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Environmental fate & pathways

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VCl4 is the least stable vanadium chloride. Its decomposition is driven by heat, dissolved Cl2 concentration, humidity or light, following the equation:

VCl4 + H2O => VOCl2 + 4 HCl

The dissolution reaction in water is exothermic, instantaneous (half-life of VCl4 < 1 hour – VCl4 can be considered as a hydrolytically unstable substance), gives white fumes of HCl and the pH of the solution is < 2.

Then the decomposition with the humidity of air follows the equations:

 

2VOCl2+ 2H2O => 2VO2+ 4HCl

 

That is why no data is available for VCl4 as such and in solution V2O4 is stable in the form of V4 +.

Released into water, vanadium is expected to exist primarily in the tetravalent and pentavalent forms (the pentavalent state is the most usual form of the metal in the environment). Both species are known to bind strongly to mineral or biogenic surfaces by adsorption or complexing. The chemical formulas of the vanadyl species most commonly reported in water are VO(2+) and VO(OH)(1+), and the vanadate species H2VO4(1-) and HVO4(2-). 

 

In aquatic compartment, this vanadate ion is then the most common form and its partition coefficients have been deeply studied. The conclusions given in the adsorption section (high Kp) showed a very strong affinity of vanadium to particulate matter, and then a low mobility in soils. These coefficients are very important and will be used for the environmental assessment.

 

The references pointed in the following endpoints support this vanadium behaviour in environment and allowed a good assessment of this compound regarded its ecotoxic potential.

 

Vanadium behaviour in soil compartment was also studied and showed that vanadium is found in rocks and soil in the relatively insoluble trivalent form and can also be present in the pentavalent form as vanadates. 

 

Weathering decomposes parent rock and increases vanadium availability in soils. Jacks (1976) found that the bulk of vanadium deposited in the environment is retained in the soil, mainly in association with organic matter. 

 

The mobility of vanadium in soils is affected by pH. Vanadium is fairly mobile in neutral or alkaline soils relative to other metals, but its mobility decreases in acidic soils. In the presence of humic acids, mobile metavanadate anions can be converted to the immobile vanadyl cations resulting in local accumulation. Under oxidizing, unsaturated conditions some mobility is observed, but under reducing, saturated conditions vanadium is immobile. The pentavalent cation is considerably more soluble than the trivalent cation, is readily dissolved by groundwater, and can be transported over long distances.

 

References: CICAD 29: VANADIUM PENTOXIDE AND OTHER INORGANIC VANADIUM COMPOUNDS, World Health organization Geneva, 2001 and Ecological Soil Screening Levels for Vanadium, U.S. Environmental Protection Agency, April 2005

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