Probing the TiO2 photocatalytic mechanisms in water purification by use of quinoline, photo-fenton generated OH. radicals and superoxide dismutase

In an attempt to improve our understanding of the basic mechanisms of the degradation of aromatic pollutants in water by TiO2 photocatalysis, quinoline (benzo[b]pyridine) was selected as a molecular probe, principally because of the difference in electron density over its two rings. This study was based on the identification and quantification of the primary products or principal secondary products of quinoline degradation either by TiO2 photocatalysis at pH 3 and 6 or by OH. radicals generated via the photo-Fenton reaction (Fe(II/III)H2O2-UV) at pH 3. In this latter case, the three major products were those expected from the preferential electrophilic attack of OH. radicals on the electron-richer benzene moiety, viz., 5-, and 8-hydroxyquinolines and quinoline-5,8-dione derived from them. TiO2 photocatalysis did not yield this dione, and at the same percentages of degraded quinoline, the amounts of 5-hydroxyquinoline were lower by a factor of ca. 2 at pH 3 and ca. 10 at pH 6 (those of the 8-isomer were also decreased but no accurate measurements were obtained). In addition, at pH 6, we observed marked increases in the amounts of products corresponding to the oxidation of the pyridine moiety, viz., 4-quinolinone and especially 2-aminobenzaldehyde (the major product) and its N-formyl derivative. These results show that oxidative steps in TiO2 photocatalysis do not involve only OH. radicals. It was also observed that, at pH 6, superoxide dismutase (SOD), which catalyzes the elimination of O2(.-) species, decreased the TiO2 photocatalytic rate of quinoline disappearance, almost suppressed the formation of 2-aminobenzaldehyde, and lowered the amount of 4-quinolinone. The SOD and pH effects suggest a mechanism involving quinoline activation by hole transfer, followed by superoxide addition to the resulting radical cation. The nucleophilic character of superoxide implies addition to the pyridine moiety, i.e., with a regioselectivity opposite that of the OH. radical pathway.