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

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=1.0(±0.3)pH-18.5(±3.4)R²=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[}/{dt}=k×[×[ and k=k₃×{K₁}/{K+[H⁺]}=k₄×{K₂}/{K+[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.