Niobium oxide

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Batch equilibrium test using natural soil samples.

GLP compliance:

no

Remarks:

Peer reviewed scientific study

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

yes

Test temperature:

Not reported, but room temperature can be reasonably assumed.

Analytical monitoring:

yes

Details on sampling:

Sorption tests:
- 95Nb tracer concentration in sorption experiments: 3.2E-13 M
- Sampling interval: Samples were taken after 1 to 9 weeks (7 to 63 days) of equilibration

Details on matrix:

COLLECTION AND STORAGE
- Geographic location: Excavator pit (code OL-KK20) dug at Olkiluoto Island, on the coast of the Baltic Sea in southwestern Finland
- Collection procedures: Shovel
- Sampling depth (cm): 70-340
- Storage conditions: At ambient room temperature in laboratory without pre-treatment
- Storage length: No data
- Soil preparation (e.g.: 2 mm sieved; air dried etc.): Yes, grain size fractions of < 0.01 mm and 1.0–2.0 mm were separated from the bulk samples
by sieving

PROPERTIES: See Table 2

Details on test conditions:

TEST CONDITIONS
- Buffer: no
- pH: 4-9 in experiments determining the soption as function of pH; for pH values of soil samples see Table 2
- Suspended solids concentration: not applicable

TEST SYSTEM
- Type, size and further details on reaction vessel: 50 mL Sorvall® polypropylene centrifuge tube
- Water filtered (i.e. yes/no; type of size of filter used, if any): no
- Amount of soil/sediment/sludge and water per treatment (if simulation test):
* 1 g soil and 25 ml model soil solution and soil solution or MilliQ water in tests with 95Nb and sorption experiments as function of pH, respectively
* Sorption experiments on inividual minerals: 0.25 g (0.075-0.30 mm grain size) plus 25 ml of liquid
- Number of reaction vessels/concentration: No data
- Measuring equipment:
* 95Nb activity was measured by means of a gamma spectrometry using Wizard ™ 3''
* c(Nb) was measured using ICP-MS
- Test performed in closed vessels due to significant volatility of test substance: no
- Test performed in open system: yes
- Method of preparation of test solution: No data
- Are the residues from the adsorption phase used for desorption: no

Type:

Kd

Remarks:

[ml/g]

Value:

ca. 184 000 other: mL/g

% Org. carbon:

0.83

Remarks on result:

other: soil depth: 0.7 m

Type:

Kd

Remarks:

[ml/g]

Value:

ca. 54 000 other: mL/g

% Org. carbon:

0.45

Remarks on result:

other: soil depth: 3.4 m

Type:

Kd

Remarks:

[ml/g]

Value:

>= 430 - <= 800 other: mL/g

% Org. carbon:

14.2

Remarks on result:

other: humus layer

Transformation products:

no

Niobium sorption in mineral soil

The Kd values decreased from 184,000 mL/g to 54,000 mL/g from 0.7 m to 3.4 m soil. depth, respectively (7 d incubation, filtration). This decrease is strongly correlated to soil SSA, and possibly to the clay fraction content and the CEC. Results from the sorption tests indicate high Nb retention in soil.

Sorption on humus

Nb sorption on humus ranged between 430 mL/g and 800 mL/g. Therefore, organic matter and humus play only a minor role in sorption of Nb onto soils.

Niobium sorption as function of pH

The Kd values that were determined in soil samples from 0.7 m soil depth equilibrated with pure water were at high levels, 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.

Sorption on individual minerals

Niobium was best adsorbed on the soil mineral kaolinite both in pure water and model soil solution experiments at pH 8, the respective Kd value was > 500,000 mL/g in both solutions. Potassium feldspar and quartz showed poorest Nb soprtion (120–220 mL/g), see Table 4 below.

Validity criteria fulfilled:

not applicable

Conclusions:

Results from the sorption tests indicate high Nb retention in soil.

Executive summary:

Batch equilibrium experiments were conducted to investigate the sorption of Niobium (Nb) on 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, 48,400 g) and filtration (0.2 µm). Kd values were calculated by using the follwing 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 on 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.

Reference 2

Endpoint:

adsorption / desorption: screening

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

For details and justification of read-across please refer to the attached report in section 13 of IUCLID.

Reason / purpose for cross-reference:

read-across source

Analytical monitoring:

yes

Type:

Kd

Remarks:

[ml/g]

Value:

ca. 184 000 other: mg/g

% Org. carbon:

0.83

Remarks on result:

other: soil depth: 0.7 m

Type:

Kd

Remarks:

[ml/g]

Value:

ca. 54 000 other: mg/g

% Org. carbon:

0.45

Remarks on result:

other: soil depth: 3.4 m

Type:

Kd

Remarks:

[ml/g]

Value:

>= 430 - <= 800 other: mg/g

% Org. carbon:

14.2

Remarks on result:

other: humus layer

Transformation products:

no

Niobium sorption in mineral soil

The Kd values decreased from 184,000 mL/g to 54,000 mL/g from 0.7 m to 3.4 m soil. depth, respectively (7 d incubation, filtration). This decrease is strongly correlated to soil SSA, and possibly to the clay fraction content and the CEC. Results from the sorption tests indicate high Nb retention in soil.

Sorption on humus

Nb sorption on humus ranged between 430 mL/g and 800 mL/g. Therefore, organic matter and humus play only a minor role in sorption of Nb onto soils.

Niobium sorption as function of pH

The Kd values that were determined in soil samples from 0.7 m soil depth equilibrated with pure water were at high levels, i.e. > 55000 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.

Sorption on individual minerals

Niobium was best adsorbed on the soil mineral kaolinite both in pure water and model soil solution experiments at pH 8, the respective Kd value was > 500000 mL/g in both solutions. Potassium feldspar and quartz showed poorest Nb soprtion (120–220 mL/g), see Table 4 below.

Validity criteria fulfilled:

not applicable

Conclusions:

Results from the sorption tests indicate high Nb retention in soil.

Executive summary:

Batch equilibrium experiments were conducted to investigate the sorption of Niobium (Nb) on 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, 48,400 g) and filtration (0.2 µm). Kd values were calculated by using the follwing 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 on 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.

This information is used in a read-across approach in the assessment of the substance to be registered.

For details and justification of read-across please refer to the attached report in section 13 of IUCLID.

Description of key information

Based on a read-across approach from Niobium, it can be stated that Niobium(II) oxide 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:

log Kp (solids-water in soil)

Value in L/kg:

4.73

Other adsorption coefficients

Type:

other: Kd (solids-water in soil)

Value in L/kg:

54 000

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)₅to 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.