Synthetic and commercial TiO2 (Degussa P-25) have been used in antibiotic photodegradation processes, due to certain features such as the wide bandgap interval in the presence of ultraviolet radiation, as well as activity under visible light following modifications, including metal or non-metal doping or heterojunction formation (DE MATOS RODRIGUES et al., 2019; DÍAZ-SÁNCHEZ et al., 2021a; DURÁN-ÁLVAREZ, et al., 2016; LIN; WANG; XU, 2017; SHEHU IMAM; ADNAN; MOHD KAUS, 2018; TEIXEIRA et al., 2018ª; WANG; JIAN, 2015; YU et al., 2020).
In one study, a heterogeneous photocatalysis process employing TiO2-P25 in the presence of UVA irradiation was applied to the degradation of 40 mg. L-1 CIP, achieving 91% removal within 60 minutes of the reaction (ESTRADA-FLÓREZ; SERNA-GALVIS; TORRES-PALMA, 2020). The high efficiency of the photocatalytic process was associated with antibiotic adsorption on the catalyst surface, along with the amount and reactivity of singlet oxygen species (1O2) and the superoxide anion radical (O2•-) formed by the absorption of the applied UVA light (DÍAZ‐SÁNCHEZ et al., 2021). Performance was also associated to physicochemical CIP properties, as this compound contains molecules such as 4-quinolone, which converts CIP to the triplet state when absorbing light (260 – 300 nm), with a high molar absorption coefficient (4000 – 14000 M-1.cm-1), thus, contributing to antibiotic degradation efficiency (ALBINI; MONTI, 2003). The absorption spectra of quinolone antibiotics, especially CIP, exhibit a longer tail that overlaps the UVA lamp emission spectrum, favoring degradation by UVA radiation (ESTRADA-FLÓREZ; SERNA-GALVIS; TORRES-PALMA, 2020). High CIP degradation rates are also associated to photocatalytic behavior, as, quinolones, like CIP, can form a triple excitation state after light absorption, forming 1O2 and O2•- (ESTRADA-FLÓREZ; SERNA-GALVIS; TORRES-PALMA, 2020). The photocatalytic process can also be favored by the generation of excited electrons and holes (ESTRADA-FLÓREZ; SERNA-GALVIS; TORRES-PALMA, 2020).
Doping may be implemented to increase semiconductor photocatalytic activity, (DÍAZ‐SÁNCHEZ et al., 2021). The most applied dopants comprise cationic or anionic dopants or a mixture of both, thus taking advantage of the best characteristics of each (ISMAEL, 2020). Doping can cause an overlap of the 'p' orbitals present in the system, thus increasing the transport of charges to the semiconductor surface, increasing its photocatalytic capacity (DÍAZ‐SÁNCHEZ et al., 2021). In one study in this regard, heterogeneous photocatalysis for CIP degradation at 20 mg.L-1 was applied by synthesizing the catalyst F-TiO2-IL-[HT]-500, fluorine-doped titanium dioxide, achieving 90% degradation within 10 minutes (DÍAZ-SÁNCHEZ et al., 2021). The catalyst was prepared from mesoporous TiO2 nanoparticle aggregates doped with 5% F formed by the hydrolysis of titanium isopropoxide (TTIP) in an HNO3 solution mixed with an NaF solution (5% fluorine), forming F-TiO2-IL[HT]-500 following a calcination step at 500 °C (0.8 °C/minute for 16 h). The results indicated high F-TiO2-IL[HT]-500 potential for photocatalytic CIP degradation, with the synthesized material presenting an organic structure with good affinity for the catalyst. The catalyst preparation through a simple F-doping method increased the material’s surface area, with a higher reaction constant observed for the F-doped catalyst (1.4x 10-3 s-1) in relation to the non-doped one (5.2 x 10-4 s-1). Doping with F formed ultra-reactive facets that, in turn, increase photocatalytic activity, in addition to forming Ti-F groups on the catalyst surface that act as traps for photogenerated electrons, facilitating their transfer to molecules that adsorb O2 (DÍAZ-SÁNCHEZ et al., 2021).
Different TiO2 materials doped with F or with Cu/F were prepared in another study and applied to CIP (20 mg. L-1) photodegradation, through the modified sol-gel method, based on the acid hydrolysis of titanium isopropoxide (TTIP) (DÍAZ‐SÁNCHEZ et al., 2021). The results indicated 80% CIP degradation within 15 minutes employing TiO2-Cu in the presence of UV light (300 W). A correlation was observed between adsorption capacity, which indicates pollutant affinity to the catalyst surface, and photocatalytic activity. Catalyst characterization revealed an ideal morphology of the TiO2 nanoparticles produced by doping with Cu, as well as ultra-reactive faces with a decrease in the band gap value formed by the F doping. In addition, a synergistic Cu and F effect allowed for the stabilization of the anatase phase, favoring photocatalytic activity, resulting in high a degradation rate (DÍAZ‐SÁNCHEZ et al., 2021). Using the same doping principle, Ag, Au and Cu were deposited in mono and bimetallic forms on a TiO2 semiconductor in another study to degrade CIP (SHEHU IMAM; ADNAN; MOHD KAUS, 2018). In the beginning, 0.5 g.L-1 of pure TiO2 led to a CIP degradation rate of 0.057 min-1. Following doping with Au, Ag and Cu, achieved rates were 0.06, 0.117 and 0.072 min-1, respectively, with greater activity observed for the Ag and Cu doping compared to pure TiO2. This is due to the electronegativity of each metallic dopant, with Ag and Cu less electronegative, therefore displayin better potential to produce superoxide radicals when compared to Au (SHEHU IMAM; ADNAN; MOHD KAUS, 2018).
In another doping study, TiO2 was synthesized with a boron-doped diamond, forming the photocatalyst TiO2/BDD, which was then applied to the photodegradation of SMX (50 mg. L-1) in an aqueous solution under UV light (λ=222 nm) (SUZUKI et al., 2022). When the photocatalytic process was applied alone, SMX degradation reached 80% in 1 h, while simultaneous photocatalytic treatment with the TiO2/BDD, SMX increased SMX degradation to 100% within the same reaction time. This demonstrates the synergism efficiency of both two processes, in which photo-induced fragments were decomposed by the electrochemical treatment and the fragments formed during the electrochemical treatment were decomposed by the photocatalytic system. The photoelectrochemical doping treatment allowed for ROS formation, responsible for SMX degradation (SUZUKI et al., 2022). In another study, an association between titanium dioxide with zinc, and charcoal treated with acid (Zn-TiO2/pBC) was applied to evaluate SMX photodegradation (10 mg.L-1), reaching 81.21% SMX removal within 30 minutes. Compared with pure TiO2 or TiO2/pBC, the zinc-doped composite (Zn-TiO2/pBC) presented better photocatalytic activity, as zinc effectively inhibits TiO2 agglomeration, and hinders the recombination of photogenerated electrons and holes (XIE et al., 2019).
The deposition of metallic nanoparticles on the surface of catalysts has been proven a favorable method for the degradation of organic compounds in aqueous matrices (DURÁN-ÁLVAREZ, et al., 2016). Metallic nanoparticles are important in capturing visible spectrum radiation, due to the resonance effect on the surface and the fact that nanoparticles act as electron traps, dragging photo-produced electrons into the conduction band, thus decreasing electron-hole recombination. The electrons trapped in metallic nanoparticles are then able to reduce O2, producing ROS, such as superoxides and hydroxyl radicals (WANG et al., 2012).
Photocatalytic TiO2-P25 nanoparticles immobilized with polyvinylidene fluoride (PVDF) have also been applied to the degradation of CIP (5 mg L-1 ) (TEIXEIRA et al., 2018a), reaching 95% degradation in 72 hours, with poorly fixed nanoparticles on the coating surface contributing to decreased photocatalytic efficiency (TEIXEIRA et al., 2018b). Furthermore, the improved hydrophilicity of TiO2/PVDF nanocomposites in comparison to the pure polymer is caused by increased catalyst surface roughness (RAHIMPOUR et al., 2012), due to the presence of hydrophilic functional groups on TiO2 nanoparticles, such as the oxygen formed on the surface in the presence of light (CUI et al., 2021; KRISHNAN et al., 2022).
Another investaigtaion applied reduced graphene oxide nanocomposites with TiO2 (TiO2-rGO) in the presence of sunlight and H2O2 for SMX degradation (100 µg.L-1), (LIN; WANG; XU, 2017). The addition of H2O2 to the system containing only TiO2 increased SMX degradation about 4-fold compared to pure TiO2, as H2O2 captures and reacts with photoinduced surface electrons, thus suppressing electron/hole recombination (MOREIRA et al., 2018a), also reacting with superoxide anion radicals, leading to the formation of additional hydroxyl radicals and, therefore, favoring degradation (IKHLEF-TAGUELMIMT et al., 2020). Concerning the TiO2-rGO system, the removal efficiency decreased when H2O2 was added to the photocatalytic system. Degradation of TiO2-rGO can take place by H2O2 attacks to the underlying C-C bonds on the GO surface, due to surface defects, thus reducing degradation efficiency (WANG et al., 2018). The results indicate that the P25/H2O2 system followed by the P25 and TiO2-rGO photocatalytic processes presented the best SMX antibiotic removal efficiency rates in less than 4 hours of reaction (MOREIRA et al., 2018a).
To avoid one more step of catalyst separation from the effluent, reduced graphene oxide nanocomposites with TiO2 (TiO2-rGO) can be immobilized on different means of support, including optical fibers (LIN; WANG; XU, 2017). In this regard, one study degraded SMX (5 mg. L-1) under different catalyst concentrations according to rGO variations (0.3 to 2.7%) immobilized on an optical fiber. A 92% degradation rate was achieved for SMX in 180 minutes under UV irradiation (160 w) using 2.7% rGO. This good photocatalytic performance can be attributed to the adsorption of SMX molecules on the catalyst surface, due to the large specific surface area of TiO2-rGO immobilized onto the optical fiber, as surface areas can increase with the presence of rGO, thus promoting higher organic molecule adsorption (LIN; WANG; XU, 2017). Mixing rGO with TiO2 nanoparticles reduces the bandgap energy, leading to absorption of a wider UV and visible spectrum radiation range, resulting in greater photoactivity (MOREIRA et al., 2018a). Furthermore, the formation of a heterojunction interface in TiO2-rGO nanocomposites in the space charge separation region can increase degradation efficiency (WANG et al., 2018). After TiO2 photoactivation, electrons can transfer to rGO nanosheets and the photoinduced holes migrate to TiO2, causing e- and h+ recombination, increasing photon yields (LIN; WANG; XU, 2017).
In another study, SMX was degraded using a photocatalyst prepared by preparing heterojunction between TiO2and CeO2 doped with gadolinium (CeGdO2/TiO2) (DE MATOS RODRIGUES et al., 2019). The catalyst (CeGdO2/TiO2 0.1 g.L-1) applied to heterogeneous photocatalysis mediated by UV irradiation (Hg 15 W) at a 1:1 ratio capable of degrading 97% of SMX in an aqueous solution (25 mg.L-1) within 120 minutes. This performance can be attributed to the formation of a heterojunction characterized by the presence of mesopores, which aid the adsorption processes that take place on the surface of the photocatalyst, as well as the high formation of active sites resulting from the formation of the electron pair promoted by the excitation of electrons from the valence band to the conduction band through photons (HU et al., 2019b). The estimated activation energies of the materials were 3.12, 3.07 and 2.84 eV for Ce0.8Gd0.2O2-&, TiO2 and Ce0.8Gd0.2O2-&/TiO2, respectively, indicating a decreased pure sample value for the composite, characteristic of the formation of an intermediate phase between the valence and conduction bands, with the presence of two crystalline structures, due to the heterojunction of both structures (DE MATOS RODRIGUES et al., 2019).
In another assessments, a BiOCl/TiO2/sepiolite composite was developed for TET degradation (50 mg.L-1) in the presence of visible light (Xenon 400W) (HU et al., 2019b). The heterogeneous BiOCl/TiO2/sepiolite structure presented a reaction constant 9.20-fold greater than BiOCl, and 2.08-fold greater than TiO2, the pure catalysts. The surface area (BET) of the BiOCl/TiO2/sepiolite composite was 134.0 m2 g-1, and of the pure BiOCl and TiO2 catalysts, 2.9 and 59.4 m2 g-1, respectively. The synthesized composite presented greater activity compared to pure catalysts, indicating that the formation of a ternary heterostructure can increase adsorption, attributed to a greater surface area and pore volume, which can, in turn, provide more reactive sites (HU et al., 2019b).
A nanocomposite (FeNi3/SiO2/TiO2) was synthesized in another study to degrade TET (ranging from 10 to 30 mg.L-1) in a synthetic effluent in the presence of UV-Vis light. At 10 mg. L-1, TET was 100% degraded within 200 minutes (KHODADADI et al., 2018a). The surface morphology of the nanocomposite displayed a significant agglomeration trend due to its magnetic properties, aiding in higher TET catalyst adsorption, and consequently, greater degradation efficiency (KHODADADI et al., 2018b). In another assessment, porous catalysts formed by TiO2/Fe3O4 modeled with corn straw were synthesized and used for TET degradation (50 mg.L-1) in the presence of UVC light (254 nm, 10 W) at a catalyst concentration of 0.3g.L-1 (YU et al., 2019). A 98% TET removal rate was achieved in 60 minutes of reaction in the presence of the TiO2/Fe3O4 heterojunction and 83% removal was obtained employing the Fe3O4-H2O2 system under the same conditions. This indicates that the heterojunction interface significantly contributed to the photo-Fenton reaction and, consequently, to TET removal. An electron-spin resonance (ESR) spectrometer analysis indicated the formation of the reactive species OH• and •O2- in the system, responsible for TET degradation (YU et al., 2019).
In another study, a catalyst was immobilized onto the chitosan biopolymer with the aim of degrading TET and also contributing to cost reductions by eliminating TiO2(P25) suspended in water (IKHLEF-TAGUELMIMT et al., 2020). An 87% TET removal rate was achieved within 60 minutes of UVA irradiation reaction (360 nm) at 30 W, and a TiO2 (P25)/chitosan ratio of 2. A material characterization analysis revealed no change in bandgap potential comparing TiO2-P25 with the formed composite (TiO2 (P25)/chitosan). Further assessments employing DRX, FTIR, and DRS spectra also indicated no compound structure changes following immobilization with chitosan, maintaining the same TET degradation value of 87%. Chitosan is a very attractive biodegradable support from an environmental point of view, comprising an interesting alternative in the immobilization of TiO2-P25 under UV radiation (IKHLEF-TAGUELMIMT et al., 2020).
Semiconductor-based photocatalysis is superior to other alternatives, as it is an economical renewable clean technology, and may require only solar energy and suitable semiconductors, employed in various applications, including organic pollutant degradation (XUE et al., 2019a). In this context, tungsten oxide (WO), a semiconductor with a bandgap of 2.4 and 2.8 eV (WO3) has been recognized as a promising candidate, due to its physical characteristics and chemical properties, such as the acidity of the WO3 layer, which controls electron transfer rates, as well as an effective adsorption capacity, due to a high surface area (CABRERA et al., 2012). In this sense, one study applied the WO6 semiconductor in the formation of the photocatalyst g-C3N4/ Bi2WO6/AgI to degrade CIP (10 mg.L-1) in distilled water in the presence of visible Xe light (300 W) (XUE et al., 2019b). A photocatalytic performance of 77% was achieved within 120 minutes, attributed to a number of factors, such as the presence of g-C3N4 nanosheets, which allowed for the dispersion of Bi2WO6 and AgI nanoparticles, preventing agglomerations, as well as the addition of g-C3N4 (60%) and AgI (20%), promoting greater UV-Vis light absorption (420 nm, 300 W), and contact between Bi2WO4, g-C3N4 and AgI, favoring the separation and transfer of photogenerated electrons (WANG et al., 2019). These electrons resulted in a greater redox capacity of the generated composite, increasing its photodegradation efficiency concerning CIP (XUE et al., 2019b). The degradation of this same antibiotic (30 mg.L-1) has also been studied in distilled water and in drinking water (50 µg.L-1) employing AgBr/Ag/Bi2WO6 as the catalyst, achieving 100% CIP removal in drinking water and 57% in distilled water within 5 h under UVC irradiation (254 nm, 13, 5W) (DURÁN-ÁLVAREZ et al., 2019a). The high CIP degradation efficiency achieved in that study is related to the generation of ●OH radicals formed from the photocatalyst structure, confirming that the heterostructure acted as a completely solid state Z scheme, with highly oxidative holes capable of breaking water molecules to generate hydroxyl radicals (ZANELLA et al., 2018). The use of the WO semiconductor has also been applied to the formation of g-C3N4/Bi2WO6 (1 g.L-1) aiming at CIP degradation (15 mg.L-1). A 98% degradation rate was achieved in 120 minutes employing 40% (g-C3N4) and 60% (Bi2WO6) UV-Vís irradiation (MAO et al., 2019). This CIP degradation efficiency was linked to the nanocompound heterojunctions, which facilitated effective separation and accumulation of photogenerated electrons in the conduction band of g- C3N4 and spaces in the Bi2WO6 valence band, thus forming hydroxyl radicals and superoxide for CIP oxidation (MAO et al., 2019; XIA et al., 2018a).
Concerning SMX (10 mg.L-1), the WO semiconductor has also been applied to the formation of the photocatalyst SiO2/1,2-bis(triethoxysil)ethane (BTESE)/Na4W10O32 under UV-Vis light irradiation, (15 W.cm-2), reaching 90% degradation within 4 h of reaction (PASTI et al., 2018). The ability of the heterogeneous photocatalyst to degrade SMX is associated to the BTESE-induced hydrophobicity of the heterogeneous photocatalytic system (BLASIOLI et al., 2014). Furthermore, about 80% of the total pore volume is classified as mesopores, which are suitable as supports, with the decatungstate anion encapsulated (MOLINARI et al., 2013). This may facilitate the photoproduction of ●OH radicals, contributing to antibiotic photodegradation. Thus, pore structure, active site distribution, and hydrophilic-hydrophobic interactions with the substrate all comprise key factors for the catalytic activity and selectivity of heterogeneous catalysts (PASTI et al., 2018). Another study evaluated the photocatalytic activity of multi-walled carbon nanotubes (CNT) containing the semiconductor WO (W03-CNT) in the degradation of an SMX solution (10 mg.L-1) carried out in the presence of artificial sunlight (300 W, Xe) (ZHU et al., 2018). An 88.5% SMX degradation rate was achieved within 3 h of reaction. A photocatalytic activity improvement was obtained using the W03-CNT composite containing 4 mg of CNT (WT-4) compared to pure W03, detected by analyzing the UV-Vis spectra acquired from XRD analyses, where pure W03 presented a low absorption intensity in the range of 450 nm, while stronger composite absorption intensity was noted, demonstrating that the incorporation of CNT extended the light absorption capacity, improving the photocatalytic performance of the composite formed in the degradation of SMX. The band-gap of the composite (2.30 eV) was significantly reduced compared to pure W03 (2.80 ev), indicating a strong interaction between W03 and CNT (NATARAJAN et al., 2017). In addition to acting in CIP and SMX degradation, WO has also been used in TET degradation (20 mg.L-1), as a WO3 (5%)/ZnS catalyst in the presence of UV-Vis irradiation (300 – 1100 nm, 300 W, Xe) (MURILLO-SIERRA et al., 2021). The TET removal percentage reached 100% within 90 minutes of reaction, and WO3/ZnS photoactivity improved when both semiconductors were coupled, forming a highly efficient direct Z scheme (XIA et al., 2018b), as the catalyst morphology experienced a strong interaction between WO3 and the ZnS particle, as well as a better ZnS particle distribution due to WO3 sheet surface deposition (ZHU et al., 2018). Another important photocatalytic material characteristic is pore volume, as high pore volume content improves light and reagent penetration, increasing the mobility of charge carriers within photocatalysts (MURILLO-SIERRA et al., 2021). Improvements in the photocatalytic process of TET degradation (10 mg. L-1) have also been observed with the junction of semiconductors WO4 and g-C3N4 (Ag2WO4/g-C3N4) under visible light (SHI et al., 2019). A 76.9% TET removal rate was achieved within 180 minutes of reaction using a heterojunction formed by 3% Ag2WO4/g-C3N4 (3AW-CN), while only pure g-C3N4 or Ag2WO4 achieved only 36.8%, indicating that the composite formation significantly improved photocatalytic activity (KARUNAKARAN et al., 2014). The heterojunction catalyst exhibited a stronger absorption in the UV-Vis region compared to the pure compounds, and (3AW-CN) catalyst absorption in the visible light region was significantly increased compared to pure g-C3N4 after heterojunction formation, thus improving degradation efficiency (SHI et al., 2019). Similarly, another study assessed the formation of g-C3N4 (Bi2WO6/BiFeO3/g-C3N4) heterojunctions aiming at TET degradation (0.01 g.L-1) in a pharmaceutical effluent under UV-Vis irradiation (357 nm, Xe lamp, 500 W) (WANG et al., 2021). A higher degradation rate (83.68%) was obtained within 45 minutes of the reaction applying 0.1 g (BiFeO3/g-C3N4) added to the heterojunction (3AW/CN). This degradation efficiency is related to the photo-Fenton activity of the catalyst formed by the heterojunction with g-C3N4, which displays good conductivity, facilitating the transformation between Fe+2 and Fe+3 in the photo-Fenton system, forming radicals, and, thus, contributing to more efficient TET degradation (SHI et al., 2019). Due to the good conductivity of BiFeO3/g-C3N4, photogenerated charge carriers migrate rapidly between the valence band and conduction band in opposite directions, and, consequently, photogenerated electrons can produce O2-• radicals, which also contribute to TET degradation (WANG et al., 2021). Photogenerated electrons were also reported as being formed in TET degradation (20 mol.L-1) by a hybrid photocatalyst (AgBr/Bi2WO6) containing 30% AgBr in the presence of visible light using a Xe lamp (420 nm) (HUANG et al., 2019a). Photocatalyst characterization indicated a spherical structure similar to a flower, about 4 µm in diameter, mounted by numerous nanoplates presenting a single crystal structure and smooth, forming internal spaces of varying sizes that increased the catalyst surface area (8,286 m2 g-1) contact with TET (ZHANG et al., 2011). The high percentage (87.5%) of degradation within 60 minutes can be attributed to the synergistic effect between the electron mediator Ag nanoparticles and the heterojunction charge transfer mechanism, which increased light collection capacity, photogenerated electron separation efficiency and, consequently, the system’s redox capacity (WANG et al., 2021).
Metal vanadates exhibit unique chemical and catalytic properties (LIU et al., 2018). The anion VO4- bonded to a metal can facilitate photocatalyst radiation absorption from the visible spectrum, due to its narrow bandgap (approximately 2.0eV) (ZHANG et al., 2021). In this context, vanadate anions have been applied in catalyst doping in order to modify the crystalline lattice and the charge density of photocatalysts (LIU et al., 2018). In this sense, a photocatalyst synthesized with VO4- (CuS/BiVO4) was applied to CIP degradation (10 mg.L-1) in the presence of visible light employing a Xe lamp (300 W) (LAI et al., 2019). A CIP degradation rate of 86.7% within 90 minutes of the reaction was obtained, with the degradation efficiency associated to the employed photocatalyst (CuS/BiVO4) which made it possible to increase the range of visible light absorption, characteristic of the presence of V04- (LIU et al., 2018), as well as increased contact surface and more active sites, thus increasing photobleaching efficiency compared to pure BiVO4 (LAI et al., 2019).
A higher visible light absorption range due to the presence of the semiconductor VO4- was also observed in SMX degradation (20 mg.L-1) by a photocatalyst formed by peroxymonosulfate (PMS) plus FeVO4 (PMS/ FeVO4/VL) (ZHANG et al., 2021). A high degradation of 96.6% was achieved after 1 h, explained by the reaction system that mainly involves two reaction mechanisms, one comprising the transformation of Fe and V ions into FeVO4 (bandgap 2.03 eV), which responds well to visible light, and can be even more efficient in the presence of VL (LAI et al., 2019; LIU et al., 2018), producing photogenerated electrons, and h+ holes, followed by activation of PMS by the formed electrons, generating active species, such as SO4•-, O2•-, OH•, and 1O2, directly contributing to SMX degradation (ZHANG et al., 2021).
The same active species generation principle (SO4•-, O2•-, OH•, e 1O2 ) from the PMS present in the CuO-BiVO4+PMS photocatalyst participating in pollutant degradation was applied to TET degradation (80 mg.L-1) in the presence of UV light (300 W) (CHEN et al., 2021). Total TET degradation (100%) was observed within 50 minutes when photocatalysis occurred coupled to the oxidation process (CuO-BiVO4-2+Vis+PMS). This high degradation rate is associated to the significant synergistic effect between the photocatalysis and the PMS oxidation process, forming photogenerated electrons, and holes (h+), with the formed electrons activating the PMS and generating active species, such as SO4•-, O2•-, OH•, e 1O2 (CHEN et al., 2021; ZHANG et al., 2021).
Zinc oxide (ZnO) has been applied as an efficient semiconductor material in photocatalytic processes and oxidative applications in water due to its wide bandgap (~3.4 eV), high photosensitivity, stability and non-toxicity (LEE et al., 2019; ZHOU et al., 2020). Pure ZnO is catalytically active only in the presence of UV light, but its activity is considerably low due to the rapid recombination of photoinduced carrier charges. In this regard, the formation of heterostructures and their doping with other materials are currently being applied to reduce this deficiency (KOUTAVARAPU et al., 2021a). The addition of plasmonic gold nanoparticles to ZnO, forming ZnO/Au, for example, was evaluated in one study concerning CIP photodegradation (10 mg.L-1) (BOJER et al., 2017). A 1.4-fold higher reaction rate was observed in the presence of ZnO/Au compared to pure ZnO nanotubes. The best degradation efficiency was associated to the positively charged ZnO surface alongside doping with Au nanoparticles, which have a synergistic effect on photocatalytic CIP degradation due to the combination of the favored electrostatic CIP adsorption to the surface of the positively charged ZnO, along with the addition of Au nanoparticles to the structure (SUN et al., 2011). This increases the reaction speed compared with pure ZnO nanotubes, which present rapid photoinduced carrier charge recombination (BOJER et al., 2017; KOUTAVARAPU et al., 2021b).
Aiming to improve ZnO features, this compound was synthesized by a hydrothermal process assisted in acidic conditions and then applied to the degradation of SMX (10 mg.L-1) under UVA irradiation in ultrapure water (MAKROPOULOU et al., 2020). The photocatalytic process was also carried out with commercial ZnO, resulting in 99% SMX degradation in 40 min, while 84% degradation in 60 min was achieved with the synthesized ZnO (MAKROPOULOU et al., 2020). The application of ZnO nanostructures synthesized under acidic conditions improved the photocatalytic result, due to the higher relative concentration of lattice defects, enabling better charge transport, and, consequently, better degradation efficiency (NKOSI et al., 2014). However, the observed differences in photocatalytic activity may be linked to the greater adsorption capacity of the antibiotic rather than that of commercial ZnO compared to the synthesized ZnO, as the synthesized ZnO underwent a thermal treatment, reducing its active surface area (BOJER et al., 2017).
The effect of ZnO doping was evaluated concerning TET degradation (4 mg. L-1) in distilled water using a synthesized ZnO nanocomposite (NaBiS2/ZnO) in the presence of sunlight (KOUTAVARAPU et al., 2021a). The percentage of TET degradation reached 100% in 90 min, and the increase in the mesoporous channels of the formed nanocomposite facilitated the rapid transport of ions, improving photocatalytic activity and TET degradation (KOUTAVARAPU et al., 2021a; LEE et al., 2019; ZHOU et al., 2020). This high degradation efficiency can also be attributed to the separation of the photogenerated electron-hole pairs, due to the formation of the heterostructure, which may have improved charge transport through synergistic interfaces (LEE et al., 2019; ZHOU et al., 2020).