Equilibrium, Thermodynamic, and Kinetic Studies of the Adsorption of

Article pubs.acs.org/jced

Equilibrium, Thermodynamic, and Kinetic Studies of the Adsorption of 2,4-Dichlorophenoxyacetic Acid from Aqueous Solution by MIEX Resin Xian Lu,† Yisheng Shao,*,†,‡ Naiyun Gao,† and Lei Ding§ †

State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, P. R. China ‡ China Academy of Urban Planning & Design, Beijing, P. R. China § School of Civil Engineering and Architecture, Anhui University of Technology, Maanshan, P. R. China ABSTRACT: The study investigated the performance of 2,4D adsorption on MIEX resin. The different temperatures affecting the adsorption performance were specifically evaluated. Equilibrium data followed the Freundlich and Langmuir isotherm model for all investigated temperatures (283 K to 333 K). The Langmuir model was slightly better than the Freundlich model at a lower temperature (283 K); however, with the increase of temperature the Freundlich model could well describe the isotherm data. The solute coefficient of distribution (Kd) could be better used to estimate the thermodynamic parameters compared with the Langmuir equilibrium constant (KL). The thermodynamic parameters (ΔH0, ΔS0, and ΔG0) showed that the adsorption of 2,4-D was feasible, endothermic, and spontaneous at 283 K to 313 K. The activation energy (Ea, 47.70 kJ·mol−1) determined by using the rate constant k2 of pseudo-second order model (the suitable kinetic model) and the mean free energy value (Efe, 6.0348 kJ·mol−1) evaluated by using the Dubinin−Radushkevich (D−R) model indicated that chemical ion-exchange was the predominant mechanism of 2,4-D adsorption. The calculated film diffusion coefficient value (Df) suggested film diffusion was the kinetic rate-limiting step. The adsorbent displayed excellent reusability with 0.1 mol·kg−1 NaCl as the regeneration agent.

1. INTRODUCTION

MIEX resin is a strong-base anion (chloride ion) exchange resin which can be used to absorb weak organic acidic ions from water.6 The smaller size (150−180 μm) of MIEX resin provides a much greater external surface area that allows for faster ion exchange kinetics and decreased resin fouling due to shorter NOM diffusion paths within the resin.6,7 In addition, the exchangeable chloride ion of MIEX resin could be effectively regenerated by the sodium chloride salt.8,9 To our knowledge, although the MIEX resin treatment is considered as a potential process for 2,4-D removal,5 the studies focusing specifically on 2,4-D adsorption performance by MIEX resin at different temperatures, and evaluating equilibrium, thermodynamic, and kinetic parameters based on the temperature of the system are very limited. These are significantly important in the design of treatment systems. Because of the fact that less information has been reported about adsorption performance of 2,4-D onto MIEX resin at different temperatures, and it is essential for interpreting the adsorption mechanism of 2,4-D to explore the equilibrium, thermodynamics, and kinetics characteristics. The objective of

Along with industrial activities, agricultural activities also contribute in water pollution to the great extent. The herbicide 2,4-dichlorophenoxyacetic acid (abbreviated as 2,4-D) is one of numerous agrochemicals in use today, which is commonly used in agriculture field for controlling the broad leaf weeds and grasses, such as cocoa, rubber, and oil palm or something because of its good selectivity and low cost.1 On the other hand, it has been frequently detected in water bodies in various regions of the world due to its poor biodegradation.2 Moreover, 2,4-D is one of the widely known endocrine disrupting chemicals (EDCs) as well as defined as a possible B-2 carcinogen and mutagen by the International Agency for Research on Cancer in 1987. Other than cancer, increase in abnormal sperms, sperms immobility and death, increase of lymphocytes, probability of immune deficiency disorders, and incidence of nervous, kidney, and respiratory diseases are among the concerns associated with using this herbicide.3 The toxicity of 2,4-D and its degradation products has become a potential hazard by contaminating our water environment.4 Therefore, eliminating 2,4-D from raw water is extremely essential. The magnetic ion exchange (MIEX) treatment has been proven to be a good option for 2,4-D elimination.5 © XXXX American Chemical Society

Received: September 28, 2014 Accepted: April 10, 2015

A

DOI: 10.1021/je500902p J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

2.2. Preparation and Analysis of Chemicals. The adsorbate 2,4-D (analytical grade, C8H6O3Cl2, pKa = 3.55) was supplied by Aladdin Chemistry Co., Ltd., China, and the details of 2,4-D employed in the experiment are listed in Table 1. By dissolving 2,4-D in methanol (HPLC grade, ≥ 99.9 %) solution, the 2,4-D containing stock solution (1 g·L−1) was obtained. The demanded 2,4-D concentrations were prepared after necessary dilutions with deionized water for batch mode adsorption studies. To quantify the loss of 2,4-D adsorbed on MIEX resin accurately, the HPLC method was adopted to analyze the concentration of residual 2,4-D in the adsorption medium. The UFLC-2010 PLUS HPLC apparatus (Shimadu, Japan) equipped with a C18 column was calibrated and tested prior to injection of the samples. Mixtures of acetonitrile (HPLC grade, ≥ 99.9 %)/ultrapure water (25 %/75 %, the ultrapure water contained 0.1 % of formic acid before mixing) were used as the mobile phase, and its flow rate was 1.0 mL·min−1.5 2.3. Batch Adsorption Studies. In batch adsorption experiments (observed from Figure 1), 2,4-D and MIEX resin particle were mixed and stirred by using the magnetic stirrer (78HW-1). For kinetics and equilibrium experiments related with the temperature, every sample was placed in the thermostat incubator for the purpose of keeping the temperature of adsorption system constant. The experimental temperature was maintained at ± 0.1 K. In this study, samples of each experiment were measured at different temperatures and replicate samples (two or more samples at each temperature) were used to determine the precision of the measurement. 2.4. Adsorption Equilibrium and Kinetic Experiments. Complete adsorption isotherms of 2,4-D were obtained by placing 0.25 mL of MIEX resin in a series of beakers containing 500 mL of 2,4-D at definite concentrations (2 mg·L−1 to 20 mg· L−1). The agitation speed was controlled at 150 rpm for 3 h while keeping the temperature at (283, 303, 313, or 333) K. For kinetic adsorption experiments, 500 mL of 10 mg·L−1 2,4-D solution and 0.5 mL of MIEX resin were agitated in a 1000 mL beaker with magnetic stirrer placed in the thermostat incubator. The mixture was agitated at 150 rpm at (293, 303, and 323) K, and the contact time was varied from (0 to 150) min. After each experiments, the supernatant containing residual 2,4-D was collected and analyzed. In these experiments, no attempt was made to maintain a constant pH during the adsorption process. The amount of 2,4-D adsorption at equilibrium (qe,

this study is to determine and compare the adsorption performances of the MIEX resin in the removal process of 2,4-D from aqueous solutions at different temperatures in view of equilibrium, kinetic, and thermodynamic studies. The corresponding mean free adsorption energy (Efe) and adsorption activation energy (Ea) were calculated to interpret the mechanism of 2,4-D removal; meanwhile, the intraparticle diffusion coefficient (Dp) and film diffusion coefficient (Df) were separately calculated to describe the kinetic diffusion process of 2,4-D adsorption. Also the thermodynamic parameters like ΔG0, ΔH0, and ΔS0 were calculated both using·Langmuir equilibrium constant (KL) and solute distribution coefficient (Kd), respectively, in order to compare the different thermodynamic calculation method. Furthermore, the reusability of MIEX resin for 2,4-D removal was examined.

2. MATERIALS AND METHODS 2.1. Adsorbents. The virgin MIEX resin was obtained from Orica Watercare of Victoria (Australia). Table 1 showed the Table 1. Details of the Adsorbate and Adsorbent Used in This Study adsorbate 2,4-Da adsorbent MIEXc resin

source Aladdin Chemical Co., Ltd. source Orica Watercare of Victoria

initial mass fraction purity

purification method

analysis method

0.97

none

HPLCb

matrix structure acrylic

functional group −N (CH3)3

exchangeable ion Cl

a 2,4-D = 2,4-dichlorophenoxyacetic acid. bHigh-performance liquid chromatography. cMIEX = magnetic ion exchange.

chemical composition of the MIEX resin used in this study. The resin was washed three times and stored in ultrapure water before being used as a adsorbent. Thereafter, MIEX resin was accurately measured out in a 5 mL glass centrifuge tube with ± 0.1 mL uncertainty. The following using details for MIEX resin were reported by Ding et al. previously.5 After adsorption, the used MIEX resins were separated using a Nd−Fe−B permanent magnet, and the resins collected were regenerated using brine solutions for reuse (seen in Figure 1).

Figure 1. Schematic diagram of batch adsorption experiments. B



Article

mg·mL−1) and at time t (qt, mg·mL−1) were calculated by eq 1 and eq 2, respectively. qe =

(C i − Ce)V M

qt =

(C i − Ct )V M

3.1.1. Adsorption Equilibrium and Calculation of Mean Free Sorption Energy. In this investigation, the most frequently used equations, Langmuir and Freundlich isotherm models, were used to analyze the isotherm data for the purpose of optimizing the design of an adsorption system. It is also an important step to establish the suitable correlation for equilibrium conditions. Originally Langmuir isotherm model was valid for the gas− solid-phase adsorption onto activated carbon.12 This empirical model is based on the following assumptions involving homogeneous adsorption situation. First, the sorption takes place at specific homogeneous sites within the adsorbent. Second, no further sorption can take place at that site once a 2,4-D molecule occupies a site. Third, the adsorption capacity of the adsorbent is finite. Fourth, the size and shape of all sites are identical and energetically equivalent.13 The Freundlich model is suitable for a highly heterogeneous surface composed of different classes of adsorption sites. This model mainly has two assumptions:13 First, with the increase of surface coverage of adsorbent, the binding strength gradually decreases. Second, the adsorption energies of active sites on the surface of adsorbent are different. The nonlinear form of Langmuir and Freundlich isotherm can be expressed as Langmuir model (nonlinear form):

(1)

(2) −1

where Ci = initial 2,4-D concentration (mg·L ), Ce = equilibrium 2,4-D concentration (mg·L−1), Ct = liquid-phase concentration of 2,4-D at time t (mg·L−1), V = the volume of solution (L), and M = the volume of adsorbent (mL). 2.5. Thermodynamics of Sorption. Three fundamental thermodynamic parameters (ΔG0, ΔH0, and ΔS0) associated with the adsorption process were calculated for the evaluation of thermodynamic nature of the adsorption process. 2.6. Regeneration Studies. The 2,4-D adsorption− desorption cycles were repeated 10 times at pH 6 to evaluate the reusability of MIEX resin. In the desorption experiment, a certain dose of MIEX resin (20 mL·L−1) loaded with 2,4-D was placed into NaCl solution (0.1 mol·kg−1) and was constantly stirred on a digital display stable temperature magnetic stirrer at 150 rpm for 90 min at 293 K. Then, 1.0 mL·L−1 recovered MIEX particles were used as the adsorbent to remove the 2,4D. In the adsorption experiment, MIEX resin was treated in higher concentration of 2,4-D solution to obtain 2,4-D-loaded adsorbent. For every desorption experiment, the desorbed samples were washed with deionized water and reconditioned for adsorption in the succeeding cycle. 2.7. Statistical and Data Analysis. All of the figures and model parameters (equilibrium and kinetic model) displayed in this paper were accomplished employing the nonlinear regression method of the Origin 8.5 program (Origin Lab, USA). Moreover, the determination coefficients (R2), residual root-mean-square error (RMSE), and sum of the squares of the errors (ERRSQ) were used as a measure of goodness-of-fit of the models. The calculated expressions of R2, RMSE, and ERRSQ are as follows:10,11 R2 =

qe =

n

RMSE =

1 n−2

RL =

i=1

∑ (Q calc − Q meas)2 i=1

1 1 + KLC0

qe = KFCe1/ n

(7)

(8)

where qm = the maximum adsorption capacity (mg·mL−1), KL = the Langmuir equilibrium constant related to the free energy of adsorption (L·mg−1), C0 = the initial concentration of adsorbate (mg·L−1), KF = Freundlich constant or the value of qe at Ce = 1 mg·L−1 (mg·mL−1 (L·mg−1)1/n), and n = heterogeneous factor related to intensity of adsorption. The magnitude of RL and n determines the feasibility of the sorption process.15 If RL = 1, the process is linear. And if n = 1, the process is homogeneous. RL = 0, the process is irreversible. n > 1, the process is favorable. RL > 1, the process is unfavorable. n < 1, the process is unfavorable. 0 < RL < 1, the process is favorable. Another common isotherm model, the Dubinin−Radushkevich (D−R) model, was always used to test the equilibrium data. The model is given as the following equations: D−R model (nonlinear form):

(3)

(4)

n

ERRSQ =

(6)

Freundlich model (nonlinear form):

n

∑ (Q meas − Q calc)2

1 + KLCe

Separation factor:14

(Q meas − Q̅ calc)2 ∑i = 1 (Q meas − Q̅ calc)2 + (Q meas − Q calc)2

qmKLCe

(5)

where Qmeas = the measured experiment data, Qcalc = the calculated data with isotherm or kinetic model, Q̅ calc = the average of Qcalc, and n = the number of data points.

3. RESULTS AND DISCUSSION 3.1. Effect of Temperature. In general the temperature is known to be the important factor to determine the process of adsorption. It may influence the adsorption rate behavior (kinetics process) and the maximum adsorption capacity at equilibrium (equilibrium process). Moreover, the dependence of the adsorption on temperature provides thermodynamic information which can be used to evaluate the nature of the adsorption system.

qe = qD − R exp −(KD − R ε 2)

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝ C

(9)

(10) DOI: 10.1021/je500902p J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Table 2. Parameters of Langmuir, Freundlich, and Dubinin−Raduskhevich Isotherm Models, for the Adsorption of 2,4-D onto the MIEX Resin (for All of the Nonlinear Isotherms)a T/K

a

283

KL (L·mg−1) qm (mg·mL−1) RL R2 ERRSQ RMSE

0.3254 60.6521 0.1332 to 0.9884 9.0629 1.0644

KF (mg·mL−1·(L·mg−1)1/n) n 1/n R2 ERRSQ RMSE

14.5487 1.5046 0.6646 0.9845 12.0719 1.2284

qD−R (mg·mL−1) KD−R (mol2·kJ−2) Efe (kJ·mol−1) R2 ERRSQ RMSE

32.3737 −0.0207 4.9147 0.9407 46.3315 2.4065

303 Langmuir isotherm constants 0.5552 48.0489 0.6058 0.0826 to 0.4738 0.9794 15.9361 1.4114 Freundlich Isotherm Constants 16.3523 1.7589 0.5685 0.9850 11.5694 1.2026 Dubinin−Raduskhevich Isotherm Constants 29.9008 −0.0122 6.4018 0.8883 86.2888 3.2842

313

333

0.7352 42.6229 0.0637 to 0.4048 0.9857 10.6888 1.1559

0.5918 44.4453 0.0779 to 0.4579 0.9842 11.5489 1.2015

17.0316 1.9919 0.5020 0.9859 10.5055 1.1459

15.8618 1.8866 0.5301 0.9927 5.3079 0.8145

29.3237 −0.0106 6.8680 0.8966 77.2561 3.1076

29.6406 −0.0141 5.9549 0.8920 78.9148 3.1407

Standard uncertainties u are u(T) = 0.1 K.

Figure 2. Adsorption isotherm of the adsorption of 2,4-D on MIEX resin (initial concentration (2 to 20) mg·L−1, agitation speed 150 rpm, adsorbent 0.5 mL·L−1, adsorption time 3 h, (283 to 333) K, pH without any adjustment). The lines of left insert figure: nonlinear fitting curves with Langmuir model; the lines of right insert figure: nonlinear fitting curves with Freundlich model. ■, 283 K; ●, 303 K; ▲, 313 K; ▼, 333 K.

Efe =

1 −2KD − R

adsorption nature is physical for Efe value no higher than 8 kJ· mol−1.16−18 All relative parameters derived for the three isotherm equations, the values of R2, ERRSQ, and RMSE are listed in Table 2, respectively. Figures 2 and 3 also showed the experimental equilibrium data and the fitted equilibrium curves. Compared with the D−R model, Langmuir and Freundlich models seem to fit well with higher R2 values (0.9794 to 0.9884 and 0.9845 to 0.9927) and smaller error values (ERRSQ 9.0629 to 15.9361, RMSE 1.0644 to 1.4114 for Langmuir model and ERRSQ 5.3079 to 12.0719, RMSE 0.8145 to 1.2284 for rgw

(11)

where qD‑R = the maximum adsorption capacity (mg·mL−1), KD−R = the D−R constant (mol2·J−2) related to the sorption energy, ε = the Polanyi potential (J·mol−1), R = the gas constant (8.314 J·mol−1·K−1), T = the absolute temperature (K), and Efe = the sorption mean free energy (kJ·mol−1). The Efe magnitude can determine the adsorption type whether it is physical or chemical. For chemical adsorption processes the Efe value is between (8 and 16) kJ·mol−1, and the D



Article

fitting the adsorption type onto ion-exchange resin, it is restricted to some harsh terms: it assumes that a monolayer adsorption takes place on homogeneous surface of adsorbent, and there is no interaction between neighboring adsorbed species.5,19 Since 2,4-D has a pKa of 3.55, 10 % of the neutral molecules will exist besides 90 % of the anion form at pH 4.55 (original solution, pH without any adjustment). The different forms of 2,4-D probably exist interaction due to competing for the active adsorption sites at a higher temperature. In addition, the increase in the temperature can make the pores of resin particles become wider and increase the activity of sorption sites to some extent. The surface morphology photos of MIEX resin particle observed via a scanning electron microscope also indicated the heterogeneous surface of MIEX resin at a higher temperature. The above results can explain the phenomenon that the isotherm of 2,4-D adsorption deviates from the Langmuir model at higher temperatures in this study. On the other hand, it is clear from Table 2 that the values of RL calculated at (283, 303, 313, and 333) K for all concentrations of 2,4-D solutions were in range between 0 and 1, and the values of n were larger than 1.0, which suggested that the 2,4-D is adsorbed favorably by the MIEX resin. Namely, the process of 2,4-D adsorption is favorable. In this study, the R2 value of the D−R isotherm model ranged from 0.8883 to 0.9407. The mean free energy (Efe) obtained using the D−R constant (KD−R) were 4.9147 kJ·mol−1 for 283 K, 6.4018 kJ·mol−1 for 303 K, 6.8680 kJ·mol−1 for 313 K, and 5.9549 kJ·mol−1 for 333 K, indicating that the adsorption process of 2,4-D occurs via a physical mechanism for all the temperatures. However, the D−R isotherm is similar to the implication of Langmuir isotherm, which assumes the adsorption of single type of uniform pores.20 The results of specific surface area, pore volumes, and average pore diameter determined by a multipoint BET model indicated that the MIEX resin is composed of small pore (micropore) and larger pore (mesopore), which suggested that the pores of MIEX resin particle are not the single type of uniform pores. Namely, the D−R isotherm model is inadequate to describe the equilibrium adsorption behavior of 2,4-D. Besides, the calculated mean value of Efe (6.0348 kJ·mol−1) approached the energy ranges of ion-exchange chemisorption ((8 to 16) kJ· mol −1 ) for temperature from (283 to 333) K. This

Figure 3. Measured and correlated adsorption isotherms of 2,4-D on MIEX resin for initial 2,4-D concentration changing from (2 to 20) mg·L−1 at different temperatures of T = (283, 303, 313, and 333) K and at t = 3 h: the points are experimental data, and the curves are Dubinin−Radushkevich (D−R) isotherms. ■, 283 K; ●, 303 K; ▲, 313 K; ▼, 333 K.

Freundlich model) respectively. But the Langmuir model fitted the experimental data slightly better than Freundlich model at a lower temperature (283 K) when comparing the values of R2, ERRSQ, and RMSE. MIEX is a macro-porous anion exchange resin with magnetic properties. However, the decrease of the temperature can make the pores of resin particles become narrow and can scarcely induce the creation of new active sites of the surface. In this case, the surface of MIEX resin is approximately homogeneous. Furthermore, the lower temperature may inhibit the process of agglomeration in a very determined sense to a certain extent, so the 2,4-D adsorption only occurs in a finite number of definite localized sites of MIEX resin surface. These are the reasons why the Langmuir model could describe the equilibrium data of 2,4-D adsorption at a lower temperature well. It can be seen from Table 2 that the Freundlich model gives slightly better fitting than the Langmuir model with the increase in temperature (from (303 to 333) K) when comparing the R2 and error values. Although the Langmuir model is typically considered to be suitable for

Table 3. Calculation of Thermodynamic Parameters Using Different Equilibrium Constant (KL or Kd) by Researchers serial no.

journal

adsorbents

KL Kd Kd

22 23 24

graphene ethylenediamine-modified cross-linked magnetic chitosan resin diethyl phthalate

Bisphenol A chromium(VI)

Kd KL

25 26

magnetic poly(EGDMA-VP) beads cationic dye

KL

27

Kd

28

Remazol Brilliant Orange 3R dye cAMP 2,4-D

KL

29

KL KL and Kd

30 present study

6

Journal of Hazardous Materials

7

Journal of Chemical & Engineering Data Chemical Engineering Journal

graphene oxide nanosheets

Chemical Engineering Journal

anion-exchange resin MIEX resin

9 10

ref

Zn(II) cesium catechol and resorcinol

4 5

8

equilibrium constant

activated carbon fiber (ACF) walnut shell granular activated carbon

Chemosphere Water Research Journal of Chemical & Engineering Data Langmuir Journal of Hazardous Materials

1 2 3

adsorbate

coffee husk-based activated carbon

E



Article

demonstrates the adsorption by MIEX resin has been of ionexchange.21 Accordingly, it is inferred that the adsorption of 2,4-D onto MIEX resin is involved in the physical (such as weak van der Waals forces) and chemical processes (ion-exchange), and the chemisorptions are the predominant process. 3.1.2. Adsorption Thermodynamics. Thermodynamic parameters can provide necessary information to design an adsorption process. Usually, thermodynamic parameters (ΔH0, in kJ·mol−1, ΔS0, in J·mol−1 K−1 and ΔG0, in kJ·mol−1) are calculated using the Langmuir equilibrium constant KL or Kd (the solute coefficient of distribution)22−30 which changes with temperature (shown in Table 3). In this study, thermodynamic parameters were calculated by eq 12 to 15 both using the KL and Kd in order to compare the thermodynamic parameters with different calculation methods. ln KL =

ΔS° ΔH ° − R RT

(12)

ln Kd =

ΔS° ΔH ° − R RT

(13)

Kd =

qe Ce

(14) (15)

ΔG° = ΔH ° − T ΔS°

According to eqs 12 and 13, the ΔH and ΔS values can be determined from the slope and intercept of the plot between ln KL or ln Kd versus 1/T (Figure 4a and b). Then, with a wellknown equation, the ΔH0 and ΔS0 values were used to calculated ΔG0 value. All of the calculated thermodynamic results were summarized in Table 4. The positive ΔS0 values indicated the increased disorder at the solid−solution interface for the adsorption system.31 The positive ΔH0 values suggested the endothermic nature of the adsorption reaction of 2,4-D, and the adsorption capacity increased with increasing temperature. Moreover, the negative ΔG0 values showed the feasible and spontaneous adsorption process when the thermodynamic equilibrium constant was replaced by the solute coefficient of distribution (kd), and the decrease in the ΔG0 values indicated the proportional extent of spontaneity as temperature was increased. The above results were consistent with that reported by Ding et al.5 However, from Table 4 the positive value of ΔG0 revealed the nonspontaneous adsorption process within the investigated temperature range from (283 to 313) K when using the Langmuir equilibrium constant (KL) as the thermodynamic equilibrium constant. As discussed in section 3.1.1, the Langmuir model provided the slightly better fitting than Freundlich model at a lower temperature (283 K), and with the increase of temperature the Freundlich model could give a little better fit than Langmuir isotherm model. Namely, the Langmuir isotherm model was unsuitable to depict the adsorption behavior of 2,4-D at equilibrium for the temperature varying from (283 to 313) K. Therefore, using the Langmuir equilibrium constant to estimate the adsorption thermodynamic parameters was practically inappropriate. This might be the reason why ΔG0 has a positive value when using the KL as the thermodynamic equilibrium constant. For that reason, it can be concluded that the 2,4-D adsorption is thermodynamically spontaneous reaction at the investigated temperatures, and the solute coefficient of 0

0

Figure 4. van’t Hoff plots for the sorption of 2,4-D onto MIEX resin using (a) Langmuir equilibrium constant KL and (b) solute coefficient of distribution at equilibrium Kd, Kd = qe/Ce. ■, C0 = 2 mg·L−1; ●, C0 = 4 mg·L−1; ▲, C0 = 6 mg·L−1; ▼, C0 = 8 mg·L−1; ⧫, C0 = 10 mg·L−1; ◀, C0 = 12 mg·L−1; ▶, C0 = 14 mg·L−1; ⬢, C0 = 16 mg·L−1; ★, C0 = 18 mg·L−1; ⬟, C0 = 20 mg·L−1.

distribution is a better method to estimate the thermodynamic parameters for the 2,4-D adsorption onto MIEX resin compared with the Langmuir equilibrium constant. 3.1.3. Adsorption Kinetics and Calculation of Activation Energy. Figure 5a showed the effects of different temperatures ((293, 303, and 323) K) on the 2,4-D adsorption. It can be observed that the varied temperature (between (293 and 323) K) affected the rate of adsorption at initial period to a certain extent. Fast 2,4-D adsorption happened in the first 30 min with the increase of temperature, indicating that the sorption of 2,4D was endothermic process. Then the 2,4-D adsorption gradually slowed down and reached equilibrium after 50 min. Besides the difference of concentration gradient, the interaction forces between solute and adsorbent become stronger than those between the solute and the solvent, leading to the fast adsorption at the initial stage.32 As time proceeded, the sorption rate decreased, and temperature variation influencing the final adsorption capacity is not significant at the later equilibrium stage. To investigate the kinetic mechanism of adsorption and characteristic constants of adsorption, pseudo-first order, and F



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Table 4. Thermodynamic Parameters for the Adsorption of 2,4-D by MIEX Resin at Different Temperaturesa T K

L·mg

283 303 313 C0 mg·L 2 4 6 8 10 12 14 16 18 20 a

−1

L·mL

21.1081−37.4499 15.5297−31.8066 15.2684−25.9928 16.1409−22.5147 15.4076−18.5430 13.4339−14.2110 10.5053−12.3818 10.8770−11.1686 9.7466−10.8951 9.6872−10.2017

−1

J·K ·mol

kJ·mol

19.85

ΔH0 −1

ΔS0

−1

0.3254 0.5552 0.7352 Kd

−1

ΔH0

KL

−1

J·K ·mol

kJ·mol

12.93 17.81 13.87 6.96 1.66 1.21 3.84 0.65 2.51 1.27

kJ·mol−1

60.76

2.65 1.44 0.83 ΔG0/(kJ·mol−1)

ΔS0

−1

ΔG0 −1

−1

70.71 85.79 71.97 47.38 28.86 25.82 33.06 22.17 27.87 23.37

283 K

303 K

313 K

−7.08 −6.47 −6.49 −6.45 −6.51 −6.09 −5.52 −5.62 −5.38 −5.34

−8.50 −8.18 −7.93 −7.39 −7.09 −6.61 −6.18 −6.06 −5.94 −5.81

−9.21 −9.04 −8.65 −7.87 −7.38 −6.87 −6.51 −6.28 −6.22 −6.04

Standard uncertainties u are u(T) = 0.1 K, and u(C0) = 0.01 mg·L−1.

the pseudo-second order models were used to simulate the kinetics experimental data. The two models are generally expressed as follows:33,34 pseudo-first order model (nonlinear form): qt = qe(1 − e−k1t )

(16)

pseudo-second order model (nonlinear form): qt =

k 2qe 2t 1 + k 2qet

(17) −1

−1

The initial adsorption rate h (mg·mL ·min ) at t = 0 is given as35 h = k 2qe 2

(18)

By using the values of the k2 from the pseudo-second order kinetic model, the activation energy of sorption can be calculated. The linear relationship (Arrhenius type relationship) between the rate constant and temperature is expressed as the following equation: Arrhenius equation (linear form): ln k 2 = ln A −

Ea RT

(19)

where qe and qt = the amount of 2,4-D adsorbed at equilibrium and at time t, respectively (mg·mL−1), k1 = the rate constant of pseudo-first order kinetics (min−1), k2 = the overall rate constant of pseudo-second order sorption (mg·mL−1·min−1), A = the frequency factor (mg·mL−1·min−1), and Ea = Arrhenius activation energy (J·mol−1). The nonlinear fitting plots of pseudo-first order and pseudosecond order model are shown in Figures 5b and 6a, respectively. Table 5 gives the values of the parameters for two kinetic models. As can be seen, the pseudo-second order kinetic model had higher correlation coefficients (R2 > 0.98) and low error values at various temperatures (293 K to 323 K), in contrast to those of pseudo-first order model. The obtained data showed a better fit with the pseudo-second order model. These results also revealed that the chemisorption process (covalent forces through sharing or exchange of electrons)

Figure 5. (a) Kinetic adsorption curve of MIEX resin for 2,4-D at various temperatures: initial 2,4-D concentration = 10 mg·L−1; adsorbent dosage = 1.0 mL·L−1; agitation speed = 150 rpm; pH without any adjustment. (b) Nonlinear fitting curves with pseudo-first order kinetic model. ■, 293 K; ●, 303 K; ▲, 323 K.

G



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Weber−Morris model: qt = k idt 1/2 + Ci

(20)

where kid (kid1, kid2, and kid3) = the intraparticle diffusion rate constant (mg·mL−1·min−1/2), kid1 is constant of the first stage involving external surface adsorption, kid2 is constant of the second stage involving gradual adsorption, and kid3 is constant of the third stage involving final equilibrium stage, and Ci = the intercept reflecting the thickness of boundary layer. According to the theory of Weber−Morris model, the plot of qt versus t1/2 should be linear when adsorption complies with the intraparticle diffusion mechanism and the intraparticle diffusion should be the only rate-determining step if the line passes through the origin. Otherwise, there are two or more rate-limiting steps involving in the adsorption process if the plots are multilinear.42 The values of kid1, kid2, kid3, and C1, C2, C3 for 2,4-D adsorption at temperatures of (293, 303, and 323) K are listed in Table 6. Figure 7 (qt versus t1/2) showed the 2,4-D adsorption process was not linear over the entire time range and the adsorption was controlled by three different stages:13 (1) instantaneous adsorption stage due to the external mass transfer; (2) intraparticle diffusion controlled gradual adsorption stage; (3) final equilibrium stage due to the extremely low 2,4-D concentration in the solution. For the above three stages, the second and third stage involved the intraparticle diffusion process. Figure 7 illustrated that intraparticle diffusion was not the rate controlling mechanism for all linear lines of stages 2 and 3 without passing through the origin. Moreover, the kid1 values of the first portion for different temperature were (1.54, 1.08, and 0.68) mg·mL−1·min−1/2, respectively, which was greater than the kid2 and kid3 (seen in Table 6). It indicated that external surface adsorption was faster compared with the intraparticle diffusion. The results further proved intraparticle diffusion was involved in the adsorption process but was not the only rate-limiting step throughout the adsorption process. Namely, other mechanisms (boundary layer diffusion or film diffusion) might contribute to the rate-determining step. The intraparticle diffusion coefficients Dp (m2·s−1) and film diffusion coefficients Df (m2·s−1) have also been calculated to confirm the above results. intraparticle diffusion coefficient:

Figure 6. (a) Simulated pseudo-second order kinetics for the adsorption of 2,4-D onto MIEX resin at T = 293 K, 303 K, and 323 K. ■, 293 K; ●, 303 K; ▲, 323 K. (b) Arrhenius plots for 2,4-D onto MIEX resin (k2, the rate constant of pseudo-second order sorption).

might be the rate-limiting step.36,37 Moreover, it was observed from Figure 6a that the curves fitted by the pseudo-second order kinetic model were well matching with the experimental points. These findings further indicated the pseudo-second order kinetic model was suitable for modeling the adsorption of 2,4-D on MIEX resin. From Table 5, the values of h (implying the initial adsorption rate, h = 1.1533 (293 K), 2.8803 (303 K), and 6.6863 (323 K)) increased with the increase in temperature, indicating the adsorption process of initial phase became faster with increasing temperature due to the endothermic diffusion process.10 As demonstrated in Figure 6b, the slope of the linear plot of ln k2 versus 1/T was constructed to calculate the adsorption activation energy. The magnitude of activation energy for 2,4-D adsorption was found to be 47.70 kJ·mol−1 (shown in Table 5). The weak forces usually represent the characteristic of physical adsorption involving low activation energies ((5 to 40) kJ· mol−1),38 while chemical sorption process has the characteristic of higher activation energies ((40 to 800) kJ·mol−1).39 Thus, the 2,4-D adsorption might be conformed to chemisorption mechanisms, which was in keeping with the fitted results of pseudo-second order kinetic model. 3.1.4. Diffusion Mechanism Study. Three major ratelimiting steps involving the kinetic diffusion mechanism are generally cited:40 (a) film diffusion; (b) intraparticle diffusion; (c) interior surface diffusion; (d) adsorption or ion exchange on the pore surface. The intraparticle diffusion model (Weber−Morris model) is applied to analyze the empirically found functional relationship (qt versus t1/2).41

Dp =

0.03R p2 t1/2

(21)

film diffusion coefficient:

Df =

0.23R pεCs t1/2C L

(22)

Table 5. Kinetics Parameters of Pseudo-First Order and Pseudo-Second Order Models for 2,4-D Adsorption on MIEX Resina pseudo-first order model

a

−1

2

T/K

k1 (min )

R

293 303 323

0.0804 0.1728 0.2992

0.9815 0.8898 0.8556

Ea

pseudo-second order model −1

−1

ERRSQ

RMSE

k2 (mg·mL ·min )

h

R

0.6019 1.2749 0.5065

0.2743 0.3992 0.2516

0.0108 0.0299 0.0712

1.1533 2.8803 6.6863

0.9824 0.9844 0.9897

2

ERRSQ

RMSE

(kJ·mol−1)

0.5736 0.1808 0.0359

0.2678 0.1503 0.0671

47.70

Standard uncertainties u are u(T) = 0.1 K. H



Article

Table 6. Intraparticle Diffusion Model Parameters for the Adsorption of 2,4-D on MIEX Resina intraparticle diffusion model

a

T/K

kid1 (mg·mL−1·min−1/2)

C1


C2


C3

293 303 323

1.5412 1.0786 0.6800

0.2181 3.5798 6.0868

0.6222 0.2775 0.0921

4.7508 7.3690 8.8275

0.0719 0.0326 0.0281

8.5762 9.0889 9.2155

Standard uncertainties u are u(T) = 0.1 K.

was the rate-determining step in the adsorption process. Namely, the film diffusion is the primary mechanism contributing to the adsorption process of 2,4-D on MIEX resin. 3.2. Recycling of Adsorbent Regeneration. The regeneration and reuse of MIEX resin for 2,4-D adsorption is quite crucial for economic costs. It was found that the spent MIEX resin could be well desorbed by 0.1 mol·kg−1 NaCl solutions. Figure 8 showed the 2,4-D removal percentage in

Figure 7. Plot of Weber−Morris intraparticle diffusion model for 2,4D adsorption on MIEX resin at T = 293 K, 303 K, and 323 K: kid1, the first stage diffusion rate constant; kid2, the second stage diffusion rate constant; kid3, the third stage diffusion rate constant. ■, 293 K; ●, 303 K; ▲, 323 K.

where t1/2 (s) = the time required to complete half of the adsorption, the value of t1/2 is calculated by using the following equation:43 t1/2 =

1 k 2qe

Figure 8. Reusability of the MIEX resin (1.0 mL·L−1 MIEX resin in 500 mL of 10 mg·L−1 2,4-D solutions at pH 6.0 were applied; agitation speed = 150 rpm; temperature = 293 K; 0 cycle: the fresh MIEX resin).

(23)

Rp (m) = the average radius of the adsorbent particles, ε = the film thickness (10−5 m),44 and Cs and CL = the concentration of adsorbate in solid and liquid phase, respectively. Michelsen et al.44 assumed that the intraparticle diffusion will be the rate-limiting step if the calculated intraparticle diffusion coefficient (Dp) value is in the range 10−15 to 10−18 m2·s−1. For the calculated film diffusion coefficient (Df) value ranging from 10−10 to 10−12 m2·s−1 the rate-limiting step is controlled by film diffusion. In this study, the calculated Dp values ranged from 1.81·10−12 to 11.2·10−12 m2·s−1, and the calculated values of Df were found to be in the order of 10−11 m2·s−1 (shown in Table 7). The results indicated intraparticle diffusion is not the only rate-limiting step of 2,4-D adsorption while the film diffusion

cycles of adsorption−desorption. It can be seen that only 2.77 % loss in the removal of 2,4-D was observed and there was still more than 91 % of the 2,4-D adsorption percentage after ten adsorption−desorption cycles. The results indicated that MIEX resin can be repeatedly used for 2,4-D removal without a significant reduction in adsorption performance. Thus, it can be concluded that the MIEX resin could be regenerated and used for many times on separation applications.

4. CONCLUSIONS The present study investigated the adsorption performance of 2,4-D on MIEX resin at different temperatures. Equilibrium studies showed the 2,4-D adsorption process fit the Langmuir and Freundlich isotherm model for the temperatures from (283 to 333) K, and the Langmuir model is slightly better than that of the Freundlich model at a lower temperature (283 K); however, with the increase of temperature, the Freundlich model gives slightly better fitting than the Langmuir model. The calculated thermodynamic parameters showed the feasible, endothermic, and spontaneous nature of 2,4-D adsorption at (283 to 313) K, and the solute coefficient of distribution (Kd) could be better used to estimate the

Table 7. Intraparticle Diffusion Coefficient (Dp) and the Film Diffusion Coefficient (Df) of the Adsorption Process at (293, 303, and 323) Ka T/K 293 303 323 a

Rp/m −4

1.8·10

t1/2/s 537.11 204.25 86.95

Dp (m2·s−1) −12

1.81·10 4.76·10−12 11.2·10−12

Df (m2·s−1) 1.42·10−11 4.32·10−11 11.14·10−11

Standard uncertainties u are u(T) = 0.1 K. I



Article

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thermodynamic parameters of 2,4-D adsorption compared with the Langmuir equilibrium constant (KL). Kinetic evaluation of experimental data showed that the adsorption processes of 2,4D followed well pseudo-second order kinetics. The calculated activation energy (Ea = 47.70 kJ·mol−1) and average adsorption energy (Efe = 6.0348 kJ·mol−1) evaluated from the D−R model indicated that the adsorption of 2,4-D onto MIEX resin predominantly followed the chemical ion-exchange mechanism. The calculated film diffusion coefficient value (Df) suggested the kinetic diffusion process of 2,4-D was controlled by film diffusion. Moreover, the recovery tests indicated that 2,4-D from the spent MIEX resin could be desorbed by NaCl solutions, and the reusability of the MIEX resin was good after ten consecutive adsorption−desorption cycles. Further study is also required to verify and obtain a more theoretical foundation for the ion-exchange interactions between 2,4-D and MIEX resin.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Funding

The authors acknowledge the support provided by research team of the Analysis and Testing Central Facility of Anhui University of Technology, as well as the research grant provided by the National Natural Science Foundation of China (Nos. 51178321 and 51308001). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors also would like to give thanks to all anonymous reviewers for their valuable suggestions.

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REFERENCES

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K


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