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Separation and Purification Technology

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Heterogeneous catalytic ozonation of oxalic acid with an effective catalystbased on copper oxide modified g-C3N4

Jing Liua, Jie Lia, Sen Hea, Lei Suna, Xiangjuan Yuana,b,⁎, Dongsheng Xiaa,b,⁎

a School of Environmental Engineering, Wuhan Textile University, Wuhan 430073, Chinab Engineering Research Center for Clean Production of Textile Dyeing and Printing, Ministry of Education, Wuhan 430073, China

A R T I C L E I N F O

Keywords:Copper oxideGraphitic carbon nitrideOxalic acidCatalytic ozonationCatalytic mechanism

A B S T R A C T

Various amounts of copper oxide (CuO) modified graphitic carbon nitride (g-C3N4) denoted as CuO-CNx com-posites were successfully fabricated via a reflux condensation method and used as efficient ozonation catalystsfor oxalic acid (OA) degradation in aqueous solution. The BET, FT-IR, HRSEM, TEM, XPS, etc. characterizationsrevealed the chemical composition, bond structure and morphologies of as-prepared CuO-CNx composites. Theresults indicated that the introduction of CuO into g-C3N4 relying on the chemical interaction rather than simplephysical mixture. Moreover, the increased active sites and the intense chemical bonds could play important rolesin improving the catalytic ozonation performances of CuO-CNx composites. In this study, the practical appli-cations of catalysts at different conditions were also investigated. When the mass ratio of CuO/g-C3N4 was 10,the CuO-CN10 composite exhibited superior catalytic ozonation activity with 97.9% OA removal efficiency after30min at pH 5.0. Meanwhile, the CuO-CN10 showed excellent stability and reusability for contaminant de-gradation during multiple consecutive cycles. Furthermore, hydroxyl radical and superoxide radical as the majorreactive radicals were identified and the catalytic mechanism for OA degradation improvement was also elu-cidated. Overall, this work would provide an efficient and eco-friendly catalyst for water treatment as well as anew idea for future environmental purification.

1. Introduction

Advanced oxidation processes (AOPs), such as Fenton, ozonation,photocatalysis, have been extensively applied in water and wastewatertreatment, especially for the degradation of persistent and toxic organicpollutants [1,2]. Heterogeneous catalytic ozonation, as an alternativeand efficient technique of AOPs, has been widely employed for re-calcitrant organic pollutant removal [3,4] with several advantages ofhigh-efficiency ozone (O3) utilization, shorter operation time, improvedmineralization rate, and more powerful reactive radicals generation[5,6]. Up to present, metal oxides (including MnO2 [7], TiO2 [8], Al2O3

[6], CuO [9], etc.) and metal on supports (such as Al2O3, MCM-41, andSBA-15) [10,11] were frequently used in heterogeneous catalytic ozo-nation system owing to their high oxygen vacancies, stable structure,and abundant surface hydroxyl groups [12,13].

Amongst, copper oxide (CuO) was a well-known p-type semi-conductor with the bandgap of 1.3–1.7 eV [14]. Due to its low toxicity,high security, and earth-richness over other materials, CuO has beenextensively studied in various fields such as solar energy conversion,batteries, gas sensors and catalysis [15]. In addition, as a heterogeneous

catalyst, CuO has played a positive role in O3 decomposition and re-fractory organics (e.g. oxalic acid (OA), phenol, and benzene) de-gradation [9,16–18]. However, the practical applicability of CuO forwater treatment was obstructed resulting from less active sites on thesurface, which was attributing to the releasing of Cu ion [19,20].Therefore, in order to further improve the catalyst stability, solve thecatalyst regeneration and the metal dissolution problems, the im-mobilization of CuO on catalyst supports as a viable approach hasaroused great concerns of researchers.

Graphitic carbon nitride (g-C3N4), a 2-D π-conjugated polymeric,metal-free semiconductor with a mild band gap (2.7 eV) has becomeone of the most promising materials in various scientific exploits in-cluding electrochemistry, organic synthesis, gas storage, and sensors[21,22]. With the advantages of excellent chemical stability, good in-trinsic electron mobility, low cost, easy availability, and photo-response[23,24], g-C3N4 has gained accumulation of attention in heterogeneouscatalysis as well [25]. Moreover, g-C3N4 could facilitate the uniformdispersion of active components and provide more active sites acting asa stable catalyst support due to its graphene-like structure [26].Therefore, many researchers were committed to couple g-C3N4 with

https://doi.org/10.1016/j.seppur.2019.116120Received 14 August 2019; Received in revised form 21 September 2019; Accepted 21 September 2019

⁎ Corresponding authors at: School of Environmental Engineering, Wuhan Textile University, Wuhan 430073, China.E-mail addresses: yuanxiangjuan1986@outlook.com (X. Yuan), dongsheng_xia@wtu.edu.cn (D. Xia).

Separation and Purification Technology 234 (2020) 116120

Available online 23 September 20191383-5866/ © 2019 Elsevier B.V. All rights reserved.

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metal oxides to improve the catalytic activities. Duan et al reported thatthe combination of CuO and g-C3N4 would contribute to the transferand aggregation of electrons on the catalyst surface, thus promotingsalicylic acid degradation in photocatalytic process [16]. Comparingwith pure g-C3N4, Li et al also revealed that the hydrogen evolution ratewas improved more than 100% by catalyst modified via CuO [27].Furthermore, the strong co-ordination mode between N and Cu atomsat the heterojunction interface of the composite that was beneficial tothe immobilization of CuO in the g-C3N4 framework to improve itsstability and catalytic activity [28]. Nowadays, the report on CuOmodified g-C3N4 (CuO-CN) for enhancing the catalytic ozonation per-formance towards pollutants degradation is still rare.

In this study, the major works were carried out in the followingaspects: (1) to fabricate and characterize a series of CuO-CN compositeswith various amounts of CuO; (2) to systematically evaluate the catalystperformance under different experimental conditions (e.g. catalyst do-sage, pollutant concentration, solution pH, and natural organic matter(NOM)) in catalytic ozonation process; (3) to identify the contributionof reactive oxygen species (ROS) in O3/CuO-CN process via radicalscavenger experiments and electron spin resonance (ESR); (4) to elu-cidate the possible mechanism for enhanced catalytic ozonation withCuO-CN composites. Here, OA was selected as the model pollutant toevaluate the catalytic ozonation performance of CuO-CN compositesbecause of its low reaction and mineralization rate with O3 [7].

2. Materials and methods

2.1. Chemicals and reagents

The chemicals including copper(II) chloride dihydrate(CuCl2·2H2O), melamine, tert-butanol (TBA), p-benzoquinone (BQ),sodium thiosulfate (Na2S2O3), etc. were at least in analytical grade andpurchased from Sinopharm Chemical Reagent, China. Humic acid (HA)sodium salt was obtained from Sigma Aldrich, USA. HPLC-grade acet-onitrile was provided by Fisher Scientific, Belgium. HPLC-grade phos-phoric acid (H3PO4) and 5,5-dimethyl-1-pyrolin-N-oxide (DMPO,purity > 97%) were supplied by Aladdin, China. The ultrapure waterwith a resistivity of 18.2 MΩ cm was obtained from a Milli-Q system(A10, Millipore, USA) and used for all the synthesis and oxidation ex-periments.

2.2. Preparation of the catalysts

The g-C3N4 was synthesized by direct polycondensation of mela-mine [29]. Briefly, 10 g melamine was put into a covered crucible,transferred into a muffle furnace, and heated to 550 °C in air for 4 hwith a ramp rate of 5 °Cmin−1. In addition, CuO was prepared by asimple hydrothermal method. Typically, 0.2M CuCl2·2H2O solution(50mL) was mixed with 2.25M NaOH solution (10mL). After vigorousstirring for 30min, the mixture was transferred to an autoclave, heatedat 130 °C for 24 h, and washed several times until neutral pH. Subse-quently, the resultant solid was dried in an oven at 60 °C for 24 h.

CuO-CN composites were fabricated via the reflux condensationmethod. The composites were denoted as CuO-CNx, in which “x” re-presents the mass ratio of g-C3N4/CuO (x= 1, 5, 10, and 15). For in-stance, 1 g g-C3N4 and different amount of CuO (e.g. 0.1 g CuO for CuO-CN10) were dispersed and ultrasonically stirred for 10min in 150mLultrapure water. Then, the mixture was treated by oil bath at 96 °C for5 h. After cooling down to the room temperature, the slurry was cen-trifuged, and finally dried at 60 °C for 24 h to obtain the final compo-site.

2.3. Characterization of the catalysts

The Brunauer–Emmett–Teller (BET) surface area of the compositeswas evaluated on the basis of nitrogen adsorption-desorption isotherm

using a Micromeritics ASAP 2010 porosimeter, USA. The Fouriertransform infrared (FT-IR) spectra was recorded on a Thermo ScientificNicolet iS5 spectrometer, USA. The analysis of X-ray diffraction (XRD)patterns were carried out on a PANalytical X’Pert Pro MPD X-ray dif-fractometer (Netherlands) with Cu Kα radiation (λ=1.54178 Å). AThermo Scientific ESCALAB 250Xi instrument (USA) was employed tocollect the X-ray photoelectron spectroscopy (XPS) data. The micro-structures were observed by scanning electron microscope (SEM,Quanta FEG 250, FEI, USA) and transmission electron microscopy(TEM, JEOL JEM-2100F, Japan). The surface charge of the samples inaqueous solution was measured using a zeta-potential analyser(Malvern Zetasizer Nano-ZS 90, UK).

2.4. Catalytic performance study

The catalytic performance of CuO-CN composites was investigatedby the degradation of OA and all of the experiments were conducted ina semi-continuous glass reactor (500mL). In a typical procedure, acertain amount of CuO-CN composites was added to OA solution bystirring in thermostatic bath. Diluted NaOH or HCl solution was em-ployed to adjust initial pH to the expectant values. O3 gas was con-tinuously sparged into the reactor through a microporous aerator with aflow rate of 0.5 Lmin−1. Samples were taken out at certain time in-tervals, quenched with 20 μL (0.05M) Na2S2O3, and filtered by a0.45 μm filter for further determination. All the experiments wereconducted at 20 °C at least in duplicates. More description involvingozonation system could be found in our previous study [30].

The Waters e2695-2489 HPLC system (USA) equipped with a WatersSunfire C18 column (USA) was employed to determine the OA con-centration. Acetonitrile and phosphoric acid at pH 3.0 (5:95, v/v) wereused as the mobile phases and the samples were analyzed at a constantflow rate of 0.85mLmin−1. The total organic carbon (TOC) wasmonitored by Hanau Elementar vario TOC analyzer, Germany. The Cuions in the solution were quantified by an inductively coupled plasma-atomic emission spectrometry (Thermo iCAP 7000, USA).

3. Results and discussion

3.1. Catalytic performance and characterization of CuO-CNx composites

To evaluate the catalytic ozonation performances of the g-C3N4,CuO, and the CuO-CNx composites, the degradation of OA was com-pared in Fig. 1a. Obviously, more than 95% of OA removal wasachieved in ozonation with various CuO-CNx composites after 30minreaction, which was higher than that of g-C3N4 (11%) and CuO (88%).The pseudo-first-order rate constant (k) with different mass ratio of g-C3N4/CuO from 1 to 15 was 0.1209 (R2=0.9229), 0.1295(R2= 0.9681), 0.1349 (R2=0.9615), and 0.1263 (R2= 0.9428)min−1, respectively. The risen k constants might be ascribed to themore surface active sites supplied by larger surface area. However, ascontent of CuO increased gradually, the CuO particles may aggregateinto bigger particles and the pores of g-C3N4 could be more covered bythem, which probably inhibited the availability of actives sites leadingto the declined reaction rate [31].

Meanwhile, XRD patterns were utilized to investigate the phasestructure of CuO-CNx composites as presented in Fig. 1b. The typicaldiffraction peak (0 0 2) of g-C3N4 was observed at 27.4°, which wasassociated with the interlayer stacking of conjugated carbon nitridelayers [32]. And the main peaks of CuO were almost detected at2θ=32.5°, 35.6°, 38.7°, 48.7°, 58.3°, 61.8°, 68.1° and 74.7° corre-sponding to (1 1 0), (−1 1 1), (1 1 1), (−2 0 2), (2 0 2), (−1 1 3),(2 2 0) and (−2 2 2) planes (PDF48-1548) [33]. For CuO-CNx compo-sites, the XRD patterns revealed the coexistent structure of g-C3N4 andCuO, suggesting an effective attachment on the surface of g-C3N4 byCuO particles and that the g-C3N4 crystal phase did not change aftermodification with CuO particles.

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Moreover, the BET specific surface areas, pore volumes, and averagepore sizes of as-prepared samples were further summarized in Table 1.The specific surface area was reduced from 21.7850 to 12.5141 m2 g−1

and the pore volumes also decreased from 0.051209 to0.026374 cm3 g−1 with the increasing amount of CuO doping, whichwas attributed to the surface coverage and pore filling of the excess CuO[34]. The consequences were in agreement with catalytic performanceresults of CuO-CNx composites indicating that the change of specificsurface area of catalyst was not the dominant factor of ozonation ac-tivity enhancement.

According to the catalytic activity and structure characterization,the CuO-CN10 exhibited the optimal catalytic ozonation performancetowards OA degradation, which was simplified as CuO-CNop in thefollowing content.

3.2. Characterization of CuO-CNop

The pore size distributions of g-C3N4 and CuO-CNop were in-vestigated by N2 adsorption-desorption analysis. As displayed inFig. 2a, the as-prepared composites both exhibited type-Ⅳ isothermwith an obvious hysteresis loop, suggesting the presence of mesoporousstructure [35]. In addition, the CuO-CNop had a more narrow pore sizedistribution from 5 nm to about 30 nm than that of g-C3N4 from 2 nm toabout 30 nm (Fig. 2a inset), which further confirmed that the pores ofcomposite were filled by the increased CuO particles [34].

The FT-IR spectroscopy was employed for analyzing the bondstructures of CuO, g-C3N4, and CuO-CNop (Fig. 2b). For the typicalcharacteristic peaks of g-C3N4, the broad band located between 3000and 3300 cm−1 was assigned to NeH stretching vibration, and the bandat 808 cm−1 was corresponded to the breathing mode of tri-s-triazineunits [36]. The strong absorption band in the range of 1240–1650 cm−1

was attributed to the stretching vibrations of the aromatic CeN het-erocycles [37]. In the case of CuO particles, the tiny peaks at 529 and

630 cm−1 were due to the CueO stretching vibrations [17], and otherpeaks at 1627 and 3422 cm−1 were derived from water species andOeH stretching [14,30]. These characteristic absorption peaks of CuOand g-C3N4 were all revealed in the spectra of CuO-CNop further in-dicating the coexistence of the two structures.

The morphologies and structures of CuO-CNop were observed bySEM and TEM. As SEM images illustrated in Fig. 3c and d, it was ob-vious that the CuO-CNop composite was made up of the wrinkled sheet-like structure (g-C3N4) (Fig. 3a) covered by CuO particles which wereirregular flaky intermixed with small strip (Fig. 3b). In addition, thelayer structure of g-C3N4 with irregular crystals and larger plates wasfurther observed in TEM images, as presented in Fig. 4a. A typicallattice fringe of g-C3N4 with spacing of 0.32 nm was in accordance withthe theoretical value as the (0 0 2) facet (Fig. 4b). As shown in Fig. 4c,the surface of g-C3N4 had been covered with blocky structures withoutdistinct boundary. And the CuO particles with lattice spacing of0.25 nm (−1 1 1) and 0.23 nm (1 1 1) were clearly detected in CuO-CNop (Fig. 4d) implying the successful introduction of CuO into g-C3N4,which was in agreement with XRD results.

To verify the elemental composition and bond configuration of CuO-CNop composite, the XPS spectrum was carried out. As exhibited inFig. 5a (the survey spectrum), C, N, O, and Cu were all detected in CuO-CNop composite and the high resolution XPS spectrum of C 1s, N 1s, O1s, and Cu 2p were given in Fig. 5b–e. In the case of C 1s spectrum(Fig. 5b), two peaks centered at 284.8 and 288.1 eV could be obviouslydistinguished. The former peak belonged to sp2 CeC or/and C]Cbonds and the latter was ascribed to sp-bonded carbon in N-containingaromatic rings (NeC]N), which represented the major carbon speciesin the g-C3N4 [38]. The N 1s spectrum in Fig. 5c revealed three mainpeaks at 398.7, 400.6, and 404.6 eV, corresponding to triazine rings(NeC^N), amino functional group (NeH), and special charging effectsof heterocycles, respectively [39]. Meanwhile, the O 1s peak sited at530.3 eV in Fig. 5d was attributed to O2− ions in the CueO bonds andanother peak (531.7 eV) was assigned to the chemisorbed oxygen dueto hydroxyl radicals [40]. Finally, as shown in Fig. 5e, the peaks locatedat 934.6 eV (Cu 2p3/2) and 954.5 eV (Cu 2p1/2) were assigned to theCu2+ species due to the presence of Cu 2p satellites (962.8 eV), whichdemonstrated the unfilled electron state of Cu 3d9 orbitals [37].Moreover, the existence of CuO was further confirmed by the weaksatellite peaks between 940 eV and 950 eV which were the character-istic of CuO [41].

With the combination of FT-IR, XRD, TEM, SEM, and XPS studies,the results testified that the sheet-like structure CuO-CNop was suc-cessfully fabricated depending on chemical interactions between CuOand g-C3N4 rather than simple physical mixture. Furthermore, the

Fig. 1. (a) OA removal with g-C3N4, CuO, and CuO-CNx composites in different doping ratios and inset represents the rate constant; (b) XRD patterns of CuO, g-C3N4,and different CuO-CNx composites. Experimental conditions: [OA]0= 50mg L−1, [CuO]=50mg L−1, [CuO-CNop]= 500mg L−1, [O3]= 5mgmin−1,pH0=6.0 ± 0.3.

Table 1The BET surface areas, pore volumes, and mean pore diameters of g-C3N4 andCuO-CNx composites.

Catalysts Surface area (m2 g−1) Pore volume (cm3 g−1) Pore diameter (Å)

g-C3N4 10.1852 0.022530 88.4795CuO 2.1447 0.011475 47.6311CuO-CN1 12.5141 0.077073 84.3013CuO-CN5 16.3768 0.095237 84.6102CuO-CN10 16.4318 0.096925 85.1233CuO-CN15 21.7850 0.132936 94.0260

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increased active sites and the intense chemical bonds could play im-portant roles in improving the catalytic ozonation performance of CuO-CNop which was in less relation with the change of specific surface area.

3.3. Applicability of CuO-CNop in catalytic ozonation

The catalytic performances of O3 alone, O3/CuO, and O3/CuO-CNop

processes were evaluated via the removal rate of OA and TOC in Fig. 6.As depicted in Fig. 6a, the efficiencies of OA degradation in O3 alone,O3/CuO, and O3/CuO-CNop processes were 2%, 56%, and 91%, re-spectively after 15min at pH 6.0. The TOC removal of O3 alone, O3/CuO and O3/CuO-CNop processes after 30min was 7%, 58% and 73%,respectively (Fig. 6b). Simultaneously, the Cu2+ release could be ne-glected in O3/CuO and O3/CuO-CNop processes (< 0.2mg L−1). Be-sides, the impacts of catalyst dosage, initial OA concentration, solutionpH, and NOM on OA degradation were taken into consideration to

explore the applicability of CuO-CNop.The dosage of catalyst, as a crucial factor in heterogeneous catalytic

reactions, has great influence on effective use of the catalyst for furtherpractical applications. As depicted in Fig. 7a, the catalytic ozonationactivities of various CuO-CNop dosages towards OA degradation wereinvestigated and the corresponded k constants also displayed (Fig. 7a-inset). The OA removal at the CuO-CNop dosage of 100, 300, 500, 700,and 1000mg L−1 was 59%, 77%, 96%, 96%, and 96% after 15minreaction, respectively. Meanwhile, the k constant raised from 0.0963(R2= 0.8678) to 0.1443 (R2=0.8506) min−1 as the dosage increasedfrom 100 to 1000m L−1. It was noticed that the highest OA degradationefficiency was achieved at the highest CuO-CNop dosage, mainly be-cause increasing dosage could improve the amount of active sites fur-ther inducing more ROS [9].

Considering the required O3 quantity and the treatment time inpractical applications, it is essential to explore the influence of pollutant

Fig. 2. (a) N2 adsorption–desorption isotherms and BJH pore-size distribution plot (inset) of CuO-CNop and g-C3N4; (b) the FT-IR spectra of CuO, CuO-CNop, and g-C3N4.

Fig. 3. SEM images of (a) g-C3N4 and (c) CuO-CNop; HRSEM images of (b) CuO and (d) CuO-CNop.

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concentration on catalyst efficiency. The impacts of initial OA con-centrations (10–100mg L−1) towards the catalytic ozonation activity ofCuO-CNop were displayed in Fig. 7b. More than 80% of OA was de-graded in all O3/CuO-CNop systems and the k constant was 0.1779(R2=0.8797), 0.1431 (R2=0.9911), 0.1329 (R2=0.8799), 0.0766(R2=0.9504), and 0.0711 (R2= 0.9562) min−1 after 30min, respec-tively (Fig. 7b-inset). The results implied that the CuO-CNop could keepgood catalytic efficiencies in ozonation with different initial OA con-centrations.

The pH value of the solution could greatly influence the catalyticozonation process owing to its effects on surface properties of catalystsand O3 decomposition [42]. As shown in Fig. 7c, the OA degradationefficiencies with different pH values (3.0–10.0) in O3/CuO-CNop processwere investigated. Compared with sole O3, the improvement (13–95%)of OA removal rates were achieved in the presence of CuO-CNop duringozonation. The pH value at point of zero charge (pHpzc) was determinedto be 4.86, therefore, its surface hydroxyl groups were in deprotonatedstatus ([CuO-CNop]-O−) when pH > 4.86. Zhao et al had reported thatnegatively charge surface has a strong reactivity towards O3 to generatemore hydroxyl radical (%OH) [43], which could promote OA degrada-tion. However, when the pH value further increased, the decompositionof O3 would generate more intermediate radicals which could reactwith %OH leading to the termination of radical reaction [44]. Theslightly decrease of OA degradation via CuO-CNop catalytic ozonation atpH 10.0 showed an agreement with the conclusion.

NOM was a key component in aquatic environment as a complexmatrix of organic materials [45]. Approximately 80% of NOM com-prised of high molecular weight humic acids (HAs) which could reactwith O3 directly and/or indirectly [10]. As illustrated in Fig. 7d, theeffects of HA on OA degradation in catalytic ozonation via CuO-CNop

was exploited. As the concentration of HA increased from 2 to10mg L−1, the OA degradation was declined from 95% to 93%. Theslight decrease could be ascribed to the tiny competition between theNOM and OA for active sites [46], indicating that CuO-CNop had a greatpotential to be used in domain of water purification.

Taking into account the practical application, it is important toevaluate the reusability and stability of the catalyst during catalyticozonation. The reusability of as-prepared CuO-CNop was examined withfive consecutive operations runs (Fig. 8). It was worth noting that theOA removal efficiency was up to 95% after 30min at each cycle with noobvious loss of catalytic activity of CuO-CNop during the ozonation.Furthermore, the maximum release of Cu2+ was determined less than0.23mg L−1 after 30min in each run, which showed excellent stability.Based on aforementioned results, the CuO-CNop revealed great reusa-bility and stability in the catalytic ozonation system, showing a greatpotential in actual application for water environmental remediation.

3.4. Mechanism for enhanced catalytic ozonation with CuO-CNop

To clarify the degradation mechanism of OA in O3/CuO-CNop pro-cess, scavenging experiments were carried out to identify the con-tribution of ROS in the reaction system as illustrated in Fig. 9. The BQand TBA were employed for scavenging the %OH and superoxide radical(O2

%−) in this study. In the presence of TBA, the OA degradation ef-ficiency was 95% with slightly inhibition compared to optimum activityof CuO-CNop, while, the BQ presented remarkable suppression with38% of OA removal. The results suggested that the presence of O2

%−

and %OH would promote the oxidation reaction. Moreover, ESR tech-nique (Fig. 9-inset) displayed the signals of DMPO-%OH and DMPO-O2

%− adducts with the characteristic intensity ratios of 1:2:2:1 and1:1:1:1 in four-peak spectrum [47], which further confirmed that O2

%−

and %OH as the main actives species were generated in O3/CuO-CNop

process.Based on aforementioned conclusions in the study and reports from

other literatures, the plausible mechanism of OA degradation in O3/CuO-CNop process was proposed (Scheme 1). With the introduction ofCuO, the lattice oxygen (O2−) in CueO of CuO-CNop composite hasbeen detected by XPS, which was unstable that would escape to thesurface of CuO-CNop to form defects and release oxygen molecule (O2)(Eqs. (1) and (2)) [48]. Besides, the extrinsic Cu2+ could be entrapped

Fig. 4. TEM images of (a, b) g-C3N4 and (c, d) CuO-CNop.

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in the nitrogen-based macrocyclic units (each contains six long pairs)for formation of stable chemical coordination due to the orbital hy-bridization [26], which could strongly modify the electronic propertiesof g-C3N4, and provide the catalyst with additional new functionalities[21,49]. Wang et al reported that electron transfer (e−) rate could beincreased via the synergistic effect of electronic connection betweenCu2+ and N [50]. Then, O2 accepted e− on the surface of CuO-CNop andfurther transformed into O2

%− (Eqs. (3) and (4)). Meanwhile, the O3

could selectively attack the defect and active sites inducing a series ofchain reactions (Eqs. (5)–(7)), which could improve the adsorption andmass transfer efficiency of O3 on the interface [23]. Furthermore, thecharacterization of FT-IR and XPS confirmed the existence of functionalgroups such as C]C and surface-OH, which could directly or indirectlyreact with O3 to generate more ROS (%OH and/or O2

%−) enhancing theOA degradation efficiency. Consequently, the strengthen production of

%OH and O2%− via the solid-liquid interface reaction of O3 with CuO-

CNop was the main cause responsible for the enhanced ozonation.

[CuO-CN]−O2−→ [CuO-CN]− defects+O2 (1)

[CuO-CN]− defects+O3→ [CuO-CN]−O2−+O2 (2)

[CuO-CN]n+→ [CuO-CN](n+1)++ e− (3)

O2+ e−→O2%− (4)

O2%−+O3→

%O3−+O2 (5)

e−+O3→%O3

− (6)

%O3−+H2O→HO3

%+OH−→ %OH+O2 (7)

Fig. 5. XPS spectra for CuO-CNop: (a) XPS survey spectra of the CuO-CNop, (b) C 1s, (c) N 1s, (d) O 1s, and (e) Cu 2p.

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4. Conclusions

In summary, the CuO-CNx composites were successfully obtained byreflux condensation method with different loading amounts of CuO andused for catalytic ozonation towards OA degradation. A series ofcharacterization techniques such as BET, XPS, XRD, SEM, and TEMwere applied for exploiting the morphology and physicochemicalstructure of CuO-CNx composites. The results indicated that the irre-gular flaky CuO-CNx composites combined via CuO and g-C3N4 wasascribed to the chemical interaction. The optimum catalytic activity

with 98% OA removal efficiency after 30min was achieved at the g-C3N4/CuO mass ratio of 10 (CuO-CNop). Moreover, radical scavengerexperiments and ESR characterization described that O2

%− and %OHwere the dominant reactive radical species. It was worth pointing outthat the solid-liquid interface reaction of O3 with CuO-CNop could im-prove the production of O2

%− and %OH, which was the main causeresponsible for the enhanced ozonation. Furthermore, the CuO-CNop

was stable and could be reused at least for five runs with quite lessactivity loss. The present study may provide a new insight into theapplication of CuO-CNop in the degradation of persistent organic

Fig. 6. (a) OA degradation efficiency and (b) the removal rate of TOC in the selected processes. Experimental conditions: [OA]0= 50mg L−1, [CuO]=50mg L−1,[CuO-CNop]= 500mg L−1, [O3]= 5mgmin−1, pH0=6.0 ± 0.3.

Fig. 7. The effects of operational parameters on OA degradation in O3/CuO-CNop process: (a) CuO-CNop dosage; (b) initial OA concentration; (c) solution pH; (d)natural organic matter. Experimental conditions: [CuO-CNop]= 500mg L−1 (except for (a)), [OA]0= 50mg L−1 (except for (b)), [O3]= 5mgmin−1,pH0=6.0 ± 0.3 (except for (c)).

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contaminants with catalytic ozonation.

Declaration of Competing Interest

The authors declared that they have no conflicts of interest to thiswork. We declare that we do not have any commercial or associativeinterest that represents a conflict of interest in connection with thework submitted. All authors of this manuscript have directly partici-pated in planning, execution, and/or analysis of this study. The con-tents of this manuscript are not under consideration for publicationelsewhere.

Acknowledgement

This work was financially supported by the National Natural ScienceFoundation of China (No. 51808412), the Natural Science Foundationof Hubei Province (Nos. 2017CFA026 and 2018CFB266), and the

Science and Technology Project of Educational Commission of HubeiProvince (No. Q20181706).

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Fig. 8. The recycling experiments of CuO-CNop. Experimental conditions:[OA]0=50mg L−1, [CuO-CNop]= 500mg L−1, [O3]=5mgmin−1,pH0=5.0 ± 0.3.

Fig. 9. The effects of different scavengers on the degradation of OA. Inset re-presents the ESR spectra of DMPO in aqueous. Experimental conditions:[OA]0=50mg L−1, [CuO-CNop]= 500mg L−1, pH0= 5.0 ± 0.3,[TBA]= [BQ]=50mg L−1, [DMPO]= 100mM.

Scheme 1. Proposed mechanism for OA degradation in O3/CuO-CNop process.

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