Homogeneous photocatalytic Fe3+/Fe2+ redox cycle for simultaneous Cr(VI) reduction and organic pollutant oxidation: Roles of hydroxyl radical and degradation intermediates

Abstract

The sustained oxidation of aqueous organic pollutants using hydroxyl radicals (HO) generated in the UV-irradiated solution of ferric ions was investigated in the presence of Cr(VI). The synergistic effect of simultaneous 4-chlorophenol (4-CP) oxidation and Cr(VI) reduction is explained in terms of the various roles of OH radical, degradation intermediates, and Fe³⁺/Fe²⁺ redox cycle. The photolysis of Fe^III(OH)²⁺ generates OH radical which degrades the organic substrate. The reduction of Cr(VI) was inhibited by the OH radical-induced re-oxidation of Cr(III) in the absence of 4-CP. The complete removal of Cr(VI) was achieved only in the presence of phenolic substrates which not only reacts with OH radical (hence inhibiting the reoxidation of Cr(III)) but also generates reducing intermediates which effectively reduce Cr(VI). Fe²⁺ also converted Cr(VI) to Cr(III) with regenerating Fe³⁺, which makes the overall process photocatalytic. The photocatalytic activity for the simultaneous removal of 4-CP and Cr(VI) was largely maintained up to five cycles. Such simultaneous and synergic photoactivity was also observed for other phenolic compounds (4-bromophenol, 4-nitrophenol, phenol). The simultaneous and synergic removal of phenolic compounds and Cr(VI) can be enabled through the redox couple of Fe³⁺/Fe²⁺ working as a homogeneous photocatalyst.

Keywords: Cr(VI) reduction; Homogeneous photocatalysis; Hydroxyl radical; Iron redox cycle; Photooxidation.