Effects of reactive oxidants generation and capacitance on photoelectrochemical water disinfection with self-doped titanium dioxide nanotube arrays

[Display omitted] •Self-doped anatase (bl-TNT) and amorphous TiO2 nanotube (bk-TNT) were interrogated.•Lower double layer capacitance of bl-TNT led to higher PEC disinfection of E. coli.•PEC bactericidal rates in tap water were similar or greater than in Na2SO4 solution.•Electrostatic interaction between photoanode and bio-solids determined the kinetics.•Commercial applicability for point-of-use disinfection was demonstrated. We herein provide photoelectrochemical (PEC) disinfection activities of anodically prepared TiO2 nanotube (TNT) arrays (diameter ˜ 100 nm, length ˜ 16 μm on average) that were electrochemically self-doped before (bk-TNT) and after (bl-TNT) an atmospheric annealing at 450 °C. The X-ray diffraction indicated predominating anatase TiO2 signal on bl-TNT, while substantial lattice distortion was noticed for bk-TNT. Although the X-ray photoelectron spectra indicated negligible Ti3+ on surface of both TNTs, linear sweep (cyclic) voltammetry and electrochemical impedance spectrometry confirmed the bk-TNT to show greater double layer capacitance and overall photocurrent, coupled with lower charge transfer resistance. Nevertheless, the PEC disinfection of E. coli was significantly invigorated on bl-TNT, while the bactericidal rates in tap water were comparable or even far greater than those in 0.1 M Na2SO4 solutions, depending on [E. coli]0 (105 or 107 CFU/mL). Under a presumed diffusion-controlled kinetic regime in this study, observed effects of capacitance and electrolyte could be interpreted in terms of electrostatic interaction between the electrical double layer of photoanodes and charged bio-solids, such as repulsion by co-ions (SO42−) and adsorption/surface blocking. Analogous PEC experiments on model organic compounds degradation (4-chlorophenol and methylene blue) corroborated a long-term stability of the bl-TNT (up to 30 consecutive cycles) and the role of surface hydroxyl radical as the primary oxidant.