Synthesis and Properties of Magnetic Carbon Nanocages Particles for Dye Removal

Magnetic carbon nanocages (MCNCs) with multiform pore structure have been synthesized by a simple low temperature carbonization process. Biorenewable lignin was used as a cheap and carbon-rich precursor for the first time. The products were characterized by X-ray diffraction (XRD), nitrogen adsorption-desorption, energy dispersive X-ray spectroscopy (EDS), vibrating samplemagnetometer (VSM), transmission electronmicroscopy (TEM), and Raman spectrum. XRD pattern and Raman spectrum showed that the product has a high degree of graphitization crystallinity. TEMmicrograph indicated that the synthesized MCNCs have the hierarchical pore and cage structure. Due to these characteristics, the obtained magnetic carbon nanocages can be used as efficient and recycled adsorbents in the removal of dye staff from textile wastewater.

To date, various synthesis methods, for instance, chemical vapor deposition (CVD) [25], arc discharge, template approach [26], pyrolysis of organic precursors [27], and solgel method [28], had been developed to fabricate carbon nanocages.However, in most of these synthesis approaches, acetylene, ethanol, and pyridine were used as carbon sources, which are expensive and unsustainable fossil fuels; they may not be sufficient in near future.Furthermore, energy and environmental concerns have initiated and energized the research on the development of CNCs from renewable carbon sources.As a result, it is crucial to switch over to the cheap and biorenewable carbon source.
Lignin, a cheap, nontoxic, and carbon-rich renewable resource, is the second most abundant biopolymer after cellulose in the nature [29].Lignin is usually a waste discharged from paper and biomass bioethanol factories.However, the molecular structure of lignin consists of repeating units of the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) (Figure 1) [30], which had a highly cross-linked structure close to the network of phenol resin [31,32].Thus, the reaction activity of lignin is similar to phenol.Phenol is a kind of toxic chemical intermediates, which was used to synthesize carbon materials [33][34][35].Clearly, nontoxic and carbon-rich renewable lignin could replace the phenol as the precursor of carbon materials.Therefore, there are numerous reports of the use of lignin for the preparation of carbon fibers [36], activated carbons [37], carbon nanotube [38], and porous carbon materials [39].Nevertheless, to the best of our knowledge, the carbon nanocages prepared by lignin have not yet been reported.
Herein, in this paper, for the first time we report a simple, low-cost, and environment-friendly method for preparing magnetic carbon nanocages (MCNCs) by a low temperature carbonization process using lignin as the precursor.The structural properties of the obtained samples were characterized by nitrogen adsorption-desorption, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy.Furthermore, lignin-based multiform pore structure and magnetic carbon nanocages used as the adsorbents of methyl orange were investigated.The pore size distributions were derived from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method.VSM measurements were performed by using a vibrating sample magnetometer (Lake Shore 7410 VSM), and the magnetic property of nanocomposite is measured by a strong magnet.The absorbance of methyl orange was measured by 752 type spectrophotometer whose operating voltage was AC 220 V/50 Hz.

Adsorption of Methyl Orange.
In order to investigate the effect of important parameters (e.g., contact time and initial dye concentration) for the removal of MO dye, batch experiments were conducted.For each experimental run, the adsorbent (50 mg) and MO (50 mL) stock solutions were taken in a 250 mL stopper conical flask.This mixture was stirred at a constant speed for 4 hours at room temperature and then the mixture was separated by magnetic separation and the obtained supernatants were tested using a spectrophotometer at wavelength 461.0 nm.Particularly, the representative adsorbent (MCNCs) was withdrawn at different time intervals (0-200 min), separated, and analyzed for remaining dye concentration.
According to the decolorization rate formula for calculation and analysis, the corresponding curve was drawn and then the saturated adsorption capacity was calculated.The removal percentage, defined as the ratio of difference in initial MO concentration before and after adsorption ( 0 − ) to the initial MO concentration in the aqueous solution ( 0 ), was calculated using the following equation, while  0 and , respectively, represent the absorbance of MO before and after the adsorption [40,41]: Besides, the equilibrium adsorption capacities (  ) were determined according to the following formula [42]: wherein   is the initial concentration,   is the residual concentration,  is the volume of the solution, and  is the mass of the adsorbent.
In research on magnetic separability, 25 mg of adsorbent was added to 25 mL of MO dye solution (50 mg/L) in a bottle.The resulting mixture was stirred at room temperature for 60 min to reach adsorption equilibrium.Subsequently, a conventional laboratory magnet was placed near the glass bottle, and the solid-liquid separation was achieved within few minutes [41].

Results and Discussion
The magnetic carbon nanocages (MCNCs) with graphitic framework were evidenced by TEM analysis as shown in Figure 2. As can be seen, the sample consists of nanocages.The size and shape of the nanocages were not uniform, and the size of the nanocages was estimated to be 30-70 nm (Figure 2(a)).After acid treatment, a great deal of iron phase was eliminated, leaving a graphitic carbon nanocages structure as shown in Figure 2(b), but a small part of iron still residue in the graphitic carbon nanocages.Particularly, the Fe 3 C particles remain within the cages as it is not in an ionic form after the acid treatment of Fe 3 C by HNO 3 .The magnetic carbon nanocages had an average size of about 50 nm and a shell thickness ranging from 5 to 10 nm.HRTEM image illustrates that the nanocage shell was composed of about 28 well-defined graphitic layers with a spacing of 0.34 nm (Figures 2(c) and 2(d)), which is consistent with the (002) plane of graphite (Figure 4(a) JCPDS Card number 41-1487).The graphitization degree of the present MCNCs (Figure 2(d)) corresponded well to the results of XRD pattern and Raman spectrum (see below), which are similar to the previous work [43].
The structure of the magnetic carbon nanocages was studied using energy dispersive X-ray spectrometry (EDX).The resulting spectrum is shown in Figure 3.As shown in this figure, the iron particles are present at 0.69, 6.41, and 6.38 keV before and after acid treatment, respectively.The peak at 6.38 keV (Figure 3(b)) can be attributed to the phase of iron particles after acid treatment.Energy dispersive Xray spectroscopy (EDS) analysis (Figure 3  Figure 4(a) shows the wide-angle X-ray diffraction (XRD) pattern of magnetic carbon nanocages before and after acid treatment; the peaks at around 26.3 ∘ , 42.2 ∘ , and 44.3 ∘ can be indexed as (002), (100), and (101) diffraction planes of the graphite (JCPDS Card number 41-1487), respectively.The weak peaks located at about 37.4 ∘ , 42.7 ∘ , and 43.5 ∘ can be, respectively, indexed to the (121), (211), and (102) reflections of Fe 3 C phases (JCPDS Card number 65-0393), while the other intense peaks at around 44.6 ∘ , 65.1 ∘ , and 82.3 ∘ can be indexed as (110), (200), and (211) diffraction planes of Fe (Figure 4(a), I, JCPDS Card number 65-4899).Compared with the original sample without acid treatment, the peaks of 44.6 ∘ , 65.1 ∘ , and 82.3 ∘ corresponding to Fe phases were no longer observed in the acid-treated sample, suggesting that the Fe was removed, but graphitic layers on the outer surface remained after acid treatment.The reflections of graphitic and Fe 3 C planes were still visible on the XRD pattern of the after acid-treated sample; meanwhile, the peaks of 26.3 ∘ were intense (Figure 4(a), II).Maybe the iron carbides in graphite layers (Fe 3 C@C) are difficult to be acid-treated.
To further investigate the graphitization state of magnetic carbon nanocages before and after acid treatment, Raman spectroscopy test was carried out to define the vibration of carbon species.As shown in Figure 4(b), the magnetic carbon nanocages exhibited a D band at 1338 cm −1 and a G band at 1586 cm −1 ; the D band is attributed to an A 1g vibration mode of carbon atoms with dangling bonds in plane terminations of disordered graphite [44].The G band corresponds to the E 2g mode of hexagonal graphite and is related to the vibration of sp 2 hybridized carbon atoms in the graphite layer.The relative intensity of these two bands (I D /I G ) can be used to reflect the degree of graphitization, which are calculated to be 1.72, 1.10 for before and after acid treatment, respectively.The greater the ratio of I D /I G is, the higher the disorder degree for graphite is.The relatively lower I D /I G value (1.10) combined with a broad D band confirms that the after acid-treated magnetic carbon nanocages had better graphitization degree [45], which can be confirmed by its XRD pattern (Figure 4(a), II).
Nitrogen sorption isotherms were recorded to investigate the before and after acid treatment of the samples.Figure 5 gives the N 2 adsorption-desorption isotherm and corresponding pore size distribution curve of MCNCs.The isotherms (Figure 5(a)) exhibit a clear H 1 -type hysteresis loop at a relative pressure of 0.5-0.7,indicating mesoporous characteristics with a capillary condensation phenomenon.The pore size distribution obtained from an analysis of desorption branch of the isotherms is shown in Figure 5(b).It can be seen that the MCNCs sample has bimodal pore size distributions centered at about 4.1, 30-65 nm and 0.57, 3.1, 30-65 nm of magnetic carbon nanocages before and after acid treatment, respectively.These sizes are close to the diameters of the nanocages observed by TEM (Figure 2).Specifically, the pores size 0.57 nm may be produced by the acid treatment for removal of the metallic iron nanoparticles.These magnetic carbon nanocages generally have high BET surface areas (307.9, 420.5 m 2 ⋅g −1 ) and large pore volumes (0.69, 0.71 cm 3 ⋅g −1 ); both are increasing with the removal of iron nanoparticles by acid treatment.
The magnetic properties of the synthesized carbon nanocages particles (Fe 3 C) were characterized by magnetic hysteresis loops with varying magnetic field at room temperature and the obtained magnetization hysteresis loop is shown in Figure 6(a).The hysteresis loop of the carbon nanocages particles (Fe 3 C) measured at 300 K shows typical soft magnetic behavior of the composite with a high saturation magnetization up to 19.1 emu⋅g −1 , low coercivity of 136.4 Oe, and low remanence of 2.5 emu⋅g −1 .Figure 6(a) (inset) shows the thermogravimetric analysis (TGA) of the magnetic carbon nanocages materials under the air atmosphere.From TGA results we obtained the iron oxide content which is 15.6% in MCNCs.Based on calculation, the Fe 3 C content is 17.5% in MCNCs.
Pollutants, especially textile dyestuffs, have caused overwhelming environmental pollution worldwide, which have triggered many studies' attention.Recent studies showed that magnetic materials with high surface area have a wide potential application for the removal various pollutants in wastewater, notably for textile dyestuffs [46][47][48].In order to evaluate the adsorption behavior of magnetic carbon nanocages, we investigate their capacities by adsorbing MO dye from its aqueous solutions.Figure 6(b) presents the adsorption kinetics of MO dye onto the representative magnetic carbon nanocages with a given initial MO dye concentration of 50 mg/L.MCNCs have obviously better removal ability than that of the MCNs before acid, with adsorption rates of 43.6%, 63.6% and 28.4%, 39.8% in the first 3, 5 min, respectively.Then the dye was rapidly adsorbed in 15 min (92.6%), and subsequently the adsorption rate decreased gradually and reached equilibrium in about 60 min (97.7%).However, compared with commercial activated carbon, MCNCs adsorption rate is not fast enough, which the adsorption rates of 98.8% in 10 min.
Magnetic carbon nanocages (MCNCs) exhibit a higher adsorption capacity, which may be attributed to the triple synergy effect based on the special hierarchical pore and nanocages structure of MCNCs.Firstly, dye molecules are easily adsorbed or condensed on the exterior surface of MCNCs.Secondly, When the adsorption of the exterior surface reached saturation, the dye molecules extended into the microporous and were adsorbed by the interior surface of MCNCs.Furthermore, it is conjectured that a certain amount of dye molecules may be adsorbed partially or fully filled in the hollow core of MCNCs.
As displayed in Figure 6(b) (inset), the magnetic carbon nanocages can be well-dispersed in MO solution; there was a change from black to colorless within few minutes by placing a conventional laboratory magnet near the glass bottle.That is to say, the black particles of magnetic carbon nanocages were attracted towards the magnet, and the clear solution could be easily decanted off or removed by pipette.It is proved that magnetic carbon nanocages possessed magnetism and could be potentially used as a magnetic adsorbent to remove organic dyes in liquid phase.

Conclusion
In conclusion, we demonstrate a simple route for the synthesis of magnetic carbon nanocages via a direct carbonization process by using biorenewable lignin as a carbon source.XRD, N 2 adsorption-desorption, EDS, VSM, TEM, and Raman results all consistently reveal that the synthesized magnetic carbon nanocages have a multiform pore structure, relative high specific surface area, and relative large pore volume, which are beneficial for the dye molecules diffusion/transfer in the absorbents and thereby result in the improvement of adsorption and separation properties.In addition, the experimental results indicate that the magnetic carbon nanocages exhibited a high capacity for methyl orange adsorption; the excellent adsorption performance of MCNCs can be attributed to its nanocage structure and hierarchical porosity.

Figure 2 :
Figure 2: (a) TEM and (c) HRTEM image of magnetic carbon nanocages before acid treatment; (b) TEM and (d) HRTEM image of magnetic carbon nanocages after acid treatment.

Figure 3 :
Figure 3: Energy dispersive X-ray spectroscopy (EDS) analysis of magnetic carbon nanocages before (a) and after (b) acid treatment, respectively.
(b)) confirmed the TEM result of the few iron nanoparticles residue in MCNCs (Figure 2(b)).

Figure 4 :
Figure 4: (a) XRD pattern and (b) Raman spectrum of the magnetic carbon nanocages before (I) and after treatment (II).

Figure 5 :Figure 6 :
Figure 5: (a) N 2 adsorption-desorption isotherms and (b) pore size distribution curves of the magnetic carbon nanocages before (I) and after acid treatment (II).