Role of the active chlorine generated in situ on the photoelectrocatalytic inactivation of bacteria and fungi with TiO2 nanotubes

Immobilised TiO2 nanotube (TiO2-NT) electrodes were grown via electrochemical anodisation in an aqueous solution containing fluoride ions at 10, 20 and 30 V. The photocatalytic (PC) and photoelectrocatalytic (PEC) activity of TiO2-NTs electrodes in the oxidation of methanol and the inactivation of bacteria and fungi was studied in different chloride salts electrolytes. Low concentrations of electrochemically generated oxidising species, such as free chlorine, were measured in experiments at pH 8.5 and +1 V of applied potential. Increasing the anodising potential results in longer nanotubes with higher photoactivity. The TiO2-NT electrode anodised at 30 V (TiO2-NT30V) generates free chlorine with an average concentration of 0.03 mg·L-1 upon illumination with UV-A at +1 V of potential bias. This concentration was enough to achieve 99.99 % of inactivation of a 106 CFU·mL-1 Gram-negative bacteria (Escherichia coli) in <3 min and Gram-positive bacteria (Enterococcus faecalis) after 7 min, whereas fungi (Candida albicans) required 15 min. The low production of chlorine was found to have a big impact on the bacteria and fungi inactivation even in not favourable chlorine generation conditions. An in situ investigation of the most influential parameters in the inactivation of some microorganisms with PEC and NT30V electrode has been done. It was found that free chlorine production increases with the length of TiO2-NT, with Cl- concentration up to 15 mmol·L-1 and with the application of potential bias. TiO2-NT30V photoanode has been demonstrated to produce active chlorine at levels compatible with the water disinfection process, suggesting that the present method could be considered a promising alternative for in situ chlorine-based disinfection.