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Environmental fate & pathways

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Read-across approach

In the assessment of the environmental fate and behaviour of divanadium pentaoxide, a read-across approach is applied based on all information available for inorganic V substances. This grouping of vanadium substances for estimating their properties 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 compounds to the environment, it is the potentially bioavailable metal ion that is liberated (in greater or lesser amounts) upon contact with water and that is the moiety of toxicological 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 all 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 5 vanadium substances with different valence states (FeV, 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) (as tabulated below). 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.

Additional information

Table. Redox speciation of dissolved vanadium after dissolution of various V substances in standard environmental media.

 Substance  Original redox state     After 24 h     After 7 days     After 28 days
     V (IV)  V (V)  V (IV)  V (V)  V (IV)  V (V)
 pH 6              
 NaVO3  V 39*  319 31*  269   <18** 260 
 V2O5  V  76 380  67  329  61  313 
 VOSO4  IV  28* 269  27*  195  <18**  185 
 V2O3  III  5* 79  87  3*  100 
 V  0  <2.1** 16  2*  29  3*  39 
 FeV  0  <2.1** <2.1**  <2.1**  18  28  85 
 pH 8              
 NaVO3  V  37* 336  33*  248  <18**  240 
 V2O5 V 53* 442  33*  368  20*  327 
 VOSO4  IV  <18** 280  69  206  20*  214 
 V2O3  III  4* 65  4*  76  4*  97 
 V  0  <2.1** 22  <2.1**  20  6*  31 
 FeV  0  <2.1** 11  19  78  32  235 

*: extrapolated value below level of quantification (66 µg V/L for NaVO3, V2O5 and VOSO4, 6.6 µg V/L for V2O3, V and FeV); **: below level of detection (18 µg V/L for NaVO3, V2O5 and VOSO4, 2.1 µg V/L for V2O3, V and FeV).