*6.1. Eco-Friendly Materials Used to Treat "Model Pollutants"*

Several materials have already been investigated for the degradation of "model pollutants". They show promise for treating OMWW efficiently, and the scientific community could draw inspiration for appropriate evaluations. By way of example, magnetic bismuthbased photocatalysts have been largely used in the wastewater remediation field, and they could also find successful application in OMMW treatment, on which only preliminary studies have been reported.

In general, magnetic bismuth-based materials can be classified as magnetic bismuthbased oxyacid salt, magnetic oxyhalides, magnetic sulfides, and magnetic oxides.

Bismuth-based oxyacid salts (commonly labeled as BiaAOb) have gained attention for their excellent visible-light absorption, band potential, and interesting chemical stability [165]. Their specific crystal phase confers good electron transport ability [166]. The introduction of proper magnetic components makes them easily recoverable and reusable for real applications.

In more detail, bismuth ferrite materials (BiFeO3) are characterized by ferroelectricity and ferromagnetic features [167]. They have been explored as nanofibers [168], nanoparticles [169–171], nanosheets [172], nanotubes [173], microspheres [174], and nanorods [175], exploiting their magnetic properties and the 2.2 eV bandgap. Li et al. [173] compared the photocatalytic behaviour of BiFeO3 in the form of nanoparticles, nanofibers, and hollow nanotubes, discovering the superior photoactivity of the latter due to the ultra-thin wall thickness and unique material structure. BiFeO3 nanosheets of 140–230 nm side length and 30 nm thickness were synthesized by Zhu et al. [172] by hydrothermal procedures, demonstrating their high capability to degrade 89% rhodamine B (RhB) under 180 min of visible light irradiation. Bharathkumar et al. [176] prepared BiFeO3 mat and mesh nanostructure materials by an electrospinning method, discovering that the photocatalytic degradation of the mesh sample was greater than that of the mat sample, probably due to the decrease of band gap energy. However, a limitation of the photocatalytic activity of BiFeO3 is related to the fast photogenerated electron-hole recombination. In this context, some studies pointed out that metal deposition and doping have a positive effect, reducing the charge recombination and improving their resulting photocatalytic performance [177].

Other bismuth-based oxyacid salts with narrow band gaps exist, such as BiVO4 (2.26–2.51 eV), Bi2WO6 (2.56–2.92 eV), Bi2MoO6 (2.49–2.66 eV), and Bi2O2CO3 (2.8–3.4 eV), which can be combined with magnetic components to obtain interesting and advanced materials with enhanced photocatalytic activity [178,179]. By way of example, Cam et al. introduced MnFe2O4 on BiVO4, obtaining an innovative material with good photocatalytic activity and magnetic recovery [180]. Sakhare et al. [181] produced BiVO4/NiFe2O4 composites able to degrade 98% methylene blue in 240 min of collected sunlight illumination and to maintain excellent stability even after four cycles. Bastami et al. [182] prepared magnetic Fe3O4/Bi2WO6 nanohybrids to degrade ibuprofen under solar light. Xiu et al. [183] developed 3D magnetic Fe3O4/Ag/Bi2MoO6 spheres, obtaining an advanced photocatalytic-Fenton coupling system, which exhibited excellent photocatalytic behaviors in the Aatrex degradation.

Bismuth oxyhalides (BiOX, X = Br, Cl, I) represent another family of bismuth-based materials, which have recently attracted scientific research due to their band gap, high stability, and non-toxicity [184,185]. They exhibit a tetragonal matlockite structure interlaced with [Bi2O2] 2+ flat plates and double halogen atomic layers, which reduce the electron-hole pairs' recombination, producing good photocatalytic behaviour [186,187]. In this context, the combination of BiOX and magnetic components represents an interesting perspective to obtain easily recoverable photocatalytic compounds on which many researchers are working. Briefly, Cao et al. [188] investigated the performances of BiOBr/Fe3O4 composites, prepared by solvothermal method, under visible light irradiation to degrade glyphosate. Li et al. [189] produced BiOBr/NiFe2O4 materials of different mass ratios according to a conventional hydrothermal approach, and their photocatalytic performances were explored in the photodegradation of methylene blue and phenol. The authors additionally synthesized BiOBr nanosheets decorated with NiFe2O4 nanoparticles and tested the samples in the rhodamine-B photodegradation [190], observing that the BiOBr/NiFe2O410 (having 10 wt.% NiFe2O4) composite was able to degrade rhodamine-B more efficiently than the pure BiOBr and NiFe2O4 (99.8% rhodamine-B degradation after 30 min radiation). Sin et al. [191] prepared N-BiOBr/NiFe2O4 composites by a hydrothermal strategy, demonstrating the enhanced photocatalytic behaviour towards phenol and Cr(VI) removal.

Moreover, systems based on BiOCl and BiOI were additionally developed, and their photocatalytic performances have been properly investigated. In particular, Ma et al. [192] prepared magnetic BiOCl/ZnFe2O4 samples, showing their high photocatalytic activity towards penicillin-G degradation (99% penicillin-G degradation within 180 min under visiblelight irradiation). Zhou et al. [193] studied ternary magnetic Ag2WO4/BiOI/CoFe2O4 hybrid compounds, evaluating their photocatalytic activity towards toxic elemental mercury Hg(0) removal. In addition, BiOI/CoFe2O4 composites modified with AgIO3 [194] and Ag2CO3 [195] were found to be highly efficient in the photocatalytic reduction of Hg(0).

Finally, magnetic sulfides and oxides deserve to be also mentioned. The former (labeled as Bi2S3) is described by the 1.3 eV energy bandgap and complete visible light region response [196]. They can be combined with materials with magnetic features to promote charge separation and guarantee good recyclability. For example, Li et al. explored the potentialities of Fe3O4/Bi2S3/BiOBr samples in the photodegradation of diclofenac and ibuprofen, observing *ca.* 94 and 97% conversion of the studied pollutants, respectively, after 40 and 30 min under visible light irradiation [197]. On the other hand, Zhu et al. tested Fe3O4/Bi2S3 microspheres towards Congo red removal, discovering good stability for continuous tests. The latter (commonly named Bi2O3) is an attractive material possessing high redox reversibility, bandgap spanning from 2.6 to 2.8 eV, and good electrochemical stability [198]. Several researchers combined it with magnetic compounds to obtain final easily recoverable materials. In particular, Abbasi et al. prepared 3D flowerlike Fe3O4@Bi2O3/g-C3N4 nanocomposites, successively evaluating their photocatalytic activity towards indigo carmine degradation [199]. In this case, introducing the conductive

C layer in the nanocomposite sample could improve the photocatalytic behaviour. In addition, Gao et al. first obtained a C/Fe3O4 composite and then a double conductive C/Fe3O4/Bi2O3 photocatalyst. In this case, electron-hole pairs' recombination and the reverse electron transfer to Bi2O3 can be prevented [200].
