Environ. Sci. Technol. 2005, 39, 3308-3313
Sorption Enhancement of Aromatic Sulfonates onto an Aminated Hyper-Cross-Linked Polymer BINGCAI PAN,* QUANXING ZHANG, FANWEI MENG, XIAOTIAO LI, XIAO ZHANG, JIANZHONG ZHENG, WEIMING ZHANG, BINGJUN PAN, AND JINLONG CHEN School of the Environment, and State Key Laboratory of Pollution Control and Resource Reuse, Nanjing University, Nanjing 210093, People’s Republic of China
A macroreticular resin adsorbent CHA-101 was aminated by dimethylamine, and a novel sorbent named M-101 was obtained. Several industrially important aromatic sulfonates including sodium benzenesulfonate (BS), sodium p-toluenesulfonate (TS), and sodium 2-naphthalenesulfonate (NS) were selected as general solutes to evaluate the performance of the newly synthesized resin particles. X-ray photoelectron spectroscope (XPS) analyses was used to determine the protonation degree of amino group at different solution pH, and the effect of pH on the sorption of these solutes onto M-101 can be explained by the ion exchange mechanism. The experimentally observed sequence of the sorption capacity of the tested organic sulfonates onto M-101 indicates that the π-π interaction between the solute molecule and the polymer matrix plays an important role in uptake of organic sulfonates from aqueous solution. Sodium sulfate was selected as a typical competitive inorganic anion, and improved selectivity of BS sorption over sulfate on M-101 was observed by comparison with a common macroporous weak base anion exchanger D-301. In addition, both sorption and desorption kinetics of M-101 were also found to be faster than that of D-301. Analyses of sorption isotherms and thermodynamics proved that BS sorption on M-101 was an exothermic and more selective process than on D-301. Both column tests and field applications proved M-101 to be an effective sorbent that can be used to remove aromatic sulfonates from aqueous solution.
Introduction HIOCs are hydrophobic ionizable organic compounds (1). One type of HIOC, the organic sulfonates that are widely used as industrial intermediates, is of particular interest due to its good solubility in water and negative environmental impacts once discharged into the receiving water system with industrial waste streams. Usually, these hydrophilic organic pollutants cannot be effectively removed by activated carbon and conventional synthetic polymeric adsorbents except for polymeric anion exchangers, which are able to remove effectively aromatic anions such as pentachlorophenate and benzenesulfonate from aqueous solution (210). Though many inorganic anions coexist with aromatic * Corresponding author phone: +011 86 25 3326433; fax: +011 86 25 3707304; e-mail:
[email protected]. 3308
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anions in industrial waste streams at concentrations several orders of magnitude greater than target organic solutes (7), previous studies have shown very favorable sorption behaviors of aromatic sulfonates and other organic anions in preference to many inorganic anions on strong or weak base anion exchanger (2-8). In general, such high sorption selectivities have been mainly attributed to the hydrophobic interactions resulting from the nonpolar moiety (NPM) of the aromatic anions, namely, the NPM-solvent and NPMmatrix interaction (3, 8). In many cases, strongly basic anion exchangers show a much higher selectivity for aromatic sulfonates sorption than weakly ones mainly due to their stronger electrostatic or Columbic interactions between the aromatic anion and the fixed positive charge on strong base anion exchangers. However, because of high sorption affinity of aromatic sulfonates on strong base anion exchangers, regeneration of the spent exchanger becomes a particularly challenging and costly task. A satisfactory regeneration process for these exchangers requires large volumes of concentrated brine or alkaline solution or even organic solvent (3). On the contrary, regeneration of a weak anion exchanger was much easier. For instance, in many cases, sodium hydroxide solution can achieve a complete regeneration process (7). The main problem with weak anion exchangers is their relatively low selectivity in removal of aromatic anions, such as sulfonates, in the existence of inorganic anions in aqueous solution (7). The objective of the current study is to synthesize a novel sorbent with higher capacity and selectivity and fast sorption and desorption kinetics for aromatic sulfonates. This was achieved by chemical modification of a commercial macroreticular adsorbent CHA-101 through amination with dimethylamine. Batch sorption experiments were conducted, and thermodynamic analyses were used to elucidate the sorption mechanism on the synthesized sorbent M-101. Sulfate ion was selected as a typical competitive anion in aqueous solution to evaluate the selectivity of M-101 for aromatic sulfonates in the existence of other inorganic anions. NaOH solution was used throughout the experiments to evaluate the regeneration efficiency of the spent sorbents.
Materials and Methods Materials. Sodium benzenesulfonate (BS), sodium p-toluenesulfonate (TS), sodium 2-naphthalenesulfonate (NS), and sodium methylsulfonate (MS) were used in this study. All chemicals are of analytical grade and were purchased from Shanghai reagent station. CHA-101, a macroreticular polymeric adsorbent, and D-301, a macroporous weakly basic anion exchanger, were provided kindly by Langfang Electrical Resin Co. Ltd. (Hebei Province, China). Both resins were obtained in spherical bead forms with sizes ranging from 0.4 to 1.0 mm. Physiochemical properties of the solutes and sorbents used in this study are presented in Tables 1 and 2. Resin Synthesis. Adsorbent CHA-101 is a macroreticular polystyrene resin adsorbent with some residual chlormethyl groups on its polymeric matrix during the synthetic process, and the chloro atom can have its place taken by other functional groups such as an amino group. To aminate CHA101 (10, 11), the resin particles were first soaked in benzene solution at 298 K for 12 h. The swollen resin particles were then filtered out of the suspension. Dimethylamine was then gradually introduced to a container containing the resin particles. After 12 h of amination at 318 K, extra dimethylamine was removed by filtering the reaction mixture. To remove residual benzene left in the resin pore space, the 10.1021/es048548j CCC: $30.25
2005 American Chemical Society Published on Web 03/10/2005
TABLE 1. Salient Properties of Polymeric Adsorbents matrix and porosity cross-link density (%) BET surface area (m2/g) macropore volume (cm3/g) mesopore volume (cm3/g) micropore volume (cm3/g) total anion-exchange capacity (TAEC, mequiv/g)a quaternary ammonium groupb (mmol/g) a Determined according to ref 19, produced.
b
CHA-101
M-101
D-301
polystyrene, hyper-cross-linked macroporous >35 721.5 0.36 0.028 0.42 0 0
polystyrene, hyper-cross-linked macroporous >35 671.5 0.16 0.028 0.40 1.53 0.027
polystyrene, macroporous ∼8 31.8 0.28 0.0002 0.0028 3.78 0.45
Brought by reaction with tertiary amino groups and chloromethyl groups on polymeric matrix when
TABLE 2. Properties of Organic Sulfonates
a Data from Lide, D. R., Ed. Handbook of Chemistry and Physics; CRC Press: London, 1991 and Buckingham, J., Ed. Dictionary of Organic compounds, 6th ed.; Chapman & Hall: London, 1996, corresponding to the acid type in aqueous solution; b Determined experimentally at 298 K except for methylsulfonate.
aminated resin particles (M-101) were finally subjected to steam distillation for 4 h. Prior to use, all the resins were packed in column and first rinsed with 10 bed volumes (BV) of 1.0 N NaOH followed by DI (diionized) washing until neutral pH was achieved. Then the column was subjected to acidic flushing by introducing 10 BV of 1.0 N HCl and again DI flushing to neutral pH. Finally, the resins were extracted with ethanol for 2 h in a Soxlet apparatus and vacuum desiccated at 325 K for 8 h before use. Surface area and pore size distribution of the adsorbents were determined by using a Micromertics 2010C automatic analyzer (Australia). Infrared spectra of CHA-101 and M-101 in the range of 650-4000 cm-1 were collected with a Nexus 870 FT-IR spectrometer (USA). Batch Sorption Experiments. Batch sorption tests were carried out in 250-mL glass bottles. To start the experiment, 0.250 g of resin particles was introduced to a 100 mL solution with known solute concentration. The flask was then transferred to a G25 model incubator shaker with thermostat (New Brunswick Scientific Co. Inc.) and shaken under 200 rpm for 24 h at desired temperature to ensure that the sorption process reached equilibrium. Sulfuric acid or sodium hydroxide was used to adjust the solution pH throughout the experiment. A 0.5 mL solution at various time intervals was sampled from the flasks to determine sorption kinetics. Resins from sorption kinetic study were transferred to another flask after filtering, and 100 mL of 0.5 M NaOH solution was used to study desorption kinetics. The amount of solute loaded on the resin particles is calculated by conducting a mass balance on the solute before and after the test.
Column Tests. Column experiments were carried out with a glass column (12 mm diameter and 230 mm length) equipped with a water bath to maintain a constant temperature. A Lange-580 pump (China) was used to ensure a constant flow rate. All column runs were performed under the same hydrodynamic conditions: the superficial liquid velocity (SLV) and the empty bed contact time (EBCT) were identical and equal to 1.0 m/h and 4 min, respectively. Analyses. X-ray photoelectron spectroscope (XPS) was used to determine the protonation degree of amino group on the polymeric matrix when the sorbent was placed in solution with different pH values. The sorbent samples, after being separated from the solutions, were vacuum-dried first to remove residual water and then dried in an oven at 45 °C until constant weights before the analysis was conducted. The analysis was made with an VG ESCALAB MK_spectrometer (USA) equipped with Mg KR X-ray source (1253.6 eV protons). The software package Scalab was used to fit the spectra peaks. Concentrations of the tested aromatic sulfonates were determined spectrophotometrically at the following wavelengths: BS, 264 nm; TS, 266 nm; NS, 252 nm using a Helious Betra UV-Vis spectrophotometer (U.K.). The concentration of MS was determined by analyzing the content of organic carbon with a TOC analyzer (Shimadzu 5000). The content of sulfate was analyzed by a Dionex 300 ion chromatograph (USA) with Na2CO3-NaHCO3 solution as elution reagent.
Results and Discussion Characterization of Sorbent M-101. Some important properties of the weak base exchanger D-301, CHA-101, and its aminated derivative M-101 are presented in Table 1. After amination, the average particle size of resin M-101 was found to be identical to the original adsorbent CHA-101. However, the BET surface area of the product decreased from 721.5 to 671.5 m2/g of adsorbent. Significant loss in macropore volume from 0.36 to 0.16 cm3/g was observed as well as negligible variation in meso- and micropore volume. This indicates that the amination reaction may occur mainly in the macroporous region of the resin particle. The presence of tertiary amino group on M-101 was further supported by the absorbance bands at 2772 and 2816 cm-1 in the IR spectra (not shown). Effect of Solution pH on Sorption. Figure 1a,b illustrates the observed effect of solution pH on sorption of organic sulfonates on M-101. For all four organic sulfonates studied, the optimized equilibrium sorption pH values were found to be around pH 3.0. A substantial decrease in sorption capacity was observed when equilibrium solution pH is larger than 3.0. When the aqueous pH is brought to below pH 2.5, the sorption capacities were decreased except for solute NS, which has the same sorption on M-101. VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effect of initial sulfate concentration in solution on BS sorption onto M-101 and D-301 at 298 K (0.250 g of sorbent added in 100 mL of solution, initial pH ) 2.5, and BS ) 5.0 mmol/l). (2) M-101, (O) D-301.
FIGURE 1. Correlation of equilibrium solution pH with equilibrium sorption capacity of organic sulfonates (a) and initial solution pH (b) on M-101 at 298 K. (0) BS, (b) TS, (4) NS, (1) MS, (g) percentage of protonated amino groups.
To explain the correlation of pH on such sorption behaviors, the following reactions were required to hypothesize (6):
+ RN(CH3)2 + OS- + H2O a RN(CH3)2‚OS- + OH- (1) H + + RN(CH3)2 + OS- a RN(CH3)2‚OSH H
(2)
where R represents the polymeric matrix of sorbent M-101; -N(CH3)2 is the tertiary amino group introduced onto the polymer matrix by animation; and OS- is the organic sulfonate. To elucidate the effect of solution pH on sorption capacity of organic sulfonates, a necessary step is to make clear how the protonation degree of the amino groups on polymeric matrix varies with the solution pH. Due to the electric double layer and Donnan exclusion effect existing in intraparticle environment, H+ concentration is much less in intraparticle environment than in extraparticle environment (solution). In the present study, XPS analysis was used to determine how many functional groups on the sorbent surface have been protonated in solution with different pH values (13). The percentages of protonated amine group at different solution pH are also presented in Figure 1a. For sites originally in free base form that correspond to the region of equilibrium pH larger than approximately 9.0, sorption occurs by hydrolysis mechanism as shown in eq 1. This hydrolysis 3310
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reaction releases hydroxyl ions to the solution, resulting in the observed larger equilibrium pH than initial pH (Figure 1b). As H+ concentration in the bulk (extraparticle) aqueous solution increases, the fraction of the protonated amine groups increases and the sorption of organic sulfonate is based on the ion exchange mechanism as demonstrated in eq 2. In fact, the sharp increase for sorption capacity of all the aromatic adsorbates from equilibrium pH 9 to pH 3 corresponds exactly to the similar increase tendency for protonated amino groups. The decrease in sorption capacity for all the adsorbates in equilibrium pH domain less than 3 would be expected by the competitive sorption between the adsorbate molecules and the sulfate ions that are introduced into the system due to pH adjustment. It was also observed the adsorption capacities on M-101 vary substantially for the four organic sulfonates following the order of NS > TS ≈ BS . MS. Close examination found that this sequence is consistent with the sequence of their corresponding solubility and aromaticity (4, 6). Previous studies indicated that NPM-matrix is dominated by π-π interaction between aromatic rings of the solute molecule and the resin matrix when adsorption of aromatic anions on the resin occurs (3, 8, 12). Hence, solute molecules with higher degree of aromaticity will have a stronger π-π interaction with resin surface; thus, higher sorption capacities are expected on M-101. Introduction of a methyl group on an aromatic ring of solute leads to negligible effect on the sorption capacity on M-101, as observed for TS and BS and also found in the literature (6). Sorption Selectivity of BS and NS onto M-101. To understand the selectivity of the synthesized resin M-101, sorption of BS and NS in the existence of sulfate was conducted for both M-101 and the reference sorbent D-301. As illustrated in Figure 2, increasing initial sulfate concentration of solution will lead to a sharp decrease in BS sorption capacity on D-301 while only a slight decrease on M-101 can be observed. This effect is consistent with the ion exchange mechanism of sorption, and similar results are expected when other competing anions are present. To quantify the selectivity of the two resins, the distribution ratio Kd (in mL/g) was determined by the following equation (14):
Kd )
mmol of sulfonate/1 g of dry resin mmol of sulfonate/1 mL of solution
(3)
The Kd value thus defined provides a measure of the sorptive ability for organic sulfonates per gram of resin. Alternatively,
TABLE 3. Effect of Initial Sulfate Concentration on BS and NS Distribution Coefficient (Kd) and Kd per Exchange Site (Kdeq) for M-101 and D-301a BS
sorbent M-101 D-301 a
collected equilibrium sorption date were fit into the Freundlich equation as
log Qe ) log Kf + n log Ce
(4)
NS
initial sulfate concn (mmol/L)
Kd (mL/g) (24 h)
Kdeq (mL/ mequiv)
Kd (mL/g) (24 h)
Kdeq (mL/ mequiv)
5 45 5 45
0.46 0.32 0.31 0.16
0.33 0.23 0.083 0.043
3.78 3.02 3.21 2.58
2.47 1.97 0.85 0.68
Initial pH ) 2.5.
where n and Kf are parameters to be determined. All the relative coefficients of sorption isotherms are larger than 0.98, indicating that Freundlich’s equation can describe the sorption behavior of BS on both adsorbents reasonably. The free energy change for sorption process is given by
∆G ) ∆H - T∆S
(5)
According to previous study (3), when swelling/shrinking of an adsorbent is negligible, the overall free energy change for an ion exchange involving a counterion with the nonpolar moiety (NPM) constitutes electrostatic (el), NPM-solvent, and NPM-matrix interactions as given in
∆Goverall ) ∆Gel + ∆GNPM-solvent + ∆GNPM-matrix
(6)
This concept is applicable to the sorption on M-101 because of the negligible swelling/shrinking effect arising from its high cross-link density. But this is not the case for D-301. In column work to be described later, as much as 15% swelling ratio was observed after BS sorption onto D-301. Thus ∆Gswelling should be incorporated into ∆Goverall as
∆Goverall ) ∆Gel + ∆GNPM-solvent + ∆GNPM-matrix + ∆Gswelling (7) At low solute concentration, ∆Goverall can be determined as follows (15):
-RT ∆Goverall )
∫ Q(x) d ln x x
0
(8)
Q(x)
where x is the mole fraction of the adsorbed solute in the solution. Since the adsorption capacity Q(x) for this study follows the Freundlich equation
Q(x) ) Kfx1/n
(9)
Incorporating eq 9 into eq 8 yields
∆Goverall ) -nRT
(10)
If isosteric sorption enthalpy change (∆H) can be assumed to be approximately constant, the van’t Hoff equation gives
d(ln Ce) FIGURE 3. BS sorption isotherms on M-101 (a) and D-301 (b) at different temperatures. Initial pH value in each test point is equal to 2.5. (9) 298 K, (O) 313 K, (2) 328 K.
Kdeq (in mL/mequiv), defined as the ratio between Kd and the total anion exchange capacity (TAEC) of a given resin, can be used to quantify the relative affinity per exchange site for organic sulfonates and, therefore, a measure of relative selectivity. Note that Kd is controlled by two factors: TAEC and relative selectivity (Kdeq). As shown in Table 3, Kd and Kdeq values of BS sorption on M-101 are larger than on D-301 at a wide range of sodium sulfate concentration, which proves that M-101 has higher selectivity than D-301. Larger Kd and Kdeq for NS than BS are believed to be attributed to the enhancement of π-π interaction with polymeric matrix. Sorption Isotherms and Thermodynamic Analysis. Shown in Figure 3a,b is the BS sorption isotherms at three different temperatures for M-101 and D-301. Experimentally
d(1/T)
)
∆H R
(11)
where T is the absolute temperature in K. The enthalpy change can be computed from the slope of the ln Ce versus 1/T plot. Previous study indicates that enthalpic change thus determined agrees well with that obtained independently using microcalorimetric technique (12). Then the entropic contribution can be subsequently determined according to eq 5. Figure 4 shows the van’t Hoff plots (ln Ce vs 1/T) with calculated thermodynamic parameters for the sorption isotherms as discussed above. The calculated ∆G value of BS sorption on M-101 is more negative than on D-301 with less cross-link density. This explains the improved selectivity of BS sorption on M-101 than on D-301. This result is also consistent with previous reports that increasing the crosslink density enhances the selectivity for the less hydrated anions (4, 16-18). The contribution of swelling on ∆Goverall of the sorption process, however, is unknown and needs to VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. van’t Hoff plots (ln K versus 1/T) for different types of isotherm presented in Figure 3 (Qe ) 0.8 mmol/g of sorbent). be further investigated. But, this topic is out of the scope of the current study. Another interesting observation is that BS sorption processes on both weak anion exchangers are exothermic (i.e., ∆H is negative). This is contradictory to generally accepted for the sorption of organic anions on strong anion exchangers by ion exchange, which is usually endothermic (i.e., ∆H is positive) (3). According to many other research results (3, 18), it can be believed that expelling water molecules imbibed into the matrix (especially around the functional group) is a necessary step before binding of BS on the adsorbents. Because of more polar functional groups on strongly anion exchanger and higher affinity with water molecules, such an expelling process is energetically more difficult than a weak one. Thus, a lesser amount of heat needs to be adsorbed for the latter one, such as M-101 or D-301 in the expelling process, when other conditions remain identical. Experimental evidence for similar results is available in the literature (18). Sorption and Desorption Kinetics. A practical resin for aromatic sulfonate sorption was a resin possessing both satisfactory selectivity and rapid sorption kinetics. As previously reported (19), increasing the cross-link density of a resin adsorbent results in lower sorption kinetics. Since the cross-link density of M-101 is much higher than the reference sorbent D-301, it is necessary to investigate the kinetic property of M-101 for its practical purposes. The kinetic behavior of BS sorption and desorption on M-101 as compared to D-301 is presented in Figure 5, and the fraction F(e) was defined as (20):
F(e) )
Qt Qe
(12)
where Qt (mmol/g) is the sorption capacity at time t. As shown in Figure 5a, contradictory to a previous report on the effect of cross-link density on sorption kinetics (18), BS sorption on M-101 reaches equilibrium quickly, and even a little faster sorption kinetics on M-101 can be observed than on D-301. As we know, higher cross-link density always results in a more meso- or micropore volume of a polymeric adsorbent, where sorption of hydrophobic compounds mainly occurs by pore-filling and other mechanisms (7, 11). Increasing cross-link density will enhance the rigidity of polymeric matrix and hinder the diffusion of solute molecules in the intraparticle environment of resin (especially for mesoor micropore environment due to lower pore diameter) and, thus, lead to lower sorption kinetics. But for BS sorption on M-101, in fact, for almost all the amino groups existing on macroporous matrix of M-101 as proven by porous volume 3312
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FIGURE 5. Batch kinetic curves of BS sorption on M-101 and D-301 (a) and desorption of BS-loaded sorbents by 0.5 M NaOH solution (b) at 298 K. (a) 0.250 g of sorbent added in 100 mL of solution, initial pH ) 2.5, and BS ) 5.0 mmol/L. (b) Sorbents from sorption kinetic experiment were BS-loaded at about 90% anion-exchange capacity. (2) M-101, (O) D-301. change values presented in Table 1, sorption can be believed to occur mainly in the macroporous region. The effect of the meso- or micropore region can be negligible in BS sorption kinetics, and the higher cross-link density of M-101 than D-301 does not play a negative role reasonably. Desorption efficiency of BS by NaOH solution (0.05 M) was studied with adsorbents loaded with BS. As presented in Figure 5b, BS anions were rapidly desorbed on M-101 by addition of NaOH solution. Higher desorption efficiency than D-301 may partly depend on the lower capacity of the quaternary ammonium group of M-101, where BS loaded cannot be readily desorbed by NaOH solution. Fixed-Bed Column Experiments and Field Application. Figure 6a,b illustrates a complete effluent history of a fixedbed column packed with either M-101 or D-301 for a feeding solution containing both BS and sulfuric acid. Note that BS breaks through fairly earlier on D-301 than M-101, although TAEC of D-301 is about 2.5 times that of M-101. Again, this proves sorption enhancement on M-101 than D-301. C/C0 in excess of unity on breakthrough curve for BS sorption on D-301 (Figure 6a) is caused by the elution effect of BS by sulfate. In other words, some BS anions initially loaded on D-301 can be replaced by sulfate molecules when insufficient sorption sites are available. This was, however, not observed for M-101 due to its improved selectivity for aromatic sulfonate over sulfate as indicated in Figure 6b. Earlier breakthrough for sulfate ions and larger than unity of C/C0
People’s Republic of China (Grant 20274017) and Natural Science Funding of Jiangsu Province (Grant BK 2004415).
Literature Cited
FIGURE 6. Comparison of BS (a) and sulfate (b) breakthroughs during two separate fixed-bed column runs with M-101 and D-301at 298 K. 1.0 g of each sorbent in column test; influent: pH ) 2.5, BS ) 2.79 mmol/l, sulfate ) 3.14 mmol/L; SLV ) 1.0 m/h; EBCT ) 4 min. Data in panels a and b for a specific sorbent were obtained from the same experiment. (2) M-101, (O) D-301. infers higher selectivity of BS sorption over sulfonate ions on M-101. To date, M-101 has been successfully used commercially in recovery of aromatic sulfonate species from many industrial waste streams with various inorganic ions of high level ionic strength. For example, M-101 was successfully adopted to separate NS from a waste stream from the 2-naphthol production process in a chemical plant in Chongqing city (People’s Republic of China). This stream contains about 5000 mg/L NS and as high as 10% sodium sulfate (7). Field application results indicate that about 95% of NS can be recovered. The regeneration efficiency near 99% for the spent resin can be achieved with 1 M NaOH as elute solution. Its superior properties of both sorption capacity and selectivity as manifested by more than 2 years of successful performance on industrial scale make M-101 a useful polymeric adsorbent for recovering useful organic sulfonates from associated industrial waste streams.
Acknowledgments We thank Dr. Aimin Li (Nanjing University) and his research group for the help on resin synthesis. This research was financially funded by National Natural Science Funding of
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Received for review September 17, 2004. Revised manuscript received January 28, 2005. Accepted February 9, 2005. ES048548J
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