A nitrogen-doped reduced graphene oxide/Fe3O4 composite (NGO-Fe3O4) was prepared through the simplified hydrothermal and deposition-precipitation method and characterized by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. The degradation efficiency of oxytetracycline (OTC) by NGO-Fe3O4 activated peroxodisulfate (PDS) under visible light irradiation was studied. The degradation efficiency reached 100% within 32.5 min (the initial OTC concentration 50 mg L−1 and PDS 1 mM; [NGO-Fe3O4]:[ PDS] = 4:1; pH = 3.0). No apparent decrease in degradation efficiency was observed after five cycles. SO4−· and ·OH were the main active oxides for OTC degradation in this system. Moreover, four degradation pathways were proposed, namely hydroxylation, dehydration, decarbonylation and demethylation according to the analysis results of high-performance liquid chromatography mass spectrometry.
Oxytetracycline (OTC) has been widely used in the treatment and prevention of human diseases (Watkinson et al. 2009). Trace antibiotics in the environment interfere with the development of ecosystems and human health by generating drug resistance, which has caused widespread concern. OTC is not readily biodegradable and difficult to be degraded by traditional biological treatment processes due to its hydrophilic and stable tetracene ring structure (Watkinson et al. 2007). OTC can cause water pollution through biological enrichment, so research on effective treatment techniques for removing these antibiotics is very important.
In order to control the sustained release and accumulation of antibiotics, it is necessary to improve the status quo through efficient degradation techniques, such as membrane filtration (Sharma et al. 2017; Wang et al. 2017), activated carbon adsorption (Huang et al. 2014; Zhu et al. 2014) and advanced oxidation processes (AOPs) (Oturan et al. 2013; Luu & Lee 2014). Since adsorption treatment hardly decomposes pollutants, which require proper subsequent treatment, more research methods for destructive degradation of OTC need to be explored. AOPs are generally regarded as excellent alternatives, especially for the degradation of emerging pollutants in the environment. AOPs based on SO4−· are a potential in situ chemical oxidation technology, and are widely used for remediation of groundwater and soil due to the strong oxidation capacity, long half-life (half period = 4 s) (Shiraz et al. 2017) and high free radical stability of SO4−·. Common methods for producing SO4−· by activated persulfate (PDS) or peronosulfate include the use of ultraviolet (Bi et al. 2016), ultrasonic (Chen & Zheng 2015), heat (Olmezhanci et al. 2013), organic matter (Pu et al. 2017), carbon materials (Duan et al. 2017) and homogeneous transition metals or heterogeneous forms (Xiong et al. 2014). Heterogeneous activation is superior to homogeneous activation due to its mild reaction conditions and low risk of secondary pollution. In addition, magnetite (Fe3O4) is a heterogeneous catalyst that can exist stably at ambient temperature, and can be reused through magnetic separation to achieve cost savings (Ding et al. 2017). However, Fe3O4 particles tend to agglomerate into large particles leading to a decrease in dispersibility and catalytic activity (Peng et al. 2018; Zhu 2019). Moreover, graphene oxide (GO) has a unique two-dimensional layer structure of sp2-bonded carbon atoms (Zhen et al. 2014), showing the advantage of a huge specific surface area. Therefore, The GO can serve as a carrier to prevent aggregation of the transition metal oxide. Some research results have shown doping the carbon network of reduced graphene oxide (RGO) with N atoms can introduce catalytic active sites (Wang et al. 2015). There is no influence on the lattice of the carbon material since the N atoms and the C atoms are adjacent in the periodic table and the atomic radii are similar. Moreover, a lone pair of electrons formed by doping N atoms can increase the charge density of the carbon material and correspondingly enhance its electron transfer ability and chemical activity (Xinran et al. 2009). In this study, a N-doped reduced graphene oxide/Fe3O4 composite (NGO-Fe3O4) was successfully synthesized by a hydrothermal and coprecipitation method. The visible-light (VIS)/NGO-Fe3O4/PDS process can effectively overcome the problems existing in the current use of such catalysts, improve the catalytic activity, and quickly and effectively remove antibiotic pollutants in wastewater. In addition, the catalyst can be recycled and costs reduced through magnetic recovery (Peng et al. 2018). Further research on its influencing factors, catalytic reaction mechanism, degradation kinetics and degradation pathways will provide technical support for the advanced treatment of bio-refractory wastewater in the actual production process. The main objectives were to: (a) study the effect of OTC degradation by Vis/NGO-Fe3O4/PDS process and optimization; (b) analyze the catalytic mechanism and main active substances of NGO-Fe3O4 on PDS; (c) explore the OTC degradation intermediates and degradation pathways.
Oxytetracycline (C22H28N2O11, 98.0%) was purchased from Huamaike Biotechnology Co., Ltd (Beijing, China). Graphite powder (C), ferrous sulfate (FeSO4·7H2O, 99.0%) and potassium persulfate (K2S2O8) were supplied by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Hydrochloric acid (36–38%), sulfuric acid (95–98%), phosphate (≥85%), ammonia (25–28%), acetonitrile (CH3CN) and methanol (CH3OH) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Other reagents used in this study were provided by Luoyang Chemical Reagent Factory. All chemicals were of analytical grade or higher. Ultra-pure water was employed throughout the experiments.
Preparation of NGO-Fe3O4
GO was synthesized by a modified Hummers method. After ultrasonication for 30 min, 3 mL of NH3·H2O (25–28%) was added, transferred to a high-pressure reaction vessel lined with polytetrafluoroethylene (PTFE), and then heated at 180 °C for 24 h. A suspension of N atom doped reduced graphene oxide (NGO) was obtained. The pH of the NGO suspension was adjusted to 11 with NH3·H2O (25–28%). Four millilitres of FeSO4·7H2O (0.35 mg L−1) was added and quickly stirred for 2 min, then heated in a water bath at 85 °C for 6 h to obtain NGO-Fe3O4 suspension. NGO-Fe3O4 was washed with deionized water to neutral, and the precipitate was dried in a petri dish.
Batch trials were performed in glass bottles with OTC solution. Adsorption experiments were started by adding NGO-Fe3O4 into solution and the mixture was placed on a mechanical stirrer (350 r/min) for 15 min. After the adsorption–desorption equilibrium reached, PDS was added and irradiated with a xenon lamp (500 W/50 Hz). Samples were taken out at predetermined time intervals and filtered through a 0.22 μm PTFE syringe filter. Methanol was added to quench the reaction for the antibiotic determination by high-performance liquid chromatography mass spectrometry (LC-MS).
The surface morphology was observed by a field emission scanning electron microscope (SEM, SU-8010, Hitachi, Japan) equipped with an energy-dispersive X-ray analyzer. The crystal structure was characterized by X-ray diffraction (XRD, Rigaku S2, Japan). The Fourier transform infrared (FTIR) spectrum was recorded by a GX spectrophotometer (Perkin Elmer, USA) with the KBr wafer technique.
RESULTS AND DISCUSSION
Characterization of NGO-Fe3O4
The SEM photographs of GO, NGO, Fe3O4 and NGO-Fe3O4 are shown in Figure 1. GO was an ultra-thin graphene layered deposit. Due to the reduction of oxygen-containing functional groups and the increase of surface defects after the hydrothermal reaction, NGOs show a clear and large number of interconnected 3D porous structures in Figure 1(b). Fe3O4 was an irregular particle with a small particle size, in agglomerated or dispersed state. For NGO-Fe3O4, Fe3O4 particles were uniformly dispersed on the NGO porous structure, avoiding agglomeration of Fe3O4 particles and NGO materials.
In Figure 2, the functional groups of the NGO, Fe3O4 and NGO-Fe3O4 were identified by FTIR spectroscopy. The peak intensities of NGO relative to GO oxygen-containing groups decreased significantly, indicating that some oxygen-containing groups were eliminated after hydrothermal reaction (Donghui et al. 2010). In the FTIR spectrum of NGO, the representative peaks of 3,144.47, 1,626.47 and 1,052.94 cm−1 correspond to O–H stretching vibration of –OH group, C=O stretching vibration of -COOH group and C=O stretching vibration of -RO group, respectively (Peng et al. 2018). The representative peak of 1,223.53 cm−1 was attributed to the stretching vibration of C=N, indicating that amination function-modified GO material was formed by introducing a nitrogen-containing functional group (Lai et al. 2011).
Figure 3 shows the XRD patterns of GO, NGO, Fe3O4 and NGO-Fe3O4. The main broad diffraction peak of NGO appeared at 26.82° and 44.34°, while the characteristic peak of GO was 21.5°. In comparison with GO, the interlayer distance of NGO had become larger, indicating that the NGO sheet material had been effectively separated to a large extent. Moreover, the positions of the individual peaks of the NGO were similar as those of GO, revealing that the introducing N atom did not destroy the structure of GO (Liu et al. 2019). The diffraction peak at 26.8° of NGO supported by Fe3O4 disappeared, which was attributed to the fact that the crystal growth of Fe3O4 inhibited the restacking of GO layers, resulting in the decreasing of the crystal structure integrity of GO (Peng et al. 2018). In NGO-Fe3O4, the diffraction peaks at 30.06°, 35.38°, 43.06°, 53.50°, 56.98° and 62.70° can be attributed to the surface-centered cubic crystal plane of the Fe3O4 particles (Hu et al. 2012), and were almost the same as the Fe3O4 standard data (Christgau et al. 2004).
Catalytic activity comparison
Effects of different parameters on the catalytic activity of NGO-Fe3O4
Ratio of NGO-Fe3O4 to PDS
Initial OTC concentration
OTC concentration was a vital factor for the degradation efficiency and the practical application of the process. Figure 7 shows the effect of different initial OTC concentrations on the degradation efficiency. The results showed that the OTC degradation efficiency was 100%, 97.01%, 83.01%, 59.01% and 42.84%, with initial OTC concentration of 20, 50, 100, 200 and 300 mg L−1, respectively. The kobs decreased from 0.112 min−1 to 0.008 min−1. This can be explained by the following three reasons. Firstly, intermediates that accumulate rapidly during the reaction process compete with OTC in the system for a limited number of active substances, leading to a decrease in the OTC degradation efficiency. Secondly, while the concentration of OTC increased, the amount of OTC adsorbed on the surface of NGO-Fe3O4 increased, resulting in a decrease in the active site on the surface of NGO-Fe3O4. Finally, high OTC concentration increased the difficulty of light penetration, making it difficult for photons to reach and act on the catalyst, which evidently diminished the OTC degradation efficiency (Li et al. 2018).
Based on the abovementioned reactions (Equations (1) and (4)), both ·OH and SO4−· may be present in the Vis/NGO-Fe3O4/PDS system. To estimate their contribution to OTC degradation in the Vis/NGO-Fe3O4/PDS system, methanol (MA, scavenger of both ·OH and SO4−·) and tert-butanol (TBA, scavenger of ·OH alone) were added to the reaction system at a ratio of 500:1 to PDS. As shown in Figure 8, the corresponding suppression ratios (kobs OTC(Blank)/kobs OTC(Scavenger)) were obtained as 14.4 and 10.1, respectively. MA could lead to a higher suppression ratio than TBA. ·OH would be the main but not the only reactive oxygen species for the OTC degradation, suggesting that SO4−· would also participate in oxidizing OTC.
Reusability of NGO-Fe3O4
In order to evaluate the stability of NGO-Fe3O4, NGO-Fe3O4 separated from the solution was washed three times with deionized water and reused in the next cycle. Five successive catalytic degradation experiments were carried out under the optimal conditions (i.e. PDS = 1 mM, [NGO-Fe3O4]:[PDS] = 4:1, pH = 3.0). Figure 9 shows that the OTC degradation was not significantly deteriorated after five successive cycles.
Analysis of OTC degradation pathways
The intermediate products of OTC degradation by Vis/NGO-Fe3O4/PDS system under optimal condition (i.e. PDS = 1 mM, [NGO-Fe3O4]:[PDS] = 4:1, pH = 3.0) were detected by LC-MS. The molecular formula and molecular structures are listed in Table 1 and the OTC degradation pathway is proposed. Generally, as an electrophilic agent, SO4−· had a similar reaction mechanism to that of ·OH, i.e., (1) hydrogen abstraction, (2) hydroxyl addition to unsaturated carbon, and (3) more significant electronic transfer (He et al. 2014; Khan et al. 2014). The aromatic ring (such as ring D, in Figure 10), C11=C12 and C1=C3 keto/enol moieties, hydroxyl group at C5 and dimethylammonium group at C4 in the OTC structure were potential targets for SO4−· attack (Liu et al. 2016a). Due to the low bond energy of N-C and C-O in the OTC structure, the living radicals can react with the OTC and cause the loss of N-methyl, amino, carbonyl, formyl and hydroxyl groups, and finally OTC is turned into intermediates with a similar structure (Chen et al. 2016).
|Product .||m/z detection value .||Possible structural formula .||Molecular formula .||Molecular weight .|
|Product .||m/z detection value .||Possible structural formula .||Molecular formula .||Molecular weight .|
According to the above discussion, four degradation pathways were proposed, namely hydroxylation, dehydration, decarbonylation and demethylation. Hydroxylation (pathway (1)) was a significant reaction of the OTC degradation process in the Vis/NGO-Fe3O4/PDS reaction system. It seemed that OTC was hydroxylated by adding unsaturated carbon or hydrogen abstraction on the saturated carbon of the hydroxyl or amino group to obtain product I (Table 1) (Ji et al. 2016). The dehydration reaction pathway (pathway (2)) can occur at C5, C6 or C12a. The dehydration at C6 could lead to the formation of a stable second aromatic C ring making it to be a more likely reaction site, resulting in the formation of a stable aromatic ring (product II(1)) at C based on the tautomerization of the C11–C12 keto/enol (Liu et al. 2016b). Decarbonylation (pathway (3)) was the reaction to a loss of =CO on the ring structure (Liu et al. 2016a). The decarbonylation byproduct m/z 418 (product III(2)) would more likely be a diradical intermediate produced by the cleavage of C1–C12a in SO4−· attacking OTC. Demethylation (pathway (4)) refers to the removal of one methyl group from the dimethylammonium group at C4 resulting in the generation of the byproduct m/z 447 (product II(2)). Liu et al. (2016b) pointed out that SO4−· could induce the demethylation reaction starting with hydrogen abstraction at the methyl moiety. The methyl group on the nitrogen atom in product II(2) was further substituted with a hydrogen radical to generate product II(3).
A high OTC removal efficiency could be reached within 32.5 min by the Vis/NGO-Fe3O4/PDS system, indicating a significant synergistic effect. The degradation constants followed the first-order kinetics. Four degradation pathways (hydroxylation, dehydration, decarbonylation and demethylation) were proposed. The Vis/NGO-Fe3O4/PDS system can maintain high OTC degradation efficiency over a wide pH range. Moreover, NGO-Fe3O4 exhibited excellent stability in repeated recycling experiments. This study has revealed a distinguished homogeneous–heterogeneous reaction system for the degradation of recalcitrant pollutants.
The authors acknowledge Henan Provincial Department of Science and Technology Research Project (172102310562).