Oxidation of Sb(III) to Sb(V) by O2 and H2O2 in aqueous solutions

The rates of Sb(III) oxidation by O-2 and H2O2 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 X 10(-3) M. Pseudo-first-order rate coefficients, k(obs), ranged from 3.5 X 10(-8) s(-1) to 2.5 X 10(-6) s(-1) at pH values between 10.9 and 12.9. The relationship between kobs and pH was: logk(obs) = 1.0(+/- 0.3)pH - 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 H2O2 was found to be first order with respect to the total Sb(III), H2O2 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[Sb(III)](lol)/dt = k X [Sb(III)(tot) X [H2O2](tot) and k = k(3) X K-1/Kal + [H+] = k(4) X K-2/K-a2 + [H+] where K-a1 and K-a2 are the deprotonation constants of Sb(OH)(3) and H2O2, 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 HO2-. Since the pK(a)-values of Sb(OH)(3) and H2O2 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 H2O2 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 H2O2 concentrations between 10(-8) M and 10-6 M, respectively. Copyright (c) 2005 Elsevier Ltd.