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Adsorption of chlorinated organic compounds from water with cerium oxide-activated carbon composite
Accepted Manuscript Adsorption of chlorinated organic compounds from water with cerium oxideactivated carbon composite Khalid R. Alhooshani PII: DOI: Reference:
S1878-5352(15)00115-X http://dx.doi.org/10.1016/j.arabjc.2015.04.013 ARABJC 1628
To appear in:
Arabian Journal of Chemistry
Received Date: Accepted Date:
21 October 2014 18 April 2015
Please cite this article as: K.R. Alhooshani, Adsorption of chlorinated organic compounds from water with cerium oxide-activated carbon composite, Arabian Journal of Chemistry (2015), doi: http://dx.doi.org/10.1016/j.arabjc. 2015.04.013
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Adsorption of chlorinated organic compounds from water with cerium oxide-activated carbon composite Khalid R. Alhooshani* *
Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
* Tel.: +966 13 860 3065; fax: +966 13 860 4277. E-mail address: hooshani@
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Abstract Adsorptive removal of dichloromethane, chloroform, and carbon tetrachloride from aqueous solutions at 25 °C by activated carbon (AC) that was loaded with cerium oxide nanoparticles (CeO2-NP/AC) was investigated. The developed adsorbent
was
characterized by scanning electron microscope (SEM), FTIR spectrophotometer, X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). The effect of contact time, initial concentration, and the adsorbent dosage were also studied. The equilibrium and kinetics of adsorption were studied in a batch-type adsorption system, and the equilibrium experimental data were analyzed using Langmuir, Freundlich, and Temkin isotherm models. Freundlich adsorption isotherm showed the best fit for the equilibrium adsorption data. Three adsorption kinetic models, pseudo first- and second-order, and intraparticle diffusion models were applied to test the kinetic data. Kinetic characterization of the adsorption process onto CeO2-NP/AC is well-described by the pseudo second-order model, and the adsorption best-fit the intraparticle diffusion model. Our study shows that at optimum conditions, 82.72%, 99.40% and 89.42% of dichloromethane, chloroform, and tetrachloride, respectively, were removed by CeO2NP/AC, at concentration between 0.25- 5.00 g/L. Keywords: Adsorption, cerium oxide-activated carbon composite, dichloromethane, chloroform, carbon tetrachloride.
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Introduction Treatment of wastewaters remains a serious challenge because of the severity of environmental challenges posed by water pollution through chemical discharge. Because of the direct and indirect impact of such discharge pollutants, it is of great concern for countries to uphold environmental protection policies and to invest in methods that can reduce the hazardous byproducts of wastewater treatment. Consequently, wastewaters containing toxic materials such as dyestuffs, organic pollutant, and metal ions must be treated carefully before introduction to the environment, and the methods for removing these toxic materials need to be further researched and improved [1]. The usage of chlorinated hydrocarbons (CHCs) in industry has a significant impacts on the environment, for they can be used either as reactants or solvents for the synthesis of herbicides and plastics [2], industrial automotive and aerospace solvent degreasing, dry cleaning solvents in garment industries, precision solvent cleaning in electronic industries [3, 4], petroleum refining [5], and chlorine-bleach-containing household products [6]. Additionally, the emission from industrial activities or wastewater streams of CHCs lead to their dispersion into the atmosphere [7], aquatic systems, and soil which pose major threats to human and aquatic life [4, 8]. In addition, the presence of these chlorinated solvents were found to cause dizziness, irregular heartbeat, reduced cognition, and nervous system impairment [9-11]. Due to their high volatility in ambient conditions [12], toxicity, strong recalcitrance to degradation, and carcinogenic effects, CHCs have been listed as one of the priority pollutants by United States Environmental Protection Agency (U.S. EPA) and European Commission (EC) [13]. Furthermore, these hydrocarbons can cause ozone layer depletion and formation of photochemical smog [14, 15]. Due to direct and indirect harmful effects associated with CHCs, it is of great importance to develop efficient methods for the removal of such pollutants and toxins. The classical remediation approach employed for the removal of CHCs is thermal incineration that involves operation at elevated temperature (1000 oC). Moreover, incineration could lead to the production of byproducts such as phosgene, dioxins, and furans that are more harmful than the original contaminants [16, 17]. Other methods for removing CHCs involve catalytic oxidation or hydrodechlorination (HDC). This method utilizes different catalysts such as Pt or Pd and usually leads to a 3
high conversion of most reactive chloromethanes, in the order of trichloromethane, dichloromethane and chloromethane, (TCM > DCM > MCM). However, major issues associated to this approach is the catalyst deactivation and the use of high cost noble metal [18-20]. To offset these limitations, there have been major advancements in the HDC approach by the introduction of transition metals because they are more resistant to deactivation when used as catalysts for the decomposition of chlorinated organic compounds [20, 21]. Nevertheless, these transition metal catalysts are not completely ideal because they lead to the formation of toxic residues such as chromium oxychloride. Adsorption, absorption, and condensation techniques are non-destructive techniques frequently used for the removal or recovery of volatile organic carbons (VOCs) from gas streams [22, 23]. The adsorption process is commonly used as a final step in the treatment of wastewater from industrial byproducts for the removal of contaminants that are significantly resistant to biodegradation even at very low concentration levels [19, 24]. Generally, volatile, semi-volatile and non-volatile chlorinated organic pollutants can be efficiently removed from drinking water, surface water, and aqueous industrial sludge by adsorption technology on the surface of activated carbon (AC) [25-27], due to its unique microporous structure with high surface area, large pore volume, and high adsorption efficiency [28]. Although activated carbon lends to great efficiencies, the commercial source for activated carbon can be very expensive. Consequently, different alternative sources for the production of activated carbon-like material such as peanut shell, bagasse, sawdust, bamboo, cherry stones, and waste tire rubber, have been developed by researchers [1, 29-33]. Obtaining AC from the waste tire is of great importance since it can lower the cost of commercial ACs, eliminate fire hazard and address pollution problems associated with the disposal process of old tires [34]. Earlier reports have described the usage of modified ACs. For example, Hongyan et al. [35] reported the adsorption and desorption of dichloromethane over metal modified activated carbons. Jiun-Horng et al. [36] studied adsorption characteristics using commercial activated carbon, sludge adsorbent, and activated carbon fibers, where fibers showed much more adsorption. Furthermore, Milchert et al. [37] and Borkar et al. [38], use the unmodified activated carbons for the adsorption of carbon tetrachloride & dichloromethane, respectively. 4
Recently, there has been an increasing interest in the use of nano-cerium oxide, CeO2, for diverse applications, such as electrolytes in solid oxide fuel cells, insulators, high refractive index materials, UV blockers, polishing materials, gas sensors, hightemperature oxidation resistance, free-radical scavenger and ultimately as catalysts, catalyst promoters or catalyst supports [39]. The use of nano-cerium oxide as catalysts is due to its high oxygen storage capacity and oxygen donation caused by partial reduction of Ce4+/Ce3+ on the surface [40]. Cerium oxide, In addition to the aforementioned advantages, CeO2 is inexpensive, highly stable, environmentally benign, and efficient for decomposition of chlorinated organic compounds. Incorporation of CeO2 nanoparticles onto the surface of AC is expected to improve the adsorption efficiency of CHCs compared to unmodified AC [19]. By modifying nanoparticles on ACs, this study will improve current costs and methods being used for the removal of chlorinated organic compounds. Specifically, this project focuses on the preparation of ceria nanocomposite supported on AC obtained from waste tire rubber. 2. Materials and methods 2.1 Reagents All chemical reagents (dichloromethane, trichloromethane, and carbon tetrachloride (purity FHULFDPPRQLXPQLWUDWHQLWULFDFLGDQGK\GURJHQSHUR[LGH) were obtained from Sigma Aldrich. Dichloromethane, trichloromethane, and carbon tetrachloride were used without further purification. All aqueous solutions were prepared using deionized water. The activated carbon (AC) used in this study was produced by pyrolysis process from waste rubber tires as described by Chan et al. [1]. 2.2 Preparation of CeO2-NP/AC Activated carbon (AC) was prepared by heating clean and crushed waste rubber tires at 300 o
C to isolate the granules from produced oil. In order to increase the porosity of the AC, the
granules were then carbonized by raising the temperature initially to 500 oC for 3 h and then gradually raised to 900 oC for 2 h. In addition, it was further treated with H2O2 and HNO3 to oxidize the surface of AC, producing oxygenated surface of the AC. Then, the resultant product (AC) was washed with excessive water and dried overnight at 100 ºC. The 5
composite of CeO2-NP/AC was prepared by co-precipitation method. In short, 6.0 g of AC was dispersed in 150 mL of suitable media of pH 7 by the use of a sonicator. Afterwards, an equimolar solution of ceric ammonium nitrate (50 mL) was added to the mixture. The mixture was then stirred at 50 oC for 24 h, followed by refluxing it for 12 h at 90 oC. Upon cooling to room temperature, the product formed was filtered and washed with water and dried overnight at 110 oC. The final adsorbent was calcined at 350 oC for 4 h in muffle furnace.
2.3 Characterization of adsorbent To study the surface morphology of the developed adsorbent, field emission scanning electron microscope (FE-SEM, LYRA 3 Dual Beam, Tescan) equipped with energy dispersive X-ray spectrometer (EDX, Oxford Instruments) was employed. The crystalline structure of the adsorbent was investigated through X-ray diffractometer (Shimadzu XRD 0RGHO-DSDQ XVLQJ.ĮUDGLDWLRQRI&XRSHUDWHGDWN9DQGP$Change on the modified surface of AC with CeO2 was studied with Fourier transform infrared spectroscopy (FTIR) and Thermal gravimetric analysis (TGA). FTIR spectra of adsorbents samples was obtained using a Nicolet 6700 spectrometer (Thermo electron, USA) equipped with a deuterated triglycine sulfate (DTGS) detector connected to OMNIC software. The samples were prepared via the solid KBr pellet techniques. The background correction of the spectra was performed by 16 scans with resolution 2 cm-1. Thermal gravimetric analysis (TGA) was performed using TA instruments SDT Q600 (USA). The samples were monitored at 15 ºC/min under nitrogen atmosphere flowing at a rate of 50 mL/min. Analysis was made over a temperature range of 20 ºC ± 900 ºC.
2.4 Batch sorption procedure Batch adsorption experiments were performed using a 125 mL conical, airtight Pyrex glass. Each flask contained 50 mL of a chlorinated hydrocarbons solution with varying initial concentration (C0) from 5 - 20 mg/L. The glass bottles were sealed and then mounted on a 6
shaker at 150 rpm until equilibrium was achieved. The adsorption experiment was performed at room temperature, 25 °C. Parameters such as effect of initial concentration, adsorbent dosage, and contact time were investigated. Chlorinated hydrocarbons adsorbed by CeO2-NP/AC composite were calculated as follow:
୲ ൌ ሺᩳ െ ୲ ሻ ൈ
ሺͳሻ ݉
Where qt is the amount of chlorinated hydrocarbons adsorbed (mg/g), V is the solution volume (L), Co is the initial concentrations of chlorinated hydrocarbons (mg/L) at time (t = 0), ୲ is the liquid phase concentration of chlorinated organic compounds (mg/L) at any time, and m is the weight of the synthesized adsorbent (g).
2.5 Analysis of chlorinated hydrocarbons Chlorinated hydrocarbons were determined using headspace gas chromatography (Agilent 7890A GC) equipped with mass selective detector (MS) (Agilent 5975C). Direct headspace automated injection was performed by using 2.5 ml-HS syringe with an injection of 250 µL of the headspace vapor. The 1mL sample was incubated at 60 °C for 15 min in a 10 mL vial. The agitation speed was 250 rpm, syringe temperature 50 °C, and the injection speed was 500µL/sec. HP-5 GC capillary column (30m x 0.32 mm i.d. x 0.25 µm film thickness) was used for this analysis. The GC-MS inlet was operated in split mode (50:1) at 180 °C. The column temperature was programmed at 35 °C for 10 min and then increased it to 40 °C at the rate of 5 ºC/min for 1 min, followed by raising it to 120 °C at 15 °C /min for 2 min. Finally, the temperature of GC oven was raised to 120 ºC at 20 °C /min and held for 11 min. Throughout the course of these experiments, the flow of helium was maintained at a constant flow of 1.4 mL/min. Quantifications of the samples were performed in selected ion monitoring (SIM) mode, where suitable fragments including base peaks of the compounds were boldfaced: For instance m/z 49, 84, 86, 88 for dichloromethane; m/z 83, 85, 47 for trichloromethane and m/z 117, 119, 121, 82 for carbon tetrachloride.
2.6 Regeneration study 7
The regeneration potential of spent adsorbent was conducted by using a thermal method where chlorinated adsorbed molecules were desorbed by thermal evaporation at 100 oC for 24 h, and washed with de-ionized water and dried at 105 oC overnight. Then, 0.1 g of regenerated adsorbent was dispersed in 50 mL solution containing 10 mg/L of chlorinated organic compounds and was reused for additional three cycles, following the same procedure as fresh adsorbent.
2.7 Data analysis The data collected from batch study at different adsorbent dosages, and initial concentrations (5, 10, 20 mg/L) of CHCs were analyzed by kinetic studies (pseudo-first order, pseudo-second order, and intra-particle diffusion). In addition, different adsorption isotherm models (Langmuir isotherm, Freundlich isotherm, and Temkin isotherm) were conducted to identify the adsorption capacity (qt).
3. Results and discussions 3.1 Characterization of CeO2-NP/AC The morphology of CeO2-NP/AC composite (Fig. 1a) revealed that the surface of the composite has a large number of nanoparticles. This, in turn, can provide additional active sites for surface adsorption on the surface of activated carbon. Based on SEM image, the average diameter of the ceria particles was estimated to be approximately 60 nm, which was further confirmed by taking the average size of the nanoparticles from several images. The elemental analysis by EDX spectrum (Fig. 1b) indicated the presence of carbon, oxygen, and cerium atoms which were as expected in the composite of CeO2-NP/AC adsorbent. Please insert figure 1(a and b) here Please insert table 1 here
The FT-IR spectrum of CeO2-NP/AC composite is illustrated in fig (2). Absorption frequency at 1640 cm-1 represents the enhancement in the aromatic C=C groups (carbonization). The strong and broad band at 3400 cm-1 indicated the presence of O±H of 8
H2O, physically adsorbed on the surface. The spectrum has absorption bands at 2900 (aliphatic C±H stretching) and at 1054 cm-1, attributed to C-O stretching vibration. A strong absorption at 500 cmí1 was assigned to (Ce±O) vibration mode [39], confirming the formation of cerium oxide nanoparticles. The XRD spectrum of the activated carbon (AC) showed three broad diffraction peaks at 2ș = 20±30 and 40±50, which indicated the presence of amorphous carbon disorderly stacked up by carbon rings [41]. The XRD pattern of CeO2NP/AC composite is illustrated in fig (3). At low diffraction angles, the typical broad diffraction band of ceria nanoparticles was observed, which confirmed the presence of cerium oxide nanoparticles and their high dispersion on the AC support, as was supported by SEM image. The diffraction peaks exhibited cubic fluorite structure in comparison with JCPDS file (PCPDF 34-0394) in the database [42]. The main phases of CeO2 are identified as (111), (200), (220) and (311), corresponding to 2 = 28.9, 33.6, 48, and 57, respectively. The diffraction pattern of the carbon exhibits a weak diffraction peak at 2 = 26o that is attributable to the aromatic carbon sheets oriented in a considerably random fashion.
Please insert figure 2 and 3 here. Thermal stability of CeO2-NP/AC composite was also investigated (Fig. 4). Thermal analysis of the prepared composites is very important to determine their thermal stability and optimal conditions of calcination. As shown in the figure, the CeO2/AC has shown better thermal stability than bare synthesized activated carbon. The total weight loss of 9 % was observed in the CeO2-NP/AC composite adsorbent which can be attributed to decomposition of adsorbed water molecules. On above 400 oC, almost no weight loss was further observed, confirming the presence of crystalline CeO2 on the activated carbon as the final adsorbent. In case of AC, no significant weight loss was observed at almost 500 o
C except approximately 18 % weight loss at above 100 oC, due to adsorbed water.
Please insert figure 4 here
3.2 Effect of contact time and initial concentration The effect of contact time on the removal capacity of chlorinated hydrocarbons by CeO 2NP/AC composite, as illustrated in fig. (5), revealed that the removal of chlorinated 9
hydrocarbons increased quickly with a contact time and then progressed at a slower rate until it reached the saturation step. This result is due to the large number of active sites on the external surface of the adsorbent. Consequently, the initial step of the adsorption was initially very fast which was slow down due to internal diffusion process. Moreover, the large number of vacant active sites observed after a certain time indicated that the uptake of the solute molecules was made difficult due to the repulsive forces between the solute molecules on the surface of the CeO2-NP/AC as well as in the bulk phases [43, 44]. However, the quantity of chlorinated hydrocarbons adsorbed per unit mass of CeO2-NP/AC was increased as the initial concentration of chlorinated hydrocarbons was increased. This was attributed to the increase in mass transfer between the solution and the adsorbent by virtue of concentration factor, a driving force in the adsorption process [45, 46]. The amount of chlorinated hydrocarbons adsorbed per gram of CeO2-NP/AC after 10 min increased from 1.47 to 8.80 mg/g for dichloromethane, 3.40 to 15.99 mg/g for chloroform and from 4.58 to 18.61 mg/g for carbon tetrachloride as when the initial chlorinated hydrocarbons concentration was increased from 5 to 20 mg/L. Therefore, the adsorption was very rapid in the first 10 min and then was decreased and remained constant at the equilibrium, which was reached in almost 30 min. Please insert figure 5 here 3.3 Effect of adsorbent dosage The amount of the adsorbent is considered a significant factor because it can identify the capacity of the adsorption for a given initial concentration of the adsorbate [47]. Fig (6) shows the effect of CeO2-NP/AC dosage on the removal of chlorinated hydrocarbons. Different adsorbent dosages were applied for dichloromethane, chloroform, and carbon tetrachloride at initial concentration of 10 mg/L, at 25 Û&DQGDVKDNLQJWLPHRIPLQ7KH results indicated that, the percent removal of chlorinated hydrocarbons was increased from 32.04 % to 82.72 % for dichloromethane, from 57.14% to 99.40 % for chloroform and from 71.74% to 89.42 % for carbon tetrachloride, with an increase of adsorbent dosage from 0.25 to 5.00 g/L. This increase could be attributed to the large vacant sites of the increased adsorbent mass, for a constant concentration of chlorinated hydrocarbons [48, 49]. However, when the adsorbent dose exceeded 3 g/L, the increase in percent removal of
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chlorinated hydrocarbons was insignificant [50, 51]. The removal percentage of chlorinated hydrocarbons in solution was calculated by Eq. (2),
Ψܴ݁݉ ݈ܽݒൌ
ι െ ୣ ൈ ͳͲͲሺʹሻ ι
Please insert figure 6 here 3.4 Adsorption kinetics The adsorption rate gives important information for designing batch adsorption systems. McKay et al., demonstrated that the adsorption process is composed of three steps: (1) (film resistance); which involves transfer of solute from the bulk solution to the external surface of the sorbent through a liquid boundary layer; (2) (intraparticle resistance) which is the transfer of solute from the sorbent surface to the intraparticle active sites and (3) (reaction resistance) interactions of the solute with the vacant sites on both the external and internal surfaces of the sorbent [52]. Additionally, the adsorption kinetic information describes the interaction between adsorbent and solution interface and can determine the mechanism of the adsorption processes. Such a mechanism might be explained by the pseudo first-order , pseudo second-order rate equation [53], and intraparticle diffusion models [54]. The agreement between experimental results and the kinetic model expected values is indicated by the correlation coefficient (R2).
3.4.1The pseudo-first order model The pseudo-first order equation was used to examine the kinetic adsorption data collected at three different initial concentrations of chlorinated hydrocarbons for investigating the ratecontrolling mechanism of the adsorption process [55]. The pseudo-first order kinetic model can be described by the following Lagergren rate equation [56].
ሺݍ െ ݍ௧ ሻ ൌ
݇ଵ ݐሺ͵ሻ ʹǤ͵Ͳ͵
11
Where k1 is Lagergren rate constant of pseudo-first-order adsorption (1/min), ݍ௧ and ݍ are the amounts of chlorinated hydrocarbons (mg/g) at contact time t and at equilibrium, respectively. From the plots of log ሺݍ െ ݍ௧ ሻ versus t (Fig. 7), the values of k1 (rate constant) and ݍ (adsorption quantity) at equilibrium were calculated from the slopes and intercepts. These values are given in table (2). Please insert figure 7 here The obtained correlation coefficient (R2) values from the plots were low for the three concentrations of chlorinated compounds, which indicated the lack of linearity for pseudo-first-order kinetic model. The correlation between the calculated qe from the equation (3) and observed qe was very poor. This indicated that the adsorption of chlorinated hydrocarbons did not obey the pseudo-first-order kinetic model and, consequently, the mechanism of the adsorption on the surface of the CeO 2-NP/AC may not follow the pseudo-first-order model well.
3.4.2 The pseudo-second order model The pseudo second-order adsorption kinetic rate equation was expressed by the following equation [57]: ݀ݍ௧ ൌ ݇ଶ ሺݍ െ ݍ௧ ሻଶ ሺͶሻ ݀ݐ Where ݇ଶ is the rate constant of the pseudo-second-order adsorption (g/mg.min), ݍ and ݍ௧ are the adsorbed amount of the chlorinated hydrocarbons in mg/g of adsorbate at equilibrium and time, t, respectively. When the boundary conditions are applied from t = 0 to t = t and qt = 0 to qt = qe, the equation (4) after the integration becomes: ͳ ͳ ൌ ݇ଶ ݐሺͷሻ ݍ െ ݍ௧ ݍ Equation (5) represents the integrated rate law for pseudo-second-order it can be expressed in the linear form as below: ͳ ݐ ݐ ൌ ሺሻ ݇ଶ ݍଶ ݍ ݍ௧
12
Where h is the initial adsorption rate at t = 0 , h (mg/g.min) can be described as : ݄ ൌ ݇ଶ ݍଶ ሺሻ Therefore, equation (7) can be written in a form as: ͳ ݐ ݐ ൌ ሺͺሻ ݄ ݍ ݍ௧ The plots of t/qt against t of equation (6) at different initial concentrations (5, 10 and 20 mg/L) of the chlorinated hydrocarbons shown in fig. (8). The adsorption parameters qe, and ݇ଶ were calculated from the slope and the intercept respectively as shown in table (2). The agreement between the calculated qe, and experimental q e, and the observed correlation coefficient values (R2=0.99) from all the data at different concentrations indicates that the adsorption kinetics between CeO2±NP/AC composite and the chlorinated hydrocarbons follow the pseudo-second order [58].
Please insert figure 8 here Please insert table 2 here
3.4.3 The intraparticle diffusion model The investigation for the possibility of intraparticle diffusion and prediction of the rate controlling step in the adsorption process were carried out by using the Weber and Morris model [59, 60]. The rate constants of intra-particle diffusion (kid) can be determined by using the following equation: ଵ
ݍ௧ ൌ ݇ௗ ݐଶ ܥሺͻሻ Where ݍ௧ is the amount of chlorinated hydrocarbons adsorbed at time t, t 1/2 is the square root of the time, C is the intercept refers to the thickness of the boundary layer, the larger the intercept, the greater the boundary layer effect. If intraparticle diffusion occurs, then ݍ௧ 13
versus t1/2 will be linear and if the plot passes through the origin, then the rate limiting process is only due to the intraparticle diffusion. Otherwise, some other mechanism along with intraparticle diffusion is also involved [61]. The plots of qt versus t1/2 are illustrated in fig. (9). The lines of the different concentration do not go through the point of origin which means the mechanism cannot determine the rate of the overall process, and the adsorption mechanism likely to be very complex [62]. Please insert figure 9 here
3.5 Adsorption isotherm Various isotherm models were used for investigation of adsorption capacity in adsorption of chlorinated hydrocarbon from aqueous solution. The three models introduced in this study are Langmuir, Freundlich, and Temkin isotherms.
3.5.1 Langmuir isotherm model The Langmuir explains that the adsorption process occurs on the homogeneous surface of the adsorbent by monolayer of the adsorbate which has a specific active sites and energies [63, 64, 65]. The linear equation is given by: ͳ ܥ ܥ ൌ ሺͳͲሻ ݍ ݇ ݍ ݍ Where ݇ is Langmuir equilibrium constant (L/mg), and ݍ (mg/g) is the monolayer adsorption capacity, were calculated from a plot C e/qe versus Ce (adsorbed equilibrium concentration) fig (10). The characteristic parameter of Langmuir isotherm can be illustrated in terms of dimensionless equilibrium parameter, RL, known as separation factor, defined by Weber and Chakkravorti:
ܴ ൌ
ͳ ሺͳͳሻ ͳ ܭ ܥ
14
Where Co in this case is the highest initial solute concentration. The value of separation factor gives an indication for the type of the isotherm and the nature of the adsorption process. Regarding the RL value, adsorption can be unfavorable (RL>0), linear (RL=1), favorable (0
Please insert figure 10 here Please insert table 3 here
3.5.2 Freundlich isotherm model The Freundlich model is usually used to explain the adsorption by a heterogeneous surface of an adsorbent [67]. The well-known logarithmic form of Freundlich is defined by the following equation: ͳ ୣ ൌ ୣ ሺͳʹሻ Where Kf (L/g) and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. From the slope and intercept of straight portion of the linear plot obtained by plotting ln (qe) versus ln (Ce) fig. (11). The Freundlich isotherm constants Kf and n was calculated as represented in table (3). It is interesting to see that all values of 1/n > 1 represent favorable adsorption condition [68-70]. The correlation coefficient is used as a tool to determine the best fitting parameters for each model, and to determine the adsorption process that is observed. It is observed that the linearity coefficient for all the CHCs was high,
and this gives better indication that the adsorption process is followed by the
Freundlich isotherm [71]. The correlation coefficient (R2) of 0.997, 0.998, and 0.996 for dichloromethane, chloroform and carbon tetrachloride, respectively, indicated that the adsorption process could be best described by Freundlich isotherm. Please insert figure 11 here
15
3.5.3 Temkin isotherm model The Temkin isotherm represents the quantity of heat needed for the adsorption by one layer of adsorbate on the surface of adsorbent. The linear form of Temkin model is given by:
ୣ ൌ
ୣ ሺͳ͵ሻ
where bT is the Temkin constant related to the heat of sorption (N-ÂPROí1), is the equilibrium binding constant which is equal to the maximum binding energy (/ÂJ-1), R is gas constant (-ÂPROí1ÂKí1) and T is the absolute temperature (K) [72]. The plot of qe versus ln (Ce) is illustrated in fig (12), the isotherm constants were determined from the slope and intercept table (3). The R2 in this case was low compared to the other two models described above, especially in the case of chloroform that show poor accuracy in the experimental data fitting using Temkin model. This observation suggested that the adsorption of chloroform compound on the adsorbent does not occur through a uniform distribution adsorbent-adsorbate interaction or mechanism.
Please insert figure 12 here
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3.6 Regeneration of adsorbents The disposal of adsorbent as byproducts in the adsorption process is not economical.; Therefore, regeneration of the adsorbent is highly desirable [73]. In order to be cost effective, adsorbent was regenerated by thermal treatment. Thereafter, the regenerated adsorbent material was tested for an additional three cycles, and the results showed excellent reusability with almost percentage removal of chlorinated compounds compared with fresh adsorbent, as shown in fig (13). Please insert figure 13 here 4. Conclusions An efficient and fast adsorptive removal of dichloromethane, trichloromethane and carbon tetrachloride from aqueous media at ambient conditions were achieved by using AC loaded with cerium oxide nanoparticles (CeO2-NP/AC). The adsorption removal of tested organic chlorinated compounds onto CeO2-NP/AC composite can be well described by Freunlich adsorption isotherm. Kinetic characterization of the adsorption process onto the developed adsorbent was well described by the pseudo second-order model, and the adsorption fit the intraparticle diffusion model. Among the features, fast equilibrium time, high adsorption capacity as well as efficient adsorbent regeneration indicated that the CeO2-NP/AC is a highly efficient low cost adsorbent for the removal of dichloromethane, chloroform, and carbon tetrachloride from aqueous media. Acknowledgements The financial support provided by King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia through summer research fund and facility support provided by the King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia for this work are gratefully acknowledged.
17
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22
List of figures
a
Figure 1. (a) SEM images of the surface structure of the CeO 2-NP/AC, (b) EDX analysis of CeO2± NP/AC
b
% Transmittance
1000
1500
-1
2500 Wavenumber (cm )
2000
3000
3500
Figure 2. FTIR spectrum of as-developed CeO2-NP/AC composite.
500
AC
CeO2-NP/AC
10
b
a
20
c
C
30
(111)
(200)
50
2T (degree)
40
C
C
(220)
60
(311)
70
(400)
80
AC
(331)
CeO2-NP/AC
Figure 3. X-Ray Diffractogram patterns of (a) AC (from waste rubber tire) (b) as-developed CeO2±NP/AC composite .
Intensity (a.u)
40
50
60
70
80
90
100
110
0
100
200
300
500
600
7HPSHUDWXUH Û&
400
700
Ceria NPs-AC
800
Figure 4. Thermogravimetry analysis of both bare AC and CeO2±NP/AC composite.
Weight (%)
900
AC
1000
0
2
4
6
8
0
20
40
c
Time (min)
60
15
20
80
0
5
10
0
100
20 mg/L 10 mg/L 5 mg/L
20
120
40
60
Time (min)
b
80
0
3
6
9
12
15
0
100
20 mg/L 10 mg/L 5 mg/L
20
120
40
60
Time (min)
80
100
20 mg/L 10 mg/L 5 mg/L
120
Figure 5. Effect of contact time on adsorption of (a) dichloromethane, (b) chloroform, (c) carbon tetrachloride at different concentrations, and temperature 25Û&.
q (mg/g) t
10
q (mg/g) t
a
q (mg/g) t
0
10
20
30
40
50
60
70
80
90
100
0.3
Dichloromethane
0.5
1.0
2.0
Carbon tetrachloride
CeO2±NP/AC dosage (g/L)
Chloroform
5.0
Figure 6. Effect of adsorbent dosage on the adsorption of dichloromethane, chloroform and carbon tetrachloride at different concentrations, and temperature 25Û&.
Removal %
Log(qe-qt)
0
10
20
30
50
c
Time (min)
40
-1.5
-1.0
-0.5
0.0
0.5
1.0
0
60
10
70
20
80
30
90
20 mg/L 10mg/L 5 mg/L
50
Time (min)
40
60
b
0
70
-1.5
-1.0
-0.5
0.0
0.5
1.0
80
20
90
20 mg/L 10mg/L 5 mg/L
10
30
40
50
Time (min)
60
70
80
90
20 mg/L 10mg/L 5 mg/L
Figure 7. Lagergren first order plot for adsorption of (a) dichloromethane (b) chloroform (c) Carbon tetrachloride at different concentrations and temperature 25Û&
-1.5
-1.0
-0.5
0.0
0.5
1.0
Log(qe-qt)
a Log(qe-qt)
t/g (min.g/mg)
20
20 mg/L 10 mg/L 5 mg/L
40
80
c
Time (min)
60
0
5
10
15
20
25
30 5 mg/L
20 mg/L 10 mg/L
20
100
40
120
60 80 Time (min)
b
100
0 10
10
20
30
40
120
20
30
20 mg/L 10 mg/L 5 mg/L
40
50
60
70 Time (min)
80
90 100 110 120
Figure 8. Linear regression of kinetics Plot: pseudo second order for (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at different concentrations and temperature 25Û&.
0
5
10
15
20
25
t/g (min.g/mg)
a t/g (min.g/mg)
qt (mg/g)
2
3
4
5
6 t
1/2
c
(min)
7 1/2
4
6
8
10
12
14
16
18
20
2
8
9
20 mg/L 10 mg/L 5 mg/L
4
10
b
6 t
1/2
4
6
8
10
12
14
16
(min)
2
8 1/2
4
10
t
1/2
20 mg/L 10 mg/L 5 mg/L
6
12
(min)
1/2
8
10
20 mg/L 10 mg/L 5 mg/L
12
Figure 9. Intraparticle diffusion kinetic plot for (a) dichloromethane (b) chloroform (c) carbon tetrachloride at different concentrations and temperature 25Û&.
2
3
4
5
6
7
8
9
10
qt (mg/g)
a qt (mg/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1
2
c
4
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Ce (mg/L)
3
1.0
5
1.5
6
b
Ce (mg/L)
2.0
-0.1 0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.5
2.5
1.0
1.5
2.0
Ce (mg/L)
2.5
3.0
3.5
4.0
Figure 10. Langmuir model for adsorption of (a) dichloromethane (b) chloroform (c) carbon tetrachloride at different concentrations, and temperature 25Û&.
Ce/qe
1.4
Ce/qe
a Ce/qe
-2.0
-1.5
-1.0
-0.5
0.0
0.5
c
ln Ce
1.0
-2.4 -0.2
-2.1
-1.8
-1.5
-1.2
-0.9
1.5
0.0
0.2
2.0
b
ln Ce
0.4
-6.0
-4.5
-3.0
-1.5
0.0
0.6
-4
0.8
-3
1.0
-2
ln Ce
-1
0
1
2
Figure 11. Freundlich model for adsorption of (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at different concentrations, and temperature 25Û&.
ln qe
0.5
ln qe
a
ln qe
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.5
c qe (mg/g) 0.00 -0.2
0.05
0.10
0.15
0.20
0.25
0.30
ln Ce
1.0
0.0
1.5
0.2 ln Ce
0.4
2.0
b
0.6
-0.2 -5
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.8
-4
1.0
-3
-2 ln Ce
-1
0
1
2
Figure 12. Temkin model for adsorption of (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at different concentrations, and temperature 25Û&.
qe (mg/g)
a qe (mg/g)
0
10
20
30
40
50
60
70
80
90
100
(a) CH2Cl2
Original adsorbent
(b) CHCl3
Recycle-1
Recycle-2
(c) CCl4
Recycle-3
Figure 13. Reusability of adsorbent (CeO2±NP/AC) for additional three cycles after the initial use for adsorption of (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at 0.1 g/L dosage, contact time 60 min, and temperature 25Û&.
Removal (%)
List of Tables Table .1 Chemical composition of CeO2±NP/AC adsorbent Elements
Weight (%)
Atomic (%)
C O
52.28 4.61
87.96 5.82
Ce
43.11
6.22
Total
100
Table 2 Kinetic constant parameters obtained from chlorinated hydrocarbons adsorption on CeO2±NP/AC. Pseudo-first order Compound
qeexp
CHCl3
CCl4
qecal
k2(10-3)
qecal
(g/mg min)
(mg/g)
kid
R2
0.6706
2.3938
0.9758
0.9912
0.2771
2.6065
0.9662
3.20
0.9973
0.098
1.9182
0.9685
0.989
16.08
0.9995
0.402
11.435
0.9749
0.9398
2.112
8.83
0.9992
0.1795
6.5962
0.9749
83.538
0.952
0.121
4.79
1.0000
0.0358
4.3556
0.9828
28.327
66.119
0.9758
2.551
19.81
1.0000
0.1997
17.475
0.9493
8.42
32.012
18.926
0.872
4.344
8.59
0.9999
0.0952
7.4244
0.9552
4.18
26.945
78.415
0.9732
11.527
4.25
1.0000
0.0379
3.7897
0.9844
-1
R2
Intraparticle diffusion model
C (mg/g)
Ci (mg/L)
CH2Cl2
k1(103)
Pseudo-second order
R2
(mg/g)
(min )
(mg/g)
20
9.57
23.491
172.139
0.8894
0.234
12.24
0.9906
10
5.67
18.194
85.949
0.9071
0.800
6.45
5
2.97
20.004
4.802
0.9355
3.033
20
15.64
27.406
131.033
0.9503
10
8.52
21.418
49.424
5
4.73
25.794
20
19.53
10 5
(mg/g min)
Table 3 Langmuir, Freundlich, and Temkin isotherm constants for chlorinated hydrocarbon adsorption on CeO2±NP/AC. Compound
Langmuir isotherm constants T (K)
CH2Cl2
298.16
qm
kL
(mg/g)
(L/mg)
3.476
0.750
RL
R2
Freundlich isotherm constants 1/n
N
KF
Temkin isotherm constants R2
kT (L/gm)
R2
bT (KJ/mol)
0.118
0.986
1.802
0.555
17.16
0.997
0.474
2.705
0.929
7 CH3Cl
298.16
7.283
7.151
0.014
0.983
1.049
0.954
8.602
0.998
3.170
33.008
0.666
CCl4
298.16
7.289
5.532
0.018
0.985
1.303
0.768
9.319
0.996
0.461
11.197
0.996
23
24
S1878-5352(15)00115-X http://dx.doi.org/10.1016/j.arabjc.2015.04.013 ARABJC 1628
To appear in:
Arabian Journal of Chemistry
Received Date: Accepted Date:
21 October 2014 18 April 2015
Please cite this article as: K.R. Alhooshani, Adsorption of chlorinated organic compounds from water with cerium oxide-activated carbon composite, Arabian Journal of Chemistry (2015), doi: http://dx.doi.org/10.1016/j.arabjc. 2015.04.013
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Adsorption of chlorinated organic compounds from water with cerium oxide-activated carbon composite Khalid R. Alhooshani* *
Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
* Tel.: +966 13 860 3065; fax: +966 13 860 4277. E-mail address: hooshani@
1
Abstract Adsorptive removal of dichloromethane, chloroform, and carbon tetrachloride from aqueous solutions at 25 °C by activated carbon (AC) that was loaded with cerium oxide nanoparticles (CeO2-NP/AC) was investigated. The developed adsorbent
was
characterized by scanning electron microscope (SEM), FTIR spectrophotometer, X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). The effect of contact time, initial concentration, and the adsorbent dosage were also studied. The equilibrium and kinetics of adsorption were studied in a batch-type adsorption system, and the equilibrium experimental data were analyzed using Langmuir, Freundlich, and Temkin isotherm models. Freundlich adsorption isotherm showed the best fit for the equilibrium adsorption data. Three adsorption kinetic models, pseudo first- and second-order, and intraparticle diffusion models were applied to test the kinetic data. Kinetic characterization of the adsorption process onto CeO2-NP/AC is well-described by the pseudo second-order model, and the adsorption best-fit the intraparticle diffusion model. Our study shows that at optimum conditions, 82.72%, 99.40% and 89.42% of dichloromethane, chloroform, and tetrachloride, respectively, were removed by CeO2NP/AC, at concentration between 0.25- 5.00 g/L. Keywords: Adsorption, cerium oxide-activated carbon composite, dichloromethane, chloroform, carbon tetrachloride.
2
Introduction Treatment of wastewaters remains a serious challenge because of the severity of environmental challenges posed by water pollution through chemical discharge. Because of the direct and indirect impact of such discharge pollutants, it is of great concern for countries to uphold environmental protection policies and to invest in methods that can reduce the hazardous byproducts of wastewater treatment. Consequently, wastewaters containing toxic materials such as dyestuffs, organic pollutant, and metal ions must be treated carefully before introduction to the environment, and the methods for removing these toxic materials need to be further researched and improved [1]. The usage of chlorinated hydrocarbons (CHCs) in industry has a significant impacts on the environment, for they can be used either as reactants or solvents for the synthesis of herbicides and plastics [2], industrial automotive and aerospace solvent degreasing, dry cleaning solvents in garment industries, precision solvent cleaning in electronic industries [3, 4], petroleum refining [5], and chlorine-bleach-containing household products [6]. Additionally, the emission from industrial activities or wastewater streams of CHCs lead to their dispersion into the atmosphere [7], aquatic systems, and soil which pose major threats to human and aquatic life [4, 8]. In addition, the presence of these chlorinated solvents were found to cause dizziness, irregular heartbeat, reduced cognition, and nervous system impairment [9-11]. Due to their high volatility in ambient conditions [12], toxicity, strong recalcitrance to degradation, and carcinogenic effects, CHCs have been listed as one of the priority pollutants by United States Environmental Protection Agency (U.S. EPA) and European Commission (EC) [13]. Furthermore, these hydrocarbons can cause ozone layer depletion and formation of photochemical smog [14, 15]. Due to direct and indirect harmful effects associated with CHCs, it is of great importance to develop efficient methods for the removal of such pollutants and toxins. The classical remediation approach employed for the removal of CHCs is thermal incineration that involves operation at elevated temperature (1000 oC). Moreover, incineration could lead to the production of byproducts such as phosgene, dioxins, and furans that are more harmful than the original contaminants [16, 17]. Other methods for removing CHCs involve catalytic oxidation or hydrodechlorination (HDC). This method utilizes different catalysts such as Pt or Pd and usually leads to a 3
high conversion of most reactive chloromethanes, in the order of trichloromethane, dichloromethane and chloromethane, (TCM > DCM > MCM). However, major issues associated to this approach is the catalyst deactivation and the use of high cost noble metal [18-20]. To offset these limitations, there have been major advancements in the HDC approach by the introduction of transition metals because they are more resistant to deactivation when used as catalysts for the decomposition of chlorinated organic compounds [20, 21]. Nevertheless, these transition metal catalysts are not completely ideal because they lead to the formation of toxic residues such as chromium oxychloride. Adsorption, absorption, and condensation techniques are non-destructive techniques frequently used for the removal or recovery of volatile organic carbons (VOCs) from gas streams [22, 23]. The adsorption process is commonly used as a final step in the treatment of wastewater from industrial byproducts for the removal of contaminants that are significantly resistant to biodegradation even at very low concentration levels [19, 24]. Generally, volatile, semi-volatile and non-volatile chlorinated organic pollutants can be efficiently removed from drinking water, surface water, and aqueous industrial sludge by adsorption technology on the surface of activated carbon (AC) [25-27], due to its unique microporous structure with high surface area, large pore volume, and high adsorption efficiency [28]. Although activated carbon lends to great efficiencies, the commercial source for activated carbon can be very expensive. Consequently, different alternative sources for the production of activated carbon-like material such as peanut shell, bagasse, sawdust, bamboo, cherry stones, and waste tire rubber, have been developed by researchers [1, 29-33]. Obtaining AC from the waste tire is of great importance since it can lower the cost of commercial ACs, eliminate fire hazard and address pollution problems associated with the disposal process of old tires [34]. Earlier reports have described the usage of modified ACs. For example, Hongyan et al. [35] reported the adsorption and desorption of dichloromethane over metal modified activated carbons. Jiun-Horng et al. [36] studied adsorption characteristics using commercial activated carbon, sludge adsorbent, and activated carbon fibers, where fibers showed much more adsorption. Furthermore, Milchert et al. [37] and Borkar et al. [38], use the unmodified activated carbons for the adsorption of carbon tetrachloride & dichloromethane, respectively. 4
Recently, there has been an increasing interest in the use of nano-cerium oxide, CeO2, for diverse applications, such as electrolytes in solid oxide fuel cells, insulators, high refractive index materials, UV blockers, polishing materials, gas sensors, hightemperature oxidation resistance, free-radical scavenger and ultimately as catalysts, catalyst promoters or catalyst supports [39]. The use of nano-cerium oxide as catalysts is due to its high oxygen storage capacity and oxygen donation caused by partial reduction of Ce4+/Ce3+ on the surface [40]. Cerium oxide, In addition to the aforementioned advantages, CeO2 is inexpensive, highly stable, environmentally benign, and efficient for decomposition of chlorinated organic compounds. Incorporation of CeO2 nanoparticles onto the surface of AC is expected to improve the adsorption efficiency of CHCs compared to unmodified AC [19]. By modifying nanoparticles on ACs, this study will improve current costs and methods being used for the removal of chlorinated organic compounds. Specifically, this project focuses on the preparation of ceria nanocomposite supported on AC obtained from waste tire rubber. 2. Materials and methods 2.1 Reagents All chemical reagents (dichloromethane, trichloromethane, and carbon tetrachloride (purity FHULFDPPRQLXPQLWUDWHQLWULFDFLGDQGK\GURJHQSHUR[LGH) were obtained from Sigma Aldrich. Dichloromethane, trichloromethane, and carbon tetrachloride were used without further purification. All aqueous solutions were prepared using deionized water. The activated carbon (AC) used in this study was produced by pyrolysis process from waste rubber tires as described by Chan et al. [1]. 2.2 Preparation of CeO2-NP/AC Activated carbon (AC) was prepared by heating clean and crushed waste rubber tires at 300 o
C to isolate the granules from produced oil. In order to increase the porosity of the AC, the
granules were then carbonized by raising the temperature initially to 500 oC for 3 h and then gradually raised to 900 oC for 2 h. In addition, it was further treated with H2O2 and HNO3 to oxidize the surface of AC, producing oxygenated surface of the AC. Then, the resultant product (AC) was washed with excessive water and dried overnight at 100 ºC. The 5
composite of CeO2-NP/AC was prepared by co-precipitation method. In short, 6.0 g of AC was dispersed in 150 mL of suitable media of pH 7 by the use of a sonicator. Afterwards, an equimolar solution of ceric ammonium nitrate (50 mL) was added to the mixture. The mixture was then stirred at 50 oC for 24 h, followed by refluxing it for 12 h at 90 oC. Upon cooling to room temperature, the product formed was filtered and washed with water and dried overnight at 110 oC. The final adsorbent was calcined at 350 oC for 4 h in muffle furnace.
2.3 Characterization of adsorbent To study the surface morphology of the developed adsorbent, field emission scanning electron microscope (FE-SEM, LYRA 3 Dual Beam, Tescan) equipped with energy dispersive X-ray spectrometer (EDX, Oxford Instruments) was employed. The crystalline structure of the adsorbent was investigated through X-ray diffractometer (Shimadzu XRD 0RGHO-DSDQ XVLQJ.ĮUDGLDWLRQRI&XRSHUDWHGDWN9DQGP$Change on the modified surface of AC with CeO2 was studied with Fourier transform infrared spectroscopy (FTIR) and Thermal gravimetric analysis (TGA). FTIR spectra of adsorbents samples was obtained using a Nicolet 6700 spectrometer (Thermo electron, USA) equipped with a deuterated triglycine sulfate (DTGS) detector connected to OMNIC software. The samples were prepared via the solid KBr pellet techniques. The background correction of the spectra was performed by 16 scans with resolution 2 cm-1. Thermal gravimetric analysis (TGA) was performed using TA instruments SDT Q600 (USA). The samples were monitored at 15 ºC/min under nitrogen atmosphere flowing at a rate of 50 mL/min. Analysis was made over a temperature range of 20 ºC ± 900 ºC.
2.4 Batch sorption procedure Batch adsorption experiments were performed using a 125 mL conical, airtight Pyrex glass. Each flask contained 50 mL of a chlorinated hydrocarbons solution with varying initial concentration (C0) from 5 - 20 mg/L. The glass bottles were sealed and then mounted on a 6
shaker at 150 rpm until equilibrium was achieved. The adsorption experiment was performed at room temperature, 25 °C. Parameters such as effect of initial concentration, adsorbent dosage, and contact time were investigated. Chlorinated hydrocarbons adsorbed by CeO2-NP/AC composite were calculated as follow:
୲ ൌ ሺᩳ െ ୲ ሻ ൈ
ሺͳሻ ݉
Where qt is the amount of chlorinated hydrocarbons adsorbed (mg/g), V is the solution volume (L), Co is the initial concentrations of chlorinated hydrocarbons (mg/L) at time (t = 0), ୲ is the liquid phase concentration of chlorinated organic compounds (mg/L) at any time, and m is the weight of the synthesized adsorbent (g).
2.5 Analysis of chlorinated hydrocarbons Chlorinated hydrocarbons were determined using headspace gas chromatography (Agilent 7890A GC) equipped with mass selective detector (MS) (Agilent 5975C). Direct headspace automated injection was performed by using 2.5 ml-HS syringe with an injection of 250 µL of the headspace vapor. The 1mL sample was incubated at 60 °C for 15 min in a 10 mL vial. The agitation speed was 250 rpm, syringe temperature 50 °C, and the injection speed was 500µL/sec. HP-5 GC capillary column (30m x 0.32 mm i.d. x 0.25 µm film thickness) was used for this analysis. The GC-MS inlet was operated in split mode (50:1) at 180 °C. The column temperature was programmed at 35 °C for 10 min and then increased it to 40 °C at the rate of 5 ºC/min for 1 min, followed by raising it to 120 °C at 15 °C /min for 2 min. Finally, the temperature of GC oven was raised to 120 ºC at 20 °C /min and held for 11 min. Throughout the course of these experiments, the flow of helium was maintained at a constant flow of 1.4 mL/min. Quantifications of the samples were performed in selected ion monitoring (SIM) mode, where suitable fragments including base peaks of the compounds were boldfaced: For instance m/z 49, 84, 86, 88 for dichloromethane; m/z 83, 85, 47 for trichloromethane and m/z 117, 119, 121, 82 for carbon tetrachloride.
2.6 Regeneration study 7
The regeneration potential of spent adsorbent was conducted by using a thermal method where chlorinated adsorbed molecules were desorbed by thermal evaporation at 100 oC for 24 h, and washed with de-ionized water and dried at 105 oC overnight. Then, 0.1 g of regenerated adsorbent was dispersed in 50 mL solution containing 10 mg/L of chlorinated organic compounds and was reused for additional three cycles, following the same procedure as fresh adsorbent.
2.7 Data analysis The data collected from batch study at different adsorbent dosages, and initial concentrations (5, 10, 20 mg/L) of CHCs were analyzed by kinetic studies (pseudo-first order, pseudo-second order, and intra-particle diffusion). In addition, different adsorption isotherm models (Langmuir isotherm, Freundlich isotherm, and Temkin isotherm) were conducted to identify the adsorption capacity (qt).
3. Results and discussions 3.1 Characterization of CeO2-NP/AC The morphology of CeO2-NP/AC composite (Fig. 1a) revealed that the surface of the composite has a large number of nanoparticles. This, in turn, can provide additional active sites for surface adsorption on the surface of activated carbon. Based on SEM image, the average diameter of the ceria particles was estimated to be approximately 60 nm, which was further confirmed by taking the average size of the nanoparticles from several images. The elemental analysis by EDX spectrum (Fig. 1b) indicated the presence of carbon, oxygen, and cerium atoms which were as expected in the composite of CeO2-NP/AC adsorbent. Please insert figure 1(a and b) here Please insert table 1 here
The FT-IR spectrum of CeO2-NP/AC composite is illustrated in fig (2). Absorption frequency at 1640 cm-1 represents the enhancement in the aromatic C=C groups (carbonization). The strong and broad band at 3400 cm-1 indicated the presence of O±H of 8
H2O, physically adsorbed on the surface. The spectrum has absorption bands at 2900 (aliphatic C±H stretching) and at 1054 cm-1, attributed to C-O stretching vibration. A strong absorption at 500 cmí1 was assigned to (Ce±O) vibration mode [39], confirming the formation of cerium oxide nanoparticles. The XRD spectrum of the activated carbon (AC) showed three broad diffraction peaks at 2ș = 20±30 and 40±50, which indicated the presence of amorphous carbon disorderly stacked up by carbon rings [41]. The XRD pattern of CeO2NP/AC composite is illustrated in fig (3). At low diffraction angles, the typical broad diffraction band of ceria nanoparticles was observed, which confirmed the presence of cerium oxide nanoparticles and their high dispersion on the AC support, as was supported by SEM image. The diffraction peaks exhibited cubic fluorite structure in comparison with JCPDS file (PCPDF 34-0394) in the database [42]. The main phases of CeO2 are identified as (111), (200), (220) and (311), corresponding to 2 = 28.9, 33.6, 48, and 57, respectively. The diffraction pattern of the carbon exhibits a weak diffraction peak at 2 = 26o that is attributable to the aromatic carbon sheets oriented in a considerably random fashion.
Please insert figure 2 and 3 here. Thermal stability of CeO2-NP/AC composite was also investigated (Fig. 4). Thermal analysis of the prepared composites is very important to determine their thermal stability and optimal conditions of calcination. As shown in the figure, the CeO2/AC has shown better thermal stability than bare synthesized activated carbon. The total weight loss of 9 % was observed in the CeO2-NP/AC composite adsorbent which can be attributed to decomposition of adsorbed water molecules. On above 400 oC, almost no weight loss was further observed, confirming the presence of crystalline CeO2 on the activated carbon as the final adsorbent. In case of AC, no significant weight loss was observed at almost 500 o
C except approximately 18 % weight loss at above 100 oC, due to adsorbed water.
Please insert figure 4 here
3.2 Effect of contact time and initial concentration The effect of contact time on the removal capacity of chlorinated hydrocarbons by CeO 2NP/AC composite, as illustrated in fig. (5), revealed that the removal of chlorinated 9
hydrocarbons increased quickly with a contact time and then progressed at a slower rate until it reached the saturation step. This result is due to the large number of active sites on the external surface of the adsorbent. Consequently, the initial step of the adsorption was initially very fast which was slow down due to internal diffusion process. Moreover, the large number of vacant active sites observed after a certain time indicated that the uptake of the solute molecules was made difficult due to the repulsive forces between the solute molecules on the surface of the CeO2-NP/AC as well as in the bulk phases [43, 44]. However, the quantity of chlorinated hydrocarbons adsorbed per unit mass of CeO2-NP/AC was increased as the initial concentration of chlorinated hydrocarbons was increased. This was attributed to the increase in mass transfer between the solution and the adsorbent by virtue of concentration factor, a driving force in the adsorption process [45, 46]. The amount of chlorinated hydrocarbons adsorbed per gram of CeO2-NP/AC after 10 min increased from 1.47 to 8.80 mg/g for dichloromethane, 3.40 to 15.99 mg/g for chloroform and from 4.58 to 18.61 mg/g for carbon tetrachloride as when the initial chlorinated hydrocarbons concentration was increased from 5 to 20 mg/L. Therefore, the adsorption was very rapid in the first 10 min and then was decreased and remained constant at the equilibrium, which was reached in almost 30 min. Please insert figure 5 here 3.3 Effect of adsorbent dosage The amount of the adsorbent is considered a significant factor because it can identify the capacity of the adsorption for a given initial concentration of the adsorbate [47]. Fig (6) shows the effect of CeO2-NP/AC dosage on the removal of chlorinated hydrocarbons. Different adsorbent dosages were applied for dichloromethane, chloroform, and carbon tetrachloride at initial concentration of 10 mg/L, at 25 Û&DQGDVKDNLQJWLPHRIPLQ7KH results indicated that, the percent removal of chlorinated hydrocarbons was increased from 32.04 % to 82.72 % for dichloromethane, from 57.14% to 99.40 % for chloroform and from 71.74% to 89.42 % for carbon tetrachloride, with an increase of adsorbent dosage from 0.25 to 5.00 g/L. This increase could be attributed to the large vacant sites of the increased adsorbent mass, for a constant concentration of chlorinated hydrocarbons [48, 49]. However, when the adsorbent dose exceeded 3 g/L, the increase in percent removal of
10
chlorinated hydrocarbons was insignificant [50, 51]. The removal percentage of chlorinated hydrocarbons in solution was calculated by Eq. (2),
Ψܴ݁݉ ݈ܽݒൌ
ι െ ୣ ൈ ͳͲͲሺʹሻ ι
Please insert figure 6 here 3.4 Adsorption kinetics The adsorption rate gives important information for designing batch adsorption systems. McKay et al., demonstrated that the adsorption process is composed of three steps: (1) (film resistance); which involves transfer of solute from the bulk solution to the external surface of the sorbent through a liquid boundary layer; (2) (intraparticle resistance) which is the transfer of solute from the sorbent surface to the intraparticle active sites and (3) (reaction resistance) interactions of the solute with the vacant sites on both the external and internal surfaces of the sorbent [52]. Additionally, the adsorption kinetic information describes the interaction between adsorbent and solution interface and can determine the mechanism of the adsorption processes. Such a mechanism might be explained by the pseudo first-order , pseudo second-order rate equation [53], and intraparticle diffusion models [54]. The agreement between experimental results and the kinetic model expected values is indicated by the correlation coefficient (R2).
3.4.1The pseudo-first order model The pseudo-first order equation was used to examine the kinetic adsorption data collected at three different initial concentrations of chlorinated hydrocarbons for investigating the ratecontrolling mechanism of the adsorption process [55]. The pseudo-first order kinetic model can be described by the following Lagergren rate equation [56].
ሺݍ െ ݍ௧ ሻ ൌ
݇ଵ ݐሺ͵ሻ ʹǤ͵Ͳ͵
11
Where k1 is Lagergren rate constant of pseudo-first-order adsorption (1/min), ݍ௧ and ݍ are the amounts of chlorinated hydrocarbons (mg/g) at contact time t and at equilibrium, respectively. From the plots of log ሺݍ െ ݍ௧ ሻ versus t (Fig. 7), the values of k1 (rate constant) and ݍ (adsorption quantity) at equilibrium were calculated from the slopes and intercepts. These values are given in table (2). Please insert figure 7 here The obtained correlation coefficient (R2) values from the plots were low for the three concentrations of chlorinated compounds, which indicated the lack of linearity for pseudo-first-order kinetic model. The correlation between the calculated qe from the equation (3) and observed qe was very poor. This indicated that the adsorption of chlorinated hydrocarbons did not obey the pseudo-first-order kinetic model and, consequently, the mechanism of the adsorption on the surface of the CeO 2-NP/AC may not follow the pseudo-first-order model well.
3.4.2 The pseudo-second order model The pseudo second-order adsorption kinetic rate equation was expressed by the following equation [57]: ݀ݍ௧ ൌ ݇ଶ ሺݍ െ ݍ௧ ሻଶ ሺͶሻ ݀ݐ Where ݇ଶ is the rate constant of the pseudo-second-order adsorption (g/mg.min), ݍ and ݍ௧ are the adsorbed amount of the chlorinated hydrocarbons in mg/g of adsorbate at equilibrium and time, t, respectively. When the boundary conditions are applied from t = 0 to t = t and qt = 0 to qt = qe, the equation (4) after the integration becomes: ͳ ͳ ൌ ݇ଶ ݐሺͷሻ ݍ െ ݍ௧ ݍ Equation (5) represents the integrated rate law for pseudo-second-order it can be expressed in the linear form as below: ͳ ݐ ݐ ൌ ሺሻ ݇ଶ ݍଶ ݍ ݍ௧
12
Where h is the initial adsorption rate at t = 0 , h (mg/g.min) can be described as : ݄ ൌ ݇ଶ ݍଶ ሺሻ Therefore, equation (7) can be written in a form as: ͳ ݐ ݐ ൌ ሺͺሻ ݄ ݍ ݍ௧ The plots of t/qt against t of equation (6) at different initial concentrations (5, 10 and 20 mg/L) of the chlorinated hydrocarbons shown in fig. (8). The adsorption parameters qe, and ݇ଶ were calculated from the slope and the intercept respectively as shown in table (2). The agreement between the calculated qe, and experimental q e, and the observed correlation coefficient values (R2=0.99) from all the data at different concentrations indicates that the adsorption kinetics between CeO2±NP/AC composite and the chlorinated hydrocarbons follow the pseudo-second order [58].
Please insert figure 8 here Please insert table 2 here
3.4.3 The intraparticle diffusion model The investigation for the possibility of intraparticle diffusion and prediction of the rate controlling step in the adsorption process were carried out by using the Weber and Morris model [59, 60]. The rate constants of intra-particle diffusion (kid) can be determined by using the following equation: ଵ
ݍ௧ ൌ ݇ௗ ݐଶ ܥሺͻሻ Where ݍ௧ is the amount of chlorinated hydrocarbons adsorbed at time t, t 1/2 is the square root of the time, C is the intercept refers to the thickness of the boundary layer, the larger the intercept, the greater the boundary layer effect. If intraparticle diffusion occurs, then ݍ௧ 13
versus t1/2 will be linear and if the plot passes through the origin, then the rate limiting process is only due to the intraparticle diffusion. Otherwise, some other mechanism along with intraparticle diffusion is also involved [61]. The plots of qt versus t1/2 are illustrated in fig. (9). The lines of the different concentration do not go through the point of origin which means the mechanism cannot determine the rate of the overall process, and the adsorption mechanism likely to be very complex [62]. Please insert figure 9 here
3.5 Adsorption isotherm Various isotherm models were used for investigation of adsorption capacity in adsorption of chlorinated hydrocarbon from aqueous solution. The three models introduced in this study are Langmuir, Freundlich, and Temkin isotherms.
3.5.1 Langmuir isotherm model The Langmuir explains that the adsorption process occurs on the homogeneous surface of the adsorbent by monolayer of the adsorbate which has a specific active sites and energies [63, 64, 65]. The linear equation is given by: ͳ ܥ ܥ ൌ ሺͳͲሻ ݍ ݇ ݍ ݍ Where ݇ is Langmuir equilibrium constant (L/mg), and ݍ (mg/g) is the monolayer adsorption capacity, were calculated from a plot C e/qe versus Ce (adsorbed equilibrium concentration) fig (10). The characteristic parameter of Langmuir isotherm can be illustrated in terms of dimensionless equilibrium parameter, RL, known as separation factor, defined by Weber and Chakkravorti:
ܴ ൌ
ͳ ሺͳͳሻ ͳ ܭ ܥ
14
Where Co in this case is the highest initial solute concentration. The value of separation factor gives an indication for the type of the isotherm and the nature of the adsorption process. Regarding the RL value, adsorption can be unfavorable (RL>0), linear (RL=1), favorable (0
Please insert figure 10 here Please insert table 3 here
3.5.2 Freundlich isotherm model The Freundlich model is usually used to explain the adsorption by a heterogeneous surface of an adsorbent [67]. The well-known logarithmic form of Freundlich is defined by the following equation: ͳ ୣ ൌ ୣ ሺͳʹሻ Where Kf (L/g) and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. From the slope and intercept of straight portion of the linear plot obtained by plotting ln (qe) versus ln (Ce) fig. (11). The Freundlich isotherm constants Kf and n was calculated as represented in table (3). It is interesting to see that all values of 1/n > 1 represent favorable adsorption condition [68-70]. The correlation coefficient is used as a tool to determine the best fitting parameters for each model, and to determine the adsorption process that is observed. It is observed that the linearity coefficient for all the CHCs was high,
and this gives better indication that the adsorption process is followed by the
Freundlich isotherm [71]. The correlation coefficient (R2) of 0.997, 0.998, and 0.996 for dichloromethane, chloroform and carbon tetrachloride, respectively, indicated that the adsorption process could be best described by Freundlich isotherm. Please insert figure 11 here
15
3.5.3 Temkin isotherm model The Temkin isotherm represents the quantity of heat needed for the adsorption by one layer of adsorbate on the surface of adsorbent. The linear form of Temkin model is given by:
ୣ ൌ
ୣ ሺͳ͵ሻ
where bT is the Temkin constant related to the heat of sorption (N-ÂPROí1), is the equilibrium binding constant which is equal to the maximum binding energy (/ÂJ-1), R is gas constant (-ÂPROí1ÂKí1) and T is the absolute temperature (K) [72]. The plot of qe versus ln (Ce) is illustrated in fig (12), the isotherm constants were determined from the slope and intercept table (3). The R2 in this case was low compared to the other two models described above, especially in the case of chloroform that show poor accuracy in the experimental data fitting using Temkin model. This observation suggested that the adsorption of chloroform compound on the adsorbent does not occur through a uniform distribution adsorbent-adsorbate interaction or mechanism.
Please insert figure 12 here
16
3.6 Regeneration of adsorbents The disposal of adsorbent as byproducts in the adsorption process is not economical.; Therefore, regeneration of the adsorbent is highly desirable [73]. In order to be cost effective, adsorbent was regenerated by thermal treatment. Thereafter, the regenerated adsorbent material was tested for an additional three cycles, and the results showed excellent reusability with almost percentage removal of chlorinated compounds compared with fresh adsorbent, as shown in fig (13). Please insert figure 13 here 4. Conclusions An efficient and fast adsorptive removal of dichloromethane, trichloromethane and carbon tetrachloride from aqueous media at ambient conditions were achieved by using AC loaded with cerium oxide nanoparticles (CeO2-NP/AC). The adsorption removal of tested organic chlorinated compounds onto CeO2-NP/AC composite can be well described by Freunlich adsorption isotherm. Kinetic characterization of the adsorption process onto the developed adsorbent was well described by the pseudo second-order model, and the adsorption fit the intraparticle diffusion model. Among the features, fast equilibrium time, high adsorption capacity as well as efficient adsorbent regeneration indicated that the CeO2-NP/AC is a highly efficient low cost adsorbent for the removal of dichloromethane, chloroform, and carbon tetrachloride from aqueous media. Acknowledgements The financial support provided by King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia through summer research fund and facility support provided by the King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia for this work are gratefully acknowledged.
17
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22
List of figures
a
Figure 1. (a) SEM images of the surface structure of the CeO 2-NP/AC, (b) EDX analysis of CeO2± NP/AC
b
% Transmittance
1000
1500
-1
2500 Wavenumber (cm )
2000
3000
3500
Figure 2. FTIR spectrum of as-developed CeO2-NP/AC composite.
500
AC
CeO2-NP/AC
10
b
a
20
c
C
30
(111)
(200)
50
2T (degree)
40
C
C
(220)
60
(311)
70
(400)
80
AC
(331)
CeO2-NP/AC
Figure 3. X-Ray Diffractogram patterns of (a) AC (from waste rubber tire) (b) as-developed CeO2±NP/AC composite .
Intensity (a.u)
40
50
60
70
80
90
100
110
0
100
200
300
500
600
7HPSHUDWXUH Û&
400
700
Ceria NPs-AC
800
Figure 4. Thermogravimetry analysis of both bare AC and CeO2±NP/AC composite.
Weight (%)
900
AC
1000
0
2
4
6
8
0
20
40
c
Time (min)
60
15
20
80
0
5
10
0
100
20 mg/L 10 mg/L 5 mg/L
20
120
40
60
Time (min)
b
80
0
3
6
9
12
15
0
100
20 mg/L 10 mg/L 5 mg/L
20
120
40
60
Time (min)
80
100
20 mg/L 10 mg/L 5 mg/L
120
Figure 5. Effect of contact time on adsorption of (a) dichloromethane, (b) chloroform, (c) carbon tetrachloride at different concentrations, and temperature 25Û&.
q (mg/g) t
10
q (mg/g) t
a
q (mg/g) t
0
10
20
30
40
50
60
70
80
90
100
0.3
Dichloromethane
0.5
1.0
2.0
Carbon tetrachloride
CeO2±NP/AC dosage (g/L)
Chloroform
5.0
Figure 6. Effect of adsorbent dosage on the adsorption of dichloromethane, chloroform and carbon tetrachloride at different concentrations, and temperature 25Û&.
Removal %
Log(qe-qt)
0
10
20
30
50
c
Time (min)
40
-1.5
-1.0
-0.5
0.0
0.5
1.0
0
60
10
70
20
80
30
90
20 mg/L 10mg/L 5 mg/L
50
Time (min)
40
60
b
0
70
-1.5
-1.0
-0.5
0.0
0.5
1.0
80
20
90
20 mg/L 10mg/L 5 mg/L
10
30
40
50
Time (min)
60
70
80
90
20 mg/L 10mg/L 5 mg/L
Figure 7. Lagergren first order plot for adsorption of (a) dichloromethane (b) chloroform (c) Carbon tetrachloride at different concentrations and temperature 25Û&
-1.5
-1.0
-0.5
0.0
0.5
1.0
Log(qe-qt)
a Log(qe-qt)
t/g (min.g/mg)
20
20 mg/L 10 mg/L 5 mg/L
40
80
c
Time (min)
60
0
5
10
15
20
25
30 5 mg/L
20 mg/L 10 mg/L
20
100
40
120
60 80 Time (min)
b
100
0 10
10
20
30
40
120
20
30
20 mg/L 10 mg/L 5 mg/L
40
50
60
70 Time (min)
80
90 100 110 120
Figure 8. Linear regression of kinetics Plot: pseudo second order for (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at different concentrations and temperature 25Û&.
0
5
10
15
20
25
t/g (min.g/mg)
a t/g (min.g/mg)
qt (mg/g)
2
3
4
5
6 t
1/2
c
(min)
7 1/2
4
6
8
10
12
14
16
18
20
2
8
9
20 mg/L 10 mg/L 5 mg/L
4
10
b
6 t
1/2
4
6
8
10
12
14
16
(min)
2
8 1/2
4
10
t
1/2
20 mg/L 10 mg/L 5 mg/L
6
12
(min)
1/2
8
10
20 mg/L 10 mg/L 5 mg/L
12
Figure 9. Intraparticle diffusion kinetic plot for (a) dichloromethane (b) chloroform (c) carbon tetrachloride at different concentrations and temperature 25Û&.
2
3
4
5
6
7
8
9
10
qt (mg/g)
a qt (mg/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1
2
c
4
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Ce (mg/L)
3
1.0
5
1.5
6
b
Ce (mg/L)
2.0
-0.1 0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.5
2.5
1.0
1.5
2.0
Ce (mg/L)
2.5
3.0
3.5
4.0
Figure 10. Langmuir model for adsorption of (a) dichloromethane (b) chloroform (c) carbon tetrachloride at different concentrations, and temperature 25Û&.
Ce/qe
1.4
Ce/qe
a Ce/qe
-2.0
-1.5
-1.0
-0.5
0.0
0.5
c
ln Ce
1.0
-2.4 -0.2
-2.1
-1.8
-1.5
-1.2
-0.9
1.5
0.0
0.2
2.0
b
ln Ce
0.4
-6.0
-4.5
-3.0
-1.5
0.0
0.6
-4
0.8
-3
1.0
-2
ln Ce
-1
0
1
2
Figure 11. Freundlich model for adsorption of (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at different concentrations, and temperature 25Û&.
ln qe
0.5
ln qe
a
ln qe
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.5
c qe (mg/g) 0.00 -0.2
0.05
0.10
0.15
0.20
0.25
0.30
ln Ce
1.0
0.0
1.5
0.2 ln Ce
0.4
2.0
b
0.6
-0.2 -5
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.8
-4
1.0
-3
-2 ln Ce
-1
0
1
2
Figure 12. Temkin model for adsorption of (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at different concentrations, and temperature 25Û&.
qe (mg/g)
a qe (mg/g)
0
10
20
30
40
50
60
70
80
90
100
(a) CH2Cl2
Original adsorbent
(b) CHCl3
Recycle-1
Recycle-2
(c) CCl4
Recycle-3
Figure 13. Reusability of adsorbent (CeO2±NP/AC) for additional three cycles after the initial use for adsorption of (a) dichloromethane (b) Chloroform (c) Carbon tetrachloride at 0.1 g/L dosage, contact time 60 min, and temperature 25Û&.
Removal (%)
List of Tables Table .1 Chemical composition of CeO2±NP/AC adsorbent Elements
Weight (%)
Atomic (%)
C O
52.28 4.61
87.96 5.82
Ce
43.11
6.22
Total
100
Table 2 Kinetic constant parameters obtained from chlorinated hydrocarbons adsorption on CeO2±NP/AC. Pseudo-first order Compound
qeexp
CHCl3
CCl4
qecal
k2(10-3)
qecal
(g/mg min)
(mg/g)
kid
R2
0.6706
2.3938
0.9758
0.9912
0.2771
2.6065
0.9662
3.20
0.9973
0.098
1.9182
0.9685
0.989
16.08
0.9995
0.402
11.435
0.9749
0.9398
2.112
8.83
0.9992
0.1795
6.5962
0.9749
83.538
0.952
0.121
4.79
1.0000
0.0358
4.3556
0.9828
28.327
66.119
0.9758
2.551
19.81
1.0000
0.1997
17.475
0.9493
8.42
32.012
18.926
0.872
4.344
8.59
0.9999
0.0952
7.4244
0.9552
4.18
26.945
78.415
0.9732
11.527
4.25
1.0000
0.0379
3.7897
0.9844
-1
R2
Intraparticle diffusion model
C (mg/g)
Ci (mg/L)
CH2Cl2
k1(103)
Pseudo-second order
R2
(mg/g)
(min )
(mg/g)
20
9.57
23.491
172.139
0.8894
0.234
12.24
0.9906
10
5.67
18.194
85.949
0.9071
0.800
6.45
5
2.97
20.004
4.802
0.9355
3.033
20
15.64
27.406
131.033
0.9503
10
8.52
21.418
49.424
5
4.73
25.794
20
19.53
10 5
(mg/g min)
Table 3 Langmuir, Freundlich, and Temkin isotherm constants for chlorinated hydrocarbon adsorption on CeO2±NP/AC. Compound
Langmuir isotherm constants T (K)
CH2Cl2
298.16
qm
kL
(mg/g)
(L/mg)
3.476
0.750
RL
R2
Freundlich isotherm constants 1/n
N
KF
Temkin isotherm constants R2
kT (L/gm)
R2
bT (KJ/mol)
0.118
0.986
1.802
0.555
17.16
0.997
0.474
2.705
0.929
7 CH3Cl
298.16
7.283
7.151
0.014
0.983
1.049
0.954
8.602
0.998
3.170
33.008
0.666
CCl4
298.16
7.289
5.532
0.018
0.985
1.303
0.768
9.319
0.996
0.461
11.197
0.996
23
24