Synthesis of a Magnetic Co@C Material via the Design of a MOF Precursor for Efficient and Selective Adsorption of Water Pollutants

3D metal–organic frameworks (MOFs) can be appropriate templates for the fabrication of nanomaterials due to they have active sites exposed on the channel or surface, which thus provide them with improved catalytic performance. In this study, a 3D cobalt-based MOF [Co(H2bpta)]n (Co-MOF), where H4bpta denotes 2,2′,4,4′-biphenyltetracarboxylic acid, has been constructed with the use of a ligand with a high carbon content. On this basis, a 2D magnetic carbon-coated cobalt nanoparticle composite (Co@C) was prepared by using the title MOF under different temperatures. Magnetic Co@C can readily absorb dye from the solution and can thus act as an inexpensive and fast-acting adsorbent. Moreover, we have explored the adsorption isotherms, kinetics and thermodynamics of the anion dyes in detail. The adsorption capacity of the Co@C-800 for investigated methyl orange (MO) and congo red (CR) dyes were 773.48 and 495.66 mg  g−1, respectively. It is noteworthy that MO adsorption is higher in existing materials. Thermodynamic studies suggest that the adsorption processes are spontaneous and exothermic. This study opens a new insight into the synthesis and application of carbon-based materials that enable the selective removal of organic dyes.


Introduction
With the growth of the printing, textile, and other industries in recent years, the discharge of dye wastewater has increased each year [1][2][3][4]. Methyl orange (MO) and Congo red (CR) are two common anionic dyes which have been widely used in the industries and research laboratories [5,6]. Nowadays, a variety of methods such as oxidation, ozonation or degradation have been exploited to remove anionic dyes from wastewater [7][8][9][10]. However, aforementioned techniques have poor selectivity and are relatively expensive because they cannot be reused. Therefore, there is an urgent need for an efficient way to selectively remove anionic dyes.
Carbon-based materials are regarded as ideal candidates for the adsorption heavy metal ions and organic pollutants because of their good stabilities and high specific surface areas [11][12][13][14]. However, these materials have only offered modest anionic dye removal performance [5,[15][16][17]. Therefore, achieving selective, efficient, and reversible anionic dye adsorption and desorption using a carbonbased material continues to pose a daunting challenge.
Metal-organic frameworks (MOFs) as sacrificial precursors or templates for preparing highly dispersed inorganic, organometallic, and composite materials with good stabilities and high specific surface areas after the calcined or pyrolysis process have been widely used [18][19][20][21]. Notably, MOF-derived materials can remain the original morphologies of the MOFs under appropriate conditions. Among them, Co-doped carbon-based materials synthesized by the pyrolysis of Co-containing MOF precursors are considered as good adsorbents [8,[22][23][24]. Carboncoated cobalt composites (Co@C-800) have been regarded as adsorbents for the removal of organic and inorganic pollutants from solutions, and they have been found to offer performances comparable to commercial adsorbents [25][26][27][28][29]. Furthermore, the outstanding adsorption capabilities of Co@C-800 relative to other adsorbents may be caused by strong interactions between Co@C-800 and the targeted compounds through π-π interactions, which result from the presence of delocalized electrons in hexagonal arrays of carbon atoms that thus facilitate adsorption onto the surfaces of Co@C-800 materials [30,31].
In this work, [Co(H 2 bpta)] n (Co-MOF), where H 4 bpta denotes 2,2′,4,4′-biphenyltetracarboxylic acid, was synthesized under solvothermal conditions (Tables S1 and S2). The Co-MOF was thermally annealed under an N 2 atmosphere to form carbon-based composites (Co@C-600, Co@C-800, Co@C-1000) (Scheme 1). The dye adsorption performances of the Co-MOF and Co@C-600, Co@C-800, Co@C-1000 have been studied. Moreover, the effects of growth temperatures of Co@C-800 composites and the kinetics and thermodynamics parameters of the dye adsorption were investigated in detail to gain insight regarding the adsorption mechanism.

Materials and Methods
The Co-MOF was synthesized by the methods described in the literature [32]. All other reagents and solvents were reagent grade, which employed from commercial sources and used without further purification.

Synthesis of the Carbon-Based Composites of [Co(H 2 bpta)] n (Co@C-600, Co@C-800, Co@C-1000)
Co@C-600: Firstly, the Co-MOF complex was placed in a quartz boat and situated in a tubular furnace. Secondly, the furnace temperature was increased to 600 °C under the N 2 flow (200 mL/min, 99.999% purity) with the heating rate of 20 °C min −1 . Thirdly, the carbon-based material was prepared with N 2 (200 mL min −1 ) for 60 min. Finally, it was cooled to room temperature with N 2 flow, the carbon-based material had been synthesized. Co@C-800: Except that the calcination temperature was changed into 800 °C, the synthesis method was the same as that of Co@C-600.
Co@C-1000: Except that the calcination temperature was changed into 1000 °C, the synthesis method was the same as that of Co@C-600. Scheme 1 Schematic illustration for the synthesis of Co-MOF and Co@C-800 derived from Co-MOF

Characterization
The elemental analyses (C, H, and N) of the samples were performed with a Perkin-Elmer 240C elemental analyzer. FT-IR spectra (KBr pellets) were recorded with a Varian 640-IR spectrometer over the spectral from 500 to 4000 cm −1 . Powder X-Ray diffraction (PXRD) data was collected using a Rigaku diffractometer with CuKα radiation. Meanwhile, the morphology of the carbon-based material was characterized via scanning electron microscopy (SEM, Nova Nano SEM 430) and high-resolution transmission electron microscopy (HRTEM, JEOL2010 at 200 kV). The thermal stabilities of Co-MOF and the carbon-based composite of Co@C-600, Co@C-800, Co@C-1000 were measured with a thermogravimetric analyzer (NETZSCH STA 449C). The specific surface area and pore structure of Co@C-600, Co@C-800, Co@C-1000 was measured by an automatic volumetric sorption analyzer (ASAP 2020 M) using N 2 as the adsorbate at − 196 °C. UV-Vis absorption spectra were recorded using an SP-1900 UV-Vis spectrophotometer.

Preparation and Characterization of the Cobalt-Coated Carbon-Nanoparticle Composite
It should be noted that the Co-MOF is the same that had been reported previously by Pan and co-workers [32]. According to reported research, the MOF-derived carbonbased materials could improve the energy conversion and storage capabilities as well as the adsorption properties of the resultant composite materials [33]. As a result, researchers have investigated the in situ growth carbon-based composites using MOFs as precursors, which is an efficient strategy for enhancing the properties of the MOF-derived materials [34,35]. On this basis, the Co@C-800 was synthesized via the pyrolysis method with the use of Co-MOF as a precursor. The morphology of the Co@C-800 was observed by SEM and TEM characterization. After calcined, the Co-MOF formed Co nanoparticles, and the surrounding organic ligands were pyrolyzed into a Co-doped carbon skeleton which surrounded the nanoparticles. It can be seen that Co@C-800 maintains a 2D layered structure, and Co particles are loading on the carbon surface ( Fig. 1). The diameter of the Co particles is in the range of 30-40 nm, and the Co nanoparticles in the carbon matrix are well dispersed without exhibiting significant agglomeration. The morphological analyses of the as-prepared nanomaterials, Co@C-600 and Co@C-1000 nanocomposite, are examined by SEM technique and presented in Fig. S1. It is evident that Co@C-600 and Co@C-1000 exhibit irregular morphologies with carbon coating and without pore distribution. Thus, the structure and properties of Co@C-800 was studied in detail. The FT-IR spectrum of the Co@C-800 was recorded in the frequency range of 500-4000 cm −1 , as shown in Fig. 2a. The band at 3437 cm −1 corresponds to the stretching and bending vibrations of the hydroxyl groups of Co@C-800, while the peaks observed at 1653 cm −1 correspond to the asymmetric and symmetric vibrations of the carboxyl groups of Co@C-800 [34,35]. Moreover, we have investigated the thermal stability and the components in Co@C-800 by thermogravimetric analysis (TGA) measurements in air at temperatures reaching up to 800 °C (Fig. 2b), and found that the weight loss of Co@C-800 occurred via a single step. This weight loss of 25% in the range of 220-310 °C is the decomposition of amorphous carbon. Moreover, the residual catalyst content is 74% after oxidation, suggesting that the majority of the catalyst residue remains in the sample. The PXRD pattern of the Co@C-800 material prepared via the calcine of Co-MOF is demonstrated in Fig. 2c. The materials show metallic Co and weak CoO and Co 3 O 4 diffraction peaks, the latter two sets of peaks can be attributed to the oxidation of the surface Co nanoparticles undergoing oxidation in the air [36]. Additionally, the cobalt particles gradually aggregate and separate from the carbon layer, then they are oxidized during the high temperature. Meanwhile, the Co@C-800 sample has a specific surface area of 340.54 m 2 g −1 and a pore size distribution from 2 to 10 nm, suggesting that it possesses mesoporous structures (Fig. 2d). It is well known that the pores with variety of sizes is an appropriate way to increase active centers and can yield high-efficiency sorbents [22].

Dye Adsorption Properties of Co-MOF and Co@C
MOFs are excellent adsorbents and are generally used on account of their specific surface areas, diverse range of functional groups, and controllable channel sizes [37,38].
Herein, the adsorption properties of dyes from wastewater brought about widespread attention [39], 40]. In this study, three cationic dyes [methylene blue (MB), rhodamine B (RhB) and gentian violet (GV)] and two anionic dye (MO and CR) were chosen as models to investigate the adsorption properties of Co-MOF. More details of adsorption experiments are given in the supporting information. As illustrated in Fig. 3, the adsorption capacities of the Co-MOF for MB, RhB, CR, and GV at room temperature are 44.93, 3.92, 81.4, and 10.10 mg g −1 at 360 min, respectively. However, Co-MOF exhibited an adsorption capacity of 235.64 mg g −1 for MO in only 10 min, thus demonstrating it is highly selective toward MO and offers rapid and efficient capture of this dye (Fig. 3). According to the literatures, the mechanism of dye adsorption is host-guest interactions with electrostatic interaction, hydrogen bonding or π-π stacking [41]. However, the existing dye adsorption techniques are expensive due to their poor recyclabilities. It is of great significance to find an inexpensive adsorbent with reversible adsorption capabilities. On this basis, we have synthesized Co@C-800 using the Co-MOF as a combined catalyst and precursor via the pyrolysis method. At time intervals of 360 min, the dye concentration was measured via UV-Vis spectroscopy (Fig. 4a- (Fig. 4f). In comparison with other adsorbents, the adsorption capacities of the Co@C-800 for MB, RhB and GV are not the highest [42]. However, we note that the adsorption capacities of our Co@C-800 adsorbent for CR and MO are much higher than those of other dyes. The presence of -OH and -COOH in the Co@C-800 and amino groups of MO and CR might promote the formation of numerous  [43]. Moreover, the favorable adsorption of MO and CR could be attributed to the electrostatic attraction between the negatively charged aqueous solution of MO and CR and the positively charged surfaces of the Co@C-800 [44]. At the same time, π-π interaction between the surface of the carbon-based material and the aromatic compounds of dyes was considered to be the main contributor of the adsorption mechanism.
Hence, these features demonstrate that the title Co@C-800 can be a highly effective sorbent for the selective removal of dye species from water. Interestingly, the magnetic separation of Co@C-800 was further tested with a magnet (Fig. S2). Co@C-800 quickly to external magnetic fields and can be absolutely isolated from an aqueous solution by a magnet. This feature facilitates the recycling of this adsorbent.  Comparatively, at time intervals of 240 min, the dye concentration was measured via UV-Vis spectroscopy (Figs. S3 and S4). The dye adsorption capacities of the Co@C-600 adsorbent for MB, RhB, MO, CR, and GV at room temperature are 0.48, 1.93, 14.78, 10.88, and 40.95 mg g −1 , respectively (Fig. S3f). The dye adsorption capacities of the Co@C-1000 adsorbent for MB, RhB, MO, CR, and GV at room temperature are 1.26, 0.96, 17.28, 5.66, and 27.82 mg g −1 , respectively (Fig. S4f). It can be clearly seen that the adsorption capacity is much lower than that of C@ Co-800, which is found that the adsorption properties of the Co@C materials depending on their structures.

Effect of Contact Time on Adsorption Performance
The effect of contact time on the adsorption of MO and CR by Co@C-800 is displayed in Fig. 4f. The initial concentration of MO and CR are 40 and 80 mg L −1 , respectively, at room temperature. It can be seen that the adsorption capacities of Co@C-800 for MO and CR increase rapidly as the contact time is increased from 0 to 180 min, due to the large number of vacant adsorption sites and the large surface area of this adsorbent. The adsorption rate began to slow down gradually during 180 min, mainly due to the lower MO and CR concentration, resulting in a weaker adsorption driving force while the number of surface active adsorption sites also decreased. After 360 min, the adsorption capacity reached equilibrium, and the equilibrium adsorption capacities were 733.33 and 495.66 mg g −1 for MO and CR, respectively.

Effect of Temperature on Adsorption Performance
Temperature is one of the significant factors influencing the adsorption process. In particular, it affects the physical and chemical properties of the adsorbent and the diffusion rate of the adsorbed molecules and determines the adsorption capacity [45]. In this experiment, the effect of temperature on the adsorption of MO and CR by Co@C-800 was studied at 298, 303, 313, and 323 K, respectively. It can be seen that the initial concentration of MO and CR is 40 mg L −1 and 80 mg L −1 and the adsorption capacity reaches 733.33 and 495.66 mg g −1 (Figs. 5 and S5). These results show that the adsorption capacity increases at higher temperatures. It may be caused by the electrostatic interactions between Co@C-800 and dye molecules at higher temperatures. The results suggest that the adsorption of MO and CR on Co@C-800 is an endothermic process [10].

Effect of Concentration on Adsorption Performance
The initial concentration of the solution is another crucial factor affecting the adsorption process. It affects adsorption by changing the adsorbent and the protonation of the adsorbent's surface functional groups. The effect of the concentration of the solution on the MO and CR adsorption capacity of Co@C-800 is shown in Figs. 6 and S6, and it can be seen that the adsorption capacity increases at higher dye concentrations. As the concentration of the dyes is increases, eventually the adsorption sites on Co@C-800 become occupied, so that a certain adsorption capacity is eventually reached.

Adsorption Isotherms
Many mathematical models have been used to describe the relationship between the adsorption capacity of adsorbents and the residual concentration in solution. The choice of isotherm model depends on the properties and types of adsorbents and adsorbates under investigation [46]. In this work, the Langmuir and Friedrich models were used to further investigate the adsorption performance of Co@C-800 and the reaction between adsorbents and adsorbates.  Table 1, all R L values are between 0 and 1, indicating that Co@C-800 has a higher adsorption capacity for MO and CR than for the other dyes [47]. As shown in Figs. S9 and S10, the values of K F and 1/n can be respectively determined from the slope and intercept of a Freundlich plot. The decision coefficient of Freundlich equation is R 2 > 0.99, thus suggesting that the adsorption of MO and CR by Co@C-800 is in accordance with the Freundlich model ( Table 1). The Langmuir and the Freundlich isotherm charts and parameters indicate that the adsorption conformed to the two models. According to the calculations performed via the Langmuir equation, the biggest adsorption capacity of Co@C-800 reaches 773.33 and 495.66 mg g −1 for MO and CR, respectively, which indicates that Co@C-800 is an excellent adsorbent calculated by the Freundlich equation, 1/n is less than 1 which indicates that it is conducive to the adsorption reaction [48].

Adsorption Kinetics
Several kinetic models, such as the quasi-first-order kinetic model, the quasi-second-order kinetic model and the internal particle diffusion model, are often used to evaluate adsorption data [49]. As shown in Table 2, the values of k 1 and q e can be determined from the intercept and slope of the plots shown in Fig. 7. Although the coefficient of determination (R 2 ) is close to 1, there is a significant difference between the calculated q e (2.97 and 0.97 mg g −1 ) and the experimental q e values (773.33 and 495.66 mg g −1 ), indicating that the quasifirst-order kinetic model is unsuitable for this experiment. A polt of t/q t versus t is shown in Fig. 7c and d, and the values of k 2 and q e are acquired from the slope and intercept of this  (Table 2) indicating that the pseudo-second-order model more accurately describes the kinetics of adsorption of the investigated MO and CR on Co@C-800. Therefore, the adsorption rate is controlled by chemical adsorption, and the adsorbent and the adsorbed material exchange electrons through shared electrons or covalent forces [49].
The internal particle diffusion model is usually used to determine the rate control steps in porous structures [49]. As shown in Fig. S11 and described in Table 2. The determination coefficients R 2 of the internal particle diffusion model are 0.98510 and 0.98923, which are higher than those of the quasi-second order equation. The curve is nonlinear and the intercept is unequal to zero and does not pass through the origin, which indicates that the internal particle diffusion is the only rate determining step, and the adsorption of MO and CR on Co@C-800 are simple process [50].

Adsorption Thermodynamics
Due to the impact of temperature on the adsorption of MO and CR by Co@C-800, the thermodynamic parameters of Co@C-800, MO, and CR were studied at different temperatures. The values of ΔH and ΔS are calulated according to the slope and intercept of the corresponding van't Hoff plot (Fig. S12) [51]. ΔG is negative, suggesting that the adsorption process is spontaneous (Tables 3 and S4). Additionally, as the temperature is ranged from 298 to 323 K, the ΔG of MO cut down about 2.00 kJ mol −1 , while the ΔG of CR reduces from − 5.47 to − 5.99 kJ mol −1 , suggesting that a higher temperature is more beneficial for adsorption. Otherwise, the negative value of ΔH (3.87 and 0.79 kJ mol −1 ) indicated that the interactions between Co@C-800 and MO or CR are endothermic processes. The positive ΔS suggests that the randomness of the system increases as the MO and CR are adsorbed onto Co@C-800 [51].

Conclusions
In summary, a magnetic Co@C-800 material was successfully synthesized with the use of Co-MOF as a catalyst precursor via the pyrolsis method. The as-grown Co@C-800 shown a highly selective adsorption capacity toward anionic dyes, and the adsorption capacities were 773.33 mg g −1 and 495.66 mg g −1 for MO and for CR, respectively. Moreover, a viable adsorption mechanism has been proposed, which is including that hydrogen-bonding interactions and π-π stacking interactions along with the effect of mesopores. The internal particle diffusion is the only rate determining step which the adsorptions of MO and CR on Co@C-800 are simple processes. Thermodynamic parameters suggest that the adsorption is an exothermic and spontaneous process. The resulting Co@C-800 was discovered to be an efficient adsorbent material for the removal of anionic dyes from aqueous solutions. This research may open up new routes toward.