The suitability of Ce³⁺-modified ZnO photocatalyst for the mineralization of monochlorophenol isomers in sunlight exposure

Abstract

The photocatalytic activity of Ce³⁺-modified hexagonal ZnO for the degradation/mineralization of monochlorophenol isomers (2-chlorophenol, 3-chlorophenol and 4-chlorophenol), in natural sunlight exposure, is reported. Compared to bare ZnO, the modified catalysts showed superior activity for the mineralization of MCP isomers. The identification of the intermediates disclosed that the mode of degradation of chlorophenol substrates also varies with increased Ce³⁺ loading. Increased mineralization was discovered with increasing concentration of Ce³⁺ at the surface of ZnO. The correlation of the results obtained by various analytical tools revealed that the photocatalytic removal of MCP isomers initially proceeds with the cleavage of the aromatic ring, the release of chloride ions and the formation of oxygenated intermediates. Finally, the intermediates are oxidized further by the oxidizing species to mineralization. The efficacy of the synthesized catalysts was tested for the mixture of chlorophenol isomers. Based on the intermediates formed, the major contribution of superoxide anion radicals was evidenced in the removal process. The Ce³⁺ impregnation protected the surface of ZnO against photocorrosion.

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Introduction

The hazards and toxicity of chlorophenols, even in very low concentrations, are well established.^1–7 The petrochemical and pesticide industries are identified as the major sources of chlorophenol-polluted industrial wastewater. On the other hand, the excessive use of chlorophenols as sanitizers contributes to the contamination of domestic wastewater. Moreover, the formation of chlorophenols during the disinfection of water is also documented as a potential source for contamination of water.^5–10

The chemical stability and transformation of chlorophenols into more toxic intermediates hinders their large-scale and efficient removal from wastewater by chemical and biological methods.^9,10 Adsorption is viewed as a viable approach in this context; however, the storage and disposal of large quantities of adsorbents loaded with toxic chlorophenols brings into question the large-scale use of this technology.^10–15

The light-mediated generation of highly reactive oxidizing radicals for the oxidation of toxic pollutants label the advanced oxidation processes as low-cost, efficient and environmental friendly processes.^16–18 Among these processes, heterogeneous photocatalysis is a versatile approach, as it is associated with the complete conversion of organic carbon to CO₂ without leaving any secondary intermediate. The affluent availability of nontoxic photocatalysts, such as TiO₂ and ZnO, makes this approach more attractive for environmental remediation, particularly for the decontamination of water.^19–27

TiO₂ was found to be the most studied photocatalyst for the degradation of chlorophenols, and researchers have proposed the mechanism of photocatalytic degradation of chlorophenols based on their experiments.^21,28–35 The principal pathway for the photocatalytic degradation of chlorophenols is through its interaction with hydroxyl radicals, which is produced during the course of the photocatalytic reaction. The position of chlorine atoms in the chlorophenols was reported to play a significant role in the formation of the intermediate compounds during the course of reaction.³⁶ Pyrocatechol was formed as an intermediate compound when 2-chlorophenol was subjected to photocatalytic degradation, whereas resorcinol was formed with 3-chlorophenol. In the case of 4-chlorophenol, it resulted in the generation of a mixture of intermediates such as hydroquinone, benzoquinone and benzenetriol.^37–39 It was widely reported in the literature that degradation pathways, product yields and reaction rates strongly depend on the photocatalytic reaction conditions.⁴⁰

ZnO, with a bandgap of 3.2 eV, is an important substitute of TiO₂ and sometimes preferred due to its higher photon absorption cross section.^41,42 However, ZnO suffers from problems such as fast recombination rate and low stability against photo-corrosion. It is well established that the photocatalytic activity and chemical stability of a photocatalyst can be enhanced by modifying its surface.^20,43,44

Being renewable, the use of sunlight as an excitation source for the generation of highly energetic reactive radicals, as well as for the mineralization of organic contaminants, is an option and can expand the scope of this technology. However, the non-availability of sunlight-active photocatalysts limits the use of this low-cost, natural photon source in photocatalysis. Therefore, for the future and widespread commercial use of this technology, it is highly desirable to develop either new sunlight-responsive, active photocatalysts or to modify the existing active photocatalysts for better sunlight response. Various strategies are reported in the literature to make the existing active photocatalysts such as TiO₂ and ZnO responders in the visible region. These strategies include metal ion doping, composite formation, surface modification by metal impregnation and insertion of non-metals such as S and N.

We have recently reported the synthesis, characterization and preliminary results regarding the enhanced photocatalytic activity of Ce³⁺-impregnated ZnO photocatalysts for the degradation of 2-chlorophenol (2-CP) as a short communication.⁴⁵ The present study is a detailed investigation of the photocatalytic activity of a series of synthesized catalysts with different loadings of Ce³⁺ (0.5%, 1%, 3%, 5% and 10%) in natural sunlight for the degradation/mineralization of individual monochlorophenol (MCP) isomers and to a mixture of these isomers. HPLC and TOC were brought together for estimating the progress of the degradation and mineralization process. Ion chromatography measured the ions released during the course of the photocatalytic process, whereas GC-MS identified the intermediates formed during the course of degradation, as well as mineralization. The results obtained by various analytical tools were correlated for predicting the plausible mechanism of degradation and mineralization.

Experimental details

Ce³⁺-modified ZnO catalysts were synthesized by impregnating the pre-synthesized ZnO with Ce³⁺ ions. The details of the synthesis and the optical characterization of the synthesized powders by solid-state diffuse reflectance spectroscopy (DRS), as well as photoluminescence (PL) spectroscopy, is discussed in our recent communication.⁴⁵ The assessment of the possible variation in the oxidation states of Ce³⁺ after being impregnated on hexagonal ZnO was investigated by X-ray photoelectron spectroscopy (XPS). An X-ray photoelectron spectrometer (PHI 5000 VersaProbe II, ULVAC-PHI) recorded the XPS profiles of Ce³⁺-impregnated ZnO (Ce³⁺@ZnO). The morphological and structural characterizations of the impregnated photocatalysts were performed by FESEM and XRD analysis. The details about these techniques are available elsewhere.⁴⁵

The photocatalytic activity of bare and Ce³⁺-impregnated ZnO in sunlight irradiation was performed by exposing 150 cm³ of 30 ppm of respective MCP solution containing the optimized amount (100 mg) of the respective catalyst. The procedure adopted for the optimization of the catalyst for maximum degradation is detailed elsewhere.^46,47 Prior to sunlight exposure of the catalyst–MCP suspension, it was kept under darkness for the evaluation of adsorption. In sunlight exposure comparative studies, after a series of experiments, it was realized that the important parameter is the intensity of light rather than the angle of incidence. Therefore, all of the experiments were performed in sunlight illumination of 1000 ± 100 × 10² lx and a fixed period of daylight. The progress of the degradation process was monitored by drawing samples every 20 min in the first hour and after 30 min in the next 1.5 h. In the next series of experiments, for most active catalysts, the concentration of MCP was increased to 50 ppm and the photocatalytic degradation studies were performed by adopting the procedure previously described. For the experiments with a mixtures of MCP isomers, a stock solution containing 17 ppm of each isomer was prepared by the dilution method. The overall MCP content of the solution was 51 ppm. The mixture of MCP isomers was also subjected to a similar procedure as previously described. After removing the catalyst by 0.20 μm Whatman syringe filter, the samples collected at various intervals were subjected to high-performance liquid chromatography (HPLC), ion chromatography (IC), and TOC and GC-MS analyses for estimating the decrease in phenolic substrate and the formation of intermediates, the estimation of anions in solution, the removal of organic carbon and the identification of intermediates, respectively. The details of the equipment and the parameters are given in the ESI.†

Results and discussion

Characterization of Ce³⁺-impregnated ZnO

The comparison of the PL analysis of the pure and Ce³⁺-loaded ZnO is presented in Fig. S1 (ESI†). The results are discussed in detail in our previous communication.⁴⁵ In the present study, the possible variations in the oxidation state of Ce during the synthesis were estimated by XPS analysis. The XPS survey scans of Ce³⁺-impregnated ZnO samples, in which the peaks related to all of the components are observable (Fig. 1). As expected, a decrease in the intensity of the peaks arising due to ZnO (Zn3p, Zn2p and O1s) and an increase for the peaks originated by the increasing surface density of Ce³⁺ was noticed. The C1s, O1s and Zn2p could be easily observed; however, because of low intensity, the peaks of Ce4d and Ce3d are less prominent. For all of the samples, there was no shift in the position of C1s peaks that appeared at 285.1 eV. Fig. 2(a) and (b) shows the split Ce3d levels of 1% and 10% Ce³⁺-loaded ZnO. In 1% Ce³⁺ loading, the two Ce3d split levels appeared at 881.8 eV and 885.5 eV, whereas for 10% Ce³⁺-loaded sample the same were shifted to lower binding energy of 880.6 eV and 884.4 eV due to the matrix effect. The obtained binding energy values, in accordance with the literature,⁴⁸ confirmed the formation of surface Ce₂O₃ and Ce in +3 oxidation states. As shown in Fig. 2(c) and (d), the Zn2p3/2 peaks of 1% and 10% Ce³⁺-loaded samples appeared at the binding energies of 1022.4 and 1022.2 eV, respectively, which were as per the literature values.⁴⁹ The appearance of additional shoulder in the fitting of Zn2p3/2 at 1020.22 eV (Fig. 2(d)) revealed the surface changes due to the presence of Zn²⁺–O–Ce³⁺-type structures. The comparison of O1s peaks of 1% and 10% Ce³⁺-loaded ZnO is presented in Fig. 2(e) and (f). For both of the samples, the principal O1s peak arising due to surface Zn²⁺–O⁻ appeared at around 530.9 eV, whereas the shoulder, centered at 532.6 eV, corresponds to oxygen-deficient sites in the ZnO matrix. The emergence of additional peak in O1s spectra of 10% Ce³⁺-impregnated ZnO sample at 529.4 eV corresponds to surface Ce₂O₃.^50,51

Fig. 1 The comparison of the XPS survey scans of 1%, 3%, 5% and 10% Ce³⁺-impregnated ZnO.

Fig. 2 The comparison of the core–shell XPS spectra of Ce3d (a and b), Zn2p3 (c and d) and O1s (e and f) in 1% and 10% Ce³⁺-impregnated ZnO, respectively.

The FESEM analysis (Fig. 2) revealed no change in the morphology of ZnO, rather an even distribution of impregnating Ce³⁺ entities was noticed. With increase in the surface concentration, deposition in overlaying layers was observed, which affected the sharpness of the edges of hexagonal ZnO. Moreover, no significant change in particle size was observed.

The comparison of the XRD patterns of Ce³⁺-impregnated ZnO is presented in Fig. 3a. The main reflections appeared at 2θ values of 31.957°, 34.603°, 36.437°, 47.711°, 56.758°, 63.012°, 68.098°, 69.236°, 72.714°, and 77.102° were corresponding to the hexagonal ZnO (JCPDS-36-1451). The growth of reflection at 2θ = 28.85°, with the increasing surface deposition of Ce³⁺ ions, represented the hexagonal Ce₂O₃ (002) phase (JCPDS-23-1048). The other characteristic reflections of surface Ce₂O₃ were entrapped in intense ZnO reflections. The extrapolation of reflections at 2θ = 28.85° revealed the crystallite size of ∼6.6 nm for surface Ce₂O₃ without the evidence of any change in oxidation state (Fig. 4b).

Fig. 3 High-resolution (120 000×) FESEM images of (a) 1% Ce³⁺@ZnO, (b) 3% Ce³⁺@ZnO, (c) 5% Ce³⁺@ZnO and (d) 10% Ce³⁺@ZnO.

Fig. 4 (a) The comparison of the XRD of pure (black), 1% Ce³⁺@ZnO (red), 3% Ce³⁺@ZnO (green), 5% Ce³⁺@ZnO (blue) and 10% Ce³⁺@ZnO (pink); (b) the XRD patterns of pure (black) with Ce³⁺-impregnated ZnO in 2θ = 20° and 2θ = 31° range, showing the growth of Ce₂O₃ with increase in the concentration of Ce³⁺.

Photocatalytic degradation/mineralization of MCP isomers

The absorption of photons by the semiconductor particles leads to the generation of a cascade of reactive oxygen species (ROS) in the aqueous medium. The formation and population of these reactive entities depend entirely on the electrochemical potential of band edges and the pH of the system. The adsorbed water molecules are oxidized by valence band holes at the potential positive rather than +1.23 V, whereas the reduction of adsorbed/dissolved oxygen requires a minimum conduction band potential of −0.28 V. The oxidation process leads to the formation of H⁺ ions and hydroxyl radicals (HO˙), whereas the reduction of adsorbed/dissolved oxygen generates superoxide anion radicals (O₂˙⁻). The diffusion of these entities in the bulk initiates the formation of an additional stream of ROS such as H₂O˙, H₂O₂, and OH⁻. In such a complex system, due to the presence of a wide variety of reactive species, the extent of the interaction of an individual entity with the substrate is inconceivable. Similarly, in such a complex situation, it is difficult to estimate the exact role of each reactive entity in the degradation/mineralization of the substrate. Recently, superoxide anion radicals (O₂˙⁻) have replaced hydroxyl radicals as major oxidizing entities in photocatalytic degradation/mineralization processes. A complex route has been proposed for the generation of hydroxyl radicals. It has also been established that the formation of hydroxyl radicals is pH dependent, and higher pH values favor the formation of superoxide anion radicals. The termination of the degradation activity in the absence of oxygen was used as supporting evidence in this regard.^{41–44,52–56} As per our perception, in a photocatalytic degradation/mineralization process, the role of oxidizing species can be classified based on the careful identification of intermediates, estimation of total organic carbon removal and measurement of released ions.

Initially, the effect of various Ce³⁺ loadings (1%, 3%, 5% and 10%) on photocatalytic activity was estimated for the degradation/mineralization of 2-CP. The comparison of the HPLC degradation/mineralization profiles, showing the degradation of 2-CP and the formation of intermediates, in the sunlight exposure, over bare and Ce³⁺ impregnated ZnO is presented in Fig. 5(a)–(e). All of the Ce³⁺-loaded catalysts showed significantly higher activity for the degradation of 2-CP compared to bare ZnO. Although 2-CP was degraded in the presence of pure ZnO, the formation of intermediates was significantly higher, whereas for Ce³⁺ impregnated catalysts, the simultaneous degradation of the 2-CP substrate and removal of intermediates was observed. The percentage degradation of 2-CP as a function of sunlight exposure time is presented in Fig. 6(a). Compared to ∼40% degradation of 2-CP for bare ZnO, ∼80%, ∼77%, ∼75% and ∼86% of 2-CP substrate is removed in the presence of 1%, 3%, 5% and 10% Ce³⁺-loaded ZnO, respectively, in the initial 20 min of sunlight exposure. All of the catalysts completely (≥99.5%) degraded 2-CP substrate in 90 min of exposure, whereas for bare ZnO, ∼95% of 2-CP was removed in 150 min. Compared to impregnated catalysts, a low TOC removal, as presented in Fig. 6(b), was observed for bare ZnO. Compared to ≥90% TOC removal for all of the catalysts in 150 min of exposure, 49% removal was observed for bare ZnO, which was even lower than that observed for Ce³⁺-impregnated catalysts in 20 min of exposure. The highest TOC removal of ∼96% was observed for 10% Ce³⁺-loaded ZnO. The rate profiles for the degradation and mineralization of 2-CP obtained by plotting ln(C_o/C) versus the sunlight exposure time are presented in Fig. 7(a) and (b), from which it can be inferred that the Langmuir–Hinshelwood kinetic model does not hold well for all catalysts. For 1% and 10% Ce³⁺-loaded ZnO, during the degradation process, the rate of degradation exponentially increases with decrease in the concentration of 2-CP substrate. A relatively better correlation was observed for pure, 3% and 5% Ce³⁺-loaded ZnO samples. Based on the results, it can be generalized that in photocatalytic degradation processes, the L–H kinetic model can only be applied for catalysts with low or moderate activity.

Fig. 5 The comparison of HPLC degradation profiles of 2-CP (30 ppm) over (a) pure ZnO, (b) 1% Ce³⁺@ZnO, (c) 3% Ce³⁺@ZnO, (d) 5% Ce³⁺@ZnO and (e) 10% Ce³⁺@ZnO.

Fig. 6 The comparison of the percentage degradation of 2-CP (30 ppm) in sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 1%, 3%, 5% and 10% Ce³⁺@ZnO. The inset shows the TOC removal as a function of sunlight exposure time.

Fig. 7 The rate profile of (a) 2-CP (30 ppm) degradation and (b) 2-CP mineralization in sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 1%, 3%, 5% and 10% Ce³⁺@ZnO.

The comparison of HPLC degradation profiles of 3-CP in standard (3-CP, 30 ppm) and exposed samples and on bare and Ce³⁺-impregnated ZnO is presented in Fig. S2 (ESI†). In the presence of bare ZnO, a low degradation (38% in 20 min) compared to Ce³⁺-impregnated catalysts (≥68% in 20 min) was observed. All of the impregnated catalysts exhibited the ability to remove ∼99.5% of 3-CP substrate in 120 min of sunlight exposure; however, the extent of degradation of 3-CP was lower than that of 2-CP. A similar behavior was observed in TOC removal (mineralization of intermediates). Compared to ∼41% for bare ZnO, ∼93% of organic carbon produced in 3-CP degradation was mineralized in 150 min of sunlight exposure.

As observed for 2-CP and 3-CP, the degradation of 4-CP in sunlight exposure over bare ZnO also generated a significant concentration of intermediates in the solution (Fig. S3, ESI†). A significantly higher degradation of 4-CP over Ce³⁺-impregnated ZnO catalyst as compared to bare ZnO was observed. Similarly, a high removal of TOC was also observed for all of the catalysts. In 4-CP degradation, compared to ∼36% for bare ZnO, ∼97% of TOC removal was observed for 10% Ce³⁺-loaded catalyst in 150 min of sunlight exposure. However, a lower degradation, compared to 2- and 3-CP, was observed under similar conditions.

The comparison of the HPLC degradation profiles of 2-CP, 3-CP and 4-CP clearly elaborates that ZnO is capable of degrading MCP isomers in the natural sunlight; however, it fails to carry out efficient mineralization (total organic carbon removal). On the other hand, all Ce³⁺-impregnated catalysts showed excellent ability of simultaneous degradation and mineralization of MCP substrates. ZnO is a wide bandgap (3.1 eV) photocatalyst and can harvest only ≤5% of total incident photons (UV region) of the sunlight. In an aqueous medium, the conduction band edge potential (−0.31 V) of ZnO is highly supportive for the reduction of dissolved oxygen to superoxide anion radicals. Moreover, pH_ZPC of 9.2 for ZnO also favors the formation of superoxide anion radicals in the photocatalytic degradation process. The negative aspect associated with ZnO is the high recombination of charge carriers. The high luminescence intensity of bare ZnO predicts the similar situation.⁴⁵ It is evident that the higher recombination rate of charge carriers (e⁻–h⁺) restricts the prolific use of excited states and fails to deliver the photo-excited electrons to adsorbed/dissolved O₂, thus generating fewer superoxide anion radicals, compared to the number of absorbed photons. In the degradation process, for pure ZnO, the majority of the available superoxide anion radicals is consumed in the degradation of MCP substrates, whereas very few are left in the system for interaction with intermediates, thus affecting the mineralization ability significantly. The modification of the ZnO surface by impregnating Ce³⁺ ions enhanced the photocatalytic degradation ability of ZnO tremendously, which led to significantly higher degradation and mineralization of MCP substrates. The varying extent of decrease in the luminescence intensity with respect to the concentration of Ce³⁺ confirmed at least the trapping capability of surface Ce³⁺ ions Fig. S1 (ESI†). On the other hand, the significantly high simultaneous removal of MCP substrates and intermediates lead to the conclusion that a trap-and-transfer synergy exist between ZnO and impregnated Ce³⁺ ions. The impregnating Ce³⁺ ions bind with the ZnO surface oxygen to form surface Ce₂O₃ nanostructures maintaining hexagonal geometry. The existence of surface Ce₂O₃ is also validated by XRD analysis. By the analysis of the HPLC degradation profiles of 2-CP, 3-CP and 4-CP, as presented in Fig. 3, S4 and S5,† respectively, compared to 3% and 5% loadings, a noticeably high activity could be perceived for 1% and 10% Ce³⁺ loaded ZnO. However, a significantly high mineralization was observed for 10% Ce³⁺ loaded ZnO as compared to 1% loading, which suggested that charge trapping and its transfer to the reductants is accomplished by more than one mechanism. It is proposed that at a lower surface density of Ce³⁺, the photoexcited electrons are trapped by Ce³⁺ defects generated at the surface in the form of Zn²⁺–O–Ce³⁺-type structures. One possibility is the direct transfer of excited electrons from the conduction band of ZnO (4s⁰) to the closely spaced 5d + 4f orbital of Ce³⁺. The presence of the additional electron imparts instability to surface Ce³⁺, which is recuperated by the immediate transfer of electrons to adsorbed O₂ molecules that form superoxide anion radicals (O₂˙⁻) as presented in the following pictorial representation given in the Scheme 1.

Scheme 1 The plausible charge-trapping and transfer mechanisms for 1% Ce³⁺-loaded ZnO in sunlight exposure.

The layer-by-layer impregnation of Ce³⁺ with increasing surface density hinders the charge-transfer ability, thus causing a decrease both in degradation and mineralization for 3% and 5% Ce³⁺-loaded ZnO catalysts. For 10% Ce³⁺-impregnated ZnO, the deposition of Ce³⁺ leads to the formation of surface composites or a heterojunction of ZnO and Ce₂O₃. The formation of these heterostructures is well documented in the literature.^57–59 However, in this particular case, the structures are being formed by the layer-by-layer surface deposition of Ce³⁺ ions, and the junction boundaries were difficult to identify even by HRTEM. The presence of additional absorption edges in the visible region absorption for 10% Ce³⁺-impregnated ZnO along with the bandgap edge of ZnO also supports the same.⁴⁵ Owing to two distinct absorption edges, in the absorption spectra, the two components of the composite, i.e. ZnO and Ce₂O₃, behave as independent photocatalysts under illumination and support each other in suppressing the unwanted charge carrier recombination. The substantial decrease in the luminescence intensity further augments this approach. ZnO has the valence band edge at +2.89 eV, whereas Ce₂O₃ possesses the same at +1.90 eV. Similarly, the conduction band edges of ZnO and Ce₂O₃ lie at −0.31 and −0.5 eV, respectively.⁶⁰ Based on the suitability of their band edges, it can be predicted that both the components mutually facilitate each other through electrochemically allowed transitions to avoid charge carrier recombination and enhance the lifetime of the excited states. Both ZnO and Ce₂O₃ have suitable band edges for the generation of superoxide radicals. Consequently, the absorption of photons by any component of the composite results in the enhanced yield of O₂˙⁻ radicals, hence resulting in increased activity for 10% Ce³⁺-loaded catalysts for the degradation of MCP isomers. The same is true for 3% and 5% Ce³⁺-impregnated catalysts, however, the amount of Ce₂O₃ that is formed is probably insufficient for bringing about any significant change; rather, a retarding effect was observed. The energetically favored transitions, which determine the enhanced activity of 10% Ce³⁺-loaded ZnO, are pictorially explained in Scheme 2.

Scheme 2 The plausible charge-trapping and transfer mechanisms for 10% Ce³⁺ loaded ZnO in sunlight exposure.

The comparison of 2-CP, 3-CP and 4-CP degradation over 10% Ce³⁺-impregnated ZnO is presented in Fig. 8. The inset of the same figure gives an overview of the TOC removal efficiency of the same catalyst for the three isomers. Due to the rapid degradation of 30 ppm of the MCP substrates, it was difficult to estimate the effect of the position of the chloro group in respective MCP isomers on the degradation process. Moreover, the degradation of the intermediates was extremely fast. To determine the plausible role of the position of the Cl group in the degradation process and identification of the intermediates, the experiments were performed at a higher concentration, i.e. 50 ppm, of MCPs in the presence of the most active (10% Ce³⁺-loaded ZnO) catalyst by maintaining the amount of the catalyst constant. The monitoring of the degradation process was carried out by drawing the samples at a regular interval of 30 min in the first two hours and 60 min in the final hour. The samples drawn after 30 min of sunlight exposure were extracted with dichloromethane for GC-MS analysis.

Fig. 8 The comparison of the percentage degradation 2-CP, 3-CP and 4-CP, 30 ppm, in sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 10% Ce³⁺@ZnO. The inset shows the time scale TOC removal of the three chlorophenol isomers.

The HPLC degradation profiles of 2-CP, 3-CP and 4-CP (50 ppm) are presented in Fig. 9(a)–(c). Due to the decrease in the oxidizing species to substrate ratio, the expected lower rate of degradation of MCP was observed. This relatively lower degradation/mineralization compared to 30 ppm concentration was found to be advantageous in the identification of intermediates, estimation of released ions, the influence of the position of chloride group on degradation process and the prediction of the degradation/mineralization mechanism. The comparison of the percentage degradation of 2-CP, 3-CP and 4-CP for a 50 ppm concentration is presented in Fig. 10(a), where a significantly high degradation of 2-CP compared to 3-CP and 4-CP is observable. The variation in the time-scale percentage degradation of the MCP isomer revealed that the ease of degradation of the respective MCP isomer depends on the position of attachment of the Cl group with the aromatic system. The hindrance due to the stereo-chemical orientation of Cl group, inter- and intra-molecular hydrogen bonding, inductive and resonance effects may be the decisive factors in this regard. The more pronounced inductive effect and intra-molecular hydrogen bonding in 2-CP makes it a soft target for interacting ROS as compared to 3-CP and 4-CP.⁶¹ The low magnitude of the inductive effect due to the position of the Cl group in 3-CP and 4-CP results in the low degradation of these substrates. The inter-molecular hydrogen bonding, leading to stabilized polymeric structure in 4-CP additionally contributes to the low degradation of 4-CP. Keeping in view the electron-withdrawing nature of the Cl group, the role of the inductive effect (−I) in generating the particular sites for the interaction of reactive species cannot be ignored. The inset of Fig. 10(a) shows the graphical evaluation of the rate of degradation of the three isomers. A linear relationship was observed in the degradation and sunlight exposure, which further strengthened the earlier comment that Langmuir–Hinshelwood kinetic model for pseudo first order reactions holds well for moderate reactions. The percentage mineralization (TOC removal) of the three substrates is presented in Fig. 10(b), whereas the inset shows the rate of TOC removal. A trend similar to that observed in the experiments with 30 ppm concentration was prevalent at higher concentrations, as well. The rate of TOC removal decreases with the decrease in the concentration of substrates, as well as the intermediates.

Fig. 9 The comparison of the degradation profiles of (a) 2-CP, (b) 3-CP and (c) 4-CP, 50 ppm, in sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 10% Ce³⁺@ZnO. The inset shows the time scale TOC removal of the three chlorophenol isomers. Arrows on each figure mark the major intermediates.

Fig. 10 The comparison of (a) the percentage degradation of 2-CP, 3-CP and 4-CP, 50 ppm, (b) percentage mineralization in sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 10% Ce³⁺@ZnO. The inset shows the plots of ln(C_o/C) versus time. The insets of (a) and (b) show the graphical evaluations of the rates of degradation and mineralization, respectively.

The ion chromatography (IC) profiles of the three isomers are presented in Fig. S4 (ESI†), where the release of chloride ions in solution, corresponding to degradation, was observed. The release of Cl⁻ ions in substantial concentration revealed the involvement of superoxide anion radicals in the degradation of MCP isomers and clearly predicted that the degradation of the MCP isomers is strictly associated with the displacement of Cl⁻; moreover, the same is not achievable with the interaction of radical species. In MCP isomers, Cl atoms, which are electronegative in nature, induce the positive charge on the attached carbon atom by their electron-withdrawing effect, thus generating a positive center for superoxide attack. The appearance of additional peaks evidenced further interaction of Cl⁻ ions with the reactive oxygen species, particularly superoxide anion radicals. The formation of ClO₂⁻, ClO₃⁻ and ClO₄⁻ was identified and plotted in comparison to Cl⁻ ion for each isomer and presented in Fig. 11.

Fig. 11 The comparison of the release of various ions during the course of chlorophenol (50 ppm) isomers degradation and mineralization (a) 2-CP, (b) 3-CP and (c) 4-CP.

The degradation of each isomer was also investigated in the mixture of three isomers containing ∼17 ppm of each isomer. The percentage degradation profiles of the three isomers are presented in Fig. 12, whereas the inset shows the HPLC profile for the simultaneous degradation of three isomers in the mixture at various time intervals. It was observed that the degradation pattern of the isomers in the mixture was similar to that of the individual isomer, which depicted the selectivity of the oxidizing species for the respective isomer.

Fig. 12 The comparison of the percentage degradation of 2-CP, 3-CP and 4-CP in the mixture of three isomers (17 ppm each) in sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 10% Ce³⁺@ZnO. The inset shows the HPLC degradation profile of three isomers in the mixture.

In the HPLC analysis, 2-CP, 3-CP and 4-CP were eluted at 6.5, 8.1 and 7.8 min, respectively, under our experimental conditions previously described. The common intermediates, having same retention time for the three isomers, appeared at the retention times of 2.05, 2.8 and 3.6 min. The intermediate at 4.6 min was common both for 2-CP and 3-CP and absent for 4-CP. An additional peak appeared at 3.3 min for 4-CP. The comparison of the HPLC chromatograms, recorded after 30 min of sunlight exposure, show the retention time of the three substrates, and the intermediates are presented in Fig. 13.

Fig. 13 The comparison of (a) HPLC chromatograms of 2-CP, 3-CP and 4-CP, 50 ppm, after 30 min of sunlight exposure (1000 ± 100 × 10² lx) in an aqueous suspension (100 mg/150 ml) over 10% Ce³⁺@ZnO. The retention times of the substrates and major intermediates are presented in the figure.

The above-mentioned samples were subjected to GC-MS analysis, and the identification of the respective isomer, in each analysis, was used as a marker. 2-CP, 3-CP and 4-CP were eluted at 11.408, 18.708 and 18.625 min, respectively. The comparison of the GC chromatograms for the three isomers is presented in Fig. 14. Interestingly, the aliphatic oxygenates were identified as the major components of the intermediates in GC-MS analysis. It is important to mention here that no chloro group bearing an aromatic compound was identified for the three isomers. The compounds eluted at retention times of 2.06 and 2.8 min in HPLC analysis were identified as C₄H₈O₂ (RT = 3.142 min) and C₆H₁₂O₂ (RT = 4.042 min) with mass numbers 88 and 116 g mol⁻¹, respectively. Another compound, in appreciable quantity, also having the mass number 88 g mol⁻¹, was eluted at RT = 3.558 min and identified as C₅H₁₂O. The above-mentioned intermediates were found commonly in the degradation of isomers. Another common compound, common for both 2-CP and 3-CP was observed at 8.150 min and identified as C₇H₁₂O₂ with the mass number of 128 g mol⁻¹. C₅H₁₂O₂ (RT = 5.317 min) was observed only for 4-CP. A number of intermediates ranging from C₁–C₆ (such as formic acid, acetaldehyde, acetic acid, and propionaldehyde) were also observed, in addition to the above-mentioned major intermediates. Moreover, aliphatic intermediates, as well as aromatic compounds like xylene and benzaldehyde, were also spotted in the analysis. Note that the majority of identifying compounds were oxygenates, which led to the assumption that the degradation of the MCP proceeds with the insertion of oxygen in the aromatic ring, leading to the cleavage and formation of oxygenates.

Fig. 14 GC-MS profiles of 2-CP, 3-CP, 4-CP and their mixture thereof over 10% Ce³⁺-impregnated ZnO after 30 min of sunlight exposure. The inset shows the separation of 3-CP and 4-CP peaks.

In the present study, the theoretical and experimental evidences, such as the conduction band edge position of ZnO (−0.31 eV), pH_ZPC of ZnO (9.2), rapid mineralization, release of chloride ions in solution and the presence of aliphatic oxygenated intermediates, strongly supports the involvement of superoxide anion radicals in the degradation and mineralization process. Because superoxide anion radicals (O₂˙⁻) are negatively charged entities, they seek positively charged locations to initiate the oxidation process rather than non-selective interaction, as in the case of hydroxyl radicals. The release of Cl⁻ ions in the solution supports the view that Cl-bearing carbon in MCPs facilitates the superoxide attack.

Based on the above-mentioned experimental evidences, it can be inferred that the mineralization of MCP isomers is furnished mainly by the superoxide anion radicals. Although occurring simultaneously, the MCP isomers are initially degraded by the loss of aromaticity and the formation of oxygenated intermediates. The presence of chloro groups on the aromatic ring facilitates the degradation by imparting partial positive charge to the attached carbon atom. The intermediates, mostly polar oxygenates, are further interacted by superoxide anion radicals to mineralization. The identification of C₁, C₂ and C₃ products in GC-MS analysis revealed that larger (C₄–C₆) intermediates undergo further fragmentation before being mineralized. The presence of electronegative oxygen in the oxygenated intermediates facilitates the mineralization process. The plausible mechanism for the degradation of MCP isomers over 1% Ce³⁺-loaded ZnO and considering 4-CP as a model substrate can be further elaborated by Scheme 3 as follows.

Scheme 3 The anticipated mechanism of degradation/mineralization of 4-CP over 1% Ce³⁺-loaded ZnO.

The monitoring of released Zn²⁺ ions in the solution under sunlight illumination for bare and Ce³⁺ impregnated ZnO catalysts revealed ∼13% corrosion of bare ZnO with respect to the weight of the loaded catalyst. Up to 5% Ce³⁺ loading an average of 4% photocorrosion was noticed, whereas it was less than 1% for 10% Ce³⁺-loaded ZnO. The above-mentioned results are based on the measurement of Zn by ICP-OES after sunlight exposure of 150 min.

Conclusion

The surface presence of Ce³⁺ ions not only suppresses the unwanted e⁻–h⁺ recombination but also improves the delivery of captured electrons to reductants that substantially enhances photocatalytic activity. The correlation of the data obtained by various analytical techniques revealed the superoxide anion radicals as major contributors in the mineralization process. The process of mineralization occurs in successive steps, starting from the loss of aromaticity and the formation of C₄–C₆ intermediates. The intermediates are further subjected to superoxide anion radical attack, which leads to the complete mineralization of MCP isomers. The presence of chloro groups facilitates the mineralization process by imparting an additional positive charge to the attached carbon atom. The study provides a base for sunlight photocatalytic mineralization of stable hazardous contaminants to complete detoxification. The surface presence of Ce³⁺ not only enhances photocatalytic activity but significantly suppresses the photocorrosion of ZnO.

Acknowledgements

Iqbal M. I. Ismail, A. Hameed and M. Aslam are greatly indebted to the Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, and the Ministry of Higher Education (MoHE), Kingdom of Saudi Arabia, for their support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07038k

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