CHAPTER 5 |
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Kinetics, Thermodynamics, and Regeneration of BTEX Adsorption in Aqueous Solutions via NaOCl-Oxidized Carbon Nanotubes
Fengsheng Su, Chungsying Lu, Kelvin R. Johnston and Sukai Hu
Department of Environmental Engineering, National Chung Hsing University, Taichung, Taiwan
Contents
2. Experimental/Materials and Methods
2.3. Batch Adsorption Experiments
2.4. Adsorption/Desorption Experiments
3.1. Effect of Contact Time and Temperature
3.4. Comparison with Literature Results
3.5. Adsorption Thermodynamics
3.6. Adsorption/Desorption Study
3.7. Cyclic BTEX Adsorption on CNTs and GAC
1. INTRODUCTION
The BTEX volatile organic compounds benzene (B), toluene (T), ethylbenzene (E), and p-xylene (X) are important industrial solvents frequently encountered as industrial contaminants. Every year, these volatile organic compounds, used in gasoline, printing industry, leather industries, and rubber manufacture [1], produce a large amount of BTEX contaminated wastewater that is discharged into the aqueous environment. According to the US Environmental Protection Agency (EPA), the most likely source of BTEX pollution in the environment is due to leakage from underground gasoline storage tanks into ground water [2], leaching from landfills, and discharge from factories and refineries [1]. Even though BTEX is relatively insoluble in water, levels have been recorded at up to 1000 mg/L [3]. This is much higher than the allowed maximum contaminant level of 0.005 mg/L for B, 1 mg/L for T, 0.7 mg/L for E, and a total of 10 mg/L for all three forms of X [1–4]. Since BTEX solutions are flammable, toxic, and carcinogenic substances, the presence of excessive amounts in aqueous surroundings may lead to an adverse impact on water quality and thus endanger public health and welfare [5]. Therefore, the removal of BTEX before releasing into the environment is necessary to meet the growing demand for cleaner water, which has necessitated the development of innovative, cost-effective alternatives for treatment.
There are many conventional methods that are being used to remove the BTEX in wastewater treatment including adsorption, aeration, biological oxidation, and chemical oxidation. Among them, the promising process for the removal of BTEX from wastewater is adsorption, because the used adsorbent can be regenerated by suitable desorption process and it is highly effective and economical. The most widely used adsorbents for BTEX removal are activated carbon [6–8] and zeolite [9]. However, these adsorbents suffer from slow kinetics and low adsorption capacities of BTEX. Therefore, researchers carried out investigation for new promising adsorbents.
Carbon nanotubes (CNTs), a new and exciting part of nanotechnology, are unique and one-dimensional macromolecules that possess thermal and chemical stability [10]. They have been proven to possess great potential as superior adsorbents for removing many kinds of organic pollutants such as dioxin [11] and volatile organic compounds [12–16] from air streams or natural organic matters [17–21] and trihalomethanes [22, 23], xylene [24], resorcinol [25], dyes [26–28], 1,2-dichlorobenzene [29, 30], chlorophenol [31], poly aromatic hydrocarbons [32–42], and herbicides [43] from aqueous solutions, which are summarized in Table 5.1. Compared with other adsorbents such as activated carbon by the foregoing researchers, it is suggested that the CNTs are a promising adsorbent for the removal of organic compounds. Recently, Lu et al. [44] conducted the surface modification of CNTs by NaOCl, HNO3, H2SO4, and HCl solutions and found that the NaOCl-oxidized CNTs are efficient BTEX adsorbents and that they possess good potential applications in wastewater treatment.
Table 5.1 Adsorption of organic compounds on CNTs
Although the NaOCl-oxidized CNTs show promising for BTEX removal in wastewater treatment, the relatively high unit cost of CNTs currently restricts their practical use. The production and the use of nonfunctionalized multiwalled CNTs have now reached industrial levels, but the price of CNTs is still too high to be used in the field. Therefore, if the regeneration of CNTs is not carried out, the practical use of CNTs in wastewater treatment is not feasible. If the regeneration of CNTs is performed, the CNTs should be successfully regenerated and reused through a number of adsorption and regeneration cycles and then can be cost-effective adsorbents.
In this article, NaOCl-oxidized CNTs were used as novel adsorbents to study kinetics, thermodynamics, and regeneration of BTEX adsorption in aqueous solutions. A comparative look at cyclic BTEX adsorption between CNTs and granular activated carbon (GAC) was conducted to evaluate their repeated availability in wastewater treatment. A statistic analysis on the replacement cost of these adsorbents was also given.
2. EXPERIMENTAL/MATERIALS AND METHODS
2.1. Adsorbents
CNTs were fabricated by the catalytic chemical vapor deposition method [45]. The catalyst was prepared by dissolving 2.5 wt% of iron acetate (Fe(CH3COO)2) and 2.5 wt% of cobalt nitrate (Co(NO3)2) into 25 mL of deionized water and then mixed with a commercially available zeolite (CVB100, Zeolyst International, Vally Forge, USA). The mixture was constantly stirred to obtain the resulting semisolid mixture, which was subsequently dried overnight at 140 °C and grounded into a fine powder (as-prepared catalyst).
The physical properties of as-prepared catalyst revealed that surface area, pore volume, and average pore diameter of micropores (MPs) are 1458 m2/g, 0.332 cm3/g, and 0.23 nm, respectively. Most pore volumes are in the size range of 0.2–0.3 nm. Because of its high surface area, small pore diameter, and narrow pore size distribution, the as-prepared catalyst appears beneficial to the production of good-quality CNTs.
The as-prepared catalyst (~1.5 g) was evenly distributed over quartz boats (3.5 cm i.d. and 16.5 cm in length), which were placed in a quartz tube and located in the central position of a horizontal furnace. The quartz boat was activated by passing N2 gas through the reaction chamber at 120 mL/min for 15 min. The reaction chamber was heated to 600 °C at a heating rate of 10 °C/min and then followed by passing acetylene (C2H2) gas at 40 mL/min and N2 gas at 295 mL/min through the reaction chamber for 1 h.
After the thermal reaction, the furnace was cooled to room temperature by the passage of N2 gas at 295 mL/min. The black powders (as-grown CNTs) were collected from the quartz boat. In this way, the CNTs can be fabricated in gram quantities (~3 g of CNTs per hour).
The as-grown CNTs were treated by 30% NaOCl (70 mL of H2O + 3 mL of NaOCl) solution, which has been found to possess better performance for BTEX adsorption than those oxidized by HCl, HNO3, and H2SO4 solutions in a previous study [44]. The mixture was heated at 100 °C and refluxed for 2 h to remove metal catalysts (Fe and Co nanoparticles). After cooling to room temperature, the mixture was filtered through a 0.45 µm fiber filter. The filtered solid was washed with deionized water until the pH of filtrate was 7 and then dried at 200 °C for 2 h. The weight loss of CNTs after the treatment was <5%.
In order to compare the performance of BTEX adsorption on CNTs with other adsorbents, a commercially available GAC (Filtrasorb 400, Calgon Carbon Co., Tianjia, China) was chosen because of its wide use for removal of organic compounds in wastewater treatment.
2.2. Adsorbates
The used B, T, E, and X were analytical grade with >99% purity and purchased from Merck (Darmstadt, Germany, for B and T; Hohenbrunn, Germany, for E and X). These chemicals were diluted using deionized H2O to the desired concentrations.
2.3. Batch Adsorption Experiments
The batch adsorption experiments were conducted using 110 mL glass bottles with addition of 60 mg of adsorbents and 100 mL of BTEX solutions at initial concentrations (C0) of 20–200 mg/L, which were chosen to be representative of BTEX concentration range in an industrial wastewater. The glass bottles were sealed with 20 mm stopper and mounted on a shaker, which was operated at 160 rpm. The experiments were carried out at 5, 25, and 45 °C in a temperature-controlled box (CH-502, Chin Hsin, Taipei, Taiwan). The choice of temperature range is to simulate possible water temperature in the field. All the experiments are repeated two times, and only the mean values were reported. The maximum deviation was <3%. Blank experiments, without the addition of adsorbents, were also conducted to ensure that the decrease in BTEX concentration was actually caused by the adsorption on adsorbents rather than by the adsorption on glass bottle wall or via volatilization.
The adsorption capacity of BTEX on adsorbents (q, mg/g) was calculated as follows:
where C0 and Ct are the BTEX concentrations at the beginning and after a certain period of time (mg/L), V is the initial solution volume (L), and m is the adsorbent weight (g).
2.4. Adsorption/Desorption Experiments
To evaluate the repeated availability of BTEX adsorption on adsorbents, 60 mg of adsorbents was added to 100 mL of solution at a C0 of 200 mg/L. As adsorption reached equilibrium, the adsorption capacity of BTEX on adsorbents was measured (qe), and then, the solution was filtered using a 0.45 µm nylon fiber filter to regenerate adsorbents. The filtered adsorbents were added into 50 mL of 10–50% NaOCl solution and 0.01–0.05 M HNO3 solution to determine the optimum regeneration agent, strength, and time for effective BTEX desorption. The adsorption/desorption process was repeated for 10 cycles. The weight loss of CNTs and GAC after regeneration in each cycle was measured, while the pore structure and the surface functional groups of CNTs before and after 10 cycles of adsorption and regeneration were characterized by their physical properties and infrared (IR) spectra. The recovery is defined as the percentage ratio of the qe of the regenerated adsorbents to that of the virgin adsorbents.
2.5. Analytical Methods
2.5.1. Benzene, Toluene, Ethylbenzene, and p-Xylene
BTEX samples were analyzed by headspace solid phase microextraction [46] and quantified via gas chromatograph (GC) (SRI 8610C, SRI Instruments, CA, USA) with flame ionization detection (FID). Microextraction fiber (Supelco, Bellefonte, USA), which was coated with polydimethylsiloxane/divinylbenzene/carboxen film, was pushed out and exposed directly to the headspace above the sample for 20 min. After extraction, the fiber was immediately inserted into GC injector for desorption and analysis. The GC-FID was operated at injection temperature of 260 °C and detector temperature of 200 °C. The following temperature program was used: 50 °C for 8 min and 4 °C/min to 100 °C for 5 min.
2.5.2. Characterization of Adsorbents
The physical properties of adsorbents were determined by N2 adsorption/desorption at 77 K using Micromeritics ASAP 2020 volumetric sorption analyzer (Norcross, GA, USA). N2 adsorption/desorption isotherms were measured at a relative pressure range (P/P0) of 0.0001–0.99 and then used to determine surface area, pore volume, and average pore diameter of adsorbents via the Barrett–Johner–Halenda equation for pore size range of 1.7–100 nm and the MP equation for pore size below 1.7 nm. The surface functional groups of adsorbents were detected by a Fourier transform infrared ray spectrometer (Spectrum One, Perkin Elmer, MA, USA).
3. RESULTS AND DISCUSSION
3.1. Effect of Contact Time and Temperature
Figure 5.1 shows the effect of contact time and temperature on the BTEX adsorption at a C0 of 200 mg/L. For all experiments, the adsorption capacity of BTEX (q) increased quickly with time and then slowly reached equilibrium. The equilibrium times at 5, 25, and 45 °C are, respectively, 180, 180, and 120 min for B; 240, 120, and 90 min for T; 180, 120, and 90 min for E; and 240, 180, and 120 min for X. It is observed that the equilibrium would be reached faster at higher temperatures, which could be explained by the fact that increasing temperature results in a rise in the diffusion rate of the adsorbates across the external boundary layer and within the CNT pores due to the result of decreasing solution viscosity. A contact time of 240 min, which can assure the attainment of adsorption equilibrium for all the tests, was thus selected in the following tests.
Figure 5.1 Effects of contact time and temperature on BTEX adsorption on CNTs.
The equilibrium adsorption capacities (qe) at 5, 25, and 45 °C are, respectively, 226, 213, and 197 mg/g for B; 240, 225, and 205 mg/g for T; 276, 256, and 240 mg/g for E; and 299, 275, and 261 mg/g for X. The qe decreased with temperature, indicating the exothermic nature of adsorption process. Favorable adsorption of order of X > E > T > B was observed in all tested temperatures which may be due to the increase in molecular weight (B, 78 g < T, 92 g < E, X, 106 g), the decrease in solubility (B, 790 mg/L > T, 530 mg/L > E, 152 mg/L > X, insoluble) [47], and the increase in boiling point (B, 80.1 °C < T, 110.7 °C < E, 136.2 °C < X, 144 °C) [25].
3.2. Adsorption Kinetics
To analyze the rate of BTEX adsorption on CNTs, Lagergren’s first-order rate equation was used [48]:
where qt is the adsorption capacity of BTEX on CNTs at time t (mg/g) and k1 is the first-order rate constant (1/min).
The k1 at 5, 25, and 45 °C, determined from the slope of a linear plot of ln [(qe − qt)/qe] versus t, are, respectively, 0.029, 0.034, and 0.046 min−1 for B; 0.022, 0.031, and 0.041 min−1 for T; 0.027, 0.034, and 0.038 min−1 for E; and 0.022, 0.031, and 0.032 min−1 for X. The correlation coefficients (r2) are in the range of 0.972–0.987 for B, 0.919–0.953 for T, 0.973–0.995 for E, and 0.926–0.962 for X, reflecting that the kinetics of BTEX adsorption on CNTs follows the first-order rate law.
The k1 increased with temperature which is inconsistent with the temperature dependence of the qe. This could be explained by the fact that the adsorption rate is faster than desorption rate at lower temperatures, but desorption rate is more sensitivity to temperature change, and it becomes greater at high temperatures. Thus, adsorption would dominate at lower temperatures, while desorption would dominate at higher temperatures. Similar results have been reported in the literature for adsorption of trihalomethanes [23] and natural organic matters [19] on CNTs.
The temperature dependence of the k1 has been found in practically all cases to be well-represented by the Arrhenius equation [49]
where k0 is the frequency of adsorption (1/min), Ea is the activation energy of the reaction (J/mol), R is the universal gas constant (8.314 J/mol/K), and T is the absolute temperature (K). A plot of ln k1 versus 1/T as a straight line, the corresponding k0 and Ea are determined from the intercept and the slope, respectively.
The k0 of BTEX are 0.911, 1.473, 0.408 and 0.494 min−1, respectively. The Ea of BTEX are 8.018, 9.701, 6.229 and 7.07 kJ/mol, respectively. Notably the Ea are all less than 20 kJ/mol, showing that adsorption of BTEX on CNTs is controlled by diffusion mechanism [50].
3.3. Adsorption Isotherms
The qes of BTEX at multiple temperatures are correlated with the Langmuir model (Eq. 4) and Freundlich model (Eq. 5):
where Ce is the equilibrium concentration (mg/L), a and b are the Langmuir constants, and Kf and n are the Freundlich constants. The constants of Langmuir and Freundlich models were obtained from fitting the isotherm model to the qe of BTEX and are given in Table 5.2. The correlation coefficients (r2) are in the range of 0.703–0.975 for the Langmuir model and 0.861–0.999 for the Freundlich model, indicating that the qe is better correlated with the Freundlich model than with the Langmuir model. The constants a and Kf, which represent the maximum adsorption capacity of BTEX, decreased with temperature.
Table 5.2 Constants of Langmuir and Freundlich models
Figure 5.2 Freundlich isotherms of BTEX adsorption on CNTs at multiple temperatures.
Freundlich isotherms (Kf) of BTEX adsorption on CNTs at multiple temperatures are presented in Fig. 5.2. It is evident that the qe increased with equilibrium concentration (Ce) but decreased with temperature. As the temperature increased from 5 to 45 °C, the Kf decreased, going from 29.56 to 24.77 (mg/g)(L/mg)1/n for B, 45.21 to 29.64 (mg/g)(L/mg)1/n for T, 50.27 to 40.29 (mg/g)(L/mg)1/n for E, and 58.33 to 49.49 (mg/g)(L/mg)1/n for X.
3.4. Comparison with Literature Results
The comparisons of qe via CNTs in this study, with those conducted via many types of raw and oxidized adsorbents such as GAC, single-walled carbon nanotube (SWCNT), powdered activated carbon, montmorillonite, and zeolite, are summarized in Table 5.3. The present CNTs show a better BTEX adsorption performance than other carbon or silica adsorbents, suggesting that the present CNTs are promising BTEX adsorbents in wastewater treatment.
Table 5.3 Comparisons of qe via various adsorbents
3.5. Adsorption Thermodynamics
The thermodynamic parameters, free energy change (ΔG0), enthalpy change (ΔH0), and entropy change (ΔS0) for BTEX adsorption were calculated using the following equations [51]:
where Ka is the thermodynamic equilibrium constant. As the BTEX concentrations in the solution decrease and approach 0, the value of the constant Ka is obtained by plotting a straight line of capacity divided by the equilibrium concentration (qe/Ce) versus qe based on a least-square analysis and extrapolating qe to 0. The intercept of vertical axis gives Ka. The enthalpy change ΔH0 is determined from the slope of the regression line after plotting ln Ka against 1/T. The free energy ΔG0 and entropy ΔS0 change are determined from Eqs. 6 and 7. Table 5.4 summaries the values of these thermodynamic parameters. Negative ΔH0 indicates the exothermic nature of adsorption process. This is supported by the decrease in qe with temperature as shown in Fig. 5.2. Negative ΔG0 suggests that the adsorption process is spontaneous with a high preference for BTEX molecules. Positive ΔS0 reflects the affinity of CNTs for BTEX and the increase of randomness at the solid/liquid interface during adsorption process [52].
Table 5.4 Thermodynamic parameters
3.6. Adsorption/Desorption Study
To test the repeated availability of BTEX adsorption on CNTs, the optimum conditions to reach effective BTEX desorption, such as the regeneration agents, the strength of regeneration solution, and the regeneration time, must be determined.
Figure 5.3 shows the BTEX recoveries of CNTs under various strengths of NaOCl solution. Desorption experiments were conducted for 12 h to assure the attainment of a desorption equilibrium. As the NaOCl strength increased from 10 to 30%, the BTEX recoveries increased from 67.40 to 88.81% for B, 52.17 to 88.82% for T, 40.3 to 88.15% for E, and 40.1 to 89.53% for X, respectively. Figure 5.4 shows the BTEX recoveries of CNTs under various strengths of HNO3 solution. It is observed that as the HNO3 strength increased from 0.01 to 0.04 M, the BTEX recoveries increased from 53.40 to 70.28% for B, 42.17 to 68.77% for T, 36.47 to 69.76% for E, and 33.53 to 70.24% for X, respectively. This could be explained by the degradation of more BTEX molecules on the CNT surface at higher regeneration concentrations where more BTEX molecules can be adsorbed into aqueous solutions again and thus lead to higher BTEX recoveries. The BTEX recoveries via NaOCl solution are much higher than those via HNO3 solution and reached a maximum for NaOCl solution of greater or equal to 30%. Thus, a 30% NaOCl solution was chosen to evaluate the optimum regeneration time.
Figure 5.3 BTEX recoveries of CNTs under various NaOCl strengths.
Figure 5.4 BTEX recoveries of CNTs under various HNO3 strengths.
Figure 5.5 BTEX recoveries of CNTs under various regeneration times.
Figure 5.5 displays the BTEX recoveries of CNTs under various regeneration times. It is evident that the BTEX recoveries increased with regeneration time and reached maximum for 3, 4, 2, and 4 h of operation, respectively. Thus, a regeneration time of 4 h was chosen in the cyclic BTEX adsorption.
It should be noted that the optimum strength of regeneration agent and the regeneration time employed in the desorption experiment depend on the initial BTEX concentration (C0). A larger qe was obtained with a higher C0, which makes desorption of BTEX molecules from the CNT surface more difficult. Thus, a higher strength of regeneration agent or a longer regeneration time is needed with a higher C0 to reach effective BTEX desorption.
3.7. Cyclic BTEX Adsorption on CNTs and GAC
A comparison of cyclic BTEX adsorption on CNTs and GAC was conducted. The physical properties of CNTs and GAC are given in Table 5.5. The surface area, pore volume, and average pore diameter for pore size below 5 nm are larger for GAC, whereas the surface area, pore volume, and average pore diameter for pore size above 5 nm are larger for CNTs.
Table 5.5 Physical properties of CNTs and GAC
Figures 5.6 and 5.7 present the qe and the BTEX recoveries of CNTs and GAC under various cycles of adsorption and regeneration (n). It is apparent that as the n increased, the qe and the BTEX recovery slightly decreased for CNTs but sharply decreased for GAC. This could be explained by the fact that the GAC has a porous structure in which BTEX molecules have to move from the inner surface to the exterior surface of the pores and thus makes desorption of BTEX more difficult.
As n increased from 1 to 10, the qe of BTEX via CNTs decreased from 199.71 to 172.67 mg/g, 208.43 to 184.17 mg/g, 229.2 to 200.66 mg/g, and 263.30 to 232.93 mg/g; the qe of BTEX via GAC decreased from 108.16 to 36.68 mg/g, 143.24 to 50.23 mg/g, 147.88 to 50.63 mg/g, and 159.78 to 54.88 mg/g; the BTEX recoveries of CNTs decreased from 89.49 to 77.38%, 89.16 to 78.78%, 89.4 to 78.3%, and 91.6 to 80.7%; the BTEX recoveries of GAC decreased from 62.35 to 21.15%, 63.15 to 22.15%, 61.4 to 22.3%, and 60.8 to 21.7%. It is apparent that the CNTs not only possess higher adsorption capacity of BTEX but also show better BTEX recovery through 10 cycles of adsorption and regeneration, suggesting that the CNTs are promising BTEX adsorbents in wastewater treatment.
Figure 5.6 Equilibrium capacities of BTEX adsorption on CNTs and GAC under various n.
Figure 5.7 BTEX recoveries of CNTs and GAC under various n.
3.8. Stability of Adsorbents
Figure 5.8 shows the percentage weight loss ratios of CNTs and GAC during the regeneration process of various n. As can be seen, the CNTs have less weight loss in each cycle of adsorption and regeneration than the GAC, reflecting that the CNTs appear rather stable after being repeated use than the GAC. This could be due to a high degree of graphitization of CNTs. The average percentage weight loss ratios of CNTs and GAC are 3.05 and 8.53%, respectively.
Figure 5.8 Percentage weight loss ratios during regeneration of CNTs and GAC under various n.
The pore structure and the surface functional groups of CNTs before and after 10 cycles of adsorption and regeneration were characterized by their physical properties and IR spectra, respectively. Figure 5.9 shows the N2 adsorption/desorption isotherms of virgin and regenerated CNTs. Both samples have very similar N2 isotherms that exhibit a type II shape [53], with a rounded knee at a very low P/P0 (about 0.01) representing some MPs in CNTs. After a very slow increase up to a P/P0 0.9, the isotherms display a sharp increment with P/P0 showing largely mesoporous nature of CNTs. A small closed adsorption/desorption hysteresis loop is also observed with a P/P0 above 0.7 probably due to the mesopores with a capillary condensation.
Figure 5.10 shows the pore size distributions of virgin and regenerated CNTs. It is noted that the MP size distribution of virgin CNTs only appears in the pore size range of 0.26–0.32 nm which become board (0.22–0.36 nm) after regeneration. Both samples have a similar distribution for pore size range of 1.7–100 nm. The pore volume below pore size 20 nm is larger for regenerated CNTs probably due to the fact that more end caps of CNTs were opened up during repeated regeneration via NaOCl solution, whereas the pore volume above pore size 20 nm is larger for virgin CNTs. The physical properties of regenerated CNTs are also given in Table 5.5. The surface area, pore volume, and average pore diameter of CNTs have no significant changes before and after regeneration.
Figure 5.9 N2 adsorption (solid line) and desorption (dash line) isotherms of virgin and regenerated CNTs.
Figure 5.10 Pore size distribution of virgin and regenerated CNTs.
Figure 5.11 IR spectra of virgin and regenerated CNTs.
Figure 5.11 shows the IR spectra of virgin and regenerated CNTs. It is evident that both samples have similar IR spectra. The band at 2350 cm−1 is assigned to −OH stretch from strongly H-bonded COOH [54]. The bands at 1700 and 1530 cm−1 are related to the stretching variations of C=O groups [6] and carboxylate anion stretch mode [54], respectively. The band at ~970 is associated to C–O stretching of alcoholic compounds [55]. The abundance of these surface oxygen groups on the external and internal surface of CNTs provides numerous chemical sites for BTEX adsorption.
It is apparent that the pore structure and the surface oxygen groups of CNTs were preserved through 10 cycles of adsorption and regeneration, suggesting that the CNTs can be used in the prolonged cyclic operation.
3.9. Cost-Effective Analysis
To evaluate the replacement cost of CNTs and GAC in wastewater treatment, a statistical analysis based on the best-fit regression of qe versus n was conducted (Fig. 5.6), and the regression equations are given in Table 5.6. The r2 of BTEX is 0.913–0.995 for CNTs and 0.898–0.918 for GAC.
Figure 5.12 shows the predicted n of BTEX adsorption via CNTs and GAC at a qe range of 60–200 mg/g. As can be seen, the predicted n rapidly decreased with qe. At a qe of 60 mg/g, the predicted n’s of BTEX are, respectively, 2.46 × 109, 4.03 × 109, 1.19 × 1010, and 1.09 × 1012 for CNTs and 6, 12, 9, and 10 for GAC. At a qe of 120 mg/g, the predicted n’s of BTEX are, respectively, 7545, 1.71 × 104, 7.2 × 104, and 2.65 × 106 for CNTs and 1, 2, 2, and 2 for GAC.
Table 5.6 Regression equations for cost-effective analysis
Figure 5.12 Predicted n of BTEX adsorption on CNTs and GAC under various qe.
It is clear that the CNTs can be reused for BTEX removal through a number of adsorption and regeneration cycles. A significant cost for the replacement of adsorbents can be thus reduced. This is the key factor whether a novel but expensive adsorbent can be accepted by the field or not. It is expected that the unit cost of CNTs can be further reduced in the foreseeable future. Therefore, the NaOCl-oxidized CNTs possess good potential for BTEX removal in wastewater treatment in the near future.
It should be noted that the predicted number of cycles (n) was estimated based on the qe of 10 adsorption and regeneration cycles since it is quite time-consuming to regenerate adsorbents for over thousands of tests. Furthermore, the adsorbent weight loss was neglected in the estimation of n. Thus, a departure of predicted results from real conditions may possibly occur, and a prolonged test on the adsorption performance of regenerated CNTs is required.
4. CONCLUSIONS
The CNTs were oxidized by NaOCl solution and were used as novel adsorbents to study kinetics, thermodynamics, and regeneration of BTEX adsorption in aqueous solutions. The adsorption kinetics follows the first-order rate law, while the adsorption thermodynamics indicates the exothermic and spontaneous nature. The CNTs show better BTEX adsorption performance than other carbon or silica adsorbents documented in the literature. A comparative study on cyclic BTEX adsorption on CNTs and GAC revealed that the CNTs have a better adsorption performance and show less weight loss through 10 cycles of adsorption and regeneration. This suggests that the CNTs are promising BTEX adsorbents in wastewater treatment. A statistical analysis on the replacement cost of CNTs and GAC revealed that the CNTs can be reused through a number of adsorption/regeneration cycles and thus be possibly a cost-effective BTEX adsorbent despite their initial high unit cost at the present time.
ACKNOWLEDGMENT
Support from the National Science Council, Taiwan, under a contract no. NSC-97-2221-E-005-036-MY3 is gratefully acknowledged.
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