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Photocatalytic degradation of nonylphenol ethoxylate and its degradation mechanism
Journal of Molecular Liquids 302 (2020) 112567
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Photocatalytic degradation of nonylphenol ethoxylate and its degradation mechanism Huiqin Liang a, Xiumei Tai a,⁎, Zhiping Du a,b,⁎⁎ a b
Shanxi Key Laboratory of Surfactants, China Research Institute of Daily Chemistry Co., Ltd., Taiyuan, Shanxi 030001, China Institute of Resources and Environmental Engineering, Shanxi University, Taiyuan, Shanxi 030006, China
a r t i c l e
i n f o
Article history: Received 6 November 2019 Received in revised form 7 January 2020 Accepted 23 January 2020 Available online 24 January 2020 Keywords: Nonylphenol ethoxylate (NPE10O) TiO2@g-CQDs Photocatalysis Path Mechanism
a b s t r a c t Photocatalyst TiO2@g-CQDs were prepared by one-step sol-gel method using tetrabutyl titanate, anhydrous ethanol, hydrochloric acid and graphite carbon quantum dots and characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectrum (FT-IR), X-ray diffraction (XRD), UV– visible diffuse reflectance spectra (UV-DRS) and X-ray photoelectron spectroscopy (XPS). The photocatalytic degradation of nonylphenol ethoxylate (NPE10O) by TiO2@g-CQDs and the degradation path of NPE10O by time-of-flight mass spectrometry (TOF-MS) were investigated. In addition, the photocatalytic mechanism of TiO2@g-CQDs was discussed in detail and the major active species were identified. The results showed that when the doping amount of g-CQDs is 5%, the amount of TiO2@g-CQDs is 0.1 g·L−1, the initial concentration of NPE10O is 50 mg·L−1, and the power of the light source is 500 W, the degradation efficiency of NPE10O can reach 100% at 60 min, which indicates TiO2@g-CQDs have excellent photocatalytic properties. TOF-MS showed that NPE10O was mainly degraded by nonylphenol polyethoxycarboxylate (NPExC) and carboxylated alkyl oxidation products of nonylphenol ethoxy (CAyPEC) intermediates during photocatalytic degradation. Trapping experiment of active species demonstrated that •OH is the major active component for photocatalytic degradation of contaminant. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Alkylphenol ethoxylates (APEOs) are the second largest commercial nonionic surfactants in the world, among which octylphenol ethoxylates (OPEO) and nonylphenol ethoxylates (NPE10O) are the most widely used [1,2]. NPE10O is the main component of textile auxiliaries such as emulsifiers, penetrants and detergents. It is widely used in leather and textile printing and dyeing [3]. After entering the water environment, NPE10O wastewater can be converted into short chain nonylphenol ethoxylates and more toxic nonylphenol (NP) major metabolites. Studies have shown that compared with the parent compound, these refractory, intermediate biometabolism with strong bioaccumulation and toxicity can interfere with human endocrine secretion, which is carcinogenic and teratogenic to humans [3–6]. At present, researchers are actively looking for alternatives to NPE10O, and more promising are fatty alcohol ether and isomeric alcohol ether, but from a completely alternative point of view, fatty alcohol ether and isomeric alcohol ether still have some difficulties. By 2017, NPE10O still ⁎ Corresponding author. ⁎⁎ Correspondence to: Zhiping Du, Shanxi Key Laboratory of Surfactants, China Research Institute of Daily Chemistry Co., Ltd., Taiyuan, Shanxi 030001, China. E-mail addresses: [email protected] (X. Tai), [email protected] (Z. Du).
https://doi.org/10.1016/j.molliq.2020.112567 0167-7322/© 2020 Elsevier B.V. All rights reserved.
accounts for a large proportion of the nonionic surfactant market. Therefore, it is extremely important to study the deep degradation and degradation mechanism of NPE10O. In recent years, processes for the advanced treatment of NPE10O wastewater, such as activated carbon [7,8], resin adsorption [9,10], photocatalytic degradation [1,11–13] and biodegradation [14–16], have attracted much attention. The relative cost of activated carbon and resin is relatively high, and biodegradation takes a long time, while photocatalytic degradation with high selectivity, wide application range, has become a research hotspot in sewage treatment in recent years. Semiconductor materials that have been studied as photocatalytic materials include TiO2 [17,18], WO3 [19], ZnO [20], Nb2O5, α-Fe2O3 [21], g-C3N4 [22,23], BiOX [24], BiVO4 [25], Ag3PO4 [26], Ag2CO3 [27], AgBr, AgI, etc. Among them, TiO2 is the earliest photocatalyst used to degrade organic pollutants, and has been enduring even after years of research based on low price, good selectivity, and high photocatalytic activity. However, the large band gap and the high recombination rate of photogenerated electron-hole pairs of pure TiO2 will severely restrict its use, therefore it needs to be modified to improving photocatalytic activity. To date, it can effectively control the electron transport of TiO2 by means of metal/non-metal doping [28,29], noble metal surface deposition [30], semiconductor recombination, and organic dye [31]/quantum dot sensitization [32], thereby reducing the recombination rate of charge carriers. Herein, the emphasis
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is on the sensitization of carbon quantum dots, which is called the most promising semiconductor photosensitizer. Carbon quantum dots (CQDs), as an excellent photosensitive material, can prevent the recombination of energy in addition to the sensitive visible light response. In addition, there is an electronic coupling between the π orbit of CQDs and the conduction band of TiO2, and electrons will be transferred at the interface between CQDs and TiO2, which demonstrates unique superiority in inhibiting the recombination of electrons and holes. CQDs can be more detailedly divided into graphene quantum dots (GQDs), carbon nanodots (CNDs) and polymer dots (PDs) [33,34]. Rajender et al. [35] reported that the graphene quantum dots (GQD) were hybridized with TiO2 nanoparticles to form heterojunctions and used for photocatalytic degradation of MB. The results show that the TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation. Al-Kandari et al. reported the preparation of nanocomposites (rGOTi) by loading 0.33 wt% of reduced graphene oxide (rGO) on commercial TiO2 nanoparticles using a hydrothermal method, and used to study photocatalytic degradation of phenol, p-chlorophenol and p-nitrophenol. The result shows that rGOTi with much higher photocatalytic activity degrade efficiently a mixture of three phenolic compounds [36]. Therefore, it is more wise to choose CQDs as photosensitizers for TiO2 [37–41]. In the work, TiO2@g-CQDs photocatalyst was synthesized by one-step sol-gel method. The sample was characterized by HRTEM, XRD, FT-IR, UV-DRS and XPS and applied to degrade NPE10O simulated wastewater. The effects of photocatalyst concentration, g-CQDs doping amount, NPE10O initial concentration and source power factor on the degradation rate of NPE10O were discussed. At the same time, the photocatalytic degradation path and mechanism of NPE10O was speculated.
2. Experimental 2.1. Chemicals and materials Graphite (99.95%) and tetrabutyl titanate (99.8%) were respectively purchased from Aladdin and Tianjin Kermel Chemical Reagent Co., Ltd. Nonylphenol ethoxylates was purchased from Beijing Sage Chemical Co., Ltd. Hydrochloric acid and absolute ethanol were attained from Shanghai Sinopharm Chemical Reagent Co., Ltd. Isopropanol (IPA), vitamin C (VC) and ethylenediaminetetraacetic acid (EDTA) were bought from Sinopharm Chemical Reagent Co. Ltd. The above reagents were of analytical grades without further purification.
2.2. Synthesis of TiO2@g-CQDs Graphite carbon quantum dots (g-CQDs) were prepared and optimized by the reported mixed acid reflux method [42]. Typically, 5.0 mL g-CQDs aqueous solution with concentration of 40 mg·mL−1 was added into the mixture solution containing 2 mL hydrochloric acids and 15 mL absolute ethanol under strong electromagnetic stirring in succession to form a pellucid and homogeneous solution. Then, moderate tetrabutyl titanate (the calculated amount of 9 mL, 18 mL and 27 mL was selected in order to attain required mass ratios of TiO2 to g-CQDs like 10:1, 20:1 and 30:1) was dissolved in 30 mL absolute ethanol followed by an addition of the previously prepared transparent mixed solution with the drop rate to 2 drops·s−1. The above operation was carried out at 35 ± 2 °C constant temperature water baths under vigorous stirring and the system turned into a gel within about 5 min after the addition was completed. The obtained gel was sealed with plastic wrap, naturally aged for 24 h, then dried at 60 ± 2 °C for 24 h in a vacuum oven, and ground to fine powders with an agate mortar. The final product X-TiO2@g-CQDs was obtained by calcination in a muffle furnace at 250 ± 2 °C for 1.5 h then 500 ± 2 °C for 2 h.
2.3. Characterization High-resolution transmission electron microscopy (HRTEM, JEM2010, JEOL, Japan) was utilized to observe the geometric morphology and particle size of the synthesized samples. The X-ray diffractometer (XRD-6000, Shimadzu, Japan) was used to identify the phase composition and crystallinity of the synthesized sample. Chemical state and chemical composition analysis was carried out using X-ray photoelectron spectroscopy (XPS, XSAM800) with a beam current of 15 mA and operating voltage of 12 kV. The Fourier transform infrared spectrum (FT-IR) of the sample was determined by a tabulating method mixed with KBr to VERTEX 70 spectrometer. The optical absorption range and the band gap energy were measured by UV–visible diffuse reflectance spectra (UV–vis DRS, Cary 300–800, Varian, American). Photoluminescence spectroscopy (PL, Hitachi, F-4600) and transient fluorescence spectrometer (Edinburgh, EdinburghFLS1000) were utilized to characterize the fluorescence properties and transient fluorescence lifetime of g-CQDs, respectively. Time-of-flight mass spectrometry (Ultraflex MALDI-TOF/TOF-MS, Bruker daltonics) was used to detect photodegradation intermediates of samples Photocatalytic degradation of NPE10O was studied using XPA-7 photocatalytic device and UV–Vis spectrophotometer (UV-1601, Beijing). 2.4. Photocatalytic degradation The photocatalytic activity of the as-prepared samples was tested under sunlight-simulated radiation source (Xe arc lamp, λ N 420 nm). Initially, 0.1 g TiO2, 10-TiO2@g-CQDs, 20-TiO2@g-CQDs, and 30-TiO2@ g-CQDs photocatalyst was dispersed in 100 mL of 50 mg·L−1 NPE10O solution, respectively. The suspension was kept by 120 min strong vibration in the dark to reach the adsorption-desorption equilibrium between NPE10O and photocatalysts before irradiation. Then placing under a 500 W Xe arc lamp, 5 mL of the solution was taken after an interval of 5 min, centrifuged for 10 min at 8000 rpm. The supernatant was analyzed using a UV–Vis absorbance spectroscopy at 223 nm of NPE10O maximum absorption peak. The concentration and absorbance of the NPE10O solution were in accordance with Lange-Beer's law throughout the test range. The effects of photocatalyst concentration, g-CQDs doping amount, initial concentration of NPE10O, and source power factor on the degradation rate of NPE10O were tested in parallel. 3. Results and discussion 3.1. HRTEM studies The morphology and particle size of the prepared samples were studied by HRTEM. Fig. 1(a–b) showed HRTEM images and particle size distribution of g-CQDs prepared by mixing acid reflux after optimization. It can be seen that the g-CQDs have good dispersibility, the particle size ranges from 1 to 3 nm, and the average diameter is 1.83 ± 0.36 nm. Fig. 1(c–d) represented the HRTEM image of TiO2, which exhibited morphology of irregular block-like with size of 30–40 nm. And it can be seen from Fig. 1(e), lattice image of TiO2 shows a lattice spacing (d) of 3.89 Å, which corresponds to the TiO2 (101) planes. Fig. 1 (f) showed the lattice image of TiO2@g-CQDs, which indicated that the simultaneous presence of lattices of TiO2 and g-GQDs to prove that gCQDs are successfully supported on TiO2. 3.2. XRD and FTIR studies Fig. 2(a) displayed the XRD patterns of TiO2 and X-TiO2@g-CQDs. The diffraction peaks at 25.3, 37.9, 48.1, 53.9, 55.1, 62.7, and 68.8 correspond to crystal plane (101), (004), (200), (105), (211), and (116) of anatase phase TiO2 (JCPDS NO.84-1285), respectively. The diffraction peaks at 70.4 and 75.2 should be assigned to the (220) and (215) crystal plane of rutile phase TiO2, which means that the synthesized TiO2 by
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Fig. 1. (a, b) HRTEM and particle size distribution images of g-CQDs; (c–e) HRTEM image and lattice images of TiO2; (f) the HRTEM lattice image of TiO2@g-CQDs showing simultaneous presence of TiO2 and g-CQDs.
one-step sol-gel method is a combination of anatase phase and rutile phase, which is more conducive to photocatalytic reaction. At the same time, the above-mentioned diffraction peaks can be observed in XRD patterns of x-TiO2@g-CQDs, indicating that the effect of doping gCQDs on the overall crystal form of TiO2 is not apparent. However, the diffraction peak of g-CQDs did not appear in this process, which may be explained by too little doping of g-CQDs. Interestingly, peak intensity of these diffraction peaks in X-TiO2@g-CQDs is strongly enhanced compared to pure TiO2. Studies have shown that this is caused by a strong interaction between TiO2 and g-CQDs similar to hydrogen bonding or van der Waals forces [43]. Fig. 2(b) showed the FT-IR spectra of g-CQDs, TiO2 and 20-TiO2@gCQDs. Peaks at 3440, 1632 and 1270 cm−1 that appeared in all samples were caused by stretching vibration and bending vibration of O\\H [44,45] and the bending vibration of C\\O bond [46]. The weak double peaks appearing at 2922 and 2854 cm−1 in all samples were assigned to the stretching vibration of _C\\H. In addition, the absorption peaks at 1381 and 1112 cm−1 in g-CQDs were attributed to the bending vibration of C\\O in carboxy and alkoxy [47], respectively. A broad absorption peak of 470 cm−1 belonging to Ti\\O should be the
characteristic absorption peak of TiO2. It should be noted that all absorption peaks in the 20-TiO2@g-CQDs can be found in the FT-IR spectra of gCQDs and TiO2, indicating that the presence of both g-CQDs and TiO2 in the nanocomposite. Moreover, the characteristic absorption peak of 20TiO2@g-CQDs below 1000 cm−1 showed red shifts compared with that of TiO2, which can be ascribed to the vibration coupling of Ti\\O\\Ti. 3.3. XPS studies The chemical element composition and bonding environment of TiO2@g-CQDs were determined by XPS. Fig. 3(a–d) showed the full spectrum and the core layer C1s, O1s, Ti2p spectra of TiO2@g-CQDs in succession. The results show that TiO2@g-CQDs contains Ti, C, and O three elements, and the percentages are 24.63%, 6.24%, and 69.13%, correspondingly. From Fig. 3(b), the two fitted peaks at 458.4, 464 eV observed in the Ti2p XPS pattern correspond to Ti2p3/2, Ti2p1/2 [35,48], which demonstrates the presence of Ti4+ in TiO2@g-CQDs. The two Gaussian fitting peaks of 529.82 and 532. 1 eV in the O1s spectrum are assigned to Ti\\O, C\\O, relatively (Fig. 3(c)). Peak at C1s region can be fitted into two peaks at 284.8, 288.4 eV, which is attributed to the
Fig. 2. (a) XRD pattern of TiO2 and x-TiO2@g-CQDs; (b) FTIR spectra of g-CQDs, TiO2 and 20-TiO2@g-CQDs.
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Fig. 3. (a–d) XPS full spectra and core level for Ti2p, O1s, and C1s of 20-TiO2@GQDs.
Fig. 4. (a)–(d) The effect of g-CQDs doping amount, photocatalyst concentration, initial concentration of NPE10O, and source power on the degradation rate of NPE10O.
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C_C, O\\C_O [49], respectively (Fig. 3(d)). And there is no obvious peak near 282 eV, which indicates the absence of Ti\\C bond. In our study, g-CQDs did not enter the TiO2 lattice in a binding manner, but adsorbed on the surface of TiO2 in a deposited manner. In addition, XPS spectra of pure TiO2 and g-CQDs are presented in Fig. S1.
3.4. Photocatalytic degradation of NPE10O 3.4.1. Effect of doping amount of g-CQDs on degradation rate of NPE10O Fig. 4(a) showed the degradation of NPE10O by x-TiO2@g-CQDs photocatalysts with different g-CQDs doping amount. During 120 min dark adsorption treatment, the doping amount of different g-CQDs has no significant difference in the degradation of NPE10O, which explains that g-CQDs do not enhance the photocatalytic activity of TiO2 under dark conditions. While under the irradiation of Xe arc lamp, the photocatalytic activity of X-TiO2@g-CQDs was significantly higher than that of pure TiO2, and the photocatalytic activity of 20-TiO2@g-CQDs was the highest. The relative residual concentration of NPE10O decreased from 0.445 to 0.014 as the doping amount of g-CQDs increased from 0 wt% to 5 wt% with 60 min illumination. When the doping amount of gCQDs continued to increase to 10 wt%, the relative residual concentration of NPE10O increased to 0.12, indicating that the doping of g-CQDs can indeed improve the photocatalytic activity of TiO2, but excessive doping amount will inhibit its activity. This can be explained by the fact that the doping amount of g-CQDs exceeds the binding site with the surface of TiO2, and the excess g-CQDs will self-agglomerate losing electron transfer ability, and may also cover the surface of TiO2@gCQDs blocking effective combination of contaminants with TiO2@gCQDs [50,51], thereby reducing the photocatalytic activity of TiO2@gCQDs. So a suitable doping amount of g-CQDs is the key factor to control the photocatalytic activity.
3.4.2. Effect of TiO2@g-CQDs concentration on degradation rate of NPE10O The effect of the concentration of 20-TiO2@g-CQDs with the best photocatalytic activity on the degradation rate of NPE10O was selected. Fig. 4(b) showed the results of degradation of NPE10O by different concentrations of 20-TiO2@g-CQDs. Illumination for 60 min after 120 min dark adsorption, it was found that the concentration of 20-TiO2@gCQDs did not observably affect the degradation of NPE10O. At the first 20 min of illumination, the degradation rate of NPE10O was markedly higher than that of the last 40 min. With the increase of 20-TiO2@gCQDs concentration from 0.6 g·L−1 to 1.2 g·L−1, the relative residual concentration of NPE10O decreased from 0.09 to 0.004. It is worth noting that the degradation curves of 1.0 g·L−1 and 1.2 g·L−1 almost coincide, which indicates that the photocatalyst concentration plays an indecisive role in the photocatalytic degradation of NPE10O by TiO2@g-CQDs.
3.4.3. Effect of initial concentration of NPE10O on its degradation rate Pursuing the principle of energy saving and high efficiency, 20TiO2@g-CQDs photocatalyst with a concentration of 1.0 g·L−1 was selected to investigate the effect of initial concentration of NPE10O on its degradation rate. Fig. 4(c) represented a photocatalytic degradation curve of NPE10O at four different initial concentrations. The initial concentration of NPE10O increased from 30 mg·L−1 to 50 mg·L−1, and the degradation rate increased significantly. However, in this process of increasing unceasingly initial concentration, the degradation rate decreased. There is competition between NPE10O and NPE10O and intermediate products, which reduces the degradation rate. Therefore, TiO2@g-CQDs is effective for low concentration NPE10O photocatalytic degradation. Fig. 5. (a)–(c) TOF-MS of NPE10O water solution under 0 min, 30 min, 60 min illumination; (d) schematic diagram of photocatalytic degradation of NPE10O.
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3.4.4. Effect of light source power on the degradation rate of NPE10O The light source power represents the energy intensity. The stronger the light intensity, the higher the energy obtainable by the photocatalyst and the higher the photocatalytic activity. Fig. 4 (d) showed a graph showing the results of the degradation of NPE10O by different source power. Reaction conditions: 20-TiO2@g-CQDs photocatalyst at a concentration of 1.0 g·L−1, and an initial concentration of NPE10O of 50 mg·L−1. As the power of the light source increases, the degradation rate of NPE10O is significantly improved. And the 700 W source power has the best degradation effect on NPE10O, and it can be almost completely degraded in 45 min. This also confirms the research of Chen et al. [52] that the degree of absorption of light depends on the loading of TiO2 and the intensity of light. And the intensity of incident photons is proportional to the output of the power supply. About stability and recyclability of photocatalyst and the XRD pattern after three and six cycles are shown in Fig. S4.
3.5. Study on the degradation path and mechanism of NPE10O In order to determine the intermediates produced by NPE10O in the photocatalytic degradation process, NPE10O solutions under 0 min, 30 min and 60 min illumination were used for TOF-MS. Fig. 5(a) showed the TOF-MS of NPE10O without illuminated, a set of peaks at m/z 264 + 44n + 1 (n = 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18) were checked, which turns out to be the peaks of [NPEnO + H+]. The m/z corresponding to each peak differs by n EO units and is distributed as Poisson with the number of EO as 10, which proves that the tested sample is NPE10O. Fig. 5(b) presented the TOF-MS of NPE10O solutions after photocatalytic degradation for 30 min. It is not significantly different from the original NPE10O solutions. The peak intensity of the smaller molecular weight is slightly increased. The overall spectrum shows a distinct peak at m/z 278 + 44n1 + 1, and there is still a difference of n EO units between these peaks. It is inferred that this should be a set of peaks of [NPEn−2C + H+] according to the molecular weight corresponding to m/z, and each peak has a Poisson distribution with an EO number of 8. This process degradation begins into the short chain carboxylated EO. When the illumination time is 60 min (Fig. 5(c)), the peak intensity of the smaller molecular weight is significantly increased, and the peak intensity of NPE6C is the strongest. The main peak is still Poisson distribution with the EO number of 6. At 60 min degradation, new distinct peaks appeared at m/z 241.1, 269.1, 297.2, 325.2, 353.2, and 397.2, which may be attributed to CAn1PEC [(HOOC-CnH2n-C6H4(OC2H4)1–2OCH2COOH) (n = 0, 2, 4, 6, 8)] and H+ addition. During this process, the oxidation of the nonyl chain proceeds concomitantly with the degradation of the EO chain, leading to metabolites having both a carboxylated ethoxylate and an alkyl chain of varying lengths
[53]. However, the spectral results did not show signs of opening of the benzene ring, which may be that the benzene ring itself is difficult to degrade or the final product CO2 is not detected below the detection limit. Therefore, according to structures of parent compounds and their degradation products detected in the analyzed samples, we speculate that the degradation path of photocatalytic degradation of NPE10O by TiO2@g-CQDs is shown in Fig. 5(d): In order to clarify the photocatalytic mechanism of TiO2@g-CQDs, active oxygen component capture experiment was performed. As shown in Fig. 6(a), isopropanol (IPA, 0.01 M), vitamin C (VC, 0.01 M) and disodium ethylenediamine tetraacetic acid (EDTA, 0.01 M) were introduced as scavenger for hydroxyl radicals (•OH), superoxide radicals + (•O− 2 ) and photogenerated holes (h ), respectively [26,54,55]. It is worth mentioning that the addition of IPA has the greatest effect on the removal rate of NOE10O, which is because IPA, as a scavenger of •OH, can rapidly decreases the •OH content, indicating that •OH is the main active species in photocatalytic reactions. In contrast, the addition of VC and EDTA demonstrated little impact on degradation. In other words, •O–2 and h+ do not contribute much to the degradation of NPE10O. Fig. 6(b) is a schematic diagram of interfacial charge transfer and free radical formation during photocatalytic oxidation of TiO2@gCQDs. As photosensitizers, g-CQDs have a certain photosensitivity under UV–vis light, which can increase the number of photogenerated electrons and holes. In addition, there is an electronic coupling between the π orbit of g-CQDs and the conduction band of TiO2, and electrons will be transferred at the interface from TiO2 to g-CQDs, which effectively inhibits the recombination of electrons and holes. After loading g-CQDs on the surface of TiO2, both of the aforementioned situations will occur. Once the charge carriers are separated, photogenerated holes will capture H2O or OH– forming •OH in the system. At the same time, photogenerated electrons will adsorb the O2 of the system and eventually lead to the appearance of •O− 2 . In addition, photogenerated holes with strong oxidizing properties can also directly oxidize NPE10O. From the results of active oxygen capture experiments, •OH, •O–2 and h+ can decompose the NPE10O into other micromolecule compounds and CO2. And •OH is the major active species for photocatalytic degradation of contaminant [26,55].
TiO2 @g CQDs þ hν→TiO2 @g CQDs þ hþ þ e− ðEλ NEg Þ
ð1Þ
H2 O→OH– þ Hþ
ð2Þ
hþ þ H2 O=OH– →•OH þ Hþ
ð3Þ
Fig. 6. (a) Trapping experiment of active species; (b) schematic diagram of interfacial charge transfer and free radical formation during photocatalytic oxidation of TiO2@g-CQDs.
H. Liang et al. / Journal of Molecular Liquids 302 (2020) 112567
e− þ O2 þ Hþ →•O2 –
ð4Þ
•OH=O2 – =hþ þ NPE10 O→NPExC→CAyPEC→CO2 þ H2 O
ð5Þ
4. Conclusion In this study, successful synthesis of photocatalyst TiO2@g-CQDs was confirmed by HRTEM, XRD, FT-IR and XPS, UV-DRS characterization. In the process of photocatalytic degradation of NPE10O, the results show that TiO2@g-CQDs has excellent photocatalytic performance, and it can completely degrade NPE10O within 60 min. Compared with pure TiO2, 20-TiO2@g-CQDs improves the degradation efficiency of NPE10O by 55.6%. The photocatalyst concentration, g-CQDs doping amount, NPE10O initial concentration and light source power have different effects on the degradation rate of NPE10O. Among them, the doping amount of g-CQDs and light source power play a decisive role in the degradation process of NPE10O. In contrast, the photocatalyst concentration and the initial concentration of NPE10O have slight effect on the degradation rate of NPE10O. The TOF-MS of NPE10O solutions with different illumination periods showed that NPE10O was degraded by EO chain attenuation and alkyl chain attenuation, and NPEn-2C and CAnPEC are important intermediates in the degradation process of NPE10O. •OH, •O–2 and h+ are responsible for the NPE10O degradation, but •OH plays a dominant role in the degradation process. This research work will provide theoretical guidance for NPE10O wastewater treatment and show a promising future in the field of organic pollution in the water. CRediT authorship contribution statement Huiqin Liang: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xiumei Tai: Resources, Writing - review & editing, Supervision, Data curation. Zhiping Du: Resources, Supervision, Data curation. Acknowledgements The work has been supported by the National Natural Science Foundation of China (No. U1610222) and the National Key Research & Development Plan (2017YFB0308704). We would like to thank the project of JALA Research Funds (JALA2017). Appendix A. Supplementary data Supporting information describes the XPS spectra of pure TiO2 and g-CQDs and the photoluminescence spectrum, the UV–vis absorption spectrum and the transient fluorescence lifetime of g-CQDs; the UV– visible diffuse reflectance spectra and the calculation band gap of TiO2 and x-TiO2@g-CQDs, and stability and recyclability of TiO2@g-CQDs. Supplementary data to this article can be found online at doi:https:// doi.org/10.1016/j.molliq.2020.112567 References [1] L. De La Fuente, T. Acosta, P. Babay, G. Curutchet, R. Candal, M.I. Litter, Degradation of nonylphenol ethoxylate-9 (NPE-9) by photochemical advanced oxidation technologies, Ind. Eng. Chem. Res. 49 (15) (2010) 6909–6915. [2] M. Castillo, G. Peñuela, D. Barceló, Identification of photocatalytic degradation products of non-ionic polyethoxylated surfactants in wastewaters by solid-phase extraction followed by liquid chromatography-mass spectrometric detection, Fresenius J. Anal. Chem. 369 (7–8) (2001) 620–628. [3] A. Priac, N. Morin-Crini, C. Druart, S. Gavoille, C. Bradu, C. Lagarrigue, G. Torri, P. Winterton, G. Crini, Alkylphenol and alkylphenol polyethoxylates in water and wastewater: a review of options for their elimination, Arab. J. Chem. 10 (2017) S3749–S3773.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Photocatalytic degradation of nonylphenol ethoxylate and its degradation mechanism Huiqin Liang a, Xiumei Tai a,⁎, Zhiping Du a,b,⁎⁎ a b
Shanxi Key Laboratory of Surfactants, China Research Institute of Daily Chemistry Co., Ltd., Taiyuan, Shanxi 030001, China Institute of Resources and Environmental Engineering, Shanxi University, Taiyuan, Shanxi 030006, China
a r t i c l e
i n f o
Article history: Received 6 November 2019 Received in revised form 7 January 2020 Accepted 23 January 2020 Available online 24 January 2020 Keywords: Nonylphenol ethoxylate (NPE10O) TiO2@g-CQDs Photocatalysis Path Mechanism
a b s t r a c t Photocatalyst TiO2@g-CQDs were prepared by one-step sol-gel method using tetrabutyl titanate, anhydrous ethanol, hydrochloric acid and graphite carbon quantum dots and characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectrum (FT-IR), X-ray diffraction (XRD), UV– visible diffuse reflectance spectra (UV-DRS) and X-ray photoelectron spectroscopy (XPS). The photocatalytic degradation of nonylphenol ethoxylate (NPE10O) by TiO2@g-CQDs and the degradation path of NPE10O by time-of-flight mass spectrometry (TOF-MS) were investigated. In addition, the photocatalytic mechanism of TiO2@g-CQDs was discussed in detail and the major active species were identified. The results showed that when the doping amount of g-CQDs is 5%, the amount of TiO2@g-CQDs is 0.1 g·L−1, the initial concentration of NPE10O is 50 mg·L−1, and the power of the light source is 500 W, the degradation efficiency of NPE10O can reach 100% at 60 min, which indicates TiO2@g-CQDs have excellent photocatalytic properties. TOF-MS showed that NPE10O was mainly degraded by nonylphenol polyethoxycarboxylate (NPExC) and carboxylated alkyl oxidation products of nonylphenol ethoxy (CAyPEC) intermediates during photocatalytic degradation. Trapping experiment of active species demonstrated that •OH is the major active component for photocatalytic degradation of contaminant. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Alkylphenol ethoxylates (APEOs) are the second largest commercial nonionic surfactants in the world, among which octylphenol ethoxylates (OPEO) and nonylphenol ethoxylates (NPE10O) are the most widely used [1,2]. NPE10O is the main component of textile auxiliaries such as emulsifiers, penetrants and detergents. It is widely used in leather and textile printing and dyeing [3]. After entering the water environment, NPE10O wastewater can be converted into short chain nonylphenol ethoxylates and more toxic nonylphenol (NP) major metabolites. Studies have shown that compared with the parent compound, these refractory, intermediate biometabolism with strong bioaccumulation and toxicity can interfere with human endocrine secretion, which is carcinogenic and teratogenic to humans [3–6]. At present, researchers are actively looking for alternatives to NPE10O, and more promising are fatty alcohol ether and isomeric alcohol ether, but from a completely alternative point of view, fatty alcohol ether and isomeric alcohol ether still have some difficulties. By 2017, NPE10O still ⁎ Corresponding author. ⁎⁎ Correspondence to: Zhiping Du, Shanxi Key Laboratory of Surfactants, China Research Institute of Daily Chemistry Co., Ltd., Taiyuan, Shanxi 030001, China. E-mail addresses: [email protected] (X. Tai), [email protected] (Z. Du).
https://doi.org/10.1016/j.molliq.2020.112567 0167-7322/© 2020 Elsevier B.V. All rights reserved.
accounts for a large proportion of the nonionic surfactant market. Therefore, it is extremely important to study the deep degradation and degradation mechanism of NPE10O. In recent years, processes for the advanced treatment of NPE10O wastewater, such as activated carbon [7,8], resin adsorption [9,10], photocatalytic degradation [1,11–13] and biodegradation [14–16], have attracted much attention. The relative cost of activated carbon and resin is relatively high, and biodegradation takes a long time, while photocatalytic degradation with high selectivity, wide application range, has become a research hotspot in sewage treatment in recent years. Semiconductor materials that have been studied as photocatalytic materials include TiO2 [17,18], WO3 [19], ZnO [20], Nb2O5, α-Fe2O3 [21], g-C3N4 [22,23], BiOX [24], BiVO4 [25], Ag3PO4 [26], Ag2CO3 [27], AgBr, AgI, etc. Among them, TiO2 is the earliest photocatalyst used to degrade organic pollutants, and has been enduring even after years of research based on low price, good selectivity, and high photocatalytic activity. However, the large band gap and the high recombination rate of photogenerated electron-hole pairs of pure TiO2 will severely restrict its use, therefore it needs to be modified to improving photocatalytic activity. To date, it can effectively control the electron transport of TiO2 by means of metal/non-metal doping [28,29], noble metal surface deposition [30], semiconductor recombination, and organic dye [31]/quantum dot sensitization [32], thereby reducing the recombination rate of charge carriers. Herein, the emphasis
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is on the sensitization of carbon quantum dots, which is called the most promising semiconductor photosensitizer. Carbon quantum dots (CQDs), as an excellent photosensitive material, can prevent the recombination of energy in addition to the sensitive visible light response. In addition, there is an electronic coupling between the π orbit of CQDs and the conduction band of TiO2, and electrons will be transferred at the interface between CQDs and TiO2, which demonstrates unique superiority in inhibiting the recombination of electrons and holes. CQDs can be more detailedly divided into graphene quantum dots (GQDs), carbon nanodots (CNDs) and polymer dots (PDs) [33,34]. Rajender et al. [35] reported that the graphene quantum dots (GQD) were hybridized with TiO2 nanoparticles to form heterojunctions and used for photocatalytic degradation of MB. The results show that the TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation. Al-Kandari et al. reported the preparation of nanocomposites (rGOTi) by loading 0.33 wt% of reduced graphene oxide (rGO) on commercial TiO2 nanoparticles using a hydrothermal method, and used to study photocatalytic degradation of phenol, p-chlorophenol and p-nitrophenol. The result shows that rGOTi with much higher photocatalytic activity degrade efficiently a mixture of three phenolic compounds [36]. Therefore, it is more wise to choose CQDs as photosensitizers for TiO2 [37–41]. In the work, TiO2@g-CQDs photocatalyst was synthesized by one-step sol-gel method. The sample was characterized by HRTEM, XRD, FT-IR, UV-DRS and XPS and applied to degrade NPE10O simulated wastewater. The effects of photocatalyst concentration, g-CQDs doping amount, NPE10O initial concentration and source power factor on the degradation rate of NPE10O were discussed. At the same time, the photocatalytic degradation path and mechanism of NPE10O was speculated.
2. Experimental 2.1. Chemicals and materials Graphite (99.95%) and tetrabutyl titanate (99.8%) were respectively purchased from Aladdin and Tianjin Kermel Chemical Reagent Co., Ltd. Nonylphenol ethoxylates was purchased from Beijing Sage Chemical Co., Ltd. Hydrochloric acid and absolute ethanol were attained from Shanghai Sinopharm Chemical Reagent Co., Ltd. Isopropanol (IPA), vitamin C (VC) and ethylenediaminetetraacetic acid (EDTA) were bought from Sinopharm Chemical Reagent Co. Ltd. The above reagents were of analytical grades without further purification.
2.2. Synthesis of TiO2@g-CQDs Graphite carbon quantum dots (g-CQDs) were prepared and optimized by the reported mixed acid reflux method [42]. Typically, 5.0 mL g-CQDs aqueous solution with concentration of 40 mg·mL−1 was added into the mixture solution containing 2 mL hydrochloric acids and 15 mL absolute ethanol under strong electromagnetic stirring in succession to form a pellucid and homogeneous solution. Then, moderate tetrabutyl titanate (the calculated amount of 9 mL, 18 mL and 27 mL was selected in order to attain required mass ratios of TiO2 to g-CQDs like 10:1, 20:1 and 30:1) was dissolved in 30 mL absolute ethanol followed by an addition of the previously prepared transparent mixed solution with the drop rate to 2 drops·s−1. The above operation was carried out at 35 ± 2 °C constant temperature water baths under vigorous stirring and the system turned into a gel within about 5 min after the addition was completed. The obtained gel was sealed with plastic wrap, naturally aged for 24 h, then dried at 60 ± 2 °C for 24 h in a vacuum oven, and ground to fine powders with an agate mortar. The final product X-TiO2@g-CQDs was obtained by calcination in a muffle furnace at 250 ± 2 °C for 1.5 h then 500 ± 2 °C for 2 h.
2.3. Characterization High-resolution transmission electron microscopy (HRTEM, JEM2010, JEOL, Japan) was utilized to observe the geometric morphology and particle size of the synthesized samples. The X-ray diffractometer (XRD-6000, Shimadzu, Japan) was used to identify the phase composition and crystallinity of the synthesized sample. Chemical state and chemical composition analysis was carried out using X-ray photoelectron spectroscopy (XPS, XSAM800) with a beam current of 15 mA and operating voltage of 12 kV. The Fourier transform infrared spectrum (FT-IR) of the sample was determined by a tabulating method mixed with KBr to VERTEX 70 spectrometer. The optical absorption range and the band gap energy were measured by UV–visible diffuse reflectance spectra (UV–vis DRS, Cary 300–800, Varian, American). Photoluminescence spectroscopy (PL, Hitachi, F-4600) and transient fluorescence spectrometer (Edinburgh, EdinburghFLS1000) were utilized to characterize the fluorescence properties and transient fluorescence lifetime of g-CQDs, respectively. Time-of-flight mass spectrometry (Ultraflex MALDI-TOF/TOF-MS, Bruker daltonics) was used to detect photodegradation intermediates of samples Photocatalytic degradation of NPE10O was studied using XPA-7 photocatalytic device and UV–Vis spectrophotometer (UV-1601, Beijing). 2.4. Photocatalytic degradation The photocatalytic activity of the as-prepared samples was tested under sunlight-simulated radiation source (Xe arc lamp, λ N 420 nm). Initially, 0.1 g TiO2, 10-TiO2@g-CQDs, 20-TiO2@g-CQDs, and 30-TiO2@ g-CQDs photocatalyst was dispersed in 100 mL of 50 mg·L−1 NPE10O solution, respectively. The suspension was kept by 120 min strong vibration in the dark to reach the adsorption-desorption equilibrium between NPE10O and photocatalysts before irradiation. Then placing under a 500 W Xe arc lamp, 5 mL of the solution was taken after an interval of 5 min, centrifuged for 10 min at 8000 rpm. The supernatant was analyzed using a UV–Vis absorbance spectroscopy at 223 nm of NPE10O maximum absorption peak. The concentration and absorbance of the NPE10O solution were in accordance with Lange-Beer's law throughout the test range. The effects of photocatalyst concentration, g-CQDs doping amount, initial concentration of NPE10O, and source power factor on the degradation rate of NPE10O were tested in parallel. 3. Results and discussion 3.1. HRTEM studies The morphology and particle size of the prepared samples were studied by HRTEM. Fig. 1(a–b) showed HRTEM images and particle size distribution of g-CQDs prepared by mixing acid reflux after optimization. It can be seen that the g-CQDs have good dispersibility, the particle size ranges from 1 to 3 nm, and the average diameter is 1.83 ± 0.36 nm. Fig. 1(c–d) represented the HRTEM image of TiO2, which exhibited morphology of irregular block-like with size of 30–40 nm. And it can be seen from Fig. 1(e), lattice image of TiO2 shows a lattice spacing (d) of 3.89 Å, which corresponds to the TiO2 (101) planes. Fig. 1 (f) showed the lattice image of TiO2@g-CQDs, which indicated that the simultaneous presence of lattices of TiO2 and g-GQDs to prove that gCQDs are successfully supported on TiO2. 3.2. XRD and FTIR studies Fig. 2(a) displayed the XRD patterns of TiO2 and X-TiO2@g-CQDs. The diffraction peaks at 25.3, 37.9, 48.1, 53.9, 55.1, 62.7, and 68.8 correspond to crystal plane (101), (004), (200), (105), (211), and (116) of anatase phase TiO2 (JCPDS NO.84-1285), respectively. The diffraction peaks at 70.4 and 75.2 should be assigned to the (220) and (215) crystal plane of rutile phase TiO2, which means that the synthesized TiO2 by
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Fig. 1. (a, b) HRTEM and particle size distribution images of g-CQDs; (c–e) HRTEM image and lattice images of TiO2; (f) the HRTEM lattice image of TiO2@g-CQDs showing simultaneous presence of TiO2 and g-CQDs.
one-step sol-gel method is a combination of anatase phase and rutile phase, which is more conducive to photocatalytic reaction. At the same time, the above-mentioned diffraction peaks can be observed in XRD patterns of x-TiO2@g-CQDs, indicating that the effect of doping gCQDs on the overall crystal form of TiO2 is not apparent. However, the diffraction peak of g-CQDs did not appear in this process, which may be explained by too little doping of g-CQDs. Interestingly, peak intensity of these diffraction peaks in X-TiO2@g-CQDs is strongly enhanced compared to pure TiO2. Studies have shown that this is caused by a strong interaction between TiO2 and g-CQDs similar to hydrogen bonding or van der Waals forces [43]. Fig. 2(b) showed the FT-IR spectra of g-CQDs, TiO2 and 20-TiO2@gCQDs. Peaks at 3440, 1632 and 1270 cm−1 that appeared in all samples were caused by stretching vibration and bending vibration of O\\H [44,45] and the bending vibration of C\\O bond [46]. The weak double peaks appearing at 2922 and 2854 cm−1 in all samples were assigned to the stretching vibration of _C\\H. In addition, the absorption peaks at 1381 and 1112 cm−1 in g-CQDs were attributed to the bending vibration of C\\O in carboxy and alkoxy [47], respectively. A broad absorption peak of 470 cm−1 belonging to Ti\\O should be the
characteristic absorption peak of TiO2. It should be noted that all absorption peaks in the 20-TiO2@g-CQDs can be found in the FT-IR spectra of gCQDs and TiO2, indicating that the presence of both g-CQDs and TiO2 in the nanocomposite. Moreover, the characteristic absorption peak of 20TiO2@g-CQDs below 1000 cm−1 showed red shifts compared with that of TiO2, which can be ascribed to the vibration coupling of Ti\\O\\Ti. 3.3. XPS studies The chemical element composition and bonding environment of TiO2@g-CQDs were determined by XPS. Fig. 3(a–d) showed the full spectrum and the core layer C1s, O1s, Ti2p spectra of TiO2@g-CQDs in succession. The results show that TiO2@g-CQDs contains Ti, C, and O three elements, and the percentages are 24.63%, 6.24%, and 69.13%, correspondingly. From Fig. 3(b), the two fitted peaks at 458.4, 464 eV observed in the Ti2p XPS pattern correspond to Ti2p3/2, Ti2p1/2 [35,48], which demonstrates the presence of Ti4+ in TiO2@g-CQDs. The two Gaussian fitting peaks of 529.82 and 532. 1 eV in the O1s spectrum are assigned to Ti\\O, C\\O, relatively (Fig. 3(c)). Peak at C1s region can be fitted into two peaks at 284.8, 288.4 eV, which is attributed to the
Fig. 2. (a) XRD pattern of TiO2 and x-TiO2@g-CQDs; (b) FTIR spectra of g-CQDs, TiO2 and 20-TiO2@g-CQDs.
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Fig. 3. (a–d) XPS full spectra and core level for Ti2p, O1s, and C1s of 20-TiO2@GQDs.
Fig. 4. (a)–(d) The effect of g-CQDs doping amount, photocatalyst concentration, initial concentration of NPE10O, and source power on the degradation rate of NPE10O.
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C_C, O\\C_O [49], respectively (Fig. 3(d)). And there is no obvious peak near 282 eV, which indicates the absence of Ti\\C bond. In our study, g-CQDs did not enter the TiO2 lattice in a binding manner, but adsorbed on the surface of TiO2 in a deposited manner. In addition, XPS spectra of pure TiO2 and g-CQDs are presented in Fig. S1.
3.4. Photocatalytic degradation of NPE10O 3.4.1. Effect of doping amount of g-CQDs on degradation rate of NPE10O Fig. 4(a) showed the degradation of NPE10O by x-TiO2@g-CQDs photocatalysts with different g-CQDs doping amount. During 120 min dark adsorption treatment, the doping amount of different g-CQDs has no significant difference in the degradation of NPE10O, which explains that g-CQDs do not enhance the photocatalytic activity of TiO2 under dark conditions. While under the irradiation of Xe arc lamp, the photocatalytic activity of X-TiO2@g-CQDs was significantly higher than that of pure TiO2, and the photocatalytic activity of 20-TiO2@g-CQDs was the highest. The relative residual concentration of NPE10O decreased from 0.445 to 0.014 as the doping amount of g-CQDs increased from 0 wt% to 5 wt% with 60 min illumination. When the doping amount of gCQDs continued to increase to 10 wt%, the relative residual concentration of NPE10O increased to 0.12, indicating that the doping of g-CQDs can indeed improve the photocatalytic activity of TiO2, but excessive doping amount will inhibit its activity. This can be explained by the fact that the doping amount of g-CQDs exceeds the binding site with the surface of TiO2, and the excess g-CQDs will self-agglomerate losing electron transfer ability, and may also cover the surface of TiO2@gCQDs blocking effective combination of contaminants with TiO2@gCQDs [50,51], thereby reducing the photocatalytic activity of TiO2@gCQDs. So a suitable doping amount of g-CQDs is the key factor to control the photocatalytic activity.
3.4.2. Effect of TiO2@g-CQDs concentration on degradation rate of NPE10O The effect of the concentration of 20-TiO2@g-CQDs with the best photocatalytic activity on the degradation rate of NPE10O was selected. Fig. 4(b) showed the results of degradation of NPE10O by different concentrations of 20-TiO2@g-CQDs. Illumination for 60 min after 120 min dark adsorption, it was found that the concentration of 20-TiO2@gCQDs did not observably affect the degradation of NPE10O. At the first 20 min of illumination, the degradation rate of NPE10O was markedly higher than that of the last 40 min. With the increase of 20-TiO2@gCQDs concentration from 0.6 g·L−1 to 1.2 g·L−1, the relative residual concentration of NPE10O decreased from 0.09 to 0.004. It is worth noting that the degradation curves of 1.0 g·L−1 and 1.2 g·L−1 almost coincide, which indicates that the photocatalyst concentration plays an indecisive role in the photocatalytic degradation of NPE10O by TiO2@g-CQDs.
3.4.3. Effect of initial concentration of NPE10O on its degradation rate Pursuing the principle of energy saving and high efficiency, 20TiO2@g-CQDs photocatalyst with a concentration of 1.0 g·L−1 was selected to investigate the effect of initial concentration of NPE10O on its degradation rate. Fig. 4(c) represented a photocatalytic degradation curve of NPE10O at four different initial concentrations. The initial concentration of NPE10O increased from 30 mg·L−1 to 50 mg·L−1, and the degradation rate increased significantly. However, in this process of increasing unceasingly initial concentration, the degradation rate decreased. There is competition between NPE10O and NPE10O and intermediate products, which reduces the degradation rate. Therefore, TiO2@g-CQDs is effective for low concentration NPE10O photocatalytic degradation. Fig. 5. (a)–(c) TOF-MS of NPE10O water solution under 0 min, 30 min, 60 min illumination; (d) schematic diagram of photocatalytic degradation of NPE10O.
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3.4.4. Effect of light source power on the degradation rate of NPE10O The light source power represents the energy intensity. The stronger the light intensity, the higher the energy obtainable by the photocatalyst and the higher the photocatalytic activity. Fig. 4 (d) showed a graph showing the results of the degradation of NPE10O by different source power. Reaction conditions: 20-TiO2@g-CQDs photocatalyst at a concentration of 1.0 g·L−1, and an initial concentration of NPE10O of 50 mg·L−1. As the power of the light source increases, the degradation rate of NPE10O is significantly improved. And the 700 W source power has the best degradation effect on NPE10O, and it can be almost completely degraded in 45 min. This also confirms the research of Chen et al. [52] that the degree of absorption of light depends on the loading of TiO2 and the intensity of light. And the intensity of incident photons is proportional to the output of the power supply. About stability and recyclability of photocatalyst and the XRD pattern after three and six cycles are shown in Fig. S4.
3.5. Study on the degradation path and mechanism of NPE10O In order to determine the intermediates produced by NPE10O in the photocatalytic degradation process, NPE10O solutions under 0 min, 30 min and 60 min illumination were used for TOF-MS. Fig. 5(a) showed the TOF-MS of NPE10O without illuminated, a set of peaks at m/z 264 + 44n + 1 (n = 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18) were checked, which turns out to be the peaks of [NPEnO + H+]. The m/z corresponding to each peak differs by n EO units and is distributed as Poisson with the number of EO as 10, which proves that the tested sample is NPE10O. Fig. 5(b) presented the TOF-MS of NPE10O solutions after photocatalytic degradation for 30 min. It is not significantly different from the original NPE10O solutions. The peak intensity of the smaller molecular weight is slightly increased. The overall spectrum shows a distinct peak at m/z 278 + 44n1 + 1, and there is still a difference of n EO units between these peaks. It is inferred that this should be a set of peaks of [NPEn−2C + H+] according to the molecular weight corresponding to m/z, and each peak has a Poisson distribution with an EO number of 8. This process degradation begins into the short chain carboxylated EO. When the illumination time is 60 min (Fig. 5(c)), the peak intensity of the smaller molecular weight is significantly increased, and the peak intensity of NPE6C is the strongest. The main peak is still Poisson distribution with the EO number of 6. At 60 min degradation, new distinct peaks appeared at m/z 241.1, 269.1, 297.2, 325.2, 353.2, and 397.2, which may be attributed to CAn1PEC [(HOOC-CnH2n-C6H4(OC2H4)1–2OCH2COOH) (n = 0, 2, 4, 6, 8)] and H+ addition. During this process, the oxidation of the nonyl chain proceeds concomitantly with the degradation of the EO chain, leading to metabolites having both a carboxylated ethoxylate and an alkyl chain of varying lengths
[53]. However, the spectral results did not show signs of opening of the benzene ring, which may be that the benzene ring itself is difficult to degrade or the final product CO2 is not detected below the detection limit. Therefore, according to structures of parent compounds and their degradation products detected in the analyzed samples, we speculate that the degradation path of photocatalytic degradation of NPE10O by TiO2@g-CQDs is shown in Fig. 5(d): In order to clarify the photocatalytic mechanism of TiO2@g-CQDs, active oxygen component capture experiment was performed. As shown in Fig. 6(a), isopropanol (IPA, 0.01 M), vitamin C (VC, 0.01 M) and disodium ethylenediamine tetraacetic acid (EDTA, 0.01 M) were introduced as scavenger for hydroxyl radicals (•OH), superoxide radicals + (•O− 2 ) and photogenerated holes (h ), respectively [26,54,55]. It is worth mentioning that the addition of IPA has the greatest effect on the removal rate of NOE10O, which is because IPA, as a scavenger of •OH, can rapidly decreases the •OH content, indicating that •OH is the main active species in photocatalytic reactions. In contrast, the addition of VC and EDTA demonstrated little impact on degradation. In other words, •O–2 and h+ do not contribute much to the degradation of NPE10O. Fig. 6(b) is a schematic diagram of interfacial charge transfer and free radical formation during photocatalytic oxidation of TiO2@gCQDs. As photosensitizers, g-CQDs have a certain photosensitivity under UV–vis light, which can increase the number of photogenerated electrons and holes. In addition, there is an electronic coupling between the π orbit of g-CQDs and the conduction band of TiO2, and electrons will be transferred at the interface from TiO2 to g-CQDs, which effectively inhibits the recombination of electrons and holes. After loading g-CQDs on the surface of TiO2, both of the aforementioned situations will occur. Once the charge carriers are separated, photogenerated holes will capture H2O or OH– forming •OH in the system. At the same time, photogenerated electrons will adsorb the O2 of the system and eventually lead to the appearance of •O− 2 . In addition, photogenerated holes with strong oxidizing properties can also directly oxidize NPE10O. From the results of active oxygen capture experiments, •OH, •O–2 and h+ can decompose the NPE10O into other micromolecule compounds and CO2. And •OH is the major active species for photocatalytic degradation of contaminant [26,55].
TiO2 @g CQDs þ hν→TiO2 @g CQDs þ hþ þ e− ðEλ NEg Þ
ð1Þ
H2 O→OH– þ Hþ
ð2Þ
hþ þ H2 O=OH– →•OH þ Hþ
ð3Þ
Fig. 6. (a) Trapping experiment of active species; (b) schematic diagram of interfacial charge transfer and free radical formation during photocatalytic oxidation of TiO2@g-CQDs.
H. Liang et al. / Journal of Molecular Liquids 302 (2020) 112567
e− þ O2 þ Hþ →•O2 –
ð4Þ
•OH=O2 – =hþ þ NPE10 O→NPExC→CAyPEC→CO2 þ H2 O
ð5Þ
4. Conclusion In this study, successful synthesis of photocatalyst TiO2@g-CQDs was confirmed by HRTEM, XRD, FT-IR and XPS, UV-DRS characterization. In the process of photocatalytic degradation of NPE10O, the results show that TiO2@g-CQDs has excellent photocatalytic performance, and it can completely degrade NPE10O within 60 min. Compared with pure TiO2, 20-TiO2@g-CQDs improves the degradation efficiency of NPE10O by 55.6%. The photocatalyst concentration, g-CQDs doping amount, NPE10O initial concentration and light source power have different effects on the degradation rate of NPE10O. Among them, the doping amount of g-CQDs and light source power play a decisive role in the degradation process of NPE10O. In contrast, the photocatalyst concentration and the initial concentration of NPE10O have slight effect on the degradation rate of NPE10O. The TOF-MS of NPE10O solutions with different illumination periods showed that NPE10O was degraded by EO chain attenuation and alkyl chain attenuation, and NPEn-2C and CAnPEC are important intermediates in the degradation process of NPE10O. •OH, •O–2 and h+ are responsible for the NPE10O degradation, but •OH plays a dominant role in the degradation process. This research work will provide theoretical guidance for NPE10O wastewater treatment and show a promising future in the field of organic pollution in the water. CRediT authorship contribution statement Huiqin Liang: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xiumei Tai: Resources, Writing - review & editing, Supervision, Data curation. Zhiping Du: Resources, Supervision, Data curation. Acknowledgements The work has been supported by the National Natural Science Foundation of China (No. U1610222) and the National Key Research & Development Plan (2017YFB0308704). We would like to thank the project of JALA Research Funds (JALA2017). Appendix A. Supplementary data Supporting information describes the XPS spectra of pure TiO2 and g-CQDs and the photoluminescence spectrum, the UV–vis absorption spectrum and the transient fluorescence lifetime of g-CQDs; the UV– visible diffuse reflectance spectra and the calculation band gap of TiO2 and x-TiO2@g-CQDs, and stability and recyclability of TiO2@g-CQDs. Supplementary data to this article can be found online at doi:https:// doi.org/10.1016/j.molliq.2020.112567 References [1] L. De La Fuente, T. Acosta, P. Babay, G. Curutchet, R. Candal, M.I. Litter, Degradation of nonylphenol ethoxylate-9 (NPE-9) by photochemical advanced oxidation technologies, Ind. Eng. Chem. Res. 49 (15) (2010) 6909–6915. [2] M. Castillo, G. Peñuela, D. Barceló, Identification of photocatalytic degradation products of non-ionic polyethoxylated surfactants in wastewaters by solid-phase extraction followed by liquid chromatography-mass spectrometric detection, Fresenius J. Anal. Chem. 369 (7–8) (2001) 620–628. [3] A. Priac, N. Morin-Crini, C. Druart, S. Gavoille, C. Bradu, C. Lagarrigue, G. Torri, P. Winterton, G. Crini, Alkylphenol and alkylphenol polyethoxylates in water and wastewater: a review of options for their elimination, Arab. J. Chem. 10 (2017) S3749–S3773.
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