Magnesium diniobate

Endpoint summary

Additional information

Magnesium diniobate

The solubility of magnesium diniobate (MgNb2O6) in environmental media is expected to be low since dissolution in water resulted in Mg concentrations < 25 microg/L and Nb concentrations < 0.2 microg/L after 34 days. The dissolution of magnesium diniobate results in Mg (2+) and Nb(OH)5 ions. Thus, the ecotoxicological moieties of concern are magnesium and niobium ions and the environmental fate of magnesium and niobium ions are separately assessed.

In order to evaluate (eco-)toxicological properties of the substance magnesium diniobate, information on the assessment entities magnesium and niobium were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for magnesium diniobate.

 

Abiotic degradation

The endpoints "Phototransformation of an element in water, soil or air" are not relevant for substances that are assessed on an elemental basis, i.e., expressed as elemental magnesium or elemental niobium. The term ‘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). The chemical safety assessment of magnesium diniobate is based on elemental magnesium and niobium concentrations, i.e., a separate assessment of magnesium and a separate assessment of niobium are conducted regardless of the respective (pH-dependent) speciation in the environment. Hence, as the assessment is based on elemental concentration, physico-chemical processes such as decomposition and degradation by reaction with water are not relevant. In general, (abiotic) degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

 

Biotic degradation

For an inorganic substance such as magnesium diniobate for which the chemical assessment is based on elemental concentrations (i.e., pooling all inorganic magnesium species or all inorganic niobium), biotic degradation is irrelevant in all environmental compartments since biotic processes may affect elemental speciation but will not eliminate the respective element from the environmental compartment by degradation or transformation. This elemental-based assessment can be considered as a worst-case for the chemical assessment.

 

Niobium

The pentavalent form of niobium is the only oxidation state in the environment. Pentavalent niobium is expected to be present in solution as Nb(OH)5 (or HNbO3) in slightly acidic to neutral media. At higher solution pH, Nb(OH)6- or NbO3- species form which is expected to decrease the adsorption of niobium. Niobium hydrolyses readily, is fairly insoluble and displays a very low mobility under all but the most extreme environmental conditions due to the high stability and very low solubility of niobium oxides. However, the presence of citric, tartaric and oxalic acids may increase the solubility of Nb through chelation (Salminen et al. 2015 and references therein). It may be assumed that the behaviour of dissociated niobium ions in the environment determine its fate 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. Thus, the chemical safety assessment is based on elemental niobium concentrations and read-across of environmental fate and toxicity data available for niobium substances as well as monitoring data of elemental niobium concentrations in the environment. The reliable data selected for the assessment of environmental fate of niobium, including adsorption/desorption, are based on elemental niobium concentrations of water, soil and sediments.

Adsorption / desorption: A sediment-water partition coefficient of 6.54 (log Kp) has been derived for niobium based on the FOREGS EU monitoring survey (Salminen et al. 2015). Niobium is relatively immobile in soils due to its strong sorption on mineral particles. The Kd values reported in the literature range from 10 L/kg to 41 000 L/kg in soils (Söderlund et al. 2013 and references therein). Söderlund et al. 2013 studied niobium sorption in Finnish sandy till soils and in humus. The Kd values of niobium for humus are lower than for mineral soil in all soil depths. In mineral soils, the Kd values decrease with soil depth. In the pH range 2.8-8.1 pH was not found to be a dominant factor affecting the adsorption of niobium in soil. A mean Kd value of 153,621 (corresponding to a log Kd of 5.19) was calculated from Kd values in sandy till after 7, 21 42 and 63 days (pH ranged from 5.3 to 7.4 during the test).

 

Magnesium

Magnesium is highly mobile under all environmental conditions, occurs in solution as dissociated Mg2+ ions (Salminen et al. 2015). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as magnesium, 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, magnesium does not form strong complexes with fulvic or humic acids. In sum, magnesium 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 dissociated magnesium ions in the environment determine its fate 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.  Thus, the chemical safety assessment is based on elemental magnesium concentrations and read-across of environmental fate and toxicity data available for soluble magnesium substances as well as monitoring data of elemental magnesium concentrations in the environment. The reliable data selected for the assessment of environmental fate of magnesium, including adsorption/desorption, are based on elemental magnesium concentrations of water, soil and sediments.  

Adsorption / desorption: A sediment-water partition coefficient of 2.96 (log Kp) has been derived for magnesium based on the FOREGS EU monitoring survey. Magnesium is expected to also be highly mobile in suspended matter and soil.