Vanadium geochemistry in the biogeosphere –speciation, solid-solution interactions, and ecotoxicity

Vanadium is a metal that receives increasing attention due to its possible toxicity and its increased use in society, i.e. in high-grade steel and in vanadium redox-flow batteries. Already today, the global biogeochemical cycle of vanadium is heavily impacted by human activities, and these impacts will probably increase. The total V concentration in the upper part of the Earth's crust, and in soils, is near 100 mg V kg−1. Usually, the dissolved V concentration is low. In seawater the mean dissolved V concentration is 1.8 μg L−1, and in freshwaters the concentration is commonly below 1 μg L−1 although in areas with volcanic and sedimentary rocks it may be much higher, i.e. at the slopes of Mt. Etna, Italy, concentrations of up to 180 μg V L−1 have been recorded. Vanadium is a redox-sensitive element, which occurs in three oxidation states (+III, +IV and +V) in the environment. Whereas vanadium(V) usually occurs as the oxyanion vanadate(V) under most environmental conditions, vanadyl(IV) is an oxocation that is stable at low pH and/or mildly reducing conditions, particularly when the organic matter concentration is high. Vanadium(III), which is the least studied form of vanadium, occurs under strongly reducing conditions. All vanadium forms are strongly bound to environmental sorbents: vanadate(V) is bound as a bidentate complex to iron, aluminium, and titanium (hydr)oxides, and with a stronger affinity than that of orthophosphate (o-phosphate). Vanadyl(IV) is strongly complexed to natural organic matter, while vanadium(III) may substitute for other trivalent ions in mineral structures. Despite this, vanadium may be mobilized to the aqueous phase, for example under high-pH conditions. Studies with V K-edge XANES spectroscopy have shown that most oxic soils usually contain a mixture of vanadium(IV) that is octahedrally coordinated in primary minerals, and surface-bound vanadate(V) on iron and aluminium (hydr)oxides, although acid organic soils are dominated by organically complexed vanadyl(IV). In reduced environments, such as in sediments and black shales, available evidence suggests that the V consists of a mixture of organically complexed vanadyl(IV) and unknown vanadium(III) species. However, considerable uncertainty exists on the V speciation under reducing conditions, and additional research is recommended. Vanadium is essential for some species of cyanolichens and algae due to its presence in vanadium nitrogenase, which can be important for N fixation in boreal ecosystems, and in vanadium haloperoxidases, which mediate the oxidation of halides, particularly iodine and bromine. In certain organisms vanadium is accumulated for unknown reasons, e.g. in ascidians, where V accumulates as a vanadium(III) complex with organic S, and in Amanita mushrooms, in which amavadin, a stable vanadium(IV)-organic complex, is accumulated. However, at high concentrations vanadium is toxic to many organisms. This is mostly due to its interference with o-phosphate in a number of biomolecules. Available evidence shows that toxic effects appear in the mg V L−1 range for most studied species. However, some organisms, i.e. algae and possibly some soil bacteria, are more sensitive. In soils, the toxic response is related to the soil solution V concentration, rather than to the solid-phase concentration. The o-phosphate concentration has been identified as a parameter that influences toxicity, but the relationship between the P status and the environmental risk of V toxicity is not yet well determined – as a result risk-based guidelines remain uncertain. There is urgent need for more research on this topic. Vanadium, being a redox-sensitive element, responds to sudden environmental change such as flooding that leads to decreased redox potential. In most, but not all, cases, an increased solubilisation of vanadium is observed after flooding, which can be attributed to reductive dissolution of vanadate(V)-sorbing iron (hydr)oxides and to vanadate(V) reduction to vanadyl(IV) that forms stable complexes with dissolved organic matter. The vanadium redox conversions are carried out by a large number of genera of bacteria. Bioremediation methods are being developed that may reduce vanadate(V) to vanadyl(IV), which may reduce the bioavailability of vanadium in many soils.