Photo-catalytic degradation of polyphenolic tannins in continuous-flow reactor using titanium dioxide immobilized on a cellulosic material.

Tannins are recalcitrant polyphenolic molecules that resist microbial attack. Their main environmental damage is due to their low biodegradability. This work aims to investigate the photo-catalytic degradation of two commercial tannin extracts, chestnut (hydrolysable tannin) and mimosa (condensed tannin). The experiments were carried out under UV-light irradiation in a continuous-flow reactor using titanium dioxide (TiO2) immobilized on cellulosic fibers. It was highlighted that photo-catalytic degradation is unfavourable in acidic medium and when the pH is too high (pH above 12); it reaches its maximum efficiency at pH 7.5 (99 and 97% for chestnut and mimosa, respectively). Nearly complete degradation of tannins requires an irradiation period of 6 h. The process efficiency is inversely affected by the concentration of tannins essentially above 75 mg/L for chestnut and 60 mg/L for mimosa. Above 240 mL/min, any increase in feed flow negatively affects the performance of the process. Furthermore, a significant decrease of treatment efficiency was seen when increasing the concentration of ethanol and salts in the medium. Obtained results suggest that UV-light irradiation in a continuous-flow photo-reactor using immobilized TiO2 may be considered as an adequate process for the treatment of water containing recalcitrant tannin molecules.