RADIOLYTIC AND TIO2-ASSISTED PHOTOCATALYTIC DEGRADATION OF 4-CHLOROPHENOL - A COMPARATIVE-STUDY

Mechanistic and kinetics details of 4-chlorophenol (4-CP) oxidation ha ve been elucidated by radiolytic and photocatalytic techniques. When 4 -chlorophenol is oxidized by gamma-radiolysis in conditions favoring h ydroxyl radical oxidation ((OH)-O-.), significant concentrations of 4- chlorocatechol (4-CC) and hydroquinone (HQ) are formed as intermediate s. Phenol is the only major intermediate when conditions favoring redu ction by hydrated electrons are employed. 4-CC and HQ are not detected when 4-chlorophenol is oxidized with azide radicals. Hydroxyl radical mediated oxidative degradation rates of 4-CP are similar at pH 3.0 an d pH 6.1. The 4-CP degradation rate is relatively slower at pH 9.1, an d no aromatic intermediates are detected. These results confirm the pr oposal that hydroxy-mediated 4-CP oxidation follows at least three sep arate degradation pathways, forming 4-CC, HQ, and unidentified mostly nonaromatic compounds, as reaction intermediates enroute to complete m ineralization. Pulse radiolysis experiments have also been carried out to characterize the radical transient species formed during the oxida tion of 4-CP (pH 6 and 10) and 4-chlorocatechol (pH 6). The second-ord er rates for scavenging of (OH)-O-. radicals at pH 6 are measured as 9 .3 x 10(9) M(-1) s(-1) for 4-CP and 7.0 x 10(9) M(-1) s(-1) for 4-CC. Photocatalytic degradation produces intermediates consistent with hydr oxyl radical oxidation, but the concentration of aromatic intermediate s is lower than in the radiolysis experiments, especially at higher co ncentrations of TiO2. This indicates that the course of photocatalytic transformation of 4-CP does not involve hydroxyl radical oxidation ex clusively. Direct electron transfer and surface chemical reactions als o contribute significantly to the disappearance of 4-CP and its reacti on intermediates in TiO2 slurries.