Enhanced mineralization of oxalate by highly active and Stable Ce(III)-Doped g-C3N4 catalyzed ozonation

Chemosphere 239 (2020) 124612

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Enhanced mineralization of oxalate by highly active and Stable Ce(III)Doped g-C3N4 catalyzed ozonation Yu Xie a, b, Shuhan Peng a, b, Yong Feng c, **, Deli Wu a, b, * a

State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science & Engineering, Tongji University, Shanghai, 200092, PR China b Shanghai Institute of Pollution Control Ecological Security, Shanghai, 200092, PR China c Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Dual reaction sites of Ce(III)/≡OH on g-C3N4 was used for catalytic ozonation.
Stable Ce(III) doping was realized by g-C3N4 coordination and ≡OH formation.
Complexation between ≡Ce(III) and OA is crucial step for enhanced mineralization.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 May 2019 Received in revised form 27 July 2019 Accepted 17 August 2019 Available online 31 August 2019

The degradation of carboxylic acid has been identified as one of the rate-determining steps in the mineralization of organic pollutants by ozonation. In this study, Ce(III)-doped graphitic carbon nitride (Ce eCN) composites with different Ce(III) contents were synthesized and used as catalysts for the ozonation of oxalate. The morphology and structure of the CeeCN were comprehensively characterized using various techniques such as SEM, XRD, FTIR, and XPS. The results show that the structure of g-C3N4 provided an ideal site for the accommodation of Ce(III) and thus facilitated the formation of surface hydroxyl groups. With 2.5%CeeCN as a catalyst, the degradation efficiency of oxalate was increased by 47.1% after reaction for 30 min. The decomposition of ozone was accelerated in the presence of CeeCN. Hydroxyl radicals were recorded by electron spin resonance and identified as the major actives species. Under the catalysis of 2.5%CeeCN, the production of hydroxyl radicals was increased by 40%. The Ce(III) and surface hydroxyl groups that distributed uniformly on the surface of CeeCN were speculated as the dual catalytic sites for the complexation of oxalate and activation of ozone, respectively. CeeCN had a high stability and reutilization capability. It is proposed that a complex was formed between surface Ce(III) and oxalate, and this complex could be more easily attacked by the surrounding ozone and hydroxyl radicals than free oxalate. As oxalate is a typical recalcitrant carboxylic acid, the findings from this study are expected to promote the application of ozonation in the removal of organic pollutants. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: Catalytic ozonation Mineralization Cerium Surface hydroxyl groups Graphitic carbon nitride

* Corresponding author. State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science & Engineering, Tongji University, Shanghai, 200092, PR China. ** Corresponding author. E-mail address: [email protected] (D. Wu). https://doi.org/10.1016/j.chemosphere.2019.124612 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

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1. Introduction Owing to the high redox potential of ozone (E0 ¼ 2.07 V) and its environmentally friendly nature, ozonation has become one of the most popular oxidation processes for water decontamination (Kasprzyk-Hordern et al., 2003). However, O3 has a high selectivity and reacts very slowly with some organic compounds such as inactivated aromatics (Mehrjouei et al., 2015). To overcome the drawbacks with O3, catalytic ozonation has been investigated to convert O3 to highly oxidizing and nonselective hydroxyl radicals (
OH) (Nawrocki and Kasprzyk-Hordern, 2010; Huang et al., 2015). As various oxidation processes developed, heterogeneous catalytic ozonation can easily recycle and reuse solid catalysts and is thus currently the most commonly pursued strategy to enhance the oxidation capacity of O3 (Li et al., 2009). Although the removal of certain aromatic compounds that are refractory to O3 can be enhanced by the effective transformation of O3 to
OH using common ozone catalysts, the effective mineralization of these compounds remains difficult. The difficulty in the mineralization can be related to the accumulation of carboxylic acids, which are the typical ozonation intermediates of aromatic compounds and are low reactive toward both O3 and
OH (Faria et al., 2008). Oxalate (OA), a typical carboxylic intermediate, does not readily react with O3 (0.04 M1 s1) and reacts relatively slowly with
OH (7.7  106 M1 s1), which is 2e3 orders of magnitude slower than the reaction of
OH with common electronrich organic compounds (Pines and Reckhow, 2002). In fact, the degradation of OA and other short-chain carboxylic acids are known as the “last mile” in the complete mineralization of organics (Shan et al., 2018). Therefore, the effect to improve the degradation of these compounds will help to promote the application of catalytic ozonation in water treatment (Shahidi et al., 2015). Heterogeneous catalytic ozonation is often associated with the enhanced decomposition of ozone and the generation of reactive oxygen species initiated by the introduced surface hydroxyl group, which have been widely accepted as the active sites for
OH production for most of the ozone catalysts (Qi et al., 2008; Zhang et al., 2008; He et al., 2017). The modification of materials with transition metals (e.g. Mn, Cu) has been proved to be an effective method to enhance the density of surface hydroxyl groups (Lei et al., 2009; Zhao et al., 2010). However, the carboxylic intermediates after preliminary oxidation are hard to further be degraded by O3/
OH. It has been proposed that formation of a metal-carboxyl acid complex is an effective strategy to transform the origin carboxyl acid to the form with increasing reactivity toward both O3 and
OH (And and Reckhow, 2002; Zhang et al., 2011, 2012b). An efficient ozone catalyst should theoretically equip with dual active sites for both O3 decomposition and OA complexation. Additionally, the design of dual reaction sites that greatly reduce the migration distance between radical species and organics, which has been proven as an effective strategy to improve the utilization of radical species in Fenton-like system (Li et al., 2018). On the other hand, the leaching of metal not only leads to the potential contamination of metals but also results in the deactivation of catalysts. Therefore, the leaching of metals from metal-based heterogeneous catalysts, particularly under acidic conditions, is also a critical problem remaining to be solved (Yang et al., 2017, 2018). In view of the above challenges regarding the development of catalytic ozonation, the main purpose of this study was to develop an efficient and stable heterogeneous catalyst with functional surfaces for ozonation. Graphitic carbon nitride (g-C3N4), a novel

low cost nitrogen-rich material widely investigated in photocatalysis (Ong et al., 2016), was selected as the catalyst carrier. The unique structure of g-C3N4 that contains heptazine rings with pyridinic nitrogen groups and six lone-pair electrons not only enables g-C3N4 as an electron-rich donor but also provides ideal sites for the accommodation of metal ions (Wang et al., 2010; Deng et al., 2015; Zhang et al., 2019). As cerium (Ce)-containing materials had previously shown high reactivity in the catalysis of ozone and displayed strong complexing capability toward oxalate (Zhang et al., 2011; Xu et al., 2016), Ce was doped into the framework of g-C3N4 (CeeCN) and used to provide structural catalytic sites for ozonation. Combining stable materials with efficient catalysts is the focus of heterogeneous catalysts research. CeeCN composites with different Ce contents were synthesized using a facile approach and their structural and surface properties were fully characterized. The catalytic performance of CeeCN toward O3 was comprehensively investigated in terms of the degradation and mineralization of OA. The oxidizing species in the catalytic systems were identified, and the reusability and stability of CeeCN were also evaluated. Finally, catalytic mechanisms with CeeCN were proposed. 2. Experimental section 2.1. Chemicals and materials All chemicals were of analytical grade and used without further purification. Cerium (III) nitrate hexahydrate, cerium (IV) oxide, cerium oxalate, melamine, oxalate, hydrochloric acid, sodium hydroxide was purchased from Aladdin Reagent (Shanghai, China). Ultrapure water was used for all experiments. Polytetrafluoroethylene syringe filters (0.22 mm) were obtained from ANPEL Scientific Instrument (Shanghai, China). Pristine g-C3N4 was synthesized according to a previously reported method with slightly modification (Liu et al., 2016; Bicalho et al., 2017). In brief, 50 mmol of melamine (6.3 g) was added to 100 mL of water solution that contains 5 mL of hydrochloric acid (37%), which was stirred in a water both for 30 min at 100  C. The suspension with white precipitates obtained was then dried at 85  C for 12 h to remove the liquid. The resulting solids were placed in an alumina crucible, heated to 500  C at 10  C/min in a tube furnace under the protection of N2 (100 mL/min), and held for 2 h. The products obtained were then ground to powders for further use. CeeCN was synthesized using a similar approach to that described above for the pristine g-C3N4, except that a desired amount of Ce(NO3)3$6H2O was added to the melamine solution. The obtained yellow products were donated as X%CeeCN, where X is the molar percentage of Ce added into the melamine solution. 2.2. Catalytic ozonation procedure The catalytic ozonation of OA was carried out in semicontinuous mode in a cylindrical borosilicate glass reactor (2.4 L) equipped with mechanical agitation equipment at room temperature. At predetermined time intervals, solution samples were withdrawn and immediately mixed with a diluted Na2S2O3 solution to terminate the reaction caused by residual O3. The ozone generator (KT-OZ-10 g, CONT) was pre-operated for 1 h to reach a steady flow of ozone. During each reaction process, the ozone/oxygen gas mixture bubbled into the reactor was maintained at 2 L/min (101 kPa, 25  C) through a porous sand core filter plate (0.22 mm)

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located at the bottom of the reactor. To study the effect of adsorption, control experiments using pure oxygen instead of ozone were carried out under identical conditions. In each experiment, the reactor was filled with 2 L of OA solution (0.5 mM) at its intrinsic pH (3.5). To evaluate the stability of the catalyst, the CeeCN after each catalytic cycle was separated, washed five times with ultrapure water, dried at 60  C for 12 h, and tested for OA degradation. 2.3. Analytical methods The concentration of OA was measured using high performance liquid chromatography (HPLC, Agilent 1260, USA) equipped with a DAD detector at 210 nm. Elution for OA was performed on an Agilent Eclipse XDB C18 column (5 mm; 4.5  250 mm). A mixture of methanol and H3PO4 solution (pH 2.5) at a volume ratio of 5:95 as the mobile phase. The flow rate was fixed at 0.8 mL/min. The Ce leached from the solids was measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Agilent 720 ES, USA). The concentration of O3 in the gaseous and aqueous phases was quantified with the iodometric method (Rakness et al., 1996)
, 1981), respectively. The and the indigo method (Bader and Hoigne mineralization of organics was measured with a total organic carbon analyzer (TOC, Shimadzu TOC 5000, Japan). Scanning electron microscope coupled with an energy dispersive spectrometer (SEM-EDS, Hitachi S-4800, Japan), X-ray diffractometer (XRD, Bruker, Germany), X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alphaþ, USA), and Fourier transform infrared-spectroscopy (FTIR, Nicolet 5700, USA) were used to characterize the catalysts. The N2 adsorption/desorption analysis was performed on a Micromeritics ASAP 2460 surface area analyzer. The zeta potential of CeeCN was measured using Zetasizer Nano ZS (Malvern, UK). DMPO was employed as the spintrapping agent for the electron paramagnetic resonance (EPR) study to detect the radical species. The detailed operating parameters are provided in Text S1. 3. Results and discussion 3.1. Structure and morphology of the catalyst The XRD patterns of CeeCN with different Ce-doping contents are displayed in Fig. 1a. The diffraction peak at 12.89 was ascribed to the (100) facet of the ordering of the tri-s-triazine units. The peak located at 27.32 was ascribed to the (002) facet of the stacking of the conjugated aromatic complex. The intensity of both these peaks

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decreased with the increase of Ce content, indicating the disappearance of rigid layered structures of g-C3N4. In addition, no peaks of Ce-containing species were observed in all the patterns. These results suggest that Ce was successfully doped into the framework of g-C3N4 (Ma et al., 2017). The FTIR spectra further illustrate the structure of CeeCN with different Ce contents (Fig. 1b). For the sample without Ce doping, it reveals the typical molecular structure of g-C3N4. In the case of the CeeCN composites, all the characteristic absorption peaks of gC3N4 were observed, which confirmed the framework of g-C3N4 in the composites. The intense absorption band at 807 cm1 was assigned to the out of plane bending modes of triazine units (Liu et al., 2011). The broad absorption band at around 3200 cm1 was ascribed to the stretching vibration of the NeH bond from the uncondensed terminal NH2/NH groups and the OeH bond from the surface adsorbed H2O (Wang et al., 2012; Zhang et al., 2012a). In addition, the multiple bands in the range of 1200e1600 cm1 were attributed to the stretching vibration from CN heterocyclic (Tian et al., 2015). CeO2 is the substance usually generated in the preparation of Ce-containing catalysts, and this substance has two typical stretching vibration adsorption peaks at around 624 cm1 and 3400 cm1 (Li et al., 2017). However, no such peaks are observed in all the CeeCN composites, which is consistent with the XRD results that the Ce was successfully coordinated into the framework of g-C3N4 (Bing et al., 2011; Jin et al., 2015). Like other transition metals, the Ce in CeeCN probably formed CeeN bonds with the N (Wang et al., 2010; Yuan et al., 2018). SEM analysis were conducted to investigate the morphology of the catalysts. Both of g-C3N4 and 2.5% CeeCN consisted of irregular surface with aggregated structures at a micron scale (Fig. 2a and b; Figs. S1eS2). Although the aggregated g-C3N4 crystals dispersed to many irregular particles, the surface of g-C3N4 presented smooth and flat layers (Kumar et al., 2014). However, an obvious increase in the surface roughness was observed after doping of Ce, which usually occurs with the other common transition metal-doped gC3N4. It is noted that the CeeCN shows a well-connected pore morphology, which is assumed to be caused by the enhanced release of gases during the thermal polymerization of melamine and the stacking of irregular layers. It is reported that the presence of metal could induce the decomposition of the precursor melamine to gases and consequently resulted in an increase in the specific surface area (Ma et al., 2017). Due to its firmly stacked structure, the BET specific surface area of pristine g-C3N4 (11.7 m2 g1) was small. After doping of Ce (2.5%), the specific surface area was increased to 20.2 m2 g1, which was probably due to the relatively less ordered structure (Table S1). In addition, Ce2O3 is

Fig. 1. (a) XRD patterns and (b) FT-IR spectra of CeeCN with different Ce contents.

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Fig. 2. SEM images of (a) g-C3N4; (b) 2.5%CeeCN, N2 adsorption/desorption isotherm and BJH pore-size distribution plot (insert) of (c) g-C3N4 and (d) 2.5%CeeCN.

easily oxidized to CeO2 by oxygen. Therefore, If the materials were simply compounded by g-C3N4 and Ce2O3, two different particles should appear in the SEM image of the material. As the structure of CeO2 was spherical (Fig. S3) and the structure of g-C3N4 was lamellar, there does not exist the above two kinds of particles in the SEM of synthesized materials, but more complex lamellar structure. The BJH pore-size distribution and N2 adsorption/desorption isotherm of g-C3N4 and 2.5%CeeCN are displayed in Fig. 2bed. BET analysis data suggest that the surface of 2.5%CeeCN was made up of slits of polymer-flake particles. Combining SEM and BET data, cerium was doped into the material rather than simply mixed together. The isoelectric points of the catalyst before and after Ce doping were measured as a function of solution pH. The point of zero charge of g-C3N4 declined from 5.28 to 4.10 after doping (Fig. S4). Therefore, the presence of Ce during the thermal polymerization of melamine influenced the surface properties of g-C3N4. The increased specific surface area and pore structures in 2.5%CeeCN were expected to be favorable to the diffusion, adsorption, and desorption of the pollutants and ozone molecular during the catalytic ozonation (Hu et al., 2014). The energy dispersive spectroscopic analysis (EDS) was used to confirm the element in 2.5%CeeCN (Fig. S5). Five elements including C, N, O, Ce, and Cl were detected in 2.5% CeeCN with the atomic proportion of 42.11%, 55.63%, 1.84%, 0.19%, and 0.23%, respectively. The ratio of C/N in 2.5%CeeCN was approximately 0.757, which is consistent with the atomic ratio of C3N4. The ratio of Ce/O was approximately 0.10, which suggests that the existing form of Ce was neither Ce2O3 (Ce/O ¼ 0.67) nor CeO2 (Ce/O ¼ 0.5). The

elemental mapping results further indicate that both Ce and O species were uniformly distributed in the framework of g-C3N4 (Fig. S6). The XPS analysis was used to confirm the chemical composition of 2.5%CeeCN composite and the valent state of elements. Elements including Ce, C, N, and O are observed in 2.5%CeeCN (Fig. 3a). The high resolution XPS spectrum of C 1s could be deconvoluted into two peaks (Fig. 3b). The peaks with binding energies of 284.5 eV and 288.0 eV were assigned to the sp2 hybridized C-(N)3 and N] CeN group of the triazine structure inside g-C3N4 (Tian et al., 2015). The main features of N 1s are presented in Fig. 3c. The peaks at 400.2 eV, 398.6 eV, and 404.8 eV correspond with the sp2-bonded nitrogen atom of N-(C)3, C]NeC of the tertiary nitrogen in triazine, and the peexcitations, respectively (Jourshabani et al., 2017). The spectrum of Ce 3d could be spitted into five peaks, in which two states of Ce are observed (Fig. 3d). The binding energies located at 904.1 eV, 899.4 eV, 885.6 eV were associated with the presence of the Ce(III), while the signals appearing at 900.5 eV and 881.9 eV ^che et al., 2010). Based on the indicate the presence of Ce(IV) (Be XPS fitting results, the fraction of Ce(IV) on the surface of 2.5% CeeCN was only 8.9%, while the surface Ce species were dominated by Ce(III) with a content of 91.1%. The peak located at 531.9 eV in the spectrum of O 1s suggested the presence of surface hydroxyl groups on the catalyst surface (Fig. 3e) (Qi et al., 2008). No peaks were observed at 530.6 eV and 533.8 eV, which suggests the absence of lattice oxygen (O2) and adsorbed H2O. According to XPS data, the peak of oxygen element in the undoped g-c3n4 appears, while the peak of oxygen element appears after doping. The hydroxyl group on the surface of the material was 0.49 mmol/g. The results showed

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Fig. 3. XPS survey spectra of g-C3N4 and 2.5%CeeCN composite (a) and XPS spectra of the 2.5%CeeCN: (b) C 1s, (c) N 1s, (d) Ce 3d, and (e) O 1s.

that the Ce-doped material had more surface hydroxyl groups, which improved the performance of the material to catalyze ozone. 3.2. Catalytic performance The optimal doping dosage of Ce was measured by investigating the catalytic reactivity of CeeCN toward OA degradation. As shown in Fig. 4a, the catalytic reactivity of CeeCN was influenced by the molar ratio between Ce(NO3)3 and melamine. Pristine g-C3N4 was an inert material and showed no obvious catalytic performance in the removal of OA. When CeeCN was used as the catalyst instead, the degradation of OA was greatly increased. With the increase of Ce content from 0 to 5%, the reactivity of the composite first increased and then decreased. The optimal Ce/melamine ratio was

measured as 2.5%. With 2.5%CeeCN as the catalyst, the degradation of OA was increased by 37.4%, 62.9%, 42.5% after reaction for 10, 20 and 30 min, respectively. It has been reported that the presence of metal ions in g-C3N4 induces the formation of surface defects such as the bond of eC^N, which has a characteristic peak at around 2174 cm1 in the FTIR spectrum (Bing et al., 2011). As shown in Fig. 1b, the adsorption intensity at 2174 cm1 was enhanced with the increase of Ce content. As the eC^N bond is generated from the collapse of the triazine ring (especially from the sp2 hybrid CeN bond), it is expected to have a negative effect on the doping process. The excessive collapse of the triazine ring resulted in the reduction of the lone pair electrons formed by the vacancy of pyridine ring, which weakened the complexation of metal with g-C3N4 and made Ce difficult to be effectively doped. In addition, an

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Fig. 4. (a) Effect of Ce contents and (b) comparison of common catalysts on the catalytic ozonation of OA. Reaction conditions: [OA]0 ¼ 0.5 mM, [CeeCN] ¼ 100 mg/L, [O3] ¼ 6.6 mg/ min, pH0 ¼ 3.5, and t ¼ 30 min.

increased level of eC^N has a strong quenching effect toward radicals (Oh et al., 2017). On the one hand, an increase in the amount of metal doped is expected to improve the catalytic reactivity via increasing the density of active sites. On the other hand, the metal excessively was not conducive to the formation of g-C3N4, which resulted in the loss of doping sites and hence reduced the catalytic reactivity. In the doping range of 0e2.5%, the enhancement effect resulting from the increase of the active sites was stronger than the adverse effect. With the further increasing of Ce doping, the Ce species probably existed as unstable large grains instead of the amorphous or microcrystalline morphology with higher stability and activity, thus leading to the decline in the catalytic performance (Fei et al., 2016). MnO2 is one of the most commonly used catalysts for ozonation. To further evaluate the reactivity of 2.5%CeeCN, the degradation of OA by MnO2/O3 was tested. As seen in Fig. 4b, the overall degradation rate of OA with 2.5%CeeCN was more than 35% greater than that with MnO2. CeO2 is usually used as a support for ozone catalysts and its reactivity toward O3 was also evaluated. As expected, no promotion effect from CeO2 on the catalysis of O3 was observed (Fig. 4b). In fact, studies on Ce-based solid catalysts shown that the Ce(III) instead of Ce(IV) was the core species responsible for the catalysis of O3 (Yan et al., 2013; Shan et al., 2018). The high content of Ce(III) in 2.5%CeeCN (91.1%) explains the efficient performance of this composite catalyst. The dosage of 2.5%CeeCN is also an important parameter in heterogeneous catalytic ozonation. With a constant concentration of ozone, the effect of catalyst dosages on the degradation of OA (0.5 mM) was investigated (pH0 ¼ 3.5) (Fig. S7). When the dosage of 2.5%CeeCN was increased from 0 to 100 mg/L, the overall degradation of OA was increased from around 54.9%e96.1%. However, a further increase in the dosage did not have any obvious effect on the catalytic degradation (Fig. S7a) and TOC removal (Fig. S7b). Considering the cost of the catalyst, the optimal dosage used in the subsequent experiment was fixed at 100 mg/L. In addition, the final pH values of the reaction suspensions were increased under all the dosages tested (Fig. S7c), which could be ascribed to the decarboxylation of OA and consumption of Hþ during the catalytic ozonation. 3.3. Effect of initial pH values pH value is one of the key parameters that is expected to significantly influence the ozonation performance. To further explore the applicability of 2.5%CeeCN, the degradation of OA by 2.5%CeeCN/O3 was studied under different initial pH conditions

(3.5e9) (Fig. S8). Under all the pH conditions investigated, the degradation of OA by O3 alone and 2.5%CeeCN/O3 followed pseudofirst order kinetics. Hydroxyl ion (OH) is one of the most common initiators for ozone decomposition, and therefore more rapid decomposition of ozone and removal of OA was observed under alkaline conditions (pH0 ¼ 7 and 9) in the absence of CeeCN. Under all pH conditions investigated, the apparent rate constants for OA degradation with 2.5%CeeCN/O3 (0.081e0.129 min1) were all notably higher than those with O3 alone (0.005e0.043 min1), suggesting the significant catalytic effect of 2.5%CeeCN. The optimal pH value was found to be 3.5. This observation was probably resulted from the surface charge property of the catalyst. It has been reported that ozonation usually has the best performance at near-neutral conditions, in which O3 molecular can easily combine with the H of the surface hydroxyl groups (Ikhlaq et al., 2012; Ren et al., 2012). As the pHpzc of 2.5%CeeCN was around 4.10 (Fig. S4), it is speculated that the neutral surface of 2.5%CeeCN (MeeOH) was more active than the deprotonated (MeeOHþ 2 ) or protonated (MeeO-) surface to enhance the generation of ROS. Moreover, the prepared 2.5%CeeCN had a strong electrostatic effect and its zero-point charge fell in the acidic region, which expanded the effective range of catalytic ozonation to acidic conditions. As OA was completely removed after 30 min, the mineralization rates achieved after reaction for 20 min were measured and compared. Results from Fig. 5b show that the mineralization rates of OA under all the pH conditions were around 80%, which indicates that 2.5% CeeCN had good catalytic performance in a wide range of pH values. The reaction follows pseudo-first-order reaction kinetics, which indicates that under certain conditions, the effect of oxalic acid degradation rate is only related to oxalic acid concentration. This is another evidence indicate that the active site of the material is stable. The ability of 2.5%CeeCN in promoting the decomposition of ozone was reflected by the variation of dissolved ozone during each process. As the decomposition of ozone induced by OH could not be neglected under alkaline conditions, the degradation of OA and the corresponding dissolved concentration of O3 at two different pH values (3.5 and 7) were measured (Fig. 6). As the O3 gas was continuously bubbled into the reactor, the concentration of dissolved O3 raised rapidly and gradually reached a steady state. The steady concentrations of O3 in the absence and presence of CeeCN were 18.32 and 15.57 mg/L, respectively. The relatively higher level of dissolved O3 in the absence of CeeCN could be attributed to the difficulty in the decomposition of O3. With the addition of 2.5% CeeCN, the dissolved concentration of O3 was reduced by about 16.1%. With a higher proportion of ozone molecules effectively

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Fig. 5. (a) Kinetics of catalytic ozonation of OA under different initial pH conditions and (b) mineralization of OA by different processes after reaction for 20 min. Reaction conditions: [OA]0 ¼ 0.5 mM, [2.5%CeeCN] ¼ 100 mg/L, and [O3] ¼ 6.6 mg/min.

decomposed and utilized, the removal of OA was enhanced. The attenuation of dissolved ozone occurred in both systems at the late stage of the reaction. It was mainly because the decarboxylation of OA gradually increased the solution pH, which was beneficial for the homogeneous decomposition of dissolved ozone initiated by OH. In the case of neutral condition, the overall level of O3 concentration was lower than that under acidic conditions and the steady concentration of O3 in catalytic ozonation (13.23 mg/L) was 11.0% lower than that in sole ozonation (14.87 mg/L). In addition, the difference of the steady dissolved O3 concentration between catalytic system and sole ozonation system was minor in neutral condition (11.0%) than in acidic condition (16.1%). In the preliminary experiment, ozone was introduced into the water containing g-C3N4 material, and the dissolution curve of ozone was basically the same as that of sole ozone. This indicated that g-C3N4 material could not effectively adsorb ozone molecules. From XPS analysis, surface hydroxyl group appeared after doping Ce. Surface hydroxyl group could combine with ozone molecules quickly and catalyzed ozone molecules to form free radicals immediately. It suggested that the adsorption site of ozone on the material is the catalytic site. The catalytic effect of 2.5%CeeCN in promoting the decomposition of O3 was inhibited to some extents under neutral conditions, which was caused by electrostatic repulsion. The morphological distribution of oxalic acid at different pH values is shown in Fig. S9. When the pH value of solution was 3.5, the main form was hydrogen oxalate, and it was negative divalent oxalate under

neutral condition. As the point of zero charge of CeeC3N4 was 4.10, the surface was positively charged at a pH of 3.5 and negatively charged at neutral condition. Due to electrostatic interaction, the transfer rate between different phases was reduced under neutral conditions, which slightly affected the removal efficiency of oxalic acid. By combining the catalytic ozonation behavior and the variation of O3 under different pH conditions, it can be concluded that 2.5%CeeCN was capable of facilitating the decomposition of ozone molecular in the liquid-solid phase. 3.4. Stability of 2.5%CeeCN The structural stability and catalytic performance of catalyst are important for practical applications. Herein, the reutilization capability of 2.5%CeeCN was investigated (details were listed in Text S2). The removal and mineralization of OA in different catalytic cycles were shown in Fig. 7. In all cycles, the overall degradation rates of OA were more than 92.4% and the corresponding mineralization degrees were more than 90%. The TOC removal was almost synchronously to the removal of OA, suggesting the direct mineralization of OA. Compared with the performances in the 1st cycle, the removal rates of OA and TOC in the 5th cycle declined by only 7.6% and 3.5%, respectively. The leaching of Ce during each run was also monitored. As shown in Fig. S10, less than 0.02 wt% of Ce in the 2.5%CeeCN was leached out in the 1st run, and the concentration of Ce was below the quantification limit (5 mg/L) in the next four runs, indicating the high stability of CeeCN. In addition, the morphology

Fig. 6. (a) Removal of OA by O3 alone and 2.5%CeeCN/O3 under different pH conditions and (b) the corresponding dissolved concentrations of O3 in the reaction systems. Reaction conditions: [OA]0 ¼ 0.5 mM, [2.5%CeeCN] ¼ 100 mg/L, [O3] ¼ 6.6 mg/min, and t ¼ 30 min.

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is no lattice oxygen at catalyst, which makes it difficult for cerium to change its price through gain and loss of lattice oxygen. In addition, ozone is catalyzed by surface hydroxyl, which is easy to regenerate in aqueous solution. In a word, it suggests that the Ce(III) are protected by surface hydroxyl and A stable lattice without lattice oxygen. The structural stability provided by the affinity between Ce(III) and g-C3N4 is considered as a prerequisite to guarantee the sustainable catalytic performance. 3.5. Identification of radical species and active sites

Fig. 7. Catalytic ozonation of OA by 2.5%CeeCN/O3 at different catalytic cycles. Reaction conditions: [OA]0 ¼ 0.5 mM, [CeeCN] ¼ 100 mg/L, [O3] ¼ 6.6 mg/min, pH0 ¼ 3.5, and t ¼ 30 min.

of the catalyst after reusing was investigated (Figs. S11eS12). After the catalytic experiments, the defects on the surface of 2.5%CeeCN were intensified, which led to more exposure of Ce. the XPS of materials after several rounds shown that Ce(III) were the dominant species, and it were stable. Cr(III) was embedded in the stable CN structure, and very few cerium elements were dissolved after the catalytic experiment. This indicates that cerium is difficult to dissolve in the material. It can be seen from the XPS data that there

ESR was used to investigate the generation of reactive oxygen species during the catalytic ozonation of OA. The ESR spectrum of 2.5%CeeCN/O3 (Fig. 8a) recorded the typical signal of
OH adduct (DMPO-OH), the 4-fold peak with the intensity ratio of 1:2:2:1, and no signal of
OH was recorded when 2.5%CeeCN was absent. These results suggest that 2.5%CeeCN could catalyze the decomposition of O3 even under strongly acidic conditions to produce
OH. The generation of
OH was further evidenced by investigating the effect of tert-butanol (TBA), which is known as a typical
OH scavenger (5.9  109 M1 s1). When TBA was present at a concentration of 5e50 mM, the degradation of OA by 2.5%CeeCN/O3 was largely inhibited (Fig. 8b). On the other hand, the degradation of OA by 2.5% CeeCN/O3 in 30 min were still 10% higher than the control group, which indicated that the addition of 2.5%CeeCN also increased the proportion of OA removed by direct oxidation of ozone molecules. The ESR spectra and scavenging results of TBA suggest that
OH was the major active species produced by 2.5%CeeCN/O3 and responsible for the degradation of OA. The oxidation of benzoic acid to p-hydroxybenzoic acid (p-HBA) is a classical method to quantify
OH in advanced oxidation

Fig. 8. (a) ESR spectra of the 2.5%CeeCN/O3/DMPO system and (b) effects of TBA and (c) PO3 4 on the catalytic ozonation of OA. Reaction conditions: [OA]0 ¼ 0.5 mM, [2.5% CeeCN] ¼ 100 mg/L, [PO3 4 ] ¼ 10 mM, [O3] ¼ 6.6 mg/min, pH0 ¼ 3.5, and t ¼ 30 min.

Y. Xie et al. / Chemosphere 239 (2020) 124612

processes. In this study, benzoic acid was used as a trapping agent and the
OH produced by CeeCN/O3 was quantified by measuring the generated p-HBA. The cumulative amounts of p-HBA after reaction for 30 min suggest that the presence of 2.5%CeeCN could increase approximately 40% of the
OH production (Fig. S13). Although the key role of
OH in the degradation of OA by 2.5% CeeCN/O3 was confirmed above, the source for
OH production remains unclear. The potential role of O2 was investigated because the input gas was a mixture of O2 and O3 and the activation of dissolved O2 by transition metals could generate
OH (Zhao et al., 2018). However, very limited removal of OA (1.2e2.3%) was observed when pure O2 gas instead of the mixture was used. Such limited removal was due to the adsorption of OA on the surface of 2.5%CeeCN (Fig. S14). As geC3N4ebased materials are widely investigated as photocatalysts, the effect of natural light illumination on the degradation of OA by 2.5%CeeCN/O3 was studied by investigating the degradation under dark conditions. As shown in Fig. S15, no obvious difference (<3%) could be observed between the degradation with and without light illumination, which consistently suggests that the removal of OA was caused by the activation of O3. During the catalytic degradation under acidic conditions (pH 3.5), Ce ions with a maximum concentration of around 8.3 mg/L were detected after reaction for 30 min. These dissolved metal ions might activate O3 homogenously to degrade OA. To evaluate this, Ce3þ ions were spiked into the OA solution with a final concentration of 8.3 mg/L and their catalytic effect toward O3 for OA degradation was tested. The results shown that around 55.1% of the OA was degraded in Ce3þ/O3 (Fig. S16). This value was very close to the degradation rate achieved (54.9%) by O3 alone. Therefore, the degradation of OA by the homogeneous activation of O3 was negligible. To identify the active sites on the material surface, phosphate was used to evaluate the role of surface hydroxyl groups and the exposed Lewis acid sites of metals. PO3 4 is a strong Lewis base that has strong affinity toward Lewis acid sites and can form complexes with the metal on solids by replacing the surface hydroxyl groups (Sui et al., 2010). In the presence of 10 mM PO3 4 , the degradation of OA decreased from 99.0% to 22.3%, which suggests the essential role of exposed metal Lewis acid sites and hydroxyl groups in the catalytic ozonation (Fig. 8c). The XPS spectra of the 2.5%CeeCN after different catalytic cycles were recorded to reveal the functional

9

sites on the catalyst surface (Fig. S17). The proportions of Ce(III) by semi-quantitative on the 2.5%CeeCN after 1st, 2nd, 3rd, 4th, and 5th runs were 89.8%, 92.3%, 98.7%, 100%, and 93.0%, respectively. These values were very close to the content of Ce(III) on the pristine g-C3N4 (91.1%). As the content of Ce(III) greatly influence the catalytic performance of the Ce-based catalyst, and the fraction of Ce(III) reported in literatures are often at a low or trace level (Abiaad et al., 1993; Faria et al., 2008), it is believed the remain unchanged of the high Ce(III) content on 2.5%CeeCN help to maintain its efficient catalytic performance. In addition, the existing form of O also remained almost unchanged as surface hydroxyl groups. Therefore, it is likely that the 2.5%CeeCN provided the surface complexation/oxidation sites that could adsorb both pollutants and ozone molecules. It is speculated that the Ce(III) were stabilized not only by the coordination of g-C3N4 but also the formation of surface hydroxyl groups (≡OH) on its exposed surface. That is, the presence of Ce(III) promoted the formation of ≡OH, which in turn prevented the Ce(III) from oxidation. Furthermore, surface complexation was previously confirmed as the main mechanism for the adsorption of OA onto ≡Ce(III) (Zhang et al., 2011; Shan et al., 2018). Therefore, the generated
OH was likely come from the surface complexation between O3, ≡Ce(III), ≡OH, and OA and the further oxidation of OA complexes (And and Reckhow, 2002), instead of the traditional mechanism that relies on the redox circulation of metal.

3.6. Proposed mechanisms Based on the results presented, the enhanced catalytic ozonation of OA by 2.5%CeeCN involved complicated interactions between dissolved O3 molecule, OA, Lewis acid sites of the exposed Ce(III), and surface hydroxyl groups. The degradation of OA by 2.5% CeeCN/O3 is proposed to contain four stages (Scheme 1). In stage 1, the dissolved O3 molecule adsorbed onto the sites of ≡OH and decomposed to the precursor of ROS (e.g. O2
- and O3
-) to generate
OH. In stage 2, the diffusion
OH along with the O3 further attack the aromatic organics in the bulk solution accompanied by the fast generation of carboxylic acid intermediates like OA. In stage 3, the carboxyl of OA adsorbed onto the ≡Ce(III) site via surface complexation, which further rearrange the electron cloud of carboxyl organics and polarize CeO bond, thus making it easier to be degraded. Finally, the O3/
OH from nearby ≡OH site further attack the chemisorption of Ce(III)-OA into CO2/H2O to achieve

Scheme. 1. Proposed catalytic and degradation mechanisms.

10

Y. Xie et al. / Chemosphere 239 (2020) 124612

complete mineralization. Considering the extremely short life time of
OH, the relative short distance between oxidants and adsorbed OA from nearby dual reaction sites might provide huge advantage in promoting the utilization efficiency of the enhanced
OH production (Li et al., 2018). According to such a mechanism, the catalytic ozonation by CeeCN should not be selective. To clarify this, several aromatic pollutants including acetaminophen, p-chloroaniline, and p-chlorobenzoic acid were selected as target contaminants to evaluate the oxidation capacity of the CeeCN/O3 system by measuring the mineralization rate and the formation of OA during the catalytic ozonation (Figs. S18eS20). Results show that the mineralization of these aromatic compounds was promoted by at least 40%. In addition, compared with the situation with O3 alone, both the formation and degradation of OA were accelerated with CeeCN/O3, which resulted in the significant decrease in the pH value at the initial stage of the catalytic reactions and the stage-characterized enhanced mineralization. 4. Conclusions An efficient and stable catalyst with a high content of Ce(III) was synthesized and used to promote ozonation. The successful surface modification of g-C3N4 induced by Ce(III) led to the enhanced mineralization of OA and various aromatic compounds. The obtained CeeCN not only provided abundant ≡OH sites for O3 composition to produce ROS, but also afforded high content of nearby ≡Ce(III) sites for instant complexation of carboxylic acid. Typical carboxylic organics, such as OA, likely adsorbed on ≡Ce(III) and formed the complex that could be easily oxidized by O3 and
OH, thereby promoting the utilization efficiency of O3 and mineralization degree. The fraction of Ce(III) on the 2.5%CeeCN surface remained almost constant during the catalytic ozonation, which could probably be related to the stabilizing effects from the framework of g-C3N4 and ≡OH. In addition, the Ce(III) was well entrapped in the pyridinic nitrogen groups of g-C3N4, which makes the composite catalyst highly stable without noticeable loss of Ce and thus ensures the good reusability of the catalyst even under acidic conditions. Declarations of interest None. Acknowledgements This study was financially supported by National Key R&D Program of China (2018YFC1903202), the National Natural Science Foundation of China (No. 41572211; No. 21776223). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.124612. References Abi-aad, E., Bechara, R., Grimblot, J., Aboukais, A., 1993. Preparation and characterization of ceria under an oxidizing atmosphere. Thermal analysis, XPS, and EPR study. Chem. Mater. 5, 793e797. And, D.S.P., y, Reckhow, D.A., 2002. Effect of dissolved cobalt(II) on the ozonation of oxalic acid. Environ. Sci. Technol. 36, 4046.
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