In-situ carbonization of ZIF-67 to fabricate magnetically Co/N-mC with high adsorption capacity toward water remediation

Co and N co-modified mesoporous carbon composites (Co/N-mC) have been simply prepared at low cost from the carbonization of ZIF-67 wherein not only providing surface N-modification for promoting the adsorption behavior of acidic dyes, but also constructing a magnetic property for convenient separation. Co/N-mC composite presented the adsorption ability of MO (178 mg g−1) > RhB (141 mg g−1) (T = 25 °C, pH = 7.0, C0 = 20 mg L−1) because of its spacious porous structure (BET: 250.8 m2 g−1; pore size: 3.9 nm) and functional basic character (pyrrolic-N and pyridinic-N). Compared with basic dyes (RhB), mesoporous C doped with N improves the adsorption of acid dyes (MO). In addition, the magnetic properties introduced in the Co/N-mC make it easier to separate after adsorption and regeneration by an external magnetic field.


Introduction
Water is an important resource for all living things on our planet. However, water quality was affected by human activities, leading to a serious threat to water source. Water pollution and water recycling have become global concerns in recent years [1]. At present, a great quantity of techniques and methods have been explored and applied to remediate the polluted water, including photocatalytic degradation [2], adsorption [3] and electrochemical precipitation [4]. Among them, adsorption method which adopting adsorbent to adsorb target pollutants to achieve the purpose of pollutant removal to remediation of wastewater to obtain purified drinking water has been determined as high efficiency, easy operation, energy economic, environmentally benign and cost-effective [5]. Many adsorbents have been investigated, such as activated carbon [6], polymers [7], MOF/ZIF-based nanomaterials [8][9][10], and so on. Commercial activated carbon is most commonly used in sewage treatment projects, but due to its rich microporous nature, there are great challenges and difficulties in the transport and adsorption for macromolecular pollutants. And the MOFs powder suspended in solution are not favor for its separation recyclability [10]. Therefore, choosing the right adsorbent is an important prerequisite and guarantee for realizing an efficient adsorption process.
High-efficiency and low-cost adsorbents are the cornerstones of an ideal adsorption process. And the structure of adsorbents are one of the main factors affecting adsorption efficiency. Magnetic-containing mesoporous carbon composites are widely used in the field of pollution control for the benefits of mesopore structures, tunable pore sizes, easy separation and reusability of adsorption materials [11]. For the introduction of magnetic component, the commonly used layer-by-layer assembly method has many complicated steps, consumes too long time, and thus restricts industrial production [12]. In addition to the internal pore structure of the adsorbent, the external surface characteristics of the adsorbent also play an irreplaceable role during the process of adsorption because the outer surface of the adsorbent is in direct contact with the target pollutants [13]. Various strategies including chemical vapor deposition [14], ammonia post-treatment [15], high-temperature carbonization of nitrogen-containing molecules [16] and so on have been implemented for the formation of nitrogen-containing groups. Thus, exploring simple synthesis and introducing magnetism and N-functionalization at the same time will greatly contribute to the broad application prospects of mesoporous carbon.
Metal organic frameworks (MOFs) have a regular interval between metal atoms (clusters) due to the existence of organic ligand frameworks, so there is a tendency to generate easily dispersed metal nanoparticles, which can avoid the aggregation of nanoparticles during calcination, resulting in high dispersed porous carbon@metal nanoparticles [17,18]. Particularly, by using magneticcontaining MOFs as precursors, Li et al. directly obtained magnetic-containing metal/carbon composites through a simple one-pot in-situ pyrolysis method, which exhibits largely enhanced electrocatalytic ORR performances [19]. Moreover, MOFs can be converted into N-doped structures through direct pyrolysis. Zhou et al. developed a feasible and effective strategy to fabricate hollow MOF-derived N-doped carbon microspheres with fascinating structural performance and outstanding properties as supercapacitors [20]. Based on the above-mentioned characteristics, it can be seen that the MOF-based method has broad application prospects in the design of composites which containing layered carbon and magnetic nanoparticle. Zeolitic imidazole frameworks (ZIFs), a subclass of MOFs, composed of Co ion and imidazole linker has caused extensive research in recent years and have now become an efficient matrix for dispersed magnetic metal Co nanoparticles [21]. Thus, magnetic porous carbon derived from Co-based ZIF-67 hold great promise for remediation of wastewater because of its excellent stabilities in water, rich in carbon, large surface area, N functionalities and magnetic Co metal sites.
In this contribution, Co and N co-modified mesoporous carbon composites (Co/N-mC) have been simply prepared at low cost from the carbonization of ZIF-67 wherein not only providing surface N-modification for promoting the adsorption behavior of acidic dyes, but also constructing a magnetic property for convenient separation. Co/N-mC was comprehensively applied to the restoration of simulated dye wastewater projects wherein acid methyl orange (MO) and basic rhodamine B (RhB) dyes were selected, including the effect of time, solution pH and dye concentration. In addition, the magnetic properties introduced in the Co/N-mC make it easier to separate after adsorption and regeneration by an external magnetic field.

Adsorbent preparation
0.4 g Co(NO 3 ) 2 ·6H 2 O and 1.8 g 2-methylimidazole were respectively dissolved into 40 mL methanol. After completely dissolved, mixed the two solutions above and kept stirring for 12 h. Soon afterwards, the solid products were separated by filtration, dried and then calcined in Ar at 900 °C for 3 h. The obtained sample was designated as Co/N-mC.

Characterization
Scanning electron microscopy (SEM) tests were recorded on a Hitachi Su-8010. X-ray photoelectron spectroscopy (XPS) tests which using Al Kα X-ray as the excitation source were performed on Phi Quantera spectrometer. Transmission electron microscopy (TEM) tests were carried out on a JEM-2010 electron microscope. Nitrogen isotherms were collected on ASAP 2020 volumetric analyzer at 77 K. Powder X-ray diffraction was conducted on Bruker D8 Advance with Cu-Kα radiation under 40 kV and 40 mA. The magnetization curve analysis was tested on a Quantum Design MPMS-7 SQUID at 300 K. Water contact angle (WCA) were measured with an OCA20 system to consider the hydrophilic and hydrophobic properties of samples. UV-vis spectroscopy analysis were performed on a Perkin Elmer Lambda 750.

Adsorption tests
Co/N-mC was comprehensively applied to the restoration of simulated dye wastewater projects wherein acid methyl orange (MO) and basic rhodamine B (RhB) dyes were selected as target molecules. The adsorption experiments were carried out in flasks containing 100 mL of dyes with different initial concentrations and 10 mg of adsorbent. Adjust temperature and pH, take solutions at different times. Then filter the solution and determine the residual concentration by UV-Vis spectroscopy. The adsorbed capacity was calculated through equation of wherein, C 0 and C t is the initial and remaining solution at each time (mg/L), respectively, V is the volume of dye solution (L) and m s is the weight of adsorbent (g).

Adsorbent regeneration
In order to explore the regeneration ability of the adsorbent further, the Co/N-mC adsorbent carrying the dye is separated with an external magnet and transferred to an ethanol solution to release the adsorbed dye. Specifically, the dye-loaded Co/N-mC adsorbent was dispersed into 100 mL ethanol and kept stirring for 30 min. Repeat this step more times until the solution is colorless, clear and transparent. Subsequently, the Co/N-mC adsorbent was washed, dried, weighed quality and then reused for another run.

Results and discussion
The morphology of ZIF-67 and Co/N-mC samples was firstly clarified through SEM and TEM, respectively. Both the images of SEM and TEM of ZIF-67 show that the ZIF-67 nanocrystals in polyhedral shape with sizes basically from ≈50 to ≈100 nm ( Fig. 1a and c). Co nanoparticles were well-dispersed on the surface of Co/N-mC, but the morphology was not changed compared with ZIF-67 (Fig. 1b).
The uniform distribution of Co nanoparticles has been further proven by TEM for one representative Co/N-mC, in which Co species are existed and highly dispersed (Fig. 1d). Moreover, in the magnified TEM (Fig. 1e), it was obviously seen that the sizes of spherical Co particles are 10-20 nm. The elemental mapping of C, N and Co screened and given by the spectroscopy of EDX on Co/N-mC shows that the N and Co elements are evenly distributed on the matrix of porous carbon (Fig. 1f ).
Powder XRD pattern which 2θ between 5° to 80° were performed to disclose the compositions of ZIF-67 and Co/N-mC composites. The characteristic peaks of ZIF-67 (Fig. 2a) [22]. After calcined at 900 °C in Ar atmosphere, as revealed in Fig. 2b, the characteristic diffraction peak situated at 2θ of 26° is assigned to (002) diffraction of graphitic carbon, suggesting that the synthesized ZIF-67 material has been carbonized to form graphitic carbon. [23]. And the ones at 2θ of 44.3°, 51.7° and 76.0° are assigned to (111), (200) and (220) diffractions of metallic cobalt originated from the Co nanoparticles embedded in carbon shell (JCPDF No.15-0806) [24], respectively. The results implying that Co/N-mC composites have been simply produced by high-temperature calcination of ZIF-67.
The surface element composition of Co/N-mC composites and the bonding between elements are tested and analyzed by XPS technology. As the XPS survey scan spectrum displayed in Fig. 3a, the characteristic signal peaks of C, N as well as Co can be clearly observed at relevant specific electron binding energy. The high-resolution C 1 s spectrum presents five important types of carbon species at 284.6, 285.4, 286.2, 287.1 and 288.6 eV, which respectively corresponded to delocalized C=C bond, localized C=C bond, C-N (or C=N), C=N (or C=O) and carbonyl C-O [25]. The high-resolution XPS spectra for N 1 s in Co/N-mC were fitted into three peaks by using XPSPeak software at round 400.8, 399.2 and 398.5 eV, assignable to pyrrolic-N, Co-N x and pyridinic-N [26]. It has been reported that the pyridinic or pyrrolic N which successful involvement into carbon materials were very important chemically active sites thereby can play crucial roles in the adsorption process. As demonstrated obviously from Fig. 3d, the characteristic signal peaks at electron binding energy of 793.4 eV and 778.5 eV were owing to Co 2p1/2 and Co 2p3/2 [27]. In particular, for Co 2p3/2, it is worth noting that three characteristic signal peaks at 778.5, 780.5 and 782.2 eV were fitted in which the peak of 778.5 eV ascribed to the existed metallic Co, the peak of 780.5 eV according to the existence of CoO x or CoC x N y and the peak of 782.2 attributed to Co-N x [28,29]. This results suggested that the composed Co nanoparticles were partly doped into the N-mC although it seem only to be able to identify metallic Co phase in the XRD experiments. Hence, Co existed in the Co/N-mC composites by the forms of the composed granular Co and doped Co.
BET surface area and the characteristic parameter of porous structure for ZIF-67 and Co/N-mC composites were conducted and supplied through the measurement of N 2 adsorption-desorption isotherms as revealed in Fig. 4, which shows type I-IV hybrid isotherms according to the IUPAC classification, reveals the coexistence of micropore and mesopore [30]. For ZIF-67, as shown in Fig. 4a, the total surface area is 1254.5 m 2 g −1 including the micropores (1163.4 m 2 g −1 ) and mesopores (91.1 m 2 g −1 ), simultaneously the pore size is distributed at 0.63 nm and 0.99 nm, which is very unfavorable for the adsorption of macromolecular dyes. The ZIF-67 as templates/precursors underwent pyrolysis at 900 °C under Ar to afford Co/N-mC (Fig. 4b). And the BET surface area for mesopores of Co/N-mC was 250.8 m 2 g −1 which would be beneficial to the exposure of active sites and the mesoporous sizes of Co/N-mC emerged at 2.3 and 3.9 nm which would be conducive to the rapid adsorption of macromolecular dyes. The magnetic behavior of the present Co/N-mC composites in presence of other Co-containing phases was recorded and displayed in Fig. 5. The saturation magnetization of the present Co/N-mC composites was 34.1 emu g −1 , which is conducive to the separation of adsorbent through an external magnetic field from the solution after adsorption as well as attractive in actual sewage treatment projects [31]. In addition, a photo (inset in Fig. 5  present Co/N-mC composites dispersed in water is shown, which presents the macroscopic magnetic behavior under an external magnetic field. Combined the results above, Co/N-mC composites with N-modification and magnetic property have been simply produced by high-temperature calcination of ZIF-67. Water contact angle (WCA) were measured to consider the hydrophilic and hydrophobic properties of Co/N-mC composites shown in Fig. 6. The contact angle of water droplets on the surface of the Co/N-mC composites is 65 ± 3°, indicating that the Co/N-mC composites is hydrophilic, which is conducive to the adsorption of hydrophilic dyes.
Contact time is an vital factor for evaluating the adsorption performance of a material [32]. The relationships between contact time and capacities adsorbed onto Co/N-mC are shown in Fig. 7. Both MO and RhB dyes were absorbed with a extremely high speed by Co/N-mC composites within 20 min. And then, adsorption capacity over Co/N-mC increased extremely slow and reached the equilibrium within 60 min. The adsorption behavior of Co/N-mC can be analyzed from three aspects, including active sites, dye concentration and steric hindrance. Specifically, the large surface area (250.8 m 2 g −1 ) for mesopores and the wide pore size (3.9 nm) of Co/N-mC could be beneficial to the exposure of active sites, thus effectively and rapidly adsorption of dyes within the first 20 min. As the adsorption time increases (from 20 to 60 min), the amount of adsorbed dye over Co/N-mC increases, and the dye concentration in the solution decreases, resulting in a smaller driving force for dye to be absorbed by the Co/N-mC adsorbent. At the same time, the adsorbed dye molecules over Co/N-mC not only occupy a large number of active sites but also cause a certain steric hindrance, which further limit the rapid adsorption of dye molecules. After 60 min, almost no change can be observed in the adsorption capacity (from 60 to 180 min) over Co/N-mC, indicating that the adsorption had reached equilibrium. The maximum adsorption capacity of Co/N-mC adsorbent for MO and RhB respectively is 178 mg g −1 and 141 mg g −1 . As listed in Table 1, compared with magnetic porous carbon [33], CMK-3 [34], magnetic biochar [35], N-doped carbons [36] and other types of adsorbents [37][38][39], Co@N-mC composites presented higher adsorption activity for RhB or MO. The adsorption capacity over Co@N-mC composites was sequenced by MO > RhB might be caused by (1) small sized MO need shorter time but adsorption amount might be increased which the porosity of carbon plays a leading role. The higher mesoporosity over Co@N-mC composites supplies supernumerary paths for the migration and diffusion of small-scale MO, as well as the larger surface area over Co@N-mC composites furnishes more abundant adsorption sites to anchor MO; (2) Alkaline Co/N-mC adsorbent is good for adsorbing acid MO dyes. Specifically, the pyridine or pyrrole N existed in Co@N-mC can produce H-bonds, thereby interact with the adsorbed dyes, which is one of the most important mechanisms for the removal of organic pollutants [40]. Moreover, the pyridinic/pyrrolic N which presence in Co@N-mC composites would adjust the chemical stability of carbon by increasing electron density around carbon thereby generate more adsorption sites on Co@N-mC for anchoring dyes [41]. The pH of the solution is a vital parameter during the adsorption process and significantly affected the adsorption extent [42]. The existence form of dyes are closely related to the acidity of the aqueous solution, thus cannot be ignored in the adsorption experiment. Figure 8 exhibited the adsorption behavior as a function of pH over Co/N-mC composite. For cationic dyes RhB, the adsorption capacity is positively correlated with pH, that is, it has a high adsorption capacity under high pH conditions. This phenomenon is due to the different protonation degree of the cationic dye at different pH. In particular, excess H + at low pH makes RHB protonate, thereby repelling Co/N-mC adsorbents that are also positively charged on the surface, resulting in a relatively low adsorption capacity [43]. The increase in pH makes the surface of the Co/N-mC adsorbent become electronegative, attracting each other with the cationic dye RhB under the electrostatic action, resulting in a relatively high adsorption capacity [44]. Nevertheless, the observed adsorption tendency of anionic dyes MO at different pH is opposite to above mentioned RhB. The maximum adsorption capacity to MO over Co/N-mC adsorbent emerged at low pH, in which Co/N-mC adsorbents are positively charged on the surface, thus attracting with the anionic MO under the electrostatic action. With the pH increase, the enhanced OHand more electrostatic repulsion decreased the removal efficiency to MO [45].
As discussed above, the initial degree of the dye is an important factor as well as a key driving force in the adsorption process where dye to be absorbed by the adsorbent. Thus, we further investigated the effects of dye concentration to obtain the adsorption behavior over Co/N-mC composite (Fig. 9). It is very obvious that with the initial dye concentration in ascending order from 10 to 50 mg L −1 , the adsorption capacity at equilibrium were corresponding enrichment from 98 to 193 mg g −1 for RhB and 123 to 242 mg g −1 for MO adsorptions. However, the adsorption rate and concentration do not show a proportional linear relationship. Adjust and change the initial concentration to 20 from 10 mg L −1 , the rapid increase in adsorption capacity corresponds to a higher adsorption rate, but when the  initial concentration is greater than 20 mg L −1 , the slow increase in adsorption capacity corresponds to a lower adsorption rate. This may be related to the diffusion of the dye in the adsorbent and the adsorption kinetics.
In the actual sewage restoration project, the recovery and reuse of adsorbents is a core technology. [46]. Therefore, the cyclic experiment of Co/N-mC composite was further explored and the results were shown in Fig. 10a, the adsorption capacity to RhB and MO remained basically unchanged after 5 cycles, which demonstrated a outstanding recycling stability of Co/N-mC composite. Moreover, the XRD of Co/N-mC after 5 cycles stability testing was given in Fig. 10b. XRD analyses demonstrate that the transform of crystalline structure for Co/N-mC was negligible. Further we examined the morphology change of Co/N-mC after the 5 cycles measurement. As presented in Fig. 10c, the morphologies are well retained, indicating the good structural stability. Co/N-mC composite material currently has high-efficiency adsorption performance, easy separation, good regeneration ability and good structural stability thus would be a promising adsorbent for organic dyes in the polluted water.

Conclusions
Co/N-mC composites have been simply prepared by carbonization of ZIF-67 wherein not only providing surface N-modification (pyrrolic-N and pyridinic-N) for promoting the adsorption behavior of acidic dyes, but also constructing a magnetic property (34.1 emu g −1 ) for convenient separation. The adsorption performance of a Co/N-mC was affected by contact time, pH and initial dye concentration. Co/N-mC composite presented the adsorption ability of MO (178 mg g −1 ) > RhB (141 mg g −1 ) (T = 25 °C, pH = 7.0, C 0 = 20 mg L −1 ) because of its spacious porous structure (BET: 250.8 m 2 g −1 ; pore size: 3.9 nm) and functional basic character (pyrrolic-N and pyridinic-N). The high pH is beneficial for RhB but the opposite phenomenon was investigated for MO. The high initial dye concentration resulting in a stronger driving force for dye to be absorbed by the Co/N-mC adsorbent. In addition, the magnetic properties introduced in the Co/N-mC make it easier to separate after adsorption and regeneration by an external magnetic field.