An overview report on the application of heteropoly acids on supporting materials in the photocatalytic degradation of organic pollutants from aqueous solutions

Photocatalytic activity

The photocatalytic degradation activity of synthesized nanophotocatalysts was compared with that of other photocatalysts reported in the literature for photocatalytic degradation of various pollutants (Table 1). Based on the obtained results, the synthesized nanophotocatalysts could represent a reasonable photocatalytic degradation activity of organic pollutants, compared to other photocatalysts.

In some studies, the optical properties of the composite including a photocatalyst in combination with HPA were better than the case of using a photocatalyst and HPA alone (Changgen, Gang & Xia, 2013; Shi et al., 2012; Yang et al., 2004; Zhang et al., 2015). In most cases, the composite had a stronger photon absorption capability compared with its ingredients (Feng, Shang & Liu, 2014; Lu et al., 2012; Zhang et al., 2015). A red shift was observed in the UV-Vis diffuser reflectance spectra (DRS) for various composites, due to the effect of the Keggin unit and other doping materials on the electronic properties of the photocatalyst (Changgen, Gang & Xia, 2013; Lu et al., 2013; Salavati, Tavakkoli & Hosseinpoor, 2012; Shi et al., 2012; Yang et al., 2004). Doping the photocatalyst by HPA has resulted in an increase in the absorption in the visible region along with a high light harvesting efficiency by expanding the light response region of composite (Lu et al., 2013).

HPA can influence the electron transformation and act as the electron shuttle in light illuminating photocatalyst (Shi et al., 2012). Thus, HPA doping compared with supporting materials can lead to the outstanding photocatalytic activity of a composite photocatalyst. The results of the photocatalytic activity of Keggin-type heteropoly acids on supporting materials in the degradation of organic pollutants in aqueous solutions have been presented in Table 1.

Mechanism of degradation

Electron–hole pairs are produced and separated when they receive the energy needed for overcoming their mutual electrostatic attraction. Therefore, electrons move to the surface of the photocatalyst where the hole is transferred to the adsorbed hydroxide to create ^∘OH, and accordingly the electron reacts with O₂ to form superoxide anion radical (^∘O₂⁻) (Ghaneian et al., 2014b; Mahmoodi et al., 2016; Norzaee et al., 2017; Shi et al., 2012).

The use of HPA as a doping agent creates a new electronic state in the middle of the photocatalyst band gap, leading to a change in the band gap energy (Shi et al., 2012). Thus, it has some advantages such as a decrease in the chance of recombining the electrons and holes. The electrons excited from the valence band of composite because of absorbing UV/visible photon are transferred to the surface of HPA in the composite. In other words, HPA acts as an electron scavenger that prevents the fast recombination of electron–hole pairs, resulting in an increase in the degradation efficiency of the system (Shi et al., 2012). In other words, the photogenerated electron (e⁻) at the semiconductor (SEMI) is trapped into an unoccupied W_5d state of the Keggin unit by HPA, which leads to a reduction in heteropoly blue (HPA⁻) (Eqs. (1) and (2)). Given its reducing ability, the HPA⁻ can sensitize the photochemical reduction of O₂ to produce supperoxides (^∘O₂⁻) (Eq. (3)). Specifically, HPA accelerates electron transfer from the semiconductor to O₂ and retards the recombination electron–hole pairs on the semiconductor. Hence, the enhanced quantum efficiency is achieved in composite photocatalyst, compared to the pure semiconductor. Further, the generated ^∘O₂⁻ can interact with the adsorbed water to produce hydroxyl radical (^∘OH) and oxidize the organic target pollutant Eqs. (4) and (5) (Tabatabaee & Abolfazl Mirrahimi, 2011; Zhang et al., 2013). Furthermore, the photogenerated holes on the semiconductor react with the adsorbed water and hydroxyl ions to yield more ^∘OH or oxidize the organic pollutant Eqs. (6)–(8) (Ghaneian et al., 2014a; Zhang et al., 2013). As a result, the ^∘OH as an active species can degrade the organic pollutant Eq. (9) (Tabatabaee & Abolfazl Mirrahimi, 2011; Zhang et al., 2013). Some of these reactions for the degradation of aniline as an organic pollutant model, using the synthesized nanophotocatlyst, are presented in Fig. 1.

(1)${\displaystyle \text{SEMI}\stackrel{\mathrm{h}\nu }{\to }\text{SEMI}\left({\mathrm{h}}^{+}+{\mathrm{e}}^{-}\right)}$ (2)${\displaystyle \text{SEMI}\left({\mathrm{e}}^{-}\right)+\mathrm{HPA}\to \text{SEMI}+{\mathrm{HPA}}^{-}}$ (3)${\displaystyle {\mathrm{HPA}}^{-}+{\mathrm{O}}_2\to \mathrm{HPA}{+}^{\circ }{\mathrm{O}}_2^{-}}$ (4)${\displaystyle {}^{\circ }{\mathrm{O}}_2^{-}+{\mathrm{H}}_2\mathrm{O}{\to }^{\circ }\mathrm{OH}+{\mathrm{OH}}^{-}}$ (5)${\displaystyle {}^{\circ }{\mathrm{O}}_2^{-}+\mathrm{Org}\to {\mathrm{Org}}_{\mathrm{ox}}\to \to \text{degradation products}}$ (6)${\displaystyle \text{SEMI}\left({\mathrm{h}}^{+}\right)+{\mathrm{H}}_2\mathrm{O}\to \text{SEMI}{+}^{\circ }\mathrm{OH}+{\mathrm{H}}^{+}}$ (7)${\displaystyle \text{SEMI}\left({\mathrm{h}}^{+}\right)+{\mathrm{OH}}^{-}\to \text{SEMI}{+}^{\circ }\mathrm{OH}}$ (8)${\displaystyle \text{SEMI}\left({\mathrm{h}}^{+}\right)+\mathrm{Org}\to \text{SEMI}+{\mathrm{Org}}_{\mathrm{ox}}\to \to \text{degradation products}}$ (9)${\displaystyle {}^{\circ }\mathrm{OH}+\mathrm{Org}\to {\mathrm{Org}}_{\mathrm{ox}}\to \to \text{degradation products}}$

Furthermore, the direct irradiation of HPA on supporting materials also leads to the creation of some reactions. The irradiation of H₃PW₁₂O₄₀ results in charging transfer from O²⁻ to W⁺⁶ in W − O − W and creating a pair of the hole (O⁻), along with a trapped electron center (W⁵⁺), as shown in the following equation (Antonaraki et al., 2010; Yang et al., 2004): (10)${\displaystyle \left[{\mathrm{W}}^{6+}-{\mathrm{O}}^{2-}-{\mathrm{W}}^{6+}\right]\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{W}}^{5+}-{\mathrm{O}}^{-}-{\mathrm{W}}^{6+}\right]}^{\mathrm{\ast }}\mathrm{i}.\mathrm{e}.,{\mathrm{HPA}}^{\mathrm{\ast }}}$

The strong oxidation ability of HPA^∗, charge transfer-excited state, has been confirmed for degrading organic pollutants (Yang et al., 2004). The hole (O⁻) formed in the charge transfer-excited state of HPA has a strong oxidation ability, which is responsible for oxidizing pollutants (Mahmoodi et al., 2016; Yang et al., 2004). The general reactions involved in degrading organic pollutants in the photocatalytic system containing HPA can be shown by the following equations (Antonaraki et al., 2010; Wei et al., 2012; Zhang et al., 2013):

(11)${\displaystyle {\mathrm{HPA}}^{\mathrm{\ast }}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HPA}}^{-}{+}^{\circ }\mathrm{OH}+{\mathrm{H}}^{+}}$ (12)${\displaystyle {}^{\circ }\mathrm{OH}+\mathrm{Org}\to {\mathrm{Org}}_{\mathrm{ox}}\to \to \text{degradation products}}$ (13)${\displaystyle {\mathrm{HPA}}^{\mathrm{\ast }}+\mathrm{Org}\to {\mathrm{HPA}}^{-}+{\mathrm{Org}}_{\mathrm{ox}}\to \to \text{degradation products}}$ (14)${\displaystyle {\mathrm{HPA}}^{-}+{\mathrm{O}}_2\to \mathrm{HPA}{+}^{\circ }{\mathrm{O}}_2^{-}}$ (15)${\displaystyle {}^{\circ }{\mathrm{O}}_2^{-}+\mathrm{Org}\to {\mathrm{Org}}_{\mathrm{ox}}\to \to \text{degradation products}}$

In the presence of hydrogen peroxide, the HPA^∗ can produce hydroxyl radicals in reaction with H₂O₂, which is transferred to the reduced state based on the equations as follows (Wei et al., 2012):

(16)${\displaystyle \mathrm{HPA}\stackrel{\mathrm{h}\nu }{\to }{\mathrm{HPA}}^{\mathrm{\ast }}}$ (17)${\displaystyle 2{\mathrm{HPA}}^{\mathrm{\ast }}+{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{HPA}}^{-}+2^{\circ }\mathrm{OH}}$

Ultraviolet irradiation of H₂O₂ is another pathway for generating hydroxyl radical in the presence of H₂O₂. (18)${\displaystyle {\mathrm{H}}_2{\mathrm{O}}_2\stackrel{\mathrm{h}\nu }{\to }{2}^{\circ }\mathrm{OH}}$

Some studies emphasized the role of reactive species and its contribution to each in the presence of scavengers, (Lu et al., 2013; Norzaee et al., 2018). For example, Mahmoodi et al. (2016) evaluated the effect of some anions such as NaCl, NaHCO₃, and Na₂SO₋₄ on the photocatalytic decolorization of Acid Orange 95, Acid Red 18, and Direct Red 81 by using H₃PW₁₂O₄₀/modified cobalt ferrite composite and observed the retarding effect of anions taken place through their reaction with ^∘OH as well as hole based on the Eqs. (19)–(22). They interpret the dominant effect of holes and ^∘OH radicals by the fact that owing to the synergistic effect of POM and cobalt ferrite, photoinduced electrons in cobalt ferrite are excited and captured by POM. As a result, a decrease occurs in the production of superoxide anion radical produced via the interaction between photoinduced electrons and oxygen molecules adsorbed on the surface of composite (Mahmoodi et al., 2016).

(19)${\displaystyle \mathrm{OH}+{\mathrm{Cl}}^{-}{\to }^{\circ }\mathrm{Cl}+{\mathrm{OH}}^{-}}$ (20)${\displaystyle {}^{\circ }\mathrm{OH}+{\mathrm{HCO}}_3^{-}{\to }^{\circ }{\mathrm{CO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}}$ (21)${\displaystyle {}^{\circ }\mathrm{OH}+{\mathrm{SO}}_4^{2-}{\to }^{\circ }{\mathrm{SO}}_4^{-}+{\mathrm{OH}}^{-}}$ (22)${\displaystyle {\mathrm{Cl}}^{-}+{\mathrm{h}}^{+}\to {\mathrm{Cl}}^{\circ }}$

Furthermore, an order of SO₄²⁻>Cl⁻>NO₃⁻ has been reported for the influential extent of the anion in reducing degradation Rhodamine B (Hua et al., 2014). The hydroxyl radical also has been represented as the main reactive species in photocatalytic degradation of bisphenol A by H₃PW₁₂O₄₀/TiO₂ film (Lu et al., 2013).

On the contrary, Shi et al. (2016) rejected the significant role of ^∘OH species on photocatalytic degradation of methyl orange by Ag/Ag_xH_3−xPMO₁₂O₄₀ under visible light irradiation and introduced ^∘O₂⁻ and h⁺ radicals as the main reactive species in the process. Niu & Hao (2011) also considered the most contribution in degradation Methyl orange for holes.

In some studies, the heteropoly acid leachate from composites also investigated. The use of H₃PW₁₂O₄₀/In₂O₃ composite photocatalyst in the photocatalytic degradation of methylene blue during four cycles resulted in a leaching of 0.2–1.4% W. Accordingly, a strong coordination communication was observed between the HPA and the In₂O₃ facade. The leaching of W was decreased with the increase of application cycle (Salavati & Saedi, 2014). A 2% leaching or loss of POM was reported for H₃PMo₁₂O₄₀/MnO₂ composite photocatalyst after using the fifth cycle (Kannan et al., 2011). Qu, Guo & Hu (2007) found W concentration within the range of 1.1–3.3 mg/L in the final treated solution by H₃PW₁₂O₄₀/ZrO₂, which was negligible with respect to its initial concentration. Finally, a strong interaction was reported between the Keggin unit and ZrO₂ support with the unchangeable photocatalytic activity of composite for the three-time cycle.

Effect of initial pH of the solution

The pH of the solution plays an important role in the photocatalytic degradation process (Taghi Ghaneian et al., 2016). Most studies in this regard report a good photocatalytic activity of composite photocatalysts including HPA toward organic pollutant in an acidic medium in a pH range of 1–2.5 (Table 1). Hence, we can infer the partial change of the HPW structure from PW₁₂ into PW₁₁ at high pH values (Qiu, Zheng & Haralampides, 2007). The surface of the composite including PW₁₂ in the outermost layer carries a negative charge due to PW₁₂, which results in accelerating the transfer rate of holes and facilitating the separation of electron–hole pairs. As a result, the photogenerated electrons could have enough time to react with the adsorbed O₂ on the composite surface to yield reactive oxygen species (^∘O₂ or H₂O₂). As previously mentioned, holes can react with the adsorbed water to generate ^∘OH. Therefore, decomposition of HPW is responsible for the relatively low photocatalytic activity at neutral and alkaline media (Niu & Hao, 2011).

However, there are some reports with conflicting results in published works, with some of them discussed in detail below.

Hua et al. (2014) observed that the degradation efficiency of rhodamine B increased with an increase in the initial pH ranging 2.5–7 in the homogeneous system (PW₁₁Mn), but the vice versa happened in the heterogeneous system (PW₁₁Mn/D301R resin). Considering a red shift in the maximum visible absorption peak of rhodamine B in presence of PW₁₁Mn, this result is related to the interaction between the catalyst and rhodamine B in the homogeneous system.

Moreover, Changgen, Gang & Xia (2013) reported that the initial pH of the solution within a range of 1–5.88 plays no significant role in degrading imidacloprid by H₃PW₁₂O₄₀/La-TiO₂ nanocomposite. Further, Wei et al. (2012) confirmed the negligible effect of pH on degradation of methyl orange using activated clay-supported HPW. This effect was observed as a decrease in degradation efficiency with an increase of pH in a range of 1.0–7.0.

Through using H₃PW₁₂O₄₀/Ag-TiO₂, an increase took place in the degradation of sulfamethoxazole after increasing the pH up to neutral to alkaline conditions (pH = 6.8–8.7), probably due to the changes in sulfamethoxazole acid–base species and an increase in its adsorption on composite photocatalyst (Xu et al., 2012).

HPW composites in combination with H₂O₂ is found in a wide range of initial pH of the solution in the photocatalytic degradation of organic pollutants (Wei et al., 2012).

The maximum degradation of bisphenol A by H₃PW₁₂O₄₀/TiO₂ composite catalyst was found at pH 8.2. The increase in photocatalytic activity of the composite catalyst at an alkaline condition (pH = 8.2) compared with acid solution is attributed to an increase in the number of OH reacting with photoinduced holes on the surface of the composite and a consequent increase in hydroxyl radicals generated for the degradation of bisphenol A. On the other hand, the decrease in efficiency at higher pHs (pH = 10.2) is attributed to ionization of bisphenol A and generation of bisphenolate anion (pKa = 9.6–10.2), which leads to the electrostatic repulsion between negatively surface charged composite (pH_pzc of TiO₂ = 6.25) and bisphenolate anion (Lu et al., 2013). Elsewhere, this composite photocatalyst was used in degrading rhodamine B in which the best results were observed in acidic solutions (pH = 4.4) (Lu et al., 2012).

Effect of photocatalyst dosage

Photocatalyst dosage is regarded as another factor playing a significant role in the photocatalytic process. This dosage should be considered in optimizing the operational conditions. Some studies indicated that an increase in photocatalyst dosage could result in increasing the photocatalytic efficiency (Yang et al., 2012). Therefore, an increase in the photocatalyst dosage does not always lead to the enhancement of photocatalytic activity (Hua et al., 2014; Wang, Huang & Yang, 2010; Yang et al., 2012). An excessive dosage increase, however, may decrease the photocatalytic efficiency. It seems that surplus photocatalyst results in scattering the photons in the solution and decreasing the photons that reach the surface of photocatalyst (Feng & Shang, 2012; Xu et al., 2012; Yang et al., 2012). Some other researchers reported the same results for various composites (Feng & Shang, 2012; Wei et al., 2012; Xu et al., 2012). In contrast, (Changgen, Gang & Xia, 2013) did not report a decrease in photocatalytic activity H₃PW₁₂O₄₀/La-TiO₂ composite with an increase of photocatalyst dosage within the range of 200–800 mg/L. An explanation for this result is that the maximum applied dosage is still in the optimum range of photocatalyst dosage based on other reports (Wei et al., 2012; Xu et al., 2012).

Effect of pollutant concentration

Several studies have shown that an increase in pollutant concentration leads to a decrease in photocatalytic efficiency through different ways (Feng & Shang, 2012; Lu et al., 2013; Mahmoodi et al., 2016; Niu & Hao, 2011; Norzaee et al., 2017; Xu et al., 2012; Yang et al., 2012).

More pollutant and intermediate products molecules are accumulated on the surface of photocatalyst when an excessive increase takes place in the initial concentration. Accordingly, the generation of reactive oxygen species is reduced because of inhibiting the adsorption of the incident photon for active sites (Lu et al., 2013; Niu & Hao, 2011). As for the dyes, the higher concentration of dye makes the color of solution very deeper and thus results in limiting the penetration of light to the solution depth and reaching the surface of photocatalyst (Feng & Shang, 2012). Besides, a constant intensity of light source and illumination time leads to a constant amount of the generated radicals (Norzaee et al., 2017). Hence, surplus dye molecules cannot be degraded and are represented as a low degradation efficiency (Niu & Hao, 2011).

Nevertheless, the low concentration of target pollutant may lead to a weak interaction between pollutant and composite photocatalyst. Wang, Huang & Yang (2010) reported this point as a reason for low efficiency in degradation of nitrobenzene by H₃PW₁₂O₄₀/TiO₂ composite photocatalyst in initial concentration 10 mg/L and increasing the efficiency through enhancing the initial concentration of nitrobenzene.

Effect of oxidants

In some studies, oxidants such as H₂O₂ have been applied to improve photocatalytic activity. Increasing the H₂O₂ concentration from 0.2 to 0.7 mol/L in a UV/ H₃PW₁₂O₄₀/activated clay system could raise the degradation of methyl orange. However, a larger increase in the oxidant dosage results in decreasing the degradation efficiency of methyl orange (Wei et al., 2012).

Cao et al. (2011) reported that the photocatalytic degradation process is accelerated in the presence of H₂O₂. In addition, it has been evidenced that addition of H₂O₂ to the photocatalytic system leads to an increase in the hydroxyl radicals produced in the system, which is regarded as a reason for enhancing the degradation rate (Wei et al., 2012).

H₂O₂ at high concentrations has a negative effect on photocatalytic degradation rate by decreasing the number of hydroxyl radicals in solution because of acting as a hydroxyl radical scavenger at a higher level of concentration. A decrease in hydroxyl radical due to the high concentration of H₂O₂takes place through the following reactions (Wei et  al.,  2012):

(23)${\displaystyle {\mathrm{H}}_2{\mathrm{O}}_2{+}^{\circ }\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}{+}^{\circ }\mathrm{H}{\mathrm{O}}_2}$ (24)${\displaystyle \mathrm{H}{\mathrm{O}}_2{+}^{\circ }\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2}$

Effect of heteropoly acid loading

Various concentrations of heteropoly acid can change the photocatalytic activity of composites (Changgen, Gang & Xia, 2013; Lu et al., 2013). The HPW loading on La-TiO ₂nanoparticle (0.3%) within the range of 10–20% resulted in increasing the photocatalytic activity of nanocomposite while the further increase in HPW loading led to a slight decrease (Changgen, Gang & Xia, 2013). Zhang et al. (2015) found same results in preparing H₃PW₁₂O₄₀-TiO₂/Bentonite and the highest photocatalytic activity achieved at a molar ratio of 0.5:100 for HPW and TiO₂, respectively. In addition, the rate constant k for an HPW loading of 20% in H₃PW₁₂O₄₀-TiO₂ composite was 1.4 times that for an HPW loading of 30%, and approximately 3 times that of an HPW loading of 10% and 40% (Feng & Shang, 2012).

Other studies reported similar results for various composite photocatalysts (Lu et al., 2013; Zhang et al., 2013). The first increase in photocatalytic activity along with an increase in a doping of HPA is attributed to the increase in captured electrons by HPA, as an electron scavenger, a consequent delay in recombination electron–hole pairs, and an increase in the photocatalytic activity (Lee, Dong & Dong, 2010; Zhang et al., 2015). However, a high increase in the doping intensity of HPA is not always favored. A high amount of HPA causes to increase the available electron traps provided by HPA and the distance between electron traps will decrease. Therefore, an excessive amount of HPA in the composite can provide a place for recombining the photogenerated electron–hole pairs, leading to a decrease in photocatalytic activity of composite photocatalyst (Zhang et al., 2013; Zhang et al., 2015). Another reason for this phenomenon might be a reduction in specific areas of composite photocatalyst and accordingly less active sites for light and pollutant introduction (Feng & Shang, 2012; Lu et al., 2013; Zhang et al., 2013).

Photocatalyst recovery

The H₃PW₁₂O₄₀/TiO₂ composite synthesized by combining sol–gel technology using a nonionic surfactant P123 as a structure directing agent with solvothermal treatment demonstrated a constant photocatalytic activity after three sequent cycles. Accordingly, the prepared composite photocatalyst using this method has a high stability (Feng, Shang & Liu, 2014). Shi et al. (2016) reported the same result for Ag/Ag_xH_3−xPMO₁₂O₄₀ after degrading methyl orange for four cycles. Further, Xu et al. (2012) reported the use of H₃PW₁₂O₄₀/Ag-TiO₂ composite in degrading Sulfamethoxazole with an insignificant decrease in photocatalytic activity after three cycles. The degradation rate for the first, second, and third cycles were 97.6, 91.7, and 87.7%, respectively. Similar results were reported for H₃PW₁₂O₄₀/modified cobalt ferrite composite in the degradation of some dyes (Mahmoodi et al., 2016). This result is consistent with those reported in (Feng, Li & Liu, 2012; Zhang et al., 2013). However, Niu & Hao (2011) obtained different results for five cycles by reusing H₃PW₁₂O₄₀/TiO₂ in the decomposition of methyl orange. They reported that an increase could take place in decomposition efficiency after an increase in reusing cycles, due to the change in photocatalyst surface property and the bound water produced on the recycled photocatalyst.

Outlook on challenge and perspective

From the catalytic activity point of view, the previous sections showed that application of heteropoly acids on supporting materials in the photocatalytic process for the removal of organic pollutants has been an effective move in this field of study. Increasing photocatalytic activity for composite materials and overcoming problems in regard to the separation of heteropoly acids from the final solution are the most significant achievements of the application of heteropoly acids on supporting materials. Although some researchers also used SiO₂, BiVO₄, Au, Mg₃Al-LDH, ZrO₂, MnO₂ as supporting materials, most works to date have focused on TiO₂ as supporting materials. Future works should focus on materials that have not yet been used such as Fe₂O₃ to develop composites with desirable separation properties as well as the biotoxicity of treated wastewater, the changes in the environmental toxicity of synthesized composites, and applying facilitating methods for the composite photocatalysts such as HPA in a wider range of pH.