Oxidation of Sb(III) to Sb(V) by O 2 and H 2O 2 in aqueous solutions

Abstract

The rates of Sb(III) oxidation by O 2 and H 2O 2 were determined in homogeneous aqueous solutions. Above pH 10, the oxidation reaction of Sb(III) with O 2 was first order with respect to the Sb(III) concentration and inversely proportional to the H + concentrations at a constant O 2 content of 0.22 × 10 −3 M. Pseudo-first-order rate coefficients, k obs, ranged from 3.5 × 10 −8 s −1 to 2.5 × 10 −6 s −1 at pH values between 10.9 and 12.9. The relationship between k obs and pH was: log ⁡ k o b s = 1.0 ( ± 0.3 ) p H − 18.5 ( ± 3.4 ) R 2 = 0.89 No significant Sb(III) oxidation by O 2 was observed between pH 3.6 and 9.8 within 200 days. Oxidation of Sb(III) by H 2O 2 was found to be first order with respect to the total Sb(III), H 2O 2 and inversely proportional to the H + concentrations in a pH range of 8.1 to 11.7. Above pH 11.7 no pH dependence was observed. The rate law for the pH range 8.1 to 12.9 was determined to be: − d [ S b ( I I I ) ] tot d t = k × [ S b ( I I I ) ] tot × [ H 2 O 2 ] tot and k = k 3 × K 1 K a 1 + [ H + ] = k 4 × K 2 K a 2 + [ H + ] where K a1 and K a2 are the deprotonation constants of Sb(OH) 3 and H 2O 2, respectively, and k 3 (=365 M −1 s −1) and k 4 (=342 M −1 s −1) are the specific second-order rate coefficients. The results indicate that the rate-limiting step below pH 11.7 involves one deprotonated species, either Sb(OH) 4 − or HO 2 −. Since the pK a-values of Sb(OH) 3 and H 2O 2 are very close (11.8 and 11.6, respectively), it was not possible to determine which species is involved. Varying the ionic strength between 0.001 and 1.0 M at pH 10 and between 0.01 and 1.0 M at pH 12 resulted in a less than twofold increase in k obs. In both cases, the increase was attributed to a shift in pK a-values as a function of the ionic strength. It could be concluded that O 2 was unlikely to be a significant oxidant in homogenous solution, but that H 2O 2 might be responsible for the oxidation of Sb(III) in natural waters since half-lives could vary from 32 yr to 117 days at pH 8 with H 2O 2 concentrations between 10 −8 M and 10 −6 M, respectively.