2.3.3. Phenol Removal over Various Catalytic Materials

Titanium dioxide is by far a benchmark for numerous photocatalytic applications due to its high chemical stability, low cost, possibility to tune the band gap with other semiconductors, and biological inertness. However, photocatalysis on TiO2 is still limited by its UV band wavelengths, weak adsorption of hydrophilic pollutants, and agglomeration

of nano-sized particles. Therefore, it is a permanent rush to develop new light harvesters which aim to outperform titanium dioxide. The most common cleaning technologies are (i) biological (aerobic/anaerobic), (ii) physical (sedimentation, screening, filtration, floating, flocculation, and aeration), and (iii) chemical (neutralization, ozonation, precipitation, adsorption, and oxidation) processes.

A novel Z-scheme-based photocatalyst composed of Bi2O3/Bi2MoO6 heterojunction was proposed by Fu et al. [77] for efficient photodegradation of phenol with 96.4% degradation efficacy. In this system, a TOC removal efficiency of 75.5% was achieved. The spatially separated redox charge carriers, the excellent light harvesting capability, and the fast-charge transportation features of the catalysts were the factors determining their enhanced performances.

Zhang and co-workers [78] prepared a Bi2S3/Bi2WO6 composite by a hydrothermal method which efficiently weakens the recombination rate of photoinduced charge carriers and narrows the photoresponse range. The composite photocatalyst exhibited enhanced photocatalytic activity in the degradation of phenol under visible light irradiation, being 6.2 times higher compared to bare Bi2WO6. The photodegradation rate of phenol (Figure 11a) reached 51.6% in the presence of the Bi2S3/Bi2WO6 composite after 2 h of visible light irradiation, whereas only 12% of phenol was degraded by Bi2WO6. The photogenerated electrons (Figure 11b) were introduced from the conduction band (CB) of Bi2S3 to the CB of Bi2WO6 due to the intimate contact between the two semiconductors. Simultaneously, holes on the valence band (VB) of Bi2WO6 were transferred to that of Bi2S3 under the band energy potential difference. Therefore, efficient separation of the photoinduced e<sup>−</sup>/h+ pairs took place while their recombination was hindered.

**Figure 11.** (**a**) Photocatalytic degradation of phenol under visible light irradiation as a function of time; and (**b**) Diagram for energy band levels of Bi2S3/Bi2WO6 composites and the possible charge separation process. Reproduced and adapted with permission from ref. [78]. Copyright 2012 American Chemical Society.

Different BiFeO3 morphologies synthesized by three synthesis routes (e.g., co-precipitation CP, hydrothermal HT, sol-gel SG) were investigated by Chien et al. [79] for the photodegradation of phenol as a model organic pollutant. The SG-BiFeO3 sample exhibited remarkable direct sunlight photocatalytic degradation of phenol (98.95%), superior to those of the HT-BiFeO3 (77.4%) and CP-BiFeO3 (66.9%) in 120 min. The radical scavenger studies implied that the photogenerated hole (h+), hydrogen peroxide (H2O2) and hydroxyl (•OH) radicals were the dominant reactive species. Under direct solar irradiation, the photogenerated electron on the BiFeO3 surface migrated from the filled VB to the CB band and left an equal number of holes in VB (Figure 12). The promoted CB electrons of BiFeO3 (+0.53 eV) reacted with the oxygen molecules to generate H2O2 (E0(O2/H2O2) = +0.695 eV). Simultaneously, the holes situated in the VB band of BiFeO3 (+2.59 eV) would react with the OH<sup>−</sup> (E0(•OH/OH−) = +1.99 eV) to generate the •OH radicals. Subsequently, these

radicals (h+, H2O2 and •OH) react with the surface-adsorbed phenol, converting it into mineralized products.

**Figure 12.** Mechanism of the phenol photocatalytic removal over BiFeO3 under direct sunlight irradiation. Reproduced with permission from ref. [79]. Copyright 2022 Elsevier.

Jiang et al. [80] fabricated a series of BiOI-ZnO nanocomposites with various BiOI contents and tested their photoreactivity for phenol degradation under simulated solar irradiation. The phenol photodegradation rate reached 99.9% within 2 h, whereas only 40% of phenol removal took place over pristine ZnO. In the BiOI-ZnO system, the internal electric field formed between n-p heterojunctions of two oxide phases forced the electron and hole charge carriers to move in the opposite direction. Thus, the internal electric field between the component oxides facilitates the separation and transfer of the photocarriers. Since the CB of BiOI is much more negative than that of ZnO, the generated electron in BiOI favors the diffusion through into the CB of ZnO. Concomitantly, the photogenerated h+ in VB of ZnO moves to p-type BiOI. As a result, more photogenerated carriers migrated to the catalyst surface, contributing to the reaction. Figure 13 illustrates the proposed mechanism for the enhanced removal of phenol by the BiOI/ZnO photocatalyst. In this research study, the superoxide species (•O2 −) and holes were established as the main reactive species in the photocatalytic reaction.

Zhang et al. [81] take a look at the photocatalytic mineralization of phenol over a single BiPO4 under UV-C irradiation. After studying the influence of several operating parameters, it was established that the mineralization of phenol was favorable in acidic conditions; the catalytic process decreased with increasing initial phenol concentration, and the chloride ions promoted the rate of mineralization. The BiPO4 photocatalyst mineralized more than 95% of phenol (10 mg L<sup>−</sup>1) after 5 h of illumination. Further after, Wang et al. [82] investigated the photodegradation of phenol under simulated solar irradiation of CeO2, Bi4O7 and 10% CeO2/Bi4O7 photocatalysts. The authors indicated that for the individual CeO2 and Bi4O7, the phenol removal rates were only 12% and 40%, respectively. The 10% CeO2/Bi4O7 photocatalyst degrades 92% phenol within 120 min, corresponding to the TOC value of 53%. Since the Fermi energy level of CeO2 is higher than that of Bi4O7, the electrons in CeO2 will be transferred to Bi4O7 until the Fermi levels of Bi4O7 and CeO2 are equalized. The energy band of Bi4O7 bends downward, and the energy band of CeO2 bends upward. Hence, a built-in electric field is formed at Bi4O7/CeO2 interface. The electrons flow from Bi4O7 to CeO2 and holes from CeO2 to Bi4O7, forming thus a typical type II heterojunction. The trapping experiments of active species evidenced that h<sup>+</sup> and •O2 − played significant

roles in phenol removal. However, the single Bi4O7 was almost deactivated, while the 10% CeO2/Bi4O7 demonstrated improved stability after three cycling experiments.

**Figure 13.** Photocatalytic pathway for the separation and transfer of the photogenerated carriers under simulated solar irradiation over BiOI/ZnO photocatalyst. Reproduced with permission from ref. [80]. Copyright 2017 Elsevier.

A series of ternary Bi7O9I3/g-C3N4/Bi3O4Cl photocatalysts were synthesized via the oil bath method by Yuan et al. [83] and tested for phenol photocatalytic removal. The optimal TOC removal rate reached up to 93.57% under visible irradiation within 160 min. After performing the trapping-species experiments and EPR characterization, the authors indicated that •OH and •O2 − were the oxidizing species responsible for the pollutant removal. The same study indicated that a dual S-scheme charge migration was generated at the interface of Bi7O9I3, g-C3N4, and Bi3O4Cl, which favors efficient charge separation.

Table 3 summarizes various studies regarding the experimental conditions and the main reactive species participating in phenol photomineralization over some photocatalysts.




Hydrothermal synthesis of BiOCl-activated carbon (AC) was reported by Sharma et al. [99] as an efficient photocatalyst for phenol removal. After 120 min of UV-light irradiation, more than half of the phenol photodegraded by the sample with 1 wt.% AC/BiOCl. The results showed a TOC value of 11.92 mg L−<sup>1</sup> for phenol mineralization. SnO2:Sb nanoparticles (with 0.2%, 0.4%, and 0.6% concentration of Sb) turned out to be competent catalysts for phenol removal under UV and solar light irradiation [100]. The authors claimed that the change in the phenol concentration influenced the solution pH due to the formation of byproducts during the reaction. The degree of mineralization reached 97% over 0.6% SnO2:Sb nanoparticles within 120 min reaction time, while 71% and 45% values were achieved for 0.4% and 0.2% SnO2:Sb samples, respectively.

Sandulescu et al. [101] showed a comprehensive view of the photocatalytic oxidation of phenol under sunlight irradiation over bare and noble metal-loaded TiO2. Experiments indicated that the supported noble metals act as a visible light absorber, assisting the separation of photo-charges and reduction of O2 to O2 −. The O2 − oxidizes mildly phenol to oxygenated products. In a parallel process, •OH radicals yielded by TiO2, mineralized phenol to CO2 by fast reaction sequences.

Photocatalytic removal of phenol under UV light was investigated by Mendoza−Damian et al. [102] by studying the effect of Sn4+ content on the SnO2-ZnAl LDH photocatalytic properties. The 0.3 mol% of the Sn4+−containing ZnAl LDH displayed the highest photocatalytic activity, with a phenol mineralization efficiency of 90.98%. The improved efficiency was due to a higher light absorption capacity and synergistic effect between the SnO2 and ZnAl LDH heterostructure.

Raciulete et al. [103] developed a multi-step ion-exchange methodology by exchanging Rb+ with Cu2+ spacer in the layered RbLaTa2O7 host to achieve photocatalysts capable of wastewater depollution. The photocatalytic degradation of phenol under simulated solar irradiation, employed as a model reaction, showed that Cu−modified layered perovskites displayed an increased photocatalytic activity compared to the RbLaTa2O7 host. Experiments demonstrated that the product intermediates over Cu-modified perovskites were hydroquinone (HQ), 1,2-di-hydroxy-benzene (1,2-DhBZ), and benzoquinone (BQ). Among the Cu−modified layered perovskites, the sample reduced at 800 ◦C was the most effective photocatalyst regarding the efficiency of phenol mineralization, yielding 2.82 μmoles h−<sup>1</sup> of CO2 and 1.78 μmoles h−<sup>1</sup> of H2.
