Photocatalytic Treatment of Cork Wastewater Pollutants. Degradation of Gallic Acid and Trichloroanisole using Triphenyl(thia)pyrylium salts

Abstract

The photocatalytic degradation of two typical pollutants present at the wastewaters of cork industry, namely gallic acid (GA) and 2,4,6-trichloroanisole (TCA) has been investigated using triphenylpyrylium (TPP+) and triphenylthiapyrylium (TPTP+) salts as organic photocatalysts. Fast removal of GA and slow but efficient degradation of the reluctant TCA were observed. The role of the transient species involved in the photodegradation has been investigated by means of photophysical experiments. Participation of singlet excited states was safely disregarded since lifetimes of both photocatalysts remained invariable upon addition of GA or TCA. Formation of photoactive ground state complexes was observed, and the complexation constants were estimated as KTPP-GA=468.4M−1, KTPP-TCA=528.0M−1 and KTPTP-TCA=657.1M−1. Moreover participation of the triplet excited states in the electron transfer oxidation of GA was very efficient, while no reaction occurred from the triplet excited states in the case of TCA. In addition, thermodynamic calculations support the participation of the triplet excited state in the case of GA, while the lack of triplet quenching in the case of TCA is associated with a thermodynamically disfavored process. Overall, photophysical experiments indicate that for GA, oxidative electron transfer occurs mainly from the triplet excited state with marginal contribution of the ground state complexes, while for TCA formation of ground state complexes constitutes the only mechanistic pathway.