Rapid Adsorption Removal of Triclosan and p-chloro-m-xylenol In Water By Nitrogen Doped Magnetic Porous Carbon

Contamination of water resources with organic substances like phenolic fungicides is undesirable due to the improvement of living standards, huge production and consumption of daily chemicals, and an increase in the population. In this study, ZIF-67(Co) was synthesized using the “one pot method”, and the Co-magnetic porous carbon (Co-NPC) was prepared by ZIF-67 (Co) carbonization in an atmosphere of N 2 . The materials were tested using a X-ray diffractometer (XRD), scanning electron microscope (SEM), infrared spectroscopy (IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), N 2 adsorption-desorption and magnetization analysis. These methods indicated that the Co-NPC was successfully prepared. With the original morphology of ZIF-67 (Co) crystal, the Co-NPC also has good porosity, magnetic properties and a large specic surface area. In water, Co-NPC-800 has a good adsorption capacity for triclosan (TCS) and p-chloro-m-xylenol (PCMX), which are kinds of aromatic fungicides. The adsorption of Co-NPC-800 on both reached equilibrium within 3 min, which is in accordance with the quasi-second-order kinetic model. At 298 K, the maximum adsorption capacity of Co-NPC-800 for TCS and PCMX was 163 and 39 mg·g -1 , respectively. The adsorption of TCS and PCMX by Co-NPC-800 is a spontaneous endothermic process with reduced entropy. The combination of Co-NPC-800 and phenols come from multiple action of electrostatic, π-π and hydrogen bond effects. Moreover, Co-NPC-800 can be regenerated through simple washing and can be reused several times. Therefore, Co-NPC-800 has great potential to be applied across sewage treatments and other environmental elds.


Introduction
In recent years, the environment has faced a massive threat due to the continuous growth of the world population, high energy consumption and toxic waste emissions (Bao &Fang 2012). Various contaminants have been detected in aquatic environments, including pesticide residue, heavy metals, phenolic fungicides amongst others. Phenolic fungicides, widely used in daily chemicals, are usually aromatic phenols (Yoo et al. 2018). Triclosan (TCS) and p-chloro-m-xylenol (PCMX) are two common phenolic fungicides containing chlorine, which is believed to cause continuous pollution to the environment because they contain chlorine atoms (Ng et al. 2019). Furthermore, chlorine compounds are harmful to the human body (Tan et al. 2021). As such, the removal of phenolic fungicides from the environment has become a hot research topic (Yu et al. 2020).
In previous studies, there many methods have been used to remove phenolic fungicides in the liquid phase, including biological oxidation (Martín et al. 2008 incineration (Oturan et al. 2009), coagulation (Wada et al. 1995) and adsorption (Ahmaruzzaman 2008).
In particular, adsorption has been commonly used due to the fact it is simple to operate, requires low energy and is highly effective (Zhang et al. 2015). In the eld of environmental science, carbon-based materials including activated carbon, carbon nanotubes, grapheme and biomass-derived carbon have been actively developed for highly e cient absorbency (Apul et al. 2013, Meng et al. 2021). Due to the excellent properties of porous carbon materials, nanoporous carbon materials (NPC) in particular have attracted the attention of many researchers (Lee et al. 2004, Yao et al. 2017. NPC have a high speci c surface area, which is bene cial for the loading of nanoparticles and the modi cation of functional groups (Vakili et al. 2021). This leads to improvements in the catalytic performance and adsorption performance of the carbon material (Liu et al. 2015). In order to achieve high adsorption performance, the researchers used all kinds of precursors to prepare the carbon materials including biomass, agricultural residue, industrial waste and metal-organic frameworks (MOFs)(Sule &Mishra 2020, Teimouri et al. 2019). MOFs are the most widely studied advanced porous materials due to their high speci c surface area and porosity. After high-temperature carbonization of MOFs, porous carbon materials can be derived (Chaikittisilp et al. 2013). The porous carbon material prepared with MOFs as template retained the porosity and large speci c surface area of MOFs, and was a good adsorbent and catalyst carrier ).
Although MOFs-derived porous carbons are promising materials, their performance can be further improved by introducing dopants such as nitrogen. The most commonly used method to obtain N-doped carbons is to carbonize MOFs loaded with nitrogen-containing compounds (Chen et al. 2018). Bio-MOF-1derived carbon, based on Zn 8 (adenine) 4 (biphenyldicarboxylate) 6 O, has shown the highest adsorption capability of bisphenol-A (BPA) in water (Bhadra et al. 2018). ZIF-mediated N-doped hollow porous carbon, hardly affected by humic acid, has been used as a high performance adsorbent to remove tetracycline from water (Liang et al. 2020). However, adsorption technology has several downfalls such as having an insu cient capacity development, a high cost and being relatively di cult to recycle (Crini et al. 2019).
In order make carbon materials more easily recyclable, researchers have developed a variety of methods including incremental weight (Kasmi et al. 2017), use of core-shells (Gupta et al. 2014, Lv et al. 2019) and adding magnetism (Oladipo et al. 2018). In N-MCFs (magnetic particle-decorated N-doped carbon foams), it was found that the distorted framework caused by N-doping led to the distortion of graphite lattice and the change in the local density of the carbon atomic state. This signi cantly contributed to the magnetic moment (Lou et al. 2021). In this work, ZIF-67 (Co), a MOF material with a zeolite structure was prepared. Co-NPC, a porous carbon material mixed with nitrogen elements, was prepared using ZIF-67 (Co) carbonization at a high temperature within a N 2 atmosphere. By controlling the carbonization temperature, the cobalt element in the ZIF-67 (Co) framework remained in the porous carbon material obtained, in the form of cobalt oxide and this created the material's magnetism. The resultant magnetic porous carbon material was used for the adsorption and removal of TCS and PCMX in the water. Notably, after nitrogen doping, the material had more adsorption sites and excellent adsorption performance. The magnetic properties of the material facilitated its recycling, which has the potential for broad application within the environmental eld.
2. Experiment 2.1 Synthesis of ZIF-67(Co) A total of 5.5 g of 2-methylimidazole and 20 mL of ultrapure water were added to a 50 mL beaker, stirred until dissolved and recorded it as solution I. A total of 0.45 g of Co(NO 3 ) 2 Ÿ6H 2 O and 3 mL of ultrapure water were added into a 10 mL centrifuge tube, dissolved and recorded it as solution II. Solution II was slowly added to solution I and stirred with high speed for 6 h at room temperature. Subsequently, the centrifuge tube was centrifuged at 8000 rpm for 5 min. Then, the product was taken out and washed alternately with ultrapure water and ethanol. After that, the product was soaked in an ethanol bath at 60°C for 6 h, washed with ethanol for 3 times, dried and activated under vacuum at 100°C for 24 h.
2.2 Synthesis of Co-NPCs 0.5 g of the obtained ZIF-67 (Co) sample was placed in a 15 ml porcelain crucible, and then the crucible was calcined in a quartz tube furnace in N 2 or Ar atmosphere. The sample was heated up to 600°C, 700°C, 800°C with 5 o C·min −1 heating rate and calcined at the target temperature for 3 hours. After it was cooled down, the obtained porous carbon materials (Co-NPCs) were named as Co-NPC-600, Co-NPC-700, Co-NPC-800, Co-NPC-800 (Ar) according to the atmosphere and temperature.
2.3 Adsorption performance and reusability of Co-NPCs 400 µL of different materials (Co-NPC-600, Co-NPC-700, Co-NPC-800, Co-NPC-800 (Ar)) suspension, 1 mL of phosphate buffered saline solution (PBS, pH = 7.0), 200 µL of TCS stock solution and 400 µL ultrapure water (or 100 µL of PCMX stock solution and 500 µL ultrapure water) were added in a 2 mL centrifugal tube. The adsorption was carried out for 20 min in a 298 K water bath. Then, the supernatant was separated with a magnet. The concentrations of TCS/PCMX in the supernatant were analyzed by an UVvisible spectrophotometer at 280 nm. The amount of TCS/PCMX adsorbed on different Co-NPCs were calculated from the expression: Where q e was the amount adsorbed at equilibrium (mg·g −1 ), C 0 and C e were the initial and equilibrium concentrations of pollutant in solution (µg·mL −1 ), respectively, m was the dosage of sorbent (mg), and V was the volume of the solution (mL).
Washed the separated Co-NPC-800 with ultrapure water and ethanol alternately until TCS or PCMX was no longer detected in the washing solution. Dried the recovered Co-NPC-800 under vacuum for 12 h at 100°C. Afterward, repeated the adsorption procedures for 4 times, and calculated the adsorption capacity in different cycle.

Adsorption kinetics
Two conventional kinetic models (quasi-rst-order and quasi-second-order) were applied to analyze the experimental data. The quasi-rst-order model can be expressed as: ln q e − q t = lnq e − k 1 t Where q e and q t were the adsorbed pollutant amounts on Co-NPC at equilibrium and at various time t (mg·g −1 ), respectively; t was the adsorption time (min), and k 1 was the rate constant of the quasi-rstorder model of adsorption (min −1 ). The values of q e and k 1 could be determined from the intercept and slope of the linear plot of ln (q e -q t ) versus t.
The quasi-second-order model includes all the steps of adsorption including external lm diffusion, adsorption, and internal particle diffusion, which is described as: Where q e and q t were the adsorption capacity (mg·g −1 ) at equilibrium and at any time t (min), respectively; k 2 was the rate constant of the quasi-second-order model of adsorption (g·mg −1 ·min −1 ). The values of q e and k 2 were determined from the slope and intercept of the plots of t/q t against t.

Adsorption thermodynamics
The thermodynamic parameters such as standard free energy change (ΔG 0 ), enthalpy change (ΔH 0 ) and entropy change (ΔS 0 ) were determined based on the following equations: Where K was the adsorption equilibrium constant, C s was the amount of pollutant adsorbed by per mass of Co-NPC-800 (mg·g −1 ), C e was the equilibrium concentration of the target pollutant (mg·L −1 ), R was the universal gas constant (8.314 J·mol −1 ·K −1 ), and T was the absolute temperature (K).

Adsorption isotherms
To further investigate the mechanism of equilibrium adsorption, the results were subjected to Langmuir and Freundlich isothermal models for simulation. The Langmuir model assumes that adsorption occurs on a homogeneous surface by monolayer coverage and no subsequent interaction between adsorbed species. The equation was as follows: ( ) C e q e = C e q max + 1 K L q max Where q e was the adsorbed pollutant amount on Co-NPC at equilibrium (mg·g −1 ), C e were the initial and equilibrium concentrations of pollutant in solution (mg·L −1 ); K L was the Langmuir constant (L·mg −1 ), which was related to the a nity of the binding sites; and q max represents the maximum adsorption capacity of the adsorbent (mg·g −1 ). The values of q max and K L were calculated from the slope and intercept of the linear plot of C e /q e against C e .
The Freundlich model was an empirical model based on multilayer adsorption on heterogeneous surfaces. The equation was commonly described as: Where q e and C e were de ned as in the Langmuir isotherm; K F and n were the Freundlich constants that represent the adsorption capacity and adsorption strength, respectively. K F and n could be obtained from the intercept and slope of the linear plot of ln q e versus ln C e . Moreover, the magnitude of 1/n quanti es the favorability of adsorption and the degree of heterogeneity of the surface. If n > 1, suggested favorable adsorption.

Characterization of ZIF-67 (Co)
The lattice data of synthesized ZIF-67 (Co) was analyzed using XRD. As shown in Figure  The morphology of ZIF-67 (Co) was scanned using SEM. At 10000 times magni cation ( Figure S3A), it could be clearly seen that a large number of ZIF-67 (CO) particles were closely packed together. At a greater magni cation, the shape of ZIF-67 (Co) is similar to a rhombic dodecahedron or a 'chamfered cube' (Figure S3B-C). It was seen that the morphology of ZIF-67 can be changed by adjusting the molar ratio of ligand to metal ion, and the synthesis time. The rhombic dodecahedron grew when the molar ratio of metal ion to ligand was low (Cravillon et al. 2012). In this study, 1:45 was the molar ratio of metal ion to ligand used to form the rhombic dodecahedron ( Figure S3D). This resulted in a favorable formation of ZIF-67 (Co) crystal. Additionally, the choice of metal ion salt could directly affect the size of ZIF (Shao et al. 2014). When cobalt nitrate was used as the source of cobalt, the ZIF-67 (Co) obtained in our work was an average size of approximately 0.5 µm.
The functional groups on the surface of ZIF-67 (Co) were analyzed using infrared spectroscopy ( Figure   S4). 2-methylimidazole, a ligand found exclusively in the ZIF-67 (Co) structure, was used as a standard comparison because it provided most of the infrared absorption signal of ZIF-67 (Co) (Lashgari et al.

Characterization of Co-NPCs
As shown in Figure S5, the XRD characterization was used to obtain the diffraction signal of the synthesized Co-NPC-800 and Co-NPC-800 (Ar). Compared to the XRD spectrum of the original ZIF-67 (Co), a lower resolution and wider particle peaks were observed after carbonization. Meanwhile, there were three obvious diffraction peaks found in both Co-NPC-800 and Co-NPC-800 (Ar), which corresponded well to the elementary substance of β-Co. Among them, 2θ = 43.9°, 51.1° and 75.4° were assigned to (111), (200) and (220) of the crystal plane's diffraction, respectively. In the XRD spectra of both samples, a relatively weak diffraction peak was found at 2θ = 25°, which belonged to the characteristic peak of the typical (002) graphite laminated structure. This result suggested that the prepared Co-NPXs were partly graphitized. In addition, we observed a faint protrusion at 2θ = 37° belonging to the characteristic peak of C O O. This may be caused by the easy oxidation, in air, of Co nanoparticles on the surface of the porous carbon materials (Ma et al. 2016).
The SEM images of Co-NPC-800 and Co-NPC-800 (Ar) are shown in Figure 2. After carbonization, the original shape of the ZIF-67 (Co) crystal remained but the surfaces of the porous carbon particles were distorted and the size of the particles were shrunk. As shown in Figure 2A-B, the surface of Co-NPC-800 (Ar) was obviously roughened, while the crystals were more concentrated. Compared to Co-NPC-800 (Ar), the surface of Co-NPC-800 was smoother, and the crystals were more dispersed. This result was attributed to the fact that defects were formed on the surface of Co-NPC-800 by nitrogen doping, which separated the crystals and made the dispersive crystals di cult to agglomerate. Additionally, more adsorption sites were available after the defects formed and a larger speci c surface area was created after the crystals separated. This increased the chance of Co-NPC-800 contacting with pollutants and caused the adsorption performance to improve.
In order to investigate the changes in the functional groups of ZIF-67 (Co) after carbonization, the infrared spectra of Co-NPC-800 (Ar) and Co-NPC-800 were analyzed ( Figure S6). It was found that the infrared spectra of Co-NPC-800 (Ar) and Co-NPC-800 were similar, indicating that their structures were basically the same. Compared to ZIF-67 (Co), Co-NPC retained the normal absorption rate of the aromatic ring in the range of 1500-600 cm −1 , which was a result of the fact the imidazole ring remained stable after high temperature carbonization. The characteristic peak at 1500 cm −1 corresponded to C=C on the aromatic ring, and the peaks at 1300-700 cm −1 were the exural vibration signals of C-H on the aromatic ring.
Absorption peaks around 3000 cm −1 were observed in both Co-NPC-800 (Ar) and Co-NPC-800, which may have been caused by the reaction of C and H at the edge of the material during pyrolysis. The broad absorption peaks near 3500 cm −1 indicated the stretching vibration of N-H or O-H. The signal of O-H may have been a result of the moisture absorbed by the material from the air. Nevertheless, the N-H peak of Co-NPC-800 was sharper than the peak of Co-NPC-800 (Ar), indicating that more N elements had been successfully introduced into Co-NPC-800 by carbonization in a N 2 atmosphere (Ma et al. 2016).
In order to explain the graphitized carbon signal in the XRD spectrum of Co-NPC, Raman spectroscopy was used to further investigate the C element in Co-NPC (Yang et al. 2021). The relative ratio of the Dband area to the G-band area (ID/IG) in the Raman spectrum is an important parameter to understand the degree of crystallization of the graphitized carbon in the materials. The D-band is a region that characteristically contains structural defects and impurities, which represent the defects and disorder of the materials. The G-band is generated by the stretching vibration of C-C, which represents the internal vibration of SP 2 hybrid carbon atoms. The amorphous degree can be expressed by ID/IG, for example, a larger ratio represents a lower degree of order in the graphite materials. The Raman spectra of Co-NPC-800 are shown in Figure S7, and ID/IG=2.34 was obtained by tting. The results indicate that Co-NPC-800 has a low degree pf graphitization, low order and many defects, which could provide more sites for the adsorption of pollutants.
The XPS was used to investigate the composition and element valence of Co-NPC-800, and to further con rm the successful doping of the N element. In the full spectrum ( Figure S8A), we could clearly observe the signals of Co, C, N and O. By tting the N1s spectrum ( Figure S8B), we discovered that it could be divided into four independent peaks and that their binding energies were 398.6, 399.3, 400.7 and 404.1 eV respectively. The results suggested that N elements in Co-NPC-800 existed in four different forms; namely pyridine, pyrrole, graphite and oxidized type. By tting the N1s spectrum ( Figure S8B), it was found that Co element existed in the forms of Co (0) (at 778.3 eV), Co 2+ (at 780.8 eV) and Co-OH (at 785.7 eV). This result was consistent with the previous XRD results and results contained in the literature (Huang et al. 2015). The existence of the oxidation state Co (Co 2+ ) veri ed the partial surface oxidation of Co nanoparticles in the XRD results. The results above showed that high temperature carbonization of ZIF-67 (Co) led to the formation of N-doped magnetic porous carbon materials (Co-NPC), whose magnetic function mainly came from the Co nanoparticles.
The speci c surface area, pore type, and adsorption capacity of Co-NPCs were investigated using a N 2 adsorption-desorption curve (Fan et al. 2020). N 2 adsorption-desorption analysis of Co-NPC-600, Co-NPC-800 and Co-NPC-800 (Ar) were carried out at 77 K. The speci c surface areas of all three materials exceeded 200 m 2 ·g −1 . Obvious H3 mesoporous hysteresis loops in the N 2 adsorption-desorption curves ( Figure S9A), indicated that mesopores formed on ZIF-67 (Co) during the carbonization. The pore size distribution of all three materials were mainly below 10 nm ( Figure S9B), and the average pore sizes were all less than 10 nm (Table 1); this con rmed that Co-NPCs were mesoporous materials. Comparing Co-NPC-600 with Co-NPC-800, it was found that the pore size, speci c surface area and total pore volume all increased alongside the increase in the carbonization temperature. This implied that the higher temperature led to the greater carbonization of ZIF-67 (Co) and more defects on Co-NPCs. Comparing Co-NPC-800 with Co-NPC-800 (Ar), it was found that the pore size, speci c surface area and total pore volume of Co-NPC-800 (Ar) were all smaller than those of Co-NPC-800, which showed that Ar was nonreactive with porous carbon materials. Fewer defects and lower parameters were obtained in Ar when compared to N 2 . Therefore, we concluded that Co-NPC-800 had greater potential in adsorption applications. As shown in Figure 3, the magnetization curves of Co-NPCs were obtained in the magnetic eld range of ±20,000 Oe, at room temperature. It can be seen that four kinds of Co-NPC each had small hysteresis loops. The magnetization of the Co-NPCs rose slowly when the magnetic eld exceeded ±10,000 Oe, which explained why Co-NPCs were soft magnetic materials. As the temperature increased, the saturation  (Figure 4). It can be seen that the in uence of temperature and atmosphere on the adsorption of TCS and PCMX were almost the same, which related to the same charge properties of both under the experimental conditions (pH=7.0). As the temperature rose, the adsorption capacity of Co-NPC-600, Co-NPC-700 and Co-NPC-800 for TCS and PCMX gradually increased, which mainly because the pore size, speci c surface area and total pore volume of Co-NPC all increased with the carbonization temperature. The contact area with pollutants and volume containing pollutants was also increased. Compared with Co-NPC-800 (Ar), the adsorption capacity of the Co-NPC-800 was signi cantly enhanced. Synthesized in a N 2 atmosphere, the Co-NPC-800 doping of N obtained more adsorption sites, which was conducive to the adsorption of TCS and PCMX. Out of all the materials, Co-NPC-800 was selected for follow-up experiments because it had the best adsorption capacity.

Effect of pH
The pH of the solution can affect the existing form of the adsorbate in the solution, and it could also affect the charge properties on the adsorbent's surface (Bernal et al. 2017). The effect of the pH on the adsorption of TCS and PCMX by Co-NPC-800 was investigated. With the increase of pH, it could be seen that the adsorption capacity of Co-NPC-800 for TCS and PCMX both increased rstly and then decreased ( Figure 5A&C). The results implied that electrostatic attraction might be the main mechanism Co-NPC-800 for the adsorption of pollutants. In order to verify this conjecture, the existing forms of TCS and PCMX were analyzed at different pH levels ( Figure 5B&D). The pKa of TCS and PCMX were 7.68 and 9.21, respectively. The TCS was mainly in form of neutral molecules in the solution when the pH was less than 7.68. Most of the molecules would carry one unit of negative charge after losing a proton where the pH was greater than 7.68. The PCMX was mainly in form of neutral molecules in the solution when the pH was less than 9.21, and most of molecules would carry one unit of negative charge after losing a proton, when the pH was greater than 9.21.
At the same time, the Zeta potential of Co-NPC-800 was investigated when the pH was between 4.0-9.0 ( Figure S10). It could be seen that the Co-NPC-800 was always negative in the whole range, and the negative charge showed a slight downward trend when the pH increased between the range of 4.0-7.0.
The negative charge showed an obvious up-trend when the pH increased through the range of 7.0-9.0. Considering the form of the pollutants and the concept of Zeta potential, if there was only an electrostatic effect impacting upon the process, the optimal adsorption pH of Co-NPC-800 for TCS and PCMX should be around 7.0. At this time, Co-NPC-800 had the lowest negative charge and the pollutants were not charged, which are the most bene cial conditions for an electrostatic effect. However, whilst the experimental results showed that best adsorption pH of Co-NPC-800 for PCMX was around 7.0, the best adsorption pH of Co-NPC-800 for TCS was around 6.0. It could be assumed that electrostatic effect was not the only mode of action in the adsorption of Co-NPC-800 to TCS, but also there were π-π and hydrogen bond effects. Under acidic conditions, the π-π effect appeared between the imidazole ring on the surface of Co-NPC-800 and the benzene ring of the TCS molecule, which benzene group could be used as a π donor or acceptor. What's more, the hydroxyl group of TCS molecules could be used as an H donor to form a hydrogen bond with the N atom on the surface of Co-NPC-800. The chlorine/hydroxyl group of TCS molecules could also be used as an H receptor to form a hydrogen bond with the H atom of Co-NPC-800. In conclusion, the optimal adsorption pH of TCS was 6.0 due to the multiple factors above.
Because of the large steric hindrance caused by two methyl groups on the surface of PCMX, π-π and hydrogen bond effects were in the minority during PCMX adsorption. As a result, the pH of PCMX adsorption remained at 7.0.

Effect of ionic strength
A high concentration of salt in industrial waste water might affect the removal of pollutants by Co-NPC.
So, the effect of solution ionic strength on the adsorption of TCS and PCMX was we studied by Co-NPC-800. As shown in Figure S11A, the ionic strength had only had a small impact on TCS adsorption. When the NaCl concentration was above 0.2 molŸL −1 , the adsorption capacity increased slightly as the NaCl concentration rose. According to the existing form of TCS and the surface charge of Co-NPC-800, the Na + between the Co-NPC-800 and TCS molecules would generate a shielding effect, which was bene cial to the interaction between adsorbate and adsorbent, and enhanced the adsorption of Co-NPC-800 to TCS.
On the contrary, the adsorption capacity of Co-NPC-800 for PCMX became weaker and tended to be stable when more NaCl was added ( Figure S11B). This result might be attributed to the fact that the adsorption of Co-NPC-800 for PCMX had, for the most part, an electrostatic effect rather than π-π effect, while NaCl would compete with PCMX for the adsorption sites on Co-NPC-800. Therefore, the binding sites of PCMX and Co-NPC-800 were partially occupied by NaCl, resulting in a decrease in its adsorption capacity. Nevertheless, under experimental conditions, the salt ion concentration had a very little effect on the adsorption of TCS and PCMX by Co-NPC-800.

Study on the Adsorption Kinetics of Co-NPC-800 on TCS and PCMX
By investigating the change of absorbance between Co-NPC-800, TCS and PCMX, the time taken to reach adsorption equilibrium could be determined. In addition, the adsorption mechanism could be better understood by tting the kinetic model. As shown in Figure 6A&D, the equilibrium time of adsorption on Co-NPC-800 (0.1 mg·mL −1 ) with TCS (20 mg·L −1 ) and PCMX (10 mg·L −1 ) were both 3 mins. Although both TCS and PCMX were highly concentrated, Co-NPC-800 showed excellent adsorptive power for these two targets.
Fitting the experimental data with the quasi-rst-order and the quasi-second-order kinetic equation respectively, the tting curves obtained are showing in Figure 6B-C&E-F, while the kinetic parameters and correlation coe cients for TCS and PCMX adsorbed by Co-NPC-800 are summarized in Table S1. It can be seen that the adsorption kinetics of Co-NPC-800 on TCS and PCMX were more in line with quasisecond-order kinetic model (R 2 >0.99), the adsorption capacity calculated by the quasi-second-order kinetic equation were closer to the experimental value as well. Above results suggested that the quasisecond-order kinetic model could be applied to the adsorption process of TCS and PCMX.

Adsorption isotherms of TCS and PCMX
The adsorption isotherms of TCS and PCMX by Co-NPC-800 were studied at 298, 308, and 318 K. The concentration range of TCS and PCMX were 8-40 and 1-25 mg·L −1 , respectively. As the concentration of TCS and PCMX increased, the adsorption capacity increased rapidly and tended to balance. Under experimental conditions, the adsorption capacity of Co-NPC-800 for TCS and PCMX also increased as the temperature rose ( Figure 7A&D). In line with the Langmuir adsorption isotherm equation and Freundlich adsorption isotherm equation, it was possible to obtain tted linear diagrams ( Figure 7B-C&E-F). The related parameters are shown in Table S2.
Only single-layer with physical adsorption was assumed in the Langmuir model. However, in the Freudlich model, single-layer and multi-layer adsorption were combined and both chemical and physical adsorption was considered. According to the tted results, it could be seen that the adsorption of TCS and PCMX by Co-NPC-800 were more in line with the Langmuir isotherm adsorption model (R 2 >0.99), while the Freundlich model t relatively poorly. The results suggested that the type of adsorption of TCS and PCMX by Co-NPC-800 was physical adsorption with a single-layer covering, which was also in line with the results of kinetics (q m ≈ q 2e ). As the temperature increased, the adsorption constant of the Langmuir model (K L ) gradually increased. The maximum adsorption capacity (q m ) of TCS increased from 163 to 263 mg·g −1 , and the maximum adsorption capacity of PCMX increased from 39 to 56 mg·g −1 . These results show that the adsorption of TCS and PCMX by Co-NPC-800 is an endothermic process.
Additionally, the adsorption constant of the Freundlich model (K F ) also increased as the temperature rose, and the value of n was greater than 1 at all the temperature, indicating that the adsorption of both pollutants by Co-NPC-800 was a preferential adsorption process.

Study on the adsorption thermodynamics of TCS and PCMX
In order to further understand the adsorption mechanism of TCS and PCMX on Co-NPC-800, we calculated thermodynamic parameters including Gibbs free energy change (ΔG 0 ), enthalpy change (ΔH 0 ) and entropy change (ΔS 0 ). The ΔG 0 value indicated the priority in the adsorption process and the magnitude of the adsorption driving force. The adsorption process could not be carried out spontaneously when ΔG 0 >0, but the adsorption process could be carried out spontaneously when ΔG 0 <0 (Tran et al. 2020). As listed in Table 2, the ΔG 0 of Co-NPC-800 adsorbing TCS and PCMX were both negative in the range of 298-318 K. This indicated that that both processes could proceed spontaneously.
Meanwhile, the ΔG 0 decreased as the temperature rose, suggesting that the temperature rise enhanced the driving force of adsorption and promoted the adsorption progress. This result also inferred that the adsorption of TCS and PCMX on Co-NPC-800 was an endothermic process. Further, the ΔH 0 >0 also proved that the two processes were endothermic. The results calculated for ΔG 0 and ΔH 0 con rmed the results of adsorption isotherm. In general, having ΔG 0 between -80 and 400 kJ·mol −1 was considered to be chemical adsorption, while ΔG 0 between -20 and 0 kJ·mol −1 was regarded as physical adsorption (Özcan et al. 2005). In this study, the ΔG 0 values of the two adsorption processes were in the range of -20-0 kJ·mol −1 , suggesting that both of them were physical adsorption. These results were also in line with the results of kinetics and adsorption isotherms. The ΔS 0 value re ects a change in system disorder. It was a process of entropy increase when ΔS 0 >0, while it was a process of entropy decrease when ΔS 0 <0. The ΔS 0 was below 0 in the adsorption process of Co-NPC-800 on TCS and PCMX, indicating that, after adsorption, the disorder of the system was reduced.
The massive desorption of water molecules arranged on the surface of Co-NPC-800 would increase the disorder of the entire system and increase the entropy in the adsorption process. However, when TCS and PCMX were adsorbed from the solution to the solid-liquid interface of the adsorbent, some translational and rotational degrees of freedom were lost, which led to a decrease in entropy. The entropy decrease in this process was greater than the entropy increase caused by the massive desorption of water molecules. Therefore, overall, the entire adsorption process showed a decrease in entropy.

Adsorption mechanism
As mentioned above, the adsorption of Co-NPC-800 to TCS and PCMX was mainly physical adsorption, which included electrostatic, π-π and hydrogen bond effects. Electrostatic interaction is one of the most common. In order to further verify the role of π-π and the hydrogen bond, an infrared spectrometer was used to investigate Co-NPC-800 after adsorption of TCS and PCMX. As shown in Figure 8A&B, obvious infrared absorptions at 1500-700 cm −1 , 3000 cm −1 and 3500 cm −1 were observed. Compared with the spectra of pure TCS and PCMX, the characteristic peaks were weaker, despite the fact it implied that TCS and PCMX were successfully adsorbed on Co-NPC-800. The slight shift of the characteristic hydroxyl peak at 3500 cm −1 was attributed to the formation of a hydrogen bond, which is between the N atom on the surface of Co-NPC-800 and the hydroxyl groups in the TCS/PCMX molecules. The characteristic peak of the C=C skeleton vibration in the aromatic ring (about 1600 cm −1 ) also shifted slightly, and became wider after adsorption. This could be attributed to the π-π effect between TCS/PCMX molecules and the imidazole ring that remained after carbonization. Due to the large steric hindrance in the PCMX molecule caused by multiple substituents on the benzene ring, the π-π effect between Co-NPC-800 and PCMX was suppressed. So, the adsorption capacity of Co-NPC-800 for PCMX was inferior to that of TCS, which meant the steric hindrance was lesser.
In addition, the hydrogen bond acceptors and donors of TCS and PCMX at different pH were analyzed, and the results are shown in Figure 8C&D. There were one hydrogen bond acceptor and two hydrogen bond donors in TCS when pH<6.0. This was consistent with the best adsorption capacity of Co-NPC-800 to TCS at pH=6. The results con rmed that the adsorption of Co-NPC-800 to TCS had several modes of action. For example, there was one hydrogen bond acceptor and two hydrogen bond donors in PCMX when pH<7.0. These were consistent with the best adsorption capacity of Co-NPC-800 to PCMX at pH=7.0. The results indicated that the hydrogen bond was also important in the adsorption of Co-NPC-800 to PCMX. The mechanism diagram of the adsorption on Co-NPC-800 to TCS and PCMX was drawn according to above-mentioned experimental and theoretical results (Figure 9). Co-NPC-800 was also highly able to remove TCS and PCMX due to the combined of electrostatic, π-π and hydrogen bond effects.

Investigation on the reusability
As shown in Figure S12, the adsorption capacity of Co-NPC-800 for TCS remained above 150 mg·g −1 even after being recycled 5 times, showing that Co-NPC-800 has good reusability. However, the adsorption capacity of Co-NPC-800 for PCMX decreased from 40 to 25 mg·g −1 after being recycled 5 times. This could be explained by the fact that the adsorption capacity of Co-NPC-800 for PCMX was originally small, and the sites where Co-NPC-800 adsorbed PCMX were partially deactivated during the cycles.

Application of Co-NPC in actual water samples
In order to verify the ability to practically apply this method in this work, we investigated the removal performance of Co-NPC-800 for TCS and PCMX in actual water samples. Both TCS and PCMX were not detected in the collected lake water, river water and tap water, so TCS with spiked concentration of 20 mg·L −1 and PCMX with spiked concentration of 10 mg·L −1 were added in the three water samples. The results of pollutant absorption are shown in Figure 10. The maximum adsorption capacity of Co-NPC-800 for TCS and PCMX in the three water samples was close to the theoretical values, which were about 160 and 40 mg·L −1 . Co-NPC-800 could be used as an adsorbent for the removal of TCS and PCMX in sewage, and it has a great potential to be applied more broadly.

Conclusion
In this work, ZIF-67 (Co), a kind of MOF material with a zeolite structure, was synthesized using the 'onepot method'. Co-NPC, the porous carbon material mixed with nitrogen element, was prepared causing ZIF- the Langmuir isotherm adsorption model. Moreover, the adsorption of TCS and PCMX by Co-NPC-800 was a spontaneous endothermic process with reduced entropy. The excellent adsorption capacity of Co-NPC-800 is a result of multiple properties including electrostaticity, π-π and hydrogen bond effects. Moreover, Co-NPC-800 can be regenerated through simple washing and can be reused several times. Co-NPC-800 is a magnetic and recyclable adsorbent with high adsorption capacity and stability, thus is valuable and may be broadly used in sewage treatment and other environmental elds.
Declarations 49. Yao Y, Zhang J, Wu G, Wang S, Hu Y, Su C, Xu T (2017) Iron encapsulated in 3D N-doped carbon nanotube/porous carbon hybrid from waste biomass for enhanced oxidative activity.Environmental         The infrared spectrum of Co-NPC-800 after adsorption of (A) TCS or (B) PCMX; The number of hydrogen bond donors and acceptors of (C) TCS or (D) PCMX at different pH.

Figure 9
The adsorption mechanism of TCS and PCMX on Co-NPC-800.

Figure 10
Co-NPC-800 application in the treatment of actual water samples

Supplementary Files
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