Visible/Solar Photocatalysis

As discussed so far, each step of the industrial sector for olive oil production implies high operational costs. In this context, any improvements introduced to reduce treatment costs must be carefully considered. Among these, for photocatalytic remediation, solar energy has to be properly developed, especially in the Mediterranean countries, with the final aim of cost-effectiveness.

Visible/solar photocatalytic strategies employ adequately designed heterogeneous and homogeneous photocatalysis, photo-Fenton, and solar-Fenton reagents. Some examples are reported in Table 3.

Gernjak et al. investigated OMWW from Portugal and Spain by solar photocatalysis [105]. In more detail, two solar reactors i were employed at pilot scale: (i) a conventional compound parabolic collector type (CPC); (ii) an open non-concentrating falling film reactor (FFR). Different solar photocatalytic systems were tested, but the photocatalyst with the higher amount of Fe (10 mM) showed the most increased activity.


**Table 3.** State of the art in visible/solar photocatalytic processes for OMWW treatment. Adapted with permission from Reference [135].

Ruzmanova et al. studied the photocatalytic treatment of a three-phase OMWW photodegradation process using reusable N-doped TiO2 sol-gel compounds, demonstrating the higher activities of doped-catalysts compared to the non-doped ones, reaching a COD removal more elevated than 60% [140]. Additionally, N-doped materials maintain high efficiency when used for several cycles.

In addition, the role of photochemistry in the Fenton-like process is gaining attention thanks to ultraviolet and/or visible light to reduce the catalyst loading, enhancing the catalytic behaviour. In particular, Gernjak et al. investigated OMWW treatment processes by solar-photo Fenton approach on a pilot-plant scale, successfully removing up to 85% COD and 100% phenols [105].

Andreozzi et al. proposed an OMWW treatment based on a three-phase method exploiting (i) centrifugation followed by solar photolysis, (ii) centrifugation and solar photo-Fenton, and (iii) centrifugation coupled with solar photo-Fenton and ozonation. In this context, the ferric catalyst is responsible for COD and phenol removal (up to *ca.* 30% and 64%, respectively) [136].

Rizzo et al. investigated OMWW treatment by photo-Fenton, preceded by coagulation. In this case, the maximum efficiency of organic matter removal was *ca.* 95% in 1 h [137].

Justino et al. studied the combination of fungi *Pleurotus sajor caju* and photo-Fenton oxidation [99]. The treatment by fungi confirmed the reduction of OMWW toxicity towards *Daphnia longispina* and resulted in 72.9% total phenolic compounds removal and 77% COD reduction. When the treatment is preceded by photo-Fenton oxidation, the biological treatment with fungi is more efficient.

Papaphilippou et al. proposed a treatment process for OMWW by coupling coagulation– flocculation and Fenton oxidation. Following the photo-Fenton oxidation, COD and phenol removals were approximately 73% and 87%, respectively [18].

Finally, Aytar et al. reached 99% phenol and 90% total organic content reduction using adsorption, biological (*T. versicolor*), and photo-Fenton treatment in sequence [138].

Considering the depicted scenarios, it emerges that a proper comparison among the performances of the studied technologies to treat OMWW is not a trivial task. Indeed, the numerous variables in play (i.e., OMWW origin, process type and operative conditions, used scale) do not allow identification of a method that guarantees the best results in terms of OMWW removal. Only a rough evaluation in terms of COD removal can be done, but in this case, all the advantages and/or drawbacks of each strategy must be considered. In general, looking at the COD removal values reported in Table 3, interesting results were

obtained when working on a laboratory scale and in pilot plants, suggesting promising avenues that deserve to be investigated.

### **5. From Conventional to Easily Recoverable Magnetic Photocatalysts**

As described in the previous sections, many approaches have been investigated for OMWW treatment [124,141–146]. Still, most of them suffer from not trivial and not negligible drawbacks (i.e., expensive maintenance, lateness in the separation time, high retention time).

In this regard, technologies based on photocatalysis can be advantageous for their environmental friendliness and high oxidation efficiency [147–149]. To develop even more efficient photocatalytic systems for real applications, research continuously moves the efforts toward exploring different materials.

Conventional nano-or micro-powder photocatalysts are developed for continuous, safe, and efficient photocatalytic reactions. Still, at the same time, their use is limited by the difficult separation and recovery from the reaction mixture for their sustainable reuse [150,151]. The recovery cost could invalidate the technology from an economic viewpoint [152]. To overcome this issue, the introduction of magnetic features in photocatalytic systems seems to be one of the best solutions, giving the possibility to maintain the catalytic performances of samples while making their separation from the reaction a more accessible medium.

Several approaches have been recently explored to develop advanced magnetic photocatalytic materials for wastewater remediation. However, unfortunately, few studies have mainly focused on applying these materials in the treatment of OMWW.

For this purpose, different magnetic nanoparticles (i.e., γ-Fe2O3, Fe3O4, MFe2O4, where M = Mg, Ni, Zn, Cu, Co) have been introduced in photocatalysts, giving rise to composite materials with magnetic features [153–156]. In this context, electron and hole migration between the magnetic and semiconductor components results in the separation of the photo-induced charge carriers, enhancing the light absorption ability [153–156]. This class of innovative materials has been studied regarding several pollutants in wastewater decontamination. Shen et al. prepared Fe3O4@TiO2@Ag-Au microspheres with promising magnetic and photocatalytic properties [157]. Singh et al. immobilized BiOI/Fe3O4 photocatalyst on graphene oxide to degrade 2, 4-dinitrophenol [158]. Furthermore, the potentialities of other magnetic composite photocatalysts have been explored, such as Cu2V2O7/CoFe2O4/g-C3N4 [159], MnFe2O4/SnO2 [160], MoO3/CoFe2O4 [161]. As already mentioned by Ma et al., the research efforts in this field have resulted in the development of several simple and magnetic photocatalytic materials, such as magnetic bismuth-based photocatalysts [162]. In addition, Ruzmanova et al. developed magnetic core TiO2/SiO2/Fe3O4 nanoparticles to degrade organic compounds in OMWW. 1.5 g·L−<sup>1</sup> of catalyst dosage optimized the photodegradation process, providing high efficiency and an easy catalyst recovery [140]. Successively, Vaiano et al., using ferromagnetic N-TiO2/SiO2/Fe3O4 nanoparticles, achieved 64% phenol removal and 55% TOC reduction after an irradiation time of 270 min, as well as good stability of the photocatalytic materials after four operation/regeneration cycles [163]. Hesas et al. explored a magnetically separable Fe3O4 on modernite zeolite to purify OMWW from Kermanshah. They identified the key parameters influencing COD and BOD removal: pH (optimized at the value of 7.8) and turbidity of the treated solution. In addition, in this case, the regenerated Fe3O4/mordenite zeolite could be reused for five consecutive cycles [164].

In addition, the research community is currently working hard on novel alternatives.

### **6. Perspectives**

Considering the high impact of OMWW treatment on the environment and human health, all the sustainability and circular economy principles should be adequately assessed. In this context, perspectives related to the development of efficient, sustainable alternatives to nano- or micro-sized photocatalysts to treat OMWW (Figure 5) can be mainly divided

into two categories: (i) eco-friendly materials (mainly characterized by magnetic features) already investigated in the treatment of several "model pollutants"; and (ii) other emerging eco-friendly materials (floating devices, membranes).

**Figure 5.** Proposed eco-friendly alternatives to nano- or micro-sized photocatalysts to treat OMWW.
