Evidence for superoxide-radical anion, singlet oxygen and OH-radical intervention during the degradation of the lignin model compound (3-methoxy-4-hydroxyphenylmethylcarbinol)

Semiconductor slurries of TiO2 were used during the photodegradation of the lignin model compound 3-methoxy-4-hydroxyphenylmethylcarbinol (from now on 3M-4HC). The TiO2 P25 Degussa suspensions (1 mg/ml) of 3M-4HC were irradiated with a medium pressure mercury lamp in the pH range 3-8. The 3M-4HC (0.85 mM) disappeared within 40 min when using TiO2 P25 Aeroxide Degussa and within 60 min with TiO2 P25 Degussa as monitored by high pressure liquid chromatography (HPLC). The mineralization of 3M-4HC was observed to proceed within 120 and 180 min, respectively, indicating the existence of long-lived intermediates under light irradiation in solution. The addition of H2O2 accelerated the decomposition of 3M-4HC to almost completion within 30 min while the mineralization was observed to be complete only after 90 min. TiO2 suspensions could be used repetitively as photocatalysts in the decomposition of 3M-4HC. The decomposition of 3M-4HC followed diffusion controlled pseudo first-order kinetics but the TOC seems to follow a mass transfer controlled process. No 3M-4HC decomposition was observed in the dark in the presence of TiO2. The abatement of 3M-4HC was observed to be sensitive to the crystallographic structure, the crystal radius and the surface area (BET) of the TiO2 used. The role of the oxygen singlet (1O2), of the superoxide anion radical HO2− and of the OH-radical during the photocatalyzed degradation of 3M-4HC was elucidated by using sodium azide (NaN3), benzoquinone (BQ) and OH-radical scavengers (methanol and tert-butanol), respectively. Evidence is presented for the intervention of these radical species produced on the TiO2 surface under light irradiation during the 3M-4HC degradation.