Adsorptive Removal of 2,4-Dichlorophenoxyacetic Acid (2,4-D) from

Article pubs.acs.org/IECR

Adsorptive Removal of 2,4-Dichlorophenoxyacetic Acid (2,4-D) from Aqueous Solutions Using MIEX Resin Lei Ding,†,‡ Xian Lu,‡ Huiping Deng,*,† and Xinxi Zhang‡ †

Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China ‡ School of Civil Engineering and Architecture, Anhui University of Technology, 59 Hudong Road, Maanshan 243002, P. R. China ABSTRACT: Batch experiments are carried out to evaluate the adsorption performances of 2,4-D from aqueous solutions by MIEX resin. The initial 2,4-D concentration, adsorption time, adsorbent dosage, inorganic ions, natural organic matter, and initial pH of solution have considerable effect on the removal of 2,4-D, but temperature has less effect. The isotherm experiments show the Redlich−Peterson and Sips models are the most appropriate models to simulate the equilibrium data at 293 K. The kinetic study demonstrates the pseudo-second-order model gives a best fitting to the adsorption of 2,4-D on the resin and the intraparticle diffusion is not the only rate-controlling step. The thermodynamic parameters are calculated. The results show that the adsorption of 2,4-D on the resin is a thermodynamically spontaneous and endothermic process. Accordingly, it can be concluded that MIEX resin can potentially be employed as an adsorbent for removing 2,4-D from raw water.

1. INTRODUCTION 2,4-Dichlorophenoxyacetic acid (2,4-D) is widely used as an herbicide on crops to control broadleaf weeds due to low cost and good selectivity.1,2 Owing to the fact that 2,4-D is an ionizable herbicide, the 2,4-D residues are weakly retained by most of the soil components3 and can easily contaminate surface water bodies by surface runoff or seepage into groundwater aquifers. Residues of 2,4-D have been detected in surface water and groundwater bodies in many countries in the world, such as the United States, Canada, Hungary, India, Russia, Germany, and Poland.4 2,4-D can be considered as a threat to the environment and human health. It has been proved to be toxic to humans and animals5 and has been classified as a possible human carcinogen and mutagen by the International Agency for Research on Cancer.6 2, 4-D is also one of the well-known endocrine disrupting chemicals which seriously affect the immune and endocrine system.3 For these reasons, the World Health Organization has recommended 70 μg L−1 as its maximum permissible concentration in drinking water.7 The permissible concentration of 2,4-D in drinking water in China is regulated below 30 μg L−1.8 However, the conventional treatment processes for drinking water, such as coagulation, sedimentation, filtration, and disinfection, are expected to be hardly effective for 2,4-D removal as this is a small and polar molecule. Consequently, it is necessary to seek alternative, more effective treatment techniques to eliminate 2,4-D from raw water. Various treatment techniques have been investigated for the removal of 2,4-D. It can be effectively eliminated by ozone oxidation technology, but the ozone degradation products may be equally harmful.9 Advanced oxidation by photochemical catalysis is a promising method for the elimination of organic pollutants such as 2,4-D in water, but the suspended solid particles in raw water may significantly affect the oxidation process.10 Also, the electrochemical method (EC) can remove 2,4-D in drinking water. But the anode is susceptible to fouling © 2012 American Chemical Society

by the intermediates, which quite lowers the current efficiency and the oxidation efficacy of 2,4-D over time.11 Fenton processes, such as the photoassisted Fenton treatment,12 electrochemical Fenton treatment,13 and cathodic Fenton treatment,14 have been investigated to remove 2,4-D from aqueous solutions, but they all have the problem of low efficiency at neutral pH values. Biological treatment has been employed to remove 2,4-D,15 but low temperature has an adverse effect on the growth of microorganisms, which decreases the removal efficacy of 2,4-D. Adsorption of 2,4-D is widely studied. In most research, activated carbon is used as adsorbent.2,6 Nevertheless, massive heat energies are consumed to regenerate the activated carbon saturated by 2,4-D, which leads to higher cost. Thus, many researchers seek other adsorbents to remove 2,4-D from water, such as clay minerals,16 Oscillatoria,17 silica gel,7 organic polymer resin,18 bioadsorbent.19 Another new type of adsorbent, magnetic ion-exchange resin (MIEX), is widely used in removing natural organic material.20 2,4-D removal by MIEX resin is focused on in this study. MIEX resin is a strong base anion exchange resin with iron oxide integrated into a macroporous, polyacrylic matrix, and is typically used with chloride as the exchangeable ion.21 The magnetic property differentiates it from other resins, for the magnetic core enables fast agglomeration and settling of resin particles.22 In addition, the MIEX resin has an increased surface due to the very small resin bead size (the average size in diameter is 180 μm, and 2−5 times smaller than traditional resins21), which may lead to fast adsorption of pollutants on MIEX surface. The saturated resins can be regenerated using brine.23 MIEX is designed specifically to remove dissolved Received: Revised: Accepted: Published: 11226

February 22, 2012 July 31, 2012 August 8, 2012 August 8, 2012 dx.doi.org/10.1021/ie300469h | Ind. Eng. Chem. Res. 2012, 51, 11226−11235

Industrial & Engineering Chemistry Research

Article

Table 1. Structural Formula and Properties of 2,4-D

system. Test solutions were prepared by diluting the standard stock solution with distilled water to the desired concentrations. 2.2. Experimental Methods. 2.2.1. Adsorption Experiments. Adsorption data were obtained from batch experiments. All experiments were performed using a set of 1000-mL beakers with a certain volume of resin and 500 mL of 2,4-D solution on a set of digital display stable temperature magnetic stirrers (78HW-1). After adsorption, the adsorbents were separated using a filter with a 0.45-μm Millipore membrane, and the 2,4D in filtrate was measured. The amount of 2,4-D adsorbed on MIEX resin at time t, qt (mg mL−1), was calculated using eq 1. The equilibrium adsorption capacity of 2,4-D, qe (mg mL−1), was calculated by eq 2, and the removal rate of 2,4-D after adsorption, E (%), is calculated by 2,4-D concentration change in solution before and after adsorption.

organic matters from water, and as such, most researchers use the DOC removal by MIEX as a parameter to assess removal efficacy of organic micropollutants.24−26 For the last two years, a few studies have been conducted to remove specific pollutants from water by MIEX resin, such as bentazone,27 estrone,28 and surfactant.29 These studies indicate that the bentazone and surfactant are predominantly removed by ion exchange, but the estrone can be removed by hydrogen bonding and nonspecific interactions as well as ion exchange. However, to our knowledge, it has not been reported yet to remove 2,4-D from raw water by MIEX. More importantly, few studies have been reported on 2,4-D removal using ion exchange resins. In this study, the removal characteristics of 2,4D by MIEX are evaluated, with the purpose to provide an alternative method to remove 2,4-D from water sources polluted suddenly or seasonally. Various factors affecting removal efficacy of 2,4-D, such as adsorbent dosage, agitation speed, solution pH, solution temperature, inorganic ions, and natural organic matter have been investigated. Also, the adsorption equilibrium, kinetics, and thermodynamics of 2,4D on MIEX have been employed, which are of significance for elucidating the removal mechanism of 2,4-D with MIEX. Accordingly, the study is aimed to (1) investigate the effects of initial 2,4-D concentration and contact time, initial solution pH, agitation speed, temperature, inorganic ions, and natural organic matter on 2,4-D removal; (2) fit the adsorption equilibrium and kinetics data using different isotherm and kinetics models and describe the adsorption state and process of 2,4-D on MIEX; (3) calculate the thermodynamics parameters such as ΔH 0 , ΔG 0 , ΔS0 and discuss the thermodynamic feasibility of 2,4-D adsorption on MIEX.

qt =

(C0 − Ct )V W

(1)

qe =

(C0 − Ce)V W

(2)

where C0, Ct, and Ce are the liquid-phase concentration of 2,4-D (mg L−1) at initial, time t, and equilibrium, respectively. V (L) is the volume of solution and W (mL) represents the volume of adsorbent. All the experiments are carried out in duplicate and the average values are reported. 2.2.2. Analysis Methods and Instruments. The pH of the 2,4-D solution was measured by a pH meter (pHS-3C model, Leici, China). The pH meter was calibrated before each use. The concentration of residual 2,4-D in the adsorption medium was determined by HPLC (Shimadu, Japan). The HPLC system was a UFLC-2010 PLUS system with a diode array PDA-detector. Isocratic separation was performed on a ShimPack VP-ODS, 250 mm ×4.6 mm, C18 column at 40 °C. Mixtures of acetonitrile/ultrapure water (70/30, V/V, 1 mL of formic acid was added to 1000 mL of ultrapure water before mixing) were used as the mobile phase. The flow rate was controlled at 1 mL min−1. The detection wavelength was 284 nm, and the sample volume injected into the HPLC was 20 μL. The correlation coefficient of calibration curve (R2) is 0.9994.

2. MATERIALS AND METHODS 2.1. Materials. 2.1.1. Adsorbent. The MIEX resin used in this study was virgin resin provided by Orica Watercare of Victoria. The resin was rinsed with organic-free deionized water to wash away the fines and stored as slurry in plastic bottles. Prior to use, several milliliters of MIEX resin slurry was taken out and transferred to a 5-mL (± 0.1 mL) glass centrifuge tube. The slurry was diluted with distilled water to a total volume of 5 mL. The 5-mL glass centrifuge tube was then placed in the test tube rack for 30 min to allow the MIEX slurry to settle. Subsequently, the supernatant and superfluous resins were removed and the resins remaining the centrifuge tubes were used in every experiment. 2.1.2. Adsorbate and Chemicals. All chemicals used in this study were obtained from Sinopharm Chemical Reagent Co., Ltd., China. 2,4-D has a chemical purity of 97%, and its physical−chemical properties are shown in Table 1. The standard stock solution of 2,4-D (1000 mg L−1) was prepared by dissolving an accurately weighed amount of 2,4-D in 1 L of methanol (HPLC grade). The samples for a calibration curve were prepared by diluting standard stock solution with ultrapure water obtained from a Millipore Super-Q plus water

3. RESULTS AND DISCUSSION 3.1. Effect of Initial 2,4-D Concentration and Contact Time. Experiments were undertaken to study the effects of varying initial concentration (5, 10, 20 mg L−1) and contact time on 2,4-D removal, and the results are presented in Figure 1. It can be seen clearly from Figure 1a that for each 2,4-D concentration used in this study, the uptake of 2,4-D on MIEX resin increased rapidly with an increase in contact time at the beginning stage of adsorption; thereafter, the adsorption rate decreased gradually, and the adsorption finally reached equilibrium with approximately constant uptake. At the initial stage of adsorption, the vacant adsorption sites and the 11227

dx.doi.org/10.1021/ie300469h | Ind. Eng. Chem. Res. 2012, 51, 11226−11235


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relatively high 2,4-D concentration gradient between aqueous solution and the surface of MIEX resin led to the rapid increase in uptake.30 Along with the extension of adsorption time, the vacant sites on the surface of MIEX resin decreased gradually. And the remaining vacant surface sites were difficult to be occupied due to repulsive forces between the solute molecules on the solid surface and the bulk phases, causing the decrease in adsorption rate.30 With the adsorption proceeding, the available adsorption sites (the working exchange capacity) had been nearly exhausted under the experimental conditions although MIEX resin has a theoretical capacity of 0.52 meq L−1. So the adsorption equilibrium is reached and the uptake of 2,4-D on MIEX resin remains approximately constant. Figure 1a also displays the fact that the higher the initial concentration of 2,4-D, the larger was the amount of 2,4-D adsorbed on MIEX resin. This may be attributed to the full utilization of available sites at high concentration. For the porous MIEX, the active sites include internal sites as well as the external sites. For a lower initial concentration of 2,4-D, the active sites are sufficient to adsorb the 2,4-D, and the 2,4-D may be adsorbed mainly on the exterior surface of MIEX resin due to the fact that compared to the internal active sites, the external sites are the easiest to be occupied by 2,4-D. So the adsorption sites in interior surface of MIEX resin may be not used effectively to some degree, leading to a lower uptake. However, for a higher initial concentration of 2,4-D, the concentration gradient of 2,4-D between the bulk solution and the surface of MIEX resin is also greater.31 The greater concentration gradient may drive the 2,4-D into the internal pores of MIEX resin.32 So the 2,4-D can be also adsorbed on the internal active sites, as well as adsorbed on the external surface. This may be the reason why the uptake of 2,4-D adsorbed on MIEX resin increases with increasing the initial concentration of 2,4-D. In addition, compared to the initial concentrations of 5 and 10 mg L−1, the time needed to reach adsorption equilibrium is longer for the 20 mg L−1 initial concentration (seen in Figure 1a). This may imply that the 2,4D diffuses into the internal pores of MIEX resin under high concentration of 2,4-D condition, which takes longer time.

Figure 1. Effect of initial 2,4-D concentration and contact time on 2,4D removal (conditions: MIEX dosage 1 mL L−1; solution volume 500 mL; temperature 293 K; agitation speed 150 rpm; pH without any adjustment).

Figure 2. Effect of MIEX dosage on the removal of 2,4-D (conditions: 2,4-D concentration 10 mg L−1; solution volume 500 mL; temperature 293 K; agitation speed 150 rpm; adsorption time 90 min; pH without any adjustment). 11228



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volume ratio and in turn removal. However, after the agitation speed is increased to more than 150 rpm, the uptake of 2,4-D on MIEX resin has no significant change and remains approximately constant. The result indicates that proceeding with increasing the agitation speed does not nearly affect the adsorption efficiency of 2,4-D on MIEX resin when the agitation speed is above 150 rpm. Therefore, it can be concluded that the agitation speed of 150 rpm is appropriate to remove 2,4-D from aqueous solutions by MIEX resin. 3.4. Effect of Initial pH of Solution. The pH of solution plays an important role in the whole adsorption system. It influences not only the surface properties of adsorbent but also the degree of ionization of solute in the solution.35 The effects of pH of solution on 2,4-D removal are evaluated by varying initial pH of solution from 3 to 11 with 0.1 M HCl or 0.1 M NaOH solutions. Figure 4 shows the results of adsorptive removal of 2,4-D by MIEX resin at different pH values.

Similar results were also reported for the adsorption of 2,4-D from aqueous solution onto activated carbon.2 Figure 1b demonstrates the percent removal of 2,4-D at different initiate concentrations. The percent removals are approximately close for 5 and 10 mg L−1 2,4-D concentrations, but they are much smaller for 20 mg L−1 2,4-D concentration. This is attributed to the fact that for 0.5 mL of MIEX resin, the available adsorption sites are sufficient to remove lower concentration 2,4-D (20 mg L−1) under the experimental conditions although MIEX resin has a theoretical exchange capacity of 0.52 meq L−1. 3.2. Effect of Adsorbent Dosage. Figure 2 shows the effects of different MIEX dosage on the removal of 2,4-D. The percent removal increases from 71.59% to 97.91% after 90 min adsorption, as the MIEX dosage is increased from 0.2 to 1.2 mL L−1. This may be attributed to the increased surface area and the more provided sorption sites with increasing MIEX resin dosage.27 However, the uptake of 2,4-D adsorbed on MIEX resin decreases from 35.73 to 8.15 mg mL−1. This may be due to the fact that considering 10 and 1.2 mL L−1 represents 7% ion exchange capacity of MIEX resin (based on MIEX capacity of 0.52 meq mL−1), some sorption sites remain unsaturated during the sorption process. 3.3. Effect of Agitation Speed. The agitation speed is an important parameter in any mass transfer phenomena. To evaluate the effects of the agitation speed on the removal of 2,4D, the agitation speed is varied from 50 to 300 rpm in this study. As can be seen in Figure 3, the uptake of 2,4-D on MIEX resin increases progressively with an increase in the agitation

Figure 4. Effect of initial pH of solution on the removal of 2,4-D (conditions: 2,4-D concentration 10 mg L−1; solution volume 500 mL; adsorbent dosage 1 mL L−1; temperature 293 K; agitation speed 150 rpm; adsorption time 90 min).

It is observed from Figure 4 that the uptakes of 2,4-D adsorbed on MIEX resin are relatively higher (changing up and down around 8.0 mg mL−1) when the pH of solution ranges from 5 to 9. At pH 5−9, 2,4-D is mainly in presence of anionic state in aqueous solution,35 thus enabling it to be removed from aqueous solution by exchanging with active sites of MIEX resin. However, the uptake of 2,4-D adsorbed on MIEX resin decreases rapidly to below 4.0 mg mL−1 at a pH of 3. As 2,4-D has a pKa of 3.55, 78% of the molecules will be in neutral form at pH 3, which cannot be removed by ion exchange. As the 2,4D uptake is still about 4.0 mg mL−1, it appears that besides ion exchange interaction, the adsorption action of 2,4-D molecules on the surface of MIEX resin is still significant due to the relatively large surface area of MIEX resin. Figure 4 also shows that the uptake of 2,4-D on MIEX resin decreases greatly to 1.71 mg mL−1 with increasing pH to 11. Although the anionic form of 2,4-D is the predominant species in strong alkaline medium due to deprotonation,36 the affinity of hydroxyl ions to the active sites of MIEX resin is much greater than that of the anions of 2,4-D. In other words, the competitive adsorption of hydroxyl ions for the active sites leads to the considerable decrease in the removal efficacy of 2,4-

Figure 3. Effect of agitation speed on the removal of 2,4-D (conditions: 2,4-D concentration 10 mg L−1; solution volume 500 mL; adsorbent dosage 1 mL L−1; temperature 293 K; adsorption time 90 min; pH without any adjustment).

speed from 50 to 150 rpm, and reaches maximum value (9.58 mg mL−1) at the stirring speed of 150 rpm. This increase in uptake can be attributed to the decrease in boundary layer thickness around the adsorbent particles being a result of increasing the degree of mixing.33 Also, it may be due to the fact that severe agitating promotes a certain turbulence which insures an intimate contact between the phases.34 In addition, the increased uptake may be related to reduced MIEX agglomeration with agitation, thus increasing surface area to 11229



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SO42 > CO32− > Cl−. Compared to the chloride, the dianions (SO42−, CO32−) have larger negative effect on the removal of 2,4-D. Further, for the SO42− and CO32− with the same charges, the sulfate has larger negative effect due to the smaller size. The reason the OH− has a more adverse effect on the removal of 2,4-D needs to be investigated in future study. 3.7. Effect of Natural Organic Matter. Usually, raw surface waters contain natural organic matter (NOM) in addition to the inorganic ions. Batch experiments were carried out to investigate the effects of different concentrations of NOM on the removal of 2,4-D on MIEX resin. Humic acid was used to prepare the different concentrations of NOM. The results are presented in Figure 6. From these results, we know

D under this strong alkaline condition. Additionally, the excess hydroxyl ions in strong alkaline medium make the surface of MIEX resin negatively charged, which restricts the anions of 2,4-D onto the surface of MIEX resin due to electrostatic repulsion force.37 This may be also a reason leading to a decrease in uptake of 2,4-D on MIEX resin in strong alkaline medium. 3.5. Effect of Temperature of Solution. The effects of temperature of solution on the removal of 2,4-D were examined at different temperatures from 283 to 333 K. The results (figure not given) show that the uptake of 2,4-D on MIEX resin fluctuates within the range of 9.27−9.48 mg mL−1. This indicates that the uptake dose not vary obviously at these temperatures investigated in this study, implying that the temperature of solution has less effect on 2,4-D removal. So the temperature of solution is not a predominant factor in the removal of 2,4-D by MIEX resin. 3.6. Effect of Coexistent Anions. Different inorganic anions, such as sulfate (SO42−), chloride (Cl−), and carbonate (CO32−), are always found in natural water bodies. They may disturb or affect the normal adsorption of 2,4-D on MIEX resin. Therefore, batch experiments were carried out to investigate the effect of different type anions on 2,4-D removal. Certain amounts of sulfate, chloride, hydroxyl, and carbonate were added into a series of 2,4-D solutions with the concentration of 10 mg L−1 to prepare these test solutions containing equivalent concentrations of anions (1 meq L−1). We fix the concentration of each anion as 1 meq L−1 in order to make a convenient comparison with effect degree of each type anion. The results are demonstrated in Figure 5.

Figure 6. Effect of HA on the removal of 2,4-D (conditions: 2,4-D concentration 10 mg L−1; solution volume 500 mL; adsorbent dosage 1 mL L−1; adsorption time 90 min; agitation speed 150 rpm; temperature 293 K; pH without any adjustment).

that the uptake of 2,4-D on MIEX resin has a small decline by 10% due to the competitive adsorption of NOM when the DOC concentration increases from 4.20 to 15.8 mg L−1. The small decrease in uptake indicates that the natural organic matter is not a predominant factor affecting the removal of 2,4D by MIEX resin. 3.8. Adsorption Isotherm. An adsorption isotherm describes the distribution of adsorbate between that on the surface of adsorbent and that in the liquid phase at equilibrium at a constant temperature, and it can be used to compare the adsorption performances of different adsorbents and illustrate adsorption behavior.38 Batch tests were conducted to study the adsorption equilibrium of 2,4-D on MIEX resin at 293 K. MIEX resins (0.25 mL) were added into a series of 500 mL portions of 2,4D solutions with various initial concentrations (2, 4, 6, 8, 10, 12, 14, 16, 18, 20 mg L−1), respectively. These adsorption experiments lasted 180 min, which was sufficient to reach equilibrium. The experimental results are depicted in Figure 7. Five isotherm models (given in Table 2), namely Langmuir,39 Freundlich,40 Temkin,41 Redlich−Peterson,42 and Sips,43 were used to fit the data of equilibrium. The model parameters and correlation coefficients (R2) were obtained by nonlinear regression using Origin Pro 8.5 software. For different isotherm models, both correlation coefficient and

Figure 5. Effect of inorganic anion on the removal of 2,4-D (conditions: 2,4-D concentration 10 mg L−1; solution volume 500 mL; adsorbent dosage 1 mL L−1; adsorption time 90 min; agitation speed 150 rpm; temperature 293 K; pH without any adjustment;).

As seen in Figure 5, the removal rate of 2,4-D is 95.50% when no other anion is present in 2,4-D solution. However, the removal rates of 2,4-D decrease to 88.57%, 87.36%, 80.36%, and 24.12% when chloride, carbonate, sulfate, and hydroxyl of equivalent concentration (1 meq L−1) are dosed, respectively. This shows that these inorganic anions more or less affect the removal outcome of 2,4-D due to competing for the active sorption sites. Figure 5 also shows that the impact degree of each type anion on 2,4-D removal is in the order of OH− > 11230



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The model parameters, correlation coefficient, and λ2 of different isotherm models are also given in Table 2. These curves fitted by different isotherm models above are also given in Figure 7 along with the experimental data. Table 2 clearly exhibits that the R2 value (0.9376) of the Temkin model is the smallest among the isotherm models used in this study, and the λ2 value (62.8598) is the largest. Additionally, the curve fitted by Temkin model considerately deviates from the experimental values (shown in Figure 7). So it can be concluded from these results that the Temkin isotherm model is not suitable to describe the equilibrium adsorption of 2,4-D on MIEX resin. Compared with the Temkin model, both Freundlich and Langmuir models give better fit with higher R2 values (0.9926 and 0.9897) and smaller λ2 values (0.4117 and 1.1480), respectively. But the fitting of the Freundlich model is slightly better than that of the Langmuir model when comparing the values of R2 and λ2. Usually, ion exchange resins are typically considered to be consistent with Langmuir model. But the Langmuir model needs to conform to some harsh terms such as homogeneous surface of adsorbent, a monolayer adsorption of adsorbate on the surface of adsorbent, and no interaction between the adsorbed species.44 However, it is very difficult for an adsorption to completely fit these assumptions due to the complexity of adsorption system. Accordingly, the adsorption isotherm for ion exchange deviates from the Langmuir model]. This may be a reason why the isotherm of 2,4-D adsorption on MIEX deviates from the Langmuir model in this study. Some researchers also found that anions removal by ion-exchange resin did not fit the Langmuir isotherm model.45 For the Freundlich model, it is an empirical model and free of any restriction on adsorption. This may be the reason why the Freundlich model gave a better simulation to the equilibrium adsorption of 2,4-D on MIEX resin than that of Langmuir model. The constant of Freundlich model, nF, reflects how favorably adsorption occurs. Adsorption is called preferential adsorption when the 1/nF ranges between 0 and 1.32 As can be seen in Table 2, 1/nF value of the fitted Freundlich isotherm equation is 0.5489, which is between 0 and 1, revealing that the adsorption behavior of 2,4-D on the MIEX resin is favorable. Langmuir, Freundlich, and Temkin isotherm models are all equations with two parameters. Redlich−Peterson and Sips isotherm models, however, contain three parameters. Table 2 demonstrates Redlich−Peterson and Sips models can best describe the equilibrium adsorption of 2,4-D on MIEX resin with approximately identical values of R2 (0.9952 and 0.9957) and λ2 (0.1828 and 0.1665), respectively. Also, the two curves fitted by Redlich−Peterson and Sips models are the closest to the experimental data, indicating that Redlich−Peterson and Sips models are the most appropriate models to simulate the adsorption equilibrium. This may be because Redlich−Peterson

Figure 7. Equilibrium isotherms of 2,4-D adsorbed on MIEX resin at 293 K (conditions: adsorbent dosage 0.5 mL L−1;solution volume 500 mL; adsorption time 180 min; agitation speed 150 rpm; pH without any adjustment).

Table 2. Isotherm Models, Parameters, Correlation Coefficient, and Chi-Square isotherm model

expression qe = qmkLCe/ (1 + KLCe) qe = kFCe1/nF

Langmuir Freundlich

qe = RTln(atCe)/ bt qe = kRPCe/(1 + ARPCeBRP)

Temkin Redlich− Peterson

qe = kSCeßs/(1 + aSCeßs)

Sips

R2

λ2

kL = 0.5160; qm = 46.883 kF = 15.5080; nF = 1.8218, 1/nF = 0.5489 bt = 298.3786; at = 9.1554

0.98968

1.148

0.99264

0.4117

0.93759

62.8598

kRP = 49.7886; ARP = 2.1417; BRP = 0.6283 ks = 19.5676; as = 0.23995; ßs = 0.72904

0.99524

0.1828

0.99569

0.1665

model parameters

Chi-square test are used to measure the goodness of fit. The Chi-square test can be defined by eq 3: m

λ2 =

∑

(qe ,exp − qe ,calc)

i=1

2

qe ,calc

(3)

−1

where qe,calc (mg mL ) is equilibrium capacity of 2,4-D adsorbed on MIEX resin and obtained from isotherm models, qe,exp (mg mL−1) is the equilibrium capacity obtained from the experimental data, and m is the number of experimental data. If the values calculated from models are close to the experimental data, λ2 will be a small number, which indicates a better fit.

Table 3. Kinetic Models, Model Parameters, Correlation Coefficients, and Standard Deviations pseudo-first-order model

pseudo-second-order model

Elovich model

ln(qe − qt) = lnqe − k1t

1/k2qe2

qt = ln(αβ)/β + lnt/β

t/qt =

C0 (mgL−1)

qe,exp (mg mL−1)

qe,cal (mg mL−1)

k1 (min−1)

R2

SD (%)

qe,cal (mg mL−1)

k2 (mL mg−1 min−1)

5 10 20

4.7889 9.4865 18.2213

3.1794 7.1199 11.7306

0.061 0.057 0.041

0.91947 0.9532 0.8625

43.6 34.3 48.63

5.03 9.98 19.49

0.0358 0.0144 0.0054

11231

+ t/qe

R2

SD (%)

ln (αβ)/β

1/β

R2

SD (%)

0.99857 0.9984 0.99777

8.31 7.52 11.22

1.1719 1.5663 1.2993

0.8188 1.7605 3.7287

0.8428 0.8428 0.87139

10.15 10.31 15.83



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and Sips models incorporate the features of both Langmuir and Freundlich models.43 This further shows that the adsorption isotherm deviates from the Langmuir model. 3.9. Adsorption Kinetics. Adsorption kinetics is mainly to study adsorption reaction rate. It is important to quantitatively describe the adsorption rate of 2,4-D on MIEX resin using different kinetic models, as the results can be used to design and predict the adsorption process. The pseudo-first-order,46 pseudo-second-order,47 and Elovich48 kinetic models (given in Table 3) are used to fit the kinetic data in this study. Figure 8a, b, and c depict the plots fitted by the three models above, respectively. Besides the correlation coefficient (R2), a standard deviation (SD) is also used to quantitatively compare the applicability of different kinetic models. A standard deviation can be calculated as follows:49

SD =

∑ [(qexp − qcalc)/qexp] n−1

2

(4)

where n is the number of data points, qt,exp is the experimental value, and qt,calc is the calculated value by model. Higher R2 and lower SD represent a better fit to the kinetic data. The model parameters R2 and SD obtained from the three models above are also listed in Table 3. As far as the pseudo-first-order model is concerned, Figure 8a does not show good fitting to the kinetic adsorption data of 2,4D on MIEX resin at different initial 2,4-D concentrations for the initial sorption period (before 60 min). Table 3 shows that the R2 values (0.8625−0.9532) of the pseudo-first-order model are relatively high. However, the standard deviations with the values of 34.30−48.63% are much greater than those of pseudosecond-order and Elovich models. In addition, Table 3 also demonstrates that the equilibrium uptakes (3.18−11.73 mg mL −1 ) calculated by the pseudo-first-order model are considerably different from the equilibrium uptakes obtained from the experiments. All these results suggest the sorption process of 2,4-D on MIEX resin does not follow the pseudofirst-order rate expression of Langergren. As for the pseudo-second-order model, Figure 8b shows that it gives a very good linear fitting to the adsorption of 2,4-D on MIEX resin at different initial 2,4-D concentrations. Also, the highest R2 values (>0.997) and the smallest SD values (8.31− 11.22%) show that the pseudo-second-order kinetic model among the models used in this study can best describe the adsorption kinetics of 2,4-D on MIEX resin. In addition, the qe,calc values calculated by the pseudo-second-order kinetic model also agree very well with the experimental data, further confirming the applicability of the pseudo-second-order equation. The best fitting to the adsorption data by the pseudo-second-order model implies that the adsorption process of 2,4-D on MIEX resin may be predominated by chemisorption involving valency forces through sharing or exchange of electrons between the active sites and 2,4-D.19 The Elovich kinetic model does not give a good fitting to the adsorption data, which can be seen in Figure 8c. In addition, although the SD values (10.15−15.83%) of the Elovich model are satisfactory, the R2 values (0.8428−0.8714) are the smallest among the models used in this study (shown in Table 3). Accordingly, it is not appropriate to describe the adsorption process of 2,4-D on MIEX resin using the Elovich kinetic model.

Figure 8. Plots using different kinetic models to fit the adsorption of 2,4-D on MIEX resin at different initial concentrations: (a) pseudofirst-order model; (b) pseudo-second-order model; (c) Elovich model.

Kinetic process is controlled by various mechanisms and steps in the adsorption or ion-exchange phenomena. Four major rate-limiting steps are generally cited:50 (1) mass transfer of solute from bulk solution to the boundary film; (2) mass transfer of solute from boundary film to surface of adsorbent; (3) internal diffusion of solute within the pores of adsorbent; 11232



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thermodynamic feasibility and spontaneous nature of 2,4-D adsorbed on MIEX resin in this study. These parameters can be obtained from the following equations:53

and (4) sorption and/or ion exchange onto sites. Among them, the fourth step is assumed to be very fast and does not represent the rate limiting step of the whole adsorption process.51 And the first and second step can be attributed to the film diffusion. The third one is an internal particle diffusion resistance step. The intraparticle diffusion model proposed by Weber and Morris52 is used to analyze the adsorption data of 2,4-D on MIEX resin in this study. The equation is as follows: qt = kidt 1/2 + C

Keq =

Cad , e Ce ΔS 0 ΔH 0 − R RT

(7)

ΔG 0 = ΔH 0 − T ΔS 0

(8)

ln Keq =

(5)

where kid (mg mL−1 min−1/2) is the intraparticle diffusion rate constant and C is a constant. According to the eq 5, if the kinetic process is dominated by the intraparticle diffusion, the plot of qt versus t1/2 should be a straight line. And if the intercept of the plot (C) is equal to 0, then the intraparticle diffusion is the only rate limiting step.52 As can be seen from Figure 9, the plots of qt versus t1/2 do not give linear forms within the whole time. This shows that the

(6)

−1

where Keq (L mL ) is the thermodynamic equilibrium constant, Cad,e (mg mL−1) is the equilibrium adsorption capacity of 2,4-D on MIEX resin at different temperature, T (K) is the absolute temperature, R (8.314 J mol−1 k−1) is universal gas constant. The thermodynamic parameters for the adsorption of 2,4-D on MIEX resin are listed in Table 4. Table 4. Thermodynamic Parameters for 2,4-D Adsorption by MIEX Resin T (K)

Keq (L mL−1)

ΔG0 (kJ mol−1)

ΔH0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

283 288 293 298

13.03 13.35 15.18 18.12

−5.945 −6.325 −6.706 −7.086

15.580

76.060

The positive value of ΔH0 (15.580 kJ mol−1) indicates the adsorption of 2,4-D on MIEX resin is an endothermic reaction, and the adsorption occurs easily at higher temperature. The positive ΔS0 (76.060 J mol−1 k−1) reflects a good affinity of 2,4D toward the MIEX resin and the increasing randomness at the solid-solution interface after the adsorption.54 The negative values of ΔG0 show the adsorption is thermodynamically feasible and spontaneous.55 Also, the ΔG0 value decreases from −5.945 to −7.086 kJ mol−1 with the temperature being increased from 283 to 298 K. This suggests the adsorption of 2,4-D on MIEX resin increases with an increase in temperature. 3.11. Comparison with Other Adsorbents. The maximum monolayer adsorption capacity (qmax) obtained from the Langmuir isotherm model is used to compare with other adsorbents reported in the literature (shown in Table 5).2,56−62 It can be observed from Table 5 that MIEX resin gives a relatively high adsorption capacity (293 mg g−1) which is a comparable sorption capacity with other adsorbents. This indicates good potential for MIEX resin to remove 2,4-D from raw water.

Figure 9. Intraparticle diffusion model of 2,4-D on MIEX resin.

adsorption of 2,4-D on MIEX resin does not follow the intraparticle diffusion model within the whole adsorption time. But the plots present a multilinearity, which indicates that two or more steps occur in the adsorption process. In addition, every linear section of these plots deviating from the origin also shows that the intraparticle diffusion is not the only rate limiting step. Accordingly, the adsorption of 2,4-D on MIEX resin may be illustrated by the following three steps:37 (1) an initial portion is attributed to rapid external diffusion and the surface adsorption; (2) the second linear portion is the gradual adsorption stage where the intraparticle diffusion is the rate limiting factor; (3) the third portion is the final equilibrium stage where the intraparticle diffusion obviously starts to slow down due to the lower 2,4-D concentration in solution. 3.10. Thermodynamic Analysis. Adsorption thermodynamics focuses on the energy change of adsorption system before and after adsorption, which is significant to better understand the thermodynamic property of adsorption behavior. The standard Gibbs free energy change (ΔG0, kJ mol−1), standard enthalpy change (ΔH0, kJ mol−1), and standard entropy change (ΔS0, J mol−1 k−1) are usually used to describe the energy change of adsorption system. The three thermodynamic parameters above are calculated to evaluate the

4. CONCLUSIONS The present study shows that MIEX resin can potentially be employed as an adsorbent for removing 2,4-D from raw water. The following conclusions can be obtained from the experimental results: 1. The uptake of 2,4-D on MIEX resin increases with increasing the initial 2,4-D concentration, but temperature of solution has less effect on 2,4-D removal. 2. The stirring intensity of 150 rpm and the pH of 5−9 are optimal to remove 2,4-D from aqueous solutions by MIEX resin. 11233



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(3) Mantilla, A.; Tzompantzi, F.; Fernandez, J. L.; Gongora, J.; Mendoza, G.; Gomez, R. Photodegradation of 2,4-Dichlorophenoxyacetic Acid Using Znalfe Layered Double Hydroxides as Photocatalysts. Catal. Today 2009, 148, 119. (4) Ignatowicz, K. Selection of Sorbent for Removing Pesticides During Water Treatment. J. Hazard. Mater. 2009, 169, 953. (5) H, G. D.; A, P. M. Review of 2,4-Dichlorophenoxyacetic Acid (2,4-D) Epidemiology and Toxicology. Crit. Rev. Toxicol. 2002, 32, 233. (6) Derylo-Marczewska, A.; Blachnio, M.; Marczewski, A. W.; Swiatkowski, A.; Tarasiuk, B. Adsorption of Selected Herbicides From Aqueous Solutions On Activated Carbon. J. Therm. Anal. Calorim. 2010, 101, 785. (7) Han, D. M.; Jia, W. P.; Liang, H. D. Selective Removal of 2,4Dichlorophenoxyacetic Acid From Water by Molecularly-Imprinted Amino-Functionalized Silica Gel Sorbent. J. Environ. Sci.-China 2010, 22, 237. (8) Ministry Of Health Of China. Drinking Water Standards; Standards Press of China: Bei Jing, 2006. (9) Chingombe, P.; Saha, B.; Wakeman, R. J. Effect of Surface Modification of an Engineered Activated Carbon On the Sorption of 2,4-Dichlorophenoxy Acetic Acid and Benazolin From Water. J. Colloid Interface Sci. 2006, 297, 434. (10) Giri, R. R.; Ozaki, H.; Ota, S.; Taniguchi, S.; Takanami, R. Influence of Inorganic Solids On Photocatalytic Oxidation of 2,4Dichlorophenoxyacetic Acid with UV and TiO2 Fiber in Aqueous Solution. Desalination 2010, 255, 9. (11) Gao, J. X.; Zhao, G. H.; Shi, W.; Li, D. M. Microwave Activated Electrochemical Degradation of 2,4-Dichlorophenoxyacetic Acid at Boron-Doped Diamond Electrode. Chemosphere 2009, 75, 519. (12) Huston, P. L.; Pignatello, J. J. Degradation of Selected Pesticide Active Ingredients and Commercial Formulations in Water by the Photo-Assisted Fenton Reaction. Water Res. 1999, 33, 1238. (13) Roe, B. A.; Lemley, A. T. Treatment of Two Insecticides in an Electrochemical Fenton System. J. Environ. Sci. Health B 1997, 32, 261. (14) Oturan, M. A. An Ecologically Effective Water Treatment Technique Using Electrochemically Generated Hydroxyl Radicals for in Situ Destruction of Organic Pollutants: Application to Herbicide 2,4-D. J. Appl. Electrochem. 2000, 30, 475. (15) Cycon, M.; Zmijowska, A.; Piotrowska-Seget, Z. Biodegradation Kinetics of 2,4-D by Bacterial Strains Isolated From Soil. Cent. Eur. J. Biol. 2011, 6, 188. (16) de Rezende, E.; Peralta-Zamora, P. G.; Abate, G. Sorption Study of Herbicides with the Clay Minerals Vermiculite and Montmorillonite. Quim. Nova 2011, 34, 21. (17) Kumar, D.; Prakash, B.; Pandey, L. K.; Gaur, J. P. Sorption of Paraquat and 2,4-D by an Oscillatoria Sp.-Dominated Cyanobacterial Mat. Appl. Biochem. Biotechnol. 2010, 160, 2475. (18) Vergili, I.; Barlas, H. Removal of 2,4-D, Mcpa and Metalaxyl From Water Using Lewatit Vp Oc 1163 as Sorbent. Desalination 2009, 249, 1107. (19) Deng, S. B.; Ma, R.; Yu, Q.; Huang, J.; Yu, G. Enhanced Removal of Pentachlorophenol and 2,4-D From Aqueous Solution by an Aminated Biosorbent B-8504-2011 F-6806-2010. J. Hazard. Mater. 2009, 165, 408. (20) Boyer, T. H.; Singer, P. C. A Pilot-Scale Evaluation of Magnetic Ion Exchange Treatment for Removal of Natural Organic Material and Inorganic Anions. Water Res. 2006, 40, 2865. (21) Hsu, S.; Singer, P. C. Removal of Bromide and Natural Organic Matter by Anion Exchange. Water Res. 2010, 44, 2133. (22) Fearing, D. A.; Banks, J.; Guyetand, S.; Eroles, C. M.; Jefferson, B.; Wilson, D.; Hillis, P.; Campbell, A. T.; Parsons, S. A. Combination of Ferric and Miex (R) for the Treatment of a Humic Rich Water. Water Res. 2004, 38, 2551. (23) Kitis, M.; Harman, B. I.; Yigit, N. O.; Beyhan, M.; Nguyen, H.; Adams, B. The Removal of Natural Organic Matter From Selected Turkish Source Waters Using Magnetic Ion Exchange Resin (Miex (R)). React. Funct. Polym. 2007, 67, 1495.

Table 5. Comparison of MIEX Resin with Other Adsorbents for the Removal of 2,4-D adsorbent

qmax(mg g−1)

ref

oil palm frond activated carbon MIEX resin DSAC carbonaceous adsorbent banana stalk activated carbon granular activated carbon HT500 black carbon OP2CEC BF sludge DP2CEC BF dust OP1CEC DP1CEC

352.89 293 238.1 212.1 196.33 181.82 179.673 (813 μmol/g) 64 42.02 30 25.77 21 12.24 9.14

56 this work 2 57 58 59 60 61 62 57 62 57 62 62

■

3. The natural organic matter and inorganic anions in aqueous solutions more or less affect the removal 2,4-D, and the impact degree of inorganic anions is in the order of OH− > SO42− > CO32− > Cl−. 4. The Redlich−Peterson and Sips models are the most appropriate models to simulate the adsorption equilibrium at 293 K. 5. The pseudo-second-order kinetic model can best describe the adsorption of 2,4-D on MIEX resin, and the intraparticle diffusion is not the only rate limiting step. 6. The adsorption of 2,4-D on MIEX resin is a thermodynamically spontaneous and endothermic reaction in nature.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-13311875226. Address: Room 215, Mingjing Building Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Foundation of Key Laboratory of Yangtze River Water Environment of Ministry of Education of P. R. China (YRWEY1005), Key Natural Science Project for University of Anhui Province (KJ2011Z042), Research Fund of Anhui University of Technology for Young Teachers (QZ201008), and Innovation Research Funds of Anhui University of Technology for graduate (2011018).

■

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Adsorptive Removal of 2,4-Dichlorophenoxyacetic Acid (2,4-D) from

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