18 Characterization and Application of Nanomaterials (2021) Volume 4 Issue 1 doi:10.24294/can.v4i1.1327 ORIGINAL RESEARCH ARTICLE Study on synthesis and adsorption property of porous carbon/Ni nanoparticle composites Sailu Xu1, Yuxin Du1, Meiqi Hui1, Zichen Wang1, Junfeng Zhao1,2*, Gang Yang1,2 1 School of Materials Engineering, Changshu Institute of Technology, Changshu 215500, Jiangsu Province, China. E-mail: jfzhao@cslg.edu.cn 2 Suzhou Key Laboratory of Functional Ceramic Materials, Changshu 215500, Jiangsu Province, China Abstract The porous carbon/Ni nanoparticle composite was prepared by a freeze-drying method using NaCl as the template. It was applied in the effect of the concentration, adsorption time, and temperature of adsorption on the adsorption be- havior. The kinetic model and the adsorption isothermic fitting results show that the adsorption behavior fits with the pseudo-secondary dynamics and the Langmuir isothermal model, indicating that the adsorption process is monolayer adsorption. Thermodynamic results indicate that the adsorption process is spontaneous physicochemical adsorption. The fitting showed that the porous carbon/Ni nanoparticle composites reach 217.17 mg·g–1, at 313 K indicates good adsorp- tion for Congo red. Keywords: Porous Carbon; Magnetic; Congored; Adsorption Performance ARTICLE INFO Received: 3 December 2020 Accepted: 23 January 2021 Available online: 30 January 2021 COPYRIGHT Copyright © 2021 Sailu Xu, et al. EnPress Publisher LLC. This work is licensed under the Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by- nc/4.0/ 1. Introduction With the steady development of the economy and the deepening of industrialization, dye wastewater has become an urgent pollution problem[1]. The adsorption method is simple and highly efficient. Po- rous carbon material (porous carbon material) is a porous material with carbon as the main body, which has the advantages of large than sur- face area, developed pores, good chemical stability, and a wide source of raw materials, rich resources, low price, and simple production pro- cess, which can carry out large-scale production. Therefore, it is widely used in energy storage and conversion, catalytic and macromolecular adsorption, and is favored by researchers at home and abroad[2,3]. Ac- cording to the internationally recognized definition, porous carbon can be divided into three types: microporous carbon material (< 2 nm), iterporous carbon material (between 2–50 nm), porous carbon material (> 50 nm). In recent years, research has found that a single type of hole structure material cannot meet the market demand of high performance and high efficiency. Therefore, artificial transformation and design of porous carbon according to performance and application requirements has become a hot research topic, and the development and application of multiporous carbon materials have emerged[4,5]. In terms of sewage treatment, although porous carbon has the advantages of large than sur- face area and good adsorption performance when porous carbon is add- ed to sewage for adsorption, it will be dispersed in water and cannot be 19 recycled, which leads to waste of resources and cost increase. In this paper, porous carbon/Ni nanoparticle complexes were prepared by the freeze-drying meth- od, using water-soluble inorganic salt NaCl as a template. The effect of adsorbent dosage, adsorption time, and temperature on the adsorption properties of porous carbon/Ni nanoparticle composites was in- vestigated respectively, and the adsorption behavior of porous carbon/Ni nanoparticle composite is ana- lyzed using kinetic and isothermic models. 2. Experiment 2.1 Preparation of porous carbon/Ni nanopar- ticle composites 0.003 mol Ni(NO3)2•6H2O and 4 g NaCl, were dissolved in 20 mL deionized water, quantity 10 mL egg white was added to the above solution, stirred with a magnetic mixer for 30 min, and mixing even- ly for standby. The mixture liquid is frozen in liquid nitrogen for 10 min to completely frozen, and the vacuum sublimation treatment for 48 h is removed to completely sublimate the water. Frozen dried sam- ples were removed and placed into the crucible. The heat-treated samples were removed at 650 ℃ for 3 h. In N2 atmosphere, ground dispersed in a certain amount of deionized water, the NaCl template was sonicated, and the samples were then extracted and separated, repeated to fully remove inorganic salt 3 times. The aspirated/Ni nanoparticle-washed samples were prepared by drying in a 60 ℃ vacuum drying tank for 8 h, to obtain a porous carbon/Ni nanoparti- cle composite. 2.2 Characterization of the material topogra- phy and structure The structure was analyzed by the X-ray diffrac- tion instrument (XRD), the scanning electron mi- croscopy (SEM, SIGMA, 20 kV) and high-resolution transmission electron microscopy (HRTEM, JEOL- 2000 CX, 200 kV), and the specific surface area and aperture size analyzer (BET) were used to analyze the specific surface area and pore structure. 2.2 Characterization of the adsorption prop- erties The sorbent properties of porous carbon/Ni nanoparticle composites were determined using a TU-1901 UV spectrophotometer. The adsorption properties of the adsorbent were studied, using Con- go Red (CR) as a pollutant model, at 20, 40, 60, 80, 100 mg/L concentrations. A solution of CR at 40 mg/ L and investigated the effects of different tempera- tures (298, 303, 308, 313 K) on the adsorption prop- erties. All the above experiments were performed at 200 r·min–1 oscillations, with 5 mL mixed solution at 15 min, 30 min, 1 h, 2 h, 3 h, placed into a centrifuge tube, centrifugation for 5 min, and the remaining CR concentration was tested using a UV spectrophotom- eter. 3. Results and discussions Figure 1 shows the XRD map of the prepared porous carbon/Ni nanoparticle composite, showing three sharp diffraction peaks at 2θ at 42.2°, at 51.9° and 76.2°, corresponding to (111), (200), (220) crys- tal surface diffraction of Ni (PDF#65-0380), respec- tively. Furthermore, a bulging, relatively weak dif- fraction peak was observed at 2θ of 25.6°, analyzed corresponding to the characteristic diffraction peak of amorphous carbon. No other miscellaneous peaks were observed, indicating that there was no impurity phase in the composite product. XRD results show that the products prepared by freeze drying-carbon- ization are C/Ni composite, the carbon component in protein converted to carbon material by high tem- perature thermolysis; nickel nitrate generates nickel oxide by high temperature, the temperature increases further, nickel oxide by carbon reduction generates magnetic nanoparticle, and eventually form porous carbon/magnetic Ni nanoparticle composite material. 20 Figure 1. XRD plot of the porous carbon/Ni nanoparticle com- posites. (a) 5,000 times; (b) 20,000 times Figure 2. XRD diagram of porous carbon /Ni nanoparticle com- posites. As can be seen from Figure 2, the material prepared by freeze-drying-carbonization presents a 3D structure similar to the block material of “frozen tofu”. Further observation found that the surface of 3D carbon material presents irregular sheet structure, in layer by layer pit gully successively filled with holes from large to small. It is believed that during the reaction precursor of liquid nitrogen freezing, NaCl cubic crystal and nickel nitrate crystal precip- itate out rapidly with the sharp drop of temperature, and a large number of NaCl cubic crystal particles are self-stacked to form a 3D structure. Nickel nitrate crystals are distributed in the 3D structure, and the proteins in the precursor are also precipitated with decreasing temperature and coated in the surface of the NaCl cubic crystals. Further vacuum sublima- tion drying completely removes the water from the system, thus forming a protein/nickel nitrate/NaCl composite powder product. Complex powder at high temperature in an inert atmosphere, NaCl structure remains stable at high temperature, the protein car- bonization on the surface is transformed into carbon material, and nickel nitrate forms nanoparticles by decomposition and reduction reaction and nanoparti- cles. The product, removed from the NaCl template with simple washing, forming a honeycomb porous carbon structure with more evenly distributed dimen- sions, interconnected and interconnected. Figure 3(a)–Figure 3(c) is a TEM plot of po- rous carbon/Ni nanoparticle complexes. It can be seen that the sample has a 3D structure of which the matrix is the porous carbon. The porous carbon structure is filled with high-density, ultra-fine nickel nanoparticles with a size of approximately 30 nm. The HRTEM plot of Figure 3(d) shows a distinct crystal surface orientation in the local region, corre- sponding to the crystal surface spacing of Ni in the composite, covered with helminth-like disordered ripples of the nickel, corresponding to the amor- phous carbon layer. This result is consistent with the XRD results, indicating that nanoparticles distributed in graded porous carbon form a composite structure. Figure 3. (a) (b) (c) TEM figures of multiwell carbon/Ni nanoparticle composites and (d) HRTEM figure. According to the BET adsorption/desorption of Figure 4(a) porous carbon/Ni nanoparticle com- posite, the sample presents a I/IV mixed adsorption model indicating the presence of a hierarchical porous structure of the material. First, the signifi- cant uptrend at P/P0 < 0.01 was attributed to type I isotherms, indicating the presence of a micropore structure in the structure. At P/P0 = 0.4–0.9, there is a distinct suction attachment hysteresis loop in the relative pressure range, in line with the IV isother- mic model, showing a large number of interpore structures in the structure. Besides, when P/P0 > 0.9, the adsorption/desorption is further increased, 21 indicating that large pore structures still exist in the structure[6–7]. Therefore, the material prepared in this experiment is a graded porous structure. The analysis of Brunauer-Emmett-Teller (BET) showed that the specific surface area of porous carbon/Ni nanoparti- cle composites is up to 511.532 m2·g–1. The aperture distribution curves of the sample are further given in Figures 4(b) and 4(c), and Barrett-Joyner-Halen- da (BJH) and Dubinin Radushkevich (DR) analysis showed that the sample showed a distinct aperture distribution in both the 2–30 nm and the micropore region of 0–2 nm. By the observation, we know that the distribution of pore diameter is relatively concen- trated, and the mesole mostly concentrated around 5 nm and the micropore around 0.3 nm, further proving that the sample has a better graded porous structure. Among them, the total adsorption pore vol- ume was 0.938 cm3·g–1, BJH adsorption cumulative interpore and total pore volume were 0.859 cm3·g–1, DR method micropore (< 2 nm) volume was 0.236 cm3·g–1. Figure 5 shows plots of the adsorption proper- ties of Congo red dye in porous carbon/Ni nanopar- ticle composites at 298, 303, 308, 313 K. As can be seen from the figure, the adsorption curve rises sharply when the adsorption time is within 60 min, and then tends to be flat. It shows that the adsorption tends to achieve saturation at 60 min. The intercept- ed 60 min time point and the adsorption concentra- tion are 40 mg·L–1 (in the order of temperature rise), respectively, are 105.37, 124.37, 158.52, 163.87 mg·g–1, indicating that the adsorption amount will in- crease as the temperature increases (the increase rate is gradually slowed and reaches saturation at a tem- perature). It shows that as the temperature increases, the concentration of the adsorption matter will also affect the size of the adsorption quantity. Figure 4. Diagram of BET and aperture distribution of Porous carbon/Ni nanoparticle composites. (a) BET adsorption/detachment map; (b) distribution map of interpore aperture map; (c) micropore aperture distribution map. Figure 5. Effects of different temperatures on the adsorption properties. To further explore the adsorption behavior of porous carbon/Ni nanoparticle composites on the or- ganic dye Congo red, a pseudo-second-order kinetic model was used to fit the adsorption data at different temperatures, with the exact formula shown in equa- tion (1)[8]. (1) The qe, qt (mg·g–1) represent the adsorption capacity of t (min) at equilibrium and at some time, respec- tively; k2 represents the pseudo-second-order rate constant (g·mg–1·min–1). Figures 6(a)–Figure 6(d) shows a pseudo-fit- ting second-order nonlinear fitting curve for the ad- sorption behavior of porous carbon/Ni nanoparticle 22 complexes at 298, 303, 308, 313 K temperatures. The kinetic parameters (k2 and qe) and regression co- efficients (R2) of the fitted model are shown in Table 1. For all four adsorption temperatures, the experi- mental data agree with the regression coefficients (R2 substantially above 0.99) at all initial concentrations. From these R2 values, the model of quasi-second order is suitable to describe the adsorption kinetic behavior. The pseudo-second-order model calcula- tions agree well with the experimental observations. These results confirm that chemical adsorption is a rate control step and depends on the concentration of contaminants exposed to the surface of the porous carbon/Ni nanoparticle composite materials[9]. Fur- thermore, the maximum adsorption of the Congo red dye at 100 mg·L–1 by the porous carbon/Ni nanopar- ticle composite at 298, 303, 308, 313 K was 119.53, 150.60, 189.04, and 206.19 mg·g–1, respectively. The interaction of adsorbent with adsorbate describes as adsorption isotherms. The well-known model Langmuir isotherm describes the adsorption process. Figure 6. Fof pseudo-second-order dynamics of the adsorption process. Langmuir isotherms are based on single-mol- ecule adsorption processes, commonly used to de- scribe equilibrium adsorption isotherms on homoge- neous surfaces. The isothermal model can represent as Equation (2)[10,11]. (2) Ce is the equilibrium concentration of the sol- ute (mg·L–1); qe indicates the adsorption amount Table 1. The fitting parameters of pseudo-second order dynamics of porous carbon/Ni nanoparticle composites to Congo red adsorp- tion 23 (mg·g–1) at equilibrium; qm is the maximum adsorp- tion amount (mg·g–1); and KL indicates the Langmuir adsorption constant (g–1). Figure 7 shows the fit diagram of the Langmuir isothermal model. The rel- evant parameters obtained from the isothermal mod- el fitting listed in Table 2 show that the regression coefficient R2 is above 0.9 at different adsorption temperatures, saying that the bright Muir model can well describe the adsorption behavior of porous car- bon/Ni nanoparticle composite adsorption agent. The isothermal fitting showed that the porous carbon/Ni nanoparticle composite is monolayer adsorption, and the adsorption is in a monolayer. Once the adsorp- tion cannot be further at that position once the dye molecule occupies the active site on the magnetic porous carbon surface[12]. By the fitting calculation, we can get that the maximum adsorption of Congo red by the porous carbon/Ni nanoparticle composite was 123.10, 144.95, 197.53, and 217.17 mg·g–1. at 298, 303, 308, 313 K, respectively. Figure 7. A fitting of the Langmuir isothermic model for the adsorption process. Table 2. Langmuir fitting parameters for the isotherm model To further evaluate the effect of temperature on the adsorption properties of porous carbon/Ni nanoparticles composites, thermodynamic param- eters, including enthalpy (ΔHo), Gibbs free energy (ΔGo), and entropy (ΔSo) were calculated, with the specific equations of: (3) (4) R is the gas constant (8.314 J·mol–1·K–1), T in- dicates the temperature (K), and K is the adsorption equilibrium constant. Figure 8 shows a linear plot of lnK and 1/T. The specific values of the thermody- namic parameter enthalpy (ΔHo), Gibbs free energy (ΔGo), and entropy (ΔSo) are calculated by fitting shown in Table 3. Figure 8. Linear plot between lnK and 1/T. Table 3. Specific values for the enthalpy of thermodynamic parameters (ΔHo), Gibbs free energy (ΔGo), and entropy (ΔSo) From Table 3, all of the ΔGo value is negative, indicating that the Congo red adsorption of the po- rous carbon/Ni nanoparticle composite is a sponta- neous process. The ΔGo tends to decrease with the increasing temperature, which indicates that the in- creased temperature is favorable for adsorption. ΔHo and ΔSo are all positive, which indicates that the ad- sorption of porous carbon/Ni nanoparticle composite to Congo red has exothermal properties[13]. ΔHo is 31.25 kJ·mol–1, higher than physical adsorption heat (2.1–20.9 kJ·mol–1) and less than chemical adsorp- tion heat (80–200 kJ·mol–1), indicating that the ad- sorption of porous carbon/Ni nanoparticle composite belongs to a physicochemical adsorption process[14]. 24 4. Conclusion The porous carbon/Ni nanoparticle composites synthesized by a freeze-drying method by NaCl as a template. SEM and TEM showed that Ni nanopar- ticles equably distributed on porous carbon carriers, which constructed into a 3D-graded porous structure. The BET and pore size distribution results show that the porous carbon/Ni nanoparticle composite is a graded porous structure composed of large holes, mesholes and micropores, with 511.532 m2·g–1 and 0.938 cm3·g–1. Compared to the surface area and the total adsorption pore volume, respectively, making the porous carbon/Ni nanoparticle composite has excellent adsorption properties. The kinetic and ther- modynamic results show that the adsorption behav- ior, which is the composite materials to the organic pollutant Congo red, accords with the pseudo-sec- ondary dynamics and Langmuir isothermal model. And it is with a maximum adsorption amount of 217.17 mg·g–1 at 313 K. Conflict of interest The authors declare that they have no conflict of interest. References 1. Lu Y, Song S, Wang R, et al. Impacts of soil and wa- ter pollution on food safety and health risks in China. Environment International 2015; 77: 5–15. 2. Chen B, Ma Q, Tan C, et al. Carbon-based sorbents with three-dimensional architectures for water reme- diation. Small 2015; 11(27): 3319–3316. 3. De S, Balu AM, Van Der Wall JC, et al. Biomass‐ derived porous carbon materials: synthesis and catalytic applications. ChemcatChem 2015; 7(11): 1608–1629. 4. Sevilla M, Ferrero GA, Fuertes AB. One-pot syn- thesis of biomass-based hierarchical porous carbons with a large porosity development. Chemistry of Materials 2017; 29(16): 6900–6907. 5. Liu R, Liu Y, Zhou X, et al. Biomass-derived high- ly porous functional carbon fabricated by using a free-standing template for efficient removal of methylene blue. Bioresource Technology 2014; 154: 138–147. 6. Gupta K, Gupta D, Khatri OP. Graphene-like porous carbon nanostructure from Bengal gram bean husk and its application for fast and efficient adsorption of organic dyes. Applied Surface Science 2019; 476: 647–657. 7. Song Y, Wei G, Kopec M, et al. Copolymer-templat- ed synthesis of nitrogen-doped mesoporous carbons for enhanced adsorption of hexavalent chromium and uranium. ACS Applied Nano Materials 2018; 6(1): 2536–2543. 8. Ho YS, Mckay G. Pseudo-second order model for sorption processes. Process Biochemistry 1999; 34(5): 451–465. 9. Pour ZS, Ghaemy M. Removal of dyes and heavy metal ions from water by magnetic hydrogel beads based on poly (vinyl alcohol)/ carboxymethyl starch-gpoly (vinyl imidazole). RSC Advance 2015; 5: 64106–64118. 10. Hemmati F, Norouzbeigi R, Sarbisheh F, et al. Mal- achite green removal using modified sphagnum peat moss as a low-cost biosorbent: kinetic, equilibrium and thermodynamic studies. Journal Taiwan Institute of Chemical Engineers 2016; 58: 482–489. 11. Liu Y, Xu H. Equilibrium, thermodynamics and mechanisms of Ni2+ biosorption by aerobic granules. Biochemistry Engineering Journal 2007; (35): 174– 182. 12. Cheng B, Le Y, Cai W, et al. Synthesis of hierarchical Ni(OH)2, and NiO nanosheets and their adsorption kinetics and isotherms to Congo red in water. Journal of Hazardous Materials 2011; 185(2-3): 889–897. 13. Zhao J, Zha J, Yang C, et al. Cauliflower-like Ni/ NiO and NiO architectures transformed from nickel alkoxide and their excellent removal of Congo red and Cr(VI) ions from water. RSC Advance 2016; 6: 103585–103593. 14. Liu S, Ding Y, Li P, et al. Adsorption of the anionic dye Congo red from aqueous solution onto natural zeolites modified with N, N-dimethyl dehydroa- bietylamine oxide. Chemical Engineering Journal 2014; 248: 135-144.