date20/06/2023
Watt, James A. J.
V is a potentially toxic metal that is gaining new interest in terms of its behaviour within environmental systems. Environmental concentrations of V are likely to increase as the consequence of the increasing production of V rich wastes from industry and need for greater extraction for the application of V within globally significant technologies such as Redox batteries for renewable energy storage. When V enters freshwater systems, it is stable in three oxidation states + 3, +4 and +5. The interconversion between these states is governed by pH, Eh, competing ions as well as the presence of solid surfaces. Our understanding of V biogeochemistry within freshwaters waters is poor. This presents a problem as the speciation of V strongly dictates its mobility and toxicity with VV being recognised as the most mobile and toxic. Due to historic V pollution from red mud, the study of Kinghorn Loch represents an opportunity to both improve our understanding of V behaviour in freshwater ecosystems as well as the processes involved in lakes recovering from metal pollution. We used a combination of: statistical and thermodynamic modelling; spatial and vertical measurements of V and other metals; sequential extractions (SE); novel incorporation experiments and X-ray absorption spectroscopy (XANES) to determine the distribution of V solid phases and their speciation within Kinghorn sediments. The use of statistical analyses of long term water quality from Kinghorn surface waters found that V concentrations exhibited seasonal behaviour. An annual peak in total V concentrations were found to occur during spring. This peak correlated with proxies of phytoplankton abundance (Chlorophyll α and SO4). It was then demonstrated that changes in pH by phytoplankton were responsible for driving the difference in aqueous V species within the Loch waters. Vertical distributions in V concentrations across a Red Mud gradient showed the V and As to be widely distributed both across the loch and through the sediment column. The spatial distribution of V could be separated into three zones largely dictated by wind driven resuspension of contaminated sediment and the deposition of calcite from phytoplankton and macrophytes. By combining total metal analyses, XRD, carbonate analysis and a novel application of the Psenner sequential extraction regime, we found that vertical distributions of V were likely governed by the presence of carbonates and the organic matter. Surprisingly, there was no evidence that Fe (oxy) hydroxides controlled the distribution of V within the loch. A set of incorporation experiments were then used to demonstrate for the first time that CaCO3 could play an important role of incorporating V within bed sediments. The application of XRD and ICP-MS analyses showed that V is capable of both sorbing to the outside CaCO3 as well as substituting inside the mineral structure at CO3 sites. XANES showed that the substituted V within the CaCO3 was present as VV. XANES was then applied for the first time to an anoxic freshwater sediment. This analysis revealed that V had an average oxidation number of 4.5 both in surface (0 – 2cm) and bottom (sediments). This has presented new evidence that potentially toxic metals can retain higher oxidation states even in highly reducing sediments (such as present at Kinghorn). Lastly a XANES linear combination fitting (LCF) analysis found that V was indeed present in the sediment in carbonate (as VV), OM (as VIV) and Fe(oxy)hroxides (as VV) but in different proportions to the SE. A set of novel validation experiments was conducted to cross validate the results of the SE and LCF. This revealed that modifications were required in the SE to prevent the co-precipitation of V, Fe and S potentially leading to an under estimation of the reducible pool of V. These research results within Kinghorn will not only allow us to better understand the biogeochemistry of V within Red Mud contaminated systems but standing bodies of freshwater generally.