Calcium wolframate

Endpoint summary

Description of key information

Calcium wolframate is a moderately soluble inorganic salt consisting of divalent calcium cations and divalent wolframate oxo-anions and thus liberates calcium cations and wolframate anions upon dissolution. The fate of calcium wolframate in the environment is most accurately evaluated by separately assessing the fate of its moieties calcium cations and wolframate anions. Thus, the chemical safety assessment is based on elemental calcium and wolfram concentrations and read-across of environmental fate and toxicity data available for soluble calcium and wolfram substances.

 

"Phototransformation in water, soil or air" is not relevant for substances that are assessed on an elemental basis, i.e., elemental calcium and wolfram.‘Hydrolysis’ refers to the “Decomposition or degradation of a chemical by reaction with water”, and this as a function of pH (i. e., abiotic degradation) and is thus not relevant for calcium wolframate as it does not contain relevant functional hydrolysable groups. Biotic degradation is not relevant for inorganic substances such as calcium wolframate consisting of calcium and wolframate ions that cannot (bio)degrade.

 

Additional information

Read-across

Calcium wolframate is a moderately soluble inorganic salt consisting of divalent calcium cations and divalent wolframate oxo-anions and thus liberates calcium cations and wolframate anions upon dissolution.The fate of calcium wolframate in the environment is most accurately evaluated by separately assessing the fate of its moieties calcium cations and wolframate anions.

Wolfram has an essentially anionic geochemistry based on the wolframate WO4(2-) ion. Although most minerals containing wolframate are generally insoluble in neutral and acid waters, the wolframate ion itself is stable and soluble above about pH 3 in the absence of divalent complexing metal cations and in oxidising conditions. Some minerals containing wolfram such as scheelite and, to a much lesser extent, wolframite are soluble under alkaline conditions (pH>8). Wolframate is thus relatively mobile in alkaline solutions. There is some evidence of strong sorption of wolfram by manganese oxides and clays in stream sediments, so that wolfram concentrations in most surface waters remain very low, typically much less than 1 μg/L. European median dissolved concentrations in streamwater and sediment are reported with 0.007 μg/L and 1.24 mg/kg, respectively. Thus, based on the prevalence of complexing cations in solutions, the solubility and mobility of wolframate appears to be low. An European median total concentration of wolfram in topsoil is reported with < 5 mg/kg and a global with 1.5 mg/kg W (Salminen et al. 2005).

 

Natural calcium minerals, such as lime, dolomite or gypsum, are ubiquitously and the major source of dissolved calcium in the pedosphere and hydrosphere. Calcium is a macronutrient in soil and the major anthropogenic contribution of calcium into the environment is the use of calcium fertilizers. The unintentional environmental contribution of calcium through the use of calcium compounds such as calcium benzoate can be considered extremely low compared to the intended input and naturally occurring levels. Dissolvedcalcium has typically a high mobility in environmental media and, except under strongly alkaline conditions, occurs in solution as Ca2+ ions (Salminen et al. 2005). Dissolved calcium forms only weak aqueous complexes with chloride and nitrate (Lindsay WL, 1979, Chemical equilibria in soils). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural layer clay minerals and organic mater, alkaline earth metals, such as calcium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011.Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, calcium does not form strong complexes with fulvic orhumic acids or phyllosilicate clays. Its mobility is mainly determined by the cation-exchange capacity of the respective soil, sediment or suspended matter which in turn is primarily determined by the quantity and quality of the clay minerals and organic matter. The binding to the cation-exchange structures is relatively weak, and one cation can easily be displaced by another cation depending on pH and the amount of other exchangeable cations like magnesium or potassium (cf textbooks on soil science, eg . Scheffer Schachtschabel 2016).

In sum, calcium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands.

Thus, it may further be assumed that the behaviour of the dissociated calcium and wolframate ions in the environment determine the fate and toxicity of calcium wolframate upon dissolution regarding (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.

Reference:

Carbonaro RF & Di Toro DM (2007) Linear free energy relationships for metal–ligand complexation: Monodentate binding to negatively-charged oxygen donor atoms. Geochimica et Cosmochimica Acta 71: 3958–3968.

Salminen et al. 2005. Geochemical Atlas of Europe. Part 1: Background Information, Methodology and Maps. ISBN: 951-690-913-2.