pH-induced mechanistic changeover from hydroxyl radicals to iron(iv) in the Fenton reaction

Abstract

A major pathway in the reaction between Fe(II) and H 2 O 2 at pH 6-7 in non-coordinating buffers exhibits inverse kinetic dependence on [H + ] and leads to oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO 2 ). This step regenerates Fe(II) and makes the oxidation of DMSO catalytic, a finding that strongly supports Fe(IV) as a Fenton intermediate at near-neutral pH. This Fe(IV) is a less efficient oxidant for DMSO at pH 6-7 than is (H 2 O) 5 FeO 2+ , generated by ozone oxidation of Fe(H 2 O) 6 2+ , in acidic solutions. Large concentrations of DMSO are needed to achieve significant turnover numbers at pH $ 6 owing to the rapid competing reaction between Fe(II) and Fe(IV) that leads to irreversible loss of the catalyst. At pH 6 and #0.02 mM Fe(II), the ratio of apparent rate constants for the reactions of Fe(IV) with DMSO and with Fe(II) is $10 4 . The results at pH 6-7 stand in stark contrast with those reported previously in acidic solutions where the Fenton reaction generates hydroxyl radicals. Under those conditions, DMSO is oxidized stoichiometrically to methylsulfinic acid and ethane. This path still plays a role (1-10%) at pH 6-7.

Standard electrode potentials involving radicals in aqueous solution: inorganic radicals

“…Given the low reactivity of O 2 · -, a Fenton-based theory of O 2 toxicity has been developed, which yielded the dread HO· ( Koppenol, 2001). However, a recent report has indicated the contrary point that a higher oxidation state of iron may be the damaging species near neutral pH (Bataineh et al, 2012). Another inorganic radical, NO·, has been generally found to have beneficial properties (Beckman and Koppenol, 1996).…”

Fenton Reaction Driven by Iron Ligands

“…However, in some of these publications have been proposed the formation of different oxidant species in dependence of reaction conditions (pH, reagents concentration, redox potential, etc.) (45)(46)(47)(48)(49) . Lee et al (48) and Bataineh et al (49) propose the pH as the main factor but these results are not concluding.…”

“…(45)(46)(47)(48)(49) . Lee et al (48) and Bataineh et al (49) propose the pH as the main factor but these results are not concluding. Recently, the singlet oxygen ( 1 O 2 ) have been postulated between the oxidant compounds generated by Fenton reaction (50,51) (9)(10)(11).…”

Fenton chemistry at aqueous interfaces

“…However, notwithstanding much work, the mechanism by which Fe 2+ 6 2+ in bulk water via a process that favors inner-sphere two-electron O-atom over outer-sphere one-electron transfers. The higher reactivity of di-iron ferryls vs. O=Fe (1)(2)(3)(4).…”