Niobium pentachloride

Adsorption / desorption

Description of key information

Niobium is highly adsorptive to the soil mineral fraction, but adsorbs to a minor degree to the organic carbon fraction.

Key value for chemical safety assessment

Other adsorption coefficients

Type:

other: Kd (solids-water in soil)

Value in L/kg:

54 000

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

4.73

Additional information

Batch equilibrium experiments (conducted at laboratory ambient temperatures) were conducted to investigate the sorption of niobium (Nb) on the humus and mineral soil samples of an excavator pit dug in Olkiluoto Island in Finland. Experiments were conducted by equilibrating samples with model soil solution and pure water after addition of Nb for 1 to 9 weeks. Solution samples were analysed for Nb concentration after centrifugation (20 min, 48400 g) and filtration (0.2 µm). Kd values were calculated by using the following equation:

Kd = (Ci-Cf)/Cf × V(ml)/m(g)

with Ci = Nb initial concentration; Cf = Nb final concentration; V = solution volume; m = sample dry mass

Results demonstrate high sorption of Nb onto mineral soil. Kd values decreased with increasing soil depth and decreasing specific surface area (SSA) from 184,000 mL/g to 54,000 mL/g at 0.7 m and 3.4 m, respectively. In comparison to mineral soil the humus layer was not found to be an important component for Nb adsorption (-> maximum Kd value ca. 800 mL/g).

The Kd values that were determined in soil samples from 0.7 m soil depth equilibrated with pure water were high, i.e. > 55,000 mL/g in the pH range 4.7–6.5. Above pH 6.5 Kd values decreased significantly, corresponding to the change in the major Nb species from the neutral Nb(OH)5 to the low-sorbing anionic Nb(OH)6– and Nb(OH)72−.

In contrast, Kd values obtained from equilibration with model soil solution at slightly alkaline pH values were an order of magnitude higher than in pure water. This is probably attributed to the formation of calcium niobate surface precipitate or electrostatic interaction between surface-sorbed calcium and solute Nb.

JUSTIFICATION FOR READ-ACROSS:

For environmental fate and distribution endpoints, it is in general the relative mobility and resulting bioavailability in various environmental compartments that determines the potential toxicity to ecological receptors.

No data on the fate and behaviour of NbCl5 in the environment are available. Terrestrial adsorption data derived from batch equilibrium experiments using the highly soluble ammonium niobate oxalate hydrate as test substance were used for read-across to niobium pentachloride (Söderlund et al., 2015).

The adsorption/desorption behaviour in environmental compartments is on the one hand dependent on the niobium ion species that is released, and on the other hand dependent on environmental conditions like e.g. soil type and pH, amount of amorphous iron oxides and aluminium oxides etc. In natural aqueous solutions molybdenum is predominantly present in the (V) form. The actual niobium ionic species depends on the pH value of the solution as well as the niobium concentration itself and concentration of possible ligands. As Nb(V) is expected to be released from niobium pentachloride and ammonium niobate oxalate hydrate as the same soluble species (exact speciation depending on pH, Nb concentration and ligands) similar behaviour in different environmental compartments can be expected.