MANGANESE AND IRON-OXIDES AS REACTANTS FOR OXIDATION OF CHLOROPHENOLS

Manganese and Fe oxides are capable of decreasing the concentration of chlorophenols, which are highly toxic pollutants in natural systems. Four di- and trichlorophenols were oxidized in oxide-buffered solution suspensions at rates depending on the concentration of reactants, the number of position of the chloro substituents, the surface charge and point of zero charge of the metal oxides, and the environmental pH. Second-order graphical plots were used to calculate the reaction constants and the half-life times. The order of reactivity of the oxides was birnessite > pyrolusite > Fe(III) oxide in all systems. Their reactivity was maximum at pH 4.0. The 2,4,6-trichlorophenol (TCP) was the substrate oxidized to the largest extent, whereas 2,3,5-TCP showed the lowest reactivity. The quantity of chlorophenols removed per unit mass of birnessite increased as the concentrations increased, without any apparent saturation effect across a wide range of concentrations, whereas it tended to be constant at high equilibrium concentration when pyrolusite and particularly Fe(III) oxide were used as oxidants. From radioactivity measurements of the C-14 recovered in liquid and solid phases after incubation of radiolabeled 2,4-dichlorophenol and 2,4,5-TCP with the metal oxides, the presence of less soluble oxidation products was shown. Oxidation products, such as quinones and traces of a dimeric compound, were detected by mass spectrometry. The effects observed support the general mechanism for a surface chemical reaction involving a metal oxide-chlorophenol complex formation followed by a phenoxyl radical generation.