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Activation of persulfate by stability-enhanced magnetic graphene oxide for the removal of 2,4-dichlorophenol
Journal Pre-proof Activation of persulfate by stability-enhanced magnetic graphene oxide for the removal of 2,4-dichlorophenol
Ya Pang, Yaoyu Zhou, Kun Luo, Zhu Zhang, Ran Yue, Xue Li, Min Lei PII:
S0048-9697(19)35651-7
DOI:
https://doi.org/10.1016/j.scitotenv.2019.135656
Reference:
STOTEN 135656
To appear in:
Science of the Total Environment
Received date:
7 September 2019
Revised date:
4 November 2019
Accepted date:
19 November 2019
Please cite this article as: Y. Pang, Y. Zhou, K. Luo, et al., Activation of persulfate by stability-enhanced magnetic graphene oxide for the removal of 2,4-dichlorophenol, Science of the Total Environment (2019), https://doi.org/10.1016/j.scitotenv.2019.135656
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© 2019 Published by Elsevier.
Journal Pre-proof Activation of persulfate by stability-enhanced magnetic graphene oxide for the removal of 2,4dichlorophenol Ya Panga, Yaoyu Zhoub*, Kun Luoa#, Zhu Zhanga, Ran Yuea, Xue Lia, Min Leia a
College of Biology and Environmental Engineering, Changsha University, Changsha, 410002,
China b
College of Resources and Environment, Hunan Agricultural University, Changsha 410128,
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China
Correspondence author: [email protected] (Y.Y. Zhou)
#
Correspondence author: [email protected]
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Journal Pre-proof Abstract A stability-enhanced magnetic catalyst, composed of -Fe2O3@Fe3O4 shell–core magnetic nanoparticles and graphene oxide (MGO), was prepared and characterized by scanning electron micrope (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Brunauer-Emmett-Teller (BET). Catalyst synthesis was used to efficiently activate persulfate for the removal of 2,4-dichlorophenol (2,4-DCP). A magnetic nanoparticle:GO mass ratio of 5
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(MGO-5) exhibited a better catalytic efficiency and could be effectively reused four times. The
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influences of the pollutant, catalyst, and oxidant concentrations were investigated, and the
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intrinsic relationships among these factors and the degradation kinetic constant were evaluated
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by a fitting method. It was found that the catalytic degradation process in the MGO-5-persulfate2,4-DCP system was most likely dominated by an interfacial catalytic reaction, with an
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activation energy of 13.88 kJ/mol. Radical quenching experiments and electron paramagnanetic
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resonance (EPR) analysis indicated that both sulfate radicals (SO4•−) and hydroxyl radicals (•OH) were responsible for 2,4-DCP removal, but surface-bounded SO4•− played a greater role.
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Chloride ions at a concentration of 0–60 mg/L had no effect on 2,4-DCP removal. The proposed
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advanced oxidation technology has potential applications for the practical removal of aqueous organic pollutants.
Keywords: Advanced oxidation technology; Sulfate radical; Magnetic nanoparticles; Graphene oxide; Wastewater treatment
1. Introduction 2,4-dichlorophenol (2,4-DCP) is widely used in the production of the commercial insecticide pentachlorophenol and herbicide 2,4-dichlorophenoxy acetic acid (2,4-D). This chemical is 2
Journal Pre-proof corrosive and toxic and may cause pathological symptoms and changes in the endocrine systems of humans (Babuponnusami et al., 2014). Due to its massive usage in agriculture and wood preservation, 2,4-DCP is found in numerous water and soil environments, posing a serious threat to humans and ecosystems (Chiron, et al., 2007; Jin et al., 2012). Therefore, the remediation of 2,4-DCP-polluted environments is necessary and urgent. Persulfate-based advanced oxidation processes (AOPs) have attracted substantial attention due
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to their ability to oxidize a variety of toxic and recalcitrant pollutants in water and soil
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environments (Zhao et al., 2017; Rodriguez et al., 2018; wan et al., 2019). These processes
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include the activation of persulfate and the production of active species. SO4•−, with a redox
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potential of 2.5–3.1 V and a lifetime of 30–40 s, is generally produced in these processes and is primarily responsible for destroying organic contaminants (Deng et al., 2011; Pang et al., 2018;
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Zhou et al., 2019). The performance of this technology is closely related to the properties of the
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catalysts employed. Various tools such as thermolysis, sonolysis, transition metals, carbonaceous materials, and reduced organic matter have been utilized to activate persulfate (Oh et al., 2010;
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Duan et al., 2015; Oh et al., 2016; Bruton et al., 2018;). Among these tools, carbonaceous
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materials, including activated carbon, carbon nanotubes, graphene oxide (GO), mesoporous carbon, nanodiamonds, and biochar, have attracted substantial attention. These materials exhibit a high thermal conductivity, large theoretical specific surface area, unique carrier mobility, lowdimensional structure, and an sp2-hybridization-dominant carbon configuration and have been demonstrated to be effective in various catalytic processes (Lee et al., 2015; Duan et al., 2018, Yu et al., 2019a; Li et at., 2019). As an innovative material, GO is a two-dimensional flat structure composed of sp2-hybridized carbon atoms and oxygen-containing functionalities, and can develop promising three-
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Journal Pre-proof dimensional graphene-based materials easily (He et al., 2018). GO has unique physical and chemical properties, excellent electron-transfer ability, and a large surface area and exhibits broad application potential in persulfate activation (Pyun 2011, Oh et at., 2016). For example, Sun et al. (2012) were the first to report the use of reduced GO (rGO) to activate peroxymonosulfate for the effective removal of aqueous organic pollutants. Subsequently, nitrogen-doped rGO was applied to activate peroxydisulfate for the degradation of bisphenol A
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and bisphenol F, which showed a significant enhancement compared with pristine rGO (Wang et
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al., 2015). Unfortunately, the high cost, poor dispersion and restacking prevent the practical
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application of single GO in activation of persulfate. GO-based composites such as TiO2-GO
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nanosheets (Genc et al., 2018) and nZVI-rGO (Ayyaz et al., 2015) have been synthesized to effectively activate persulfate for organic contaminant removal. The combination of GO with
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other materials may result in new active components and synergetic effects in the composites.
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The stability and cost of these composites are better than those of pristine GO. Hence, the preparation of GO-based nanocomposites may be a feasible choice for the development of
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efficient catalysts for persulfate activation.
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As a heterogeneous catalyst, the separation of GO-based catalysts from solution is necessary for achieve recycling and preventing possible secondary pollution. Compared with filtration or centrifugation, endowing the catalyst with magnetic properties is a more convenient and efficient approach. Magnetic nanoparticles such as Fe3O4, -Fe2O3, nanoFe0, and MFe2O4 (M = Co, Mn, Cu) are common sources of magnetism, and many of these sources can activate persulfate (Avetta et al., 2014; Zhao et al., 2015). Single magnetic nanoparticles may aggregate and been corroded (Oh et al., 2019), with a reduction in the number of effective active sites for catalysis; however, the dispersion of these nanoparticles on GO or develop core shell structure could
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Journal Pre-proof overcome this drawback. Therefore, the development of a magnetic GO (MGO) catalyst may both facilitate the separation and improve the catalytic efficiency. Herein, stability-enhanced MGO, composed of core–shell Fe3O4@-Fe2O3 magnetic nanoparticles and GO, was synthesized for the activation of persulfate. Its effect on the activation of persulfate was compared with that of individual magnetic nanoparticles and GO. The effects of solution pH, persulfate and MGO dosage, and chloride ion concentration on the degradation of
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2,4-DCP were investigated to elucidate the degradation process. Moreover, the stability and
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regeneration of the catalyst were studied, with the aim of developing the proposed technology for
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a potential wastewater treatment method.
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2. Materials and Methods
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2.1 Chemicals
Multilayer GO was purchased from Su Zhou Heng Qiu Technology Company Limited (China).
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Analytical-grade 2,4-DCP, potassium persulfate, ferric sulfate, ferrous sulfate, sodium hydroxide,
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sulfuric acid, ammonium hydroxide, 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO), methanol,
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ethanol, and potassium iodide were purchased from Sinopharm Chemical Reagent Company, and ultra-pure water was used in the experiments. 2.2 Preparation of MGO catalyst MGO samples with different magnetic nanoparticle:GO mass ratios were prepared according to a previous method, with some modifications (Pang et al., 2018). Specifically, to prepare MGO samples, 0.5 g GO was dispersed in 150 mL of ultra-pure water, and a homogeneous dispersion was obtained after ultrasonic treatment for 3 h. Then, 0.01 M Fe2+ (FeCl24H2O) and 0.015 M Fe3+ (FeCl36H2O) were dissolved in the GO solution. The mixture was transferred to a water-
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Journal Pre-proof bath mechanical stirrer at a temperature of 90 ℃ and a stirring speed of 550 rpm, followed by the addition of 4 M ammonium hydroxide to adjust the pH to 9–11. The reaction proceeded for 60 min, and the obtained black suspension was separated by an external magnetic field and washed with water. The resulting MGO solution was transferred to a PTFE digestion vessel and heated at 180 ℃ for 12 h of hydrothermal treatment; the solution was then collected, washed, and dried for use. This composite was denoted as MGO-3, with a magnetic nanoparticle:GO mass ratio of
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approximately 3. Other MGO samples denoted as MGO-0.5, MGO-1, MGO-5, and MGO-7 were
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also prepared by adjusting the dosage of GO to 3g, 1.5g, 0.3g or 0.22g, respectively, when fixed
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the amount of Fe2+ and Fe3+ as 0.01M and 0.015 M. For comparison, Fe3O4 was prepared in a
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similar manner only using Fe2+ and Fe3+ without GO.
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2.3 Analytical methods
Different methods were applied to characterize the properties of the MGO. The surface
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morphology of the GO and MGO was observed by SEM (Hitachi S4800, Japan). The
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crystallinity of the MGO was analyzed by XRD using a diffractometer with Cu Ka radiation
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(Bruker D8, Germany). The BET surface area and pore size information were obtained by a BET analyzer (Quantachrome, USA) at a temperature of -196 ℃ using liquid nitrogen and an analysis time of 16 h. The zeta potential of the catalyst was measured using a Malvern Zetasizer NanoZS90 analyzer (UK). To prepare samples for zeta potential measurement, 10 mg of MGO-5 was dispersed in 100 mL of 1 mM NaCl solution and sonicated for 30 min; then, the suspension was maintained for 24 h. The supernatant was used to determine the zeta potential. EPR experiments were also performed for a radical study using DMPO as the spin-trapping agent. The reaction system consisted of 0.5 mM persulfate, 200 mg/L MGO-5, 20 mg/L2，4-DCP and 0.1 mol/L DMPO, and the EPR spectra were obtained at room temperature through a JEOL JES-FA 200 6
Journal Pre-proof (Japan). For electrochemical impedance spectroscopy (EIS) tests, chitosan solution was used as film-forming agent to disperse GO, Fe3O4 or MGO-5, the mixture was added to glassy carbon electrode (GCE) and dried for working electrode. Pt electrode and Hg/Hg2Cl2 electrode (SCE) served as counter and reference electrode, respectively. EIS test was measured with the initial potential of 0.2 V (amplitude of 0.0005 V) and frequency ranging from 0.01 to 100000 Hz, in 5 mM Fe(CN)63-/4- solution with 0.1 M KCl.
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2.4 Batch experiments
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Batch experiments were performed in a 250-mL conical flask with 50 mL of 2,4-DCP solution
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and different concentrations of potassium persulfate and MGO-5. The flasks were placed in a
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water bath shaker at a temperature of 288 K and shaken at a speed of 150 rpm under dark
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conditions. Specifically, 10 mg of MGO-5 was added to 50 mL of 2,4-DCP solution, followed by the addition of persulfate to maintain a concentration of 0.5 mM. The solution was periodically
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removed and filtered using a 0.45-μm Millipore filter. Then, 0.2 mL of 4 M KI was immediately
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added to 10-mL filtered samples to quench the reaction. The 2,4-DCP concentration was determined by an HPLC apparatus (Waters 2695) equipped with UV–visible detection (Waters
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2489) at a wavelength of 285 nm, using a C18 column (4.6 150 mm). The injection volume was 10 μL, and the mobile phase was a mixture of 70% acetonitrile and 30% water with an injection rate of 1.0 mL/min. The column temperature was maintained at 25 °C. GC-MS was applied to detect the possible intermediates, and the specific operation conditions were described in the supporting information. Control experiments were also performed under the same conditions with GO and Fe3O4 instead of MGO or persulfate combined with 2,4-DCP solution. The degradation efficiency was expressed as
, where Ct is the 2,4-DCP concentration at
t and C0 is the initial concentration. Quenching experiments were carried out to examine the
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Journal Pre-proof formation of radical species in the MGO/persulfate system by using ethanol and KI as quenching agents for SO4•− or •OH. All of the batch experiments were conducted at least three times, with an RSD of less than 5%. 3. Results and discussion 3.1 Catalyst characterization
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As presented in Fig. 1a, the GO had a stacked layer structure with a creased surface. The GO
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acted as a good carrier, and magnetic nanoparticles with an irregular size (0.5–2 m) were
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distributed on its surface, as shown in Fig. 1b. XRD patterns of magnetic GO before and after hydrothermal treatment (Fig. 2a) were obtained and analyzed using Jade 5.0. The results
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indicated that Fe3O4 formed before the hydrothermal treatment, as typical diffraction peaks of
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Fe3O4 were observed. This composition partially transformed to non-magnetic -Fe2O3 (PDF# 33-0664) after hydrothermal treatment, with the color changing from black to dark red.
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Accompanying the formation of -Fe2O3, the magnetic intensity of MGO-5 decreased to
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approximately 42 emu/g, compared to 73 emu/g for MGO without hydrothermal treatment, and
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the two magnetic nanoparticle samples approached superparamagnetisim, as shown in Fig. 2b. Although -Fe2O3 is non-magnetic, its acid-alkali resistance and anti-oxidation properties are much better than those of Fe3O4 (Yang et al., 2011). The magnetic nanoparticles in MGO-5 were composed of an -Fe2O3@Fe3O4 shell–core structure, with the external -Fe2O3 acting as a protective layer to improve the stability of MGO-5. The leaching of iron ions for a solution pH of 1–12 was less than 0.8 mg/L (determined by atomic absorption spectrometry, Shimadzu, AA6680), which is lower than the European Union standard of 2 mg/L (Xiao et al., 2018), moreover, the Fe leaching after degradation was determined to be 0.48 mg/L, indicating a negligible environmental application risk. According to the information supplied by the GO manufacturer, 8
Journal Pre-proof the surface area and pore size were 162 m2/g and 5.22 nm, and for MGO-5, the values were 117.32 m2/g and 3.86 nm, respectively, and its N2 adsorption-desorption curve and pore distribution curve were presented in Fig. S1. The mesoporous structure of the materials results from the slit space formed by the accumulation of GO. Fig.1 here Fig.2 here
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3.2 Activation of persulfate by MGO for the degradation of 2,4-DCP
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Five types of MGO catalysts and solely GO or Fe3O4 were used to activate persulfate. As
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shown in Fig. 3, the MGO (＞60%) had a better catalytic capacity than sole Fe3O4 (6.9%) or GO
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(17.4%), demonstrating a significant synergetic effect between the magnetic nanoparticles and
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GO. MGO-5 (90.7%) showed the best catalytic removal efficiency, but the use of MGO-5 as an adsorbent and the addition of persulfate alone removed only 24% and 8% of the 2,4-DCP,
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respectively, indicating that the removal of 2,4-DCP was primarily due to the activation of
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persulfate by MGO-5. And magnetic nanoparticles in MGO were the main electron donors as
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verified by our previous study (Pang et al., 2018). Despite their magnetic properties, MGO-0.5 and MGO-1 cannot be completely separated from the solution by an external magnetic field due to their low content of magnetic nanoparticles. As the content of magnetic nanoparticles increased in the MGO, the magnetic intensity increased correspondingly, which is beneficial for simple and rapid separation. The different catalytic performances among the five types of MGO demonstrate that it is important to utilize a reasonable magnetic nanoparticle:GO mass ratio. Moreover, Electrochemical impedance spectroscopy (EIS) tests were applied to investigate the synergetic effect between GO and magnetic nanoparticles. As shown in the EIS results, the impedance of bare electrode, iron oxide nanoparticles modified electrode and GO modified
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Journal Pre-proof electrode were 4700 ohm, 1300 ohm and negligible, indicating the excellent conductivity of GO. And for the MGO-5 modified electrode, its impedance was not obvious, which was due to the existing of GO in it, and the excellent conductivity of GO has also been observed in previous studies (Genc et al., 2018; Jahan et al., 2013). The existing of GO in MGO-5 greatly facilitates transfer of electron from magnetic nanoparticles to the persulfate, and compared with other MGO-x, it was possible that dispersion of magnetic nanoparticles and transportation of electron
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kept a good balance in MGO-5, which resulted in its better catalytic performance than others.
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Fig. 3 here
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3.3 Effect of pH on degradation
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The pH value significantly influences the degradation process. As shown in Fig. 4, in the pH
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range of 3–9, the removal efficiency exceeded 60%, reached a maximum at a pH of 6, and then rapidly decreased as the pH value increased further. Interestingly, this tendency is consistent
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with the zeta potential change, indicating that the interfacial properties of the catalyst may
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substantially influence the degradation process. The isoelectric point of MGO-5 was approximately 6.5, as shown in Fig. 3; thus, the catalyst is positively charged when the pH is less
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than 6.5. Because the activation in this study is a heterogeneous process, the positively charged catalyst may be beneficial for the adsorption of S2O82- anions, following activation of the oxidant and producing radicals. The radicals were primarily distributed on the catalyst surface, (as demonstrated by the mechanism study described in Section 3.7), and the surrounding 2,4-DCP could be rapidly degraded by the radicals. A lower pH would induce a stronger electrostatic adsorption between the oxidant and catalyst, which could impede the subsequent approach and degradation of 2,4-DCP. Therefore, a pH of 6 was better for activation than a lower pH. The reaction rate constants for pH values of 6 and 4 were 0.032 min-1 and 0.0136 min-1, respectively.
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Journal Pre-proof When the pH was higher than 6.5, the catalyst surface was negatively charged, which inhibited contact between the catalyst and oxidant. OH- may also compete with S2O82- and occupy some binding sites on MGO-5 through electrostatic adsorption. In addition, the formed SO4•− can react with OH- and form hydrogen radicals, whose lifetime and potential are lower than those of SO4•−. Therefore, the degradation of 2,4-DCP in this study occurred slowly under alkali conditions. Fig. 4 here.
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3.4 Degradation of 2,4-DCP at different temperatures
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Based on the above results, the MGO-5 mass ratio and a pH of 6 were chosen as the optimal
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operation conditions, and the degradation of 2,4-DCP at 288, 298, and 308 K was evaluated.
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Considering the possible adsorption effect of the catalyst, persulfate was added after 60 min of
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adsorption. As presented in Fig. 5, approximately 30% of the 2,4-DCP was removed by adsorption, which may due to the relatively large surface area of MGO-5 (117.32 m2/g).
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Activation was initiated after the addition of 0.5 mM persulfate, and 92%, 96%, and 98% of the
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2,4-DCP were degraded within 120 min at temperatures of 288, 298, and 308 K, with a pseudofirst-order reaction rate constant of 0.0309, 0.0384, and 0.045 min-1, respectively. This result
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indicates that increasing the temperature accelerated the degradation process. The activation energy based on the Arrhenius equation was calculated to be 13.88 kJ/mol, with a correlation coefficient of R2 = 0.995. The activation energy for 2,4-DCP degradation was 49.3 kJ/mol and 91.5 kJ/mol in a mesoporous carbon CMK-3/persulfate system (Tang et al., 2018) and nanoscale zero-valent iron/persulfate system (Li et al., 2015), respectively. The lower activation energy of this study may due to the good conductivity of graphene oxide and dispersity of iron oxide nanoparticles in MGO-5. Fig. 5 here.
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Journal Pre-proof 3.5 Effect of catalyst, oxidant, and 2,4-DCP concentration on degradation Reasonable catalyst and oxidant concentrations are crucial for high-efficiency, low-cost degradation of 2,4-DCP and for investigating the inner degradation process. As shown in Fig. 6a, as the MGO-5 dosage was increased from 100 to 400 mg/L, the degradation efficiency increased, with removal efficiencies of 43%, 60%, 67%, 86%, and 95% in 30 min, indicating that increasing the catalyst concentration accelerated the degradation of 2,4-DCP. As a
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straightforward explanation, a higher concentration of catalyst corresponds to more active sites
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for the activation of persulfate and more rapid degradation. The reaction rate constants based on
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pseudo first order kinetics were 0.0122, 0.0416, 0.0771, 0.2021, and 0.7385 min-1, respectively.
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The rate constant for the 400 mg/L reaction system was approximately 60-fold greater than that
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of the 100 mg/L reaction system; therefore, increasing the catalyst loading may be an effective method for accelerating the reaction process.
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In addition, different concentrations of persulfate were added to investigate the effect of oxidant dosage on degradation. As shown in Fig. 6b, the removal efficiency increased from 70%
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to 92% as the persulfate concentration increased from 0.05 to 0.5 mM and decreased to 87.5%
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when 0.8 mM persulfate was added. However, as the oxidant concentration was increased to 1 mM, the efficiency reached 93%. Increasing the persulfate concentration can supply sufficient oxidant for activation, but persulfate also exerts a negative effect by consuming SO4•− (selfquenching phenomenon) and reducing its concentration (Zhang et al., 2019; Tang et al., 2018, Hammouda et al., 2017). For this reason, the addition of 0.8 mM persulfate led to a slight decrease in removal efficiency, and the reaction rate constant was 0.0184 min-1for 0.8 mM persulfate, smaller than the values for 0.3 mM (0.0216 min-1), 0.5 mM (0.0233 min-1), and 1 mM (0.024 min-1). For 1 mM persulfate, the reaction rate constant was higher than that of 0.8 mM, it
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Journal Pre-proof is possible that even though self-quenching phenomenon existed at this concentration of persulfate, the consumed SO4•− and persulfate could be supplemented quickly due to the relative high persulfate concentration. Therefore, degradation of 2,4-DCP did not negatively affected under1 mM persulfate. The influence of 2,4-DCP concentration on degradation is shown in Fig. 6c. As the concentration increased from 10 to 80 mg/L, the removal efficiency decreased from 100% to
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40%. After the first cycle, the MGO-5 was separated by an external magnetic field and washed
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with ethanol and water for reuse. The removal efficiencies were 93.1%, 89.2%, and 79.4% for 10
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mg/L 2,4-DCP in the second, third, and fourth cycles, respectively, which is much better than
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that of rGO, whose removal efficiency decreased to 56.5% and 25.5% in the second and third cycles (Sun et al., 2012). The reduced efficiency may due to the occupation of active sites by
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intermediates and the oxidation of active functional groups by persulfate or radicals, as well as
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the structural degradation of MGO-5 during repeated usage (Hou et al., 2019; Hussain et al., 2016). Although the 2,4-DCP was effectively removed, the TOC removal efficiency did not
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exceed 20%, indicating that a high level of intermediates was produced during the catalytic
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degradation process. And GC-MS was applied to identify the by-products. The peak at the retention time of 8.85 min which attributed to 2,4-DCP disappeared after degradation for 120 min, accompanied by the formation of new peaks (Fig. S2a). Based on the m/z values (Fig. S2b) and degradation environment, it was proposed that 4-chlorophenol, cyclohexanol, crotonic acid, butane and 2-methylbutane were the main intermediates. For a more intuitive validation and to identify the key conditions, the relationships between the reaction rate constant (kobs) and the concentrations of persulfate, catalyst, and temperature (K) were taken as logarithms for analysis. It was found that ln(kobs) against ln(Cpersulfate), ln(CMGO-5),
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Journal Pre-proof and ln(temperature) was fairly linear, with slopes of 0.4332 (R2 = 0.928), 2.8625 (R2 = 0.988), and 5.6033 (R2 = 0.993), respectively, where the slope reflects the degree of influence for each factor (Fig. 6d) (Yu et al., 2019b). The maximum slope of 5.6033 indicates that the temperature has the strongest influence on the degradation rate. The activation energy of this system was 13.88 kJ/mol, indicating that a relatively low energy can initiate the reaction, and the system was sensitive to changes in temperature, resulting in a high slope value. For the direct activation
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process in the ternary catalytic system, the MGO-5 concentration had a positive and more
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significant influence on 2,4-DCP degradation than persulfate, with the slope of ln(kobs) vs.
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ln(CMGO-5) being 6.6-fold greater than that of ln(kobs) vs. ln(Cpersulfate). If MGO-5 only reacted
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with persulfate and produced radicals for 2,4-DCP degradation, the slopes of the two factors against ln(kobs) would be similar; thus, the significant difference between the two slopes indicates
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that MGO-5 may have a direct relationship with 2,4-DCP. To confirm this possibility, ln(Qe)
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(where Qe is the equilibrium adsorption capacity mg/g, based on the first 60 min of adsorption) vs. ln(kobs) was fitted by a linear function, with a slope of -4.1745 and R2 = 0.956, as shown in
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Fig. 6d. The large absolute value of the slope indicates that adsorption is an important factor
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influencing the degradation rate. More importantly, as adsorption generally occurs on the surface of MGO-5, the adsorption-determined degradation rate here demonstrates that the catalytic process among MGO-5/persulfate was more likely to be dominated by an interfacial catalytic reaction. Fig. 6 here. 3.6 Effect of chloride ions on degradation Chloride ions (Cl-) are commonly found in water. The degradation of 2,4-DCP may produce chloride ions, and some studies have indicated that Cl- has a significant effect on SO4•−-based
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Journal Pre-proof advanced oxidation technology (Huang et al., 2018; Jorge et al., 2019). Therefore, in this study, the effect of Cl- was studied by adding 0–80 mg/L NaCl to the catalytic degradation system. As shown in Fig. 7, the degradation of 2,4-DCP remained almost constant as different concentrations of Cl- were added, indicating that the anion had no influence on 2,4-DCP removal in this system. Theoretically, Cl- could react with SO4•− and produce Cl• (E0 = 2.4 V) (Outsiou et al., 2017). It is possible here that the hydrophobic SO4•− was first distributed on the surface of
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MGO-5 and was then utilized by the surrounding 2,4-DCP, thus preventing its consumption by
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Cl-. Hence, Cl- had no obvious influence on the interfacial catalytic reaction dominated process
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in this study.
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Fig. 7 here. 3.7 Mechanism investigation
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In this study, stability-enhanced MGO-5 was applied to activate persulfate for 2,4-DCP
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removal. It was found that catalysis plays a primary role in pollutant removal. To clarify the possible activation process and radical species, quenching experiments and EPR analysis were
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applied. Ethanol is widely used as a scavenger for sulfate and hydroxyl radical quenching, as its
+ethanol
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reaction rate constants with the two radicals are as high as KSO4•− + ethanol = 1.6107 M-1S-1 and K•OH = 1.9109 M-1S-1 (Oh etal., 2017, Cátia et al., 2017). As shown in Fig. 8a, with the addition
of ethanol at concentrations of 32, 320, and 640 mM, there was only a slight change in the removal efficiency, indicating that ethanol did not suppress the reaction. If the radicals first bound to the MGO-5 surface, it was difficult to distribute ethanol on the surface of the MGO-5 due to the hydrophobicity of ethanol. Therefore, KI, which can react with the surface-bounded radicals, was used (Wang et al., 2015). It was found that with increasing KI concentration, the depression effect increased correspondingly. In consideration the possible shield effect of I- to
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Journal Pre-proof prohibit the interaction between catalyst and persulfate, dimethyl sulfoxide (DMSO), another scavenger that could be used to quench the surface-bounded reactive radicals was applied. After adding 320 mM and 640 mM DMSO, only about 10% removal efficiency was obtained, which may due to the free radicals and the adsorption of MGO-5. These results suggest that the surface bounded SO4•− instead of free radicals plays an important role in 2,4-DCP degradation. It is well known that SO4•− can react with OH- and form •OH, and EPR determination verified the
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production of •OH, as shown in Fig. 8b. These results indicate that the degradation of 2,4-DCP is
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a radical pathway, with both SO4•− and •OH contributing to 2,4-DCP removal, where SO4•− plays
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a primary role.
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Fig. 8 here.
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4. Conclusion
In this study, MGO-5 composed of magnetic shell-core -Fe2O3@Fe3O4 and GO with a
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mass ratio of 5 was synthesized in a simple manner to activate persulfate for 2,4-DCP removal.
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The results showed that a synergetic effect between the magnetic nanoparticles and GO promoted the activation of persulfate. In this manner, 10–20 mg/L 2,4-DCP could be removed
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almost completely within a 90-min degradation process. The catalytic process among MGO5/persulfate was more likely to be dominated by an interfacial catalytic reaction, according to the fitting analysis. SO4•− was formed and distributed on the catalyst surface. The surface-bound radicals were the dominant active substances for 2,4-DCP degradation. This magnetic graphene catalyst had good acid-alkali resistance, with negligible iron ion leaching at pH values of 1–12, and could be used effectively over four cycles, demonstrating a feasible method for persulfate activation in wastewater treatment. Acknowledgments
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Journal Pre-proof This work was financially supported by the National Natural Science Foundation of China (No. 51409024, 51508043, 51579096, and 51709103), the Training Program for Excellent Young Innovators of Changsha (No. kq1802022 and kq1802020), the Natural Science Foundation of Hunan
Province,
China
(2018JJ3242),
the
China
Postdoctoral
Science
Foundation
(2018M630901), and the Hong Kong Scholars Program (XJ2018029).
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transfer path and key impact factors. Appl. Catal. B Environ. 260, 118160 Zhang, M.W., Yeoh, F.Y., Du, Y.C., Lin, K.A., 2019. Magnetic cobalt-embedded carbon nitride composite derived from one-dimensional coordination polymer as an efficient catalyst for activating oxone to degrade methyltheobromine in water. Sci. Total. Environ. 678, 466-475 Zhao, Q., Mao, Q., Zhou, Y., 2017. Metal-free carbon materials-catalyzed sulfate radicalbased advanced oxidation processes: A review on heterogeneous catalysts and applications. Chemosphere, 189, 224-238.
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Journal Pre-proof Zhao, Y. S, Sun, C., Sun, J.Q., 2015. Kinetic modeling and efficiency of sulfate radical-based oxidation to remove p-nitroaniline from wastewater by persulfate/Fe3O4 nanoparticles process. Sep. Purifi. Technol. 142, 182-188. Zhou, H., Wu, S.K., Zhou, Y.Y., Yang, Y., Zhang, J.C., Wang, S.B., Tsang, C.W., 2019. Insights into the oxidation of organic contaminants by iron nanoparticles encapsulated within boron and nitrogen co-doped carbon nanoshell: Catalyzed Fenton-like reaction at
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Journal Pre-proof Conflict of interest statement We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work entitled “Activation of persulfate by stability-enhanced magnetic graphene oxide for the
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removal of 2,4-dichlorophenol” submitted to the journal of “Science of the total environment”.
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Journal Pre-proof Fig. 1 SEM images of GO (a) and MGO-5 (b). Fig. 2 XRD patterns (a) and VSM measurement (b) of MGO-5 and magnetic GO without hydrothermal treatment. Fig.3 2,4-DCP removal efficiency by different treatments (a), electrochemical impedance spectroscopy (EIS) of different glass carbon electrodes(b). Reaction conditions: persulfate concentration = 0.5 mM, MGO-x (x=0.5, 1, 3, 5, 7) concentration = 200 mg/L, 2,4-DCP
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concentration = 20 mg/L, GO concentration= 100 mg/L, Fe3O4 concentration= 200 mg/L,
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reaction time=120 min, 298K.
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Fig. 4 Effect of pH on removal efficiency and zeta potential. Reaction conditions: persulfate
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concentration=0.5 mM, 2,4-DCP concentration= 20 mg/L, MGO-5 concentration=200mg/L, reaction time=120min, 298K.
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Fig. 5 Effect of temperature on the removal of 2,4-DCP. Reaction conditions: persulfate
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concentration=0.5 mM, MGO-5 concentration=200 mg/L, degradation time=120 min, pH=6 Fig. 6 Effect of the concentrations of MGO-5 (a), persulfate (b), and 2,4-DCP (c) on removal
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efficiency; linear correlation results for ln kobs and ln (parameter) (d), where the parameters
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include temperature, persulfate concentration, MGO-5 concentration, and Qe. Reaction conditions: persulfate concentration=0.5 mM, 2,4-DCP concentration= 20 mg/L, MGO-5 concentration= 200 mg/L, pH=6, reaction time=120 min, 298K. Fig. 7 Effect of Cl-1 concentration on 2,4-DCP removal. Reaction conditions: persulfate concentration=0.5 mM, 2,4-DCP concentration=20 mg/L, MGO-5 concentration=200 mg/L, degradation time=120 min, pH=6, 298K. Fig. 8 Quenching experiments results after the addition of ethanol, KI and DMSO(a), ESPR signal of the MGO-5/persulfate/2,4-DCP system after a reaction time of 5 min (b). Reaction
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Journal Pre-proof conditions: persulfate concentration=0.5 mM, 2,4-DCP concentration=20 mg/L, MGO-5
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concentration=200 mg/L, degradation time=120 min, pH=6, 298K.
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Journal Pre-proof Graphical abstract
Highlights
A catalyst composed of -Fe2O3@Fe3O4 nanoparticles and graphene oxide was prepared.
The Ea for degradation of 2,4-DCP by activated persulfate was 13.8 KJ/mol.
The degradation was more likely dominated by interfacial catalytic process.
Surface bounded SO4•− instead of free SO4•− was responsible for pollutant removal.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
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Ya Pang, Yaoyu Zhou, Kun Luo, Zhu Zhang, Ran Yue, Xue Li, Min Lei PII:
S0048-9697(19)35651-7
DOI:
https://doi.org/10.1016/j.scitotenv.2019.135656
Reference:
STOTEN 135656
To appear in:
Science of the Total Environment
Received date:
7 September 2019
Revised date:
4 November 2019
Accepted date:
19 November 2019
Please cite this article as: Y. Pang, Y. Zhou, K. Luo, et al., Activation of persulfate by stability-enhanced magnetic graphene oxide for the removal of 2,4-dichlorophenol, Science of the Total Environment (2019), https://doi.org/10.1016/j.scitotenv.2019.135656
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© 2019 Published by Elsevier.
Journal Pre-proof Activation of persulfate by stability-enhanced magnetic graphene oxide for the removal of 2,4dichlorophenol Ya Panga, Yaoyu Zhoub*, Kun Luoa#, Zhu Zhanga, Ran Yuea, Xue Lia, Min Leia a
College of Biology and Environmental Engineering, Changsha University, Changsha, 410002,
China b
College of Resources and Environment, Hunan Agricultural University, Changsha 410128,
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China
Correspondence author: [email protected] (Y.Y. Zhou)
#
Correspondence author: [email protected]
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*
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Journal Pre-proof Abstract A stability-enhanced magnetic catalyst, composed of -Fe2O3@Fe3O4 shell–core magnetic nanoparticles and graphene oxide (MGO), was prepared and characterized by scanning electron micrope (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Brunauer-Emmett-Teller (BET). Catalyst synthesis was used to efficiently activate persulfate for the removal of 2,4-dichlorophenol (2,4-DCP). A magnetic nanoparticle:GO mass ratio of 5
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(MGO-5) exhibited a better catalytic efficiency and could be effectively reused four times. The
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influences of the pollutant, catalyst, and oxidant concentrations were investigated, and the
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intrinsic relationships among these factors and the degradation kinetic constant were evaluated
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by a fitting method. It was found that the catalytic degradation process in the MGO-5-persulfate2,4-DCP system was most likely dominated by an interfacial catalytic reaction, with an
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activation energy of 13.88 kJ/mol. Radical quenching experiments and electron paramagnanetic
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resonance (EPR) analysis indicated that both sulfate radicals (SO4•−) and hydroxyl radicals (•OH) were responsible for 2,4-DCP removal, but surface-bounded SO4•− played a greater role.
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Chloride ions at a concentration of 0–60 mg/L had no effect on 2,4-DCP removal. The proposed
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advanced oxidation technology has potential applications for the practical removal of aqueous organic pollutants.
Keywords: Advanced oxidation technology; Sulfate radical; Magnetic nanoparticles; Graphene oxide; Wastewater treatment
1. Introduction 2,4-dichlorophenol (2,4-DCP) is widely used in the production of the commercial insecticide pentachlorophenol and herbicide 2,4-dichlorophenoxy acetic acid (2,4-D). This chemical is 2
Journal Pre-proof corrosive and toxic and may cause pathological symptoms and changes in the endocrine systems of humans (Babuponnusami et al., 2014). Due to its massive usage in agriculture and wood preservation, 2,4-DCP is found in numerous water and soil environments, posing a serious threat to humans and ecosystems (Chiron, et al., 2007; Jin et al., 2012). Therefore, the remediation of 2,4-DCP-polluted environments is necessary and urgent. Persulfate-based advanced oxidation processes (AOPs) have attracted substantial attention due
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to their ability to oxidize a variety of toxic and recalcitrant pollutants in water and soil
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environments (Zhao et al., 2017; Rodriguez et al., 2018; wan et al., 2019). These processes
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include the activation of persulfate and the production of active species. SO4•−, with a redox
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potential of 2.5–3.1 V and a lifetime of 30–40 s, is generally produced in these processes and is primarily responsible for destroying organic contaminants (Deng et al., 2011; Pang et al., 2018;
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Zhou et al., 2019). The performance of this technology is closely related to the properties of the
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catalysts employed. Various tools such as thermolysis, sonolysis, transition metals, carbonaceous materials, and reduced organic matter have been utilized to activate persulfate (Oh et al., 2010;
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Duan et al., 2015; Oh et al., 2016; Bruton et al., 2018;). Among these tools, carbonaceous
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materials, including activated carbon, carbon nanotubes, graphene oxide (GO), mesoporous carbon, nanodiamonds, and biochar, have attracted substantial attention. These materials exhibit a high thermal conductivity, large theoretical specific surface area, unique carrier mobility, lowdimensional structure, and an sp2-hybridization-dominant carbon configuration and have been demonstrated to be effective in various catalytic processes (Lee et al., 2015; Duan et al., 2018, Yu et al., 2019a; Li et at., 2019). As an innovative material, GO is a two-dimensional flat structure composed of sp2-hybridized carbon atoms and oxygen-containing functionalities, and can develop promising three-
3
Journal Pre-proof dimensional graphene-based materials easily (He et al., 2018). GO has unique physical and chemical properties, excellent electron-transfer ability, and a large surface area and exhibits broad application potential in persulfate activation (Pyun 2011, Oh et at., 2016). For example, Sun et al. (2012) were the first to report the use of reduced GO (rGO) to activate peroxymonosulfate for the effective removal of aqueous organic pollutants. Subsequently, nitrogen-doped rGO was applied to activate peroxydisulfate for the degradation of bisphenol A
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and bisphenol F, which showed a significant enhancement compared with pristine rGO (Wang et
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al., 2015). Unfortunately, the high cost, poor dispersion and restacking prevent the practical
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application of single GO in activation of persulfate. GO-based composites such as TiO2-GO
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nanosheets (Genc et al., 2018) and nZVI-rGO (Ayyaz et al., 2015) have been synthesized to effectively activate persulfate for organic contaminant removal. The combination of GO with
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other materials may result in new active components and synergetic effects in the composites.
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The stability and cost of these composites are better than those of pristine GO. Hence, the preparation of GO-based nanocomposites may be a feasible choice for the development of
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efficient catalysts for persulfate activation.
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As a heterogeneous catalyst, the separation of GO-based catalysts from solution is necessary for achieve recycling and preventing possible secondary pollution. Compared with filtration or centrifugation, endowing the catalyst with magnetic properties is a more convenient and efficient approach. Magnetic nanoparticles such as Fe3O4, -Fe2O3, nanoFe0, and MFe2O4 (M = Co, Mn, Cu) are common sources of magnetism, and many of these sources can activate persulfate (Avetta et al., 2014; Zhao et al., 2015). Single magnetic nanoparticles may aggregate and been corroded (Oh et al., 2019), with a reduction in the number of effective active sites for catalysis; however, the dispersion of these nanoparticles on GO or develop core shell structure could
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Journal Pre-proof overcome this drawback. Therefore, the development of a magnetic GO (MGO) catalyst may both facilitate the separation and improve the catalytic efficiency. Herein, stability-enhanced MGO, composed of core–shell Fe3O4@-Fe2O3 magnetic nanoparticles and GO, was synthesized for the activation of persulfate. Its effect on the activation of persulfate was compared with that of individual magnetic nanoparticles and GO. The effects of solution pH, persulfate and MGO dosage, and chloride ion concentration on the degradation of
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2,4-DCP were investigated to elucidate the degradation process. Moreover, the stability and
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regeneration of the catalyst were studied, with the aim of developing the proposed technology for
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a potential wastewater treatment method.
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2. Materials and Methods
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2.1 Chemicals
Multilayer GO was purchased from Su Zhou Heng Qiu Technology Company Limited (China).
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Analytical-grade 2,4-DCP, potassium persulfate, ferric sulfate, ferrous sulfate, sodium hydroxide,
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sulfuric acid, ammonium hydroxide, 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO), methanol,
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ethanol, and potassium iodide were purchased from Sinopharm Chemical Reagent Company, and ultra-pure water was used in the experiments. 2.2 Preparation of MGO catalyst MGO samples with different magnetic nanoparticle:GO mass ratios were prepared according to a previous method, with some modifications (Pang et al., 2018). Specifically, to prepare MGO samples, 0.5 g GO was dispersed in 150 mL of ultra-pure water, and a homogeneous dispersion was obtained after ultrasonic treatment for 3 h. Then, 0.01 M Fe2+ (FeCl24H2O) and 0.015 M Fe3+ (FeCl36H2O) were dissolved in the GO solution. The mixture was transferred to a water-
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Journal Pre-proof bath mechanical stirrer at a temperature of 90 ℃ and a stirring speed of 550 rpm, followed by the addition of 4 M ammonium hydroxide to adjust the pH to 9–11. The reaction proceeded for 60 min, and the obtained black suspension was separated by an external magnetic field and washed with water. The resulting MGO solution was transferred to a PTFE digestion vessel and heated at 180 ℃ for 12 h of hydrothermal treatment; the solution was then collected, washed, and dried for use. This composite was denoted as MGO-3, with a magnetic nanoparticle:GO mass ratio of
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approximately 3. Other MGO samples denoted as MGO-0.5, MGO-1, MGO-5, and MGO-7 were
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also prepared by adjusting the dosage of GO to 3g, 1.5g, 0.3g or 0.22g, respectively, when fixed
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the amount of Fe2+ and Fe3+ as 0.01M and 0.015 M. For comparison, Fe3O4 was prepared in a
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similar manner only using Fe2+ and Fe3+ without GO.
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2.3 Analytical methods
Different methods were applied to characterize the properties of the MGO. The surface
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morphology of the GO and MGO was observed by SEM (Hitachi S4800, Japan). The
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crystallinity of the MGO was analyzed by XRD using a diffractometer with Cu Ka radiation
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(Bruker D8, Germany). The BET surface area and pore size information were obtained by a BET analyzer (Quantachrome, USA) at a temperature of -196 ℃ using liquid nitrogen and an analysis time of 16 h. The zeta potential of the catalyst was measured using a Malvern Zetasizer NanoZS90 analyzer (UK). To prepare samples for zeta potential measurement, 10 mg of MGO-5 was dispersed in 100 mL of 1 mM NaCl solution and sonicated for 30 min; then, the suspension was maintained for 24 h. The supernatant was used to determine the zeta potential. EPR experiments were also performed for a radical study using DMPO as the spin-trapping agent. The reaction system consisted of 0.5 mM persulfate, 200 mg/L MGO-5, 20 mg/L2，4-DCP and 0.1 mol/L DMPO, and the EPR spectra were obtained at room temperature through a JEOL JES-FA 200 6
Journal Pre-proof (Japan). For electrochemical impedance spectroscopy (EIS) tests, chitosan solution was used as film-forming agent to disperse GO, Fe3O4 or MGO-5, the mixture was added to glassy carbon electrode (GCE) and dried for working electrode. Pt electrode and Hg/Hg2Cl2 electrode (SCE) served as counter and reference electrode, respectively. EIS test was measured with the initial potential of 0.2 V (amplitude of 0.0005 V) and frequency ranging from 0.01 to 100000 Hz, in 5 mM Fe(CN)63-/4- solution with 0.1 M KCl.
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2.4 Batch experiments
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Batch experiments were performed in a 250-mL conical flask with 50 mL of 2,4-DCP solution
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and different concentrations of potassium persulfate and MGO-5. The flasks were placed in a
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water bath shaker at a temperature of 288 K and shaken at a speed of 150 rpm under dark
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conditions. Specifically, 10 mg of MGO-5 was added to 50 mL of 2,4-DCP solution, followed by the addition of persulfate to maintain a concentration of 0.5 mM. The solution was periodically
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removed and filtered using a 0.45-μm Millipore filter. Then, 0.2 mL of 4 M KI was immediately
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added to 10-mL filtered samples to quench the reaction. The 2,4-DCP concentration was determined by an HPLC apparatus (Waters 2695) equipped with UV–visible detection (Waters
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2489) at a wavelength of 285 nm, using a C18 column (4.6 150 mm). The injection volume was 10 μL, and the mobile phase was a mixture of 70% acetonitrile and 30% water with an injection rate of 1.0 mL/min. The column temperature was maintained at 25 °C. GC-MS was applied to detect the possible intermediates, and the specific operation conditions were described in the supporting information. Control experiments were also performed under the same conditions with GO and Fe3O4 instead of MGO or persulfate combined with 2,4-DCP solution. The degradation efficiency was expressed as
, where Ct is the 2,4-DCP concentration at
t and C0 is the initial concentration. Quenching experiments were carried out to examine the
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Journal Pre-proof formation of radical species in the MGO/persulfate system by using ethanol and KI as quenching agents for SO4•− or •OH. All of the batch experiments were conducted at least three times, with an RSD of less than 5%. 3. Results and discussion 3.1 Catalyst characterization
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acted as a good carrier, and magnetic nanoparticles with an irregular size (0.5–2 m) were
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distributed on its surface, as shown in Fig. 1b. XRD patterns of magnetic GO before and after hydrothermal treatment (Fig. 2a) were obtained and analyzed using Jade 5.0. The results
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indicated that Fe3O4 formed before the hydrothermal treatment, as typical diffraction peaks of
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Fe3O4 were observed. This composition partially transformed to non-magnetic -Fe2O3 (PDF# 33-0664) after hydrothermal treatment, with the color changing from black to dark red.
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Accompanying the formation of -Fe2O3, the magnetic intensity of MGO-5 decreased to
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approximately 42 emu/g, compared to 73 emu/g for MGO without hydrothermal treatment, and
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the two magnetic nanoparticle samples approached superparamagnetisim, as shown in Fig. 2b. Although -Fe2O3 is non-magnetic, its acid-alkali resistance and anti-oxidation properties are much better than those of Fe3O4 (Yang et al., 2011). The magnetic nanoparticles in MGO-5 were composed of an -Fe2O3@Fe3O4 shell–core structure, with the external -Fe2O3 acting as a protective layer to improve the stability of MGO-5. The leaching of iron ions for a solution pH of 1–12 was less than 0.8 mg/L (determined by atomic absorption spectrometry, Shimadzu, AA6680), which is lower than the European Union standard of 2 mg/L (Xiao et al., 2018), moreover, the Fe leaching after degradation was determined to be 0.48 mg/L, indicating a negligible environmental application risk. According to the information supplied by the GO manufacturer, 8
Journal Pre-proof the surface area and pore size were 162 m2/g and 5.22 nm, and for MGO-5, the values were 117.32 m2/g and 3.86 nm, respectively, and its N2 adsorption-desorption curve and pore distribution curve were presented in Fig. S1. The mesoporous structure of the materials results from the slit space formed by the accumulation of GO. Fig.1 here Fig.2 here
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3.2 Activation of persulfate by MGO for the degradation of 2,4-DCP
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Five types of MGO catalysts and solely GO or Fe3O4 were used to activate persulfate. As
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shown in Fig. 3, the MGO (＞60%) had a better catalytic capacity than sole Fe3O4 (6.9%) or GO
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(17.4%), demonstrating a significant synergetic effect between the magnetic nanoparticles and
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GO. MGO-5 (90.7%) showed the best catalytic removal efficiency, but the use of MGO-5 as an adsorbent and the addition of persulfate alone removed only 24% and 8% of the 2,4-DCP,
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respectively, indicating that the removal of 2,4-DCP was primarily due to the activation of
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persulfate by MGO-5. And magnetic nanoparticles in MGO were the main electron donors as
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verified by our previous study (Pang et al., 2018). Despite their magnetic properties, MGO-0.5 and MGO-1 cannot be completely separated from the solution by an external magnetic field due to their low content of magnetic nanoparticles. As the content of magnetic nanoparticles increased in the MGO, the magnetic intensity increased correspondingly, which is beneficial for simple and rapid separation. The different catalytic performances among the five types of MGO demonstrate that it is important to utilize a reasonable magnetic nanoparticle:GO mass ratio. Moreover, Electrochemical impedance spectroscopy (EIS) tests were applied to investigate the synergetic effect between GO and magnetic nanoparticles. As shown in the EIS results, the impedance of bare electrode, iron oxide nanoparticles modified electrode and GO modified
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Journal Pre-proof electrode were 4700 ohm, 1300 ohm and negligible, indicating the excellent conductivity of GO. And for the MGO-5 modified electrode, its impedance was not obvious, which was due to the existing of GO in it, and the excellent conductivity of GO has also been observed in previous studies (Genc et al., 2018; Jahan et al., 2013). The existing of GO in MGO-5 greatly facilitates transfer of electron from magnetic nanoparticles to the persulfate, and compared with other MGO-x, it was possible that dispersion of magnetic nanoparticles and transportation of electron
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kept a good balance in MGO-5, which resulted in its better catalytic performance than others.
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Fig. 3 here
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3.3 Effect of pH on degradation
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The pH value significantly influences the degradation process. As shown in Fig. 4, in the pH
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range of 3–9, the removal efficiency exceeded 60%, reached a maximum at a pH of 6, and then rapidly decreased as the pH value increased further. Interestingly, this tendency is consistent
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with the zeta potential change, indicating that the interfacial properties of the catalyst may
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substantially influence the degradation process. The isoelectric point of MGO-5 was approximately 6.5, as shown in Fig. 3; thus, the catalyst is positively charged when the pH is less
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than 6.5. Because the activation in this study is a heterogeneous process, the positively charged catalyst may be beneficial for the adsorption of S2O82- anions, following activation of the oxidant and producing radicals. The radicals were primarily distributed on the catalyst surface, (as demonstrated by the mechanism study described in Section 3.7), and the surrounding 2,4-DCP could be rapidly degraded by the radicals. A lower pH would induce a stronger electrostatic adsorption between the oxidant and catalyst, which could impede the subsequent approach and degradation of 2,4-DCP. Therefore, a pH of 6 was better for activation than a lower pH. The reaction rate constants for pH values of 6 and 4 were 0.032 min-1 and 0.0136 min-1, respectively.
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Journal Pre-proof When the pH was higher than 6.5, the catalyst surface was negatively charged, which inhibited contact between the catalyst and oxidant. OH- may also compete with S2O82- and occupy some binding sites on MGO-5 through electrostatic adsorption. In addition, the formed SO4•− can react with OH- and form hydrogen radicals, whose lifetime and potential are lower than those of SO4•−. Therefore, the degradation of 2,4-DCP in this study occurred slowly under alkali conditions. Fig. 4 here.
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3.4 Degradation of 2,4-DCP at different temperatures
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Based on the above results, the MGO-5 mass ratio and a pH of 6 were chosen as the optimal
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operation conditions, and the degradation of 2,4-DCP at 288, 298, and 308 K was evaluated.
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Considering the possible adsorption effect of the catalyst, persulfate was added after 60 min of
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adsorption. As presented in Fig. 5, approximately 30% of the 2,4-DCP was removed by adsorption, which may due to the relatively large surface area of MGO-5 (117.32 m2/g).
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Activation was initiated after the addition of 0.5 mM persulfate, and 92%, 96%, and 98% of the
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2,4-DCP were degraded within 120 min at temperatures of 288, 298, and 308 K, with a pseudofirst-order reaction rate constant of 0.0309, 0.0384, and 0.045 min-1, respectively. This result
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indicates that increasing the temperature accelerated the degradation process. The activation energy based on the Arrhenius equation was calculated to be 13.88 kJ/mol, with a correlation coefficient of R2 = 0.995. The activation energy for 2,4-DCP degradation was 49.3 kJ/mol and 91.5 kJ/mol in a mesoporous carbon CMK-3/persulfate system (Tang et al., 2018) and nanoscale zero-valent iron/persulfate system (Li et al., 2015), respectively. The lower activation energy of this study may due to the good conductivity of graphene oxide and dispersity of iron oxide nanoparticles in MGO-5. Fig. 5 here.
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Journal Pre-proof 3.5 Effect of catalyst, oxidant, and 2,4-DCP concentration on degradation Reasonable catalyst and oxidant concentrations are crucial for high-efficiency, low-cost degradation of 2,4-DCP and for investigating the inner degradation process. As shown in Fig. 6a, as the MGO-5 dosage was increased from 100 to 400 mg/L, the degradation efficiency increased, with removal efficiencies of 43%, 60%, 67%, 86%, and 95% in 30 min, indicating that increasing the catalyst concentration accelerated the degradation of 2,4-DCP. As a
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straightforward explanation, a higher concentration of catalyst corresponds to more active sites
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for the activation of persulfate and more rapid degradation. The reaction rate constants based on
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pseudo first order kinetics were 0.0122, 0.0416, 0.0771, 0.2021, and 0.7385 min-1, respectively.
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The rate constant for the 400 mg/L reaction system was approximately 60-fold greater than that
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of the 100 mg/L reaction system; therefore, increasing the catalyst loading may be an effective method for accelerating the reaction process.
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In addition, different concentrations of persulfate were added to investigate the effect of oxidant dosage on degradation. As shown in Fig. 6b, the removal efficiency increased from 70%
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to 92% as the persulfate concentration increased from 0.05 to 0.5 mM and decreased to 87.5%
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when 0.8 mM persulfate was added. However, as the oxidant concentration was increased to 1 mM, the efficiency reached 93%. Increasing the persulfate concentration can supply sufficient oxidant for activation, but persulfate also exerts a negative effect by consuming SO4•− (selfquenching phenomenon) and reducing its concentration (Zhang et al., 2019; Tang et al., 2018, Hammouda et al., 2017). For this reason, the addition of 0.8 mM persulfate led to a slight decrease in removal efficiency, and the reaction rate constant was 0.0184 min-1for 0.8 mM persulfate, smaller than the values for 0.3 mM (0.0216 min-1), 0.5 mM (0.0233 min-1), and 1 mM (0.024 min-1). For 1 mM persulfate, the reaction rate constant was higher than that of 0.8 mM, it
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Journal Pre-proof is possible that even though self-quenching phenomenon existed at this concentration of persulfate, the consumed SO4•− and persulfate could be supplemented quickly due to the relative high persulfate concentration. Therefore, degradation of 2,4-DCP did not negatively affected under1 mM persulfate. The influence of 2,4-DCP concentration on degradation is shown in Fig. 6c. As the concentration increased from 10 to 80 mg/L, the removal efficiency decreased from 100% to
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40%. After the first cycle, the MGO-5 was separated by an external magnetic field and washed
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with ethanol and water for reuse. The removal efficiencies were 93.1%, 89.2%, and 79.4% for 10
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mg/L 2,4-DCP in the second, third, and fourth cycles, respectively, which is much better than
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that of rGO, whose removal efficiency decreased to 56.5% and 25.5% in the second and third cycles (Sun et al., 2012). The reduced efficiency may due to the occupation of active sites by
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intermediates and the oxidation of active functional groups by persulfate or radicals, as well as
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the structural degradation of MGO-5 during repeated usage (Hou et al., 2019; Hussain et al., 2016). Although the 2,4-DCP was effectively removed, the TOC removal efficiency did not
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exceed 20%, indicating that a high level of intermediates was produced during the catalytic
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degradation process. And GC-MS was applied to identify the by-products. The peak at the retention time of 8.85 min which attributed to 2,4-DCP disappeared after degradation for 120 min, accompanied by the formation of new peaks (Fig. S2a). Based on the m/z values (Fig. S2b) and degradation environment, it was proposed that 4-chlorophenol, cyclohexanol, crotonic acid, butane and 2-methylbutane were the main intermediates. For a more intuitive validation and to identify the key conditions, the relationships between the reaction rate constant (kobs) and the concentrations of persulfate, catalyst, and temperature (K) were taken as logarithms for analysis. It was found that ln(kobs) against ln(Cpersulfate), ln(CMGO-5),
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Journal Pre-proof and ln(temperature) was fairly linear, with slopes of 0.4332 (R2 = 0.928), 2.8625 (R2 = 0.988), and 5.6033 (R2 = 0.993), respectively, where the slope reflects the degree of influence for each factor (Fig. 6d) (Yu et al., 2019b). The maximum slope of 5.6033 indicates that the temperature has the strongest influence on the degradation rate. The activation energy of this system was 13.88 kJ/mol, indicating that a relatively low energy can initiate the reaction, and the system was sensitive to changes in temperature, resulting in a high slope value. For the direct activation
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process in the ternary catalytic system, the MGO-5 concentration had a positive and more
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significant influence on 2,4-DCP degradation than persulfate, with the slope of ln(kobs) vs.
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ln(CMGO-5) being 6.6-fold greater than that of ln(kobs) vs. ln(Cpersulfate). If MGO-5 only reacted
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with persulfate and produced radicals for 2,4-DCP degradation, the slopes of the two factors against ln(kobs) would be similar; thus, the significant difference between the two slopes indicates
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that MGO-5 may have a direct relationship with 2,4-DCP. To confirm this possibility, ln(Qe)
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(where Qe is the equilibrium adsorption capacity mg/g, based on the first 60 min of adsorption) vs. ln(kobs) was fitted by a linear function, with a slope of -4.1745 and R2 = 0.956, as shown in
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Fig. 6d. The large absolute value of the slope indicates that adsorption is an important factor
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influencing the degradation rate. More importantly, as adsorption generally occurs on the surface of MGO-5, the adsorption-determined degradation rate here demonstrates that the catalytic process among MGO-5/persulfate was more likely to be dominated by an interfacial catalytic reaction. Fig. 6 here. 3.6 Effect of chloride ions on degradation Chloride ions (Cl-) are commonly found in water. The degradation of 2,4-DCP may produce chloride ions, and some studies have indicated that Cl- has a significant effect on SO4•−-based
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Journal Pre-proof advanced oxidation technology (Huang et al., 2018; Jorge et al., 2019). Therefore, in this study, the effect of Cl- was studied by adding 0–80 mg/L NaCl to the catalytic degradation system. As shown in Fig. 7, the degradation of 2,4-DCP remained almost constant as different concentrations of Cl- were added, indicating that the anion had no influence on 2,4-DCP removal in this system. Theoretically, Cl- could react with SO4•− and produce Cl• (E0 = 2.4 V) (Outsiou et al., 2017). It is possible here that the hydrophobic SO4•− was first distributed on the surface of
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MGO-5 and was then utilized by the surrounding 2,4-DCP, thus preventing its consumption by
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Cl-. Hence, Cl- had no obvious influence on the interfacial catalytic reaction dominated process
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in this study.
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Fig. 7 here. 3.7 Mechanism investigation
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In this study, stability-enhanced MGO-5 was applied to activate persulfate for 2,4-DCP
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removal. It was found that catalysis plays a primary role in pollutant removal. To clarify the possible activation process and radical species, quenching experiments and EPR analysis were
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applied. Ethanol is widely used as a scavenger for sulfate and hydroxyl radical quenching, as its
+ethanol
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reaction rate constants with the two radicals are as high as KSO4•− + ethanol = 1.6107 M-1S-1 and K•OH = 1.9109 M-1S-1 (Oh etal., 2017, Cátia et al., 2017). As shown in Fig. 8a, with the addition
of ethanol at concentrations of 32, 320, and 640 mM, there was only a slight change in the removal efficiency, indicating that ethanol did not suppress the reaction. If the radicals first bound to the MGO-5 surface, it was difficult to distribute ethanol on the surface of the MGO-5 due to the hydrophobicity of ethanol. Therefore, KI, which can react with the surface-bounded radicals, was used (Wang et al., 2015). It was found that with increasing KI concentration, the depression effect increased correspondingly. In consideration the possible shield effect of I- to
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Journal Pre-proof prohibit the interaction between catalyst and persulfate, dimethyl sulfoxide (DMSO), another scavenger that could be used to quench the surface-bounded reactive radicals was applied. After adding 320 mM and 640 mM DMSO, only about 10% removal efficiency was obtained, which may due to the free radicals and the adsorption of MGO-5. These results suggest that the surface bounded SO4•− instead of free radicals plays an important role in 2,4-DCP degradation. It is well known that SO4•− can react with OH- and form •OH, and EPR determination verified the
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production of •OH, as shown in Fig. 8b. These results indicate that the degradation of 2,4-DCP is
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a radical pathway, with both SO4•− and •OH contributing to 2,4-DCP removal, where SO4•− plays
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a primary role.
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Fig. 8 here.
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4. Conclusion
In this study, MGO-5 composed of magnetic shell-core -Fe2O3@Fe3O4 and GO with a
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mass ratio of 5 was synthesized in a simple manner to activate persulfate for 2,4-DCP removal.
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The results showed that a synergetic effect between the magnetic nanoparticles and GO promoted the activation of persulfate. In this manner, 10–20 mg/L 2,4-DCP could be removed
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almost completely within a 90-min degradation process. The catalytic process among MGO5/persulfate was more likely to be dominated by an interfacial catalytic reaction, according to the fitting analysis. SO4•− was formed and distributed on the catalyst surface. The surface-bound radicals were the dominant active substances for 2,4-DCP degradation. This magnetic graphene catalyst had good acid-alkali resistance, with negligible iron ion leaching at pH values of 1–12, and could be used effectively over four cycles, demonstrating a feasible method for persulfate activation in wastewater treatment. Acknowledgments
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Journal Pre-proof This work was financially supported by the National Natural Science Foundation of China (No. 51409024, 51508043, 51579096, and 51709103), the Training Program for Excellent Young Innovators of Changsha (No. kq1802022 and kq1802020), the Natural Science Foundation of Hunan
Province,
China
(2018JJ3242),
the
China
Postdoctoral
Science
Foundation
(2018M630901), and the Hong Kong Scholars Program (XJ2018029).
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Journal Pre-proof Oh, W.D., and Lim, T.T., 2019. Design and application of heterogeneous catalysts as peroxydisulfate activator for organics removal: An overview. Chem. Eng. J. 358:110-133. Outsiou, A., Frontistis, Z., Ribeiro, R.S., 2017. Activation of sodium persulfate by magnetic carbon xerogels (CX/CoFe) for the oxidation of bisphenol A: Process variables effects, matrix effects and reaction pathways. Water Res. 124:97–107. Pang, Y., Luo, K., Tang, L., Li, X., Wang, L.P., 2018. Preparation and application of
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Nanotechnology. Angewandte Chemie International Edition, 50(1), 46-48. Rodríguez-Chueca, J., Giustina, S.V., Rocha, J., Encinas, A., Marugán, J., 2018. Assessment
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Sun, H., Liu, S., Zhou, G., Ang, H.M., Tadé, M.O., Wang, S.B., 2012. Reduced graphene oxide for catalytic oxidation of aqueous organic pollutants. ACS Appl Mater Inter. 4, 5466-5471. Tang, L., Liu, Y.N., Wang, J., Zeng G.M., Deng Y. C., Dong, H.R., 2018. Enhanced activation process of persulfate by mesoporous carbon for degradation of aqueous organic pollutants: Electron transfer mechanism. Appl Catal B: Environ. 231: 1-10 Wan, Z., Sun, Y., Tsang, D.C., Yu, K.M., 2019. Sustainable biochar catalyst synergized with
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Yang, T., Di, W., Xiao, J., 2011. Core-Shell Nanostructure of α-Fe2O3/Fe3O4: Synthesis and
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Photocatalysis for Methyl Orange. J. Nanomater. 46, 29-34. Yu, J.F., Tang, L., Pang, Y., Zeng, G.M., Wang, J.J., Feng, H.P., Ren, X.Y., 2019a. Magnetic
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nitrogen-doped sludge-derived biochar catalysts for persulfate activation: Internal electron
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transfer mechanism. Chem. Eng. J. 364, 146–159. Yu, J.F., Tang, L., Pang, Y., Zeng, G.M., Wang, J.J., Feng, H.P., Ren, X.Y., 2019b.
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Hierarchical porous biochar from shimp shell for persulfate activation: A two-electron
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transfer path and key impact factors. Appl. Catal. B Environ. 260, 118160 Zhang, M.W., Yeoh, F.Y., Du, Y.C., Lin, K.A., 2019. Magnetic cobalt-embedded carbon nitride composite derived from one-dimensional coordination polymer as an efficient catalyst for activating oxone to degrade methyltheobromine in water. Sci. Total. Environ. 678, 466-475 Zhao, Q., Mao, Q., Zhou, Y., 2017. Metal-free carbon materials-catalyzed sulfate radicalbased advanced oxidation processes: A review on heterogeneous catalysts and applications. Chemosphere, 189, 224-238.
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Journal Pre-proof Zhao, Y. S, Sun, C., Sun, J.Q., 2015. Kinetic modeling and efficiency of sulfate radical-based oxidation to remove p-nitroaniline from wastewater by persulfate/Fe3O4 nanoparticles process. Sep. Purifi. Technol. 142, 182-188. Zhou, H., Wu, S.K., Zhou, Y.Y., Yang, Y., Zhang, J.C., Wang, S.B., Tsang, C.W., 2019. Insights into the oxidation of organic contaminants by iron nanoparticles encapsulated within boron and nitrogen co-doped carbon nanoshell: Catalyzed Fenton-like reaction at
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natural pH. Environ. Int. 128, 77-88.
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Journal Pre-proof Conflict of interest statement We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work entitled “Activation of persulfate by stability-enhanced magnetic graphene oxide for the
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removal of 2,4-dichlorophenol” submitted to the journal of “Science of the total environment”.
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Journal Pre-proof Fig. 1 SEM images of GO (a) and MGO-5 (b). Fig. 2 XRD patterns (a) and VSM measurement (b) of MGO-5 and magnetic GO without hydrothermal treatment. Fig.3 2,4-DCP removal efficiency by different treatments (a), electrochemical impedance spectroscopy (EIS) of different glass carbon electrodes(b). Reaction conditions: persulfate concentration = 0.5 mM, MGO-x (x=0.5, 1, 3, 5, 7) concentration = 200 mg/L, 2,4-DCP
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concentration = 20 mg/L, GO concentration= 100 mg/L, Fe3O4 concentration= 200 mg/L,
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reaction time=120 min, 298K.
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Fig. 4 Effect of pH on removal efficiency and zeta potential. Reaction conditions: persulfate
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concentration=0.5 mM, 2,4-DCP concentration= 20 mg/L, MGO-5 concentration=200mg/L, reaction time=120min, 298K.
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Fig. 5 Effect of temperature on the removal of 2,4-DCP. Reaction conditions: persulfate
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concentration=0.5 mM, MGO-5 concentration=200 mg/L, degradation time=120 min, pH=6 Fig. 6 Effect of the concentrations of MGO-5 (a), persulfate (b), and 2,4-DCP (c) on removal
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efficiency; linear correlation results for ln kobs and ln (parameter) (d), where the parameters
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include temperature, persulfate concentration, MGO-5 concentration, and Qe. Reaction conditions: persulfate concentration=0.5 mM, 2,4-DCP concentration= 20 mg/L, MGO-5 concentration= 200 mg/L, pH=6, reaction time=120 min, 298K. Fig. 7 Effect of Cl-1 concentration on 2,4-DCP removal. Reaction conditions: persulfate concentration=0.5 mM, 2,4-DCP concentration=20 mg/L, MGO-5 concentration=200 mg/L, degradation time=120 min, pH=6, 298K. Fig. 8 Quenching experiments results after the addition of ethanol, KI and DMSO(a), ESPR signal of the MGO-5/persulfate/2,4-DCP system after a reaction time of 5 min (b). Reaction
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Journal Pre-proof conditions: persulfate concentration=0.5 mM, 2,4-DCP concentration=20 mg/L, MGO-5
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concentration=200 mg/L, degradation time=120 min, pH=6, 298K.
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Journal Pre-proof Graphical abstract
Highlights
A catalyst composed of -Fe2O3@Fe3O4 nanoparticles and graphene oxide was prepared.
The Ea for degradation of 2,4-DCP by activated persulfate was 13.8 KJ/mol.
The degradation was more likely dominated by interfacial catalytic process.
Surface bounded SO4•− instead of free SO4•− was responsible for pollutant removal.
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