Photoelectrochemical catalysis on nanostructured TiO2 films: colour and emerging contaminants removal

Introduction In recent years, significant advances in wastewater (WW) treatment were demonstrated for the removal of both chromophores and other specific contaminants. In the first part of this study, innovative treatments for colour removal has been analysed. In recent years, in fact, the interest on WW colour removal become significant (Collivignarelli et al. 2019). Considering the rising of the problem related with the presence of dyes and chromophores into WW, several advanced processes are currently adopted or under study. Unlike conventional oxidation processes, the basic principle of advanced oxidation processes (AOPs) is production of highly reactive and non-selective hydroxyl radicals (OH●). Among the AOPs, one of the treatments that is attracting wide interest and is still obviously underinvestigated is the PhotoElectrochemical Catalysis (PEC) (Franz et al. 2015). This process combines conventional photocatalysis with the physico-chemical phenomena obtained in theTiO2 catalyst by electrical polarization during operation. This approach has been shown to lead to a synergistic effect, enhancing the production of OH● and therefore significantly increasing the degradation rate of chromophore molecules or other contaminants. Additional practical advantages arise from the stability and the lowcost of the TiO2 (Komtchou et al., 2016), and the possibility to synthesise highly photoactive TiO2 films by an industrial technique, namely Plasma Electrolytic Oxidation, which makes it easier a future scale up for WW applications (Franz et al., 2016). In the same years, PEC with TiO2 has been tested several times on synthetic dyes, particularly on azo dyes considered the most recalcitrant due to their N-N bonds. For instance, Franz et al. (2015) obtained 95% of RR243 after 60 min of PEC treatments. Moreover, Turolla et al. (2012) showed the removal (87.5%) of DG26 after 24 h of process. However, the experiments on a real WW are very few so far. Considering specific contaminants, since several years the scientific community has been targeting the so-called Contaminants of Emerging Concern (CECs), which are warranting attention because of their considerable spread in all environmental matrices and because they may occur in groundwater as a result of human activities, i.e. agriculture and industrial practices. Nowadays, chlorinated volatile organic compounds (VOCs) and pesticides fall into the group of CECs and are usually detected in groundwater in trace concentrations (μg L-1 or ng L-1) (McCance et al., 2018). Considering that only in recent years PEC has been tested on groundwater (Komtchou et al., 2018), the possible application of PEC to the removal of specific CECs from this type of water has been investigated.