Environ Sci Pollut Res (2014) 21:6571–6577 DOI 10.1007/s11356-014-2556-x
RESEARCH ARTICLE
Kinetic study of the removal of dimethyl phthalate from an aqueous solution using an anion exchange resin Zhengwen Xu & Ling Cheng & Jing Shi & Jiangang Lu & Weiming Zhang & Yunlong Zhao & Fengying Li & Mindong Chen
Received: 20 August 2013 / Accepted: 13 January 2014 / Published online: 9 February 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Phthalate acid esters are becoming an important class of pollutants in wastewaters. This study addresses the kinetics of removal of dimethyl phthalate (DMP) using the anion exchange resin D201-OH from an aqueous solution. The effects of various factors on the removal rate and efficiency were investigated. An overall initial removal rate (OIRR) law and a pseudo first-order kinetic (PFOK) model were also developed. The internal diffusion of DMP within the resin phase of D201-OH is the rate-controlling step. Optimization of the particle size and pore structure of the resin D201-OH, the DMP concentration, and the reaction temperature can improve the DMP removal rate. The hydrolysis reaction of DMP catalyzed by D201-OH indicates an overall reaction order of 1.76, a value that is between the first order and the second order. The apparent activation energy of the reaction is Responsible editor: Angeles Blanco Electronic supplementary material The online version of this article (doi: 10.1007/s11356-014-2556-x ) contains supplementary material, which is available to authorized users. Z. Xu : J. Lu : Y. Zhao : F. Li : M. Chen Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, Nanjing 210044, People’s Republic of China Z. Xu (*) : L. Cheng : J. Lu : Y. Zhao : F. Li : M. Chen School of Environmental Science & Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, People’s Republic of China e-mail: [email protected] J. Shi (*) School of Sciences, China Pharmaceutical University, Nanjing 211198, People’s Republic of China e-mail: [email protected] W. Zhang School of the Environment, Nanjing University, Nanjing 210093, People’s Republic of China
34.6 kJ/mol, which is below the homogeneous alkaline hydrolysis activation energy of 44.3 kJ/mol. The OIRR law can quantify the initial removal rate under different conditions. The results also show that the theoretical DMP removal efficiency predicted by the PFOK model agrees well with the experimentally determined values. Our research provides valuable insights into the primary parameters influencing the kinetic process, which enables a focused improvement in the removal or hydrolysis rate for similar processes. Keywords Dimethyl phthalate . Removal . Anion exchange resin . Catalysis . Kinetic . Hydrolysis
Introduction Phthalate acid esters (PAEs) are a group of industrial chemicals with many commercial uses, such as plasticizers, additives, and solvents (Janjua et al. 2007; Staples et al. 1997). The worldwide production of PAEs was estimated to be 6× 106 t yr−1 in 2004 (AgPU 2006). Production on this scale and utilization of PAEs cause a significant environmental diffusion of these compounds. PAEs have been detected in water, sediment, soil, air, dust, food, sewage sludge, and rainwater (Clark et al. 2003). Numerous studies have shown that many PAEs have toxic and endocrine-disrupting effects on humans and wildlife, even at low-concentration levels (Higuchi et al. 2003; Jobling et al. 1995; Zhu et al. 2009). As a result, PAEs are listed as priority pollutants in many countries, such as the USA (Saito et al. 2010) and the European Union (Bodar et al. 2003). Because the level of PAE pollution in aqueous systems is becoming a concern, the development of treatment technologies that can effectively remove PAEs from water is required. Several methods for the removal of PAEs from contaminated water have been studied, such as biodegradation (Chatterjee and Dutta 2008; Fang et al. 2007; Roslev et al. 2007; Wang
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et al. 2000), chemical oxidation (Liu et al. 2007; Yuan et al. 2008), adsorption (Murai et al. 1998; Zhang et al. 2007), and hydrolysis (Patnaik et al. 2001). Recently, anion exchange resins have received increased attention (Morwick 2006; Welzel and Morwick 2008) because they not only catalyze the hydrolysis of organic compounds, such as shikonin ester derivatives (Liu et al. 2012), esterbound biphenyl cyclooctene lignans (Ma et al. 2011), epoxides, and aziridines (Zhang and Ye 2008), but also adsorb the hydrolysis products through a subsequent anion exchange process. Therefore, anion exchange resins can remove organic pollutants from wastewater or from synthesis products and can purify the hydrolysis products simultaneously. In our previous study (Xu et al. 2010), we developed a novel approach to remove dimethyl phthalate (DMP), a type of PAE, from an aqueous solution using a macroporous OH-type strong-base anion exchange resin, denoted D201-OH. The results from that research demonstrated that D201-OH, used as a solid basic catalyst, was a highly efficient material for DMP catalytic hydrolytic degradation and for further ion exchange removal of the hydrolysis products in water. As a catalyst and adsorbent, anion exchange resins have been proposed as a new material for wastewater treatment and organic synthesis. However, previous studies were limited to the evaluation of the anion exchange resin performance; therefore, little is known about their kinetic properties. To attain a higher removal or hydrolysis rate, the primary parameters influencing the kinetic evolution must be investigated. Therefore, the objective of the present study is to investigate the primary parameters influencing the kinetics of removal of DMP using an anion exchange resin D201-OH in an aqueous solution. The significant influencing factors and the rate-controlling step of the process were investigated. Also, using the experimental data, the details of the kinetic process were explored through the overall initial removal rate (OIRR) law and the pseudo first-order kinetic (PFOK) model.
Environ Sci Pollut Res (2014) 21:6571–6577
achieved. The column was subjected to acidic flushing by introduction of 15 BV of 1.0 mol/L HCl, followed by DI flushing to a neutral pH. Finally, the resin was washed with ethanol for 2 h and vacuum-desiccated at 313 K for 24 h before use. To convert D201-Cl into a hydroxy-type anion exchange resin (denoted D201-OH), the D201-Cl beads were packed into the glass column and rinsed with 50 BV of 2.0 mol/L NaOH at a rate of 1 BV min−1, followed by DI washing until a neutral pH (pH∼7.5) was achieved and the ionic strength of the effluent reached a constant value. The primary physiochemical properties of the resin D201-OH are presented in Table 1. Different hydroxyl ion concentrations in the resin phase of D201-OH were obtained by partial neutralization with hydrochloric acid (Hanková et al. 2006). Kinetic experiments Kinetic experiments were performed in a 1,500-mL-capacity glass flask containing 1,000 ml DMP aqueous solution of known concentrations (0.063 to 0.251 mmol/L). The reaction temperature was maintained by a thermostatic shaking water bath in which the flask was immersed. After the solution reached the chosen temperature (283, 288, 293, 303, and 313 K), a desired amount of D201-OH was added to the flask, and agitation commenced at a known speed. At various time intervals, a 0.5-mL sample was removed from the flask for kinetic determination. A solution of KH2PO4 (0.1 mol/L) was used to adjust the sample to a neutral pH (Wolfe et al. 1980). Analysis
Materials and methods
Concentrations of DMP in the solution were analyzed by high-performance liquid chromatography (HPLC, Agilent, 1120, USA) equipped with a reversed-phase column (Agilent, TC-C18, 5 μm) and a UV detector. The mobile phase was composed of 55 % methanol and 45 % 0.2 mol/L potassium dihydrogen phosphate in an aqueous solution, and the solution was analyzed using a wavelength detection of 228 nm.
Materials
Table 1 Physicochemical properties of D201-OH
The chemicals used in this study, DMP, disodium tetraborate decahydrate, potassium dihydrogen phosphate, ethanol, sodium hydroxide, sodium chloride, and hydrochloric acid, were of analytical grade and were purchased from Shanghai Reagent Station (Shanghai, China). The strong-base anion exchanger D201-Cl (in chloride form, as denoted by the -Cl suffix) was provided by Jiangsu N&G Environmental Technology Co., Ltd. (Nanjing, China). Prior to use, D201-Cl was packed in a glass column and rinsed with 15 bed volumes (BV) of 1.0 mol/L NaOH, followed by washing with deionized (DI) water until a neutral pH was
Matrix
Polystyrene-divinylbenzene
Functional groupa Anion exchange capacity (mmol/g) Average pore diameter (nm) Moisture content (%) Cross-link density (%) Pore volume (cm3/g) BET surface area (m2/g) Particle size (mm)
−CH(CH3)3N+.OH− 3.4 28.6 50–60 8.3 0.50 24.5 0.3–0.9
a
The counter anion of D201-Cl is Cl− instead of OH−
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Results and discussion
The effect of agitation speed on the DMP removal efficiency and the OIRR was studied at six different speeds, 350, 570, 800, 980, 1,370, and 1,790 rpm, to examine the influence of the external mass transfer resistance, and the results are illustrated in Fig. 1a, b. Figure 1a shows that the removal efficiency of DMP did not change in the overall process when the stirring speed increased from 350 to 1,790 rmp. Therefore, the removal efficiency was independent of the agitation speed in the range of 350–1,790 rpm. The OIRR was determined using the first linear region of the removal process (within 28 min), as determined by linear regression analysis (Angulo et al. 1998; Puxty et al. 2009). Figure 1b reveals that the OIRR was consistently close to 0.63×10−3 mmol/L*min with varying degrees of agitation. These results demonstrated that there was no resistance to external mass transfer, and the external diffusion stage was not the rate-determining step. This conclusion is consistent with previous studies by Delgado et al. (2007), Izci and Bodur (2007), and Moreau et al. (1996) in which it was established that external diffusion does not control the overall reaction rate unless the agitation speed is very low or the viscosity of reactant mixture is very high. Therefore, our subsequent experiments were conducted at 800 rpm to ensure that the removal rate was not influenced by external diffusion.
80
350rpm 570rpm 800rpm 980rpm 1350rpm 1790rpm
60
40
20
0 0
100
200
300
400
500
Time (min) 1.0
3
Effect of agitation speed
Removal efficiency of DMP(%)
The course of a catalytic reaction induced by an ion exchange resin is generally considered to involve the following three stages: external diffusion, internal diffusion, and reaction (Helfferich 1962). For this study, in which the kinetic experiments were performed in a batch reactor, the external mass transfer effect is directly related to the stirrer speed (Ali et al. 2007; Yadav and Kamble 2012). In addition, the particle size of the anion exchange resin is one of the most important factors influencing internal mass transfer resistance in catalytic reactions (Ali et al. 2007; Moreau et al. 1996). Therefore, the effect of agitation speed, particle size, concentration of hydroxyl ions in the resin phase, and temperature were evaluated using varying degrees of each parameter.
Initial removal rate ×10 (mmol/(L*min))
Determination of the rate-controlling step
a
100
b 0.9 0.8 0.7 0.6 0.5 0.4 0.3 200
400
600
800
1000 1200 1400 1600 1800 2000
Speed of agitation (rpm)
Fig. 1 Effect of agitation speed on a the removal efficiency and b the initial removal rate of DMP
smaller D201-OH particles had a higher removal efficiency. The results in Fig. 2b show that the OIRR was inversely proportional to the particle diameter and increased from 0.43×10−3 to 0.81×10−3 mmol/L*min as the resin particle diameter decreased, indicating a strong pore resistance (Jogunola et al. 2010). Therefore, the DMP hydrolysis reaction was strongly influenced by internal diffusion, and the internal diffusion process may be the primary ratecontrolling step. Similar results were obtained by GD Yadav (Yadav and Krishnan 1999) for the acylation of 2methoxynaphthalene with acetic anhydride using different solid acid catalysts.
Effect of particle size Effect of hydroxyl ion concentration To gain further insight of the effect of internal diffusion resistance, D201-Cl was screened into several different particle sizes, and experiments were performed in the particle range between 0.3 and 0.9 mm. Figure 2a, b shows the effect of particle size on the DMP removal efficiency and the OIRR. Figure 2a shows that, compared with the larger particles, the
To further validate the conclusion that internal diffusion is the rate-controlling step, D201-OH with different hydroxyl ion concentrations in the resin phase was used to evaluate the effect of surface reaction on the removal efficiency and the OIRR. Figure 3a, b shows the results obtained from these
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a
100
Removal efficiency of DMP(%)
Removal efficiency of DMP(%)
100
80
60
0.3mm 0.6mm 0.9mm
40
20
80 70 60 50
2 mol/L 3 mol/L 4 mol/L 5.4 mol/L 6.8 mol/L
40 30 20 10 0
0 100
200 300 Time (min)
400
500
100
200
300
400
500
Time (min)
b
1.0
0
Initial removal rate × 10 (mmol/(L*min))
0
0.8
0.9
b 0.8 0.7
3
3
Initial removal rate × 10 (mmol/(L*min))
a
90
0.6
0.4
0.2
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.6 0.5 0.4 0.3 0.2 2
3 4 5 6 Hydroxyl ion concentraton (mol/L)
-1
1/d, mm
Effect of temperature The effect of temperature on the OIRR under otherwise similar conditions was studied in the range from 283 to 313 K. Figure 4 shows the OIRR at various temperatures and reveals that the OIRR increases from 0.24×10−3 to 1.1×10−3 mmol/
L*min with the increasing temperature in the range of 283– 313 K. A higher temperature can accelerate the apparent removal rate of DMP. The apparent activation energy (Ea) of the process was obtained from the slope of the plots lnk=f(1/
3
experiments. The data from Fig. 3a show that there was a negligible effect of the hydroxyl ion concentration (3.0– 6.8 mol/L) on the removal efficiency of DMP. However, when the hydroxyl ion concentration in the resin phase decreased to 2 mol/L, the removal efficiency dropped down to less than 80 %, which was below the value of other concentrations. It can also be seen from Fig. 3b that the OIRR remained nearly constant (approximately 0.6×10−3 mmol/L*min) for each hydroxyl concentration, except for 2 mol/L which was only about 0.5×10−3 mmol/L*min. These results provide strong evidence that the rate of DMP hydrolysis is not controlled by surface reaction when the hydroxyl concentration in the resin phase ranges from 3.0 to 6.8 mol/L.
Fig. 3 Effect of hydroxyl ion concentration on a the removal efficiency and b the initial removal rate of DMP
Initial removal rate × 10 (mmol/(L*min))
Fig. 2 Effect of resin particle size on a the removal efficiency and b the initial removal rate of DMP
7
1.2 1.0 0.8 0.6 0.4 0.2 280
285
290
295
300
305
310
315
Temperature (K)
Fig. 4 The relationship between the overall initial removal rate and temperature
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The OIRR law For a catalytic reaction controlled by internal diffusion, several parameters can influence the OIRR law, such as the particle size and pore structure of the resin, the DMP concentration, the reaction temperature, and the resin loading. As a commercially available resin, the physiochemical properties of D201-OH were constant. Therefore, the OIRR law only relates to the DMP concentration, the reaction temperature, and the resin loading. According to the reaction pathways suggested in our previous study (Xu et al. 2010), we assumed that the alkaline hydrolysis rate law of DMP catalyzed by D201-OH may be expressed using the following equation (Patnaik et al. 2001): r ¼ k ½D201−OHm ½DMPn
ð1Þ
DMP
-2.6
D201-OH
-2.8 -3.0
lgr
T, Fig. 5). The value of Ea is equal to 34.6 kJ/mol, which is below the homogeneous alkaline hydrolysis activation energy of 44.3 kJ/mol (calculated from Fig. S1 in the Supporting Information). The low activation energy value also illustrates that intraparticle diffusion of DMP controls the hydrolysis reaction (Helfferich 1962). These results illustrated that intraparticle diffusion of DMP in the catalyst particle is the rate-controlling step. To improve the DMP removal rate with a given amount of resin, the particle size and pore structure of D201-OH, the DMP concentration, and the reaction temperature, which can increase the diffusion rate, should be optimized. The effect of these parameters on the OIRR and the removal efficiency was quantified and is described in the following sections.
lgr=1.01lg[DMP]-2.16 2 R =0.993
-3.2 -3.4 -3.6
lgr=0.76lg [D201-OH] -3.57 2
R =0.999
-3.8 -1.0
-0.5
0.0
0.5
1.0
lg[D201-OH] or [DMP]
Fig. 6 Plot of logarithms of the initial rate versus the logarithm of the varying concentration of D201-OH or DMP at 303 K
Rubert IV and Pedersen 2006). Table S1 (in the Supporting Information) presents the experimental data for the initial hydrolysis rates of DMP catalyzed by D201-OH at 303 K. Figure 6 shows the logarithms of the initial rate plotted against the logarithm of the varying concentrations of D201-OH or DMP. The initial reaction orders of m=0.76 and n=1.0 were obtained from the slopes of two straight lines. Thereafter, the initial rate law for DMP hydrolysis at 303 K shows a fractional order of 0.76 for D201-OH and a first order for DMP, with the overall order of 1.76. The order of this reaction is between the first and second orders of DMP hydrolysis in NaOH solution (Wolfe et al. 1980), which may be because intraparticle diffusion is the rate-controlling step of the process (Helfferich 1962). According to the Arrhenius law, the reaction rate constant can be calculated using the following equation: k ¼ Aexpð−Ea=RT Þ
where r is the OIRR of DMP, k is the reaction rate constant, and m and n are the reaction orders to be determined. The initial rate method was employed to determine the reaction orders (Ghosh et al. 2003; Perez-Benito and Arias 1998;
ð2Þ
where k is the reaction rate constant, A is the preexponential factor, Ea is the activation energy, R is the universal gas
Removal efficiency of DMP(%)
100 -5.4 -5.6 lnk=-4166.9(1/T)+7.79
-5.8
R2 =0.996
lnk
-6.0 -6.2 -6.4 -6.6 -6.8
80 predicted data experimental data
60 2
χ =
3.3 × 10-4
40 DMP concentration 0.312mmol/L Temperature 293K Mass of resin 1.5 g
20
0
-7.0
0 0.00320
0.00328
0.00336
0.00344
1/T
Fig. 5 Arrhenius plot for determining activation energy
0.00352
200
400
600
800 1000 1200 1400 1600 1800 Time (min)
Fig. 7 Comparison between the experimental and the predicted removal efficiency of DMP
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constant, and T is the temperature. The value of A is equal to 2,416.3 L0.76 mmol0.76 min−1, which was obtained by the intercept of the plots lnk=f(1/T) in Fig. 5. Thus, we obtain the OIRR law from the following equation: r ¼ 2; 416:3 expð−34:6=RT Þ½D201−OH0:76 ½DMP
ð3Þ
According to Eq. (3), the effect of temperature, DMP initial concentration, and D201-OH loading on the initial removal rate of DMP can be evaluated.
Kinetics model Because of the large excess of D201-OH relative to DMP used in the kinetic experiments (the amount of D201-OH was 13.6 times as much as DMP), the variation in the concentration of D201-OH in the aqueous phase versus time was approximately constant, and the kinetic law can be described by the PFOK model (Helfferich 1962; Liger et al. 1999). Therefore, Eq. (2) becomes the following Eq. (4): r ¼ k 1 ½DMP ¼
dc dt
ð4Þ
where the apparent reaction rate constant k1 is defined using the following equations: k 1 ¼ k ½D201−OH0:76
ð5Þ
and k ¼ 2416:3expð−Ea=RT Þ
ð6Þ
Integrating Eq. (4), we obtain the following equation: C ¼ C 0 expð−k 1 t Þ
ð7Þ
where C0 denotes the initial concentration of DMP in the aqueous phase, and C is the concentration of DMP at time t. Table S2 (in the Supporting Information) lists the values of k1 calculated from the various operating parameters according to Eqs. (5) and (6). To validate the derived PFOK model, a comparison between the experimental and the predicted removal efficiency of DMP under different conditions is shown in Fig. 7.
The chi-square (χ2) test was adopted to evaluate the validity of the derived PFOK model and is described by the following Eq. (8): n X ðqexp −qpre Þ2 =qexp
χ2 ¼
i¼1
n−1
ð8Þ
where the superscripts pre and exp indicate the predicted and the experimental amounts of hydrolysis, respectively, and n is the total number of experimental points. The low values of χ2=3.3×10−4 proved the validity of the derived PFOK model for predicting DMP removal efficiency under different conditions.
Conclusions In the present study, the kinetics of DMP removal by D201OH was investigated. The internal diffusion of DMP within the D201-OH phase is the rate-controlling step of the process. The particle size and pore structure of the resin, the DMP concentration, and the temperature play an important role in the removal process. An OIRR law, as developed from the kinetic experiments, can quantify the effects of DMP concentration, reaction temperature, and D201-OH concentration on the initial hydrolysis rate of DMP. A recently developed and experimentally validated PFOK model can delineate the effect of these parameters on the removal efficiency of DMP. Acknowledgments This study was supported by the National Natural Science Foundation of China (grant no. 21107050) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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RESEARCH ARTICLE
Kinetic study of the removal of dimethyl phthalate from an aqueous solution using an anion exchange resin Zhengwen Xu & Ling Cheng & Jing Shi & Jiangang Lu & Weiming Zhang & Yunlong Zhao & Fengying Li & Mindong Chen
Received: 20 August 2013 / Accepted: 13 January 2014 / Published online: 9 February 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Phthalate acid esters are becoming an important class of pollutants in wastewaters. This study addresses the kinetics of removal of dimethyl phthalate (DMP) using the anion exchange resin D201-OH from an aqueous solution. The effects of various factors on the removal rate and efficiency were investigated. An overall initial removal rate (OIRR) law and a pseudo first-order kinetic (PFOK) model were also developed. The internal diffusion of DMP within the resin phase of D201-OH is the rate-controlling step. Optimization of the particle size and pore structure of the resin D201-OH, the DMP concentration, and the reaction temperature can improve the DMP removal rate. The hydrolysis reaction of DMP catalyzed by D201-OH indicates an overall reaction order of 1.76, a value that is between the first order and the second order. The apparent activation energy of the reaction is Responsible editor: Angeles Blanco Electronic supplementary material The online version of this article (doi: 10.1007/s11356-014-2556-x ) contains supplementary material, which is available to authorized users. Z. Xu : J. Lu : Y. Zhao : F. Li : M. Chen Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, Nanjing 210044, People’s Republic of China Z. Xu (*) : L. Cheng : J. Lu : Y. Zhao : F. Li : M. Chen School of Environmental Science & Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, People’s Republic of China e-mail: [email protected] J. Shi (*) School of Sciences, China Pharmaceutical University, Nanjing 211198, People’s Republic of China e-mail: [email protected] W. Zhang School of the Environment, Nanjing University, Nanjing 210093, People’s Republic of China
34.6 kJ/mol, which is below the homogeneous alkaline hydrolysis activation energy of 44.3 kJ/mol. The OIRR law can quantify the initial removal rate under different conditions. The results also show that the theoretical DMP removal efficiency predicted by the PFOK model agrees well with the experimentally determined values. Our research provides valuable insights into the primary parameters influencing the kinetic process, which enables a focused improvement in the removal or hydrolysis rate for similar processes. Keywords Dimethyl phthalate . Removal . Anion exchange resin . Catalysis . Kinetic . Hydrolysis
Introduction Phthalate acid esters (PAEs) are a group of industrial chemicals with many commercial uses, such as plasticizers, additives, and solvents (Janjua et al. 2007; Staples et al. 1997). The worldwide production of PAEs was estimated to be 6× 106 t yr−1 in 2004 (AgPU 2006). Production on this scale and utilization of PAEs cause a significant environmental diffusion of these compounds. PAEs have been detected in water, sediment, soil, air, dust, food, sewage sludge, and rainwater (Clark et al. 2003). Numerous studies have shown that many PAEs have toxic and endocrine-disrupting effects on humans and wildlife, even at low-concentration levels (Higuchi et al. 2003; Jobling et al. 1995; Zhu et al. 2009). As a result, PAEs are listed as priority pollutants in many countries, such as the USA (Saito et al. 2010) and the European Union (Bodar et al. 2003). Because the level of PAE pollution in aqueous systems is becoming a concern, the development of treatment technologies that can effectively remove PAEs from water is required. Several methods for the removal of PAEs from contaminated water have been studied, such as biodegradation (Chatterjee and Dutta 2008; Fang et al. 2007; Roslev et al. 2007; Wang
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et al. 2000), chemical oxidation (Liu et al. 2007; Yuan et al. 2008), adsorption (Murai et al. 1998; Zhang et al. 2007), and hydrolysis (Patnaik et al. 2001). Recently, anion exchange resins have received increased attention (Morwick 2006; Welzel and Morwick 2008) because they not only catalyze the hydrolysis of organic compounds, such as shikonin ester derivatives (Liu et al. 2012), esterbound biphenyl cyclooctene lignans (Ma et al. 2011), epoxides, and aziridines (Zhang and Ye 2008), but also adsorb the hydrolysis products through a subsequent anion exchange process. Therefore, anion exchange resins can remove organic pollutants from wastewater or from synthesis products and can purify the hydrolysis products simultaneously. In our previous study (Xu et al. 2010), we developed a novel approach to remove dimethyl phthalate (DMP), a type of PAE, from an aqueous solution using a macroporous OH-type strong-base anion exchange resin, denoted D201-OH. The results from that research demonstrated that D201-OH, used as a solid basic catalyst, was a highly efficient material for DMP catalytic hydrolytic degradation and for further ion exchange removal of the hydrolysis products in water. As a catalyst and adsorbent, anion exchange resins have been proposed as a new material for wastewater treatment and organic synthesis. However, previous studies were limited to the evaluation of the anion exchange resin performance; therefore, little is known about their kinetic properties. To attain a higher removal or hydrolysis rate, the primary parameters influencing the kinetic evolution must be investigated. Therefore, the objective of the present study is to investigate the primary parameters influencing the kinetics of removal of DMP using an anion exchange resin D201-OH in an aqueous solution. The significant influencing factors and the rate-controlling step of the process were investigated. Also, using the experimental data, the details of the kinetic process were explored through the overall initial removal rate (OIRR) law and the pseudo first-order kinetic (PFOK) model.
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achieved. The column was subjected to acidic flushing by introduction of 15 BV of 1.0 mol/L HCl, followed by DI flushing to a neutral pH. Finally, the resin was washed with ethanol for 2 h and vacuum-desiccated at 313 K for 24 h before use. To convert D201-Cl into a hydroxy-type anion exchange resin (denoted D201-OH), the D201-Cl beads were packed into the glass column and rinsed with 50 BV of 2.0 mol/L NaOH at a rate of 1 BV min−1, followed by DI washing until a neutral pH (pH∼7.5) was achieved and the ionic strength of the effluent reached a constant value. The primary physiochemical properties of the resin D201-OH are presented in Table 1. Different hydroxyl ion concentrations in the resin phase of D201-OH were obtained by partial neutralization with hydrochloric acid (Hanková et al. 2006). Kinetic experiments Kinetic experiments were performed in a 1,500-mL-capacity glass flask containing 1,000 ml DMP aqueous solution of known concentrations (0.063 to 0.251 mmol/L). The reaction temperature was maintained by a thermostatic shaking water bath in which the flask was immersed. After the solution reached the chosen temperature (283, 288, 293, 303, and 313 K), a desired amount of D201-OH was added to the flask, and agitation commenced at a known speed. At various time intervals, a 0.5-mL sample was removed from the flask for kinetic determination. A solution of KH2PO4 (0.1 mol/L) was used to adjust the sample to a neutral pH (Wolfe et al. 1980). Analysis
Materials and methods
Concentrations of DMP in the solution were analyzed by high-performance liquid chromatography (HPLC, Agilent, 1120, USA) equipped with a reversed-phase column (Agilent, TC-C18, 5 μm) and a UV detector. The mobile phase was composed of 55 % methanol and 45 % 0.2 mol/L potassium dihydrogen phosphate in an aqueous solution, and the solution was analyzed using a wavelength detection of 228 nm.
Materials
Table 1 Physicochemical properties of D201-OH
The chemicals used in this study, DMP, disodium tetraborate decahydrate, potassium dihydrogen phosphate, ethanol, sodium hydroxide, sodium chloride, and hydrochloric acid, were of analytical grade and were purchased from Shanghai Reagent Station (Shanghai, China). The strong-base anion exchanger D201-Cl (in chloride form, as denoted by the -Cl suffix) was provided by Jiangsu N&G Environmental Technology Co., Ltd. (Nanjing, China). Prior to use, D201-Cl was packed in a glass column and rinsed with 15 bed volumes (BV) of 1.0 mol/L NaOH, followed by washing with deionized (DI) water until a neutral pH was
Matrix
Polystyrene-divinylbenzene
Functional groupa Anion exchange capacity (mmol/g) Average pore diameter (nm) Moisture content (%) Cross-link density (%) Pore volume (cm3/g) BET surface area (m2/g) Particle size (mm)
−CH(CH3)3N+.OH− 3.4 28.6 50–60 8.3 0.50 24.5 0.3–0.9
a
The counter anion of D201-Cl is Cl− instead of OH−
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Results and discussion
The effect of agitation speed on the DMP removal efficiency and the OIRR was studied at six different speeds, 350, 570, 800, 980, 1,370, and 1,790 rpm, to examine the influence of the external mass transfer resistance, and the results are illustrated in Fig. 1a, b. Figure 1a shows that the removal efficiency of DMP did not change in the overall process when the stirring speed increased from 350 to 1,790 rmp. Therefore, the removal efficiency was independent of the agitation speed in the range of 350–1,790 rpm. The OIRR was determined using the first linear region of the removal process (within 28 min), as determined by linear regression analysis (Angulo et al. 1998; Puxty et al. 2009). Figure 1b reveals that the OIRR was consistently close to 0.63×10−3 mmol/L*min with varying degrees of agitation. These results demonstrated that there was no resistance to external mass transfer, and the external diffusion stage was not the rate-determining step. This conclusion is consistent with previous studies by Delgado et al. (2007), Izci and Bodur (2007), and Moreau et al. (1996) in which it was established that external diffusion does not control the overall reaction rate unless the agitation speed is very low or the viscosity of reactant mixture is very high. Therefore, our subsequent experiments were conducted at 800 rpm to ensure that the removal rate was not influenced by external diffusion.
80
350rpm 570rpm 800rpm 980rpm 1350rpm 1790rpm
60
40
20
0 0
100
200
300
400
500
Time (min) 1.0
3
Effect of agitation speed
Removal efficiency of DMP(%)
The course of a catalytic reaction induced by an ion exchange resin is generally considered to involve the following three stages: external diffusion, internal diffusion, and reaction (Helfferich 1962). For this study, in which the kinetic experiments were performed in a batch reactor, the external mass transfer effect is directly related to the stirrer speed (Ali et al. 2007; Yadav and Kamble 2012). In addition, the particle size of the anion exchange resin is one of the most important factors influencing internal mass transfer resistance in catalytic reactions (Ali et al. 2007; Moreau et al. 1996). Therefore, the effect of agitation speed, particle size, concentration of hydroxyl ions in the resin phase, and temperature were evaluated using varying degrees of each parameter.
Initial removal rate ×10 (mmol/(L*min))
Determination of the rate-controlling step
a
100
b 0.9 0.8 0.7 0.6 0.5 0.4 0.3 200
400
600
800
1000 1200 1400 1600 1800 2000
Speed of agitation (rpm)
Fig. 1 Effect of agitation speed on a the removal efficiency and b the initial removal rate of DMP
smaller D201-OH particles had a higher removal efficiency. The results in Fig. 2b show that the OIRR was inversely proportional to the particle diameter and increased from 0.43×10−3 to 0.81×10−3 mmol/L*min as the resin particle diameter decreased, indicating a strong pore resistance (Jogunola et al. 2010). Therefore, the DMP hydrolysis reaction was strongly influenced by internal diffusion, and the internal diffusion process may be the primary ratecontrolling step. Similar results were obtained by GD Yadav (Yadav and Krishnan 1999) for the acylation of 2methoxynaphthalene with acetic anhydride using different solid acid catalysts.
Effect of particle size Effect of hydroxyl ion concentration To gain further insight of the effect of internal diffusion resistance, D201-Cl was screened into several different particle sizes, and experiments were performed in the particle range between 0.3 and 0.9 mm. Figure 2a, b shows the effect of particle size on the DMP removal efficiency and the OIRR. Figure 2a shows that, compared with the larger particles, the
To further validate the conclusion that internal diffusion is the rate-controlling step, D201-OH with different hydroxyl ion concentrations in the resin phase was used to evaluate the effect of surface reaction on the removal efficiency and the OIRR. Figure 3a, b shows the results obtained from these
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a
100
Removal efficiency of DMP(%)
Removal efficiency of DMP(%)
100
80
60
0.3mm 0.6mm 0.9mm
40
20
80 70 60 50
2 mol/L 3 mol/L 4 mol/L 5.4 mol/L 6.8 mol/L
40 30 20 10 0
0 100
200 300 Time (min)
400
500
100
200
300
400
500
Time (min)
b
1.0
0
Initial removal rate × 10 (mmol/(L*min))
0
0.8
0.9
b 0.8 0.7
3
3
Initial removal rate × 10 (mmol/(L*min))
a
90
0.6
0.4
0.2
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.6 0.5 0.4 0.3 0.2 2
3 4 5 6 Hydroxyl ion concentraton (mol/L)
-1
1/d, mm
Effect of temperature The effect of temperature on the OIRR under otherwise similar conditions was studied in the range from 283 to 313 K. Figure 4 shows the OIRR at various temperatures and reveals that the OIRR increases from 0.24×10−3 to 1.1×10−3 mmol/
L*min with the increasing temperature in the range of 283– 313 K. A higher temperature can accelerate the apparent removal rate of DMP. The apparent activation energy (Ea) of the process was obtained from the slope of the plots lnk=f(1/
3
experiments. The data from Fig. 3a show that there was a negligible effect of the hydroxyl ion concentration (3.0– 6.8 mol/L) on the removal efficiency of DMP. However, when the hydroxyl ion concentration in the resin phase decreased to 2 mol/L, the removal efficiency dropped down to less than 80 %, which was below the value of other concentrations. It can also be seen from Fig. 3b that the OIRR remained nearly constant (approximately 0.6×10−3 mmol/L*min) for each hydroxyl concentration, except for 2 mol/L which was only about 0.5×10−3 mmol/L*min. These results provide strong evidence that the rate of DMP hydrolysis is not controlled by surface reaction when the hydroxyl concentration in the resin phase ranges from 3.0 to 6.8 mol/L.
Fig. 3 Effect of hydroxyl ion concentration on a the removal efficiency and b the initial removal rate of DMP
Initial removal rate × 10 (mmol/(L*min))
Fig. 2 Effect of resin particle size on a the removal efficiency and b the initial removal rate of DMP
7
1.2 1.0 0.8 0.6 0.4 0.2 280
285
290
295
300
305
310
315
Temperature (K)
Fig. 4 The relationship between the overall initial removal rate and temperature
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The OIRR law For a catalytic reaction controlled by internal diffusion, several parameters can influence the OIRR law, such as the particle size and pore structure of the resin, the DMP concentration, the reaction temperature, and the resin loading. As a commercially available resin, the physiochemical properties of D201-OH were constant. Therefore, the OIRR law only relates to the DMP concentration, the reaction temperature, and the resin loading. According to the reaction pathways suggested in our previous study (Xu et al. 2010), we assumed that the alkaline hydrolysis rate law of DMP catalyzed by D201-OH may be expressed using the following equation (Patnaik et al. 2001): r ¼ k ½D201−OHm ½DMPn
ð1Þ
DMP
-2.6
D201-OH
-2.8 -3.0
lgr
T, Fig. 5). The value of Ea is equal to 34.6 kJ/mol, which is below the homogeneous alkaline hydrolysis activation energy of 44.3 kJ/mol (calculated from Fig. S1 in the Supporting Information). The low activation energy value also illustrates that intraparticle diffusion of DMP controls the hydrolysis reaction (Helfferich 1962). These results illustrated that intraparticle diffusion of DMP in the catalyst particle is the rate-controlling step. To improve the DMP removal rate with a given amount of resin, the particle size and pore structure of D201-OH, the DMP concentration, and the reaction temperature, which can increase the diffusion rate, should be optimized. The effect of these parameters on the OIRR and the removal efficiency was quantified and is described in the following sections.
lgr=1.01lg[DMP]-2.16 2 R =0.993
-3.2 -3.4 -3.6
lgr=0.76lg [D201-OH] -3.57 2
R =0.999
-3.8 -1.0
-0.5
0.0
0.5
1.0
lg[D201-OH] or [DMP]
Fig. 6 Plot of logarithms of the initial rate versus the logarithm of the varying concentration of D201-OH or DMP at 303 K
Rubert IV and Pedersen 2006). Table S1 (in the Supporting Information) presents the experimental data for the initial hydrolysis rates of DMP catalyzed by D201-OH at 303 K. Figure 6 shows the logarithms of the initial rate plotted against the logarithm of the varying concentrations of D201-OH or DMP. The initial reaction orders of m=0.76 and n=1.0 were obtained from the slopes of two straight lines. Thereafter, the initial rate law for DMP hydrolysis at 303 K shows a fractional order of 0.76 for D201-OH and a first order for DMP, with the overall order of 1.76. The order of this reaction is between the first and second orders of DMP hydrolysis in NaOH solution (Wolfe et al. 1980), which may be because intraparticle diffusion is the rate-controlling step of the process (Helfferich 1962). According to the Arrhenius law, the reaction rate constant can be calculated using the following equation: k ¼ Aexpð−Ea=RT Þ
where r is the OIRR of DMP, k is the reaction rate constant, and m and n are the reaction orders to be determined. The initial rate method was employed to determine the reaction orders (Ghosh et al. 2003; Perez-Benito and Arias 1998;
ð2Þ
where k is the reaction rate constant, A is the preexponential factor, Ea is the activation energy, R is the universal gas
Removal efficiency of DMP(%)
100 -5.4 -5.6 lnk=-4166.9(1/T)+7.79
-5.8
R2 =0.996
lnk
-6.0 -6.2 -6.4 -6.6 -6.8
80 predicted data experimental data
60 2
χ =
3.3 × 10-4
40 DMP concentration 0.312mmol/L Temperature 293K Mass of resin 1.5 g
20
0
-7.0
0 0.00320
0.00328
0.00336
0.00344
1/T
Fig. 5 Arrhenius plot for determining activation energy
0.00352
200
400
600
800 1000 1200 1400 1600 1800 Time (min)
Fig. 7 Comparison between the experimental and the predicted removal efficiency of DMP
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constant, and T is the temperature. The value of A is equal to 2,416.3 L0.76 mmol0.76 min−1, which was obtained by the intercept of the plots lnk=f(1/T) in Fig. 5. Thus, we obtain the OIRR law from the following equation: r ¼ 2; 416:3 expð−34:6=RT Þ½D201−OH0:76 ½DMP
ð3Þ
According to Eq. (3), the effect of temperature, DMP initial concentration, and D201-OH loading on the initial removal rate of DMP can be evaluated.
Kinetics model Because of the large excess of D201-OH relative to DMP used in the kinetic experiments (the amount of D201-OH was 13.6 times as much as DMP), the variation in the concentration of D201-OH in the aqueous phase versus time was approximately constant, and the kinetic law can be described by the PFOK model (Helfferich 1962; Liger et al. 1999). Therefore, Eq. (2) becomes the following Eq. (4): r ¼ k 1 ½DMP ¼
dc dt
ð4Þ
where the apparent reaction rate constant k1 is defined using the following equations: k 1 ¼ k ½D201−OH0:76
ð5Þ
and k ¼ 2416:3expð−Ea=RT Þ
ð6Þ
Integrating Eq. (4), we obtain the following equation: C ¼ C 0 expð−k 1 t Þ
ð7Þ
where C0 denotes the initial concentration of DMP in the aqueous phase, and C is the concentration of DMP at time t. Table S2 (in the Supporting Information) lists the values of k1 calculated from the various operating parameters according to Eqs. (5) and (6). To validate the derived PFOK model, a comparison between the experimental and the predicted removal efficiency of DMP under different conditions is shown in Fig. 7.
The chi-square (χ2) test was adopted to evaluate the validity of the derived PFOK model and is described by the following Eq. (8): n X ðqexp −qpre Þ2 =qexp
χ2 ¼
i¼1
n−1
ð8Þ
where the superscripts pre and exp indicate the predicted and the experimental amounts of hydrolysis, respectively, and n is the total number of experimental points. The low values of χ2=3.3×10−4 proved the validity of the derived PFOK model for predicting DMP removal efficiency under different conditions.
Conclusions In the present study, the kinetics of DMP removal by D201OH was investigated. The internal diffusion of DMP within the D201-OH phase is the rate-controlling step of the process. The particle size and pore structure of the resin, the DMP concentration, and the temperature play an important role in the removal process. An OIRR law, as developed from the kinetic experiments, can quantify the effects of DMP concentration, reaction temperature, and D201-OH concentration on the initial hydrolysis rate of DMP. A recently developed and experimentally validated PFOK model can delineate the effect of these parameters on the removal efficiency of DMP. Acknowledgments This study was supported by the National Natural Science Foundation of China (grant no. 21107050) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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