Kinetics and mechanism of the advanced oxidation process of Cr(III) to Cr(VI) by SO4 −˙ free radicals in slightly acidic simulated atmospheric water

Abstract

Abstract In a previous work, we showed that the oxidation of Cr(III) to Cr(VI) by OH˙ present in the atmospheric water droplets has the potential to threaten the people’s health since non-toxic species is transformed into environmental carcinogens. The same oxidation might be initiated by the SO4 −˙ free radicals. Here, we shed some light on the detailed mechanisms of this oxidation reaction occurring in ambient atmosphere. Steady state irradiation and pulse radiolysis technique were used to generate SO4 −˙. The advanced oxidation process mechanism was investigated at pH 4 and 6 selected as typical values of cloud water acidity. Our findings showed that the oxidation is pseudo-first order with respect to Cr(III) and is pH dependent. In the suggested reaction mechanism, the electron transfer proceeds via an inner sphere mechanism, with formation of the [Cr(III)–SO4 −˙] precursor adduct, followed by an electron transfer inside the adduct, from Cr(III) to SO4 −˙, to form Cr(IV): Cr(III) + SO 4 − · ⇌ [ C r ( III ) – SO 4 − · ] → Cr(IV) + SO 4 2 − . $${\rm{Cr(III)}} + {\rm{S}}{{\rm{O}}_4}^{ - \cdot}[Cr({\rm{III}})-{\rm{S}}{{\rm{O}}_4}^{ -\cdot }] \to {\rm{Cr(IV)}} + {\rm{S}}{{\rm{O}}_4}^{2 - }.$$ At pH 4, the equilibrium constant and the rate constant are 7.52 × 104 M−1 and 2.47 × 104 s−1, respectively. At pH 6 these values become 1.90 × 105 M−1 and 1.41 × 104 s−1, respectively.