Functionalized Mesoporous Carbon Nanostructures for Efficient

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Functionalized Mesoporous Carbon Nanostructures for Efficient Removal of Eriochrome Black‑T from Aqueous Solution Pitchaimani Veerakumar,*,†,‡ Tharini Jeyapragasam,*,§ Surabhi,§ Kamaraj Salamalai,∥ Thandavarayan Maiyalagan,⊥ and King-Chuen Lin*,†,‡ †

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan § Department of Chemistry, Sethu Institute of Technology, Kariapatti 626115, Tamil Nadu, India ∥ Department of Mechanical Engineering, PSN Institute of Technology and Science, Tirunelveli 627152, Tamil Nadu, India ⊥ SRM Research Institute, Department of Chemistry, SRM Institute of Science & Technology, Kattankulathur 603203, Tamil Nadu, India

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 03/19/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Acid-functionalized mesoporous carbons (AFMPCs) have been synthesized and used as adsorbents for removal of an azo dye, Eriochrome Black-T (EBT), from aqueous solution. To generate acid surface functionalities, mesoporous carbons (MPCs) were treated with sulfuric acid. Characterization of the samples was analyzed by XRD, Raman spectra, N2 adsorption−desorption, FE-TEM, TGA, and FTIR studies. The adsorption studies were carried out under various parameters, such as pH, adsorbent dosage, contact time, initial dye concentration solution temperature, and salt concentration. The results showed that the EBT adsorption onto samples was affected by the pH of solution; the maximum EBT ion adsorption took place at pH 1; and the adsorption uptake was increased with an increase in the initial dye concentration. Moreover, the mechanism of adsorption was investigated using kinetic, diffusion, and isotherm models. The best fit was obtained by the Langmuir model with high correlation coefficients (R2 = 0.9463) with a maximum monolayer adsorption capacity of 117.0 mg·g−1. The adsorbed anionic EBT dye molecules were eluted by ethanol solvent with the recovery percentage of 98%. Moreover, this study demonstrates that AF-MPCs can be successfully used as a low-cost adsorbent for the removal of EBT from aqueous solutions.

1. INTRODUCTION With the growing world population, industrial activity leads to the contamination of air, soils, and aquatic ecosystems. Water pollution and its consequences have become a concern of major research focus. The textile industry consumes a substantial amount of water during the dyeing and finishing operations. Nearly 70% of azo dyes were currently used in the textile industry throughout the world. The important structural feature of azo dyes is the presence of one or more azo linkages (−NN−) which act as the chromophore in the molecular structure.1 Along with the azo linkage, an aromatic moiety or aromatic heterocyclic units and sulfonate groups were present as the substituents.2,3 Due to the complex structure of azo dyes, they persist in the aquatic medium, affecting the aquatic life and hence causing the destruction of ecosystems.4−6 Due to high resistance to light, thermal, chemical, and microbial attack, azo dyes are difficult to remove even at a low concentration.7 Toxic, mutagenic, and carcinogenic amines were formed as the result of anaerobic microbial reductive cleavage of the azo (−NN−) bond.8−10 Hence it is © XXXX American Chemical Society

important to remove azo dye from the wastewater effluent before discharging into the aquatic system. Eriochrome Black-T (EBT), as one of the azo dyes, finds extensive applications in the textile, dying, printing, and cosmetics industries. It is commonly used as a metal ion indicator in analytical chemistry for the determination of calcium and magnesium contents in water.11 EBT is hazardous and carcinogenic, and its degradation products like naphthaquinone are even more carcinogenic.12 Therefore, it becomes important to develop an efficient adsorbent for removing the acid EBT dye from the aqueous solution. The methods such as chemical coagulation,13 adsorption,14 photodegradation,15 biodegradation,16 and membrane separation have been investigated for the removal of dye molecules. Among these techniques, adsorption is the most effective for eliminating the dye molecules from the industrial effluent due Received: September 29, 2018 Accepted: February 27, 2019

A

DOI: 10.1021/acs.jced.8b00878 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Scheme 1. Schematic Representation for the Preparation Process of AF-MPCs and Adsorption of EBT Dye

to its low cost, time savings, and low toxicity.17,18 Various adsorbents such as activated carbon,19 β-cyclodextrins/polyurethane,20 nickel ferrite nanoparticles,21 and synthetic iron particles22 have been adopted as the efficient adsorbent for the EBT removal from the aqueous solution. Recent work has suggested that adoption of carbonaceous materials is one of the most efficient methods for dye removal and decoloration.23,24 They contain mainly carbon blacks (CBs), activated carbons (ACs), and activated carbon fibers (ACFs). In particular, ACs can efficiently remove dyes in wastewater including azo dyes which are known to be hard for biodegradation. However, the micropore sizes of these ACs restrict their practical applications. For instance, Walker et al.25 reported only 14% of pores can be occupied by the dyes in the adsorption of three kinds of acidic dyes on AC. When the molecules become larger, the utility of the micropores in ACs used for molecular adsorption is even lower. For this reason, lots of methods are inspired to increase mesopore volumes in ACs.26 For synthesis of mesoporous carbons with uniform pores, low-cost raw materials such as phenol, formaldehyde, and triblock copolymer pluronic F127 have been used to obtain highly ordered mesoporous carbon (MPC) powders with pore sizes from 3 to 8 nm. The large-scale production of MPCs can thus be facilitated and practically applied to the adsorption of bulky organic dyes27 because MPCs possess high surface areas and large and tunable pore channels, showing the merits to adsorb the large molecules. For example, Hartmann et al.28 found the larger capacity of the mesoporous carbon replica to adsorb vitamin E than ACs. Vinu and co-workers developed the MPCs with a large pore size and pore volume to adsorb protein;29 they also investigated different adsorbents for 30 L-histidine and lysozyme. In addition, Hyeon and co31 workers synthesized nanoporous carbon materials with a large surface area as an attractive way to exhibit excellent adsorption capacities for bulky dyes. This opens up a high application potential of MPC materials as novel and efficient adsorbents for dye removal from aqueous solution owing to its large surface area, high pore volume, mesoporous structure, thermal stability, nontoxicity, chemical inertness, and strong interaction with organic dye molecules.32−34 With the oxidative treatment of MPCs, numerous hydrophilic groups were formed in the mesopore channels without destroying its ordered mesostructure. The functionalization of ordered largepore mesoporous carbons possessed high densities of surface

oxygen complexes, especially carboxylic groups, which offer the merits to serve as efficient adsorbents.35,36 The main objective of this work is to synthesize AF-MPCs using the soft template method, and the nanomaterial is then characterized by using Raman spectroscopy, thermal gravimetric analysis, N2 adsorption/desorption isotherms, and differential thermal analysis. The AF-MPC thus prepared was applied to adsorb EBT molecules from aqueous solution (Scheme 1). The AF-MPC synthesized with remarkable adsorption capacity toward EBT can be attributed to its large specific surface area and highly ordered mesoporous structure. For understanding the related mechanism, its adsorption kinetics and adsorption isotherm were further characterized using various models. The involved thermodynamic properties such as Gibbs free change (ΔG°), enthalpy change (ΔH°), entropy change (ΔS°), enthalpy of activation (ΔH*), entropy of activation (ΔS*), free change of activation (ΔG*), activation energy, and Arrhenius factors were also calculated.

2. EXPERIMENTAL SECTION 2.1. Materials. Phloroglucinol (99.98%, Acros), formaldehyde (37% in water, Acros), HCl (37% Acros), triblock copolymer Pluronic F-127 (EO106PO70EO106, Mw = 12,600, Sigma-Aldrich), Erichrome Black T (C20H12N3O7SNa, SigmaAldrich), and ethanol (C2H5OH, 99%) were obtained commercially and used without further purification. The EBT dye stock solution of 1000 mg L−1 concentration was prepared and diluted to the required concentrations using deionized (DI) water. The dye solution was buffered to the alkaline or acidic pH by adding 0.1 M NaOH or HCl. All of the glassware required was cleaned thoroughly with “Aqua regia” composed of concentrated hydrochloric and nitric acids (3:1) and then rinsed with deionized water. 2.2. Preparation of Acid-Functionalized Mesoporous Carbon. For the preparation of MPCs, the Phl-F (phloroglucinol−formaldehyde) sol−gel method was employed as reported.37,38 Typically, 3.75 g of Phl, (99.98%, Acros) and 4.0 g of pluronic F127 (PF-127, Mw = 12,600, Sigma-Aldrich) were dissolved in 9.0 mL of solvent containing an equal volume of ethanol and water (C2H5OH:H2O). Aqueous Phl was mixed with Na2CO3 (0.05 g) to accelerate dehydrogenation of Phl. After stirring the solution for a few minutes, 3.7 mL of F (37% in water, Acros) was slowly added into the solution to form a sol. Ambient drying of sol was then done at B



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Figure 1. (a) XRD pattern, (b) N2 adsorption−desorption, (c) pore size distribution, (d) Raman spectra, (e) TGA, and (f) DTA curves of MPC and AF-MPCs.

where qe (mg·g−1) denotes the equilibrium adsorption capacity; C 0 and C e are the initial and equilibrium concentration (mg·L−1) of EBT; and V (mL) and W (mg) are the volume of the solution and weight of the adsorbent, respectively. 2.4. Determination of Point of Zero Charge (pHZPC). The point of zero charge of as-prepared carbon samples was determined by adding 20 mL of 0.05 M NaCl to a series of 100 mL of Erlenmeyer flasks, and the pHi values of each solution were adjusted from 1 to 10. Then, 4.0 mg of carbon materials was added to each flask solution with pHi accurately noted. After shaking the suspensions for 6 h, we measured their final pH values again. The difference of pH, ΔpH = (pHi − pHf), between the initial and final value, was plotted against. pHi and the intersection point of the resulting null ΔpH correspond to the point of zero charge. 2.5. Instrumentation. The X-ray diffraction (XRD) pattern was recorded by a PANalytical X’Pert PRO diffractometer using Cu−Kα radiation (λ = 0.15418 nm). The Raman spectra were obtained with excitation at 488 nm from an Ar+ laser which was equipped with a charge coupled device (CCD) detector (Jobin Yvon T64000). The elemental analysis was conducted using a Thermo Flash 2000 CHNS/O Analyzer. Nitrogen adsorption−desorption isotherms were recorded with a Quantachrome Autosorb-1 physisorption system at 77 K. The Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface areas and pore-size distributions of the as-prepared carbon samples. The X-ray photoelectron spectroscopy (XPS) was performed using a ULVAC PHI PHI 5000 Versa Probe apparatus. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were applied to track the thermal decomposition process using a NETZSCH TG-209 instrument under air atmosphere. pH was monitored using a SevenCompact pH meter S220. Fourier transform infrared (FT-IR) spectroscopy was applied with KBr powder to record the 4000−400 cm−1 spectrum with a spectral resolution of 2 cm−1 using a Bruker IFS28 spectrometer.

room temperature for 1 day. The polymeric gel was then loaded on a large Petri dish, dried at RT overnight, cured at 80 °C for 2 h, and then exposed to microwave radiation controlled at power of 300 W for 2−3 h, followed by curing at 100 °C. The template was removed thermally by carbonization under the conditions of N2 atmosphere at 400 °C for 3 h with a heating rate of 3 °C min−1 and then at 900 °C for 3 h with a heating rate of 5 °C min−1. After carbonization, natural cooling was processed. The resultant carbon powder was denoted as mesoporous carbons (MPCs). Acid-functionalized mesoporous carbon (AF-MPCs) was prepared as follows:39 first, MPC powder (500 mg) was dispersed in 25 mL of H2SO4 (5.0 M) (added dropwise) by ultrasonic bath for 45 min. Then, the mixture was stirred vigorously and refluxed overnight to reach complete oxidation. The resultant product was filtered and then washed with ethanol and DI water, followed by drying in an oven at 80 °C to obtain AF-MPCs. The resultant carbon was kept in a desiccator for further use. 2.3. Dye Removal Experiments. The dye removal experiments were performed in an iodine flask which contained 20 mL of dye solution at different concentration from 50 to 150 ppm and 80 mg of AF-MPCs with appropriate pH adjusted. The flask was shaken in the shaker (GENEI TMSLM-INC-OS-250) for a predefined time and then centrifuged. Finally, supernatant is analyzed for colorimetric studies. The EBT solution was detected with a visible colorimeter to determine the absorbance at 530 nm before and after the removal process. The adsorption capacity, qe (mg· g−1), indicating a mg of dye adsorbed on a g of AF-MPCs and dye removal percentage were calculated using eqs 1 and 2 qe =

(C0 − Ce)V W

Removal (%) =

(C − Ce) × 100 C0

(1)

(2) C



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Table 1. Physical Properties of MPCs and AF-MPC Samples textural properties

elemental analysis (wt %)

sample

Stota (m2·g−1)

Smicrob (m2·g−1)

Vtota (cm3· g−1)

Vmicrob (cm3· g−1)

Vmesoc

Vnd (%)

Dpe (nm)

C

H

O

ID/IG

MPCs AF-MPCs

734.2 623.9

340.6 430.8

0.43 0.41

0.1451 0.1824

0.2849 0.2276

33.7 44.5

4.73 4.39

71.12 73.08

7.38 7.25

21.37 19.28

0.99 0.98

a

Total BET surface area, Stot, and pore volume, VTot, derived at P/P0 = 0.99. bMicroporous surface area, Smicro, and pore volume, Vmicro, obtained from t-plot analyses. cMesopore volume (Vmeso = Vtot − Vmicro). dVn = Vmicro/Vtot. ePore diameter, Dp, calculated by the Barrett−Joyner−Halenda (BJH) method using adsorption branches of isotherms.

materials (Figure 1b).45 The total surface area (Stot) decreases from 734.2 to 623.9 m2·g−1 and total pore volumes (Vtot) from 0.43 to 0.41 cm3·g−1, which may be caused by abundant surface functionalities on the MPC surface. In addition, the surface area of AF-MPC is reduced much more than that of MPC; the resulting pore volume, pore diameters, and the subsequent density between AF-MPC and bare MPC are listed in Table 1. The total surface areas (Stot) of the MPC before and after acid reaction are summarized in Table 1. It is noticeable that the Stot decreases from 734.2 to 623.9 m2·g−1. Total pore volumes (Vtot) from 0.43 to 0.41 cm3·g−1 are comparable, whereas the pore size for MFC (Dp = 4.73 nm) is greatly decreased compared with Dp= 4.39 nm because the functionalization with functional groups would reduce the specific surface area and pore size of the AF-MPC sample. Furthermore, the pore diameters (Dp) were calculated based on the Barrett−Joyner−Halenda (BJH) method (Figure 1c), and the results are presented in Table 1. The Stot and Vtot of the AF-MPC sample are significantly lower than the untreated samples. The acid-treated sample (AF-MPC) shows a decreased surface area of 623.9 m2·g−1 and pore volume of 0.41 cm3·g−1. This phenomenon indicates that introducing acid treatment on the MPC might produce the oxygen-containing functional groups (e.g., −OH, −CO, −COOH) which may be located on the surface of mesopores to occupy pore spaces.46 These groups are preferred to attract with the chemical species or dye molecules. The decrease is due to the formation of functional groups in the MFC pore channels.47,48 Interestingly, the acid treatment of MPCs is likely to increases microsurface areas (Smicro) and micropore volume (Vmicro); this fact is because an increase of more new micropores is created as well as surface groups formed in the interior side of the carbons during acid treatment. Furthermore, the elemental compositions of MPCs and AFMPCs were examined by a Thermo Flash 2000 CHNS/O Analyzer. The AF-MPC sample contains carbon (71.12 wt %), hydrogen (7.38 wt %), and oxygen (21.37 wt %). The presence of heteroatoms such as O and H enhanced the adsorption properties of AF-MPCs. The increase of H and O contents may dramatically decrease the carbon contents after the H2SO4 treatment (Table 1), thus indicating the successful acid group functionalization. Raman spectroscopy was employed to inspect the electronic properties of carbon materials.49 Figure 1d shows that the MPC and AF-MPC had two common peaks located at about 1320 and 1580 cm−1 which were attributed to the characteristic D and G peaks of carbon materials. The ID/IG ratio is 0.99 and 0.98 for MPC and AF-MPC, respectively. No additional peaks were detected. All the Raman results are consistent with the following TG results (vide supra). Thermogravimetric analysis of the carbon materials yields the weight loss as a function of temperature, and the plot is displayed in Figure 1e. Weight loss from thermal desorption of

3. RESULTS AND DISCUSSION 3.1. Characterization of AF-MPCs. The crystallinity or amorphousness can be checked by powder XRD measurements. The XRD patterns of as-prepared MPC and AF-MPC samples were characterized, mainly by one peak placed around at 2θ = ∼23.3° (002) which corresponds to the (002) reflection (low degree of graphitization). A strong graphite peak was observed for MPC, denoting perfect crystallinity, which is ascribed to stacking of the graphene layers. The other two peaks at around 43.8° and 80° correspond to the (100) and (111) reflections stemming from the in-plane structure of graphitic crystallites of carbonaceous material.40 However, Figure S1 shows powder XRD patterns for the low-temperature carbon materials examined (Supporting Information, hereafter denoted as SI). Even though the measurements were taken with the same sample amounts and for the same duration, the peak intensity of the activated carbon was 2 orders of magnitude lower than that of graphite. This difference is due to the lower crystal quality of the carbon sample: in other words, the amorphousness of the carbon resulted in a lower intensity of the XRD signal. When Phl-F was heated at 80 °C, at this stage only the polymerization process was established and yielded only an amorphous or disordered nature of carbon (not like graphitic crystallites). These polymeric organic components have little or no surface area. The XRD peak signals at 29.1° and 42.3° corresponding to the (002) and (100) planes of the amorphous carbon in this work were buried in noise even for the same measuring conditions, which highlight its superior amorphousness compared to that of the MPC. No structural information could be obtained from the XRD measurements for this carbon material prepared in this work, owing to its highly amorphous character. Likewise, when the carbonization temperature was at 400 °C, the micro- and mesopore development accelerates and was also completely removed by the template (i.e., surfactant). Therefore, this type of carbon has little graphitic crystalline nature and slightly shifted to 2θ ≈ 23.3° reflection, proving that this sample consists of small sp2 platelets in agreement with literature data.41 After carbonization at 900 °C, the disordered nature of carbons was transformed into graphitic form (Figure 1a). Hence, the MPC shows that two broad peaks can be distinguished with more intensity. This peak resembles those typical of crystalline graphite in agreement with literature data.42 Oxidation was performed using H2SO4 and is known to generate a higher ratio of oxidized functionalities (e.g., −OH, −CO, −COOH) on the MPC surface, which could perform competitive complexation with the other molecules of interest.43,44 The nitrogen adsorption/desorption isotherms of two carbon materials exhibit typical Type-IV curves with indicated capillary condensation steps at P/P0 ∼ 0.4−0.7 and a welldefined H1-type hysteresis loop anticipated for mesoporous D



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Figure 2. FE-TEM images of (a−e) pristine MPCs and (f−i) AF-MPCs.

water appeared at about 150 °C. The greater the water weight removed, the higher the hydrophilic nature of the material carried. After the MPC sample was carbonized to 900 °C, its TG curve showed a very small weight loss. Figure 1f shows the corresponding differential thermal analysis (DTA) curves. The weak peak near 350 °C indicates the existence of a small amount of amorphous carbon, and another peak at high temperature is attributed to the oxidation of MPC. The weight loss recorded for the AF-MPC samples in the region of 150− 800 °C was caused by decomposition of the oxygen functional groups on the surface of these samples. The weight loss below ∼300 °C is due to the decomposition of carboxylic groups. The less pronounced curve change for the decomposition of phenolic groups appears in the range 280−400 °C,50 while the weight loss at about 450−650 °C is due to the decomposition of organic (lactone, quinine, and anhydride groups) moieties.51,52 Given increased temperature for AF-MPC oxidation, the weight losses correspond to the decomposition of a carbon skeleton. Hence, the distinct difference between two curves obtained before and after acid treatment of MPC

suggests the burning of a carbon layer with attachment of surface groups and also confirms that the number of organic groups on the carbon surface increased after acid treatment. FE-TEM images permitted identification of the structures of the samples studied and evaluation of the degree of their ordering. The FE-TEM image of pristine MPCs (Figure 2a−e) shows a hexagonal structure with high ordering long channels. The oxidation of MPCs with a 5.0 M H2SO4 solution at a high temperature probably deteriorates the carbon layers to become an uneven surface with many cavities; the results are fairly similar to the literature previously reported.53 The surface composition of the MPC material before and after acid treatment was characterized by X-ray photoelectron spectroscopy (XPS). The survey spectra of two samples were displayed in Figure 3a and 3b. In the C 1s XPS spectrum (Figure 3a1), four peaks can be seen: the C−C bond at 284.4 eV, the C−O bond at 286.0 eV, the OC−C bond at 287.4 eV, and the π−π* shakeup satellite at 289.2 eV.54 After the prolonged acid treatment, four peaks were observed in highresolution C 1s XPS (Figure 3b1). The dominant peak at 284.6 E



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Figure 3. (a, b) XPS survey spectra of the MPC before and after acid treatment. (a1, b1) C 1s and (a2,b2) O 1s core-level XPS spectra of MPCs and (f) AF-MPCs.

Scheme 2. Proposed Mechanism of the Pore Formation During MPC Oxidation in Concentrated H2SO4 at Reflux Conditions

CO2. Accordingly, the protons left in the carbon matrix render the vacancies enlarged at the sites of aggregation of hydroxyl and epoxy groups; such a result is more likely to happen for highly oxidized graphites.39 3.2. Adsorption and Removal of EBT. The structure and some chemical properties of EBT dye are listed in Table S1 (SI). In order to evaluate the adsorption process, various experimental conditions such as stirring rate, contact time, pH, adsorbent concentration, and dye concentration were optimized. The stirring rate was varied from 50 to 250 rotations per minute (rpm), and the maximum adsorption was found at 250 rpm, which has thus been used for the removal of EBT from the aqueous solution. 3.3. Effect of Initial Concentration. Figure 4a shows the effect of initial concentration on the adsorptive removal of the EBT by MPC and AF-MPC. The concentration of the dye was varied from 50 to 130 mg L−1, while the solution pH, adsorbent dosage, and contact time were kept constant at 1, 4 mg, and 60 min, respectively. The adsorption of the dye was found to decrease with increasing dye concentration for AFMPC, due to higher concentration of the EBT dye molecules which restrict the adsorption sites on the MPC and AF-MPC surfaces. Figure 4a shows that the adsorbent surfaces were

eV was assigned to the C−C bond, and the adjacent peak centered at about 286.4 eV corresponding to C−O groups appeared to decrease slightly after acid treatment (285.7 eV) for C−O. The other peak at about 287.6 eV representing O− CO was found to enhance, indicating carbon oxidation to some extent. Figure 3a2 shows the results of the high-resolution analysis of the O 1s of MPCs which was deconvoluted into two peaks corresponding to the CO bond (532.9 eV) and C− O−C bond (535.2 eV), whereas the hydroxyl group or carboxyl group was denoted as C−OH/COOH and was centered at about 533.7 eV, according to the previous report.55 Stolyarova et al. have proposed the mechanism of the pore development in MPCs, as treated with hot concentrated H2SO4.39 The schematic is illustrated in Scheme 2. Increasing the synthesis temperature to 280 °C may lead to an increase in the pore size and the oxidation of graphitic planes. In the primary stage, acid-catalyzed elimination of water (Pinacol rearrangement) from Pinacol gives epoxy bond formation, which is less stable (metastable) than the carbon hexagon in the basal plane. This reaction may occur for a variety of fully substituted 1,2-diols in the carbon matrix. Under acid reflux conditions, the epoxy bond breaks to form two carboxyl (−COOH) groups at the pores of the carbon skeleton. While further heating, the carboxyl groups may decompose to release F



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Figure 4. (a) Effect of initial EBT dye concentration, (b) effect of AF-MPC dose, (c) removal efficiency, (d) effect of contact time, (e) effect of pH, and (f) point of zero charge of the AF-MPC sample.

saturated at higher concentration such that they cannot adsorb any more. 3.4. Effect of Adsorbent Dosage. Adsorbent dose plays an important role to affect the removal efficiency of an adsorbent. The solid adsorbent’s capacity is usually determined by the adsorbent mass for a given initial concentration of adsorbate in a solution. The adsorbent dose was varied from 1 to 4 mg·L−1, while the solution pH, adsorbent dosage, and contact time were kept constant at 1, 4 mg, and 60 min, respectively. Initially, removal efficiency increased with increasing adsorbent dose up to 4 mg·L−1 with the maximum removal efficiency of 91.5% for EBT, due to the increase in active surface binding sites for a given dose of adsorbent (Figure 4b). Then, no further increase in removal efficiency was observed, showing that the condition of equilibrium is established. As the adsorbent dose increases from 0.5 to 4 mg· mL−1, the adsorption rate increases; this fact could be attributed to the higher surface area (>1000 m2·g−1), large pore volume (>1 cm3·g−1), and uniform mesoporous structure of carbon. The strong electrostatic and hydrogen bonding interaction occurs between the groups of the adsorbate and the groups on the surface of the pore wall structure coming from mesoporous carbon.56 The removal efficiency experiment was examined using 20 mL of EBT solution (50 ppm) with an adsorbent amount of 5 mg at different contact times (0−60 min). Figure 4c shows the % dye removal of EBT onto the AFMPCs. 3.5. Effect of Contact Time. For designing batch adsorption experiments, the effect of contact time was carried out to find the equilibrium conditions. Therefore, given the initial dye concentration of 50 ppm, the EBT adsorption at different temperature was investigated within a period of 60 min. Figure 4d shows the adsorption of EBT by mesoporous carbon as the function of contact time at different temperature. As the contact time increases, adsorption of EBT increases to reach equilibrium at 10 min. The initial rate of adsorption is

fast due to the adsorption of dye molecules onto the exterior surface. After that, adsorption rate becomes relatively slow due to the slow movement of molecules into the pores. The initial EBT uptake is rapid due to the large surface area and large pore volume of AF-MPCs.57 The slower adsorption rates is due to the saturation of the binding sites and attainment of equilibrium. When the temperature is increased from 298 to 383 K, the equilibrium is reached within 4 min, and adsorption capacity of EBT increases from 115 mg·g−1 to 124 mg·g−1, indicating the endothermic nature of the adsorption process. 3.6. Effect of pH. To study the influence of pH on the adsorption of EBT, pH experiment was carried out at room temperature with 5 mL of 50 ppm of aqueous EBT solution and 20 mg·L−1 of AF-MPCs. The EBT is a diprotic dye with pKa values of 6.6 and 11.6 which can be dissociated depending upon the pH values.58 Figure 4e shows the effect of pH on the adsorption of EBT from pH 1 to 9. The maximum uptake of dye apparently takes place at pH 1 and 2, whereas adsorption rate decreases significantly at the higher pH. At lower pH, the surface of AF-MPCs was positively charged, thus attracting EBT (anionic dye) via strong electrostatic interaction. At higher pH, the number of positively charged sites decreases on the surface of the AF-MPCs. In other words, at acidic conditions, protonated hydroxyl (−OH) and carbonyl (−COOH) groups present in the AF-MPCs attract the anionic EBT dye, while at the basic conditions, strong repulsion exists between the EBT dyes and AF-MPCs. Similar behavior was reported for EBT dye in the literature.58 Hence, the adsorptive removal studies throughout all EBT concentrations were carried out at pH 1 for understanding the adsorption kinetics and thermodynamics. The equilibrium exists of EBT in different pH solutions (see Figure 4e inset). 3.7. Points of Zero Charge. The pH value plays an important role with respect to the adsorption of EBT dye molecules on MPC and AF-MPC adsorbents. The surface functionalization of MPC by oxidation with H2SO4 leads to G



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Figure 5. (a) Plot of adsorption capacity vs equilibrium concentration, (b) Langmuir isotherm, and (c) Freundlich adsorption isotherm for adsorption of EBT on the AF-MPCs.

Table 2. Adsorption Parameters of the Langmuir and Freundlich Isotherm Model for the Adsorption of EBT on the AF-MPCs Langmuir isotherm

Freundlich isotherm

temp (K)

KL (mg g−1)

q0 (mg L−1)

R2

χ2

MSE

KF (mg g−1)

N

R2

χ2

MSE

298 303 313 323 343 353 363 383

58.00 57.00 56.56 55.17 54.16 53.19 52.17 51.00

172.41 174.45 176.12 178.57 180.23 188.67 193 196.07

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

0.33 0.53 0.54 0.35 0.39 0.40 0.45 0.47

0.05 0.07 0.09 0.02 0.03 0.08 0.06 0.07

148627 153340 158840 161808 165807 176401 18670 191337

11.08 10.89 10.50 10.61 10.5 10.10 9.89 9.56

0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98

0.33 0.53 0.54 0.35 0.39 0.40 0.45 0.47

0.08 0.09 0.01 0.07 0.09 0.07 0.06 0.09

constant saturation level. The highest adsorption capacity can be explained based on high specific area, mesoporous structure, uniform pore distribution, presence of functional groups, and high adsorption capacity.62 In order to understand the nonlinear adsorption process, the Langmuir and Freundlich isotherms were studied to fit the experimental data.63,64 Langmuir isotherms65 assume the monolayer adsorption of adsorbate on a structurally and energetically homogeneous active site. Each site in the adsorbent adsorbs the same amount of activation energy and only one molecule; there is no interaction between adjacent adsorbed molecules.6667 The linear form of the Langmuir isotherm is given by the eq 3

production of oxygen-containing groups such as hydroxyl, carbonyl, and carboxyl groups. Additionally, we measured pHzpc (zero-point charge of adsorbent), i.e, the pH dependence of the surface charge of the surrounding electrolyte or medium. The pH value is called “point of zero charge” (PZC), at which the surface has zero net charge. The effect of surface functionality on EBT adsorption may be associated with the charge feature of the mesoporous carbon surface. Given pH values from 1 to 8 in the experiments, a portion of surface functional groups should become deprotonated with increasing pH; i.e., a less positively charged carbon surface appears at higher pH. The conjugation of all functional groups determines pHPZC, at which the net charge on the carbon surface is zero.59 The carbon surface has a net positive charge at pH < pHPZC, while the surface becomes negative at pH > pHPZC. Figure 4f shows the pH drift tests, from which the pHPZC value of 3.52 for AF-MPC was obtained (Figure 4e). The H+ ion concentration increases in the system at pH < pHPZC, and the carbon surface becomes positive by absorbing H+ ions, thus allowing a strong electrostatic attraction with the negatively charged EBT molecules to the maximum adsorption of dye at low pH.60,61 3.8. Adsorption Isotherms. The adsorption isotherm is the relationship between the amount of a substance adsorbed at constant temperature and its concentration in the equilibrium condition. The adsorption isotherm provides information on how the dye molecules are distributed between the liquid and solid phase during the adsorption process at equilibrium condition. The adsorption thermodynamics of EBT is studied as a function of equilibrium concentration. Figure 5a shows the adsorption isotherm for various initial EBT concentrations (50, 70, 90, 110, and 130 mg·L−1) for 10 min at constant stirring speed of 250 rpm. For mesoporous carbon, the adsorption capacity of EBT increased with increasing equilibrium concentration and tends to reach a

ij 1 yz Ce 1 = + jjjj zzzzCe jq z qe q0KL k 0{

(3)

where KL is the Langmuir constant which denotes the adsorption capacity and q0 is the Langmuir maximum monolayer adsorption capacity. The values of q0 and KL were calculated from the slope and intercept of the linear plot of Ce/ qe vs Ce. Figure 5b shows the Langmuir isotherm plots for mesoporous carbon, and the parameters were exhibited in Table 2, with the correlation coefficient (R2) as a measure of the data fit to the isotherm’s model. The Langmuir equation can be expressed in terms of dimensionless separation factor, RL (eq 4) RL =

1 1 + KLCi

(4)

where Ci is the largest initial solute concentration. The feasibility of the adsorption process was evaluated from dimensionless separation factor. As RL > 1, the adsorption process is unfavorable in contrast to a favorable process in which RL is between 0 and 1. For the adsorption of EBT onto H



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Table 3. Kinetic Model Parameters for Adsorption of EBT onto AF-MPCs parameters model

temp (K)

qe,cal (mg·g−1)

qe,exp (mg·g−1)

k1 (mg·g−1·min−1)

k2 (mg·g−1·min−1)

R22

298 303 313 323 343 353 363 383 298 303 313 323 343 353 363 383

115.0 117.64 118.0 120.0 121.0 122.0 123.0 124.0 115.0 117.64 118.0 120.0 121.0 122.0 123.0 124.0

1.01 1.21 5.03 14.17 15.16 13.49 15.57 16.39 114.94 117.00 119.04 120.48 121.45 121.95 123.45 123.45

0.23 0.27 0.35 0.49 0.54 0.56 0.76 0.86 -------------------------

------------------------0.040 0.045 0.060 0.065 0.120 0.220 0.330 0.390

0.96 0.97 0.97 0.96 0.96 0.97 0.96 0.96 1 1 1 1 1 1 1 1

pseudo-first-order

pseudo-second-order

Figure 6. (a) Plot of a pseudo-second-order kinetic equations. (b) Fitting the experimental data in intraparticle diffusion equations at different temperature and (c) plot of the Van’t Hoff equation for adsorption of EBT on the AF-MPCs.

the AF-MPC, the RL value ranges from 0 to 0.2 suggesting that the adsorption process is favorable. The Freundlich isotherm is suitable for the multilayer adsorption of adsorbate on a structurally and energetically heterogeneous active site. Each site adsorbs a different amount of energy with the formation of a multilayer. The logarithmic form of Freundlich is given by the following empirical eq 568 log qe = log KF +

1 n log ce

involved in the process of adsorption. Given the amount of dye adsorbed per unit weight of the adsorbent, the kinetic data were analyzed with the regression coefficient (R2). Lagergren’s pseudo-first-order kinetic model is expressed as (eq 6)69,70 i kt y log(qe − qt) = log qe − jjj 1 zzz k 2.303 {

(6)

where k1 is the pseudo-first-order rate constant, and qe and qt are the adsorption capacity of EBT molecules onto mesoporous carbon at equilibrium and time t, respectively. The values of k1 and qe were calculated from the slope and intercept of the plot of log (qe − qt) vs t and were listed in Table 3. Note that the estimated qe was found to be much lower than the experimental value. The linear form of pseudosecond-order kinetics was proposed by Ho and Mckay and represented by eq 7. This model is based on the assumption that the adsorption follows pseudo-second-order chemisorption.

(5)

where n and KF represent the Freundlich constant, indicating the adsorption intensity and adsorption capacity between adsorbent and adsorbate affinity, respectively. The model parameters n (heterogeneity factor) and the equilibrium adsorption capacity (KF) were estimated by a plot of ln qe vs 1/ln ce. The correlation coefficient obtained for the Langmuir isotherm is almost the same as the Freundlich isotherm (Figure 5c). Hence, both models are suitable for describing the adsorption equilibrium of EBT on mesoporous carbon. However, the Langmuir plot is suitable for low concentration of EBT to avoid multiple-layer adsorption, while the Freundlich isotherm is applicable to the higher concentration, as reflected in Figure 5a and Figure 5c. 3.9. Adsorption Kinetics of EBT onto the AF-MPCs. Pseudo-first-order and pseudo-second-order rate equations and intraparticle diffusion were suggested for the kinetic behavior

t 1 t = + 2 qt qe k 2q e

(7)

where k2 is the pseudo-second-order adsorption rate constant. The k2 and qe were estimated from the slope and intercept of the plots of t/qt vs t (Figure 6a) and presented in Table 3. The table shows that the correlation coefficient (R2) of pseudo-second-order was better than the pseudo-first-order I



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kinetic model. The qe,cal values of pseudo-second-order kinetic approach are consistent with qe,exp values which are far from those based on the pseudo-first-order model. It was concluded that the pseudo-second-order model was better than the Lagergren first-order model. 3.10. Sorption Mechanism. The adsorption mechanism and its kinetics were analyzed using an intraparticle diffusion method. According to the Weber and Morris intraparticle diffusion model,70 adsorption in the porous medium occurs along three steps. They are (i) transport of the EBT molecules to the external surface of the mesoporous carbon (film diffusion), (ii) transport of EBT molecules within the pores of the mesoporous carbon (particle diffusion), and (iii) adsorption of EBT molecules on the interior surface of the mesoporous carbon. Since the third step is very fast, it cannot be the sole rate-determining step in the uptake of EBT molecules. The standard integral equation of the intraparticle diffusion model is expressed as (eq 8). qt = k it 1/2 + C

Table 4. Intraparticle Diffusion Parameters for the Adsorption of EBT Dye onto AF-MPCs intraparticle diffusion parameters

(8)

and qt qe

c for ki1 (mg·g−1·min−0.5)

298 303 313 323 343 353 363 383

11.49 12.0 13.0 14.0 15.0 17.43 19.56 21.0

80.73 83.339 84.02 80.90 90.9 108.0 113.75 117.17

ΔG° = −RT ln Kd

(13)

ΔG° = ΔH ° − T ΔS°

(14)

ΔS° ΔH ° − (15) R RT −1 where ΔG° is standard free energy in kJ·mol ; ΔS° is standard entropy in J·mol−1·K−1; ΔH° is standard enthalpy in kJ·mol−1; R is the universal gas constant (8.314 J·mol−1·K−1); T is the temperature (K); and Kd is the distribution coefficient of EBT dye. Therefore, the parameter Kd can be calculated using the following eq 1673 ln Kd =

(10)

where qe is the amount of adsorbate adsorbed at equilibrium (mg·g−1) and qt represents the amount of dyes adsorbed at time t (min); F denotes the fraction of solute adsorbed at time t; and Bt is a mathematical function of F. Substituting eq 8 into eq 7 ÄÅ ÉÑ ÅÅÅij 6 yz ÑÑ 1 − F = ÅÅjj 2 zzexp( −Bt )ÑÑÑ ÅÅÇk π { ÑÑÖ (11)

ij q yz ij C yz Kd = jjjj e zzzz/jjj e zzz j q z jk Cu z{ (16) k u{ where Ce is the equilibrium EBT dye concentration on the adsorbent (mg·L−1) and qe is the adsorption capacity of EBT dye onto mesoporous carbon at equilibrium (mg·g−1). qu is 1 mg·g−1, and Cu is 1 mg·L−1. Because Kd is dimensionless, qu and Cu should be included to make the unit consistent in eq 16. The thermodynamic parameters ΔH° and ΔS° were calculated from the slope of the linear Van’t Hoff plot of ln Kd vs 1/T as shown in Figure 6c, which shows an excellent linearity, and the data were presented in Table 5. The negative values of ΔG° indicate the feasibility and spontaneous nature of EBT adsorption onto the mesoporous carbon. The value of ΔG° decreased with increasing temperature, hence indicating the increased degree of spontaneity at higher temperatures. The positive values of ΔS° reveal an increased degree of entropy of the system increased during the adsorption of EBT.

or Bt = −0.4977 − ln(1 − F )

ki1 (mg·g−1·min−0.5)

3.11. Thermodynamic Parameters. Variations of EBT adsorption data with temperature were used to calculate the thermodynamic parameters, i.e., change in Gibbs free energy (ΔG°), enthalpy change (ΔH°), and entropy change(ΔS°). The ΔG° indicates the spontaneity of a chemical reaction. Given both ΔH° and ΔS the Gibb’s free energy of the process can be determined. However, the behavior free energy change (ΔG°) indicates the spontaneous nature of adsorption favorable at higher temperatures. The change in free energy refers to the heat evolved in the physisorption or chemisorption process; the magnitude of enthalpy indicates the exothermic or endothermic nature of the adsorption process. Similarly, the magnitude of entropy refers to the randomness of the adsorption process. The free energy of an adsorption process is related to the equilibrium constant by the classical Van’t Hoff (eqs 13−15).71,72

where ki is the intraparticle diffusion rate constant and C is a constant and ki (mg·g−1·min−1/2) can be calculated from the slope of the plot between the adsorption capacity (qt, mg·g−1) and the square root of time (t0.5, min0.5). The remaining two steps such as external transport and internal transport can be the rate-determining steps. In order to determine the exact rate determination step, quantitative treatment of sorption dynamics could be applied with the help of the Boyd kinetic model in eq 9 ÉÑ ÄÅ ÑÑ ÅÅÅij 6 yz F = 1 − ÅÅjj 2 zzexp( −Bt )ÑÑÑ ÑÑÖ ÅÅÇk π { (9)

F=

temp (K)

(12)

The Bt values at different contact times can be calculated using eq 12. Therefore, the plot of Bt vs time (t) is called a Boyd plot, which helps to differentiate between external transport and intraparticle diffusion processes. Since the curve of the Boyd model does not pass through the origin, it implies that the external mass transport is the sole rate-limiting process for the EBT dye adsorption. Table 4 shows the values of intraparticle diffusion parameters. Figure 6b shows two slopes are present in the adsorption process at different temperature (303, 323, and 353 K), hence reflecting two or more steps involved in the sorption process. According to eq 6, the intercept C in the model determines the boundary layer thickness, clearly showing a greater boundary layer thickness for region II based on a flat slope 2. J



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Table 5. Thermodynamic Parameters for the Adsorption of EBT Dye onto AF-MPCs temp (K)

ΔG°a (kJ·mol−1)

ΔH°b (kJ·mol−1)

ΔS°c (J·mol−1·K−1)

ΔG*d (kJ·mol−1)

ΔH*e (kJ·mol−1)

ΔS*f (J·mol−1·K−1)

298 303 313 323 343 353 363 383

−42.94 −44.27 −46.91 −49.56 −54.76 −57.51 −60.45 −65.46

------36.022 ---------

------0.265 ---------

−58.268 −59.96 −61.64 −65.27 −70.12 −73.26 −77.98 −81.24

------20.621 ---------

------0.266 ---------

a

Standard free energy change. bStandard enthalpy change. cStandard entropy change. dFree energy of activation. eActivation enthalpy. fActivation enthalpy.

Figure 7. (a) Arrhenius plot and (b) Eyring plot for EBT on the AF-MPCs.

Figure 8. (a) Effect of NaCl concentration on the adsorption of EBT and (b) reusability of mesoporous AF-MPCs in the removal of EBT.

The positive value of ΔH° indicates that the adsorption process is endothermic in nature. 3.12. Activation Energy. Activation energy Ea (kJ·mol−1) is the minimum energy needed by the adsorbate ion to interact with functional groups on the adsorbent surface. The activation energy (Ea) for the EBT adsorption onto mesoporous carbon can be determined from measurements of the pseudo-secondorder rate constant k2 at different temperatures according to the Arrhenius equation74 expressed by ln k 2 = ln A +

Ea RT

where Ea is the Arrhenius activation energy; A is the Arrhenius factor; R is the gas constant; and k2 is the pseudo-second-order rate constant. The values of Ea and A can be obtained from the slope and intercepts of the plot of ln k2 vs 1/T (Figure 7a). The activation energy for the adsorption of EBT onto mesoporous carbon was found to be 26 kJ·mol−1. The magnitude of Ea gives the idea about the type of adsorption. The physisorption usually has energies in the range of 5−40 kJ· mol−1, while higher activation energies 40−800 kJ·mol−1 suggest chemisorption. Hence, physisorption takes place for the adsorption of EBT onto the mesoporous carbon.

(17) K



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Figure 9. (A) FT-IR spectra of (a) MPC, (b) AF-MPC, (c) EBT, and (d) AF-MPC after sorption of EBT. (B) Schematic representation of surface interactions between the AF-MPC and EBT dye molecules. (C) Typical UV−Vis spectrum of the targeted EBT dye (inset shows the chemical structure of EBT dye) and (D) photographs of before and after adsorption of EBT dye.

out experiments using an ethanol extraction procedure. Ethanol facilitates the release of EBT from the dye-adsorbed mesoporous carbon.77 In the present experiment, 80 mg of AFMPC is used as the adsorbent for 50 ppm of EBT dye solution. The adsorbed dye adsorbent is washed three times with ethanol and dried. During the first regeneration cycle, nearly 90% of the adsorbed dye was recovered. After the second cycle, nearly 89% of the adsorbed dye was recovered. The regeneration process was repeated for four cycles. The regeneration process gradually decreases for consecutive cycles with nearly 86% recovery. This strongly supports the desorption process and makes it possible to reuse mesoporous carbon for multiple cycles, indicating that AF-MPC is a promising adsorbent for practical applications. 3.15. Adsorption Mechanism. The FT-IR spectra of MPC, AF-MPC, EBT, and AF-MPC-EBT were shown in Figure 9A. The broad band at 3445 cm−1 is characteristic of the stretching vibration of hydrogen-bonded (−OH) hydroxyl groups of the MPCs. The spectrum shows a pronounced band at 1575 cm−1 that can be assigned to the CC stretching78 vibration in the structure of the MPC as shown in Figure 9A(a). In the FT-IR spectra of AF-MPC (Figure 9A(b)), the peaks at 1569 and 1100−700 cm−1 could be attributed to C C and C−O and C−O−C groups, respectively, which confirms the presence of oxygen-containing groups on the graphene surface acid modification (AF-MPC), while peaks at 3440 cm−1 correspond to O−H vibrations in the hydroxyl group. It clearly demonstrates the formation of oxygen-containing functional groups after acid modification.79 On the other hand, the relative intensity of these bands in the AF-MPC sample was higher than those of the MPC sample, indicating that more oxygen-containing functional groups were introduced after oxidation. This fact that oxidation caused higher content of functional groups indicated that chemical reactions on the carbon surface occurred between the H2SO4 and C

The Eyring equation is used to calculate the important thermodynamic parameters such as activation enthalpy (ΔH*), activation entropy (ΔS*), and free energy of activation (ΔG*) in the adsorption process by the following expression75 ln kB ln k ΔS # ΔH # = + − T h R RT

(18)

where k is the adsorption rate constant; kB is the Boltzmann constant (1.3807 × 10−23 J·K−1); h is the Planck’s constant (6.6261 × 10−34 J·s); R is the ideal gas constant (8.314 J mol−1· K−1); and T is temperature (K). The values of ΔH# and ΔS# could be determined from the slope and intercept of the plot of ln k/T vs 1/T (Figure 7b), and the values are shown in Table 5. A negative value of Gibbs free energy (ΔG#) of activation implies the spontaneous nature of the adsorption process. The positive value of enthalpy of activation confirms that the adsorption process is endothermic. The positive value of entropy of activation reflects the increased randomness at the solid−solution interface during adsorption. 3.13. Effect of Ionic Strength. Wastewater contains significant quantities of salts along with dye molecules, and hence it is important to study the effect of electrolyte on the dye removal. The experiments were carried out using 50 ppm of EBT along with various concentrations of NaCl ranging from 0.1 to 0.5 M. Figure 8 illustrates the effect of NaCl concentration on the dye removal percentage. As the electrolyte concentration increases, dye removal percentage decreases. At the acidic conditions, the presence of a sodium ion neutralizes the negatively charged feature of the EBT dye molecules and thus decreases the adsorption efficiency of dye onto the mesoporous AF-MPCs. Hence the dye removal percentage decreases at higher concentration of NaCl.76 3.14. Regeneration of AF-MPCs. In order to check the desorption feasibility of mesoporous carbon, we have carried L



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Figure 10. (a−d) Simultaneous adsorption of the cationic dyes (RhB, CV, MB) and anionic dye (MO) with the same concentration of EBT and their photographs and (e) the adsorption capacity mixture of dye solution.

electrostatic or π−π attraction and/or hydrogen bonding (Figure 9B). Our previous study has also reported a similar behavior.69 Figure 9C exhibits the typical UV−visible spectrum, and the inset shows the chemical structure of the targeted EBT dye. Two distinct bands at 341 and 530 nm were observed in the UV−visible spectrum. Figure 9D (a and b) represents the photograph of the adsorption of EBT dye at equilibrium of AF-MPCs before and after adsorption in aqueous medium. Indeed, AF-MPCs are highly porous and possess a large number of functional groups. As can be seen in Figure 9D(b), the EBT became colorless compared to the original dark red solution (Figure 9B (inset)). This phenomenon confirmed that the AF-MPC could adsorb EBT completely from the anionic EBT solution because there are various oxygen-containing functional groups, such as −COOH and −OH on the pore channels and edges of AF-MPCs (Figure 9B). Meanwhile, AF-MPCs have a huge surface area and graphitized pore structure, allowing strong π−π interactions with the aromatic moieties present in the dye molecules, thereby increasing the adsorption capacity of EBT dyes. The BET isotherms of the EBT dyes adsorbed on AFMPCs are shown in Figure S2 (SI). It is noticed that the BET surface area of AF-MPCs decreased with increase of the dye adsorption. Some micropores could be blocked by the adsorbed pollutants which caused the reduction of surface area, and the corresponding textural properties for the AFMPC sample were depicted in Table S2 (SI). 3.16. Selectivity of the Adsorbent. The adsorbent selectivity for the EBT dye in the presence of cationic and

atoms, which were partially oxidized to form C−OH and/or − COOH. The reaction equations were proposed as follows: Oxidize

>C− ⎯⎯⎯⎯⎯⎯→ >C−OH Oxidize

>C− ⎯⎯⎯⎯⎯⎯→ >COOH

(19) (20)

Thus, the FT-IR spectral result indicates that the prepared activated carbon is rich in surface functional groups. Figure 9A(c) and Figure 9A(d) indicate the FT-IR spectra of EBT and AF-MPC after adsorption of EBT adsorption. In Figure 9A(c), the stretching vibration bands at 3432 cm−1, 1332 cm−1, 1401 cm−1, and 1215 cm−1 are attributed to −OH, −NN−, −NO2, and −SO3 groups, respectively. After EBT adsorption, an appreciable shift and an increase in the intensity and the broadness of the CC bands of vibration at 1579 cm−1 are noted. At the end of adsorption, bands at 1085 and 3432 cm−1 shifted to the higher wavelength of 1095 and 3442 cm−1 show that C−O and −OH groups of AF-MPC play a role in the adsorption. At the 400−1000 cm−1 region, there were no major shifts observed. Therefore, evidence of interactions from FT-IR allows us to propose the following mechanism to demonstrate the adsorption of organic dye (Figure 9A(e)). The acid-treated carbon (AF-MPC) architecture provides a variety of interactions such as electrostatic, covalent, hydrogen, and π−π bonds, which are the main mechanisms involved in the adsorption of organic compounds onto the surface of carbons.58 However, the hydroxyl and carboxyl groups on the surface of carbon facilitate the uptake of the azo dye, EBT, via M



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Figure 11. (a) Photographs of dye-polluted industrial wastewater, (b) after absorption, and (c) clean water (after centrifugation).

anionic dyes is inspected, including rhodamine B (RhB), crystal violet (CV), and methylene blue (MB) as cationic dyes while EBT and methyl orange (MO) as anionic dyes.63 The selectivity of the as-prepared AF-MPCs for different dye adsorption experiments was carried out in a batch mode by the same experimental procedure described above. After being still for 10 min and centrifuged, the result is displayed in Figure 10a−d. As can be seen in Figure 10a−c, when the cationic dyes (RhB, CV, and MB) were mixed with EBT (anionic dye), only the EBT dye was preferred in adsorption. This is because at the initial pH a number of positively charged sites were created on the carbon surface which may repel the positively charged dye (RhB, CV, and MB) solution due to the electrostatic repulsion. However, another solution containing two anionic dyes (EBT and MO) was competitively adsorbed at the same time. The adsorption capacities of each dye solution are shown in Figure 10e. AF-MPCs offer a positively charged surface in acidic pH and favor the adsorption of anionic dyes. It was concluded that the materials could selectively adsorb anionic dyes from the solution. 3.17. Practical Application. The water-soluble organic dyes easily contaminate water to affect human health and environment. A photograph of dye-polluted wastewater is shown in Figure 11a. After addition of the AF-MPC, the color of the solution changed to colorless within 30 min following the same experimental procedure (Figure 11b), indicating that the AF-MPC is suitable for cleaning the polluted water. Surprisingly, the absorption efficiency was evaluated ca. 95.4%. Finally, the used AF-MPC material was separated from the reaction mixture by ultracentrifugation (10,000 rpm), and clear water was obtained (Figure 11c). Therefore, the environment acidity is an important factor influencing the electrostatic interaction between adsorbent and adsorbate. Thus, the approach proposed here could offer alternative opportunities to adsorb organic dyes in lakes and industrial wastewater. 3.18. Comparison with the Literature. A number of conventional sorbents have been used for the removal of EBT dye molecules from aqueous solution.20,21,58,80−89 These findings are summarized together with our data (Table 6). The adsorption capacity using AF-MPC (qe = 117.0 mg·g−1) adsorbent is much higher than those previously reported. For instance, cyclodextrins/polyurethane (β-CD/PU) foam composite20 yielded qe = 20.17 mg·g−1, while the magnetic nanoparticles show an adsorption capacity of NiFe2O4: qe = 47.0 mg·g−121 and NiFe2O4 NPs (qe = 81.52 mg·g−1).84 Likewise, Salem et al.85 have reported magnetite/silica/pectin hybrid nanocomposites for the adsorption kinetics of EBT, yielding a maximum adsorption capacity, qe = 65.35 mg·g−1, for 120 min, which is significantly lower than our system. Recently, commercially available Nteje clay89 was used to

Table 6. Comparison of Adsorption Capacity of Various Adsorbents for EBT adsorbent β-CD/PUa NiFe2O4 magnetic NPs eucalyptus bark graphene acid-modified graphene activated charcoal untreated almond shell CPb-treated almond shell MWRc-treated almond shell HCPZAd bottom ash NiFe2O4 NPs MSPe NPs alumina titanate nanotubes MPACf Nteje clay AF-MPCs

SBET (m2·g−1)

pH

time (min)

temp (K)

qe (mg·g−1)

− −

− 6

6h 10

323 298

20.17 47.0

20 21

20.47 404 512.5

8 2 2

150 3h 3h

303 298 298

52.37 102.04 70.42

58 62 62

− −

1 10

30 180

298 303

4.71 6.02

80 81

−

10

180

303

18.18

81

−

10

180

303

29.41

81

− − − − − 250

5 3.5 2 1 4−5

20 4.5 h 10 120 40 90

298 298 298 298 298 298

15.9 94.96 81.52 65.35 62.99 31.0

82 83 84 85 86 87

189 − 623.9

2 2 1

3h 60 10

313 303 303

46.51 16.26 117.0

ref

88 89 this work

β-Cyclodextrins/polyurethane. bCold plasma. cMicrowave radiation. Hydrophobic cross-linked polyzwitterionic acid. eMagnetite/silica/ pectin NPs. fMosambi peel activated carbon. a

d

remove EBT from wastewater. However, there was no significant adsorption capacity (qe = 16.26 mg·g−1), compared to AF-MPCs (qe = 117.0 mg·g−1) with the contact time of 10 min. These results indicate that AF-MPC is a more efficient adsorbent than other nanostructured adsorbents mentioned in Table 6.

4. CONCLUSION In this study, acid-functionalized mesoporous carbon (AFMPCs) was synthesized with high surface area and tested for the removal of EBT dye from aqueous solution. The prepared carbons can be well dispersed in the aqueous solution, due to its hydrophilic nature, and easily separated from the solution using ultracentrifugation after adsorption. XRD, Raman, TGA, BET, TEM, and FT-IR characterization techniques were used to confirm the structural and morphological properties of the AF-MPCs. The estimated maximum adsorption capacity of AF-MPCs predicted by the Langmuir isotherm model was 117.0 mg·g−1, which is higher than many adsorbents used for N



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ΔH* activation enthalpy, kJ·mol−1 ΔS* activation enthalpy, J·mol−1·K−1

the absorption of EBT. AF-MPC materials have many advantages including ease of preparation, large surface area, low cost, environmental friendship, stability in solvents, and the presence of functional groups on their surface, which is suitable for further chemical modification. Additionally, the potential to reuse the AF-MPCs often makes it the more economically efficient choice. Therefore, the AF-MPC material could be considered as a promising adsorbent; it was found to be useful and valuable for controlling water pollution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00878. Experimental details and XRD (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Pitchaimani Veerakumar: 0000-0002-6899-9856 King-Chuen Lin: 0000-0002-4933-7566 Funding

This work was supported by the Ministry of Science and Technology (MOST), Taiwan (NSC 102-2113-M-002-009MY3 to KCL). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS qe equilibrium adsorption capacity, mg·g−1 C0 initial concentration of dye in the liquid phase, mg·L−1 Ce liquid-phase equilibrium concentration of EBT dye, mg· L−1 pHzpc zero-point charge of adsorbent RL separation factor KL equilibrium adsorption constant related to the affinity of binding sites, L·mg−1 Kd distribution coefficient, dimensionless Ci largest initial solute concentration Ct liquid-phase concentration of EBT dye at time t, mg·L−1 q0 Langmuir maximum monolayer adsorption capacity, mg·g−1 KF adsorption capacity of the adsorbent, (mg·g−1) (L· mg−1)1/n k1 pseudo-first-order rate constant, mg·g−1·min−1 k2 pseudo-second-order rate constant, mg·g−1·min−1 q0 solid-phase concentration at time t, mg·g−1 ki intraparticle diffusion rate constant, mg·g−1·min−1/2 C constant n constant F the fraction of solute adsorbed at time t Bt mathematical function of F activation energy of reaction, kJ·mol−1 Ea A pre-exponential factor, s−1 ΔG° standard free energy change, kJ·mol−1 ΔH° standard enthalpy change, kJ·mol−1 ΔS° standard entropy change, kJ·mol−1·K−1 ΔG* free energy of activation, kJ·mol−1 O



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