Environ Sci Pollut Res DOI 10.1007/s11356-017-9191-2 RESEARCH ARTICLE ZIF-8 derived nitrogen-doped porous carbon as metal-free catalyst of peroxymonosulfate activation Wenjie Ma 1 & Yunchen Du 1 & Na Wang 1 & Peng Miao 1 Received: 13 January 2017 / Accepted: 2 May 2017 # Springer-Verlag Berlin Heidelberg 2017 Abstract Nitrogen-doped porous carbon (NPC) is synthesized through a direct pyrolysis of zeolitic imidazolate framework (ZIF)-8 under N2 flow followed by acid washing process. It is found that NPC-800 pyrolyzed at 800 °C can inherit the perfect rhombic dodecahedron morphology of ZIF8 crystals and achieve the considerable nitrogen-doping content of 15.20%. When NPC-800 is applied as the heterogeneous catalyst in peroxymonosulfate (PMS) activation for the degradation of Rhodamine B (RhB) and phenol, NPC-800 will exhibit its better performance than some conventional transition metal-based oxides and common carbon materials. The active sites can be primarily ascribed to nitrogen modification and sp2-hybridized carbon frameworks. Besides, the influence of several parameters such as the dosage of catalyst, the concentration of oxidant, and reaction temperature is conducted systematically. More importantly, NPC-800 can maintain its considerable degradation in the presence of some anions and natural organic matters, even under some actual water background conditions. Although NPC-800 displays mild deactivation in repeated experiments, its catalytic performance can be easily recovered through heat treatment at 350 °C in air. Radical quenching tests reveal that both sulfate and hydroxyl radicals are responsible for the removal of Responsible editor: Vítor Pais Vilar Electronic supplementary material The online version of this article (doi:10.1007/s11356-017-9191-2) contains supplementary material, which is available to authorized users. * Yunchen Du yunchendu@hit.edu.cn 1 MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China organic pollutants. This research may provide a new way for the application of novel metal-free carbocatalysts in terms of PMS activation. Keywords ZIF-8 . Nitrogen-doped porous carbon . Peroxymonosulfate . Metal-free catalysis . Organic degradation . Advanced oxidation technology Introduction In the past decades, advanced oxidation processes (AOPs) have been regarded as one of the most promising strategies for disposal of various recalcitrant organics due to their high efficiency and nearly nonselective degradation (Anipsitakis and Dionysiou 2004; Tušar et al. 2012). However, it is unfortunate that classic Fenton reaction employing H2O2 always suffers from the shortages of pH adjustment, instability of peroxide, and sludge disposal, which offer it a dim prospect for large-scale applications (Neyens and Baeyens 2003). As an advanced alternative method, sulfate radical (SO4·−)-based AOPs are receiving tremendous attention as they usually demonstrate more costeffective and environmentally friendly processes (Oh et al. 2016; Hu and Long 2016). It is well known that SO4·− can be released from peroxymonosulfate (PMS) or persulfate (PS) by heating, UV irradiation, ultrasonication, and catalytic activation, where catalytic activation has its own advantages without the assistance of necessary equipment and high-energy input (Oh et al. 2016; Wang et al. 2016b; Lin and Chen 2017). Compared with PS, PMS [commercially available as Oxone (2KHSO5·KHSO4·K2SO4)] is a preferable candidate for the generation of SO4·− in catalytic system, because its asymmetrical molecular structure and relatively large O-O bond length in free molecular will Environ Sci Pollut Res facilitate the activation process, and meanwhile, the redox cycle of active sites in PMS activation of will also contribute to longer lifespan of catalysts than the irreversible conversion of active sites in PS activation (Oh et al. 2016; Duan et al. 2015b). Literature review reveals that catalytic activation of PMS has exhibited its versatile functionality in the removal of various types of recalcitrant contaminants, including volatile organic compounds, endocrine disruptors, pharmaceuticals and their metabolites, cyanotoxins, synthetic dyes, and perfluorinated compounds, and it could even be utilized for the disintegration of activated sludge, disinfection, and decontamination of pool water (Ghanbari and Moradi 2017). Although homogeneous catalytic activation of PMS are very effective, the toxicity caused by the overload of transition metal ions (e.g., Co2+, Ni2+, Fe3+) severely hinders the application of this method in water treatment (Du et al. 2016). Therefore, there are numerous interests in developing various kinds of heterogeneous catalysts, such as zerovalence metal, metal oxides, and supported composites, to alleviate the potential secondary contamination from the homogeneous systems (Lin and Chen 2016a; Zhang et al. 2013b; Zeng et al. 2015; Wang et al. 2014; Du et al. 2016; Zhao et al. 2016; Yao et al. 2014; Hu et al. 2011; Guan et al. 2013; Ren et al. 2015), while the negative effects of leaching ions from heterogeneous catalysts cannot be fully neglected. More recently, some metal-free materials appeared as a new kind of heterogeneous catalysts with excellent performance for PMS activation (Sun et al. 2012; Sun et al. 2014; Indrawirawan et al. 2015a, b). For example, Zhang et al. demonstrated that granular activated carbon could reinforce the decolorization efficiency of Acid Orange 7 by catalytic decomposition of PMS (Zhang et al. 2013a); Shao et al. employed amorphous boron (A-boron) as a metal-free catalyst for PMS activation to produce free radicals for effective degradation of bisphenol S, and they found that A-boron not only exhibited outstanding catalytic activity and superior stability in the degradation of bisphenol S as compared with metal-based and metal-free carbon-based catalysts but also worked for the degradation of many other typical organic pollutants in water (Shao et al. 2017); Lin et al. and Dong et al. reported that α-sulfur and gC3N4 were also eligible activators for the generation of powerful radicals from PMS (Lin and Zhang 2016b; Dong et al. 2016). Among these metal-free catalysts, carbon materials are always the most attractive candidates due to their large surface area, chemical stability, tunable properties, and diverse forms. Various carbon materials, e.g., carbon nanotubes (CNTs), carbon nanofibers (CNF), nanodiamonds, and reduced graphene oxide (rGO), have been extensively studied as highperformance PMS activators (Chen et al. 2016; Yang et al. 2016b; Lee et al. 2016; Peng et al. 2013). The active sites of these carbocatalysts were primarily ascribed to their zigzag edges and some oxygen-containing groups, where the conjugation of sp2 carbon network, defective sites, and proper amounts of functional groups (especially the C=O groups) played vital roles for the electron transfer efficiency in activation processes (Sun et al. 2012, 2014; Duan et al. 2015e, 2016a, b; Wang et al. 2016a). In the subsequent research, Wang’s group adopted a heteroatom-substitution (e.g., N, P, S, B) strategy to disrupt the sp2-hybridized carbon configuration, modulate the electrical properties, and create more active sites in the carbon framework (Duan et al. 2015f; Wang et al. 2016c). It turned out that after doping modifications, NCNT, N-rGO B-rGO, P-rGO, and S, N-rGO demonstrated dramatically enhanced activities for phenol removal as compared with those pristine nanocarbons (Duan et al. 2015c, d). Density functional theory (DFT) calculations further indicated that with the smaller atom radius and higher electronegativity, nitrogen atoms could act as the preferable Lewis basic sites to break the inertness of sp2hybridized carbon configuration by inducing the electron transfer from the neighboring carbon to nitrogen sites (Liang et al. 2012). Although many efforts have been devoted to constructing various N-doped carbocatalysts with considerable activities (Liang et al. 2016, 2017; Lin and Zhang 2016a; Yao et al. 2017), most of them are still stuck with tedious procedures and low N content. In the recent advances of carbon-based functional materials, in situ pyrolysis of metal-organic frameworks (MOFs) becomes a fascinating route, since the periodic arrangement of different atoms in crystalline MOFs provides congenital advantages for homogeneous chemical distribution and heteroatom substitution (Qiang et al. 2015; Torad et al. 2013; Wu et al. 2015). As a classical type of MOFs, zeolitic imidazolate frameworks (ZIFs) with ZIF-8 and ZIF-67 as representative examples [ZIF-8 for Zn(MeIm)2 and ZIF-67 for Co(MeIm)2, MeIm = 2methylimidazole], characterized by uniform polyhedral morphology, ultrahigh surface areas, and large pore volumes, have distinguished themselves in gas separation, environmental remediation, and catalysis (Song et al. 2012; Jiang et al. 2013; Jung et al. 2015; Lin and Chang 2015a). Thanks to their thermally stable carbon frameworks and nitrogenous ligand of 2-methylimidazole, ZIFs have been taken as the ideal precursors for porous N-doped carbon with excellent performance in different fields (Bai et al. 2014; Bhadra et al. 2016; Lin et al. 2015c; Zhang et al. 2014). Lin et al. fabricated Co3O4/C composite for PMS activation in the removal of Rhodamine B (RhB) and caffeine through one-step pyrolysis of ZIF-67, whereas highly catalytic efficiency could not conceal the critical drawbacks from the involvement of high-loading metal oxides (Lin et al. 2015b; Lin and Chen 2016a). When ZIF-8 is chosen as the precursor, metal-free N-doped carbon will be produced due to the low boiling Environ Sci Pollut Res point and easy etching of metal zinc, and the latest developments also validate that the carbocatalyst from ZIF-8 can also perform better efficiency in the degradation of some organic pollutants by activating PMS than those from other MOFs (Li et al. 2017; Wang et al. 2017). However, it is found that the specific relationship between the nitrogen doping and catalytic effectiveness, as well as the extended applications of this new kind of carbocatalysts, should be further addressed. In this article, we prepare N-doped porous carbon (NPC) by choosing ZIF-8 as the self-sacrificial template and evaluate their catalytic performance in the degradation of RhB and phenol. Compared with nitrogen-containing porous carbon prepared from tedious procedures, this MOFderived strategy not only endows carbon materials with rich pore structure properties but also facilitates the process of doping nitrogen into the carbon frameworks, which can alter the electronegativity of adjacent carbon atoms, thus boosting the reaction rates. The effects of the content and existence forms of doping nitrogen species on the catalytic efficiency are carefully investigated. Several influential parameters, such as catalyst dosage, oxidant concentration, and reaction temperature, are also systematically conducted during the degradation processes. Finally, the performance of the as-prepared NPC was evaluated under some actual water background conditions, and radical quenching tests are carried out to uncover the catalytic mechanism. Experimental section Chemical reagents Zinc nitrate (Zn(NO3)2·6H2O), sodium thiosulfate (Na2S2O3· 5H 2 O), sodium chloride (NaCl), sodium bicarbonate (NaHCO 3 ), sodium acetate (NaAc), sodium carbonate (Na2CO3), sodium nitrate (NaNO3), nitric acid (HNO3), sodium dihydrogen phosphate (NaH2PO4), activated carbon (AC), and hydrochloric acid (HCl) were purchased from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. Graphene oxide and carbon nanotubes were supplied from Jiangsu Daopeng Technology and Shenzhen Nanotechnologies Port Co., Ltd., respectively. RhB and phenol were obtained from Tianjin Guangfu Fine Chemical Research Institute and Dongli Chemical Reagent Factory, respectively. Methanol was supplied by Sinopharm Chemical Reagent Co., Ltd. Oxone (KHSO5· 0.5KHSO 4·0.5K 2SO4, PMS), humic acid (HA), and 2methylimidazole were of ACS reagent grade and purchased from Sigma-Aldrich. Commercial MnO2 and Fe3O4 were supplied by Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. All chemicals were used as received without any purification. Preparation ZIF-8 powders were prepared according to previous literature (Torad et al. 2013). Briefly, Zn(NO3)2·6H2O (0.5160 g) was dissolved into methanol (40 mL) to form a clear solution, which was named as solution A. Similarly, 2-methylimidazole (0.5260 g) was added to another methanol (40 mL) to form solution B. Then, solutions A and B were mixed together and stirred for 10 min. The mixed solution was aged for 24 h under room temperature, and the product was collected by centrifugation, washed by methanol for 3 times, and dried at 60 °C. As shown in Scheme 1, the as-synthesized ZIF-8 powders were placed into a tubular furnace under flowing N2 atmosphere, and calcined at given temperatures (700, 800, and 900 °C) for 300 min with the heating rate of 5 °C/ min. Subsequently, the obtained products were mixed with concentrated hydrochloric acid under vigorous stirring for 24 h to thoroughly remove the residual zinc species. Finally, the product was purified by water for several times and collected for further use. The samples were denoted as NPC-X, in which X represents the pyrolysis temperature of 700, 800, and 900 °C, respectively. For homemade CuO and Co3O4, typically, Cu(NO3)2 or Co(NO3)2 was put into a crucible boat, and then heated to 450 °C for 1 h in air at a ramp of 5 °C/min; the product was finally washed for three times and dried for use. For mesoporous carbon (MPC), 0.44 g of resorcinol, 1.50 g of F127, and 0.56 g of p-phthalaldehyde were mixed together thoroughly, and then sealed in an autoclave and heated at 250 °C for 12 h. The obtained brown product was washed with water for 3 times, dried, and then carbonized at 800 °C for 5 h under nitrogen atmosphere at a ramp of 5 °C/min (Zhang et al. 2015b). For purification of carbon nanotubes, the as-obtained carbon nanotubes was dispersed into the mixture of nitric acid and sulfuric acid (volume ratio 3:1) and heated to 80 °C for 3 h according to the previous literature (Huang et al. 2015). Then, it was collected, washed with water for several times, and dried at 60 °C. Characterization Powder X-ray diffraction (XRD) data were acquired on a Rigaku D/MAXRC X-ray diffractometer with a Cu Ka radiation source (45.0 kV, 50.0 mA). Nitrogen adsorption isotherms were recorded on a QUADRASORB SI-KR/MP (degassing at 120 °C). JEM-3000F from JEOL was utilized to obtain the transmission electron micrographs and high-resolution fieldemission TEM (HRTEM) images. The thermogravimetric (TG) analysis was carried out on a SDT Q600 TGA (TA Instruments) in the temperature range of room temperature to 800 °C at a heating rate of 10 °C/min. Total organic carbon (TOC) was determined by Analytik Jena AG MultiN/C 2100 Environ Sci Pollut Res Scheme 1 Schematic illustration of synthesizing ZIF-8 and nitrogen-doped porous carbon (NPC) TOC analyzer. The concentration of several metal elements was made with an Optima 8300 (PerkinElmer, USA) ICPOES. X-ray photoelectron microscopy (XPS) was conducted on PHI 5400 ESCA System using Al Kα (1486.6 eV) to irradiate samples. Scanning electron microscopy (SEM) data was collected by a Quanta 200S (FEI). The confocal Raman spectroscopic system (Renishaw, In Via) with a 633-nm laser was employed to obtain Raman spectra. Fourier transform infrared (FTIR) spectra were performed on an Avatar 360 FTIR using KBr for tableting. Evaluation of catalytic oxidation A series of catalytic tests were performed to investigate the performance of NPC-X for RhB/phenol degradation. Typically, in a beaker (100 mL) containing of pollutant solution at 20 °C, a fixed amount of catalyst was added to the solution under constant stirring for about 30 min to reach absorption-desorption equilibrium. After that, Oxone was added to initiate the oxidation process. During each time interval, the predetermined amount (1.0 mL) of solution was injected into a 20-mL vial, in which excessive saturated Na2S2O3 was used as quenching agent. UV spectrophotometer was employed to obtain the concentration of RhB. The concentration of phenol was determined using a highperformance liquid chromatography (HPLC) with a UV detector set at 270 nm, where AC-18 column was used to separate the organics. The mobile phase with a flow rate of 1.5 mL/min was made of 40% CH3OH and 60% water. Every catalytic test was repeated for three times and the relative standard errors were less than 5%. Results and discussion Characterization of nitrogen-doped porous carbon The structure information of pure ZIF-8 and NPC-X is primarily investigated by XRD, and as shown in Fig. S1, all diffraction peaks of ZIF-8 match well with the simulated data reported in previous literature, implying the formation of high-purity zeolite-type structure with cubic crystal system (I43m ) (Torad et al. 2013). However, these characteristic peaks disappear in NPC-X (Fig. 1a), and only one broad peak centered at ca. 22.6° can be detected, which indicates that ZIF-8 has been successfully converted into amorphous carbon materials after high-temperature pyrolysis. The absence of characteristic peaks and almost 100% of weight loss in air confirm that Zn species have been completely removed during the processes of high-temperature pyrolysis and subsequent acidic treatment (Fig. S2). With increasing the pyrolysis temperature, it is found that this diffraction peak slightly shifts toward higher 2θ degree and an additional peak at 43.5° will be identified in NPC-900. These phenomena are attributed to the fact that higher pyrolysis temperature can promote the graphitization degree of these carbon materials. Raman spectroscopy is widely used for the characterization of the structure and crystallization of carbon materials by examining the bonding state of carbon atoms within given materials. It is clear that all three samples display two distinguishable peaks assigned to D band and G band at about 1350 and 1580/cm, respectively. In general, G band corresponds to the E2g mode induced by stretching vibrations of sp2 bond, and it occurs at all sp2 sites and not only in crystalline graphite; D band originates from the breathing mode of A1g symmetry involving phonons near the K zone boundary, and it becomes active in the presence of defects/disorders (Chu and Li 2006; Ferrari and Robertson 2000). As a result, the relative intensity ratio of these two bands, ID/IG, always presents regular variation with the changes of graphitization degree. However, when the pyrolysis temperature increases from 700 to 900 °C, the ID/IG ratio of NPC-X displays an abnormal fluctuation, where the maximum is achieved in NPC-800 (0.91). It is possible that the introduction of nitrogen species in carbon frameworks upsets the established change of ID/IG ratio, because the nitrogen content and nitrogen bonding configurations are different in NPC-700, NPC-800, and NPC-900. Similar phenomena Environ Sci Pollut Res Fig. 1 XRD patterns (a) and Raman spectra (b) of NPC-700, 800, and 900 have also been observed in some previous reports on nitrogen-doped carbon materials (Lin et al. 2013; Tao et al. 2015; Zheng et al. 2014). This kind of carbon materials with abundant defects and heterogeneous atoms is highly desirable for the activation of PMS (Indrawirawan et al. 2015b). Figure S3 shows FTIR spectra of ZIF-8 and NPC-X. Pristine ZIF-8 displays a series of regular absorption bands at 600–1500/cm, which are associated with the ring stretching and in-plane/out of plane bending of the imidazole. However, for NPC-X, the bands of several functional groups appear instead, such as the bands at 3300/cm for OH group, 1594/cm for C=C groups, and 1300/cm for COOH or CHx groups. Besides, it is found that the signals of oxygen-containing groups become more and more weak as the pyrolysis temperature is raised from 700 to 900 °C. These results again verify the gradual enhancement of graphitization degree. The information of porous structure in ZIF-8 and NPC-X is determined by N 2 adsorptiondesorption isotherms (Fig. 2a). As observed, ZIF-8 performs standard type-I isotherm according to the IUPAC classification (Kruk and Jaroniec 2001), indicating its typical microporous characters (Torad et al. 2013). The small hysteresis hoop at about P/P0 = 0.9–1.0 is attributed to the interspaces among ZIF-8 particles. In contrast, NPC-800 shows isotherm between type-II and type-IV with a long and narrow hysteresis hoop at relative pressure (P/P0) from 0.45 to 1.0, which means that some disordered mesopores are created during the process of high-temperature pyrolysis. NPC-700 and NPC-900 display similar adsorptiondesorption isotherms to NPC-800 (Fig. S4). Based on these isotherms, their corresponding textural parameters, including specific surface areas and pore volumes, can be further obtained (Fig. 2b). The specific surface areas of NPC-700, NPC-800, and NPC-900 are deduced as 694.4, 676.9, and 676.4 m2/g, respectively, while these values are still much inferior to that of ZIF-8 (1164.3 m2/g). Similarly, the pore volumes of NPC-X also exhibit more or less decrease as compared with ZIF-8. The shrinkage of carbon frameworks and the destruction of well-ordered channel system at high temperature should be responsible for the decreased textural parameters. However, it is worth noting that the pore volumes of NPC-700 (0.82 cm 3 /g) and NPC-800 (0.86 cm 3 /g) are much larger than that of NPC-900 (0.72 cm3/g), suggesting that there may be a tremendous change in the microstructure of NPC-900. Figure 3 shows SEM and TEM images of ZIF-8 and NPC800. It can be observed that ZIF-8 crystals possess typical rhombic dodecahedral shape with smooth surface and good dispersion, and the average size is about 90 nm (Fig. 3a, c). Very interestingly, NPC-800 can inherit the basic polyhedral profiles of ZIF-8 and give a visible shrinkage and aggregation after high-temperature pyrolysis (Fig. 3b, d). The inset in Fig. 3d also revealed that there are many disordered mesopores in the polyhedral of NPC-800, in good agreement with the results of N2 adsorption-desorption isotherms (Fig. 2). NPC-700 demonstrates similar results to NPC-800 (Fig. S5a, c). However, for NPC-900, some dispersed particles are sintered and the original polyhedrons are completely torn up into pieces, even including many dense carbon agglomerates (Fig. S5b, d), which can directly explain its drastically decreased pore volume (Fig. 2b). It is well known that the boiling point of zinc is around 800 °C, and thus when the pyrolysis temperature reaches 900 °C, the removal of residual zinc species will inevitably cause a severe damage to the integrity of the carbon framework. Besides, element mapping indicates that carbon and nitrogen species are dispersed homogeneously throughout the whole polyhedral structures, which predicts great benefits to PMS activation because most carbon sites with catalytic inertness may be activated by the sufficient doping with nitrogen species (Fig. S6). XPS measurements are carried to analyze the elemental composition and nitrogen bonding configurations in NPC-X. The summary of element composition validates that carbon, nitrogen, and oxygen elements are the main compositions for NPC-X (Fig. 4a), and negligible zinc content can be detected in NPC-700 (0.67%) and NPC-800 (0.87%). The comparable oxygen content in NPC-700 (5.44%), NPC-800 (4.31%), and NPC-900 (5.17%) implies that the oxygen-containing groups may be involved by the surface oxidation of NPC-X. The carbon content presents a monotonous increase from 72.52% for NPC-700 to 90.99% for NPC-900 along with the Environ Sci Pollut Res Fig. 2 N2 adsorption-desorption isotherm of ZIF-8 and NPC-800 (a) and the comparison of BET specific surface areas and pore volumes (b) corresponding loss of nitrogen content from 21.37 to 3.85%, indicating that high pyrolysis temperature is unfavorable for the doping of nitrogen species. Of great significance, compared with other conventional nitrogen modifying methods employing melamine or urea as additives, this MOF-derived in situ doping strategy can easily obtain the considerable doping content, e.g., 15.20% for NPC-800 vs 8.23% for NGmelamine and 6.94% for NG-urea (Duan et al. 2015a; Yang et al. 2016a). To access bonding configurations of Ncontaining groups, the deconvolution results of highresolution XPS N1 s spectra have been shown in Fig. 4b–d, where four main peaks centered at around 398.27, 399.81, 400.86, and 403.93 eV can be assigned to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively (Jiang et al. 2016). Remarkably, with the raise of pyrolysis temperature, the contents of graphitic N and oxidized N go up sharply, accompanying with the reduction of pyridinic N and pyrrolic N, because the latter ones possess a relatively low thermal Fig. 3 SEM and TEM images of ZIF-8 (a, c) and NPC-800 (b, d) stability and are susceptible to be transformed into graphitic N. Furthermore, it is well established that graphitic N has a higher electronegativity and smaller atom radius to accelerate the electron flow from neighboring carbon atom, promoting the charge density of adjacent carbon (Zhang and Xia 2011). Therefore, PMS activation may benefit from high graphitic N content. In view of total nitrogen content and different proportion for graphitic N, the performance of NPC-700 and NPC-800 may be expected due to their relatively high density of surface graphitic N (total nitrogen content × graphitic N proportion). Catalytic performance of NPC for RhB/phenol degradation Intrigued by their structural features, the catalytic tests of various carbocatalysts in activation of PMS for RhB degradation are conducted. As shown in Fig. 5a, sole ZIF-8 or NPC-800 Environ Sci Pollut Res Fig. 4 XPS survey of NPC-700, 800, and 900 (a). XPS N 1s spectra of NPC-700 (b), NPC-800 (c), and NPC-900 (d) cannot promise any visible RhB removal, suggesting that the contribution from physical adsorption is negligible. In contrast, sole PMS accounts for a moderate RhB degradation at 48.5% in 60 min due to the direct oxidation. When both PMS and NPC-X are present, the degradation efficiency of RhB will be significantly enhanced, and it is found that the pyrolysis temperature has an obvious impact on the catalytic performance of NPC-X. The optimal degradation efficiency of about 85.0% can be realized by NPC-800, which is totally consistent with the prediction of XPS results. However, NPC-700 only performs RhB degradation at 60.3%, even less than that over NPC-900 (68.3%). According to the previous literature, sp2hybridized carbon configuration can be ideal platform for the catalytic efficiency of graphitic N (Duan et al. 2015e; Zhang and Xia 2011). Although NPC-700 contains similar density of surface graphitic N to NPC-800, its graphitization degree is relatively low, and thus there will be less sp2-hybridized Fig. 5 Degradation of RhB under various catalytic PMS systems (a); conditions: [RhB]0 = 100 mg/L, [Oxone] = 1.40 g/L, [catalyst] = 0.20 g/L, temperature = 20 °C. Phenol degradation at different conditions (b); reaction conditions: [phenol]0 = 25 mg/ L, [Oxone] = 0.80 g/L, [catalyst] = 0.20 g/L, temperature = 20 °C carbon sites in NPC-700, resulting in its inferior catalytic performance. Figure 5b presents the degradation of phenol over NPC-X obtained at different pyrolysis temperatures. It is a little different from the degradation of RhB that sole PMS is unable to degrade phenol in the absence of catalysts and NPC800 removes ca. 14% of phenol by physical adsorption. These results may be linked with the hydroxyl group and low molecular weight of phenol, where the former makes phenol as a typical radical scavenger that cannot activate PMS (Liang and Su 2009; Ghanbari and Moradi 2017), and the latter can facilitate its physical adsorption inside the micropores and mesopores of NPC-800. The considerable degradation of phenol cannot be observed unless both PMS and NPC-X are introduced into the catalytic system, and the degradation efficiencies of phenol over NPC-700, NPC-800, and NPC-900 are 73.0, 86.1, and 76.4%, respectively. Interestingly, the rich graphitic N content still endows NPC-800 with the best Environ Sci Pollut Res performance in phenol degradation among these three catalysts. Total organic carbon (TOC) measurement shows that the capacities of mineralization of NPC-800/PMS system for RhB and phenol degradation are 47.3 and 53.6%, respectively. In general, the degradations of organic dye and phenol obey the pseudo-first order kinetics (Wang et al. 2014), and thus the kinetic rate constant over various catalysts can be calculated by the following equation, . ln C C 0 ¼ −kt ð1Þ where C and C0 represent the concentration of RhB/phenol at certain time intervals and at the beginning, respectively, and t is the reaction time and k means the kinetic rate constant. The results show that the kinetic rate constants over NPC-700, NPC-800, and NPC-900 in RhB degradation are 0.0190, 0.0427, and 0.0227/min, respectively, and the corresponding kinetic rate constants in phenol degradation are 0.0250, 0.0418, and 0.0355/min, respectively. To demonstrate the superiority of nitrogen modification in terms of PMS activation, we employ homemade mesoporous carbon (MPC), commercial activated carbon (AC), graphene oxide (GO), and carbon nanotubes (CNTs) as control samples. It is found that MPC does not show any significant enhancement in RhB degradation, and AC, GO, and CNTs can stimulate RhB degradation as compared with sole PMS to a certain degree, while their performances are still less than that of NPC-800 (Fig. S7a), validating that nitrogen modification is an effective method to produce active sites in carbon framework. The gap between these carbon materials and NPC-800 will be further widened in the degradation of phenol due to its relatively low reactivity (Fig. S7b). In addition, we also compare the catalytic activity of NPC-800 in RhB degradation with those of several popular transition metal-based oxides, including commercial Fe3O4 and MnO2, and homemade CuO and Co3O4 (their XRD patterns are showed in Fig. S8). It is exciting that NPC-800 can perform superior RhB removal to those transition metalbased oxides that have been widely applied in previous studies (Fig. S9), which indicates that NPC-800 will be a promising heterogeneous catalyst for PMS activation. Effects of different parameters on RhB and phenol degradation Several potential influential factors, including catalyst dosage, oxidant concentration, and reaction temperature, related to catalytic performance, are extensively studied. Figure 6a shows the effect of NPC-800 dosage on the degradation of RhB. With increasing the dosage of NPC-800 from 0.10, 0.15, 0.20 to 0.30 g/L, 66.8, 72.3, 85.0, and 92.5% of RhB removal can be achieved, which may be attributed to the fact that more active sites can accelerate the generation of radical species. Similarly, thanks to the abundant HSO5− absorbed to the surface of carbocatalyst, when the concentration of Oxone is adjusted from 0.60 to 3.00 g/L, the degradation efficiency of RhB will be increased from 65.2 to 90.1% (Fig. 6b). In contrast, it is found that reaction temperature has limited stimulation effect on RhB degradation, where the degradation efficiencies of RhB at 20 and 30 °C are very close, and the removal efficiency of RhB is merely promoted to 92.0% as the reaction temperature is ascended to 40 °C, implying that a marginal effect appears between the temperature and the catalytic activity. The kinetic rate constants at different reaction temperature are further fitted to address the differences (Fig. S10), and the kinetic rate constants for the degradation of RhB at 20 and 30 °C are 0.0427/min (R2 = 0.98) and 0.0400/min (R2 = 0.98), respectively, and the degradation of RhB at 40 °C may account for a little higher kinetic rate constant [0.0543/min (R2 = 0.99)]. These results indicate that the positive effect of reaction temperature cannot be presented until it is obviously higher than room temperature, which may be attributed to the weak temperature dependence of carrier mobility in N-doped carbocatalysts. Similar phenomena have also been reported in the cases of N-doped graphene and Ndoped CNTs (Sun et al. 2014; Wang et al. 2016c). Moreover, the effects of these influential parameters are also examined in phenol degradation. As shown in Fig. S11a, b, the catalyst dosage and oxidant concentration still play positive roles in promoting the degradation of phenol. For example, when the catalyst dosage is 0.08 g/L, only 44.9% phenol can be removed in 60 min, far less than the 70.1% at 0.14 g/L and 86.1% at 0.20 g/L, and complete phenol degradation can be achieved in 30 min at the catalyst dosage of 0.30 g/L; meanwhile, the increase of oxidant concentration from 0.20 to 2.00 g/L will induce boost phenol degradation from 60.0% in 60 min to almost 100% in 20 min. As for the reaction temperature, it also fails to promise obvious impacts on the degradation of phenol, while the kinetic rate constants at 20 °C [0.0418/min (R2 = 0.96)], 30 °C [0.0533/min (R2 = 0.99)], and 40 °C [0.0705/min (R2 = 0.97)] indeed identify the modest contribution from reaction temperature (Fig. S11c, d). Based on these results, it can be concluded that the stimulation effects from catalyst dosage, oxidant concentration, and reaction temperature are a little stronger than those in RhB degradation, which may be ascribed to the variation in the organic molecular structures and degradation pathways (Duan et al. 2015e; Zhang et al. 2013a). However, the similar functions of these operational parameters suggest that NPC-X has same catalytic mechanism in these two degradation systems. Influence of common anions, NOM, and actual water for RhB degradation It is well known that some common anions and nature organic matter (NOM) can affect the degradation of organic pollutants Environ Sci Pollut Res Fig. 6 The influence of catalyst dosage (a), Oxone concentration (b), reaction temperature (c) to the RhB degradation. Stability tests of NPC-800 for RhB removal (d). Unless explicitly stated, conditions are as follows: [RhB]0 = 100 mg/L, [Oxone] = 1.40 g/L, [NPC-800] = 0.20 g/L, and temperature = 20 °C greatly in AOPs (Guan et al. 2013). To testify the potential application of NPC/PMS system, we also conduct the degradation of RhB in the presence of some typical anions (e.g., Cl−, HCO3−, CO32−, CH3COO−, H2PO4−, NO3−) and humic acid (HA). The influence of chloride with different concentrations (0–5.0 mM) is presented (Fig. S12a). In general, chloride will compete with organic pollutants to consume SO4·− and generate less reactive chloride radicals species like Cl· and Cl2·− (redox potentials for Cl·/Cl− and 2Cl−/Cl2·− are 2.41 and 2.09 V vs NHE, respectively) via Eqs. (2) to (5), while the active Cl2 and HOCl derived from the directly nonradical interaction between chloride ion and PMS can also cause fairly positive effect for the proceeding of the degradation through Eqs. (6) and (7) (Jaafarzadeh et al. 2017; Huie et al. 1991; Zhou et al. 2015). Thus, the dual roles of chloride could be finally reflected as the stimulation result at very low concentration and the inhibition effect for the degradation at a relatively high concentration (Wang et al. 2016b). The similar effect of chloride can be also observed SO4 − þ Cl− → SO4 2− þ Cl Cl þ Cl− → Cl2 − ð2Þ ð3Þ Cl2 − þ Cl2 − →2Cl− þ Cl2 ð4Þ Cl þ Cl →Cl2 ð5Þ − − Cl þHSO5 → SO4 2− þ HOCl 2Cl− þHSO5 − þHþ →SO4 2− þ Cl2 þH2 O ð6Þ ð7Þ in our case, where the degradation of RhB is slightly promoted at 0.5 mM of chloride and gradually weakened with higher chloride concentration. The moderate inhibition effect of chloride in RhB degradation is well consistent with those results on the degradation of organic dyes in previous studies (Lin and Zhang 2016b, Li et al. 2017; Zhang et al. 2015a). Likewise, some other anions, including CO32−, HCO3−, CH3COO−, H2PO4−, and NO3−, are also able to interact with SO4·− and produce different counterparts with lower redox potential compared to SO4·−, for instance, CO3− · and HCO3· own the redox potential of 1.59 and 1.65 V (vs NHE), which are inferior to the approximately 2.00 V for CH3COO· and 2.30–2.50 V (vs NHE) for NO 3 · . It was reported that H2PO4·could even achieve somewhat similar reactivity to SO4·− (Neta et al. 1988; Liang et al. 2006). In view of the distinguished oxidation capability of anions radicals, the addition of various anions will decrease the efficiency to a certain extent. Figure S12b, c describes the effect of some other common anions. Bicarbonate, as a recognized radical scavenger (Laat et al. 2011), displays a monotonously increased inhibition, and when its concentration is changed from 0 to 5.0 mM, the degradation efficiency of RhB will decrease from 85.0 to 72.4% (Fig. S12b). At the same concentration (3.0 mM), it can be concluded that the adverse effect of these anions on RhB degradation follows the order of CO32− > HCO3− > CH3COO− > Cl− > H2PO4− > NO3−. Humic acid (HA) is always utilized as a kind of characteristic NOM in the model reaction of AOPs, and its abundant phenolic hydroxyl and carboxyl groups may absorb onto the surface of heterogeneous catalysts and block the most active sites (Yao et al. 2015; Guan et al. 2013; Wang et al. 2016b; Oh et al. 2015). However, it is worth noting that HA in the studied concentration interval (2.0–10.0 mg/L) only causes insignificant Environ Sci Pollut Res detrimental effect on RhB degradation, which may be possibly attributed to the fact that high concentration of RhB (100 ppm) dilute the adsorption of HA on the surface of NPC-800 and primarily consume radicals released from the active sites. Three actual water bodies, tap water, surface water (Songhua River in Harbin), and mining sewage, are further employed to investigate the integrated influence of backgrounds on the degradation of RhB catalyzed by NPC-800/ PMS system. Several parameters of these actual water bodies are listed in Table S1. Although these three actual water bodies possess different compositions, especially that mining sewage and surface water have much higher total organic carbon (TOC) concentration than tap water, the degradation efficiencies of RhB in these actual water bodies are still close to that in Milli-Q water (Fig. S13). The possible reason may be that the impurities, such as NOMs, dissolved organic matters (DOMs), and other trace irons in actual water bodies, can only hinder the interaction between PMS and NPC-800 to a lesser extent (Gong et al. 2017), which validates the potential of NPC-800/PMS system for the removal of organic dyes in the practical applications. Reusability of NPC-800 for RhB degradation To investigate the stability and reusability of NPC-800, three recycling tests are conducted after washing the used catalyst for 24 h with deionized water. In Fig. 6d, it can be seen that about 68.8% of RhB is removed in the second and third run, indicating a slight decline compared with 85.0% of removal efficiency over the fresh catalyst. This phenomenon seems that carbocatalysts herein exhibit a poor stability as compared with commonly used transition metal-based heterogeneous catalysts. Nevertheless, it is found that the microstructural morphology of NPC-800 is completely maintained during the repeated catalytic processes (Fig. S14), and its catalytic activity can be entirely recovered when carbocatalyst after third run is subjected to heat treatment at 350 °C for 1 h in air. XPS results reveal that the proportion of oxygen greatly increases to 23.35% and the relative amount of nitrogen is accordingly reduced to 7.20% for NPC-800 after three runs, Fig. 7 Degradation of RhB by NPC-800 coupled with various oxidants (a), radical quenching tests using methanol and tert-butyl alcohol (TBA) as the quenching agents (b), and the molar ratio of TBA or methanol/PMS is 500 and 1000, respectively; conditions: [RhB]0 = 100 mg/L, [PMS/PS/ H2O2] = 4.56 mmol/L, [NPC-800] = 0.20 g/L, temperature = 20 °C and thus the deactivation of NPC-800 may be ascribed to the fact that the intermediates from the decomposition of RhB molecules susceptibly absorb to the delicate surface of carbon materials, and then poison the high-activity functional groups and hinder the access to the inner sp2-hybridized carbon networks, alleviating the robust performance of catalysts (Duan et al. 2015a). Fortunately, the oxygen-containing organic molecules on the surface can be removed by the high-temperature treatment in air, and the catalytic sites will be exposed again. Radical quenching tests and possible active sites for PMS activation In this study, two kinds of commonly used oxidants such as PS and H2O2 are also applied for degradation of RhB to investigate the universality of NPC-800 (Fig. 7a). Compared with PMS, it is often believed that PS is more difficult to be activated due to its symmetric molecular structure and pretty higher O-O bonding energy (Oh et al. 2016; Duan et al. 2016b). However, in our case, one can find that NPC-800 coupled with PS also accounts for the removal percentage of RhB at 73.0%, which is a little inferior to that in PMS system, indicating that NPC-800 is also effective for PS activation. When H2O2 is applied as the oxidant, the degradation efficiency of RhB is only 7.1%, and this insufficient RhB degradation may be attributed to the low adsorption energy and near zero charge transfer of H2O2 (Duan et al. 2015b). Different from the PMS activation mechanism of transition metal-based catalysts, carbocatalysts can perform both radical and nonradical pathways in terms of their structures. Duan et al. reported that the intact sp2-conjugated carbon frameworks and rich Lewis basic sites could stimulate PMS dissociation to generate SO4·− and ·OH, and the nonradical oxidations usually occurred at the defective edges of the boundary in carbon network (Duan et al. 2016a). Therefore, the classical quenching tests are carried out to figure out the specific reaction mechanism in the system of RhB degradation. It is widely accepted that tert-butyl alcohol (TBA) may be evaluated as the typical scavenger for ·OH, because the kinetic rate constant Environ Sci Pollut Res from the reaction of TBA and ·OH (3.8–7.6 × 108 /M/s) is almost three orders of magnitude higher than that from the reaction of TBA and SO4·− (4–9.1 × 105/M/s) (Liang and Su 2009; Ghanbari and Moradi 2017). In contrast, methanol is always utilized as a universal scavenger because it can rapidly react with both SO4·− (3.2 × 106/M/s) and ·OH (9.7 × 108/M/s). As shown in Fig. 7b, when the molar ratio of TBA to PMS reaches 500:1, the degradation efficiency will decrease from 85.0 to 71.6%, and this value will further decline to 65.7% with the molar ratio of TBA to PMS at 1000:1. It is obvious that methanol always plays a more aggressive role in inhibiting RhB degradation as compared with TBA, e.g., the degradation efficiencies of RhB are 58.0 and 50.7% at the molar ratios of 500:1 and 1000:1, respectively. These results indicate that both sulfate and hydroxyl radicals are involved in the degradation of RhB, which is consistent with many studies about PMS activation (Ji et al. 2013; Lin et al. 2015c). In order to rule out the contribution of radicals completely, we also perform the degradation of RhB in absolute methanol solution, and it can be found that only 6.7% of RhB is removed, confirming that radical pathway is dominant in the system of NPC/PMS. Based on the characterization and experimental data, it can be deduced that in our case, the presence of nitrogen atoms, especially graphitic N, should be responsible for breaking the inertness of sp2hybridized carbon frameworks and modulating the electronic structures of carbon materials to release the powerful radicals, and some defective sites and zigzag edges may contribute to the nonradical oxidations moderately. Conclusions In conclusion, nitrogen-doped porous carbon (NPC) with comparable nitrogen content and the regular polyhedral morphologies can be prepared via a facile ZIF-8 derived method. It is found that the as-prepared carbon materials possess high specific surface area and show good catalytic efficiency in PMS activation for degradation of RhB and phenol. In particular, NPC-800 displays better performance than some conventional transition metal-based oxides and common carbon materials. Some operational parameters, including the dosage of catalyst, the concentration of oxidant, and reaction temperature, can impact on the degradation of RhB and phenol effectively. NPC-800 can be regenerated through simple heat treatment at 350 °C in air. Radical quenching tests reveal that both sulfate and hydroxyl radicals will be released from PMS activation. It is very interesting that, in the degradation of RhB, NPC-800 can maintain good performance in the presence of some anions and natural organic matters, even under some actual water background conditions, indicating that it is a promising heterogeneous catalyst for PMS activation in the practical applications. Acknowledgements We appreciate the financial support from Natural Science Foundation of China (21676065) and the Natural Science Foundation of Heilongjiang Province (B201405). References Anipsitakis GP, Dionysiou DD (2004) Radical generation by the interaction of transition metals with common oxidants. Environ Sci Technol 38:3705–3712 Bai FH, Xia YD, Chen BL, Su HQ, Zhu YQ (2014) Preparation and carbon dioxide uptake capacity of N-doped porous carbon materials derived from direct carbonization of zeolitic imidazolate framework. Carbon 79:213–226 Bhadra BN, Ahmed I, Kim S, Jhung SH (2016) Adsorptive removal of ibuprofen and diclofenac from water using metal-organic framework-derived porous carbon. Chem Eng J 314:50–58 Chen JB, Zhang LM, Huang TY, Li WW, Wang Y, Wang ZM (2016) Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: radical versus non-radical mechanism. J Hazard Mater 320:571–580 Chu PK, Li L (2006) Characterization of amorphous and nanocrystalline carbon films. Mater Chem Phys 96:253–277 Dong HJ, Wei MY, Li J, Fang J, Gao L, Li XX, Xu AH (2016) Catalytic performance of supported g-C3N4 on MCM-41 in organic dye degradation with peroxymonosulfate. RSC Adv 6:70747–70755 Du YC, Ma WJ, Liu PX, Zou BH, Ma J (2016) Magnetic CoFe2O4 nanoparticles supported on titanate nanotubes (CoFe2O4/TNTs) as a novel heterogeneous catalyst for peroxymonosulfate activation and degradation of organic pollutants. J Hazard Mater 308:58–66 Duan XG, Ao ZB, Sun HQ, Indrawirawan S, Wang YX, Kang J, Liang F, Zhu ZH, Wang SB (2015a) Nitrogen-doped graphene for generation and evolution of reactive radicals by metal-free catalysis. ACS Appl Mater Interfaces 7:4169–4178 Duan XG, Ao ZM, Sun HQ, Zhou L, Wang GX, Wang SB (2015b) Insights into N-doping in single-walled carbon nanotubes for enhanced activation of superoxides: a mechanistic study. Chem Commun 51:15249–15252 Duan XG, Ao ZB, Zhou L, Sun HQ, Wang GX, Wang SB (2016a) Occurrence of radical and nonradical pathways from carbocatalysts for aqueous and nonaqueous catalytic oxidation. Appl Catal B Environ 188:98–105 Duan XG, Indrawirawan S, Sun H, Wang SB (2015c) Effects of nitrogen-, boron-, and phosphorus-doping or codoping on metal-free graphene catalysis. Catal Today 249:184–191 Duan XG, O’Donnell K, Sun HQ, Wang YX, Wang SB (2015d) Sulfur and nitrogen co-doped graphene for metal-free catalytic oxidation reactions. Small 11:3036–3044 Duan XG, Su C, Zhou L, Sun HQ, Suvorova A, Odedairo T, Zhu ZH, Shao ZP, Wang SB (2016b) Surface controlled generation of reactive radicals from persulfate by carbocatalysis on nanodiamonds. Appl Catal B Environ 194:7–15 Duan XG, Sun HQ, Kang J, Wang YX, Indrawirawan S, Wang SB (2015e) Insights into heterogeneous catalysis of persulfate activation on dimensional-structured nanocarbons. ACS Catal 5:4629–4636 Duan XG, Sun HQ, Wang YX, Kang J, Wang SB (2015f) N-dopinginduced nonradical reaction on single-walled carbon nanotubes for catalytic phenol oxidation. ACS Catal 5:553–559 Ferrari AC, Robertson J (2000) Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 61:14095 Environ Sci Pollut Res Ghanbari F, Moradi M (2017) Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants: review. Chem Eng J 310:41–62 Gong C, Chen F, Yang Q, Luo K, Yao FB, Wang SN, Wang XL, Wu JW, Li XM, Wang DB, Zeng GM (2017) Heterogeneous activation of peroxymonosulfate by Fe-Co layered doubled hydroxide for efficient catalytic degradation of Rhodamine B. Chem Eng J 321: 222–232 Guan YH, Ma J, Ren YM, Liu YL, Xiao JY, Lin LQ, Zhang C (2013) Efficient degradation of atrazine by magnetic porous copper ferrite catalyzed peroxymonosulfate oxidation via the formation of hydroxyl and sulfate radicals. Water Res 47:5431–5438 Hu PD, Long MC (2016) Cobalt-catalyzed sulfate radical-based advanced oxidation: a review on heterogeneous catalysts and applications. Appl Catal B Environ 181:103–117 Hu LX, Yang XP, Dang ST (2011) An easily recyclable Co/SBA-15 catalyst: heterogeneous activation of peroxymonosulfate for the degradation of phenol in water. Appl Catal B Environ 102:19–26 Huang G, Zhang FF, Du XC, Qin YL, Yin DM, Wang LM (2015) Metal organic frameworks route to in situ insertion of multiwalled carbon nanotubes in Co3O4 polyhedra as anode materials for lithium-ion batteries. ACS Nano 9:1592–1599 Huie RE, Clifton CL, Neta P (1991) Electron transfer reaction rates and equilibria of the carbonate and sulfate radical anions. Radiat Phys Chem 38:477–481 Indrawirawan S, Sun HQ, Duan XG, Wang SB (2015a) Low temperature combustion synthesis of nitrogen-doped graphene for metal-free catalytic oxidation. J Mater Chem A 3:3432–3440 Indrawirawan S, Sun HQ, Duan XG, Wang SB (2015b) Nanocarbons in different structural dimensions (0–3D) for phenol adsorption and metal-free catalytic oxidation. Appl Catal B Environ 179:352–362 Jaafarzadeh N, Ghanbari F, Ahmadi M (2017) Catalytic degradation of 2, 4-dichlorophenoxyacetic acid (2, 4-D) by nano-Fe2O3 activated peroxymonosulfate: influential factors and mechanism determination. Chemosphere 169:568–576 Ji F, Li CL, Wei XY, Yu J (2013) Efficient performance of porous Fe2O3 in heterogeneous activation of peroxymonosulfate for decolorization of Rhodamine B. Chem Eng J 231:434–440 Jiang M, Cao XP, Zhu DD, Duan YX, Zhang JM (2016) Hierarchically porous N-doped carbon derived from ZIF-8 nanocomposites for electrochemical applications. Electrochim Acta 196:699–707 Jiang JQ, Yang CX, Yan XP (2013) Zeolitic imidazolate framework-8 for fast adsorption and removal of benzotriazoles from aqueous solution. ACS Appl Mater Inter 5:9837–9842 Jung BK, Jun JW, Hasan Z, Jhung SH (2015) Adsorptive removal of parsanilic acid from water using mesoporous zeolitic imidazolate framework-8. Chem Eng J 267:9–15 Kruk M, Jaroniec M (2001) Gas adsorption characterization of ordered organic-inorganic nanocomposite materials. Chem Mater 13:3169–3183 Laat JD, Dao YH, Najjar NHE, Daou C (2011) Effect of some parameters on the rate of the catalysed decomposition of hydrogen peroxide by iron(iii)-nitrilotriacetate in water. Water Res 45:5654–5664 Lee H, Kim HI, Weon S, Choi W, Hwang YS, Seo J, Lee C, Kim JH (2016) Activation of persulfates by graphitized nanodiamonds for removal of organic compounds. Environ Sci Technol 50: 10134–10142 Li S, Zhang X, Huang Y (2017) Zeolitic imidazolate framework-8 derived nanoporous carbon as an effective and recyclable adsorbent for removal of ciprofloxacin antibiotics from water. J Hazard Mater 321:711–719 Liang J, Jiao Y, Jaroniec M, Qiao SZ (2012) Sulfur and nitrogen dualdoped mesoporous graphene electrocatalyst for oxygen reduction with synergistically enhanced performance. Angew Chem Int Edit 51:11496–11500 Liang C, Su HW (2009) Identification of sulfate and hydroxyl radicals in thermally activated persulfate. Ind Eng Chem Res 48:5558–5562 Liang C, Wang ZS, Mohanty N (2006) Influences of carbonate and chloride ions on persulfate oxidation of trichloroethylene at 20 °C. Sci Total Environ 370:271–277 Liang P, Zhang C, Duan XG, Sun HQ, Liu SM, Tade MO, Wang SB (2016) An insight to metal organic framework derived N-doped graphene towards oxidative degradation of persistent contaminants: generation of singlet oxygen from peroxymonosulfate. Environ Sci Nano 4:315–324 Liang P, Zhang C, Duan XG, Sun HQ, Liu SM, Tade MO, Wang SB (2017) N-doped graphene from metal–organic frameworks for catalytic oxidation of p-hydroxylbenzoic acid: N-functionality and mechanism. ACS Sustain Chem Eng 5:2693–2701 Lin KYA, Chang HA (2015a) Zeolitic imidazole framework-67 (ZIF-67) as a heterogeneous catalyst to activate peroxymonosulfate for degradation of Rhodamine B in water. J Taiwan Inst Chem E 53:40–45 Lin KYA, Chang HA, Chen RC (2015b) MOF-derived magnetic carbonaceous nanocomposite as a heterogeneous catalyst to activate oxone for decolorization of Rhodamine B in water. Chemosphere 130:66–72 Lin KYA, Hsu FK, Lee WD (2015c) Magnetic cobalt–graphene nanocomposite derived from self-assembly of MOFs with graphene oxide as an activator for peroxymonosulfate. J Mater Chem A 3:9480– 9490 Lin KYA, Chen BC (2016a) Efficient elimination of caffeine from water using Oxone activated by a magnetic and recyclable cobalt/carbon nanocomposite derived from ZIF-67. Dalton Trans 45:3541–3551 Lin KYA, Chen BJ (2017) Prussian blue analogue derived magnetic carbon/cobalt/iron nanocomposite as an efficient and recyclable catalyst for activation of peroxymonosulfate. Chemosphere 166:146– 156 Lin ZY, Waller GH, Liu Y, Liu ML, Wong CP (2013) Simple preparation of nanoporous few-layer nitrogen-doped graphene for use as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions. Carbon 53:130–136 Lin KYA, Zhang ZY (2016a) Alpha-sulfur as a metal-free catalyst to activate peroxymonosulfate under visible light irradiation for decolorization. RSC Adv 6:15027–15034 Lin KYA, Zhang ZY (2016b) Degradation of bisphenol a using peroxymonosulfate activated by one-step prepared sulfur-doped carbon nitride as a metal-free heterogeneous catalyst. Chem Eng J 313: 1320–1327 Neta P, Huie RE, Ross AB (1988) Rate constants for reactions of inorganic radicals in aqueous solution. J Phys Chem Ref Data 17:1027–1284 Neyens E, Baeyens J (2003) A review of classic Fenton’s peroxidation as an advanced oxidation technique. J Hazard Mater 98:33–50 Oh WD, Dong ZL, Hu ZT, Lim TT (2015) A novel quasi-cubic CuFe2O4–Fe2O3 catalyst prepared at low temperature for enhanced oxidation of bisphenol a via peroxymonosulfate activation. J Mater Chem A 3:22208–22217 Oh WD, Dong Z, Lim TT (2016) Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: current development, challenges and prospects. Appl Catal B Environ 194: 169–201 Peng WC, Liu SZ, Sun HQ, Yao YJ, Zhi LJ, Wang SB (2013) Synthesis of porous reduced graphene oxide as metal-free carbon for adsorption and catalytic oxidation of organics in water. J Mater Chem A 1:5854 Qiang R, Du YC, Zhao HT, Wang Y, Tian CH, Li ZG, Han XJ, Xu P (2015) Metal organic framework-derived Fe/C nanocubes toward efficient microwave absorption. J Mater Chem A 3:13426–13434 Ren YM, Lin LQ, Ma J, Yang J, Feng J, Fan ZJ (2015) Sulfate radicals induced from peroxymonosulfate by magnetic ferrospinel MFe2O4 Environ Sci Pollut Res (M = Co, Cu, Mn, and Zn) as heterogeneous catalysts in the water. Appl Catal B Environ 165:572–578 Shao PH, Duan XG, Xu J, Tian JY, Shi WX, Gao SS, Xu MJ, Cui FY, Wang SB (2017) Heterogeneous activation of peroxymonosulfate by amorphous boron for degradation of bisphenol S. J Hazard Mater 322:532–539 Song Q, Nataraj SK, Roussenova MV, Tan JZ, Hughes DJ, Li W, Bourgoin P, Alam MA, Cheetham AK, Muhtaseb SA, Sivaniah E (2012) Zeolitic imidazolate framework (ZIF-8) based polymer nanocomposite membranes for gas separation. Energy Environ Sci 5: 8359–8369 Sun HQ, Kwan C, Suvorova A, Ang HM, Tadé MO, Wang SB (2014) Catalytic oxidation of organic pollutants on pristine and surface nitrogen-modified carbon nanotubes with sulfate radicals. Appl Catal B Environ 154:134–141 Sun HQ, Liu SZ, Zhou G, Ang HM, Tade MO, Wang SB (2012) Reduced graphene oxide for catalytic oxidation of aqueous organic pollutants. ACS Appl Mater Interfaces 4:5466–5471 Tao GJ, Zhang LX, Chen LS, Cui XZ, Hua ZL, Wang M, Wang JC, Chen Y, Shi JL (2015) N-doped hierarchically macro/mesoporous carbon with excellent electrocatalytic activity and durability for oxygen reduction reaction. Carbon 86:108–117 Torad NL, Hu M, Kamachi Y, Takai K, Imura M, Naito M, Yamauchi Y (2013) Facile synthesis of nanoporous carbons with controlled particle sizes by direct carbonization of monodispersed ZIF-8 crystals. Chem Commun 49:2521–2523 Tušar NN, Maučec D, Rangus M, Arčon I, Mazaj M, Cotman M, Pintar A, Kaučič V (2012) Manganese functionalized silicate nanoparticles as a Fenton-type catalyst for water purification by advanced oxidation processes (AOP). Adv Funct Mater 22:820–826 Wang YX, Ao ZB, Sun HQ, Duan XG, Wang SB (2016a) Activation of peroxymonosulfate by carbonaceous oxygen groups: experimental and density functional theory calculations. Appl Catal B Environ 198:295–302 Wang GL, Chen S, Quan X, Yu HT, Zhang YB (2017) Enhanced activation of peroxymonosulfate by nitrogen doped porous carbon for effective removal of organic pollutants. Carbon 115:730–739 Wang ZL, Du YC, Liu YL, Zou BH, Xiao JY, Ma J (2016b) Degradation of organic pollutants by NiFe2O4/peroxymonosulfate: efficiency, influential factors and catalytic mechanism. RSC Adv 6:11040– 11048 Wang C, Kang J, Sun HQ, Ang HM, Tadé MO, Wang SB (2016c) Onepot synthesis of N-doped graphene for metal-free advanced oxidation processes. Carbon 102:279–287 Wang YX, Sun HQ, Ang HM, Tade MO, Wang SB (2014) Facile synthesis of hierarchically structured magnetic MnO2/ZnFe2O4 hybrid materials and their performance in heterogeneous activation of peroxymonosulfate. ACS Appl Mater Interfaces 6:19914–19923 Wu CS, Xiong ZH, Li C, Zhang JM (2015) Zeolitic imidazolate metal organic framework ZIF-8 with ultra-high adsorption capacity bound tetracycline in aqueous solution. RSC Adv 5:82127–82137 Yang F, Chi C, Wang CX, Wang Y, Li YF (2016a) High graphite N content in nitrogen-doped graphene as an efficient metal-free catalyst for reduction of nitroarenes in water. Green Chem 18: 4254–4262 Yang SY, Li L, Xiao T, Zheng D, Zhang YT (2016b) Role of surface chemistry in modified ACF (activated carbon fiber)-catalyzed peroxymonosulfate oxidation. Appl Surf Sci 383:142–150 Yao YJ, Cai YM, Lu F, Wei FY, Wang XY, Wang SB (2014) Magnetic recoverable MnFe2O4 and MnFe2O4-graphene hybrid as heterogeneous catalysts of peroxymonosulfate activation for efficient degradation of aqueous organic pollutants. J Hazard Mater 270:61–70 Yao YJ, Cai YM, Wu GD, Wei FY, Li XY, Chen H, Wang SB (2015) Sulfate radicals induced from peroxymonosulfate by cobalt manganese oxides (CoxMn3-xO4) for Fenton-like reaction in water. J Hazard Mater 296:128–137 Yao Y, Zhang J, Wu G et al (2017) Iron encapsulated in 3D N-doped carbon nanotube/porous carbon hybrid from waste biomass for enhanced oxidative activity. Environ Sci Pollut Res 24:7679–7692 Zeng T, Zhang XL, Wang SH, Niu HY, Cai YQ (2015) Spatial confinement of a Co3O4 catalyst in hollow metal–organic frameworks as a nanoreactor for improved degradation of organic pollutants. Environ Sci Technol 49:2350–2357 Zhang J, Chen MY, Zhu L (2015a) Activation of persulfate by Co3O4 nanoparticles for orange g degradation. RSC Adv 6:758–768 Zhang J, Shao XT, Shi C, Yang SY (2013a) Decolorization of Acid Orange 7 with peroxymonosulfate oxidation catalyzed by granular activated carbon. Chem Eng J 232:259–265 Zhang LJ, Su ZX, Jiang FL, Yang LL, Qian JJ, Zhou YF, Li WM, Hong MC (2014) Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions. Nano 6:6590–6602 Zhang ZZ, Wang BD, Zhu CM, Gao P, Tang ZY, Sun NN, Wei W, Sun YH (2015b) Facile one-pot synthesis of mesoporous carbon and Ndoped carbon for CO2 capture by a novel melting-assisted solventfree method. J Mater Chem A 3:23990–23999 Zhang LP, Xia ZH (2011) Mechanisms of oxygen reduction reaction on nitrogen-doped graphene for fuel cells. J Phys Chem C 115: 11170–11176 Zhang T, Zhu HB, Croue JP (2013b) Production of sulfate radical from peroxymonosulfate induced by a magnetically separable CuFe2O4 spinel in water: efficiency, stability, and mechanism. Environ Sci Technol 47:2784–2791 Zhao X, Cai ZQ, Wang T, O’Reilly SE, Liu W, Zhao DY (2016) A new type of cobalt-deposited titanate nanotubes for enhanced photocatalytic degradation of phenanthrene. Appl Catal B Environ 187:134–143 Zheng FC, Yang Y, Chen QW (2014) High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework. Nat Commun 5:5261 Zhou J, Xiao JH, Xiao DX, Guo YG, Fang CL, Lou XY, Wang ZH, Liu JS (2015) Transformations of chloro and nitro groups during the peroxymonosulfate-based oxidation of 4-chloro-2-nitrophenol. Chemosphere 134:446–451