ARTICLE pubs.acs.org/IECR
Magnetic Polyacrylic Anion Exchange Resin: Preparation, Characterization and Adsorption Behavior of Humic Acid Chendong Shuang, Fei Pan, Qing Zhou, Aimin Li,* and Penghui Li State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing 210046, People’s Republic of China
Weiben Yang College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, People’s Republic of China
bS Supporting Information ABSTRACT: Magnetic polyacrylic resin (R0) was prepared by suspension polymerization in the presence of γ-Fe2O3 and γ-methacryloxypropyltrimethoxysilane (γ-MPS). The obtained magnetic resin (R0) was modified sequentially by ammonolysis and alkylation to produce a magnetic weakly basic anion exchange resin (R1) and a magnetic strongly basic anion exchange resin (R2). Infrared (IR) spectra, elemental analysis, scanning electron microscopy (SEM), magnetization, BrunauerEmmettTeller (BET) surface area and chemical analysis were all determined to characterize these resins. The investigation of humic acid (HA) adsorption on R0, R1, and R2 showed that chemical interactions between the functional groups of the resin and HA were responsible for HA adsorption. The adsorption capacity of HA on R1 was ∼20%60% less than that of R2, but the R1 resin was easier to regenerate with NaOH aqueous solution. A mixture of 1% NaCl and 1% NaOH was found to be highly efficient for the regeneration of both R1 and R2 resins, with desorption efficiencies measuring >90%. Pseudo-second-order model and Freundlich equation fit better for HA adsorption onto R1 and R2 than other typical kinetic and thermodynamic models, respectively. The effects of pH, coexisting salts (i.e., NaCl and Na2SO4), and recyclability were also assessed.
1. INTRODUCTION Natural organic matter (NOM) commonly exists in various natural water systems and can cause unfavorable color, taste, and odor. It also contributes to adverse effects, such as membrane fouling, in water treatment processes. Furthermore, NOM can result in the formation of disinfection byproducts (DBPs) and bacterial regrowth (BRG) in drinking water systems, both of which threaten human health. Anion exchange is considered to be an effective method for NOM removal, because most NOM fractions are weakly acidic.14 The anion exchange process typically used for water treatment occurs in fixed-bed columns filled with anion exchange resin (AER); such resin beads range from 0.3 mm to 1.2 mm in diameter. However, the full-scale application of fixed-bed columns for water treatment has some inevitable shortcomings, such as high capital costs and limited flux.57 MIEX resin, developed by Orica, is a magnetic polyacrylic AER with a macroporous structure that has fast adsorption kinetics, because of its small size.8 It can be used in completely mixed contactors processes for ion exchange, in which the resin is mixed with water for adsorption and self-agglomerated for separation.9,10 Many MIEX full-scale systems have been reported for the removal of NOM in drinking water all over the world.3 The efficient removal of NOM, heavy metals, and dyes by varies magnetic resins during water purification has recently been reported.1115 For NOM removal, most works have suggested that AER should be acrylic and macroporous and should have high water content, as well as a large adsorption capacity.1 The removal of NOM in drinking water treatment by MIEX resin r 2011 American Chemical Society
reduced the formation of THMs and HAAs, both of which are typical DBPs.8 Furthermore, the MIEX pretreatment of drinking water can improve other problems in succeeding treatment, such as reducing the coagulant dosage or protecting the membrane from fouling.16,17 However, the removal of hydrophobic NOM by MIEX resin was reported to be much less efficient than hydrophilic NOM for raw waters.18 Humic acid (HA), as a hydrophobic fraction of NOM, can cause the AER fouling problem.19,20 Because commercial resins for selection are usually nonmagnetic and have different structures or sizes than MIEX, knowledge of the adsorption performance for other magnetic resins has been very limited. In general, methods for the preparation of magnetic resins can be classified into two main categories, which are in situ formation and copolymerization. For in situ formation, the magnetic particles (MPs) are typically formed within pores or active sites of the prepared resin.21,22 However, the adsorption or desorption behavior might be affected by pore blocking or site shielding by the formed MPs. Furthermore, these MPs are prone to corrosion. For copolymerization, monomers are polymerized in the presence of MPs that were previously pretreated23 or modified with a surfactant or coupling agent.2427 The use of a solid dispersant for copolymerization is an alternative path, Received: July 12, 2011 Accepted: November 28, 2011 Revised: November 26, 2011 Published: November 28, 2011 4380
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Industrial & Engineering Chemistry Research because the dispersant has an affinity for the MPs and could react with the monomer.28−30 Although the use of additional reagents might increase costs, the copolymerization process for preparing magnetic resin is more suitable for full-scale application when considering the life cycle of the resin. Until now, styrenic and acrylic monomers are widely used for the production of AERs. Acrylic monomers have caused increased attention, because the preparation of styrenic AER typically involves the use of chloromethyl methyl ether, which is carcinogenic. The primary objective of the present work was to develop a new method for the preparation of a magnetic polyacrylic AER for applications involving water treatment. The resin was designed as a magnetic, acrylic structure with ammonium groups and was prepared sequentially with three steps of polymerization, ammonolysis, and alkylation. To introduce MPs into the polymer matrix, γ-methacryloxypropyltrimethoxysilane (γ-MPS) was added during polymerization. The γ-MPS molecule has a hydrolyzable group that facilitated adhesion of the MPs and an unsaturated bond for reaction with the monomer. The predicted resins prepared after polymerization, ammonolysis and alkylation were magnetic polyacrylic resin (R0), weakly basic magnetic AER (R1), and strongly basic magnetic AER (R2). Each of these three forms were prepared and characterized. Their adsorption performances were investigated using commercial HA as a model compound, because HA is a significant component of NOM. It was reported that the charge density of NOM is the most important factor for anion exchange,31 HA could be used as a model of NOM, to some extent, because HA is similar to other NOMs that contain carboxyl and hydroxyl groups in their structure. However, the hydrophobility, molecular size, and aromaticity are commonly higher than other fractions of NOMs. The results obtained from HA adsorption provided a feasible method for the removal of HA or treatment of raw water containing high content of HA. Moreover, AER fouling caused by HA was previously reported.19,20 Hence, regeneration and recyclability of the resins were further investigated.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Methyl acrylate (MA) (g98.5%), benzoyl peroxide (BPO), (g98%), and inorganic salts were purchased from Sinopharm Chemical Reagent Company in China. Inorganic salts are all analytical reagents. Commercial γ-Fe2O3 was provided by Cathay Pigments (China), Ltd. 200# solvent naphtha (SN200) was obtained from Zhongchao Chemical Co., Ltd. (China). Industrial products of divinylbenzene (DVB) (63.3%), γ-methacryloxypropyltrimethoxysilane (γ-MPS) (g99%), poly(vinyl alcohol) (PVA) (1788), poly(vinyl pyrrolidone) (PVP) (K90, BASF Company), N,N-dimethyl-1,3-propanediamine (DMPDA) (>99%), and monochloromethane (MCM) (>98%) were all purchased from commercial sources. Alfa humic acid (HA) was purchased from J&K Chemical, Ltd. 2.2. Preparation. The preparation of magnetic anion exchange resin (AER) required polymerization, ammonolysis, and alkylation. For polymerization, 84.2 g of MA, 15.8 g of DVB, 1.0 g of BPO, and 2.0 g of γ-MPS were all mixed in a 1-L three-necked flask. Twenty grams (20.0 g) of SN200 was used as porogen. A total of 30.0 g of γ-Fe2O3 was then added to the flask while stirring. After heating at 323 K for 1 h, 450.0 g of an aqueous solution containing 2.0 g of PVA and 1.0 g of PVP was added to the flask. The mixture was allowed to polymerize at 348 K for 12 h with continuous stirring at 300 rpm to obtain the
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magnetic resin (R0). Next, the magnetic resin was separated with the aid of a magnet and washed five times with distilled water at a temperature of 353 K. Finally, the magnetic resin (R0) was dried at 323 K overnight, followed by extraction with acetone for 8 h. Both ammonolysis and alkylation were performed in a 2-L sealed pressure vessel, where 100.0 g of R0 was reacted with 800.0 g of DMPDA at 448 K for ammonolysis. After washing three times with distilled water, all of the ammonolyzed magnetic resin (R1) was alkylated by reacting with MCM at 0.5 MPa in 500 mL 15% NaOH solution. The obtained resin (R2) was rinsed repeatedly with doubly distilled water until the effluent was neutral. 2.3. Characterization. Fourier transform infrared (FTIR) spectroscopy was performed with an FTIR spectrophotometer (Nexus870, Nicolet, USA). The morphology of the magnetic AER beads was observed using scanning electron microscopy (SEM) (Model S-3400N II, Hitachi, Japan). The sample magnetization curves were measured with a vibrating sample magnetometer (VSM) (Quantum Design, Model MPMS-5S), and elemental analysis was performed with an elementary analyzer (EA, Vario Micro, Elementar, Germany). The specific surface area (SSA) and the pore diameter distribution (PDD) of particles were both determined by an automatic analyzer (Micromeritics ASAP-2010C, USA) with nitrogen as the adsorbate. The Brunauer EmmettTeller (BET) and BarrettJoynerHalienda (BJH ) method were employed for SSA and PDD analysis, respectively. The samples were dried under vacuum at 323 K for 8 h before the above-mentioned characterization. The anion exchange capacity was determined by chemical analysis. The pretreatment process proceeded as follows. A total of 30 mL of resin was washed sequentially with 250 mL of HCl aqueous solution (1 mol/L), 250 mL of distilled water, and 250 mL of NaOH aqueous solution (1 mol/L) at a flow rate of 5 mL/min in a glass column. Subsequently, 300 mL of HCl aqueous solution (1 mol/L), 250 mL of distilled water, and NH3NH4Cl aqueous solution (1 mol/L NH4Cl solution with the pH adjusted to 9.25 ( 0.15 with NH4OH solution) were passed through the column sequentially at a flow rate of 7 mL/ min. The resin was then washed with distilled water until the effluent was neutral. For strong base anion exchange capacity (SEC), the pretreated resin was soaked in NaNO3 aqueous solution (1.00 mol/L) for 1 h, and then the capacity was determined by titrating AgNO3 against the released Cl in solution. For weak base anion exchange capacity (WEC), the pretreated resin was soaked in the HCl aqueous solution (0.10 mol/L) for 2 h, and the capacity was obtained by NaOH (0.100 mol/L) titration of the residual HCl in the solution. 2.4. Adsorption. The three resins, R0, R1, and R2, were employed to study HA adsorption. Three conical flasks containing 250 mL of 200 mg/L HA solution were shaken at 293 K with 0.100 g of resin at 130 rpm. The amounts of HA in the solutions at different sampling times were obtained by measurement of the total organic carbon (TOC). Batch experiments at different temperatures (278, 293, and 308 K) were performed at 130 rpm for 100 h. To explore the effect of pH on adsorption, 50 mL of 100 mg/L HA solution and 0.05 g resin were introduced into a series of 100-mL conical flasks with the pH of the solutions varying from 3 to 11, which was adjusted by using 1 mol/L HCl or 1 mol/L NaOH aqueous solution. To determine the effect of NaCl and Na2SO4 on adsorption, the amounts of HA solution and resin were identical as those of the experiments for the pH series. The concentrations of NaCl and Na2SO4 varied 4381
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from 0 to 80 g/L and 0 to 6 g/L, respectively, and the effect of solution chemistry on adsorption was conducted at 293 K for 100 h. All experiments were repeated three times to obtain average values. The adsorption capacity of HA (qt, mg/g) was calculated using eq 1: qt ¼
V ðC0 Ct Þ W
ð1Þ
where V is the volume of solution (given in liters), and W is the weight of the dry resin (given in grams). C0 (mg/L) and Ct (mg/L) represent the initial concentration and concentration at time t (min), respectively. The variables qt and Ct are replaced, respectively, by qe and Ce to represent the adsorption capacity and HA concentration at adsorption equilibrium, respectively. 2.5. Regeneration. A series of parallel experiments investigating HA adsorption were conducted at 293 K with 50 mL of 100 mg/L HA solution and 0.050 g of resin in conical flasks. The resin was filtered after adsorption equilibrium. Distilled water, NaOH aqueous solution, and NaCl aqueous solution were used for regeneration. Distilled water was also used for comparison. The desorption efficiency (DE, %) was calculated using eq 2: Vd Cd DE ð%Þ ¼ Wqe
Figure 1. FTIR spectra of magnetic resin (R0) (spectrum a), ammonolysed magnetic resin (R1) (spectrum b), and magnetic anion exchange resin (R2) (spectrum c).
Table 1. Physicochemical Properties of the Obtained Resins R0
ð2Þ
where Vd is the volume of the desorption agent (in liters), and Cd is the concentration of HA at adsorption equilibrium (mg/L). In addition, the cycles of adsorption/desorption were repeated for 10 times. The HA concentrations of samples from adsorption and regeneration were all determined by a TOC analyzer (OI Analytical, Model Aurora 1030D TOC, USA).
R1
R2
C (%)
43.4
57.1
48.2
H (%)
5.3
9.2
7.6
N (%)
0.07
12.2
8.24
WECa (mmol/g)
0.12
4.11
0.62
SECb (mmol/g)
0.15
1.24
4.17
specific surface area (m2/g) average pore diameter (nm)
3.44 11.25
3.13 9.23
2.02 6.68
a Weak base anion exchange capacity. b Strong base anion exchange capacity.
3. RESULTS AND DISCUSSION 3.1. Preparation. The preparation of magnetic AER was carried out, at first, via suspension polymerization in the presence of γ-Fe2O3, and then functionalization. In the suspension polymerization step, the addition of γ-MPS to the reaction resulted in brick-red polymer beads that could be separated with an applied magnet, while the polymer beads obtained without γ-MPS addition could not. This phenomenon confirmed the successful combination of γ-Fe2O3 to the polymer matrix due to γ-MPS, as expected. The subsequent functionalization involved ammonolysis and alkylation; the reaction times of which had been previously optimized by a kinetic investigation (Figure S.1 in the Supporting Information). In the kinetic study, the nitrogen content (%) decreased slightly with increasing time after 5 h of ammonolysis, which may have been the result of the amide bond breakage at 448 K. For alkylation, increasing the time enhanced the reaction during our 30 h of experiment time, indicating a slow kinetic process between MCM and the ammonolyzed magnetic polymer. However, a reaction time of 18 h was chosen for alkylation, after taking production efficiency into account. 3.2. Characterization. The FTIR spectra of the obtained resins are shown in Figure 1. The absorption band at 1734 cm1 in spectrum (a) in Figure 1, corresponding to CdO, shifted to 1649 cm1 in spectrum (b) in Figure 1, which indicated the success of ammonolysis. In both spectra (b) and (c) in Figure 1, the absorption band at 1548 cm1 was attributed to a bending vibration of the NH bond. The absorption band at 1467 cm1 could be attributed to an asymmetric angular bending of the methyl groups on the quaternary group. Although the magnetic
Figure 2. Scanning electron microscopy (SEM) photograph of the magnetic anion exchange resin (R2).
AER exhibited similar spectra (see spectrum (c) in Figure 1) as the ammonolyzed resin (see spectrum (b) in Figure 1), its higher SEC confirmed the quaternization step (see Table 1). The FTIR spectrum of MIEX resin was used to compare with the obtained AER, as shown in Figure S.2 in the Supporting Information. The significant difference of FTIR spectra between MIEX resin and R2 can be seen. The elemental fractions listed in Table 1 were similar to the theoretically calculated values, which is consistent with the results previously mentioned. The average pore diameters of 4382
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R0, R1, and R2 were 11.25, 9.23, and 6.68 nm, indicating the mesoporous structure of the obtained resins. It was also found that the average pore diameter and specific surface area were reduced significantly (Table 1), which were attributable to the effect of functional groups on pore structure. The surface morphology of the magnetic AER, characterized by SEM, displayed the rough surface of the resin beads that was caused by the presence of needle-shaped γ-Fe2O3 (Figure 2). It was also observed that the resin bead diameters were all distributed around 100 μm, which is ∼38 times smaller than conventional resins. However, the separation of smaller beads is very difficult with mixed contactors. Magnetic separation is considered to be an efficient method to this problem. Figure 3 showed the magnetization curves of the obtained resins. The values of specific saturation magnetization decreased after functionalization because functional groups increased the mass fraction of the polymers. Hysteresis of the magnetization curves indicated the permanent magnetism of the obtained resins. Therefore, the individual beads of the obtained resins could agglomerate into larger aggregates, because of the weak
force interactions between individual beads that were caused by the permanent magnetism. 3.3. Adsorption Kinetics. Figure 4 shows the kinetic data for the adsorption of HA by resins R0, R1, and R2. The amount of HA adsorbed by R1 and R2 increased with time, while the adsorption of HA onto R0 was not detected. Therefore, R0 was not investigated further in this study. Moreover, the negligible amount of HA adsorbed onto R0 excluded the possibility of adsorption by the polymer backbone. This finding suggests that the functional groups cause the adsorption. The HA molecule is generally negatively charged, as a result of its carboxyl and hydroxyl groups. The anionic groups of HA could lead to chemical interactions with the positively charged AERs. To describe the kinetic behaviors of HA adsorbing onto R1 and R2, three different kinetic models were applied: a pseudo-first-order equation, a pseudo-second-order equation, and the Elovich equation. The form of each model is expressed, respectively, as follows. Pseudo-first-order equation: ð3Þ qt ¼ qe 1 expð k1 tÞ Pseudo-second-order equation: k2 tq2e 1 þ k2 tqe Elovich equation:
qt ¼
ð4Þ
qt ¼ ð1=βÞ 3 lnðαβÞ þ ð1=βÞ 3 lnðtÞ
ð5Þ
In eqs 3 and 4, qe is the amount of adsorbed HA at equilibrium (given in units of mg/g), and qt is the amount of adsorbed HA at time t (also given in units of mg/g). The variables k1 (given in units of min1) and k2 (given in units of g/(mg min)) are the rate constants for the first- and second-order equations, respectively. For the Elovich equation (eq 4), the parameter α is the initial adsorption rate (mg/(mg min)), and β is the desorption constant (g/mg). These coefficients were obtained from linear fitting and are listed in Table 2. Similar results were obtained from the kinetic fittings for both resins, indicating a similar mechanism of adsorption for each of resins. The Elovich equation is usually applied to describe chemical adsorption on highly heterogeneous adsorbents, but it does not propose any definite mechanism for adsorbateadsorbent interactions. Instead, the pseudo-secondorder equation was demonstrated to be the best model for HA adsorption, with the highest correlation coefficient between the three equations. For both resins, the calculated values of qe using the pseudo-second-order equation were more similar to the observed values than those from the other equations. The equilibrium adsorption capacity of HA onto R2 was significantly greater than that of R1, indicating that the strongly basic AER was more suitable for HA adsorption than the weakly basic AER. 3.4. Isotherms. The adsorption isotherms of HA adsorbed onto the magnetic AERs R1 and R2 at three different temperatures are illustrated in Figure 5. The equilibrium adsorption capacities of HA increased for both resins at low equilibrium
Figure 3. Magnetization curves of (R0) magnetic resin, (R1) ammonolyzed magnetic resin, and (R2) magnetic anion exchange resin.
Figure 4. Magnetization curves for HA adsorption onto R0, R1, and R2 and second-order modeling for adsorption onto R1 and R2.
Table 2. Kinetic Parameters for HA Adsorption onto Magnetic Anion Exchange Resins R1 and R2 at 293 K First-Order Kinetic Model resin
k1
R1
8.06 104
R2
4
7.88 10
Second-Order Kinetic Model 2
Elovich α
β
R2
0.996
0.5
0.022
0.977
0.997
3.2
0.019
0.979
qe
R
k2
qe
R
197.3
0.963
8.76 106
285.7
253.8
0.969
6.78 106
333.3
4383
2
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Table 3. Obtained Constants for Langmuir and Freundlich Equations at 288, 293, and 308 K Langmuir Model resin
T (K)
KL
Qm
R1
278
0.0031
450.6
293
0.0025
681.3
308 278
0.0024 0.0061
951.8 459.9
293
0.0070
721.6
308
0.0186
833.05
R2
a
Figure 5. Adsorption isotherms of R1 and R2 at 278, 293, and 308 K and their modeling by Langmuir and Freundlich equations.
concentration as the HA concentration or temperature was increased, in accordance with the principles of chemical adsorption. In addition, the equilibrium adsorption capacities of HA by R1 were 20%60% smaller than that of R2. The following Langmuir and Freundich equations were applied to the adsorption isotherm data. Langmuir equation: qe ¼
Q m K L Ce 1 þ K L Ce
ð6Þ
Freundlich equation: qe ¼ KF C1=n e
ð7Þ
In eqs 6 and 7, qe and Ce are the equilibrium adsorption capacity and equilibrium concentration of HA, respectively. (Both are given in units of mg/g.) Qm is the maximum adsorption capacity of the adsorbent (given in units of mg/g). KL is the Langmuir binding constant (expressed in units of L/mg), which is related to the energy of adsorption, and KF is the Freundlich constant, which is related to the adsorption capacity. The constant n for the Freundlich equation gives an indication of the favorableness of their adsorbent/adsorbate system. The obtained modeling constants for both resins are listed in Table 3. In general, the Freundlich equation described the data better than the Langmuir equation, suggesting a heterogeneous adsorption of HA on both magnetic AERs R1 and R2. The heterogeneous adsorption was possibly due to other interactions except ion exchange, such as backbone adsorption. However, the
Freundlich Model 2a
KF
n
R2 a
0.9698
7.911
1.741
0.9907
0.9816
7.968
1.574
0.9915
0.9979 0.9652
8.252 21.37
1.473 2.195
0.9933 0.9795
0.8947
36.22
2.195
0.9792
0.8196
155.38
3.750
0.9702
R
R2 is the correlation constant.
Figure 6. Effect of solution pH on the adsorption of HA onto R1 and R2.
lack of HA adsorption onto R0 from the kinetic study had previously revealed that the polymer backbone did not interact with HA. The heterogeneous adsorption could be explained by mechanisms including (i) electric attraction between resin and HA, (ii) cationπ bonding between resin and HA, and (iii) ππ interaction between dissolved HA and absorbed HA. This explanation is consistent with the results obtained from HA adsorption by a bifunctional resin.32 In addition, the larger values of KF for R2 than R1 indicated a larger adsorption capacity of the strongly basic AER than that of the weakly basic AER, which is consistent with the experimental observations. 3.5. Effects of Solution pH. As shown in Figure 6, the effects of solution pH on the adsorption of HA by R1 and R2 were different. R1 had a higher adsorption capacity for HA than R2 at pH >4.0. The adsorption capacity for R2 was enhanced with increasing pH, because of the ionization improvement of the HA molecules. However, the adsorption capacity for HA onto R1 was not affected as a function of pH throughout the pH range of 3.010.0. The adsorption capacity of R1 decreased significantly at pH >10.0, because the tertiary amine groups of R1 were deprotonated, which reduced the anion exchange capacity. This phenomenon was also observed during the adsorption of perfluorooctane sulfonate (PFOS) onto a weakly basic AER by Deng et al., but the adsorption capacity of PFOS onto a strongly basic resin changed very little as the pH increased.33 This difference in adsorption between PFOS and HA may result from the different degrees to which they are ionized. Because of the decrease in ionization of HA under acidic conditions, the 4384
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Figure 7. Effect of (A) NaCl and (B) Na2SO4 on the adsorption of HA onto R1 and R2.
electrostatic interaction between resin and HA should be reduced. Hence, the adsorption under acidic conditions is most likely due to other interactions, such as cationπ bonding between the resin and HA, and ππ interaction between dissolved HA and absorbed HA. The cationπ bonding is a noncovalent interaction between a cation and the π-bond moleculars. Under acidic conditions, the cationic resin is surrounded by HA molecules, which can act as π-donors. This cationπ interaction between organoclays modified with quaternary ammonium cations and polycyclic aromatic hydrocarbons (PAHs) was reported by Qu.34 The results also showed that the strongly basic AER was more suitable than weakly basic AER for NOM adsorption during water treatment. However, the enhancement of HA adsorption through increased pH indicated that alkaline solutions may not efficiently desorb HA off the strongly basic AER, while the efficiency would be higher for the weakly basic AER. In addition, the concentrations of ferric ion for solutions after adsorption were detected (see Figure S.3 in the Supporting Information). It was found that γ-Fe2O3 incorporated in resin could be eroded when the pH value was 3.0 and 4.0, suggesting that the water treatment by these resins should not occur under acidic conditions at pH
