Chemiluminescence as a New Indicator for Monitoring Hydroxylated

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Chemiluminescence as a New Indicator for Monitoring Hydroxylated Intermediates in Persulfate-Based Advanced Oxidation Processes Zenghe Li,† Xuejie Chen,† Xu Teng,*,‡,§ and Chao Lu*,† †

State Key Laboratory of Chemical Resource Engineering and ‡Beijing Advanced Innovation Center for Soft Matter Science and Engineering (BAICAS), Beijing University of Chemical Technology, Beijing 100029, China § Institute of Plant Protection, Heilongjiang Academy of Agriculture Science, Harbin 150086, China

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S Supporting Information *

ABSTRACT: The formation of hydroxylated intermediates is the first step in the persulfate-based advanced oxidation processes (AOPs) and depends directly on the oxidation degree of recalcitrant pollutants. Therefore, the monitoring of hydroxylated intermediates plays a vital role in persulfate-based AOPs for evaluating the oxidation degree and understanding the degradation mechanism. In this work, based on the attractive amplification of luminol chemiluminescence (CL) signals by hydroxylated intermediates, we developed a rapid, sensitive, and simple CL-based method to monitor hydroxylated intermediate (i.e., 3,5-diethyl1,4-benzenediol) generated in persulfate-based AOPs of 1,2-divinylbenzene (DVB). The possible mechanism of the proposed CL system was that hydroxylated intermediates could be oxidized by persulfate in alkaline solution to form superoxide anion radicals and thus lead to an increase in the luminol CL. Finally, the proposed CL system was successfully applied in monitoring hydroxylated intermediates generated during persulfate-based AOPs under different degradation conditions, whose validity and reliability were verified by liquid chromatography−mass spectrometry (LC-MS) and Fourier-transform infrared spectroscopy (FT-IR). The generality of the proposed CL system was verified by monitoring hydroxylated intermediates generated in persulfate-based AOPs of methylbenzene and benzoic acid. These results suggested that this present CL system provided a promising indicator for monitoring hydroxylated intermediates in persulfate-based AOPs. ultraviolet visible (UV−vis) spectroscopy, and fluorescence spectroscopy. Unfortunately, these reported methods are unsatisfactory for the detection of hydroxylated intermediates in AOPs due to the intrinsic background signals and the low concentrations of hydroxylated intermediates in the complex degradation solution. Moreover, mass spectrometry is a sensitive detection method to measure hydroxylated intermediates.14,15 However, the nonvolatile salts (e.g., Na2S2O8) could suppress the mass signals and even damage the mass spectrometer.16 The complex, cumbersome, and time-consuming pretreatment is necessary to remove the nonvolatile salts form the samples, which makes it impractical and inconvenient for monitoring hydroxylated intermediates in AOPs. Therefore, it is of great significance to invent a simple and rapid approach to monitor hydroxylated intermediates generated in AOPs. Chemiluminescence (CL) refers to the phenomenon that generates light emission based on the molecules in a chemically generated excited state.17 Such emission can be detected immediately by a photomultiplier tube (PMT) when the CL

1. INTRODUCTION Today there is a continuously increasing concern for the treatment of recalcitrant pollutants, which are not treatable by conventional techniques due to their high chemical stability and low biodegradability.1 Advanced oxidation processes (AOPs) are considered highly competitive water treatment technologies for the removal of those recalcitrant pollutants.2 Usually, the AOPs could degrade recalcitrant pollutants by forming hydroxyl radicals (•OH radicals) as the main oxidizing species.3 However, the discovery of persulfate, as an important precursor for the powerful sulfate radicals (SO4•− radicals) for AOPs, overshadows the •OH radicals. This is due to the higher oxidation potential and longer lifetime of SO4•− radicals (3.1 V, 30−40 μs) in comparison with those of •OH radicals (2.8 V, 0.02 μs).4−6 In addition, the degradation of recalcitrant pollutants is initiated by forming the hydroxylated intermediates in persulfate-based AOPs.7−9 Therefore, the formation and consumption of all hydroxylation intermediates depend directly on the oxidation degree, and it is vital to monitor hydroxylated intermediates in persulfate-based AOPs.10 In recent years, a variety of techniques have been performed for the detection of intermediates generated during AOPs,11−13 including Fourier-transform infrared (FT-IR) spectroscopy, © XXXX American Chemical Society

Received: July 24, 2019 Revised: August 13, 2019 Published: August 19, 2019 A

DOI: 10.1021/acs.jpcc.9b07058 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. CL Mechanism for Monitoring Hydroxylated Intermediates Generated in Persulfate-Based AOPs

reactants are mixed.18 Therefore, CL is becoming a promising technique for monitoring the generation of intermediates with ultralow background, high sensitivity, fast response, cheap instruments, and simple operation, without requiring sample pretreatment. Recently, many CL probes have been developed and applied to the monitoring of various compounds.19 Among them, luminol, regarded as a typical strong CL reagent, has been found that its light emission can be enhanced significantly when hydroxyl compounds were added into this CL reaction.20−22 In addition, persulfate is an oxidant, which can produce weak CL emission when it is mixed with alkaline luminol.23,24 These interesting results encourage us to explore a luminol probe-triggered CL measurement system for monitoring hydroxylated intermediates generated in AOPs. In this work, we chose 1,2-divinylbenzene (DVB), a typical industrial chemical, which was widely used in the manufacture of plastics and ion exchange resins, as a model recalcitrant pollutant to investigate the hydroxylated intermediates generated in persulfate-based AOPs. It was found that the luminol CL emission could be significantly enhanced when the hydroxylated intermediates are involved in this CL system. The CL spectrum, electron spin resonance (ESR), and radical scavenging methods demonstrated that the high CL signals were attributed to an increased amount of superoxide anion radicals (O2•− radicals) from the oxidation reaction of hydroxylated intermediates by Na2S2O8 in alkaline solution (Scheme 1). Therefore, we designed a rapid, sensitive, and simple CL system for monitoring hydroxylated intermediates generated in persulfate-based AOPs. The validity and reliability of this method had been verified by liquid chromatography− mass spectrometry (LC-MS) and Fourier-transform infrared spectroscopy (FT-IR). The generality of the proposed CL system was confirmed by monitoring hydroxylated intermediates generated in persulfate-based AOPs of methylbenzene and benzoic acid.

Tesque Inc. (Tokyo, Japan). DVB was purchased from Xiya Chemical Co., Ltd. (Shandong, China). 5,5-Dimethyl-1pyrroline N-oxide (DMPO) was obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Luminol, ascorbic acid, Na2S2O8, methylbenzene, benzoic acid, cresol, and terephthalic acid were obtained from J&K Scientific Ltd. (Beijing, China). 2,5-Diethyl-1,4-benzenediol was obtained from Beijing HWRK Chemical Reagent Co., Ltd. (Beijing, China). A stock solution of luminol (0.03 M) was prepared by dissolving luminol in 0.1 M NaOH solution. Additionally, the as-prepared luminol solution was left in a dark bottle for weeks prior to use to ensure the stability of luminol solution. All working solutions were prepared by deionized water (Milli-Q, Millipore, Barnstead, CA). 2.2. Degradation Experiments. In a typical experiment, 100 mL of an aqueous solution containing the desired concentration of DVB was loaded in a two-necked roundbottom flask. Then the appropriate amount of Na2S2O8 was added into the above solution, and the degradation reaction took place under the magnetic stirring in open air equilibrium. The temperature of the as-prepared degradation solution during the degradation experiment was 80 °C, which was maintained by a thermostated water bath. The pH of the asprepared degradation solution was initially adjusted to pH 6.5 by using 0.1 M NaOH or 0.1 M H2SO4 in all experiments. Samples were periodically taken from the reactor and then analyzed as follows. All experiments were performed at least in duplicate to ensure reproducibility. 2.3. Apparatus. UV−vis absorption spectra were collected with a spectrophotometer (Model UV-3600, Shimadzu, Japan). FT-IR spectra were examined by a Nicolet 6700 FTIR spectrometer (Thermo, U.S.A.). Fluorescence spectra were measured with a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan). LC-MS analyses were performed using a quattro micro triple quadrupole mass spectrometer (Waters, USA). The measurements were conducted at the positive ionization mode on an acquity UPLC HSS T3 column. ESR data were obtained on an ESR spectrometer (Bruker, Germany). The CL detection was conducted on a biophysics CL (BPCL) luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). The CL spectrum was obtained with the F-7000 fluorescence spectrophotometer by turning off the xenon lamp. Analyses of DMPO adducts were done using the Bruker Xepr software to

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solutions. All chemicals were analytical grade and used without further purification. NaOH, KOH, H2SO4, and CH2Cl2 were purchased from Beijing Chemical Reagent Company (Beijing, China). Thiourea and NaN3 were purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai B

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20 min, Na2S2O8 concentration was still more than 75% of the original concentration (Figure S3). Therefore, we evaluated the CL intensities of the proposed system when Na2S2O8 concentrations were 80% and 75% of the original concentration. As shown in Figure S4, the CL intensities showed a 4.5% decrease when Na2S2O8 concentration was 75%. These results indicated that the change of Na2S2O8 concentration during persulfate-based AOPs had little effect on the CL emission of the present system. In the following sections, more experiments would be performed to explore the potential relationship between the CL intensity and the intermediates in the DVB degradation solution during persulfate-based AOPs. 3.2. Identification and Quantitation of Intermediates in Persulfate-Based AOPs. To further clarify the origin of the attractive CL amplification from the DVB degradation solution−luminol system, we investigated intermediates in the DVB degradation solution during persulfate-based AOPs by LC-MS analysis. It was well-recognized that hydroxylated intermediates were responsible for the persulfate-based AOPs.26 As shown in Figure 2A, three degradation products clearly showed up at different retention times. Interestingly, as the degradation experiment moved forward, the degradation product at 6.8 min of retention time reached its maximum within 8 min and then disappeared after 15 min of the treatment with hot Na2S2O8. These results were consistent with the change trend of the CL signals in the DVB degradation solution−luminol system during persulfate-based AOPs. Afterward, the degradation product at 6.8 min was confirmed as a hydroxylated compound (i.e., 2,5-diethyl-1,4benzenediol) by its typical signature in the MS analysis (Figure 2B). Briefly, the hydroxylated compounds were characterized by addition of a proton to the molecule to form the molecular ion [M + H]+, followed by the loss of a water molecule [M + H − H2O]+. Therefore, the hydroxylated compound in the DVB degradation solution presented a molecular ion [M + H]+ at 167 m/z with a fragment at 149 m/z [C10H9O]+. The findings provided the compelling evidence that the proposed CL method could be used as a novel indicator for the hydroxylated intermediates in the DVB degradation solution during persulfate-based AOPs. To further confirm the formation of hydroxylated intermediates in the DVB degradation solution during persulfatebased AOPs, the DVB degradation solutions at different treatment time were extracted with CH2Cl2 for the FT-IR measurements.12 In the FT-IR spectra (Figure 2C), the transmittance of aromatic skeleton vibration band (1480 and 1624 cm−1) and stretching and bending vibrations of C−H in olefin (3086, 989, and 906 cm−1) decreased gradually with increasing treatment time. Such a phenomenon was due to the ring-opening process of DVB during persulfate-based AOPs. In addition, the change of broad peak centered at 3400 cm−1, which could be assigned to hydroxyl stretch, demonstrated that hydroxylated intermediates in the DVB degradation solution were maximum at 5 min of the treatment with hot Na2S2O8 and disappeared at 20 min of the treatment with hot Na2S2O8. These results were consistent with the change trend of LC-MS results and CL intensities in the DVB degradation solution− luminol system during persulfate-based AOPs. On the other hand, UV absorption analysis and fluorescence analysis were done immediately when the DVB degradation solutions were taken from the heated reactor at different time intervals. As shown in Figure 2D, an absorption band appeared at around 255 nm, which related to the characteristic absorbed

determine area and peak height of signals, and the identification of adducts was performed with the simulation software EPR simulator. The hyperfine splitting constants (in G) used for simulation of the spectra of the DMPO−•OH and DMPO−O2•− were as follows: (aH = 15.1; aN = 15.1) and (aN = 14.0; aH = 11.0), respectively. 2.4. CL Measurements. A 100 μL aliquot of luminol (0.3 mM, pH = 12) was mixed with the degradation solution by the injection of 100 μL of the degradation solution. The CL signals were monitored by a photomultiplier tube (PMT) adjacent to the CL quartz vial. Then the profile of CL intensity was acquired by BPCL software under Windows 7 at an interval of 0.1 s, and a work voltage of −880 V was used for the CL detection. The signals were imported to the computer for data acquisition.

3. RESULTS AND DISCUSSION 3.1. CL Performances of Intermediates in PersulfateBased AOPs. The intermediates generated in persulfate-based AOPs on the luminol CL system were examined in a static injection CL setup (Figure S1). As shown in Figure S2, the DVB degradation solution could induce a significant increase in the CL intensity of the luminol. Under the same experimental conditions, the controlled experiments were also performed including Na2S2O8, DVB, and the mixture of DVB and Na2S2O8. Interestingly, no CL enhancement was observed for the DVB solution. However, Na2S2O8 and the mixture of DVB and Na2S2O8 showed a slight enhancement on the luminol CL intensity due to the strong oxidizing property of Na2S2O8. These findings indicated that the intermediates in the persulfate-based AOPs played an important role in enhancing the luminol CL. Figure 1 shows the CL intensities

Figure 1. CL intensities of the DVB degradation solution, Na2S2O8, and DVB at different treatment time in the luminol system. The concentrations of Na2S2O8, DVB, and luminol were 2.5 mM, 35.0 μM, and 0.3 mM, respectively. The temperature of the DVB degradation solution in persulfate-based AOPs was 80 °C. The pH of the DVB degradation solution was 6.5.

of the DVB degradation solutions with the treatment of hot Na2S2O8 for different times in the luminol system. The CL intensity first increased gradually after 2.0 min of the treatment with hot Na2S2O8, reached its maximum within 8 min, and then decreased to the control level after 15 min of the treatment with hot Na2S2O8. Note that the CL responses for Na2S2O8 alone or DVB alone remained unchanged during the period of treatment under the same conditions. Furthermore, the iodometric titration was applied to determine Na2S2O8 concentrations at different treatment times.25 After as long as C

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Figure 2. (A) LC-MS of the DVB degradation solution during persulfate-based AOPs. (B) MS spectrum of the DVB degradation solution with the treatment of hot Na2S2O8 for 8 min when the retention time was 6.8 min. (C) FT-IR spectra and (D) UV absorption spectra of the DVB degradation solution with the treatment of hot Na2S2O8 for different times. The concentrations of Na2S2O8 and DVB were 2.5 mM and 35.0 μM, respectively.

Figure 3. (A) Experimental (black) and simulated ESR (red) spectra of DMPO−•OH adduct in the only Na2S2O8 system. (B) Experimental (black) and simulated ESR (red) spectra of DMPO−•OH and DMPO−O2•− adducts in the DVB degradation solution system. The DVB degradation solution was treated with hot Na2S2O8 for 8.0 min. The concentrations of Na2S2O8, DVB, and DMPO were 5.0 mM, 35.0 μM, and 90.0 mM, respectively.

spectrum for an aromatic ring.2 Interestingly, the absorbance intensity at 255 nm decreased during the degradation process which could be attributed to the ring-opening degradation of DVB during persulfate-based AOPs. Moreover, as the DVB degradation experiment moved forward, the fluorescence intensity at 330 nm decreased, without significant changes in the maximum emission wavelength and spectral shape (Figure S5). This finding further indicated that DVB was degraded under the experimental conditions. On the other hand, the hydroxylated intermediate in the persulfate-based AOPs of DVB was quantitatively analyzed. As shown in Figure S6, the CL intensity was proportional to the concentration of 2,5-diethyl-1,4-benzenediol in the range from 0.1 to 3.0 μM. The regression equation is y = 30.91x + 0.14 (R2 = 0.9925), where y is the relative CL intensity and x is the concentration of 2,5-diethyl-1,4-benzenediol. The detection limit for 2,5-diethyl-1,4-benzenediol was 0.025 μM (S/N = 3). Moreover, it had a good discrimination under low concentration conditions. The relative standard deviation for nine

repeated measurements of 1 mM 2,5-diethyl-1,4-benzenediol was 2.1%. Based on the above results, the amount of 2,5diethyl-1,4-benzenediol generated during the persulfate-based AOPs is shown in Figure S7. The results were in good agreement with those obtained by the LC-MS method (Table S1). Moreover, the detection limits of earlier reported methods for the hydroxylated intermediates were between 0.5 and 0.05 μM.27−29 Therefore, it was concluded that the proposed method was a sensitive method. 3.3. Emitting Species. ESR spectroscopy was used to confirm the exact emitting species of the present system.30,31 As shown in Figure 3, in DVB solution, two radical adducts were detected. In combination with the results from ESR spectra simulation, O2•− and •OH radicals were in the presence of the proposed system. The simulation results were consistent with those reported earlier.32−34 Moreover, the Na2S2O8 solution produced a strong DMPO−•OH signal. The reason for this phenomenon was that SO4•− radicals could be generated by Na2S2O8 (eq 1 in Scheme S1) and rapidly D

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Figure 4. (A) Effects of radical scavengers for various reactive oxygen species on the DVB degradation solution−luminol system. The DVB degradation solution was treated with hot Na2S2O8 for 8.0 min. (B) CL spectrum of the DVB degradation solution−luminol system. The concentrations of Na2S2O8, DVB, ascorbic acid, thiourea, NaN3, and NBT were 2.5 mM, 35.0 μM, 10 mM, 10 mM, 10 mM, and 1.0 mM, respectively.

converted into •OH radicals (eq 2 in Scheme S1) in the heat and base coactivated Na2S2O8 system.35 Moreover, the existence of •OH radicals and O2•− radicals was further confirmed by using radical scavenging methods.36 As shown in Figure 4A, the CL signals were almost completely quenched by the addition of ascorbic acid (a scavenger for free radicals), which indicated that free radicals in the CL reaction played a critical role in the CL emission. Thiourea was an effective radical scavenger for •OH radicals. The mechanism of the reaction between thiourea and •OH radicals is shown in Scheme S3.37 The reaction rate of thiourea with •OH was rapid (k(•OH+thiourea) = 1.0 × 1010 dm3 mol−1 s−1). There was no CL emission in the reaction between thiourea and •OH. Therefore, 10 mM thiourea induced a significant decrease in the CL intensity, indicating that •OH radicals were released in the reaction.38 Moreover, 10 mM NaN3 (a scavenger for singlet oxygen, 1O2) did not quench the CL intensity, providing strong evidence that 1O2 did not contribute to the observed CL.39 In addition, NBT was a scavenger for O2•− radicals. It could be selectively and rapidly oxidized by O2•− radicals to produce formazan without CL emission (k(O2•−+NBT) = 5.88 × 104 dm3 mol−1 s−1). The mechanism of the reaction between NBT and O2•− radicals is shown in Scheme S2.40 When 1.0 mM NBT was added to the present CL system, there was a significant decrease in the CL intensity. In conclusion, this system did exist in O2•− radicals.41 Those results showed that the CL emissions in the system were mainly originated from O2•− radicals and •OH radicals, and the contribution of the 1O2 to the strong CL was less. To gain insight into the emitting species in this CL reaction, the CL spectrum of this present system was obtained with the F-7000 fluorescence spectrophotometer by turning off the xenon lamp. As shown in Figure 4B, there was a maximum emission peak at about 425 nm, corresponding to an emission band of 3aminophthalate, indicating that the CL emission was derived from the oxidation of luminol. To explore the amount of O2•− radicals in the DVB degradation solution−luminol system during the DVB degradation reaction, NBT was added to the above system.42 Briefly, 100 μL of the DVB degradation solution was injected into 100 μL of NBT (1 mg/mL), and then luminol was added into the above solution. The supernatant was carefully removed, and the precipitate was dissolved in the mixture of KOH (2 M, 90 μL) and DMSO (110 μL). The maximum wavelength of dissolved precipitate was at 660 nm in the UV−

vis absorption spectra (Figure S8), corresponding to the formation of the formazan. Based on the absorbance of the formazan, the amount of O2•− radicals could be obtained in different DVB degradation solution−luminol systems during persulfate-based AOPs. Therefore, the amount of O2•− radicals gradually increased as the degradation experiment moved forward and reached its maximum amount of O2•− radicals when the DVB degradation solution was treated with hot Na2S2O8 for 8 min. Moreover, terephthalic acid (a scavenger for •OH radicals) could readily react with •OH radicals to produce the highly fluorescent product 2-hydroxyterephthalic acid.43 When 2.0 mM terephthalic acid was added to the DVB degradation solution−luminol system, a fluorescence peak at about 420 nm appeared, indicating that •OH radicals were indeed formed. As shown in Figure S9, fluorescence intensity was basically unchanged as the degradation experiment moved forward. This phenomenon indicated that the concentration of • OH radicals in the DVB degradation solution−luminol system was almost constant during persulfate-based AOPs. 3.4. Mechanism for Monitoring Hydroxylated Intermediates in Persulfate-Based AOPs. Based on the discussion above, the mechanism of the reaction can be illustrated in Scheme S1. Na2S2O8 was a moderate oxidizing agent. Under the experimental conditions, it could be activated to generate highly reactive SO4•− radicals. Then the DVB molecule could be degraded to form hydroxylated intermediates by SO4•− radicals and then underwent a ring-opening reaction. When the mixture of hydroxylated intermediates and Na2S2O8 was injected into the CL vial containing basic luminol, the hydroxylated intermediates could be oxidized by SO4•− radicals to produce more O2•− radicals.21 Then the generated •OH radicals and O2•− radicals reacted immediately with luminol to form an excited state of luminol, which may emit the light at 425 nm when it returned to the ground state.44 3.5. CL as a Novel Indicator for Hydroxylated Intermediates in Persulfate-Based AOPs. It has been demonstrated that for persulfate-based AOPs the degradation temperature, the degradation pH, and initial concentrations of reactants (i.e., pollutants and Na2S2O8) have significant effects on the degradation rate of pollutants.45 Therefore, we monitored hydroxylated intermediates generated during persulfate-based AOPs in the conditions of different degradation temperatures, different degradation pHs, and different initial concentrations of reactants (i.e., DVB and Na2S2O8). As shown in Figure 5, when the degradation E

DOI: 10.1021/acs.jpcc.9b07058 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. CL as an indicator for monitoring hydroxylated intermediates generated in persulfate-based AOPs under different conditions. (A) Na2S2O8 concentration. (B) Temperature of the DVB degradation solution. (C) DVB concentration. (D) pH of the DVB degradation solution. The concentrations of luminol, Na2S2O8, and DVB were 0.3 mM, 2.5 mM, and 28.0 μM, respectively. The temperature of the DVB degradation solution in persulfate-based AOPs was 80 °C. The pH of the DVB degradation solution was 6.5.

Figure 6. CL intensities of methylbenzene degradation solution (A) and benzoic acid degradation solution (B) at different treatment times in the luminol system. LC-MS of the methylbenzene degradation solution (C) and benzoic acid degradation solution (D) during persulfate-based AOPs. The concentrations of Na2S2O8, methylbenzene, benzoic acid, and luminol were 2.5 mM, 35.0 μM, 35.0 μM, and 0.3 mM, respectively. The temperature of the methylbenzene degradation solution and the benzoic acid degradation solution in persulfate-based AOPs was 80 °C. The pH of the methylbenzene degradation solution and the benzoic acid degradation solution was 6.5.

increased, the formation of SO4•− radicals and hydroxylated intermediates was enhanced, and thus the degradation of DVB was accelerated. Moreover, increasing the initial concentration of DVB tended to delay the formation and conversion time of hydroxylated intermediates in the degradation solution. Besides, variation of pH value had a significant impact on the persulfate-based AOPs. When the pH value changed from 3.0 to 6.5, the formation rate of hydroxylated intermediates

temperature was increased from 70 to 85 °C, the formation rate of hydroxylated intermediates was increased, but the existence time of hydroxylated intermediates in the degradation solution was shortened, indicating a rapid degradation of DVB. These results suggested that Na2S2O8 could be converted to SO4•− radicals by thermal energy, resulting in a considerable enhancement in the reaction rates. On the other hand, when the initial concentration of Na2S2O8 was further F

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technique. The proposed CL system can be expanded to track a wider variety of hydroxylated intermediates generated during aging of organic materials and oxidation of other organic substances.

was almost unchanged, but the conversion time of hydroxylated intermediates was slightly delayed under acidic conditions. The reasons for this phenomenon were that SO4•− radicals predominated under acidic and neutral conditions. However, it may enhance the production of less reactive species like HSO4− under a highly acidic condition. When the pH value increased to 11.0, the formation rate of hydroxylated intermediates was decreased, implying that alkalinity environments were not conducive to the formation of hydroxylated intermediates. Furthermore, as the pH value increased, the OH− could lead to a transformation from SO4•− radicals to •OH radicals, which may reduce the proportion of SO4•− radicals in the reaction system.10 Because •OH radicals (E0 = 2.80 V) had a lower oxidative capacity than SO4•− radicals (E0 = 3.10 V), such a transformation would reduce the oxidative capacity of the Na2S2O8 system, resulting in a relatively slow formation rate of hydroxylated intermediates under strong alkaline conditions.46 These results further indicated that the conversion time of hydroxylated intermediates in the degradation solution was related to the degradation effect of DVB: a shorter conversion time of hydroxylated intermediates could contribute to the better degradation effect. On the other hand, the degradation of DVB in the same conditions was further investigated by UV absorption spectra (Figures S10−S13). In comparison with UV absorption spectra, CL could more accurately monitor the formation of intermediates in the DVB degradation process. 3.6. Performances of a Universal CL Method for Monitoring Hydroxylated Intermediates in PersulfateBased AOPs. To verify the universality of this method, we investigated the other two pollutants: methylbenzene and benzoic acid. Briefly, Figure 6A,B shows the CL intensities of the methylbenzene and benzoic acid degradation solutions with the treatment of hot Na2S2O8 for different times in the luminol system. As shown in Figure 6C,D, LC-MS results of hydroxylated intermediates were consistent with the change trend of the CL signals. Moreover, the degradation products during persulfate-based AOPs were confirmed as hydroxylated compounds (i.e., cresol, hydroxybenzoic acid, and hydroquinone) by their typical signatures in the MS analysis (Figures S14 and S15). These results demonstrated that the proposed CL method exhibited the generality in monitoring hydroxylated intermediates during persulfate-based AOPs. On the other hand, we quantitatively analyzed the cresol in the persulfate-based AOPs of methylbenzene (Figures S16 and S17). The results were satisfactory in comparison with those obtained by the LC-MS method (Table S2).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b07058.

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Schematic diagram of the static injection CL analysis system; CL intensity of luminol in the presence of Na2S2O8, DVB, the mixture of Na2S2O8 and DVB, and the DVB degradation solution; concentrations of Na2S2O8 in the DVB degradation solution at different treatment times; effect of Na2S2O8 concentration on CL intensities of the DVB degradation solution in the luminol system; fluorescence spectra of the DVB degradation solution treated with hot Na2S2O8 for different times; CL intensities at different concentrations of 2,5-diethyl-1,4-benzenediol and the calibration curve for 2,5-diethyl-1,4-benzenediol; the amount of 2,5diethyl-1,4-benzenediol generated during the persulfate-based AOPs; UV−vis absorption spectra of the mixture of the DVB degradation solution and NBT; fluorescence spectra of the mixture of the DVB degradation solution and terephthalic acid; effect of Na2S2O8 concentration, temperature, DVB concentration, and pH on UV−vis absorption intensities of the DVB degradation solution; MS spectrum of the methylbenzene degradation solution; MS spectra of the benzoic acid degradation solution; CL intensities at different concentrations of cresol and the calibration curve for cresol; the amount of cresol generated during the persulfate-based AOPs; CL mechanism of luminol CL enhanced by hydroxylated intermediates generated in persulfate-based AOPs; the mechanism of the reaction between thiourea and •OH radicals; the mechanism of the reaction between NBT and O2•− radicals; determination of 2,5-diethyl-1,4-benzenediol during persulfatebased AOPs of DVB; determination of cresol during persulfate-based AOPs of methylbenzene; determination of 2,5-diethyl-1,4-benzenediol during persulfate-based AOPs of DVB; determination of cresol during persulfate-based AOPs of methylbenzene (PDF)

AUTHOR INFORMATION

Corresponding Authors

4. CONCLUSIONS In summary, we designed a novel CL system for monitoring hydroxylated intermediates generated in persulfate-based AOPs. The CL spectrum, ESR, and radical scavenging methods revealed that the DVB molecule was degraded to form hydroxylated intermediates in persulfate-based AOPs. The hydroxylated intermediates could be oxidized by Na2S2O8 in alkaline solution to produce more O2•− radicals, which resulted in an increase in the luminol CL. The validity and reliability of this method had been verified by LC-MS and FT-IR. The generality of the proposed CL system was confirmed by monitoring hydroxylated intermediates generated in persulfatebased AOPs of methylbenzene and benzoic acid. This study has successfully opened a new way to monitor hydroxylated intermediates generated in persulfate-based AOPs by the CL

*(X.T.) E-mail: [email protected]. Phone: +86 10 64411957. Fax: +86 10 64411957. *(L.C.) E-mail: [email protected]. Phone: +86 10 64411957. Fax: +86 10 64411957. ORCID

Chao Lu: 0000-0002-7841-7477 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21705035, 21521005, and 21575010), the National Basic Research Program of China (973 Program, No. 2014CB932103), the Research Project of Heilongjiang G

DOI: 10.1021/acs.jpcc.9b07058 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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Academy of Agriculture Science (2017ZC03), and the Innovation and Promotion Project of Beijing University of Chemical Technology.

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DOI: 10.1021/acs.jpcc.9b07058 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.9b07058 J. Phys. Chem. C XXXX, XXX, XXX−XXX