Carbon/CoFe2O4 Composite for the Removal of Rhodamine B from Aqueous Solution

Faculty of Civil and Environmental Engineering, ai Nguyen University of Technology (TNUT), Tich Luong Ward, ai Nguyen City, Vietnam Faculty of Natural Resources and Environment, TNU-University of Sciences (TNUS), Tan inhWard, ai Nguyen City, Vietnam 3 e Center for Technology Incubator and Startup Support, ai Nguyen University of Agriculture and Forestry, Quyet ang Ward, ai Nguyen City, Vietnam Institute of Environmental Technology (IET), Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet Street, Nghia Do, Cau Giay, Hanoi, Vietnam


Introduction
e dye pollution has always been a dramatic environmental issue lingering in developed and developing countries. Many dyes are highly toxic that not only affect the aquatic environment but also harm human health [1]. Rhodamine B dye (RhB) is a cationic xanthene dye that has been used for many industrial purposes. It presents strong fluorescent properties and high solubility in water. It is, thus, widely used in textile, paper making, painting, and lather production [2,3]. RhB is an eye, skin, and respiratory tract irritant. Furthermore, it can cause carcinogenic, neurotoxicity, and chronic poisoning when it exists in drinking water and enters the human body [4]. erefore, it is essential to discard RhB from drain water before discharging into nature.
A variety of technologies have been used to remove RhB from wastewater, including ion exchange, biological degradation, chemical oxidation, coagulation and flocculation, adsorption, membrane filtration, and electrochemical and reverse osmosis [5,6]. Emerging from these methods, adsorption proves to be one of the low-cost effective techniques to remove RhB [2]. Several materials have been applied as adsorbents to eliminate RhB from aqueous solutions such as by-agricultural products [5], activated carbon and biochar [7,8], polymers [9], and nanomaterials [2,6,10]. Among all adsorbent materials, activated carbon (AC) appears to be drastically efficient in adsorbing dyes with a high adsorption capacity [11]. However, it is difficult to separate AC powder after the adsorption process. us, some studies focused on the application of magnetically separable AC composites as a low-cost adsorbent with a high solid-liquid separation ability and significant effectiveness. Bagheri et al. [12] made an investigation to make Fe 3 O 4 magnetite nanoparticles and afterward to load them on active carbon to enhance the efficiency in the elimination of methylene blue, Sunset yellow, and Eosin b. Fayazi et al. [13] also did a study to make activated carbon/c-Fe 2 O 3 a nanocomposite Alizarin red S adsorption from aqueous samples with the maximum adsorption capacity reaching 108.69 mg/g. Feiqiang et al. [14] synthesized magnetic-activated carbons by the modified onestep method for removing malachite green from solutions. Spinel CoFe 2 O 4 is a magnetic material with drastic anisotropy and high coerciveness at room temperature. Moreover, CoFe 2 O 4 has excellent physical and chemical stability [15] for dye adsorption. erefore, combining AC and CoFe 2 O 4 nanoparticles also has been studied to make efficient adsorbent with highly magnetic separating ability. Ai et al. [16] successfully manufactured activated carbon/CoFe 2 O 4 composite (AC/CFO) by the simple onestep refluxing route for adsorbing malachite green. Reports of the study showed that AC/CFO could be utilized as a promising and effective adsorbent to adsorb malachite green with the adsorption capacity of 89.29 mg/g. Liang et al. [15] did a study to make CoFe 2 O 4 /activated carbon (CoFe 2 O 4 /AC) through a one-step low-temperature refluxing route to adsorb gentian violet.
is material presented a remarkable adsorption capacity of gentian violet with 184.2 mg/g at 303°K. Qiu et al. [17] made an investigation to make CoFe 2 O 4 /activated carbon composites by a simplified hydrothermal technique for removing Cr(VI) from wastewater. e results specified that the Cr(VI) adsorption process using CoFe 2 O 4 /activated carbon was more powerful than the pristine AC. According to the current knowledge, the effect of molar ratios of Co : Fe:AC on making CoFe 2 O 4 /AC for dye adsorption is still unclear though there were a few reports of systematic in-depth studies examining the adsorption of RhB onto CoFe 2 O 4 /AC composites.
In this study, activated carbon was produced from coconut shell (CAC), and activated carbon/CoFe 2 O 4 was prepared through a single-step refluxing router with various molar ratios of Co : Fe:CAC. e analyses of the physicochemical properties of the adsorbent material were apprehended by SEM, EDX, XRD, and FTIR. e adsorption process was investigated with parameters including molar ratios of Co : Fe:CAC, solution pH, contact time, initial Rhodamine B (RhB) concentration, and adsorbent dosage. e evaluations of isotherm, adsorption kinetics, and mechanism of RhB adsorption onto activated carbon/CoFe 2 O 4 were also investigated in batch experiments. e stock Rhodamine B (RhB) solution at a concentration of 500 mg/L was produced by dissolving 0.5 g RhB in 1000 mL of distilled water. All experimental solutions were achieved by diluting the stock solution with distilled water.

Preparation of Coconut Activated Carbon/CoFe 2 O 4 (CAC/CoFe 2 O 4 ).
e coconut shell was collected from Ben Tre province, Vietnam. At first, the coconut shell was rinsed several times with tap water and afterward with distilled water to remove surface dust before being dried at 105°C to obtain a constant weight of dry matters. en, the dried coconut shell was crushed until a particle diameter ranges from 10 to 100 mm. Next, the coconut activated carbon (CAC) was processed by heating under slow pyrolysis at 700°C for 3 h in a Nabertherm furnace (model L3/11/B170, Germany) under water stream in nitrogen at a flow rate 100 mL/min. Afterward, the furnace was cooled down to the ambient temperature and the products were ground until the size of each particle was less than 0.5 mm. CAC was, then, cleansed and dried in an oven at 105°C for 2 h. e CAC/CoFe 2 O 4 composites were synthesized by a single-step refluxing router [16]. In this synthesis process, 3.4 g NaOH was diluted in 150 mL distilled water before adding a certain proportion of CAC and stirring for 30

Characterization of CAC/CoFe 2 O 4 .
e Brunauer-Emmett-Teller (BET) surface area and the pore structure of CAC and CAC/CoFe 2 O 4 were analyzed by Micromeritics SSA-4300 surface analyzer. e surface morphology of prepared samples was determined by energy-dispersive X-ray spectroscopy (Hitachi S-4800) with EDS and SEM systems. e crystalline structures of CAC, CAC/CoFe 2 O 4 , and RhB-CoFe/CAC were determined by the X-ray diffraction technique using XRD-D8 ADVANCE with Cu Ka radiation (λ � 1.5417Å). e 2θ angle scanned from 10°to 70°at a scanning speed of 3°/min. e 2 Journal of Chemistry identification of functional groups of CAC/CoFe 2 O 4 before and after adsorbing Rhodamine B was clarified by Fourier transform infrared spectroscopy (FTIR-6300) in a range of 500-4000 cm −1 . e magnetic property of CAC/CoFe 2 O 4 was measured at room temperature using a vibrating sample magnetometer (VSM LakeShore 7404). e pH value at zero charges (pH PZC ) was obtained by the Mular-Robert titration method [18]. A known amount of CAC/ CoFe 2 O 4 (1 g) was put into 100 mL of 0.1 M KCl solution with pH adjustment in the range from 2 to 12 done by the addition of 0.1 M NaOH or 0.1 M HCl (pH is ). e flasks then were sealed and shaken in 24 hours before recording the final solution pH (pH fs ).
e ΔpH values (ΔpH � pH is −pH fs ) were orchestrated against pH is . e pH PZC represents the intersection point of the curve and ΔpH.

Adsorption Procedure.
Adsorption experiments of RhB onto CAC/CoFe 2 O 4 were conducted using a batch equilibrium technique. All experiments were performed in triplicate. A certain amount of CAC/CoFe 2 O 4 was placed in 50 mL Erlenmeyer flasks that contained 25 mL of RhB at various concentrations. e flasks were secured with paraffin and afterward agitated at 120 rpm by the PH-4A shaker machine (China) at ambient temperature. e management of solution pH was carried out by using HCl 0.1 M and NaOH 0.1 M. After the adsorption process was finished, adsorbents were filtered from the dye sample using 0.11 μm filter paper. e content of RhB in the suspended solution was determined at 554 nm using a UV-Vis spectroscopic method [19]. e RhB adsorption capacity of CAC/CoFe 2 O 4 calculations at time t (denoted as q t , division: mg/g) and equilibrium (denoted as q e , division mg/g) was identified by equations (1) and (2), respectively: where C o (mg/L), C t (mg/L), and C e (mg/L) are RhB concentrations in the solution at the beginning time, random time t, and equilibrium, respectively. V (L) is the volume of the RhB solution and m (g) is the mass of the CAC/CoFe 2 O 4 .

Data
Analysis. e obtained data from experimenting was processed for analysis by Origin software 8.1. e error bars in all figures appear for the standard deviation of a triplicate test.

Optimizing the Ratio of CAC and CoFe 2 O 4 to Produce CAC/CoFe 2 O 4 on Rhodamine B Adsorption.
e experiments were carried out in order to apprehend the evaluation of the adsorption capacity of CAC, CoFe 2 O 4 , and CAC/CoFe 2 O 4 at various molar ratios of Co : Fe : CAC (1 : 2 : 300; 1 : 2:250; 1 : 2 : 200; 1 : 2 : 150, and 1 : 2 : 100). Other parameters were set with an adsorbent dose of 0.05 g/25 mL, initial RhB concentration of 50 mg/L, and adsorption time of 60 min at room temperature. Figure 1 indicates that the adsorption capacity of CAC/CoFe 2 O 4 for RhB was better than that of CAC and CoFe 2 O 4 . e adsorption capacity of CAC and CoFe 2 O 4 only reached 8.15 mg/g and 9.67 mg/g, respectively, while it increased from 13.57 mg/g to 17.68 mg/g with increasing molar ratios of Co : Fe in making CAC/CoFe 2 O 4 adsorbent (Co : Fe : CAC at 1 : 2:300; 1 : 2 : 250, and 1 : 2 : 200).
is is due to the existence of CoFe 2 O 4 nanoparticles in the micropores of CAC that can coordinate highly with the carboxyl group in molecular Rhodamine B [20]. However, the adsorption capacity of bare CoFe 2 O 4 was also lower than of CAC/CoFe 2 O 4 composites because of its lower specific surface area and pore volume [15,21]. Figure 1 also shows that the adsorption capacity of CAC/ CoFe 2 O 4 for RhB decreased with the molar ratios of Co : Fe:CAC at 1 : 2:150 and 1 : 2:100. is may be a saturation of the active sites on CAC/CoFe 2 O 4 surfaces. us, there were not active sites sufficient for the attachment of RhB onto CAC/CoFe 2 O 4 [22]. According to the above result, CAC/CoFe 2 O 4 synthesized at a molar ratio of Co : Fe:CAC that is 1 : 2:200 (CAC/CoFe 2 O 4 200) shows the highest adsorption capacity for RhB. erefore, it was chosen for further experiments.

Characterization of Activated Carbon-CoFe 2 O 4 Composite (CAC/CoFe 2 O 4 ).
e Brunauer-Emmett-Teller (BET) results disclose that CAC has a large specific surface area of 867.449 m 2 /g, and the average pore volume was 0.381 cm 3 /g. However, the presence of CoFe 2 O 4 affected the characteristics of CAC/CoFe 2 O 4 with a slight decrease of the surface area (to 759.638 m 2 /g) and pore volume of CAC/CoFe 2 O 4 200 (to 0.321 cm 3 /g), respectively. is can explain that the CoFe 2 O 4 has loaded on the coconut activated carbon leading to the benefits of adsorbing other molecules [23]. Similar results were obtained in previous reports of magnetic oxide/activated carbon composites [16,24]. e results of SEM from Figures 2(a) and 2(b) indicated that the structure of CAC was modified after being composited by CoFe 2 O 4 . Besides, the results of EDS analysis revealed that CAC is composed of 100% C (Figure 2(b)  e result is shown in Figure 4(a). In the pH level ranging from 2 to 4, the removal efficiency and adsorption capacity for RhB increased sharply from 30.26 to 60.98% and 9.14 to 18.81 mg/g, respectively, whereas the pH level of 4 to 8 made the adsorption capacity of CAC/CoFe 2 O 4 200 to RhB relatively stable. However, there were significant declines in the sorption capacity as well as the removal efficiency of RhB corresponding to the increase in pH from 9 to 10. To make an explanation for this result, we need to consider the structure of RhB dye and the zero point charge of CAC/CoFe 2 O 4 200. At solution pH lower than 3.5, RhB ions are cationic (RhB + ) and molecules are in monomeric form. When pH is higher than 3.5, the zwitterionic form of RhB (RhB ± ) increased [8]. Moreover, pH PZC of CAC/CoFe 2 O 4 200 was 8.34. At solution pH below pH PZC (<8.34), the surface of CAC/CoFe 2 O 4 200 was positively charged with more H + ions while RhB at pH < 3.5 was also cationic. us, H + ions might compete with RhB + cations in solution causing low adsorption [25]. At pH above 3.5, the adsorption capacity grew sharply due to the surface of RhB that was changed to zwitterionic form causing the attractive electrostatic interactions between RhB ± ions and the surface of CAC/CoFe 2 O 4 200 [6]. However, at solution pH ≥ 9, the deprotonation of RhB dye increased and the surface of CAC/CoFe 2 O 4 200 was negatively charged leading to the decrease in adsorption rate [26]. Similar results were reported in some studies [10,25].  Figure 5 indicates that the adsorption capacity and removal efficiency of RhB grew rapidly from 8.07 to 18.09 mg/g and from 29.65 to 66.46%, respectively, for the first 60 min of contact time. While extending the contact time, the adsorption of RhB slowed down and reached equilibrium at 150 min with the maximum adsorption capacity and removal efficiency that were 23.14 mg/g and 84.98%, respectively. is could be due to the fact that at the initial stage of the adsorption process, the surface of CAC/ CoFe 2 O 4 200 had a large number of active sites available for RhB attachment. After that, because of adsorption of RhB molecules on the CAC/CoFe 2 O 4 200 surface leading to saturation of adsorption sites, the adsorption reduced and became stable at equilibrium point [10]. Some other studies also reported a similar tendency [27,28].

Effect of Absorbent Dose.
e experiment studying the effects of applied CAC/CoFe 2 O 4 200 doses on RhB adsorption was arranged at different dosages ranging from 25 to 200 mg/ 25 mL, initial RhB concentration of 50 mg/L, solution pH of 4, and contact time of 150 min. e results are given in Figure 6. It was observed that with the CAC/CoFe 2 O 4 200 dosage increasing from 25 to 100 mg/25 mL, the RhB removal efficiency increased significantly from 62.96% to 92.34%. e result may be explained following an increase of active sites corresponding to the increased CAC/CoFe 2 O 4 200 dosage [1]. However, the percentage of adsorption did not increase and became stable as the adsorbent dose further increased from 100 to 200 mg/25 mL, whereas the RhB adsorption capacity reduced from 34.29 to 6.26 mg/g corresponding to CAC/CoFe 2 O 4 200 dose rising from 25 to 200 mg/25 mL. All experiments were investigated at constant initial RhB concentrations and volumes. erefore, the active sites on CAC/CoFe 2 O 4 200 surface may be saturated when the adsorbent dose increased causing the decrease in adsorption capacity [29]. e above results showed that the applied adsorbent dosage plays a vital role in the adsorption process, which was also found in other studies [7,28].

Effect of Initial Rhodamine B Concentration.
As a factor that considerably affects the adsorption capacity, the initial RhB concentration and its effects require clarification and evaluation. e experiment examining the influence of variations of the initial RhB concentration from 20 to 400 mg/L on its adsorption onto CAC/CoFe 2   initial RhB concentration raised to 350 mg/L ( Figure 7). However, the adsorption capacity did not increase and became stable at the initial RhB concentration of more than 350 mg/L. Otherwise, the removal efficiency of RhB decreased from 93.29% to 45.56% corresponding to the initial RhB concentration growing from 20 to 400 mg/L. is phenomenon can be due to the fact that at a higher concentration, the driving force of the concentration gradient rises leading to the increasing adsorption capacity [30]. However, with the fixed amount of adsorbent dosage, the adsorption sites were limited. us, at a higher RhB concentration, the ratio of activating sites and RhB molecules was low causing the decrease in RhB removal [10]. Some previous studies such as the study of RhB adsorption onto kaolinite [28] or MgO supported Fe-Co-Mn nanoparticles [2] also had the same tendency.

Isotherm
Modeling. e distribution of Rhodamine B molecules between the liquid state and the solid state can be provided by the isotherm parameters. Langmuir, Freundlich, and Temkin isotherm models were utilized as analyzers for the gathered experimental data. According to the Langmuir model, it is assumed that the energy of all adsorption sites is equivalent, the sorption surface is homogeneous, and molecule adsorption processes do not interact with each other [31]. Meanwhile, the Freundlich model   explains the multilayer adsorption on heterogeneous surfaces and the difference in the energy of all adsorption sites [6]. On the other hand, Temkin isotherm supposes that the heat of adsorption reduces linearly and binding energies were distributed uniformly [28]. e equations of Langmuir, Freundlich, and Temkin models are described as shown in (3) [31], (4) [6], and (5) [28], respectively: where C e and q e are the RhB concentration and the adsorption capacity (mg/g) at equilibrium (mg/L), respectively, q m is the Langmuir adsorption capacity (mg/g), K L is the Langmuir constant (L/mg), K f is the Freundlich coefficient (mg/g), n is the adsorption intensity, A T is the Temkin isotherm equilibrium binding constant (L/g), b is the Temkin isotherm constant (J/mol), R is the universal gas constant (8.314 J/mol/K), and T is the temperature at 298 K. Figure 8 and Table 1 show the results of Langmuir, Freundlich, and Temkin isotherm models for the adsorption of RhB onto CAC/CoFe 2 O 4 200. According to the calculated data, the correlation coefficient for the Freundlich isotherm model (0.985) was higher than the values obtained for Langmuir (0.978) and Temkin (0.957) isotherm. is expressed that the RhB adsorption onto CAC/CoFe 2 O 4 200 isotherm fitted well with the Freundlich model. is indicated that the mechanism of RhB adsorption was multilayer and the surface of the adsorbent was heterogeneous [10]. In addition, the 1/n value given by the Freundlich equation was 0.377 < 1, which confirmed the favorability of RhB adsorption onto CAC/CoFe 2 O 4 200 [6,32]. However, for the Langmuir model, R 2 value was 0.978 and the q m value was approximately 107.48 mg/g corresponding to q mexp (94.08 mg/g). is evidenced that the monolayer adsorption also had a vital part in the reception of RhB onto CAC/CoFe 2 O 4 200 [33]. e Langmuir maximum adsorption capacity of RhB onto CAC/CoFe 2 O 4 200 compared with other adsorbents was also reported in the references ( Table 2). It can be seen that the adsorption capacity of CAC/CoFe 2 O 4 200 for RhB is relatively higher than some adsorbents (Jute stick powder, Fe 3 O 4 /Al pillared bentonite, cobweb-mediated AgNPs, kaolinite, duolite C-20 resin, and surfactant-modified coir pith) but lower than that of Gg-Cl-P(AA-Co-AAm)/Fe 3 O 4 nanocomposites, CoFe 2 O 4 @vacancy@mSiO 2 , and Zn/CoZIF-derived carbon.
ln q e − q t � ln q e − k 1 t, where q t (mg/g) and q e (mg/g) are the adsorption capacity at time t and at equilibrium, respectively, k 1 (min −1 ) is the firstorder rate constant, and k 2 (g/mg·min) is the second-order rate constant. e results of linear models fitting the experimental data with pseudo-first-order and pseudo-second-order models are expressed in Figure 9, and the kinetic parameters are shown in Table 3. e value of correlation coefficient (R 2 ) of the RhB adsorption onto CAC/CoFe 2 O 4 200 achieved via the pseudo-second-order equation (7) was 0.934, while the correlation coefficient value of the pseudo-first-order was R 2 � 0.874. Besides, the adsorption capacity calculated (q e cal ) from pseudo-second-order was 25.26 mg/g, which was relatively fitting with the experimental result (q e exp � 23.14 mg/ g). ese indicated that the adsorption of RhB onto CAC/ CoFe 2 O 4 200 followed the pseudo-second-order kinetic model and the adsorption process inclined more toward chemisorption [6]. e same result was also reported in the RhB adsorption onto cobalt nanoparticles-embedded magnetic ordered mesoporous carbon (Co/OMC) [39], CoFe 2 O 4 @vacancy@mSiO 2 [10], and Zn/Co ZIF-derived carbon [34].

Proposed Adsorption Mechanisms.
e RhB adsorption process and functional groups available on the surface of CAC/CoFe 2 O 4 200 share a firm association. e surface characteristics of adsorbents are affected by the type and quantity of functional groups that are identified by FTIR ( Figure 10). e FTIR spectral results show the difference of the functional groups presenting in CAC, CAC/ CoFe 2 O 4 200, and CAC/CoFe 2 O 4 200-RhB. A peak at approximately 3660 cm −1 proposed the stretching of the O-H group [40,41] in all CAC, CAC/CoFe 2 O 4 200, and CAC/ CoFe 2 O 4 200-RhB. Two broad peaks at around 2981 cm −1 and 2900 cm −1 are involved in the C-H stretching in alkane groups [42]. A small peak at 2319 cm −1 pointed out the stretching vibration of C≡C [30]. e peaks at 1392 cm −1 and 1479 cm −1 corresponded to the symmetrical C-H vibration [30]. e strong peaks in the range from 1050 cm −1 to 1288 cm −1 also marked the C-O stretching vibration [15,42]. e peaks observed in the range from 698 cm −1 to 894 cm −1 were about the C-H group [42]. However, the peaks at around 1741 cm −1 and 1604 cm −1 corresponding to the C�O vibration and the C�C stretching vibration of the aromatic ring, respectively [40], appeared in CAC but disappeared in   [44]. e calculated results of isotherm models and adsorption kinetics indicated that monolayer, multilayer, and chemisorption were dominated for RhB adsorption onto CAC/CoFe 2 O 4 200. erefore, the negatively charged surface and positive charged + NH-CH 3 RhB molecules might be formed owing to the electrostatic attraction, hydrogen bonding interaction, and π-π interaction [45]. Additionally, the new peaks at 35.61°and 61.78°( Figure 11)

Regeneration of Adsorbent.
e reuse of adsorbent is the requirement to minimize the cost of adsorption processes. erefore, CAC/CoFe 2 O 4 200 should be regenerated after RhB adsorption and desorption for further cycles. Figure 12  e main adsorption mechanisms of RhB onto CAC/CoFe 2 O 4 200 included the electrostatic attraction, hydrogen bonding interaction, and π-π interaction. Finally, it is possible to conclude that the CAC/CoFe 2 O 4 200 is efficient for adsorbing RhB away from aqueous solutions and should be promoted in the future.

Data Availability
e data used to support the findings of this study are included within the article.