Development of ozonation and reactive electrochemical membrane coupled process: Enhanced tetracycline mineralization and toxicity reduction

Journal Pre-proofs Development of ozonation and reactive electrochemical membrane coupled process: enhanced tetracycline mineralization and toxicity reduction Dan Zhi, Jianbing Wang, Yaoyu Zhou, Zirui Luo, Yuqing Sun, Zhonghao Wan, Lin Luo, Daniel C.W. Tsang, Dionysios D. Dionysiou PII: DOI: Reference:

S1385-8947(19)32561-6 https://doi.org/10.1016/j.cej.2019.123149 CEJ 123149

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 August 2019 9 October 2019 11 October 2019

Please cite this article as: D. Zhi, J. Wang, Y. Zhou, Z. Luo, Y. Sun, Z. Wan, L. Luo, D.C.W. Tsang, D.D. Dionysiou, Development of ozonation and reactive electrochemical membrane coupled process: enhanced tetracycline mineralization and toxicity reduction, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.123149

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Development of ozonation and reactive electrochemical membrane coupled process: enhanced tetracycline mineralization and toxicity reduction Dan Zhia, Jianbing Wangb, Yaoyu Zhoua,c,*, Zirui Luoa, Yuqing Sunc, Zhonghao Wanc, Lin Luoa, Daniel C.W. Tsangc, Dionysios D. Dionysioud a

College of Resources and Environment, Hunan Agricultural University, Changsha 410128, PR

China b

School of Chemical and Environmental Engineering, Beijing Campus, China University of

Mining and Technology, Beijing 100083, PR China c

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University,

Hung Hom, Kowloon, Hong Kong, China d

Environmental Engineering and Science Program, Department of Chemical and Environmental

Engineering (ChEE), University of Cincinnati, Cincinnati, OH 45221, USA

 Corresponding

author: [email protected]

Abstract: The ozone-reactive electrochemical membrane (O3-REM) coupled process for tetracycline (TC) removal was developed by coupling ozonation and electrochemical oxidation in a substoichiometric titanium dioxide (Ti4O7) REM reactor. To achieve concurrent and efficient degradation-mineralization, TC was treated by ozonation and electrochemical oxidation over the Ti4O7 REM anode in a sequent manner. TC was nearly completely removed within 20 min, while the total organic carbon (TOC) removal efficiency attained 9.1% in 20 min, and then reached 77.1% in 80 min under optimal conditions of 2 mg/min ozone dosage and 2 mA/cm2 current density. TC was suspected to be oxidized into some four- and three-ring intermediates within 20 min, some two- and one-ring intermediates within 80 min, and eventually carboxylic acids, CO2 and H2O. The concentrations of NO3ˉ, NO2ˉ and NH4+ increased slightly in the first 20 min and then elevated significantly from 20 to 80 min, indicating an enhanced denitrification rate via electrochemical oxidation. Besides, the bioluminescence inhibition ratio exhibited a slight decrease from 54.5% to 50.6% in the first 20 min and then significantly decreased to 14.5% in 80 min, suggesting an augmented toxicity reduction simultaneously by the coupled technology. Overall, the O3-REM coupled process demonstrated higher TC degradation and TOC removal efficiencies, enhanced toxicity-reduction capability, and lower energy consumption for TC removal compared to individual O3 or Ti4O7 REM processes. By comprehensively analyzing the performance, mechanism and energy consumption of TC removal by the O3-REM coupled process, our research provides insights into the application potential of the coupled process to efficiently eliminate the amount and toxicity of various pharmaceuticals from wastewater. Keywords:

Reactive

electrochemical

membrane,

Advanced

Antibiotics removal, Transformation products, Energy consumption

2

wastewater

treatment,

1. Introduction Advanced oxidation processes (AOPs) have been demonstrated as one of the most effective methods for the treatment of wastewater containing bio-refractory pharmaceuticals (e.g., antibiotics, β-blockers, and estrogens), wherein the formation of strong oxidizing species such as hydroxyl radicals (•OH) enables rapid decomposition of a wide range of contaminants [1-4]. Among various AOPs, ozonation is more attractive owing to its cost-efficiency, relatively high oxidation performance, and ease of operation and scaling-up [5-7]. However, satisfactory removal efficiency of total organic carbon (TOC) cannot be readily achieved because sole ozonation tends to degrade organics into short-chain aldehydes and carboxylic acids rather than complete mineralization into H2O and CO2 [6, 7]. Thus, the treated wastewater may still contain considerable amounts of potentially toxic by-products degraded from their parent organic contaminants, posing secondary threats to human health and ecosystems. As one of the most promising AOPs for eliminating pharmaceuticals from wastewater, electrochemical advanced oxidation processes (EAOPs) have drawn much attention in the last few years due to their high oxidation performance, environmental compatibility, and mild conditions [1, 3, 4, 8]. During the EAOPs, the stability and activity of the anode material are reported to determine the electrochemical oxidation performance [4, 9, 10]. Various anode materials (e.g., BDD, PbO2, Ti/RuO2, Ti/RuO2-IrO2 and Ti/SnO2-Sb) have been developed to electrochemically oxidize organics in wastewater, among which the boron-doped diamond (BDD) thin-film electrodes are the best anode material known for electrooxidation due to its satisfactory chemical stability and efficient generation of •OH radicals; however, the critical difficulties from the perspective of production and cost (15,000-22,000 $/m2) make it less practical in field scale [4, 11-14]. Recent studies have reported that the Ti/Ti4O7 anode (a flat plate anode prepared by spraying Ti4O7 powders onto the surface of the cleaned titanium 3

plate) possesses similar oxygen evolution potential compared to the BDD anode, but its deploying price is 60-70% cheaper than that of the BDD anode [2, 4, 14]. The mechanisms of oxidizing organic compounds over the Ti/Ti4O7 anode are proven to be mainly ascribed to the high reactivity of physisorbed hydroxyl radicals (A~•OH) which form when water is oxidized at the surface of the ‘non-active’ anode [4, 14]. The Ti/Ti4O7 anode has been employed to achieve high removal efficiencies (>95%) of tetracycline and amoxicillin by electrochemical oxidation, showing great potentials in eliminating bio-refractory pharmaceuticals from wastewater [4, 14]. Although satisfactory removal efficiencies of pharmaceuticals can be obtained by sole electrochemical oxidation process using the Ti/Ti4O7 anode, the EAOPs may still operate at relatively low current efficiency [15, 16]. This is mainly ascribed to the flow-by operating mode of the traditional flat plate electrodes which are primarily used in EAOPs [15, 17]. This flow-by operating mode leads to a hydrodynamic diffusion boundary layer of about 100 μm, which impedes the rapid reaction of organic pollutants owing to the diffusional limitation [13, 15]. To overcome this limitation, the porous ceramic Ti4O7 reactive electrochemical membrane (REM) operated in flow-through mode has been developed recently [12, 13, 17-19]. Ti4O7 has been utilized for REM fabrication because it can be synthesized into porous monolithic structures at low cost and is reported to produce •OH radicals via water oxidation [17, 20]. The Ti4O7 REM has a large electroactive surface area, and its advection-enhanced mass transfer rates during the EAOPs are about 10-fold higher than those of the traditional flat plate electrodes [17]. However, only a few researches have applied the Ti4O7 REM in wastewater remediation. A tubular Ebonex® anode (a titanium oxide electrode composing mainly of Ti4O7) was used as an REM and obtained nearly 100% removal efficiencies of p-substituted phenolic organics in wastewater [15]. The Ebonex® REM also achieved satisfactory COD removal efficiency (66%) during the electrochemical oxidation process of 4

industrial wastewater [18]. Nevertheless, the application of the porous Ti4O7 REM to the electrochemical oxidation of pharmaceuticals has rarely been studied yet. Meanwhile, some studies have shown that the combination of ozonation and electrochemical oxidation can significantly improve the degradation and mineralization efficiencies of bio-refractory organics in wastewater [21, 22]. Some individual intrinsic limitations of ozonation and electrochemical oxidation, e.g., the selective oxidation of organics with ozone and the limited pollutant mass transfer efficiency of the electrodes, may be synergistically overcome in this coupled process [21, 22] By combining ozonation and electrochemical oxidation in a REM reactor, the O3-REM coupled process is hypothesized to have great potential for pharmaceuticals removal due to the following advantages: (i) the O3-REM coupled process is expected to increase the mineralization efficiency of organics due to a stronger tendency to completely mineralize organic acids (cumulative by-products difficult to be removed by ozonation) via electron transfer reaction. (ii) this process may increase the organic removal efficiency by enhancing the mass transfers of ozonation (when the ozone-containing bubbles pass through the membrane, they are more favorable to generate minute bubbles, which can strengthen the mass transfer of ozonation and improve the residual ozone utilization) and electrochemical oxidation (the mass transfer of organics from the liquid phase to the anode surface can be enhanced by aeration). (iii) this process may reduce the membrane fouling because some macromolecular organics can be rapidly degraded into small molecular organics by ozonation. (iv) refractory aldehyde or aldehyde compounds generated from pharmaceuticals degradation are prone to be degraded in this process, because these compounds have similar functional group (carbonyl) with organic acids which can be completely mineralized by the REM via electron transfer reaction. On account of aforementioned merits, an efficient and safe process for the removal of pharmaceutical contaminants from water body is expected to be evolved within. However, 5

the removal of pharmaceuticals by this ozonation and porous Ti4O7 REM coupled process has rarely been investigated. Herein, ozonation and electrochemical oxidation were combined in a Ti4O7 REM reactor to develop an O3-REM coupled process for the removal of pharmaceuticals in aqueous solution. The roles of ozone dosage and current density on the degradation kinetics of tetracycline (TC, a model antibiotic frequently detected in wastewater) and TOC removal efficiency were systematically investigated. By analyzing the transformation products and degradation pathway of TC, the mechanism of TOC change during the O3-REM coupled process was qualitatively explored. The energy consumption of the O3-REM coupled process for TC removal was calculated and compared with individual O3 or Ti4O7 REM processes. The reactivity of the coupled process toward different pharmaceutical compounds was investigated. Moreover, the implications of the O3-REM coupled process for industrial applications were also analyzed.

2. Materials and methods 2.1. Reagents and materials Tetracycline hydrochloride (C22H25N2O8Cl, AR Grade, with >99% purity) was supplied by Sigma-Aldrich (UK). HPLC-grade oxalic acid, formic acid, methanol, dichloromethane and

bis(trimethylsilyl)trifluoroacetamide

(BSTFA)

were

purchased

from

Dikma

Technologies (Beijing, China). High-purity oxygen was purchased from Beiwen Gas Manufacturing Plant (Beijing, China). The Ti4O7 REM was constructed from a tubular Ebonex® electrode (28 cm long, 20 mm inner diameter, 28 mm outer diameter; Vector Corrosion Technologies, Inc). The Ebonex® REM has properties of 1.7 μm median pore diameter, 31% porosity, and 2.78 m2/g specific surface area. 6

2.2. Removal of TC by the O3 REM coupled process The degradation experiments were conducted in a Ti4O7 REM reactor, which combined the ozonation and electrochemical oxidation, as shown in Fig. 1. The Ti4O7 REM reactor contained an Ebonex® electrode as anode, a 316 stainless steel rod (3.18 mm diameter) which was placed in the center of the Ebonex® anode as cathode, and the plexiglass (15 mm thick) as the outer wall material. Feed solution was pumped through the Ebonex® electrode center at a flow rate of 36 L/h. The feed and permeate solutions were 100% recycled to the collection tank. After 5 min of filtration, ozone was introduced into the reactor from the center of the Ebonex® electrode. In the collection tank, 1 L TC solution (5 mg/L) was used as the reactant, and 30 mmol/L Na2SO4 was added to the solution as supporting electrolyte. In the O3-REM coupled process, the goal was to rapidly degrade TC by ozonation, followed by efficient TOC removal by electrochemical oxidation over the Ti4O7 REM anode. Initially, TC ozonation process was carried out in the REM reactor without an applied current until TC was almost completely removed. Then, ozone introduction into the reactor was stopped and TC degradation experiments continued in the REM reactor with continuous current supply until TOC removal reached a plateau without any more significant change. The effects of reaction conditions on TC degradation were investigated with regard to the ozone dosage (1.5-4 mg/min) and the current density (5-25 mA/cm2). Unless noted otherwise, the reaction was carried out with the ozone dosage of 2 mg/min, ozone gas flow rate of 1 L/min, current density of 2 mA/cm2, membrane flux of 40 L/(h·m2), and reaction time of 80 min. At predetermined time intervals, 10 mL water sample was taken from the collection tank to analyze the concentrations of TC, TOC and the transformation products. After every experiment, the Ti4O7 REM was washed as reported by Zaky and Chaplin [20]. All trials were conducted in triplicates.

7

2.3. Analytical methods The concentrations of TC and other typical pharmaceuticals were analyzed by injecting 20 μL sample to a high-performance liquid chromatograph (LC-10AT, Shimadzu, Japan). The detailed conditions and procedures are shown in Supporting Information. The transformation products of TC were identified by a high-performance liquid chromatograph coupled with a triple quadrupole mass spectrometer (Shimadzu, LCMS-8040, Japan), and also a gas chromatograph-mass spectrometer (GC-MS, 6890GC/5973MSD, Agilent, USA). The detailed information of transformation products analysis is shown in Supporting Information. TOC in aqueous solutions was analyzed by the Shimadzu TOC-L CPH analyzer. Toxicity of TC solution was determined by the HACH LUMIStox 300 toxicity analyzer with Vibrio fischeri bacteria [4]. The detailed information of TOC and toxicity analysis is shown in Supporting Information. The concentrations of NH4+, NO3ˉ and NO2ˉ were analyzed by injecting 25 μL sample to an ion-chromatograph system (ICS-1000, Dionex, USA) equipped with an autosampler, a degasser, a pump, a guard column and a separation column. The detailed analytical procedure is shown in Supporting Information. 2.4. Data analysis The energy consumption was calculated as described in Equation (1), which consists of two parts: (i) the energy consumption during the process when TC ozonation took place in the REM reactor without an applied current until TC was almost completely removed; (ii) the energy consumption during the process when ozone supply into the reactor was stopped, and TC degradation experiments continued to be conducted in the REM reactor with continuous current supply until TOC removal reaches a plateau without any more significant change.

8

ECTOC (kWh / kg TOC ) 

8  60  PO3  t1 TOC1  V



1000UIt2 TOC2  V

(1)

where ECTOC (kWh/kg TOC) is the total energy consumption of TC degradation, PO3 (mg/min) is ozone dosage, t1 (h) is the time when TC is almost completely removed in the REM reactor without an applied current, ΔTOC1 (mg/L) is the change of TOC concentration during time t1, t2 (h) is the time during an applied current is provided to the REM reactor until TOC removal reaches a plateau without any more significant change, ΔTOC2 (mg/L) is the change of TOC concentration during time t2, I (A) is the applied current, U (V) is the applied voltage, and V (L) is the solution volume. It takes about 8 kWh electric energy to prepare 1 kg of ozone in the laboratory [23]. TC degradation kinetics were analyzed according to a pseudo-first-order kinetic model, and the bioluminescence inhibition ratio was calculated based on the Standard Methods for the Examination of Water and Wastewater, details are provided in Supporting Information. Results in this study were presented in mean ± standard error. Statistical analysis was carried out by the SPSS package (version 11.0) with a p-value<0.05 statistical significance.

3. Results and discussion 3.1. TC degradation and mineralization by the O3-REM coupled process 3.1.1 TC ozonation in the Ti4O7 REM reactor At initial step of the O3-REM coupled process, TC was oxidized by ozonation in the Ti4O7 REM reactor without an applied current until TC was almost completely removed. The effect of ozone dosage on TC removal was investigated during the ozonation process. All reactions followed the pseudo-first-order kinetics model (R2>0.94), and the kinetic parameters were analyzed (Fig. 2a and Table 1). When the ozone dosage ranged from 1.5 to 4 mg/min, the TC removal efficiencies 9

reached 94.2±0.2 % to 97.9±0.01 % and the k values reached (1.2±0.01)×10-2 to (1.9±0.02)×10-2 min-1 in 20 min (Fig. 2a and Table 1), respectively. The results show that ozonation in the Ti4O7 REM reactor was efficient for TC removal. The efficient TC removal by ozonation was also observed by Khan et al. (removal efficiency >95%, 10 min reaction time, 10 mg/min ozone dosage, and 0.5 mmol/L initial TC concentration) and Wang et al. (removal efficiency >90%, 25 min reaction time, 20 L/h ozone dosage, and 2.08 mmol/L initial TC concentration) in conventional column-type reactors [5, 24]. Our k values were slightly higher than those of Wang et al. (0.6-1.2 min-1), which might be due to the enhancement of ozone mass transfer and improvement of residual ozone utilization when the ozone-containing bubbles passed through the microporous membrane and generated smaller bubbles. Meanwhile, the TC degradation efficiencies and rates showed an increasing trend with the ozone dosage (Fig. 2a and Table 1), indicating a faster removal of TC at a higher ozone dosage. When the ozone dosage increased to 2 mg/min, the TC removal efficiency reached about 97% in 20 min, and the increasing trend became less obvious with further ozone dosage increase. The TC ozonation in the Ti4O7 REM reactor achieved 9.1% TOC removal efficiency in 20 min when TC was nearly completely removed (Fig. 2b). When extending the reaction time to 120 min, the TOC removal efficiency reached 24.5%, a little higher than that reported by Khan et al. (20%, 120 min) [24]. Noting that the TOC removal efficiency showed little change when further extending the reaction time, indicating that ozonation was less efficient for TC mineralization. This result was consistent with those of TC ozonation concluded by Wang et al. [5] and Khan et al. [24]. Khan et al. [24] suggested that ozonation could degrade TC into many refractory oxidation intermediates but not completely into CO2 and H2O irrespective of the efficient TC removal. 3.1.2 TOC removal by the Ti4O7 REM anode 10

Although TC was nearly completely removed in 20 min by ozonation in the Ti4O7 REM reactor, the TOC removal efficiency could only reach 9.1%. The TC solution was further treated by electrochemical oxidation over the Ti4O7 REM anode in the absence of ozone supply into the reactor to promote the unfavorable TOC removal. As Fig. 3 shows, the TOC removal efficiency reached 54.0%-77.9% after 60 min electrochemical oxidation reactions when the applied current density was ranging from 1 to 5 mA/cm2, indicating that the Ti4O7 REM process has high efficiency for TOC removal during the electrochemical treatment of TC. The TOC removal efficiency showed little change when further extending the reaction time to 90 min. Moreover, the TOC removal efficiency increased with the current density increasing from 1 to 5 mA/cm2 (Fig. 3). The TOC removal efficiency could reach about 77.2% in 60 min under the current density of 2 mA/cm2, and the increasing trend of TOC removal efficiency became insignificant when the current density was further increased. 3.1.3 Transformation products of TC degradation during the O3-REM coupled process To elucidate the low TOC removal efficiency (9.1%, 20 min) obtained by sole ozonation, the TC ozonation intermediates were identified and the results are shown in Table 2. In total, 33 compounds (the MS spectra are shown in Supporting Information) were detected. Among these, 12 compounds with relatively high molecular weight were identified by HPLC-MS. Compounds with m/z=400, 415, 431, 448 and 477 have four-ring molecular structures similar to TC, and compounds with m/z=396, 412, 451, 480, 496, 509 and 525 are three-ring intermediates formed by ring opening of TC molecule. Typical compounds with m/z=448, 477, 480, 496, 509 and 525 have also been found in a related TC ozonation process [24]. Moreover, 21 compounds with relatively low molecular weight were identified by GC-MS, none of which has been previously reported in TC ozonation studies. Most of these compounds are short-chain compounds, demonstrating that the ozonation in the Ti4O7 REM reactor was ineffective towards the removal of the derivative short-chain compounds, which 11

might lead to a low TOC removal efficiency. Transformation products of TC degradation were also identified when the TOC removal efficiency reached about 77.2% after 60 min electrochemical oxidation reactions (Table 2). In total, 18 compounds (the MS spectra are shown in Supporting Information) were detected. Among those, 8 compounds with relatively high molecular weight were identified by HPLC-MS. Integrated peak areas of typical compounds with m/z=396, 412, 451 and 496 were significantly reduced, while compounds with m/z=367, 298, 351 and 253 were newly detected and their molecular weights were smaller than those identified by HPLC-MS after TC ozonation. Moreover, 10 other compounds with relatively low molecular weight assigned to carboxylic acids with short-chains were also identified by GC-MS; these were also detected after TC ozonation and their integrated peaks were significantly reduced. Based on the analysis of transformation products of TC degradation, a possible TC degradation pathway during the O3-REM coupled process is proposed (Fig. S5). Studies have reported that TC has three types of functional groups with relatively high electron density, which are easily attacked by ozone and •OH radicals, including double bond, phenolic group and amine group [4, 24]. Among these functional groups, the double bond is the most reactive to ozone and •OH radicals attack [4, 24]. In the first 20 min of TC degradation by O3-REM, TC may be oxidized into compound with m/z=477 by ozonation of double bonds at position C11a-C12 and C2-C3, and compound with m/z=509 by ozonation of double bonds at position C11a-C12 and C6a-C7. Then, compound with m/z=477 may be oxidized into compounds with m/z=448 and 525 by ozonation at position C4 and C6a-C7, respectively. Compound with m/z=509 may be oxidized into compounds with m/z=480 and 525, and compounds with m/z=525, 480 and 448 may be oxidized into compound with m/z=496 which may be further oxidized into compounds with m/z=451, 412 and 396. Additionally, compound with m/z=431 may be formed via loss of N-methyl group due to the low bond 12

energy of N–C [25]. It may form compound with m/z=415 via the loss of amino, and further degradation may lead to the generation of compound with m/z=400 via loss of hydroxyl. Compounds with m/z=400 may be also oxidized into compound with m/z=451 by ozonation at position C4, C2-C3, C11a-C12 and C6a-C7. Khan et al. also reported a degradation pathway of TC oxidizing into compounds with m/z=477, 509 and 525, and they indicated that •OH radicals can non-specifically react with the ozonation products of TC and are able to attack any site of TC structure, producing a number of final low molecular weight products [24]. In 20 to 80 min of TC degradation by O3-REM, compounds with m/z=451, 412 and 396 may be further oxidized into some short-chain compounds by •OH radicals at different positions. These intermediates might be gradually mineralized into CO2, H2O and inorganic ions such as nitrate or ammonium. 3.2. Comparison of TC removal and energy consumption by O3-REM, O3 and Ti4O7 REM To assess the superiority of the O3-REM coupled process, the TC and TOC removal efficiencies were compared when TC was treated by O3-REM, O3 (in the Ti4O7 REM reactor) and Ti4O7 REM, as illustrated in Fig. 4. Under optimum conditions, the O3-REM, O3 and Ti4O7 REM achieved favorable 97.2%, 96.8% and 97.2% TC removal efficiencies in 120 min respectively (Fig. 4a). Accordingly, the TOC removal efficiencies were 77.8% (O3-REM), 24.5% (O3) and 76.8% (Ti4O7 REM) respectively (Fig. 4b), suggesting that the O3-REM coupled process and Ti4O7 REM achieved higher efficiencies for TC mineralization than TC ozonation. Thus, both O3-REM coupled process and Ti4O7 REM could achieve satisfactory TC and TOC removal efficiencies compared to sole TC ozonation. The energy consumptions of the three processes when achieving similar TOC removal efficiencies (about 77%) were also analyzed, as shown in Fig. 4c. The O3-REM coupled process and the Ti4O7 REM achieved about 77% TOC removal efficiency in 80 min and 120 min, respectively (Fig. 4b). Since the TOC removal efficiency reached 24.5% in 120 min for 13

sole TC ozonation and exhibited little change with further prolonging the reaction time, the energy consumption for TC removal by sole ozonation was calculated in 120 min. The energy consumption of the O3-REM coupled process was 101.5 KWh/kg TOC, significantly lower than those of O3 (241.2 KWh/kg) and Ti4O7 REM (121.5 KWh/kg) (Fig. 4c). Comprehensively considering the TC removal efficiency, TOC removal efficiency and energy consumption, the O3-REM coupled process is more favorable for TC removal than the ozonation in the Ti4O7 REM reactor and electrochemical oxidation over the Ti4O7 REM anode. Additionally, the TC removal efficiency can be affected by initial pH value when TC was treated by O3-REM, O3 and Ti4O7 REM, showing that the TC removal efficiencies increased with the increase of initial pH value from 3.0 to 7.0 (Fig. S6). TC is an amphoteric organic compound which has four kinds of TC ions under different pH conditions, including TCH3+ (pH<3.3), TCH20 (3.3
by ozonation, and the TOC removal efficiency increased from 9.1% in 20 min to 77.1% in 80 min by electrochemical oxidation over the Ti4O7 REM anode (Fig. 4a and Fig. 4b). The TC and TOC removal efficiencies barely elevated by extending the reaction time to 100 min, indicating that some carbonaceous organics in other forms were still present in spite of the enhanced TOC removal, which was consistent with the results of transformation products analysis in section 3.1.3. To further evaluate the level of TC mineralization, the concentrations of NO3ˉ, NO2ˉ and NH4+ were analyzed during the process of TC removal by O3-REM. Fig. 5a shows that the concentrations of NO3ˉ, NO2ˉ and NH4+ increased throughout the process of TC removal by O3-REM. The increasing trend was insignificant in the first 20 min, indicating the slow denitrification of TC molecules in the TC ozonation process, which was consistent with the results of transformation products analysis of TC ozonation (many nitrogen-containing degradation intermediates were detected). Then, the concentrations of NO3ˉ, NO2ˉ and NH4+ significantly increased from 20 to 80 min, indicating the occurrence of continuous denitrification of TC molecules, which was consistent with the results of transformation products analysis when the TOC removal efficiency reached about 77.2% (the integrated peak areas of the detected nitrogen-containing degradation intermediates were reduced). Throughout the O3-REM coupled process, the NH4+ concentration increased more rapidly than NO3ˉ and NO2ˉ concentrations, suggesting that amino group (-NH2) on TC structure might be attacked primarily to form NH4+. Subsequently, the released NH4+ might be further oxidized into NO3ˉ and NO2ˉ, which are less harmful towards the water body. Moreover, the change of bioluminescence inhibition ratio during the TC removal process by O3-REM is shown in Fig. 5b. The bioluminescence inhibition ratio showed a slight decline from 54.5% to 50.6% in the first 20 min. This result may be ascribed to similar toxicities of TC and the by-products retaining the three-ring structure, which corresponded to the results 15

of transformation products analysis of TC ozonation (many macromolecular intermediates with similar molecular structures of TC were detected). Then, the bioluminescence inhibition ratio significantly decreased from 50.6% to 14.5% in reaction time from 20 to 80 min, implying that TC was further degraded into less toxic intermediates by TC electrochemical oxidation over the Ti4O7 REM anode. 3.4. Removal of other pharmaceutical compounds by the O3-REM coupled process To further evaluate the performance of O3-REM coupled process on the removal of other pharmaceuticals in aqueous solution, the degradation of propranolol (PPN, a typical β-blocker), ciprofloxacin (CIP, a typical quinolone antibacterial), and sulfamethoxazole (SMX, a typical sulfonamide) was also investigated, as shown in Fig. 6. After 80 minutes reaction in the O3-REM coupled process, the PPN removal efficiency reached 98.8% and the corresponding TOC removal efficiency was 80.8%, which were close to those of PPN electrochemical oxidation (degradation efficiency 98.0 %, TOC 82.0 %) over a Ti/Ti4O7 anode with applied current of 60 mA [27]. The O3-REM coupled process also achieved 96.2% CIP removal efficiency and 81.1% TOC removal efficiency, which were higher than those of CIP electrochemical oxidation (degradation efficiency 95.1%, TOC 70.3%) over a Ti/SnO2-Sb anode with current density of 100 mA/cm2 [1]. Moreover, the O3-REM coupled process obtained 92.5% SMX removal efficiency and 75.1% TOC removal efficiency, which were close to those of SMX electrochemical oxidation (degradation efficiency 90.4%, TOC 75.1%) over a Ti/SnO2-Sb/Ce-PbO2 anode with current density of 2 mA/cm2 [28]. Therefore, the O3-REM coupled process has shown excellent performance for the degradation of various pharmaceutical pollutants (i.e., TC, PPN, CIP, and SMX) with less energy input and facile coupled electrochemical technology, which shows a great potential for the treatment of pharmaceutical-contaminated wastewater.

16

4. Implications for industrial applications In previous studies, ozonation and EAOPs have achieved satisfactory applications to the treatment of real wastewater including coking water [29, 30], landfill leachate [31, 32], municipal wastewater [33, 34], tannery waste liquors [35, 36], olive oil wastewater [37, 38], and textile dye effluents [39, 40]. Recently, the Ti4O7 REM has also been proposed as a satisfactory anode for EAOPs because of its outstanding properties and its promising performance for the removal of organic contaminants from industrial wastewater [16, 18]. In this study, we explored the combination of ozonation and electrochemical oxidation in a Ti4O7 REM reactor, which could achieve favorable degradation and TOC removal efficiencies for various pharmaceutical pollutants (i.e., TC, PPN, CIP, and SMX), and induced lower energy consumption simultaneously compared to the individual ozonation or electrochemical oxidation process. Based on the overall results, the O3-REM coupled process stands out as a promising method for the treatment of water body contaminated with various pharmaceutical residues. Given the ozonation and electrochemical advanced oxidation reactions in real wastewater are complex, more studies are still needed for comprehensive investigations into the applications and mechanisms of the O3-REM coupled process in future.

5. Conclusions In this work, we developed an energy-effective O3-REM coupled process with satisfactory performance in TC degradation and mineralization, and analyzed the mechanism of TOC change during the O3-REM coupled process by exploring the transformation products and degradation pathway of TC. At the initial 20 min, TC was nearly completely removed by ozonation in the Ti4O7 REM reactor and the corresponding TOC removal efficiency was 9.1%, when 33 intermediates including some nitrogen-containing compounds and 17

macromolecular compounds were still detected. With ceased ozone supply phase from 20 to 80 min, the TOC removal efficiency reached 77.1% by electrochemical oxidation over the Ti4O7 REM anode, and 18 intermediates including some short-chain carboxylic acids were identified. TC may be oxidized into three-, two- and one-ring intermediates, and eventually carboxylic acids, CO2 and H2O. In the O3-REM coupled process, the concentrations of NO3ˉ, NO2ˉ and NH4+ increased and the bioluminescence inhibition ratio decreased simultaneously, showing a better denitrification efficiency via electrochemical oxidation. Compared to individual O3 or Ti4O7 REM process, the O3-REM coupled process exhibited an enhanced TC and TOC removal efficiencies and lower energy consumption for TC removal. Additionally, the O3-REM coupled process also showed excellent performance in the degradation of PPN, CIP, and SMX, which suggested the great potential of the O3-REM coupled process for the remediation of water body contaminated with various pharmaceutical residues. This study tested the performance, mechanism and energy consumption of TC removal by a O3-REM coupled process, providing insights into the application potential of the O3-REM coupled process to efficiently eliminate pharmaceuticals from wastewater.

Acknowledgements The study was financially supported by the National Natural Science Foundation of China (Grant No. 51709103), National Key R&D Projects of China (Grant No. 2016YFC0403002), Hunan Provincial Key R&D (Grant No. 2018WK4007), Training Program for Excellent Young Innovators of Changsha (Grant No. kq1802020), Double First-Class Construction Project of Hunan Agricultural University (Grant No. kxk201801005 and SYL201802005), and Hong Kong Scholars Program (Grant No. XJ2018029).

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Figure Caption Fig. 1. Device of the TC removal experiments by the O3-REM coupled process Fig. 2. Effect of the ozone dosage on TC removal (a), and evolution of TOC removal (b) during TC ozonation in the Ti4O7 REM reactor. The experimental condition was: 2 mg/min ozone dosage, 1 L/min zone gas flow rate, 40 L/(h·m2) membrane flux, and 5 mg/L initial TC concentration. Fig. 3. Evolution of TOC removal during TC mineralization by the O3-REM coupled process after TC was nearly completely removed with the applied current densities ranging from 1~5 mA/cm2 and the membrane flux of 40 L/(h·m2). Fig. 4. Comparisons of TC (a) and TOC (b) removal efficiencies, and energy consumption (c) by O3-REM, O3 and Ti4O7 REM respectively. The experimental condition was: 2 mg/min ozone dosage, 1 L/min ozone gas flow rate, 2 mA/cm2 current density, 40 L/(h·m2) membrane flux, and 5 mg/L initial TC concentration. Fig. 5. Changes of NO3ˉ, NO2ˉ and NH4+ concentrations (a) and effluent toxicity (b) during the O3-REM coupled process. The experimental condition was: 2 mg/min ozone dosage, 1 L/min ozone gas flow rate, 2 mA/cm2 current density, 40 L/(h·m2) membrane flux, and 5 mg/L initial TC concentration. Fig. 6. TC (a) and TOC (b) removal efficiencies during propranolol, ciprofloxacin and sulfamethoxazole removal by the O3-REM coupled process. The experimental condition was: 2 mg/min ozone dosage, 1 L/min ozone gas flow rate, 2 mA/cm2 current density, 40 L/(h·m2) membrane flux, and 5 mg/L initial compound concentration.

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Table 1 Efficiency and kinetics of TC ozonation in the Ti4O7 REM reactor (20 min)a.

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Rate Degradation

Parameters

efficiency

constants

Half-lives (t1/2, min)

R2

2.7

0.963

2.1

0.964

1.7

0.961

1.4

0.959

1.0

0.949

(k, min-1)

Ozone dosage （mg/min）

a

1.5

(94.2±0.2)%

2

(96.8±0.3)%

2.5

(97.2±0.12)%

3

(97.7±0.03)%

4

(97.9±0.01)%

(1.2±0.01)×1 0-2 (1.5±0.03)×1 0-2 (1.6±0.05)×1 0-2 (1.7±0.02)×1 0-2 (1.9±0.02)×1 0-2

Except specific statement, the other experimental conditions for each treatment was: 2

mg/min ozone dosage, 1 L/min zone gas flow rate, 40 L/(h·m2) membrane flux, and 5 mg/L initial TC concentration.

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Table 2 Potential intermediates of TC degradation detected during the O3-REM coupled processa. Methods

Intermediates identified by HPLC-MS

Intermediates identified by GC-MS

m/z

Molecular formula

MS2 product ion (m/z)

m/z

Molecular formula

445(TC)

C22H24O8N2

427, 410

149

C4H4O6

146 222 119 118 90b 116 46b 61b 138 166 122 230 32b 61b 106 87b 182 152 119 151 146

C4H2O6 C6H6O9 C3H3O5 C3H2O5 C2H2O4 C4H4O4 CH2O2 CH3NO2 C7H6O3 C8H6O4 C7H6O2 C8H6O4 CH4O C2H7NO C7H6O C3H5NO2 C12H22O C10H16O C7H5NO C8H9NO2 C4H2O6

149, 148, 131, 103 146, 129, 101 222, 205, 177 119, 103, 75 118, 101, 73 90, 73, 45 116, 99, 71 118, 117,103, 88 133, 118, 117, 60 138, 121, 93 166, 149, 121 122, 104, 76 230, 213, 185 104, 103, 89, 74 133, 118, 103, 60 106, 91, 71 87, 86, 69, 70 182, 167, 165 152, 137, 135 119, 77 151, 136, 105 146, 129, 101

222

C6H6O9

222, 205, 177

119

C3H3O5

119, 103, 75

118

C3H2O5

118, 101, 73

90b

C2H2O4

90, 73, 45

46b

CH2O2

118, 117,103, 88

Characteristic ions (m/z)

Time

20 min

396 400 412 415 431 448 451 477 480 496 509 525

C15H9O12N 379, 361 C21H21O7N 382, 396 C16H13O12N 394, 377 C21H22O7N2 398, 380 C21H22O8N2 414, 396, 378 C20H17O11N 430, 413 C19H14O13 433, 417 C22H24O10N2 460, 442 C20H17O13N 462, 445 C20H17O14N 478, 460 C22H24O12N2 491, 447 C22H24O13N2 507, 489

412

C16H13O12N

394, 377

451

C19H14O13

433, 417

496

C20H17O14N

478, 460

396

C15H9O12N

379, 361

367

C15H10O11

349

298

C11H7O9N

281

80 min

27

a b

351

C14H6O11

333

253

C10H4O8

235

61b

CH3NO2

133, 118, 117, 60

138

C7H6O3

138, 121, 93

166

C8H6O4

166, 149, 121

122

C7H6O2

122, 104, 76

Possible structures of these intermediates are shown in Supporting Information. Intermediates identified by GC-MS after derivatization.

Highlights 

A coupled process of ozonation and Ti4O7 reactive electrochemical membrane



Satisfied tetracycline degradation and mineralization via the coupled process



Analysis of transformation products explains tetracycline mineralization mechanism



Relatively lower energy consumption for TC mineralization by the coupled process



Implications for industrial applications to real wastewater remediation

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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