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

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.