Registration Dossier
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EC number: 235-117-8 | CAS number: 12069-94-2
Adsorption / desorption
Administrative data
Link to relevant study record(s)
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.
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