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Reaction mass of ammonium diaqua[bis(oxalate)]oxoniobate(1-) hydrate and ammonium hydrogen oxalate oxalic acid (1:1:1) dihydrate composes of 70% of oxalate-niobate ammonium salt in presence of 27% of ammonium oxalate and oxalic acid. The substance is water-soluble and dissociates into ammonium ions (NH4+) and oxalate-niobate (V) and oxalate respectively as anions.

The readily biodegradation study (CBMM, 2014, biodegradation) indicates that the organic components of oxalate (C2O22-) and ovalate-niobate (V) (Nb(O)(C2O2)2-) are fast degradable under environmental conditions and that released Nb (V) undergoes hydrolysis to form different niobium hydroxide species depending on the pH, ionic strength and temperature of media.

Babko et. al. (1963) measured the solubility of freshly precipitated, active Nb2O5 at 19°C over a pH range from 0.7 to 9.73. The solid phase was assumed to be amorphous and two step-wise hydrolysis equilibria and hydrolysis constants were proposed:

Nb(OH)4++ H2O(l)→Nb(OH)50+ H+, log10K4-5≈ 0.6

Nb(OH)50+ H2O(l)→Nb(OH)6-+ H+, log10K5-6≈ -7.8

Etxebarria (1994) and Babko et al. (1963) studied the stability boundary between Nb(OH)50and Nb6O16(OH)5−3predicted that monomers will dominate below mNb = 10−5at pH < 7.5, decreasing to almost 10−6at pH ≈ 9. The stabilities of the polynuclear anions that were investigated previously at > 10−3mol·kg−1 would decrease by a factor of 1012.

Peiffert (2010) studied the solubility of B-Nb2O5 and the hydrolysis of niobium (V) in aqueous solution as a function of temperature and ionic strength. Treatment of the solubility results indicated the presence of four species, Nb(OH)n5−n(where n = 4 – 7), so that the molal solubility quotients were determined according to:

0.5Nb2O5(cr) + 0.5(2n-5) H2O(l)→Nb(OH)n5-n+ (n-5) H+

and were fitted empirically as a function of ionic strength and temperature.

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.



Babko, A.K., Lukachina, V.V., Nabivanets, B.I. (1963). Solubility and acid-base properties of tantalum and niobium hydroxides. Russ. J. Inorg. Chem. 8, 957–961 (1963)

Etxebarria, N.E., Fernández, L.A., Madariaga, J.M. (1994). On the hydrolysis of niobium(V) and tantalum(V) in 3 mol·dm−3 KCl at 25 °C. Part 1. Construction of a thermodynamic model for NbV. J. Chem. Soc., Dalton Trans.3055–3059 (1994)

KOEI (2014). Biodegradation test of ANO.Unpublished study report, B12011