Enhanced Photocatalytic Degradation and Selective Removal of

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Environ. Sci. Technol. 2008, 42, 1687–1692

Enhanced Photocatalytic Degradation and Selective Removal of Nitrophenols by Using Surface Molecular Imprinted Titania XIANTAO SHEN, LIHUA ZHU,* GUOXIA LIU, HONGWEI YU, AND HEQING TANG* Department of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China

Received July 19, 2007. Revised manuscript received October 12, 2007. Accepted December 27, 2007.

Poor selectivity of titania (TiO2) photocatalysis is unfavorable tophotocatalyticremovalofhighlytoxiclow-levelorganicpollutants in polluted waters in the presence of other less toxic highlevel pollutants. A new strategy of increasing this selectivity is the surface modification of TiO2 via coating a thin layer of molecular imprinted polymer (MIP), which provides molecular recognition ability toward the template molecules. By using 2-nitrophenol and 4-nitrophenol as target pollutants, MIP-coated TiO2 photocatalysts were prepared via surface molecular imprinting and were observed to have high activity and selectivity toward the photodegradation of the targets. In the presence of bisphenol A (50 mg L-1) as a nontarget pollutant, the apparent rate constant for the photodegradation of the target 2-nitrophenol and 4-nitrophenol (1.8 mg L-1) over the corresponding MIPcoated TiO2 was 10.73 × 10-3 and 7.06 × 10-3 min-1, being 2.46 and 4.61 times of that (4.36 × 10-3 and 1.53 × 10-3 min-1) over neat TiO2, respectively. The enhanced photocatalytic selectivity was increased when the concentration of the target was decreased and/or when the difference in both the chemical structure and molecule size between the target and nontarget molecules was increased. The increased selectivity was mainly attributed to the special interaction between the target molecules and the footprints polymer via the functional groups (-OH and -NO2).

Introduction Phenolic compounds are widely used in various fields, leading to their increased disposal. Some of them are highly toxic and carcinogenic and can remain in the environment for a long time due to their stability and bioaccumulation. Eleven phenolic compounds have been listed as priority pollutants by the U.S. Environmental Protection Agency (U.S. EPA) (1). To remove the phenolics from polluted waters, various methods have been developed, including physicochemical treatment (2), biological degradation (3), and photocatalytic degradation (4). Photocatalytic degradation is important to the removal of highly hazardous phenolics, because it is not influenced by their toxicity. However, in many practical streams, the presence of highly hazardous and non* Address correspondence to either author. Phone: +86-2787543432. Fax: +86-27-87543632. Emails: [email protected] (L.Z); [email protected]. 10.1021/es071788p CCC: $40.75

Published on Web 02/02/2008

 2008 American Chemical Society

biodegradable phenolics at low levels is accompanied by the coexistence of other lowly toxic biodegradable pollutants at high levels. A preferable approach to treat such wastewaters is to degrade the highly hazardous phenolics first with photocatalytic treatment and then handle it biologically. TiO2 photocatalysis has been well-studied with concerns for environmental protection (4–7). It is still difficult to realize selective removal of harmful low-level pollutants from complicated wastewaters in the presence of other high-level less-harmful pollutants via photocatalytic treatment, because TiO2 has a very poor selectivity and cannot differentiate between these pollutants. Thus, it is urgently necessary to promote the selectivity of TiO2 photocatalysts. Several approaches have been proposed to enhance the selectivity of TiO2. One of them is to control the surface’s electric charge by adjusting pH. The isoelectric point for TiO2 in water is about at pH ) 6; thus, a lower pH is helpful for the degradation of negatively charged contaminants, and a higher pH is favorable for the degradation of positively charged contaminants. Using this method, Robert et al. accelerated the photodegradation of 4-hydroxybenzoic acid and benzamide in wastewater (8). However, it is not effective for high selectivity due to the fact that most contaminants are neutral. A second way is to modify the surface of TiO2 with specific molecules which can obtain a selective and effective adsorption. Makarova et al. modified TiO2 nanoparticles with arginine and enhanced the decomposition of nitrobenzene in aqueous systems (9). Inumaru and co-workers reported that n-octyl-grafted TiO2 was highly active for the decomposition of 4-nonylphenol (10). Hidaka et al. found that partially organosilicone-coated TiO2 provided better activity to the photocatalytic destruction of the pesticide permathrin (11). However, this type of photocatalyst has a poor “stability”, because nothing can keep the organic host molecules accreted directly to the catalyst from decomposing. Double-region-structured photocatalysts are proposed as the third way and are composed of one region of binding sites providing a selective adsorption and the other region of photocatalytic sites supplying an effective degradation of contaminants. Enhanced photodegradation was observed for the photocatalytic decomposition of the herbicide propyzamide by TiO2 supported on activated carbon (12) and the photodecomposition of pyridine over TiO2 loaded zeolites (13). Ghosh-Mukerji et al. developed a photocatalyst which was composed of molecular recognition sites located in the vicinity of TiO2 microdomains (14). The preparation process of this photocatalyst is too difficult for its practical application. Alternatively, Xamena et al. reported that a simple treatment with HF enhanced the activity of ETS-10 (a molecular sieve) toward the photodegradation of large aromatic molecules, such as 2,5-dichlorophenol and 2,4,5-trichlorophenol (15). The increased photoactivity was accompanied by a parallel increase of the shape selectivity (16). However, this method is not so efficient in increasing the selectivity of the photocatalysts, since the adsorption selectivity of the inert domains is fairly poor. We are trying to develop the fourth way to obtain high selectivity by imprinting cavities of target molecules on the photocatalyst’s surface. Our preliminary work reported a possibility of increasing the selectivity of TiO2 by increasing selective adsorption of target molecules on surface -modified TiO2 (17). The modified TiO2 photocatalyst was prepared by coating a thin layer of molecular-imprinted polymer (MIP) via polymerization of o-phenylenediamine (OPDA) in the presence of target molecules and TiO2 nanoparticles. The VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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novel photocatalysts were able to remove selectively lowlevel target 2-chlorophenol or 4-chlorophenol via a photocatalytic treatment. To get more efficient photocatalysts with high selectivity and confirm the MIP coating as a general way of increasing the selectivity of TiO2 photocatalysts, we will further investigate the selective photocatalytic degradation of nitrophenols on TiO2 nanoparticles coated with a thin layer of MIP. In the present work, 2-nitrophenol (2-NP) and 4-nitrophenol (4NP) are chosen as the representative members of the priority pollutants because of their environmental importance.

Experimental Section Chemicals and Materials. TiO2 nanoparticles (P25; BET surface area, 49.6 m2 g-1) were obtained from Degussa. Methanol (chromatographic purity) was purchased from Tedia, and the other chemicals were of analytical purity grade. All these chemicals were used as received without further purification. Preparation of MIP-Coated Photocatalyst. The preparation procedures were similar to that described in our previous work (17). In a typical preparation, 0.24 g of OPDA and 0.13 g of 2-NP (or 4-NP) were dissolved in 40 mL of distilled water, and the solution was stirred for 20 min, followed by adjusting the pH to 2.0 with 6 mol L-1 HCl. TiO2 particles (0.4 g) were further added, and the resultant dispersion was ultrasonicated for 3 min. The polymerization was photocatalytically initiated by UV illumination with a 250 W Philips high-pressure mercury lamp. The obtained solids were washed five times by using Na2CO3 solution and distilled water, respectively. The as-prepared photocatalysts were correspondingly referred to as 2NP-P25 or 4NP-P25. When no template was used in the preparation, the obtained product was referred to as NIP-P25. Characterization. The morphology was observed via the high-resolution transmission electron microscopy (HRTEM) on a JEM-2010FEF TEM. The MIP coatings were characterized by measuring FTIR spectra on a Bruker VERTEX 70 spectrophotometer and UV–visible solid-state reflection spectra on a Shimadzu UV-2550 spectrophotometer. Apparatus and Methods for Adsorption and Photocatalysis. Adsorption experiments were carried out in 2-NP and 4-NP solutions (5 mL) at various concentrations with a photocatalyst loading of 2 g L-1, in dark on a rocking tray to keep well the catalyst’s suspension. The photocatalytic experiments were conducted in a water-jacketed cylindrical quartz photoreactor filled with 250 mL of nitrophenol solution in the presence of the catalyst (0.1 g L-1). During the experiment, the TiO2 suspension was well-stirred by using a magnetic stirrer. A 9 W UV lamp (Philips, λmax ) 254 nm) as the light source was immersed in the photoreactor. After the suspension was stirred for 20 min to favor the organic adsorption onto the catalyst surface, the concentration of the pollutant was determined as the initial concentration c0, and then the photoirradiation started. Aliquots of the reaction solution were sampled at given time intervals. After the samples were filtered through 0.45 µm filters to remove TiO2particles, the remaineing concentrations of the pollutants and intermediates were measured by high-performance liquid chromatography on a PU-2089 HPLC (JASCO), equipped with a C18 ODS column and an ultraviolet detector. The detection wavelengths were set at 318 nm for 4-NP and 279 nm for 2-NP. At least triplicate runs were carried out for each test, and the standard deviation was generally less than 10%.

Results and Discussion Formation of an Associated Species between the Functional Monomer and Template Molecules. A high quality of MIP coating requires the formation of a precursor between the 1688

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functional monomer and the template molecules before the polymerization. The precursor may be an associated species between the monomer and template molecules via chemical bonds and/or hydrogen bonds. There are two amino groups (-NH2) in a molecule of OPDA, one of which can interact with hydroxyl group (-OH) and/or nitro group (-NO2) of the template molecule, leading to the formation of the precursor. It is yet difficult to clarify the chemical structure of the precursor, but the association was confirmed as below. UV absorption spectra were measured for the functional monomer, the template, and their mixture in aqueous solutions at pH ) 2 (Figure S1 in Supporting Information). If there was not any remarkable association between the monomer and template molecules, the absorbance of their mixture should be the sum of their individual ones at each wavelength. However, the absorption spectrum of their mixture was much below that calculated by summing the spectra of the monomer and template individuals. For example, the measured absorbance at the maximum wavelength of 191 nm was 1.80, being only 71% of that (2.54) of the calculated one for the 2-NP system, and 2.00 measured at 192 nm, being 68% of the calculated 2.94 for the 4-NP system. Compared with the calculated absorbance, the marked decreasing in the measured absorption of the mixtures is attributed to an association between the monomer and template molecules (18), and the formation of the associated species results in distinction to a great extent. Thus, we can assume that the formed precursor can copolymerize with the free functional monomers on the surface of TiO2 by photocatalytic polymerization, producing the MIP layer and then the MIP-coated TiO2 photocatalyst. Characterization of MIP-Coated Photocatalysts. The HRTEM examination (Figure S2 in Supporting Information) indicated that for the MIP-coated photocatalysts, the surface of TiO2 nanoparticles was covered by a layer of MIP, which was estimated with a thickness of about 5 nm, corresponding to a favorable one for the selectivity of the imprinted shell as reported by Stephen and Stephen (19). In the UV–visible solid-state reflection spectra of the photocatalysts (Figure S3 in Supporting Information), the MIP-coated TiO2 yielded a broad absorption peak at about 440 nm, being attributed to the absorption of the polymer coating. The MIP coating was characterized by FTIR spectroscopy (Figure S4 in Supporting Information). A sharp peak at 1617 cm-1 is associated with the CdN stretching vibration, and the strong absorption band at 1525 cm-1 is ascribed to the CdC stretching vibrations in the benzene ring. The absorptions at 1230 and 1346 cm-1 correspond to the dCsN stretching on the benzene ring. Thus, the FTIR measurements indicated that the MIP layer had a structure of the polymer of OPDA. Effects of Synthesis Conditions on the Adsorption Capacity of Photocatalysts. The effects of synthesis parameters were investigated on the adsorption capacity of photocatalysts (Table S1 in Supporting Information), where the feed ratio of template to monomer was controlled at 1:1 in mole. By comparing the adsorption capacity of the photocatalysts, the preparation conditions were optimized at the temperature of about 35 °C, the solution pH of 2.0, and the photoinitiating time of 60 min. The optimization of the solution pH at pH ) 2 is rational, because the chemical polymerization of aniline and its derivatives is known to require low pH values for the protonation of the monomer at the -NH2 group (20), and we also wanted to obtain the polyaniline-like structure, which is photochemically stable and favorable to the photocatalytic efficiency of TiO2/ polyaniline under sunlight (21). Adsorption Behavior of Template Molecules. By using 2-NP and 4-NP as a pair of template and nontemplate compounds, the selective adsorption ability of the MIP-coated TiO2 was measured as shown in Figure 1. It is clearly

FIGURE 1. Adsorption of 2-NP (a) and 4-NP (b) on photocatalysts of 4NP-P25 (1), 2NP-P25 (2), NIP-P25 (3), and P25 (4).

TABLE 1. Freundlich Isotherm Parameters for Adsorption of 2-NP and 4-NP adsorption catalyst 4NP-P25 2NP-P25 P25 NIP-P25

Qf (mg g-1)(L µmol-1)1/n 0.30 0.84 0.25 0.19

of2-NP

adsorption

1/n

K0 (mg g-1)n(L µmol-1)

rF

0.67 0.61 0.67 0.72

0.17 0.75 0.13 0.10

0.99 0.99 0.98 0.99

demonstrated that the adsorption amounts of 2-NP or 4-NP over the MIP-coated TiO2 prepared by using 2-NP or 4-NP as the template is roughly 2-4 times of that over neat TiO2 and NIP-P25. The enhanced adsorption will help the selective photodegradation of the target pollutants. The adsorption of both 2-NP and 4-NP on the photocatalysts is found to obey the Freundlich isotherm (Qe ) QfCe1/n) (22), where Q e is the amount of nitrophenol adsorbed per unit mass of the photocatalyst (mg g-1), Ce is the concentration of nitrophenol in solution (µmol L-1), Qf (the Freundlich coefficient with a dimension of (mg g-1)(L µmol-1)1/n) is an index of the adsorption capacity of the adsorbent, and the heterogeneity index (1/n) gives an indication of the degree of error from the linearity, because strongly heterogeneous surfaces have large values of n while less heterogeneous ones have values closer to unity. Moreover, the value of rF for the fitting is a more quantitative evaluation of the validity of applying the Freundlich isotherm to a particular portion of the binding isotherm. As the binding ability of the template to MIP changes from the high-affinity sites at a lower concentration to the low-affinity sites at a higher concentration, the median binding affinity K0 (K0 ) Qfn with a dimension of (mg g-1)n (L µmol-1)) is applied. The values of the four parameters (Qf, 1/n, K0, and rF) are listed in Table 1 for the adsorption of 2-NP and 4-NP on different photocatalysts. In the case of 2-NP, K0 ) 0.75 on 2NP-P25, being considerably greater than 0.10 on NIP-P25, 0.13 on P25, and 0.17 on 4NP-P25. Moreover, Qf for 2NP-P25 is 0.84, being much greater than the ones of 0.30, 0.19, and 0.25 for 4NP-P25, NIP-P25, and P25, respectively. Therefore, 2NP-P25 has the highest adsorption capacity among the four catalysts. A similar trend is also observed in the case of 4-NP. These suggest that the molecular imprinting creates a microenvironment on the surface of TiO2 for the adsorption based on shape selection and hydrogen bond recognition of the target molecules. Photodegradation Activity in Single Systems. The photocatalytic ability was evaluated in single systems (only one organic substrate in the suspension) and in binary systems (with a coexisting pollutant in the suspension). Our experimental results indicated that the direct photolysis without

Qf (mg g-1)(L µmol-1)1/n 0.61 0.22 0.23 0.13

of4-NP

1/n

K0 (mg g-1)n(L µmol-1)

rF

0.50 0.54 0.55 0.68

0.37 0.060 0.064 0.048

0.98 0.95 0.96 0.96

photocatalyst was not so efficient for both 2-NP and 4-NP (curve 1 in Figure 2). Figure 2 also gives the kinetic data for the photodegradation of 4-NP and 2-NP over different photocatalysts in the single systems, which clearly show that all the degradation processes follow a pseudo-first-order kinetics. Thus, the values of the apparent rate constant k of the pseudo-first-order reaction are also listed. The k value of the target 4-NP over 4NP-P25 is 0.045 min-1, being 346% of that over NIP-P25 (0.013 min-1) and 188% of that over P25 (0.024 min-1), respectively. The k value of 4-NP over 2NPP25 is 0.018 min-1, being only 40% of that over 4NP-P25 (0.045 min-1). Similarly, when 2-NP is selected as the target, the k value of 2-NP over 2NP-P25 is 0.040 min-1, being 333 and 160% of that over NIP-P25 (0.012 min-1) and P25 (0.025 min-1), respectively. When it is degraded over 4NP-P25, the k value is decreased to 0.019 min-1, being only 47.5% of that over 2NP-P25. Therefore, the degradation of the target pollutant is much enhanced by the MIP layer, and the degradation of the nontarget pollutant is correspondingly depressed by the MIP coating. This enhanced photocatalytic selectivity can be attributed to molecularly selective adsorption on the MIP coatings. Accumulation of Degradation Intermediates. The pathway of nitrophenol photodegradation has been investigated well (23–25). By using HPLC, it was found that the main aromatic intermediate products included 3-nitrocatechol and 1,2,4-benzenetriol during 2-NP photodegradation, whereas only 1,2,4-benzenetriol was detected in the case of 4-NP (Figure S5 in Supporting Information). The photodegradation of 2-NP over 2NP-P25 was faster than over P25, but producing the same types of intermediates. By monitoring the accumulation of the aromatic intermediates, it was observed that the accumulation levels of the major aromatic intermediates produced over the MIP-coated photocatalyst were lower than that over neat TiO2 (Figure S6 in Supporting Information). Similar accumulation behavior of the intermediates were also observed in the case of 4-NP over 4NPP25 (Figure S7 in Supporting Information). Thus, the MIPcoated photocatalysts promote not only the photodegradation of the target pollutant but also the degradation of the generated intermediates, being favorable to the complete mineralization of the organic pollutants. VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Kinetic data for the direct photolysis (1) and the photocatalytic degradation of 2-NP (a) and 4-NP (b) over 4NP-P25 (2), 2NP-P25 (3), NIP-P25 (4), and P25 (5). The initial concentration of each organic compound was 10 mg L-1.

FIGURE 3. Initial concentration dependence of the degradation rate constant of 2-NP (a) or 4-NP (b) over the corresponding MIP-coated P25 (1) and P25 (2). The inset gives the ratio of the k value of targets over MIP-coated P25 to that over P25.

TABLE 2. Rate Constants for the Photocatalytic Degradation of Target Pollutant (2-NP or 4-NP, 1.8 mg L-1) in the Presence of BPA (50 mg L-1) over Different Photocatalystsa 2-NP ktarget (10-3min-1) knontarget (10-3min-1) Rb Rb

overP25

over4NP-P25

overP25

10.73 ( 1.81 4.63 ( 1.33 2.32 4.73

4.36 ( 0.97 8.85 ( 1.45 0.49 5.10

7.06 ( 1.49 4.61 ( 0.39 1.53

1.53 ( 0.66 5.16 ( 0.40 0.30

a Total illumination time was 180 min. that over P25.

b

R is the ratio of ktarget/knontarget, and R is the ratio of the R value over MIP-P25 to

Effect of Concentration on Photocatalytic Degradation. As shown in Figure 3a, when the concentration of 2-NP was decreased from 50 to 1.8 mg L-1, the k value of 2-NP degradation was increased from 0.0044 to 0.21 min-1 over 2NP-P25 and from 0.0031 to only 0.056 min-1 over P25. The ratio of the k values over 2NP-P25 to P25 was increased from 1.42 to 3.75. When 4-NP was chosen as the target pollutant, a similar trend was observed (Figure 3b). This indicates that the selectivity of the MIP-coated TiO2 is rapidly increased as the initial concentration of the target pollutant is decreased, which will help us to remove selectively the low-level target pollutant. Photocatalytic Activity in Binary Systems. Table 2 lists the experimental results obtained from the binary systems composed of target pollutant (2-NP or 4-NP) and the nontarget bisphenol A (BPA). Here, BPA was selected as the nontarget because it is somewhat similar to the target 2-NP in chemical structure but it is more easily photocatalytically 1690

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degraded over P25. As anticipated, the rate constants for the target’s degradation over the MIP-coated TiO2 are greater than over TiO2, and that for the nontarget BPA over the MIPcoated TiO2 is smaller than that over TiO2. Moreover, the rate constant ratio (R ) ktarget/knontarget) is k2NP/kBPA ) 2.32 over 2NP-P25, being much greater than 0.49 over P25, and the ratio k4NP/kBPA (1.53) over 4NP-P25 is also considerably greater than 0.30 over P25. The large values (4.7-5.1) of R, which is defined as the ratio of the R value over MIP-coated TiO2 to that over neat TiO2, further confirm that the MIP coating enhances the photocatalytic selectivity toward the target contaminant. The enhanced selectivity has been observed in the binary system at different pH values. It was found that when the pH was changed, the k values of 2-NP photodegradation over the two photocatatlysts were varied, but both the rate constant ratio (R) over the individual photocatalyst and the ratio of the R values (R) over different photocatalysts had

FIGURE 4. Rate constants for the photodegradation of (a) the target 2-NP and (b) the nontarget pollutants over P25 and MIP-coated P25 in the binary systems. The coexisting pollutants were (I) 2,4-binitrophenol, (II) naphthol, (III) phenol, and (IV) toluene. relatively large values (about 2.5 for R and 4 for R), being almost independent from the pH in a wide range from 1 to 6. Even in weak alkaline solutions (pH ) 8-10), the values of R and R are more than 1.39 and 1.54, respectively. This suggests that in such a wide pH range, the MIP-coated photocatalysts can provide much better selectivity toward the photodegradation of the target pollutant in the presence of the nontarget one at a high level (Table S2 in Supporting Information). Generally Observation of Enhanced Photocatalytic Selectivity of MIP-Coated TiO2 for Different Coexisting Pollutants. As discussed above, when 2-NP (or 4-NP) was specified as the target, the enhanced photocatalytic selectivity of MIP-coated TiO2 was observed for 4-NP (or 2-NP) and BPA as the nontarget pollutant, respectively. In fact, this was a fairly general observation in other binary systems, where the initial concentrations were controlled at 12.9 µmol L-1 (1.8 mg L-1) for the target 2-NP (or 4-NP) and 532 µmol L-1 for the coexisted pollutant. Four benzene derivatives were selected here, including phenol (50 mg L-1), toluene (48.9 mg L-1), naphthol (76.7 mg L-1), and 2,4-binitrophenol (97.8 mg L-1). As shown in Figure 4a, the rate of the photodegradation of 2-NP (as the target) over 2NP-P25 is increased in the order of 2,4-binitrophenol < naphthol < phenol < toluene for the coexisted organics, and the k value is correspondingly estimated as 0.0027, 0.0078, 0.0081, and 0.024 min-1. The difference between the k values over 2NP-P25 and over P25 is significantly increased in the same order. During the experiment, the photodegradation of the coexisted nontarget pollutants was also monitored, and their k values were illustrated in Figure 4b. Relative to the degradation over TiO2, the degradation of naphthol and toluene as the nontarget is considerably decreased over MIP-coated TiO2, whereas the degradation of phenol and 2,4-binitrophenol is increased over MIP-coated TiO2. In the case of 4-NP as the target, similar tendencies were observed (Figure S8 in Supporting Information). Generally, MIP-coated TiO2 will enhance the degradation of target pollutants and depress that of nontarget pollutants. Relative to neat TiO2, the MIP-coated photocatalysts provide better selectivity toward the target pollutant as the difference between the target and nontarget pollutants in the chemical nature and molecular size/shape is increased. Because of this difference, the MIP-coated TiO2 can well differentiate 2-NP and 4-NP from each other during their photocatalytic degradation (Figure 2). The increased selectivity of MIPcoated TiO2 toward the target in the order of the nontarget pollutants of 2,4-binitrophenol < naphthol < phenol < toluene (Figure 4) indicates that the difference between the

target and nontarget molecules is increased in the same order. As we know, there are two hydrophilic functional groups and one benzene ring in a molecule of the target (2-NP or 4-NP). However, the number of hydrophilic functional groups in the molecule is 0, 1, 1, and 3 for toluene, phenol, naphthol, and 2,4-binitrophenol, respectively. Our observation is that the selectivity is the best between the target and toluene, and the poorest, between the target and 2,4-binitrophenol. It should be noted further that, as the nontarget pollutant, phenol and 2,4-binitrophenol are degraded over MIP-coated TiO2 more rapidly than TiO2. These interesting phenomena can be attributed to the existence of -OH and/or -NO2 groups in the molecules of these coexisting pollutants as that in the target molecules. These observations strongly indicate that the photocatalytic selectivity of the MIP-coated TiO2 is mainly determined by the chemical interaction between the target molecules and the footprints polymer via the functional groups (-OH and -NO2), and then to a lesser extent by the matching between the cavity size and the target molecule size. That is, the enhanced selectivity of the MIP-coated TiO2 primarily requires the existence of -OH and/or -NO2 groups in the molecules of the “target” as in the template used in the preparation of the MIP-coated photocatalyst, which will strengthen the chemisorption of the “target” to the MIP layer via hydrogen bonds between these functional groups in its molecule and the amino groups (-NH2) at the footprint of the MIP layer. In conclusion, by using two nitrophenols as templates, MIP-coated photcatalysts have been prepared in aqueous suspension. The MIP layer provides the photocatalyst molecular recognition ability, leading to the selective photodegradation of the target pollutant. The photodegradation experiments in both single and binary systems have confirmed that the MIP-coated photocatalysts have good selectivity toward the photocatalytic degradation of the target pollutants. Such selectivity is dependent mainly on the difference in the functional groups and then the difference in the molecular size and shape between the target and nontarget pollutants. In our preliminary work (17), we have already demonstrated that the MIP-coated photocatalysts have fairly good lifetime only if there are the coexisting organic pollutants in the photocatalytic systems. Moreover, when a nontarget pollutant has functional groups in its molecule as the target pollutant, the photocatalytic degradation of the nontarget pollutant over the MIP-coated photocatalytst will be also enhanced considerably. This is a very important finding, especially for degradation of very toxic pollutants. We could prepare a new type of MIP-coated photocatalyst by using a less toxic substance as a pseudotemplate and use VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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it to remove selectively the concerned highly toxic pollutant. The related research for this purpose is ongoing in our laboratory.

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Acknowledgments The financial support for this work from the National Natural Science Foundation of China (Grant Nos. 30571536 and 20677019) is greatly appreciated. The Analytical and Testing Center of Huazhong University of Science and Technology is thanked for its help in the characterization of the photocatalysts.

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Supporting Information Available (15)

Figures showing UV absorption spectra, HRTEM results, UV–visible solid-state reflection spectra of the photocatalysts, FTIR characterization of the MIP coating, the main aromatic intermediate products via HPLC, accumulation levels of the major aromatic intermediates, and tendencies in degradation and tables showing the effects of synthesis parameters and selectivity toward the photodegradation of the target pollutant. This material is available free of charge via the Internet at http://pubs.acs.org.

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