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

Abstract

Semiconductor slurries of TiO 2 were used during the photodegradation of the lignin model compound 3-methoxy-4-hydroxyphenylmethylcarbinol (from now on 3M-4HC). The TiO 2 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 TiO 2 P25 Aeroxide Degussa and within 60 min with TiO 2 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 H 2O 2 accelerated the decomposition of 3M-4HC to almost completion within 30 min while the mineralization was observed to be complete only after 90 min. TiO 2 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 TiO 2. The abatement of 3M-4HC was observed to be sensitive to the crystallographic structure, the crystal radius and the surface area (BET) of the TiO 2 used. The role of the oxygen singlet ( 1O 2), of the superoxide anion radical HO 2 − and of the OH-radical during the photocatalyzed degradation of 3M-4HC was elucidated by using sodium azide (NaN 3), benzoquinone (BQ) and OH-radical scavengers (methanol and tert-butanol), respectively. Evidence is presented for the intervention of these radical species produced on the TiO 2 surface under light irradiation during the 3M-4HC degradation.