Divanadium pentaoxide

Endpoint summary

Description of key information

Divanadium pentaoxide is soluble in environmental media. In a standard transformation/dissolution test according to OECD Series No 29 with a loading of 1 mg V2O5/L, a nearly complete dissolution (92-98%) was observed after 24h resulting in dissolved vanadium levels of 516 μg/L and 551 μg/L at pH 8 and pH 6, respectively. Further, the solubility of divanadium pentaoxide is expected to determine its behaviour and fate in the environment, and subsequently its bioavailability and potential for bioaccumulation and ecotoxicity. Divanadium pentaoxide at pH 6 and 8, similar to other inorganic vanadium substances, releases ions in pentavalent form upon dissolution (83-89% (V(V) after 24 hours) and dissolved vanadium ions remain in the pentavalent form (84-94% V(V) after 28 days). Therefore, a read-across approach is applied based on all information available for different inorganic vanadium substances and the fate of released vanadium ions can be expected to be similar to the common fate of vanadium ions in the environment. Further information on the applied read-across approach is summarized below.

Solubility and speciation of vanadium substances in environmental media

Regarding the solubility in environmental media based on T/D testing (OECD Series # 29), readily soluble vanadium substances (including VOSO4, NaVO3, and V2O5 and VO(C2O4)) dissolve practically completely (87 - 98%) at pH 6 and pH 8 after only a short period of time (24 h) and thus are expected to be of similar bioavailability with respective toxicity depending on the vanadium content of each substance. In contrast, poorly/moderately soluble vanadium substances (VO2, V2O3, V, VCN and VC) release only 0.1 – 16 % of vanadium into the surrounding media at pH 6 and pH 8. A pH dependency of the vanadium release was not observed. Upon dissolution, all vanadium substances more or less transform immediately to the pentavalent form and dissolved vanadium ions remain pentavalent throughout the 28-d day test at pH 6 and pH 8. A pH dependency of the redox speciation was not observed in T/D tests.

Degradation

Abiotic degradation, including photodegradation and hydrolysis, is not relevant for inorganic vanadium substances. Biodegradation in water, sediment or soil is not relevant for inorganic substances, including inorganic vanadium salts, oxides and the metal, which are considered not (bio)degradable. The respective environmental hazard assessment is based on total vanadium concentrations, pooling all vanadium species, which can be considered a worst-case.

 

Bioaccumulation and essentiality

The potential for bioaccumulation of vanadium seems to be low and there is not an indication for biomagnification of vanadium in food chains. All available BCF and BAF data for aquatic organisms (fish and invertebrates) were applied in a weight-of-evidence approach, resulting in 5th, 50th and 95th percentiles of 1.3, 12.3 and 366 L/kg wet weight, respectively. The 50th percentile (12.3 L/kg wet weight) was selected for the chemical safety assessment. Vanadium does also not appear to accumulate in terrestrial plants since BSAFs reported for several plant species are below 0.1. Vanadium is biologically active and is an essential nutrient for normal cell growth of many organisms, including green algae, fungi and nitrogen-fixing microorganism. Whereas its precise biochemical function is still in some doubt, a role in peroxidase enzymes is suggested (WHO, 2000) .

Transport and distribution

Partition coefficients of 464 European agricultural soils with varying soil properties sampled within the GEMAS project were determined for added vanadate, Log Kp values range from 1.09 to 4.13, with a median of 2.79 that was selected for the chemical safety assessment.

A total of 757 paired stream water and sediment samples from the FOREGS Geochemical Baseline Mapping Programme covering a wide range of pH, hardness and organic carbon concentrations of stream water and sediment were processed to derive a median logKpsolids-water in sediment of 5.06.

Partition coefficients of suspended matter are available for 6 water systems, with log Kp values ranging from 3.24 to 4.83 L/Kg (median 4.60) for freshwater and a log Kp value of 4.77 for marine water. Because Log Kp values for freshwater and marine water seem to be similar, the 50th percentile of 4.5 was selected for the chemical safety assessment.

 

Ubiquitousness and environmental chemistry of vanadium

Vanadium is the 20th most abundant element in the earth's crust and naturally occurring in air, soil, plants, and water. The average content of vanadium in the upper part of Earth's crust is estimated with 97 mg/kg. Baseline vanadium levels of European soils range from 1 to 281 mg V/kg with a median concentration of 33 mg V/kg. Vanadium concentrations of agricultural and grazing land soil range from < 1 to 552 mg V/kg with 50th percentiles of 25.3 and 25.8 mg V/kg, respectively. Even though baseline concentrations of European sediments can be as high as 407 mg V/kg, V concentrations in the water column of European stream waters are well below 20 μg V/L and European median concentrations were derived with 62.5 mg V/kg sediment and 0.46 μg V/L stream water.

Vanadium occurs in four different oxidation states, +II, +III, +IV (i.e. vanadyl) and +V (usually vanadate). The divalent oxidation state, i.e. V(II), is thermodynamically unstable in water. Vanadium is a highly mobile element and remains mobile under oxidising conditions but is subject to precipitation at reducing conditions (i.e. just above the sulfate/sulfide redox threshold) within the pH range of 5.0–8.0. Vanadium displays cationic character as vanadyl ions under acid conditions, and anionic character, as vanadate ions under less acid to alkaline conditions. Vanadium solubility is strongly controlled by its oxidation state. Its solubility is highest in oxic environments, where vanadyl cations predominate. Complexes with fluoride, sulfate and oxalate may increase vanadium solubility under oxidising conditions, although the presence of uranium and phosphates can result in the formation of highly insoluble pentavalent complexes. Under more reducing conditions, the relatively immobile trivalent state dominates. Vanadium speciation in (pore)water is sensitive to solution parameters, including vanadium concentration, pH, ionic strength and redox conditions. Dissolved vanadium in most natural waters is a mixture of vanadyl (IV) and vanadate (V) (Gustafsson, 2019 and references therein, Salminen et al. 2005 and references therein).

References:

Gustafsson JP, 2019. Vanadium geochemistry in the biogeosphere – speciation, solid-solution interactions, and ecotoxicity. Applied Geochemistry 102: 1-25.

WHO, 2000. Air Quality Guidelines - Second Edition. Chapter 6.12 Vanadium.

Additional information

Read-across approach

In the assessment of the environmental fate and behaviour of inorganic vanadium substances, a read-across approach is applied based on all information available for different inorganic vanadium substances. This grouping of vanadium substances for estimating their environmental fate and toxicity is based on the assumption that properties are likely to be similar or follow a similar pattern as a result of the presence of the common vanadium ion. After emission of metal (vanadium) substances into the environment, it is the potentially bioavailable metal ion (vanadium ion) that is liberated (to a greater or lesser extent) upon contact with environmental solutions, including sediment and soil porewater, and that is the moiety of ecotoxicological concern.

This assumption can be considered valid when differences in solubility among V substances do not affect the results for behaviour (adsorption, bioaccumulation etc.), and there are not any important differences in speciation of vanadium in the environment after emissions of the different V substances.

The reliable data selected for the environmental fate and behaviour of vanadium are based on monitoring data of prevailing elemental vanadium concentrations in water, soil, sediment, suspended matter and organisms or on experimental results with soluble pentavalent (V2O5, NaVO3, NH4VO3 and Na3VO4) or tetravalent (VOSO4 and VOCl2) vanadium substances.

Vanadium can exist in a multitude of different oxidation states from -2 to +5. However, being a first-row transition element, vanadium has the tendency to exist in high oxidation states (+3, +4 and +5), and vanadium ions will form oxy complexes in aqueous solutions (Cotton and Wilkinson, 1988; Crans et al., 1998). The aqueous chemistry of the metal is complex and involves a wide range of oxygenated species for which stabilities depend mainly on the acidity and oxygen level of receiving waters. Under conditions commonly found in oxic fresh waters (i.e., pH between 5 and 9; redox potential [Eh] between 0.5 and 1 V), the pentavalent forms will be the dominant species in solution (Brookins, 1988; Crans et al., 1998; Takeno, 2005, Larsson et al., 2015a). Tetravalent vanadium also may exist under some specific conditions (e.g. pH< 5). Soil organic matter, which strongly adsorbs to tetra- and pentavalent vanadium species, may potentially reduce V(V) to V(IV), therefore lowering its overall bioaccessibility (Reijonen et al., 2016). It is therefore assumed that upon dissolution of inorganic vanadium substances, the environmental conditions control the (redox) speciation of vanadium in water, soil and sediment, independently of the identity of the V substance.

 

This is confirmed by redox speciation analysis of dissolved vanadium during transformation/dissolution tests for vanadium metal and vanadium substances with different valence states (including V, V2O3, VOSO4, NaVO3, V2O5) according to OECD Series No 29 (2009). The tests were conducted at a loading of 1 mg/L over 28 days in standard OECD test media at pH 6 and pH 8 under a set of standard laboratory conditions representative of those in standard OECD aquatic ecotoxicity tests. The redox speciation of dissolved vanadium was measured by separating V(IV) and V(V) species by HPLC and analysis by ICP- MS. Regardless of the original valency of the vanadium substance, dissolved V at pH 6 and pH 8 is predominantly present in the pentavalent V form (75-97% of all V), with some traces of V(IV). Recovery of total dissolved V by measured V(V) and V(IV) was on average 96% and did not differ significantly among the substances tested.

Similar observations were made in laboratory experiments at pH 6: In a study performed by Larsson et al. (2015a) using natural soils spiked with vanadium (IV) or vanadium (V), pentavalent vanadium predominated in soil extracts after a 10-d equilibration period. Therefore, vanadium speciation in soil solution was independent of the valence state of the added salt. Accordingly, Reijonen et al. (2016) demonstrated that vanadium speciation (and bioavailability) is mainly regulated by soil organic matter (SOM) and soil pH under oxic conditions, whereas the original valence state of the added vanadium substance is negligible for controlling distribution and water solubility. In line with these findings, long-term investigations (26 years) on fate and transformation of vanadium species performed by Larsson et al. (2015b) on vanadium-containing converter lime applied to pine forest soil suggest further that the long-term speciation is governed by soil properties (pH, SOM, metal hydroxides) and less dependent on the initial valence state.

Based on this information, it was concluded that the read-across conditions as stated above are met. Therefore, all data based on monitoring studies or on tests with soluble V substances (i.e. maximal bioavailability) are used in a read-across approach and results for environmental fate and behaviour are expressed based on elemental vanadium concentrations. For further information on the applied read-across approach, please refer to the RAAF document "Read-across approach for environmental toxicity of the vanadium category, 2020" attached in IUCLID Section 13.