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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Type of information:
other: inorganic constituent
Remarks:
The substance is water-soluble, dissociating readily into ammonium ions (NH+), oxalate-niobate (V) and oxalate anions, respectively. The presented data refers to the inorganic component Nb (V) representing a dissociation product.
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Well-documented publication
Qualifier:
no guideline followed
Principles of method if other than guideline:
Solubilities of the crystalline B-Nb2O5 were determined in five NaClO4 solutions (0.1 ≤ Im/mol·kg−1 ≤ 1.0) over a pH range of 1 to 13 at 25.0±0.1 °C and at 0.1 MPa.
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
- Sampling intervals for the parent/transformation products:after 5, 10, 15 and 30 days
- Sampling intervals/times for pH measurements: every sampling
Duration:
30 d
pH:
4
Initial conc. measured:
20 g/L
Duration:
30 d
pH:
7
Initial conc. measured:
20 g/L
Duration:
30 d
pH:
9
Initial conc. measured:
20 g/L
Positive controls:
not specified
Negative controls:
not specified
Transformation products:
not measured
Remarks on result:
not measured/tested
Remarks on result:
not measured/tested

The solubility study indicated the possibleexistence of five mononuclear Nb(V) species in solution,viz., Nb(OH)32+, Nb(OH)4+, Nb(OH)50, Nb(OH)6-, Nb(OH)72-in equilibrium with B-Nb2O5, according to the generalized reaction:

0.5Nb2O5(cr) + 0.5(2n-5) H2O(l)  Nb(OH)n5-n + (n-5) H+ (n = 4 – 7)

 

However, the trihydroxo species Nb(OH)32+could only be detected in the analysis conducted at 70 °C (Im= 1.0 mol·kg−1). The sudden appearance of this species at the highest temperature study is counterintuitive, because hydrolysis of metal ions is known to increase with increasing temperature such that Nb(OH)32+should be destabilized with respect to Nb(OH)4+and Nb(OH)50with increasing temperature.

 

All of the solubility data at pHm> 9 show a discontinuity, probably favoring the transformation of Nb2O5to a more insoluble solid phase, especially when one considers that the solution speciation must follow continuously from that at pH < 9 such that the Nb(OH)72-would immediately predominate. 

 

XRD analyses of solids recovered after equilibration with acidic (10−2mol·kg−1, HNO3) and basic (10−2mol·kg−1, NaOH) solutions at room temperature showed sharp peaks which are consistent only with crystalline B-Nb2O5.The result indicated that any mixed oxide/hydroxide solid phase(s) which may be formed in the latter medium is/are either present as a thin crystalline film on the B-Nb2O5particles or as a thin amorphous layer.

Endpoint:
hydrolysis
Type of information:
other: inorganic constituent
Remarks:
The substance is water-soluble, dissociating readily into ammonium ions (NH+) and oxalate-niobate (V) and oxalate anions respectively. The presented data refers to the inorganic component Nb (V).
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Well-documented publication
Radiolabelling:
no
Analytical monitoring:
yes

Description of key information

Solubility study of crystalline Nb2O5 indicated the presence of four species, Nb(OH)5−n/n (where n = 4–7).

Key value for chemical safety assessment

Additional information

Solubility measurements of well-characterized B-Nb2O5 as a function of temperature (10 – 70 °C), ionic strength (0.1 – 0.6 mol kg−1), and pH (1 – 9) have provided a unique thermodynamic platform for determination of the speciation of Nb(OH)n5-n, where n = 4 - 7.

For acidic (pH ≤ 4) solutions, equilibrium (i.e., constant niobium concentration) was attained after 15 days. In the solutions with pH range of 4 to 9, a minimum of 30 days was necessary to reach equilibrium. The ionic strength, Im, in the range of 0.1 – 6.0 mol kg−1 seemed to have no effect on the kinetics of dissolution of niobium oxide.