Comparison of Oxidized Carbon nanotubes efficiencies for benzene and toluene removal from aqueous solution

Vietnam Journal of Science and Technology 55 (4C) (2017) 311-318 COMPARISON OF OXIDIZED CARBON NANOTUBES EFFICIENCIES FOR BENZENE AND TOLUENE REMOVAL FROM AQUEOUS SOLUTION Le Huu Quynh Anh 1, * ,Tran Thu Kieu 2 ,Tran Duy Hai 1 1 HCMC University of Natural Resources and Environment, 236B Le Van Sy Street, Tan Binh District, HoChiMinh City 2 Industrial University of Hochiminh city, 12 Nguyen Van Bao, Go Vap, Ho Chi Minh City * Email: quynhanh.lehuu@gmail.com Received: July 15, 2017 ; Accepted for publication: 18 October 2017 ABSTRACT Carbon nanotubes (CNTs) have been widely used as adsorbent in environmental treatment, especially for pollutants and volatile organic compounds (VOCs). The purpose of this work is to develop materials based on functionalized multiwalled carbon nanotubes (MWCNTs) for Benzene and Toluene removal. This will exploit adsorption propertiesof modified MWCNTS for Benzene and Toluene. In the first study, adsorption capacity of pristine MWCNTs to Benzene and Toluene in aqueous solution was investigated through isotherm study. The qe values of CNTs for Benzene and Toluene were 45,5 mg.g -1 and 56.3 mg.g -1 , respectively and a contact time of 120 min. The modification of three oxidized MWCNTs were performed with acid solution (HNO3/H2SO4), hydrogen peroxide (H2O2) and sodium hypochlorite (NaOCl). The modified MWCNTs materials (CNT-COOH,CNT-NaOCl,CNT-H2O2) were characterized by Fouriertransform infrared spectroscopy (FT-IR). The effects of functional groups on the MWCNTs on the adsorption capacity of Benzene, Toluene in aqueous solution were studied and compared to pristine MWCNTs. The results demonstrated that the efficiency of adsorption was significantly enhanced with oxidized CNTs, and following the order CNTs-NaOCl > CNTs- H2O2 > CNTs-H2SO4 > pristine CNTs. Keywords:VOCs, carbon nanotubes, waste water treatment, oxidized carbon nanotubes, functionalization. 1. INTRODUCTION In recent years, industrialization has been associated with increased pollution in various citiesall over the world, especially in developing countries. There are many contaminants discharged from industrial or agricultural lands resulting in serious problems concerning water pollution. Currently, the volatile organic compounds (VOCs) are used in common industrial processes, especially in paint, dyes, polymer and varnishing industries. Benzene and its Le Huu Quynh Anh, Tran Thu Kieu, Tran Duy Hai 312 derivative Toluene were the main solvent components involved in these industries which have a significant impact on the quality of ground water and on the public health. The most preferable treatment method for VOCs include adsorption on solid sorbents [1], such as activated charcoal [2], carbon molecular sieves, montmorillonite or porous polymers. The adsorption capacity mainly relies on the proper characteristics of the sorbent. Since the discovery of carbon nanotubes (CNTs) and recent developments have provedtheir significant properties such as the unique structure, exceptional mechanical and electronic properties. Many applications of CNTs in waste water treatment for Benzene/Toluene recovery have indicated the high adsorption efficiency, attributed to their high surface active and controlled pore size distribution [3].Furthermore, the adsorption capacity of CNTs have been successfully improved by changing the hydrophobic nature of carbon, for example the surface can be functionalized by oxidation treatment [4] or surfactant treatment. The overall goal of this paper was to compare the adsorption performance of pristine CNTs and different oxidized Carbon Nanotubes for Benzene and Toluene removal in aqueous solution. In the first study, the adsorption capacity of pristine CNTs for Benzene and Toluene was performed. The second objective focused on the modification of CNTs by oxidation and the characterization of functionalized CNTs. Afterthat, the applications of three oxidized CNTs for removal of Benzene, Toluene were discussed, and a comparative study of adsorption efficiency were performed in the lasttarget. 2. MATERIALS AND METHODS 2.1. Materials All chemicals, such as Benzene, Toluene, HCl, HNO3, H2SO4, NaOCl, H2O2 were purchased from Sigma Aldrich with > 99 % purity. Multiwalled Carbon Nanotubes (MWCNTs) was synthesized by Institute of Materials Science of Vietnam, with the fundamental physical properties as following: diameter 20-100 nm, length 1-10 µm, purity > 95 %. 0.22 μm polytetrafluoroethylene (PTFE) membrane filter was purchased from Sigma Aldrich. 2.2. Methods 2.2.1. Modification CNTs by oxidation Modification CNTs in using the mixture of HNO3 and H2SO4 (CNT-COOH) : 2 g of pristine CNTs was dispersed in 200 mL of the mixture of acid HNO3 and H2SO4 (in the volume ratio of 1:3). The solution was sonicated in an ultrasonic bath for 10 min and then heated at 80 °C during 30 min. The mixture was cooled at room temperature and filtrated on a 0.22 µm PFTE membrane. The solid obtained was washed with deionized water until the pH of filtrate became neutral and then dried in an oven at 120 °C for 8 h. Modification CNTs by H2O2 (CNT-H2O2): 1 g of CNT-COOH was dispersed in 200 mL of the solution H2O2. The mixture was stirred at ambient temperature for 10 min then sonicated for 30 min. The resulting dispersion was filtered and washed with deionized water until the pH neutral. The solid was dried in an oven at 120°C for 8 h. Modification CNTs by NaOCl (CNT-NaOCl): 1 g of CNT-COOH were oxidized by a solution of 3 % NaOCl (97 mL of H20 + 3 mL of 70 % purity NaOCl). The mixture was stirred Comparison of oxidized Carbon Nanotubes efficiencies for Benzene, Toluene removal from 313 for 8 h at ambient temperature and then filtrated and washed until the pH neutral. The resulting solid was dried in an oven at 120°C for 8 h. 2.2.2. Batch adsorption experiments All of the adsorption experiments were carried out in 1 L glass bottle with airtight cap, in order to avoid the vaporization of adsorbates.The adsorption experiments were performed with 60 mg of pristine CNTsdispersed in 400 mL of Benzene / Toluene solution (10 mg.L -1 ). The mixture was magnetically stirred at 300 rpm. Aliquots of mixture solution were analyzed by Gas Chromatography (GC) at various intervals, 20 minutes, 40 minutes, 60 minutes, 90 minutes, 120 minutes, 150 minutes to determine the concentration of the solution after adsorption process. To perform the adsorption isotherm study, the adsorbate samples were obtained by mixing 60 mg of pristine CNTs and different concentrations of VOCs in range of 1-20 mg.L -1 . The solution was stirred at 300 rpm for 120 min at room temperature and analyzed by GC. The adsorption of Benzene and Toluene (10 mg.L -1 ) on modified CNTs(CNT-COOH, CNT- H2O2, CNT-NaOCl) were performed underthe same conditions to evaluate the adsorption performance of modified CNTs. All the experiments were duplicated and the mean values are reported. A blank solution contains Benzene and Toluene without any adsorbent was used for eliminating the loss of adsorbate concentration due to the vaporization during the experiments, which were found to be negligible. The adsorbed amount on adsorbents (q, mg.g -1 ) was calculated following the equation (1): ( ) (1) where Co and Ct (mg.L -1 ) are the concentration of adsorbates at initial time and at equilibrium phase, respectively. V (L) is the volume of the solution and m (g) is the adsorbent weight. 2.2.3. Characterization and analytical methods The Brunauer, Emmett and Teller specific surface area (BET) of CNTs was performed from the adsorption-desorption isotherms of N2 at 77 K using a NOVA 3200e-Quantachrome USA instrument. The Fourier Transform Infrared (FT-IR) spectra was determined by using a FT-IR spectrophotometer 8400S, Shimadzu, Japan, in the range of 400 - 4000 cm -1 .The concentration of Benzene, Toluene was quantified by a Gas Chromatography equipped with flame ionization detector (GC-FID) (Shimazu, QP2010ULTRA). The GC-FID was operated at injection temperature of 200 °C. The following temperature program was used: 60 °C for 5min and increased with rate of 10 °C/min to 200°Cwith 5min of holdtime. The total operation time was 24 min. 2.2.4. Adsorption isotherms The adsorption experiment data were fitted using three basic types of adsorption isotherms models : Langmuir, Freundlich and Dubinin – Radushkevich (RD). The linearized Langmuir isotherms is represented [5] by equation : (2) max max 1 1e e e C C q q b q Le Huu Quynh Anh, Tran Thu Kieu, Tran Duy Hai 314 where, Ce is equilibrium concentration of adsorbate (mg.L -1 ), qe is the adsorption capacity in equilibrium (mg.g -1 ), qm (mg.g -1 ), and b (L.mg -1 ) are empirical constants, can be evaluated from the slope and intercept of the linear plot of Ce /qe against Ce. The equation of second fitting model Freundlich is presented as below : (3) where, K is the Freundlich characteristic constant [(mg.g -1 )(L.g -1 ) 1/n] and 1/n is the heterogeneity factor of sorption, obtained from intercept and slope of ln qe versus ln Ce linear plot respectively [6]. The Dubinin – Radushkevich (D-R) isotherm is calculated by fowlling equation: (4) where, qe , qs , Kad , are qe is amount of adsorbate in the adsorbent at equilibrium(mg.g -1 ); qs is theoretical isotherm saturation capacity (mg.g -1 ); KDR is Dubinin–Radushkevich isotherm constant (mol 2 /kJ 2 ) and =Dubinin–Radushkevich isotherm constant [7]. 3. RESULTS AND DISCUSSION 3.1. Adsorption performance andisotherm studies on the Benzene, Toluene removal using pristine CNTs 3.1.1. Effect of contact time The relationship between the contact time and adsorption uptake for Benzene and Toluene of pristine CNTs was shown in Figure 1. It can be clearly seen that the relative rate of adsorption increases with increasing contact time for both Benzene and Toluene during the first 20 min and the saturation was attained in approximately 120 min. In the first period, the adsorption capacity increasedsignifically due to the occupation of active sites by Benzene and Toluene molecules. At equilibrium, a saturation of active attachment sites on the CNTs surface led to theconstant adsorption capacity. Figure 1. Effect of contact time on the adsorption of Benzene and Toluene on pristine CNTs. 3.1.2. Adsorption performance ln ln lne eq n C k 2ln lne s DRq q K 0 20 40 60 80 100 120 140 160 0 10 20 30 40 50 60 Benzene Tolueneq e, m g /g time (min) Comparison of oxidized Carbon Nanotubes efficiencies for Benzene, Toluene removal from 315 Various initial concentrations of Benzene and Toluene ranged from 1 mg.L -1 to 20 mg.L -1 after 120 min contact time were evaluated in order to assess the interest of adsorption performance of pristine CNTs. The data presented in Figure 2 shows that adsorption capacity increased proportionally with initial adsorbate concentrations.This result demonstrates that CNTs display high affinity to aromatic compounds such as Benzene and Toluene. Molecular structure of Benzene and Toluene was planar with the aromatic ring and its -orbitals spanning all six carbon atoms. The adsorbate - adsorbent interaction is attributed to the strong - staking interaction where the coupling between -orbitals on structure of CNTs and C=C double bonds of aromatic rings occurs.The qe values of CNTs for Benzene and Toluene were in accordance with the other reported results. Bijan et al. [8] evaluated the equilibirum amount removed by SWCNTs (Benzene : 9.98 mg.g -1 and Toluene : 9.96 mg.g -1 ) with Co of 10 mg.L -1 and a contact time of 10 min. Figure 2. Adsorption capacity pristine CNTs for various concentrations of Benzene and Toluene. It can be seen in the figure 2 that adsorption capacity of Toluene was higher than that of Benzene. This could be explained by the rich electronic structure of Toluene due to donating effect of methyl groups, which enhance the π- π couplinginteraction.Daifullah et al. [9] indicated that the adsorption of Benzene and Toluene on activated carbon proceeds in the Toluene > Benzene order. 3.1.3. Adsorption isotherms The adsorption isotherm data was recorded and fitted with various model such as Langmuir, Freundlich and D-R model in order to characterize the correlation between interaction equilibrium of CNT andBenzene and Toluene. The linear form of these models were fitted based on experimental and fitted plots were presented in Figure 3. Figure 3. Linear fitted plot with (a) Langmuir isotherm model, (b) Freundlich isotherm model, (c) DR isotherm model for the adsorption of Benzene and Toluene on pristine CNTs. 0 2 4 6 8 10 12 14 16 18 20 22 0 10 20 30 40 50 60 70 80 90 Benzene Toluene q e, m g /g Co, mg/L 0 2 4 6 8 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0,11 y Toluene = 0,0062x + 0,0345 R 2 = 0,6898 Benzene Toluene C e/ q e, g /L Ce, mg/L y Benzene = 0,007x + 0,0474 R 2 = 0,8389 (a) -2 0 2 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 (b) (a) L n q e , m g /g y Toluene = 0,8719x + 3,1359 R 2 = 0,8553 y Benzene = 0,7433x + 2,8533 R 2 = 0,9941 Benzene Toluene LnCe, mg/L 0,0 5,0x10 6 1,0x10 7 1,5x10 7 2,0x10 7 1,5 2,0 2,5 3,0 3,5 4,0 4,5 (c) y Toluene = -1,12.10 -7 x + 3,86 R 2 = 0,8598 Benzene Toluene L n q e, g /L 2 y Benzene = -2.10 -7 x + 4,3 R 2 = 0,9765 Le Huu Quynh Anh, Tran Thu Kieu, Tran Duy Hai 316 The best fitting model was selected based on the highest correlation coefficient R 2 . From Figure 3, it can be seen that the best fits were providedby Freundlich model for Benzene adsorption and D-R model for Toluene adsorption. According to Freundlich model, the adsorption mechanism of Benzene, Toluene using Freundlich model tends to form a multilayer adsorption on a heterogeneous CNTs surface. The isotherm parameters were determined from the linear fitted plots and shown in Table 1. Table 1.The calculated parameters in the Langmuir, Freundlich and DR models for adsorption on pristine CNTs. Adsorbents Langmuir Freundlich Dubinin-Radushkevich qmax (mg.g –1 ) b (1.g –1 ) K (mg.g –1 )(mg/L) 1/n n (L.mg –1 ) qs (mol.g –1 ) KDR (mol 2 .k -1 J -2 ) Benzene 142.85 0.1476 1.3449 0.743 47.309 1×10 –7 Toluene 161.29 0.1797 23.009 0.872 73.538 2×10 –7 The Freundlich characteristic constant k and n values related to adsorption capacity and intensity of Toluene on CNTs wereconsidered more meaningfulthan that of Benzene, which is in accordance with the results of prior studies. The value of heterogeneity factor of sorption 1/n = 1,47 for Benzene and 1/n = 1.34 for Toluene indicated that the adsorption process was favorable. 3.2. Characterization of oxidized CNTs by FT-IR 4000 3500 3000 2500 2000 1500 1000 500 0 20 40 60 80 100 4000 3500 3000 2500 2000 1500 1000 500 0 20 40 60 80 100 4000 3500 3000 2500 2000 1500 1000 500 20 40 60 80 100 4000 3500 3000 2500 2000 1500 1000 500 0 20 40 60 80 100 %T %T 1/cm 1/cm a CNT 3400 %T 1/cm c CNT-H2O2 3500 2200 1634 1370 1105 618 b CNT-COOH 1643 1391 1110 978 609 2100 3190 1/cm d CNT- NaOCl 2180 1634 1386 980 %T Figure 4. Fourier Transform Infrared (FT-IR) spectra of (a) pristine CNTs, (b) CNTs-COOH, (c) CNTs- H2O2, (d) CNTs-NaOCl, respectively. The oxidation treatments of CNTs were characterized by FT-IR spectroscopy in order to determine the presence of functional groups on the surface of CNTs (Figure 4). FT-IR spectrum of pristine CNTs exhibits a broad peak at 3412 cm -1 which can be assigned to the O-H stretch of hydroxyl group.The oxidation of CNTs with H2SO4 solution has been confirmed by the O-H stretch at 2360 cm -1 from carboxylic group (COOH). The intensive band near 1650 cm -1 has been attributed to the stretching vibrations of aromatic C=C and C=O bonds. The C-O and C-C Comparison of oxidized Carbon Nanotubes efficiencies for Benzene, Toluene removal from 317 vibrations have been explained by peaks at 1000-1300 cm -1 . H2O2 oxydation leads to the presence of both acid groups and hydroxyl groups by large band at 2100 cm -1 corresponding to OH vibration from carboxy group and large band at 3400 cm -1 corresponding to the OH stretching vibration. A intense band is observed at 1100 cm -1 is observed for CNT-NaOCl which could be asigned to the presence of C-O band. It is obvious that oxydation of NaOCl provides large amount of OH groups coming from both carboxylic and hydroxy with large band from 2300 to 3500 cm -1 . The assignment of the absorption bands related to all modification of CNTs structure is also observed in the litterature by Lu et al. [11]. 3.3 Adsorption performance of oxidized CNTs The last study focused on comparison of pristine CNTs and oxidized CNTs synthesized in term of adsorption capacity.Figure 5 indicated the equilibrium amount of Benzene and Toluene adsorbed on pristine CNTs and modified CNTs.It can be obseved that the adsorption capacities of Benzene, Toluene were enhanced by using oxidized CNTs and the highest qe was obtained with CNTs-NaOCl (Benzene : 59.8 mg.g -1 ; Toluene : 74.7 mg.g -1 ). Lu et al. [10] showed that the qe of CNT (NaOCl) for Benzene and Toluene were 230.7 mg.g -1 and 252.1 mg.g -1 , respectively, which is greater than values obtained with pristine CNTs (Benzene 18.1 mg.g -1 and Toluene 80.1 mg.g -1 ). Figure 5. Adsorption capacities of pristine CNTs and oxidized CNTs for Benzene and Toluene. The oxidation of CNTs by different solutionsleads to the formation of oxygenated form which improvesthe dispersion ability of CNTs in water and in organic solvents. The modification by acid mixture created carboxyl groups (COOH) and the treatment with hydrogen peroxide formed carbonyl group (C=O) and hydroxyl group (OH). The strong oxidizing agent NaOCl causes oxygen to penetrate into the space between structural layer of CNTs and form more carbonyl groups -C=O. In addition, the oxygenated functional groups in the surface of CNT could interact with aromatic ring of Benzene and Toluene through hydrogen bonding.It is observed that the adsorption uptake of these oxidized CNTs showed more efficiency than that of pristine CNTs. Figure 5 also shows that the adsoprtion capacity (qe) is varied depending on the functionalisation approach of CNTs and it follows the order CNTs-NaOCl > CNTs-H2O2 > CNTs-H2SO4 > pristine CNTs. Afterthat, the synthesized CNTs-NaOCl showed high efficiency adsorption toward aromatic compound and could be used as adsorbents in environmental treatment, especially in water treatment. CNTs CNTs-H2SO4 CNTs-H2O2 CNTs-NaOCl 0 10 20 30 40 50 60 70 80 q e (m g .g -1 ) Adsorbents Benzene Toluene Le Huu Quynh Anh, Tran Thu Kieu, Tran Duy Hai 318 4. CONCLUSIONS Various types of carbon nanotubes including pristine CNTs and oxidized CNTs were employed as adsorbents to study their adsorption performance for Benzene and Toluene in aqueous solution. The first study have shown that the adsorption capacity of Toluene was higher than that of Benzene. The experimental isotherm data were well-fit with Freundlich model for Benzene adsorption and D-R model for Toluene adsorption. In the second objective, the modification of CNTs by differentmixtures such as CNT- COOH, CNT-H2O2, CNT-NaOCl were achieved. The presence of functional groups on CNTs surface and the structure were confirmed by FT-IR.Finally, the comparison in term of adsorption performance of three oxidized CNTs and pristine CNTs for Benzene and Toluene was carried out. The results indicated that the qe was significantly enhanced with oxidized CNTs, with the following order of qe CNTs-NaOCl > CNTs-H2O2 > CNTs-H2SO4 > pristine CNTs. 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