PHOTOREDUCTION OF IRON OXYHYDROXIDES AND THE PHOTOOXIDATION OF HALOGENATED ACETIC-ACIDS

The photolytic reduction of ferrihydrite (am-Fe2O3 . 3H(2)O), lepidocrocite (gamma-FeOOH), goethite (alpha-FeOOH), hematite (alpha-Fe2O3), maghemite (gamma-Fe2O3) and iron-containing aerosol particles (Fe-aerosol) in the presence of a series of halogenated acetic acids has been investigated. The fastest rates of photoreduction of Fe(III) to Fe(II) were achieved with ferrihydrite as an electron acceptor and fluoroacetic acid as an electron donor. The relative rates of photooxidation of the monohalogenated acetic acids with ferrihydrite in order of decreasing reactivity were as follows: FCH2CO2H > ClCH2CO2H > BrCH2CO2H > ICH2CO2H; for multiple substituents the relative order of reactivity was as follows: FCH2CO2H > F2CHCO2H > F3CCO2H. With respect to the iron oxide electron acceptors, the relative order of reactivity toward monohaloacetate oxidation was am-Fe2O3 . 3H(2)O > gamma-Fe2O3 > gamma-FeOOH greater than or equal to alpha-Fe2O3 greater than or equal to Fe-aerosol > alpha-FeOOH. Strong kinetic isotope effects observed for the photooxidation of ClCD2CO2H suggest that the oxidation of the mono- and disubstituted haloacetic acids proceeds via hydrogen-atom abstraction by surface-bound hydroxyl radicals to produce haloacetate radicals, which in turn yield the corresponding halide and glycolic acid. Fully halogenated haloacetic acids appear to be oxidized via a photo-Kolbe mechanism to yield the corresponding halo acids and CO2.