A review on heterogeneous oxidation of acetaminophen based on micro and nanoparticles catalyzed by different activators

3.1 Mechanism by nanoparticles catalyzed by UV and visible light

Semiconductor nanoparticles capable to generate a hole (h⁺) and electrons (e⁻) after illuminated with UV or visible light make it the most promising oxidation process as electrons act as reduction agents, whereas holes act as oxidation sites [2]. The mechanism of semiconductors that catalyzed by irradiation, based on photoexcited of the electrons that exist on the catalyst surface, leading to the movement of electrons (e⁻) from valance band to conduction band leaving positives holes (h⁺). Both electrons and holes can start the redox reactions and oxidize ACT. The mechanism of nanoparticles catalyzed by UV and visible light is depicted in Figure 2.

Figure 2

(a) The reaction between irradiation waves and the surface catalyst and (b) reaction leading to the movement of electrons (e⁻) from valance band to conduction band leaving positives holes (h⁺), which may react directly with ACT as shown in equation (17).

The equations (13)–(18) represent the possible chemical reactions of TiO₂ catalyzed by UV or visible light system [35].

Some disadvantages have been observed related to photodegradation in the presence of semiconductor TiO₂. For example, the high rate of recombination of electrons–holes leading to minimize the degradation of ACT, a wide bandgap of TiO₂, low ionic and electrical conductivity, slow charge transfer rate limits the quantum efficiency of TiO₂ in the photocatalytic reactions, limited adsorption capacity and porosity, and lower efficiency under solar irradiation restricts the application of this system. To decrease the bandgap and enhance the absorption of irradiation of surface catalyst, the researchers have doped semiconductors with transition metals (Co, Fe, Ni, Ag, Au, Cu, Mg, Pt, Zn, Si, and Al) and nonmetals (N and Cl) [17,36,37,38,39,40,41,42,43]. Kohantorabi et al. [38] revealed that ACT was completely degraded after 15 min of reaction, and the mineralization was 63% within 1 h when 1.0 % w / w Ag / ZnO @ NiFe 2 O 4 / PMS / UVA was applied. In addition, Yang et al. [44] examined TiO 2 nanoparticles activated by UVC to degrade ACT from a liquid medium. In this system, the initial concentration of ACT was 2.0 mM, and the catalyst concentration was 0.4 g/L. The degradation of ACT was 96% after 80 min of reaction at a pH range of 5.1–3.2. Moreover, Montenegro-Ayo et al. [45] applied TiO 2 nanoparticles/UV/0.02 M Na 2 SO 4 system to oxidize ACT. Around 72% of ACT was degraded after 300 min. Sun et al. [46] applied 0.5 mM N/S codoped ordered mesoporous carbon to catalyze peroxymonosulfate (PMS) for ACT degradation. Around 50 mg/L of ACT was totally oxidized within 30 min, and the mineralization was 27%. The kinetic reaction of ACT in this system was 2.4 × 10 − 1 /min. Wang et al. [47] examined BiOCl / UV / Persulfate system to oxidize ACT from an aqueous medium. The results showed that 50 µM of ACT was completely degraded within 150 min, and the mineralization was 83% after 180 min at pH = 5.4. The kinetic reaction was 7.13 × 10 − 4 /min. The studies that applied semiconductor as a catalyst and activated by UVA or UVC to degrade ACT from an aqueous medium have summarized in Table 1.

As the catalyzing of nanoparticles by using UV is costly, to avoid that, the researchers intensified on an alternative photocatalytic method such as visible light. As mentioned, TiO₂ powder has a limited absorption capacity of solar light: just around 5% could be absorbed by TiO₂ powder. To enhance the optical absorption of the catalyst, the researchers examined different approaches to improve the TiO₂ performance. Gómez-Avilés et al. [2] used C-modified TiO 2 nanoparticles/solar irradiation system for ACT degradation. In this system, ACT was completely removed within 60 min, and the mineralization was 20.4% after 120 min. Da Silva et al. [40] studied the degradation of ACT by using 2 g/L of 25 % MgO doped with TiO 2 catalyzed by the solar light system. About 48.3% of ACT was degraded within 1 h at pH = 7. Furthermore, Aziz et al. [43] applied 5 wt% of TiO 2 doping with SiO 2 -ZSM-5/visible light system to remove ACT from an aqueous medium. The results showed around 96% of ACT was oxidized after 180 min, and the mineralization was 77.8%. In addition, 0.1 wt% Cu-doped TiO 2 /visible light system was applied by Lin and Yang [48]. The results indicated that 50 mg/L ACT was completely decomposed after 3 h of reaction at pH = 6, and the degradation rate was 0.0243/min. Also, Feng et al. [49] examined oxygen vacancies and phosphorus codoped black titania-coated carbon nanotube composites (OVPTCN) activated by a visible light system to degrade and mineralize ACT from a liquid medium. The results were 96 and 20.4% of degradation and mineralization, respectively. Table 2 includes the studies that investigated the semiconductor particles/visible light systems to remove ACT from an aqueous medium.

3.2 Mechanism by nanoparticles catalyzed by oxidants

Many studies investigated the oxidation of ACT by using synthesized particles in the presence of oxidants. For example, Ikhlaq et al. [50] examined the oxidation of ACT by zeolite/O₃ system. They proposed that both ACT and O₃ were adsorbed on the surface of zeolite then react with each other, which supports that the nonradical mechanism was dominant. Figure 3 illustrated the adsorption of oxidant and ACT on the catalyst surface, then the oxidant attacked ACT, which resulted in the degradation of the pollutant via the heterogeneous or nonradical mechanism.

Figure 3

(a) Both ACT and O₃ were adsorbed on the zeolite surface, (b) O₃ was attacked with ACT, and (c) both O₃ and ACT were oxidized.

In addition, heterogeneous and homogeneous could happen together. Mashayekh-Salehi et al. [3] applied MgO/O₃ system to oxidize ACT in an aqueous solution. They proposed the following chemical equations (19)–(25) that might happen while the degradation reaction is running.

• Direct oxidation with O₃ molecules on MgO surface:

  (19) MgO − O 3 + ACT → H 2 O 2 + CO 2 + Intermediates

  (20) MgO − ACT + O 3 → H 2 O 2 + CO 2 + Intermediates

• Radical type catalytic oxidation on MgO surface:

  (21) MgO − S + O 3 → MgO O ˙ + O 2

  (22) MgO − S O ˙ + 2 ( H 2 O ) + O 3 → MgO − S OH ˙ + 3 ( OH ) ˙ + 2 O 2

  (23) MgO − S ( OH ˙ ) 2 + ACT → H 2 O 2 + CO 2 + Intermediates

• Direct oxidation with O₃ molecules in the bulk solution:

  (24) O 3 + ACT → H 2 O 2 + CO 2 + Intermediates

• Radical type catalytic oxidation in the bulk solution:

The symbol S in the Eqs. (21)–(23) represents Lewis acid sites on the surface of MgO composites, which were available for reacting with ozone. Hydrogen peroxide (H₂O₂) has been used as an oxidant to promote the degradation of ACT. For example, the doping of bimetallic iron–copper in the presence of S 2 O 8 2 − and H₂O₂ has been applied.

The Eqs. (26)–(31) represent the main chemical equations of generating hydroxyl radicals onto iron and copper oxide surfaces [51]:

The possible reactions in the presence of PS are presented in Eqs. (32)–(36) [52]:

The possible reactions Eqs. (37)–(39) in the presence of PMS are as follows [53]:

Furthermore, in the systems that relay on PS and PMS as a catalyst, the reaction may go further to produce hydrogen peroxide ( OH ˙ ) as in the following Eqs. (40)–(44) [5].

Table 3 lists the studies that applied metals and semiconductor particles catalyzed by oxidants to remove ACT from the aqueous medium.

4.1 The influence of ACT concentration

Major studies that have applied AOP systems in the presence of synthesized particles for ACT oxidation pointed out that when the ACT concentration increases, the degradation efficiency decreases. For systems based on oxidants such as hydrogen peroxide, PS, and PMS, high ACT concentrations may adsorb and cover a wide number of the active sites on the catalyst’s surface, consequently, suppressing the production of super oxidant radicals. Also, for the systems that depend on UV or visible lights as a catalyst, a high ACT concentration may accumulate on the catalyst surface and prevent the penetration of the irradiation, which may reduce the photocatalytic efficiency. For example, Yang et al. [44] applied TiO₂ catalyzed by UVA and UVC. They carried out different ACT initial concentrations from 2 to 10 mg/L. The removal decreased from 95% to less than 20%. Montenegro-Ayo et al. [45] studied the changes in the degradation rate when the initial concentration of ACT increased from 5 to 50 mg/L. The results showed a decrease in the oxidation rate from 2.05 × 10 − 4 to 2.86 × 10 − 5 , respectively. Also, Tan et al. [54] examined TiO₂ activated by UV system to eliminate ACT from liquid medium. The oxidation of ACT was declined from 72.7 to 40.2% when the ACT initial concentration increased from 0.017 to 0.067 mM, respectively. Furthermore, Kurniawan et al. [55] applied photocatalytic of BaTiO₃/TiO₂ composites to remove ACT; 5 and 25 mg/L of initial ACT concentration were implemented, and the results were a decline in the degradation efficiency from 81 to 19%. The same results were observed when Yaghmaeian et al. [56] carried out modified MgO nanoparticles catalyzed by ozone. When the initial concentration of ACT was 10, 50, 100, and 200 mg/L, the removal was 99.5, 99.4, 77, and 45%, respectively. However, Fan et al. [57] observed that when the initial ACT concentrations were 0.5, 1.0, and 1.5 mg/L at Ag/AgCl@ZIF-8/visible light system, the removal efficiency was stable at 99% after 60 min. They reported that when the concentration was between 0.5 and 1.5, the reaction rate reached the fastest at 1.5 mg/L. However, when the initial concentration exceeds 2.0 mg/L, the reaction rates decrease because the permeability of the photon would reduce when the substrate concentration was too high.

4.2 The influence of semiconductor and metal dosages

Most studies agreed that when the catalyst concentration increases to a certain level, it may benefit and increase the degradation reaction. However, if an excessive amount of catalyst adds, that maybe affected adversely on the degradation performance, or at least the degradation performance stays similar. For systems based on photocatalytic, that might interpret because, at high catalyst concentrations, the agglomeration and the shielding effect of the suspended catalyst is due to increased turbidity and impedes the light penetration, which reduces the accessible light to the catalyst surface resulted decreasing in the photocatalytic. For AOP-nanocomposite systems that based on irradiation activation such as hydrogen peroxide, PMS, and PS, high catalyst concentration leading to an increase in the number of activated radicals, which leads to the self-consumption of generated radicals. Below are some studies that investigated different catalyst concentrations. Hassani et al. [39] carried out different concentrations of CoFe₂O₄/mpg-C₃N₄ nanoparticles to activate PMS when the catalyst concentration increased from 10 to 40 mg/L, the removal enhanced from 53 to 85% within 25 min. Although the concentration increased from 40 to 60 mg/L, the oxidation was still the same. Zhang et al. [52] implemented iron–copper bimetallic/PS system to degrade ACT and observed improvement in the degradation percentage at catalyst concentrations between 0.1 and 0.3 g/L, the removal increased from 38.6 to 90%. When 0.4 g/L of catalyst was added, there was no significant change in the degradation performance. Thus, it might be because the excessive amount of SO 4 ˙ − increases the self-consumption as Eq. (45):

Furthermore, MnFe₂O₄/PMS and CoFe₂O₄/PMS systems were applied by Tan et al. [58], and the results were 96.7, 100, 100, and 100% for MnFe₂O₄/PMS system and 61, 99.5, 100, and 100% for CoFe₂O₄/PMS at 0.1, 0.2, 0.3, and 0.4 g/L, respectively. Cheshme Khavar et al. [59] mentioned the effect of catalyst dosage when the LED/titanium dioxide doped with graphene oxide (TiO₂@rGO) system was applied to remove ACT. They implemented TiO₂@rGO concentrations from 0.4 to 4 g/L. It observed that when the catalyst concentrations were 0.4, 1, and 2 g/L, the removal was 53, 57, and 81%, respectively. Meanwhile, when the concentration of TiO₂@rGO increased from 2 to 4 g/L, there was no improvement in the degradation of ACT. Moreover, Abdel-Wahab et al. [60] examined the TiO₂/Fe₂O₃/UV system that when the catalyst concentration increased from 0.1 to 2 g/L, the reaction rate was strongly affected, while concentrations between 0.1 and 1.2 g/L, the removal rate increased because the number of active sites and activated radicals increased. Hence, at 2 g/L of TiO₂/Fe₂O₃ was applied, the degradation rate was declined. Also, different K₂S₂O₈-doped TiO₂ dosages have been applied. From 0.25 to 0.5 g/L concentration, the oxidation of ACT increased from 90 to 100% after 9 h. When the dosage increases from 0.5 to 1.5 g/L, the removal was kept around 100% and reached the fastest reaction rate at 1 g/L. While at 2 g/L was applied, the degradation dropped from 100 to 97% [61].

4.3 The influence of pH

pH is a significant factor in the AOP based on semiconductors and metal systems. The effect of pH in (AOP/composites) system for the degradation of ACT is widely investigated. It has been observed that ACT has two chemical forms depending on the pH: (i) nonionic form when the pK _a is under 9.4 and (ii) ionic form when pK _a is more than pH 9.4. In addition, pH_zpc and the type of composite were the main variables to define the oxidation performance of ACT [62]. For example, Ziylan-Yavaş and Ince [17] studied the degradation of ACT by using Pt-supported nanocomposites of the Al₂O₃/O₃ system. The results showed that at the base and neutral pH conditions, ACT was eliminated in 7 and 10 min, respectively. The carbon mineralization increased when increasing the pH because the consumption of ozone increased with increasing pH. That might be attributed to the increase in O₃ that converted to OH ˙ at high pH atmosphere. The impact of pH between 4 and 10 on the oxidation of ACT by using 1.0% w/w Ag/ZnO0.4@ NiFe₂O₄/PMS/UVA system was investigated. The best result was obtained at pH between 6 and 7. Because at this pH, the HSO₅ ⁻ ions can be attracted to the positive surface of the catalyst, which improve the oxidation efficiency by the production of OH ˙ /SO 4 ˙ − . At acidic conditions, the activation of PMS was decreased due to the H-bond formation between H⁺ and O–O group of HSO₅, which declines the ACT removal [38]. Hassani et al. [39] studied the impact of pH from 3 to 11 on the oxidation of ACT by using the CoFe₂O₄/mpg-C₃N₄ system. They revealed that the best result was obtained when the pH was 7. Both acidic and base conditions were not favorable for degradation in this system. In the acid atmosphere, the oxidation of ACT was not good for two reasons because the activation of PMS was decreased due to the H-bond formation between H⁺ and O–O group of HSO₅ ⁻, which decreased the ACT removal, and both SO 4 ˙ − and OH ˙ react with H⁺ resulted in reducing in the degradation of ACT. In alkaline conditions, there were some possible reasons responsible for decreasing ACT degradation: (1) converting SO 4 ˙ − radicals to OH ˙ species with relatively lower redox potential by OH⁻ ions, (2) formation of metal hydroxide complexes of CFNPs and subsequently decrease of PMS decomposition reactions, and (3) self-decomposition of PMS to water and sulfate ions. Ling et al. [41] examined the influence of pH on the mineralization of ACT when (solar/4% Ag-g-C₃N₄/O₃) system was implemented. They applied the following pH: 3, 5, 7, 9, and 11. From pH 3–7, the degradation of ACT was enhanced because the increase of pH value increases the conversion of O₃ to OH ˙ , which degrades more ACT effectively when the pH increased from 7 to 11, the removal of ACT kept constant because at these pH values, the Ag-g-C₃N₄ and ACT have the same charge, and the repulsion force was dominant. Sun et al. [46] applied pH from 3.5 to 9 on N/S codoped/PMS system. The results showed that when the pH increased, the degradation was improving because the high concentration of OH⁻ caused the decomposition of PMS to produce SO 4 ˙ − . Ikhlaq et al. [50] studied the oxidation of ACT by using a zeolite/O₃ system at pH 3, 7, and 10. In this study, the optimum pH for zeolite/O₃ was 7.12 because, at this pH value, the ACT and hydroxyl groups on the zeolite surface were protonated. However, at pH 3 and 10, the oxidation of ACT decreased because ACT and zeolite at this pH have the same charge. Zhang et al. [52] studied the pH in the iron–copper bimetallic system activated by PS to remove ACT. The best pH values were between 5 and 7. The strong acid and alkaline conditions were not favorable for this system. Moreover, the effect of pH in Fe₃O₄ magnetic nanoparticles investigated by Tan et al. [53]. They reported that there were two effects of pH on the experiment: (1) Different PMS fractions would be affected by pH. In acidic conditions, HSO₅− predominated, while SO₄− predominated in alkaline conditions. (2) The electrostatic point of Fe₃O₄ had a pH effect on the catalyst surface charge of 7.3. At acidic pH, less PMS could catalyze on the catalyst surface because of the inhabitation effect of pH the H-bond formation between H⁺ and O–O group of HSO₅ and positively charged catalyst surface. Kurniawan et al. [55] examined a wide range of pH values from 3 to 11 for ACT degradation using BaTiO₃/TiO₂/UVA. From pH 3 to 7, the degradation enhanced from 7 to 95%, and the optimum value for ACT oxidation was 7; meanwhile, in alkaline conditions, the degradation decreased from 95 to 54% because both the catalyst and the ACT molecules had negative charges in alkaline conditions. As a result, the catalyst’s surface was repelled to the negatively charged ACT molecules, leading to a low ACT removal. Yaghmaeian et al. [56] investigated the influence of different pH to oxidize ACT by using modified MgO nanoparticles catalyzed by ozone. They observed that the consumption of ozone was related to the increasing of pH: when the pH increased from 2 to 8, the consumption of ozone increased from 17 to 41.5%, and from 75 to 90% when the pH increased from 9 to 10. Also, more O₃ consumption means more OH production. OH ˙ , has a high oxidation potential, which is more than O₃. The best pH value in this system was the natural pH solution close to 5.4; at this pH, the ACT molecule was mostly in its molecular form and could better interact with OH. In acidic conditions, there was no O₃ converted to OH ˙ enough. In the alkaline conditions, the isoelectric point of modified MgO and ACT were 10.4 and 9.4, respectively, which means the catalyst m-MgO significantly promoted the decomposition of O₃. In addition, Fan et al. [57] mentioned the influence of pH on the Ag/AgCl@ZIF8 system for oxidation of ACT. In this system, the optimum pH value was 5. The pH values between 7 and 9.4 were not desired because of weaker electrostatic integration between ACT and Ag/AgCl@ZIF8. For pH, more than 9.4 any pH values less than 7 were favorable for degradation ACT in this system. However, they noted that Ag/AgCl@ZIF8 dissolved in strong acid, which decreased the efficiency of this system for ACT removal. The influence of pH for decomposition of ACT in TiO₂@rGO nanoparticle system was studied by Cheshme Khavar et al. [59]. They revealed that when the pH increased from 4 to 9, the degradation of ACT promoted from 68 to 93%, respectively. Thus, it can be explained that at the natural pH solution, the surface catalyst has a negative charge, the anion species could not adsorb at the catalyst surface, allowing higher functional group interaction of OH⁻, which resulted in a high amount of OH⁻ converting to OH ˙ and finally enhanced the degradation of ACT. Zhang et al. [63] observed that when the pH increased from pH 3 to 6.5, the degradation efficiency increased from 80 to 91%, respectively. In the acidic atmosphere, the oxidation of Fe²⁺ to Fe³⁺ was slower, according to the increasing of H⁺, which inhabited the catalyst, resulting in a decrease in the degradation of ACT. When the initial pH value was 11, it has been observed that the pH decreased to be 3. They explained that because PS produced a large amount of H⁺, the pH was drastically reduced. At pH 11, the OH⁻ molecules combined with Fe²⁺ caused a rapid reduction of Fe²⁺, resulting in an insufficient coupling effect of Fe²⁺ and CuO, thus limiting the oxidation of ACT. Peng et al. [64] carried out different pH conditions. They revealed that pyrite/PDS could apply for a wide pH range, whereas pyrite/H₂O₂ in a narrow range. When pH 6 was implemented, the degradation of ACT in the pyrite/PDS system was 50%, whereas 0% when pyrite/H₂O₂ system was applied. Also, at pH 8, the results were 10 and 0%, respectively. It is believed that pH playing a central role in the determination of radical species of PDS, from pH 2 to 7 SO 4 ˙ − , from 8 to 10 OH ˙ /SO 4 ˙ − , and from 10 to 12 OH ˙ was dominant. Also, Dong et al. [65] applied pH 3, 9, and 12. The results were 98.5, 79.5, and 13.5%, respectively. They observed, when the initial pH was 12, the pH value was decreasing to 3. This, because SO 4 ˙ − would react with OH⁻ and H₂O resulting in the removal of H⁺ and consumption of OH⁻. In the base conditions, OH⁻ combined with Fe²⁺ to form oxyhydroxides and leads to precipitation. The absence of OH⁻ leading to insufficient activation of PS, which was due to a decrease in the degradation of ACT. Zhang et al. [66] also mentioned the influence of pH by using a S-doped graphene/Pt/TiO₂ system. When pH value increased from 4, 8, and 10, the removal was 99.9, 95.3, and 95.1%, respectively. In acidic conditions, hydroxyl radical behaved like a weak acid and reacted with hydroxyl ions under neutral and alkaline conditions, which was due to decreasing degradation reaction. The effect of pH on green rust coupled with Cu(ii) has been reported by Zhao et al. [67]. They investigated three systems GR_SO4/Cu(ii), GR_CO3/Cu(ii), and GR_Cl/Cu(ii), and it was observed that when pH 6 was applied on GR_SO4/Cu(ii), and GR_Cl/Cu(ii), the pH declined to 4, and 5.4, respectively. However, when pH 6 was applied to GR_CO3/Cu(ii), the pH increased to 6.4 and then decreased to 4.2. This pattern may be due to the buffering effect of CO₃ in this H⁺ system. Hydrolysis due to the rise in the pH, but as more accumulated, the buffering ability was exceeded, and then the pH decreased. When the pH decreased in the efficiency of GR_SO4/Cu(ii) and GR_CO3/Cu(ii) decreased from 100 to 82% and 84 to 28%, respectively.

4.4 The influence of scavengers

There are two mechanisms for the AOP: heterogeneous and homogeneous. In the mineral-based catalyst systems, oxidation mainly occurs on the catalyst surface; therefore, the heterogeneous reaction pathway is dominant. The mechanism of AOP is complex because some radical species are generating in parallel or series. To identify the radicals that are responsible for the degradation of ACT, the researchers added some substance called scavengers or quenching agents acting to trap the activated radicals: after adding these scavengers, the oxidation significantly declines, which means the radical trapped is responsible for the oxidation process. Table 4 represents the radicals generated in the AOP-synthesized particle-based system for degradation of ACT. Many scavengers such as isopropanol (IPA), tert butyl alcohol (TBA), methanol, salicylic acid, benzoquinone (BQ), KI (potassium iodide), triethanolamine (TEOA), ammonium oxalate, ethylenediaminetetracetic acid disodium (EDTA-2Na), ethanol (EtOH), sodium oxalate, N₂, 4-hydroxy-2,2,6,6 tertamethylpiperidinyloxy (TEMPOL), and l-histidine (l-his) have been applied.

4.5 The influence of doping ratio

The impact of the doping ratio in the synthesized particles, such as TiO₂@rGO, Ag-g-C₃N₄, ZSM-5/TiO₂, BaTiO₃/TiO₂, La-doped ZnO, TiO₂/Fe₂O₃, Fe₃O₄@SiO₂, Mg/SiO₂, and iron-copper bimetallic doped with silica (Cu/Fe₃O₄@SiO₂), for the ACT removal has been investigated. Most of these studies agreed that when the doping ratio increases, the degradation of ACT increases. But if the doping ratio increased above the threshold, it might negatively impact the degradation performance. For example, Ling et al. [41] applied the Ag-g-C₃N₄ system to degrade ACT. The excessive amount of Ag might be accumulated on the g-C₃N₄ nanoparticle surface and cover the active sites, which increase the recombination of photogenerated charges. Lin and Yang [48] examined Cu-doped TiO₂ to eliminate ACT, many doping ratios were applied, 0.1, 1, and 10 wt%. The best result was 100% of ACT degradation within 3 h by using 0.1% of Cu. They revealed that when the Cu ratio increased, it may generate isolated CuO aggregates expelled from the Cu–TiO₂ framework to the pore channels, which may act as a center of charge recombination and decline the mass transport between the pore channels. Also, Kurniawan et al. [55] prepared BaTiO₃/TiO₂ composites catalyzed by solar irradiation. Three different ratios of BaTiO₃/TiO₂ were applied, composite-A (1:3), composite-B (1:1), and composite-C (3:1). After 4 h of reaction, the results were 76, 39, and 26%, respectively. Cheshme Khavar et al. [59] applied TiO₂@rGO nanocomposites catalyzed by UVA to oxidize ACT. Different doping ratios starting from 0, 1, 3, 5, 7.5, and 10 wt% of rGO were applied, and the degradation of ACT was 53, 83, 100, 87, 76, and 70%, respectively. These clearly showed that 3 wt% was the best doping ratio in the TiO₂@rGO system. If the doping ratio exceeds 3 wt%, the degradation efficiencies begin to decrease because a large amount of ACT adsorbs on the catalyst surfaces, which due to some occupied active sites of TiO₂ resulted in decreasing the UV light that reaches to TiO₂ surface and reduces the photocatalytic activity. In addition, when the UV transmission decreased by TiO₂, it might increase the recombination rate. Thi and Lee [68] observed that the presence of La doped on ZnO nanoparticles enhanced the photocatalytic activity and reduced the bandgap energy when applied 0.5 and 1.0 wt% La-doped ZnO. Meanwhile, too much adding like 1.5 wt% of La may adversely affect the system performance and increase the bandgap energy because, in each semiconductor, there is an energy level named Fermi, which is the highest energy level occupied by electrons in a particular site. In ZnO, the Fermi level is between the conduction band and valance band. When more than 1.0 wt% of La doped onto ZnO, the bandgap was increased because Burstein–Moss effect. These states could push the Fermi level to a higher energy position, and then the Fermi level would lie in the conduction band the process depicted in Figure 4.

Figure 4

(a) Band gap narrowing of 0.5–1.0 wt% La-doped ZnO, and (b) band gap broadening for 1.5 wt% of La-doped ZnO.

Aziz et al. [69] studied TiO₂ doped onto fibrous silica ZSM-5 system catalyzed by solar light to oxidize ACT. Different doping ratios starting from 1, 3, and 5 wt%, of TiO₂ were carried out, and the results were 65, 90, and 71%, respectively. The results indicated that 3 wt% was the best doping ratio in the ZSM-5/TiO₂ system. They explained that TiO₂ might agglomerate on the surface of fibrous silica and cover the active sites, which caused low penetration of the visible irradiation. This effect was detected when electron carrier concentration exceeded the conduction band edge density of the state. However, some studies did not observe any decreases with an increase in the doping ratio. For example, Da Silva et al. [40] applied Mg/SiO₂/UV system. Many Mg concentrations experimented with to oxidize ACT, 1, 2, 10, and 25 wt%. The best result was 60% of ACT was removed within 60 min obtained when 25 wt% Mg concentration was applied. Moreover, Abdel-Wahab et al. [60] applied TiO₂/Fe₂O₃ with different TiO₂ ratios, 15, 33, and 50 wt%. They observed that when the concentration of TiO₂ increased from 15 to 50%, the degradation increased from 52.5 to 98%, respectively. The improvement of ACT degradation could be correlated to the effective separation of photogenerated electron–hole pairs accomplished by a combination of narrow bandgap Fe₂O₃ with wide bandgap TiO₂. In addition, the oxidation rate of ACT was accelerated proportionally when increasing copper concentration from 0 to 1% in Cu/Fe₃O₄@SiO₂ system, and the degradation increases from 59.2 to 100%, respectively. That was because ACT could quickly and efficiently adsorb on the catalyst and thus increase the catalyst activity. Do et al. [51] observed that when the molar ratio of Cu increased from 2.1 to 2.94, the reaction rate begins to slow down because of the dispersion morphology of Cu nanocomposites of the surface of iron doped with silica (Fe₃O₄@SiO₂).

4.6 The influence of oxidants dosage

The influence of addition and the concentrations of PMS, PS, oxygen (O₂), ozone (O₃), and hydrogen peroxide (H₂O₂) have been widely investigated. All the studies agreed that the addition of oxidants enhances the degradation and the reaction rates of ACT. However, when an excessive amount of oxidant is added, it may impact adversely on the degradation performance. This attributed to many reasons: (1) the excessive amount of oxidants generate more radicals, these radicals may consume each other as shown in the Eqs. (46–50):

(2) The limitation of active sites on the catalyst surface according to the presence of a high concentration of oxidant, and (3) if the excessive concentration of H₂O₂ was added, the generated hydroxyl radicals may react with H₂O₂ to produce HO 2 ˙ HO₂˙, which contributed less oxidation potential than OH ˙ . Several studies investigated the addition of oxidant in AOP based on composites systems. Ziylan-Yavaş and Ince [17] observed that when the ozone flow increased from 3, 6, and 9 mg/min on Pt/Al₂O₃/O₃ system, the oxidation was enhanced, and 9 mg/min contributed the best ozone flow in this system. That may refer to the excessive amount of H₂O₂ produced from the oxidation of ozone, which in turn increases the OH ˙ that attacks ACT resulting in an increased ACT degradation. Hassani et al. [39] also examined different PMS dosages in CoFe₂O₄/mpg-C₃N₄ catalyzed by PMS to degrade ACT. At 0.5 mM of PMS, the degradation efficiency was 60.9% after 25 min reaction. At 1.5 mM of PMS, the efficiency increased to 92%. They mentioned that higher PMS concentration was not favorable in this system because of the reasons mentioned above. Moreover, Sun et al. [46] examined many concentrations of PMS on N/S codoped ordered mesoporous carbon system. When 0.25, 0.5, and 1.0 mM of PMS were applied, the k values increased from 2.0 ± 0.04 × 10 − 2 to 2.4 ± 0.06 × 10 − 1 and 3.7 ± 0.2 × 10 − 1 , respectively. Also, Wang et al. [47] pointed out the addition of Na₂S₂O₈ and H₂O₂ on the BiOCl/UVA system. When Na₂S₂O₈ was added, the degradation rate and the mineralization were accelerated and enhanced. They attributed the improvement to three reasons: (1) the direct reaction between the photon and PS molecules, which results in generating sulfate radicals, (2) also, PS may react with conduction band electrons yielding the formation of sulfate radicals, and (3) sulfate radicals may generate from the reaction of O 2 ˙ − with PS. However, when H₂O₂ was added, the total organic carbon (TOC) removal was slowed down because the higher O–O the bond energy of H₂O₂ compared with the band in the free PS ions under the natural pH might interpret why PS was easier to activate than H₂O₂. Zhang et al. [52] studied the effect of different PS dosages, from 0.2 to 0.6 g/L, on the Fe₂O₃@Cu₂O system to degrade ACT. They noted that a further increase in PS concentration might be less effective because sulfate radicals consumed each other. Tan et al. [53] studied the influence of adding different PMS concentrations on the degradation rate in Fe₃O₄ magnetic nanoparticles/PMS system. They observed that when the initial concentration of PMS increased from 0.0 to 0.2 mM, the reaction rate was promoted from 0.23 × 10 − 2 to 1.22 × 10 − 2 /min. However, when the initial concentration of PMS increased from 0.2 to 0.5 mM, the reaction rate slightly decreased from 1.45 × 10 − 2 to be 1.13 × 10 − 2 /min. The increase of degradation rate in the initial concentration 0.2 mM was attributed to the availability of PMS. At this concentration, PMS acting as a limiting factor controlling the yield of radicals. Furthermore, Tan et al. [58] applied different PMS dosages on MnFe₂O₄ and CoFe₂O₄ to eliminate ACT. The dosages were 0.05, 0.1, 0.15, and 0.2 g/L, and the removal of ACT when MnFe₂O₄ and CoFe₂O₄ were used were 89, 100, 100, 100% and 55.6, 85.7, 94, 100%, respectively. They noted that when the initial concentration of PMS increased to 0.4 mM, the degradation rate started to decline. In this study, there was no adverse effect observed because the initial concentration of PMS did not reach the threshold level. Dong et al. [65] mentioned that when higher PS concentration applied, PS got in the micropores in the catalyst and would react with SO₄˙ ⁻ and led to PS consumption, thereby causing the undesired inhibiting effect. In addition, Velichkova et al. [70] demonstrated that at 153 mM of H₂O₂ in MGN1, MGN2, and MGM systems, the reaction efficiency started to decrease, according to the reaction of OH ˙ with H₂O₂ as shown in equation (51):

As mentioned above, HO 2 ˙ contributed less oxidation potential than OH ˙ , which adversely affect the degradation performance.

4.7 The influence of oxygen

The impact of the oxygen for ACT degradation by using AOP based on composites has been investigated. Moctezuma et al. [8] revealed that oxygen has a strong effect on photocatalytic degradation. Bubbling O₂ acted to trap the free electrons to inhabit the recombination of (e⁻/h⁺), which affect positively on the degradation performance. Also, Yang et al. [44] reported that O₂ increased the degradation of ACT more than six times. O₂ could inhabit electron–hole recombination as O₂ consumes conduction band electrons allowing valance band holes too, directly and indirectly as shown in Eqs. (52)–(55).

Zhang et al. [52] demonstrated that low O₂ concentration resulted in a lower ACT oxidation rate in the iron–copper/PS system. Also, the dissolved oxygen (DO) in the solution related adversely with N₂ purging, which decreased SO 4 ˙ − resulted in the decrease of ACT oxidation. It should be noted that the formation of O 2 ˙ − through the reduction of O₂ by photoinduced electrons in the conduction band and subsequent inhabitation of (e⁻/h⁺) recombination. Furthermore, O 2 ˙ − may react and generate more oxidizing species such as OH ˙ , H₂O₂, and HO 2 ˙ . Moreover, a high concentration of oxygen and a small amount of UV 185 nm induce the formation of O₃ followed by the generation of H₂O₂ to produce more OH ˙ . Abdel-Wahab et al. [60] examined two oxygenating systems: (1) static O₂ atmosphere and (2) purging 100 mL/min of O₂. The results showed that after 90 min of reaction, the removal of ACT was 75 and 99%, respectively. The enhancement in the degradation performance may be attributed to the formation of O 2 ˙ − by photoinduced electrons in the conduction band and subsequent inhabitation of (e⁻/h⁺) recombination. Furthermore, O 2 ˙ − may react and generate more oxidizing species such as OH ˙ , H₂O₂, and HO 2 ˙ . Zhang et al. [63] investigated the effect of DO in the degradation of ACT by using Fe²⁺/CuO/PS system. The result showed that the degradation decreased from 92 to 70% when the system purged with N₂, which means that oxygen played a major role in this system. As DO decreased, O 2 ˙ − , which functioned between and Cu²⁺ and Cu⁺, decreased according to Eqs. (56)–(59). Thus, resulting in a decrease in OH ˙ , which was due to a decline in the degradation of ACT.

Zhang et al. [71] pointed out, that DO plays an important role in the oxidation of ACT and for radical’s generation when Fe²⁺/CuO was applied. When Fe²⁺/CuO was added, the concentration of DO decreased from 9.48 to 4.85 mg/L in the first 10 min of reaction and then increased to 7.64 mg/L after 6 h reaction. In addition, they observed that the removal of DO completely inhibits the degradation of ACT.

4.8 The influence of temperature

According to the literature, the temperature is directly proportional to the removal of pollutant because the pollutant migrates from the bulk solution to the gas–liquid interface region where temperature and OH ˙ are high. Moreover, it has been proposed that the optimum temperature was based on the characteristic of organic matter and the kinetics of the reaction between OH ˙ and pollutant. Velichkova et al. [70] studied the effect of temperature on the degradation of ACT by three types of nanoparticles of iron oxide/H₂O₂ system. They revealed that the increase in the temperature from 30 to 60°C had a beneficial effect for all examined conditions. Also, a higher temperature increases the rate of OH ˙ formation. In contrast, high temperature increases the decomposition of H₂O₂ into O₂ and water, which reduces the removal efficiency of ACT. In addition, Im et al. [72] studied the effect of temperature on the removal of ACT by ultrasound. Two ultrasound waves were applied at 28 and 1,000 kHz, and the best temperature was 25 and 35°C, respectively. They mentioned that beyond the optimum temperature leads to an increase in bubble vapor and then bubble collapse due to the reaction of net energy and free radicles. Tan et al. [53] applied different temperatures from 30 to 70°C in Fe₃O₄ magnetic nanoparticles/H₂O₂ and PMS system. The results showed that temperature affects positively on the elimination of ACT and the kinetic increased from 1.5 × 10⁻² to 10 × 10⁻²/min, for 30 and 70°C, respectively. In addition, Sun et al. [46] reported that when N/S codoped ordered mesoporous carbon was applied for ACT degradation at 25°C, the removal was 100% after 30 min, whereas at 45°C, ACT was completely degraded after 20 min, and the kinetic at 25 and 45°C were 2.4 × 10⁻¹ and 3.5 × 10⁻¹/min, respectively. The activation energy, E _a, of the oxidation system was calculated as 13.8 kJ/mol, which suggested the reaction temperature does not have a significant effect on the oxidation reaction.