*6.2. Other Emerging Eco-Friendly Materials (Floating Devices, Membranes)*

Due to their floating properties and good visible light utilization, floating photocatalysts could be considered an excellent choice to gradually substitute conventional photocatalysts [201]. In fact, since 1993, floating TiO2-based materials have been studied [202]. In general, a floating device exploits a lightweight material to float on the water surface, and the photocatalytic performances are maximized thanks to its exposed large surface [203,204]. At the same time, due to its peculiar structure, it minimizes photocatalyst loss, avoiding the long-term contact between photocatalyst and pollutants, which can decrease photocatalytic activity. In the last decades, various supports (i.e., perlite, vermiculite, glass, cork, graphite, polymer) have been investigated as candidates for developing efficient floating photocatalysts [201].

Among them, by way of example, some of the authors studied the performance of aerogel water-floating based materials prepared by poly (vinyl alcohol) and polyvinylidene fluoride as a polymer platform and loaded with different semiconductors, such as g-C3N4, MoO3, Bi2O3, Fe2O3 or WO3, obtaining interesting results towards the reduction of Cr(VI) under visible light [204]. Moreover, Wang et al. [205] recently investigated the use of advanced spongy foam photocatalysts composed of BiOX compounds deposited onto polyurethane foams to degrade targeted pollutants, such as methyl orange, phenol, and chlortetracycline. These systems showed a high potential because they can conjugate high stability, excellent adaptability, and easy recovery, with high photocatalytic performances and good reusability.

In the present panorama, the possibility of using supports characterized by ecofriendly features (i.e., low-cost, non-toxicity, bioavailability) is a priority for further evaluation, and will require strenuous investigation efforts. Some researchers have already considered luffa cylindrica, alginate sphere, or light expanded clay aggregate (LECA), but their potentialities are still the object of study today. Following this perspective, Chawla et al. immobilized MoSe2/BiVO4 on luffa cylindrica, and then they tested it in phenol degradation, observing up to 97% removal within 2 h of visible light irradiation [206]. Huang et al. recently investigated the possibility of combining alginate spheres with magnetic components, finding exciting results. In this case, the excellent floating performance, together with the availability of reaction sites offered by the material, resulted in the degradation of the selected pollutants (e.g., methyl orange) [207].

Finally, the use of membranes deserves also to be cited. This technology has been investigated in the OMWW treatments for several advantages (simplicity, modulability, easy maintenance, high separation efficiency, small footprint, and easy scale-up) [208]. Several membrane types have been developed and produced, from the polymeric-based ones [209,210] to the inorganic-based ones [211]. All of them have shown excellent performance in the separation of targeted pollutants. However, membrane technology is characterized by some drawbacks. By way of examples, they may be limited by the high concentration of suspended solids present in the OMWW to be treated, and they suffer from foulant deposition due to contaminants separated from the feed. Thus, further treatments are usually required. In this context, Dzinun et al. [212–214] tried to develop a photocatalytic membrane to overcome the membrane fouling and use it as support for photocatalysts. In this case, the photocatalyst addition should minimize the fouling rate. Unfortunately, photocatalytic membranes are also affected by some drawbacks. For example, prolonged exposure to irradiation may ruin their structure, causing damage to the active surface area, which strongly impacts the photocatalytic efficiency [215]. In this context, many ideas are currently put into action by several researchers, as recently reported by Salim et al. [216,217].

All these interesting and promising results obtained in the decontamination of targeted pollutants present in wastewater can be a starting point to investigate more in detail what happens in the case of such complex matrices as OMWW.

### **7. Conclusions**

This review provides a critical insight into the current status and the consequent advances related to OMWW treatments, underlying their potentialities and drawbacks. A particular focus on developing innovative eco-friendly photocatalysts, which could become valid alternatives to conventional systems, if properly optimized, is provided.

Nowadays, the OMWW sector plays a fundamental role in the European economy, but at the same time, it also leads to dramatic consequences on the environment and human health. In this context, the current challenge involves optimizing well-known and conventional technologies. Still, the most captivating challenge is the development of innovative advanced strategies, such as those based on photocatalysis. These latter offer many advantages (i.e., high efficiency, low cost) but require the use of novel materials to overcome the common issues related to using slurry reactors and difficult photocatalyst recovery.

In this scenario, the potential use of easily recoverable magnetic compounds as well as floating- and membrane-based devices points to new horizons for sustainability, alternative to conventional TiO2-based systems. The application of these advanced systems still needs hard work by the research world. Their future success in real applications will create a bridge between environmental protection and a circular economy.

**Author Contributions:** Conceptualization, E.F. (Ermelinda Falletta) and C.L.B.; methodology, E.F. (Ermelinda Falletta); visualization, M.G.G., E.F. (Elena Ferrara) and E.F. (Ermelinda Falletta); literature collection and analysis, M.G.G., E.F. (Elena Ferrara) and E.F. (Ermelinda Falletta); Content design, E.F. (Ermelinda Falletta); Writing—original draft preparation, M.G.G., E.F. (Elena Ferrara) and E.F. (Ermelinda Falletta); writing—review and editing, E.F. (Ermelinda Falletta) and C.L.B.; supervision, C.L.B.; project administration, E.F. (Ermelinda Falletta) and C.L.B.; funding acquisition, C.L.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** Velux Stiftung Foundation is gratefully acknowledged for its financial support through project 1381, "SUNFLOAT—Water decontamination by sunlight-driven floating photocatalytic systems".

**Data Availability Statement:** The data that support the plots within this paper are available from the corresponding author on reasonable request.

**Acknowledgments:** This work was supported by the Department of Chemistry, Università degli Studi di Milano, Italy (Piano Sostegno alla Ricerca, PSR, grant 2021).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

