Vanadium in soils
Abstract: Vanadium is a redox-sensitive metal that is released to soils by weathering and anthropogenic emissions. Swedish metallurgical slags are naturally high in vanadium and used as soil amendments and in road materials. However, understanding of vanadium chemistry and bioavailability in soils is limited. The aim of this thesis was to provide knowledge of vanadium in soils in terms of sorption, toxicity and speciation, in order to enable improved environmental risk assessments. Vanadium sorption to ferrihydrite was evaluated in batch experiments. Toxicity assays using microorganisms and plants were conducted to measure vanadium toxicity in different vanadium soil treatments; freshly spiked, aged and blast furnace slag (800 mg V kg⁻¹). Vanadium speciation in a podzolic soil amended with converter lime (14.6 g V kg⁻¹) 26 years previously was assessed by using XANES spectroscopy and HPLC-ICP-MS. Ferrihydrite adsorbed vanadium strongly, but adsorption was reduced by large additions of phosphate. EXAFS spectroscopy revealed that a vanadate(V) edge-sharing bidentate complex formed on the ferrihydrite surface. In the toxicity assays, increasing vanadium sorption strength in the freshly spiked soils reduced the toxicity. Toxicity was also reduced by soil ageing, possibly because of vanadium incorporation into metal (hydr)oxides. No toxicity was observed when soils were amended with up to 29% blast furnace slag, probably owing to the low solubility of vanadium in slag. The variation in toxicity between soils and vanadium treatments was due to differences in bioavailability of vanadium which was explained by the vanadium concentration in soil solution. The vanadium added with converter lime was in pentavalent form, but the main fraction of the vanadium recovered from the mor layer sorbed to organic matter as vanadium(IV). In the mineral soil layers, the added vanadium sorbed to metal (hydr)oxides as vanadium(V). The most toxic vanadium form, vanadium(V), dominated in the soil solution but the concentrations were below toxic levels. In conclusion, vanadium toxicity varies between soils and treatments and is most accurately described by the vanadium concentration in the soil solution. Vanadium speciation in soil is mainly controlled by soil properties, and not by the vanadium species added to the soil.
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