UV Photocatalysis

Data summarized in Table 2 show how UV photocatalysis finds application in the OMWW treatment in the presence of both homogeneous and heterogeneous catalysts.


**Table 2.** State of the art of UV photocatalysis used to treat OMWW. Adapted from Reference [100].

Since 1972, titanium dioxide (TiO2)-based photocatalysts have been investigated [106] and then widely used for their effective semiconductor features, enabling the removal of various pollutants in environmental remediation [107–109]. Interesting properties characterize these systems, like chemical stability, long-term stability, remarkable oxidation ability, and low-cost [110–112]. Heterojunction photocatalysts based on TiO2 have been studied mainly for the mineralization of targeted pollutants into harmless products, thanks to the

generation of electron-hole (e−/h+) pairs if the semiconductor is under UV radiation [97]. In this frame, 2.80 V oxidizing power was produced by hydroxyl radicals produced during the photocatalytic step [96]. Besides the high chemical and physical stability of TiO2, this material tends to go through phase transformation from anatase to rutile [113]. This induces a detrimental effect on the resulting TiO2-materials because the rutile-phase has a lower surface area, negatively impacting the photocatalytic behaviour because of the (e−/h+) pairs' recombination [114].

In this regard, Chatzisymeon et al. explored the photocatalytic treatment of a threephase OMWW remediation approach using TiO2 in a laboratory-scale photoreactor. By properly optimizing the contact time, they observed the enhancement of COD removal. The product was a non-toxic effluent with 200 mg·L−<sup>1</sup> COD organic content [98].

In this context, the high surface/volume ratio of TiO2 nanoparticles, the possibility to dope them to increase the activation under solar irradiation, and the resistance to photocorrosion are advantages related to the use of TiO2-based photocatalysts.

This hitch can be minimized with the introduction of a second metal oxide component (e.g., MnO2, NiO, La2O3, SiO2, SnO2, ZnO, ZrO2), which has been recognized to induce significant degradation under UV irradiation [115–119], generating oxygen vacancies by the substitution of di- or tri-valent atoms by tetravalent atoms and providing particle-particle interaction [120]. In this context, very promising results have been obtained in terms of improved chemical stability and photocatalytic activities of the obtained materials, as demonstrated by many researchers in the last decades [121–123] and recently by Yaacob et al. for ZrO2-TiO2 materials [124].

However, TiO2 has been recently recognized as a carcinogenic substance [125], so an unavoidable challenge is the development of alternative systems able to maintain the same or better photocatalytic activity. In this scenario, among all the potential candidates, one could be zinc oxide (ZnO), which is able to absorb a wide fraction of the solar spectrum and more than TiO2 [126]. Many researchers have demonstrated its efficiency in the photodegradation of organic pollutants in water matrixes [127]. Additional features describe ZnO more than TiO2 [128]; by way of example, it can be used in acidic or alkaline environments through proper treatment [129,130]. Moreover, the optimum pH for the ZnO process is *ca.* 7, whereas that of TiO2 lies at acidic values, implying lower operational costs and higher efficiency than TiO2 in the advanced oxidation of pulp mill bleaching wastewater [131], phenol and 2-phenyl phenol photooxidations [132,133]. In addition, it is highly photosensitive, stable, and possesses a bandgap of *ca.* 3.2 eV [134]. However, besides the numerous studies on using this material in this field, efforts to overcome drawbacks are necessary.
