Molecularly imprinted TiO2 photocatalysts for degradation of diclofenac in water

Colloids and Surfaces A 538 (2018) 729–738

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Research Paper

Molecularly imprinted TiO2 photocatalysts for degradation of diclofenac in water

T

⁎

Cícero Coelho de Escobara, , Yolice Patricia Moreno Ruiza, João Henrique Zimnoch dos Santosb, Lei Yec a

Departament of Chemical Engineering, Federal University of Rio Grande do Sul, Luiz Englert, s/n, CEP 90040-040, Porto Alegre, RS, Brazil Institute of Chemistry Federal University of Rio Grande do Sul, Av. Bento Gonçalves, 9500, CEP 1500-000, Porto Alegre, Brazil c Division of Pure and Applied Biochemistry, Lund University, Box 124, 221 00, Lund, Sweden b

G RA P H I C A L AB S T R A C T

A R T I C L E I N F O

A B S T R A C T

Keywords: Selectivity photoctalysis Diclofenac degradation Imprinted catalyst

In order to improve the selectivity in photocatalytic process, molecularly imprinted photocatalysts containing a low loading of TiO2 (from 6.6 to 16.6% of total mass) were prepared for photocatalytic degradation of an organic pollutant, diclofenac (DIC). The photocatalytic component TiO2 (P25), with and without being doped with Cu2O, was embedded in diclofenac-imprinted polymers. The molecularly imprinted polymers (MIPs) and the composite photocatalysts exhibited a superior specific target recognition for selective degradation of DIC over non-target reference molecules, fluoxetine (FLU) and paracetamol (PARA). In contrast to the non-selective commercial sample of TiO2, the average value of selectivity of the imprinted catalysts for photocatalytic degradation of DIC was estimated to be 2.8, which suggests that the specific binding sites created by the molecular imprinting are essential for gaining high catalytic selectivity and efficiency. After 6 cycles of testing under UV-light, the imprinted catalysts maintained almost the same efficiency for photo degradation of DIC. In addition, the morphology and the structure of the imprinted catalysts remained after repeated uses. The results suggest that it is feasible to use MIPs to control the selectivity of photocatalytic degradation of organic pollutants.

1. Introduction During recent years, the preservation of groundwater and surface have been focus of increasingly restrictive legislation [1]. One group of chemicals of concerns are the compounds that encompass personal steroids, pharmaceuticals, pesticides, health care products, surfactants, ⁎

and dyes [2–5]. These so-called emerging contaminants possess high persistence and low biodegradability [6]. As a consequence, pharmaceuticals and their metabolites have increasingly been found in several water bodies, including surface water and effluents from wastewater treatment plants. These compounds are of great concern because of their potential impact on human health and the environment even at

Corresponding author. E-mail address: [email protected] (C.C. de Escobar).

https://doi.org/10.1016/j.colsurfa.2017.11.044 Received 23 September 2017; Received in revised form 14 November 2017; Accepted 15 November 2017 Available online 20 November 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.

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material was prepared via precipitation polymerization using a low TiO2 loading. Our main goal was to enhance the selectivity of the photocatalyst using a commercial sample (Degussa P25) as a bench mark. In addition, we also studied the combination of MIP with Cu2Odoped TiO2, and investigated the performance of the new, visible lightactivated photocatalyst. To the best of our knowledge, modifying Cu2Odoped TiO2 with organic MIPs has not yet been reported.

low concentration levels [7]. Diclofenac (DIC) is one of the most widely used non-steroidal antiinflammatory drugs used to reduce inflammation, and as an analgesic in conditions such as arthritis or acute injury. Previous investigations have shown that 15% of DIC was excreted unchanged after consumption [8]. Today it is still one of the most frequently detected pharmaceuticals in the water environment [9], and it has been detected in both the influents and effluents of wastewater treatment plants at concentrations up to mg/L level [6,10]. As conventional wastewater treatment systems cannot efficiently remove DIC [11], it is necessary to develop alternative methods to effectively remove DIC from contaminated aquatic environment. A condition that is necessary for complete removal is the mineralization of the contaminant, which can be achieved by using advanced oxidation processes (AOPs). Considering emerging contaminants, the heterogeneous photocatalysis is one of more promising strategies among AOPs. The use of TiO2 as catalyst encompasses several advantages, such as low cost and low toxicity [12] photocatalytic oxidation [10,11]. Recently, several studies have explored the viability of employing photocatalysis for degradation of DIC [9,13–15]. However, a serious shortcoming in heterogeneous photocatalytic oxidation is its low selectivity to the target contaminants, and the photocatalysts cannot differentiate highly toxic target pollutants from other organic compounds of low toxicity [16,17]. In real scenarios, effluent streams may contain highly toxic organic pollutants (normally non-biodegradable) that coexist with less toxic and biodegradable molecules. It turns out that usually the former is present at a lower concentration [16,18], and thus it is desirable preferential degradation toward the most toxic substance. To overcome the problem of the co-existing non-target molecules, the molecular imprinting technique can be exploited to increase the selectivity of photocatalytic degradations. Molecular imprinting is wellknown for its capability of creating template-defined molecular recognition sites in synthetic polymers [19]. With this technique, specific binding sites can be created in either organic or inorganic materials using template-directed radical polymerization or polycondensation [20,21]. Although there are some reports in the literature showing the feasibility of preparing MIPs selective for DIC [22–25], the combination of photocatalysis with molecular imprinting to enable selective removal of toxic pharmaceuticals is an emerging field [15,17]. Besides the lack of selectivity, bare TiO2 have other intrinsic drawbacks that limit its applications, such as the difficulty to be recycled [26] and its limitation of relying on UV light for photocatalysis. The dependence of bare TiO2 on UV light for catalysis is due to the large bandgap of approximately 3.2 eV [27]. In order to improve the visiblelight-active photocatalytic efficiency and to inhibit charge recombination, several research groups have doped TiO2 with special compounds that are able to decrease the band gap energy. As a result, the doped TiO2 can effectively be activated by visible light. In this sense, doping TiO2 with non-noble metals has been increasingly studied [28–30]. Among the non-noble metals, Cu2O has been found to be a good modifier for achieving both UV and visible-light responses [31–37]. Several papers have reported the use of relatively high loading of TiO2, from 200 to 1000 mg/L, to photochemically degrade DIC [13,14,38–41]. Moreover, it has been shown that lower concentration of TiO2 might be used in combination of imprinting technology aiming to concentrate the target molecule before the photodegradation [42]. Based on the existing results, we envisaged that by introducing specific DIC binding sites into TiO2-based photocatalysts, it should be possible to use the new composite to concentrate target pollutants before they are photodegraded. In this way, it should be possible to lower the consumption of photocatalyst, realize higher selectivity and reduce the treatment cost. The aim of this work was to develop a TiO2-MIP composite to realize target-selective photocatalysts. The inorganic-organic composite

2. Experimental 2.1. Materials and methods Acetonitrile (99.7%), methacrylic acid (MAA, 98.5%), azobisisobutyronitrile (AIBN, 98%), trimethylolpropane trimethacrylate (TRIM) and D-(+)-glucose were purchased from Merck (Darmstadt, Germany). AIBN was recrystallized from methanol before use. Fluoxetine (FLU), paracetamol (PARA), diclofenac (DIC) and titanium dioxide (P25) were purchased from Sigma-Aldrich (Steinheim, Germany). Hydrated copper (II) acetate was purchased from Acros Organic (Geel, Belgium). Diclofenac sodium was extracted into chloroform from an acidic solution in order to obtain its acid form. All the aqueous solutions were prepared in deionized water. 2.1.1. Preparation of Cu-doped TiO2 Cu2O-doped TiO2 was prepared using an alcohol-aqueous based chemical precipitation method as described previously [37]. A calculated amount of copper (II) acetate monohydrate (1.4 g) was dissolved in 100 mL of ethanol to form a deep green solution and was stirred for 30 min. Then, 1.0 g of TiO2 (P25-Degussa) was added. The solution was sonicated (Branson 3200) to give a uniform suspension. After this step, 100 mL of aqueous glucose solution (0.2 M) as a reducing agent was added dropwise while the solution was heated at 60 °C. Thereafter, 120 mL NaOH solution (0.3 M, prepared in 70 mL ethanol and 50 mL water) was added dropwise. The yellow precipitate formed was separated by centrifugation (3500 rpm × 20 min) (Awel MF 48R), washed for three times with ethanol and water to remove the residual acetate, glucose and NaOH. Finally, the Cu2O-doped TiO2 was dried in a vacuum chamber overnight. 2.1.2. Preparation of molecularly imprinted photocatalysts Molecularly imprinted polymer-coated photocatalyst (MIP25) was synthesized as follows. The template molecule, diclofenac (0.53 mmol), was dissolved in 40 mL of acetonitrile in a 150 mm × 25 mm borosilicate glass tube equipped with a screw cap. Methacrylic acid (1.31 mmol), trimethylolpropane trimethacrylate (2.02 mmol), azobisisobutyronitrile (28 mg) and titanium dioxide (P25, 20 mg) were then added. To get a good dispersion of P25, the solution was sonicated in an ultrasound bath for 10 min. Then, the solution was purged with a gentle flow of nitrogen for 5 min and sealed. Polymerization was carried out by rotating the borosilicate glass tube horizontally in a Stovall HO-10 Hybridization Oven (Greensboro, NC, USA) at a speed of 20 rpm, at 60 °C for 24 h. After the polymerization, the solid particles were collected by centrifugation. The template was removed by washing with methanol containing 10% acetic acid (v/v), until no template could be detected from the washing solvent using UV spectrometric measurement. The composite particles were finally washed with acetone and dried in a vacuum chamber. For the preparation of molecularly imprinted polymer photocatalyst bearing Cu2O-doped TiO2 (MIPCuP25), the same procedure was used. In this case, the Cu2O-doped TiO2 was used instead of the bare TiO2. As control systems, two types of non-imprinted materials were synthesized under the same conditions except that no template was added in the reaction: one non-imprinted photocatalyst (NIP25) was prepared by adding P25, and another non-imprinted polymer (NIP) was prepared without adding P25. Similarly, a non-imprinted, Cu2O-doped photocatalyst (NIPCuP25) containing Cu2O-doped TiO2 was also synthesized. 730

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experiments were carried out using the same procedure as described above for characterizing the photocatalysis activity. At the end of each cycle, the samples were centrifuged for 15 min at 3500 rpm to remove the supernatant. Between two consecutive cycles, the catalyst was regenerated to remove the template (using the condition for template removal as described above). For comparison reasons, similar experiments were performed using the non-imprinted NIP25 as the photocatalyst. From the results, the relative effect of imprinting was assessed by comparing the imprinted catalyst with the non-imprinted catalyst. For each cycle, the imprinting factor (IF) was calculated using the following equation:

2.1.3. Adsorption experiments For adsorption capacity measurement, 33 mg of photocatalyst was added into 50 mL of DIC solution (0.062 mmol/L) at room temperature. Aliquots (1 mL) were collected with a syringe at regular intervals. Then, the particles were sedimented by centrifugation at 12000 rpm (Biofuge) for 15 min. The concentration of free DIC was determined by UV–vis absorbance at 274 nm using a DU 800 spectrophotometer (Beckman Coulter). The experiments lasted for 2 h and were performed in triplicates. The adsorption capacity was determined by using the following equation:

qe =

(C0 − Ce)V m

(1)

IF =

(C0: initial concentration; Ce: equilibrium concentration; V: volume of solution; m: amount of adsorbent used)

⎜

2.1.7. Characterization of materials FTIR analysis was carried out on a Nicolet iS5 FT-IR Spectrophotometer (Thermo Scientific). The dry particles were transferred onto the sample plate of the FTIR instrument and directly analyzed. All spectra were collected at room temperature in the 4000 cm−1 to 500 cm−1 region using 16 scans. X-ray diffraction (XRD) analyses were performed in a Rigaku DMAX 2200 diffractometer equipped with a Cu tube and secondary monochromator, using scintillation [NaI(Tl)] detector. Scanning electron microscopy (SEM) analysis was carried out using SEM LEO 1560 microscope (Zeiss, Oberkochen) operated at a voltage of 10 kV. X-ray energy-dispersive spectroscopy (EDS) measurements were performed using a JSM5800 (JEOL) microscope, operating at 10 kV. The samples were deposited on a carbon tape and then sputtered with a thin layer of gold. Transmission Electron Microscopy (TEM) images were obtained using a JFEI Morgani D268 electron microscope operated at 100 kV with 5000 − 10000 × magnifications. Specific surface area was determined using the Brunauer-EmmettTeller (BET) method at −196 °C, in the partial pressure range of 0.2 < P/P0 < 0.9 using a surface area analyzer (Gemini 2375 Micromeritics). Prior to each measurement, samples were preheated at 110 °C for 14 h under vacuum. The total pore volume was obtained from single-point desorption at P/P0 = 0.967. The pore diameter was determined using the Barret-Joyner-Halenda (BJH) method. Small-angle X-ray scattering (SAXS) measurements were performed at the D1B-SAXS1 beamline at the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. Dried samples were put between two Kapton® foils and the sample holder was sealed. The collimated Xray beam was passed through a chamber containing the sample and the measurements were performed at room temperature. Transmission, dark current, and Kapton® foil corrections were performed on the two dimensional image before further data processing. The incident beam was detected at two different sample-to-detector distances (1549.8 mm and 2245.7 mm) to increase the range of the scattering vector q (q = (4π/λ) × sin θ; 2θ – scattering angle). The isotropic scattering patterns were radially averaged. SAXS data analysis was performed using the Irena evaluation routine [43], which was implemented using Igor Pro Software (Wave-Metrics, Portland, USA) [44]. A multilevel unified fit was utilized to describe the two levels of structural organization that were evident in the scattering data. In this method, the scattering provided by each structural level is the sum of a Guinier exponential form and a structurally limited power-law tail. A generalized equation that represents the various levels can be written as:

⎟

(2)

(Ci: initial concentration; Cf: final concentration after adsorption/ degradation. For degradation, Ci is the initial concentration after 1 h of the dark-stage). 2.1.5. Selectivity experiments To assess the selectivity, separate experiments of photodegradation of FLU and PARA were carried out using the same procedure as described above for photodegradation of DIC (concentration of test compound 0.062 mmol/L, mass of photocatalyst: 33 mg). The final concentration of FLU and PARA was determined by measuring UV–vis absorbance at 242 and 225 nm for PARA and FLU, respectively. The coefficient of selectivity (kselectivity) was calculated using the following equations. For the imprinted systems (MIP25 and MIPCuP25):

kimprinted =

Ads / DegrDIC Ads / DegrFLU / PARA

(3)

For other photocatalysts (NIP25, NIPCuP25, P25 and CuP25):

k comparison =

k selectivity =

Ads/DegrDIC Ads/DegrFLU/PARA

(4)

kimprinted kcomparison

(6)

The IF value was calculated for the adsorption stage (IFads) and the degradation stage (IFdegr). The value of IF was reported as the mean of triplicate measurements.

2.1.4. Photocatalytic degradation In a typical degradation experiment, 50 mL of DIC (0.062 mmol/L) was added to 33 mg of photocatalyst in a bath reactor (100 mL) under flowing air (3 mL/min) at room temperature. The results of the photocatalytic tests were assessed at two stages under magnetic stirring (ca. 400 rpm) as follows: (i) dark stage, which consists of one hour with the lamp off (initial adsorption), and (ii) one hour with the lamp on (degradation). An ultraviolet lamp was used (366 nm, Sylvania 8 W Blacklight Blue Fluorescent). The tests were carried out at neutral pH. The liquid samples (1 mL) were collected with a syringe and stored in Eppendorf tubes and protected from light. To remove the catalyst, the samples were centrifuged for 20 min at 12000 rpm. The final concentration was determined by measuring the UV–vis absorption at 274 nm using a DU 800 spectrophotometer (Beckman Coulter). Photolysis tests were also performed to determine the percentage of degradation caused by exposure to the UV light in the absence of the photocatalyst. All the experiments were performed in triplicate. The adsorption and degradation were calculated by using the following equation:

Ci − Cf ⎞ Ads / Degr = ⎛ × 100% ⎝ Ci ⎠

Ads / Degr MIP 25 Ads / Degr NIP 25

(5)

2.1.6. Photochemical stability of MIP25 To evaluate the stability of the photocatalyst MIP25, the catalyst was reused in several cycles of photocatalytic reactions. The 731

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n

I (q) =

∑ i= 1

Pi

2 2 2 2 3 ⎛ −q Rg(i+1) ⎞ ⎡ (erf(qRgi / 6 )) ⎤ ⎛ −q Rgi ⎞ + Gi exp ⎜ B exp i ⎟⎢ ⎜ ⎥ 3 ⎟ 3 q ⎦ ⎠⎣ ⎠ ⎝ ⎝

Chu et al. (2011) and Adamu et al. (2017), respectively, while Xiu et al. (2009) have showed that the loaded Cu2O onto the surface of TiO2 did not significantly change the surface area. Possible reason for this reduce in area could be that Cu2O did not incorporate into the skeletal structure of TiO2, or that the heating combined with the addition of the others precursors during the synthesis of MICuP25 could have affected the porosity of the surface in comparison with bare TiO2. Moreover, the addition of copper (MICuP25) have increased the pore diameter in comparison with both P25 and MIP25, which is in agreement to the literature [36]. The materials were further analyzed by SAXS. The SAXS curves obtained for NEP25 and MIP25 are shown in Supplementary Information (Fig. S1). The analysis of the curves indicated that the measured materials have multi-scale structures containing three distinct levels of organization. In the high-q region, a shoulder-type Guinier pattern can be observed, which allows for the determination of the radius of gyration (Rg). In the low-q region, it is possible to observe a power-law decay, which makes it possible to obtain the fractal dimension of the system. Comparing NIP25 and NEP25, the estimated Rg is grater for the latter. This result suggests that the presence of DIC (in NEP25) perturbed the formation of the polymer particles. For the extracted materials (i.e., MIP25 and MICuP25), the value of Rg is higher than that of NIP25, NIP and NEP. Notably, the difference between NEP25 and MIP25 is the extraction step, and the increase of Rg value could be a consequence of an overall increase in the pore size [47]. This finding is in agreement with the increase of surface area, as previously commented. The material NEP25 showed a final particle aggregation consistent with a mass fractal structure, whereas all other materials showed P values consistent with surface fractals [51]. The change of P value from NEP25 to MIP25 suggests that the extraction procedure affected the aggregation of the particles, as can be clearly seen through SEM analysis (see below). The size distribution is shown in Supplementary Fig. S2. It can be seen that the hydrodynamic diameter of NIP was found to be ca. 2 times of that obtained from NIP25 and MIP25. This finding is in agreement with others imprinted nanoparticles prepared by precipitation polymerization, in which the MIP beads were about half of the size of the reference non-imprinted beads [52]. On the other hand, the PDI values (Table 1) observed from NIP25 and MIP25 are 2-fold of the value found for NIP. This result indicates that the composite materials NIP25 and MIP25 are less uniform due to the agglomerated particles. According to the IUPAC classification, all the materials prepared in this work contain macropores [53]. The imprinted materials showed larger pore volume and smaller pore diameter than the non-imprinted materials (NIP and NIP25). Again, this observation is similar to the previous results reported in the literature, and it indicates the presence of cavities in the imprinted materials [42,54,55]. The results of EDX analysis are shown in Table 2. For all the tested materials the presence of carbon atoms are obvious. For both NIP25 and MIP25, Ti atoms were also confirmed. However, for MIPCuP25, neither Cu nor Ti atoms were detected. Considering that EDX is more sensitive to the outermost surface composition, it is possible that the majority of Cu and Ti exist deep inside the polymer network. This hypothesis is corroborated by the XRD patters observed from MIPCuP25 (Fig. 1), where the characteristic peaks attributed to cuprous oxide [34,56] and titanium dioxide [57] are clearly shown. In addition, although MIP25 was predominantly amorphous, it still displayed some characteristic peaks of TiO2 [58], such as at 2θ of 25 and 48° (anatase). As can be seen in Supplementary Fig. S3, the control system NIP25 showed similar XRD to the MP25. Moreover, NIP showed a completely amorphous pattern due the absence of P25 during the synthesis. Fig. 2 shows the SEM images of NIP, NIP25 and MIP25. Both NIP and NIP25 are spherical particles, and NIP showed a much more homogeneous distribution and smaller diameter while the composite

(7) where n is the number of observed structural levels, G is the Guinier prefactor, Rg is the radius of gyration and B is a prefactor specific to power-law scattering, which is specified as the decay of the exponent P [45,46]. The size of the synthesized nanoparticles was determined by dynamic light scattering (DLS) using a Zetasizer Nano ZS instrument equipped with the DTS Ver. 4.10 software package (Malvern Instruments Ltd., Worcestershire, UK). The DLS measurements were carried out at 21 °C with a particle concentration of approximately 20 μg/mL in acetonitrile. Thermogravimetric analysis (TGA) was performed on a SDT Q600 thermal analyzer Q20 (TA Instruments) under flowing nitrogen at a scanning rate of 20 °C/min from 25 to 800 °C. 3. Results and discussion 3.1. Characterization The surface area and pore characteristics of the photocatalysts prepared in this work are shown in Table 1. For the sake of clarity, a brief description of each material is also given. The imprinted material containing non-extracted template molecules (NEP) had a surface area 19.2- and 4-folds of the non-imprinted material NIP25 and NIP, respectively. This dramatic difference of surface area indicates that the presence of the template perturbs the system and affects the formation of the pores. Comparing the imprinted material before and after template removal (NEP25 vs. MIP25), it can be seen that the surface area increased (61%) and the pore volume decreased after the template was removed. This physical change is often attributed to the removal of template from the polymer matrix during the extraction. In this sense, the increased surface area caused by the extraction process can be attributed to the formation of cavities in the imprinted material. Similar results have been reported in the literature for MIPs based on silica matrices [47,48]. In addition, the higher surface area and pore volume of the imprinted polymers (MIP25 and MICuP25) compared to the non-imprinted NIP and NIP25 suggest that more binding sites are created during the imprinting process. In consistent with the recently reported results, the MIPs prepared in this work also have a higher specific surface area (SBET) and larger pore volume (Vp) than that of the corresponding NIPs [49,50]. After addition of copper on TiO2 (MICuP25), the value of area is reduced by 61% in comparison to the P25, and 77% in comparison to the MIP25. A decrease of area of 90% and 11.5% was also reported by Table 1 Textural properties of the synthesized materials. Material

Specific surface area [SBET] (m2/g)

Pore volume [Vp] (cm3/ g)

Pore diameter [Dp] (Å)

Rga (nn)

Pb

PDI

NEP25 MIP25 MICuP25 NIP25 NIP P25

57.7 92.9 21.0 3.0 14.4 54.0

27.50 0.16 0.05 0.01 0.03 0.07

235.7 114.4 119.0 315.6 125.5 48.0

0.86 1.93 1.47 1.40 0.81 n.d.

2.35 3.58 3.23 3.88 3.34 n.d.

n.d. 0.73 n.d. 0.69 0.36 0.39

NEP25: Non Extracted Polymer containing P25; MIP25: Molecularly Imprinted Polymer containing P25; MICuP25: Molecularly Imprinted Polymer containing Cu-doped P25, template removed; NIP25: Non-Imprinted Polymer containing P25; NIP: Non-Imprinted Polymer. PDI: Polydispersity Index. a Radius of gyration extracted from high-q region (SAXS). b power-law decay extracted from low-q region (SAXS). n.d.: not determined.

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large particles will dominate, making smaller particles difficult to observe [60]. Similar morphologies have been found for imprinted polymers prepared using MAA [22,61] and acrylamide [62] as the functional monomers (both synthetized by precipitation polymerization method). As discussed elsewhere [63], the particle diameter of the final product can be influenced by the nucleation step of the reaction and the duration of the polymerization. The presence of template could also influence the nucleation step of a precipitation polymerization. In this work, the presence of P25 and the extraction process were also found to contribute to the differences of morphologies observed among the samples. Fig. 3 shows the TGA curves of MIP25 and MIPCuP25. For MIP25 the weight loss reached 93.4% at 755 °C. This weight loss was attributed to the decomposition of the polymer layer, thus the remaining mass was assigned to the remaining TiO2. For MIPCuP25, there is a noticeable and a large weight loss at 90 °C and 275 °C, possibly due to the evaporation of water from the sample. The remaining mass after 430 °C was assigned to the thermal-resistant TiO2 and Cu2O. The remaining mass observed from TGA analysis of MIP25 and MIPCuP25 (6.6% and 16.6%, respectively) was used to estimate the amount of the photocatalytic P25 and Cu2O-doped P25 of the materials used in the remaining experiments.

Table 2 Atomic ratio of atoms detected by EDX analysis. Atomic Ratio

NIP25 MIP25 MIPCuP25

C/O 4.30 4.06 5.38

Ti/O 0.04 0.35 n.d.

Cu/O n.a. n.a. n.d.

3.2. Adsorption experiment Fig. 1. XRD patterns of MIP25 (a) and MICuP25 (b).

In order to investigate the time needed for adsorption of DIC, kinetic binding experiments were carried out first. As shown in Fig. 4, within the first 30 min, the adsorption of DIC (Q) increased with the incubation time. After 60 min, the value of DIC adsorption, Q, did not increase further, and the final value of Q for MIP25 (8.6 mg/g) is 59% higher than that of NIP25. The higher DIC binding by MIP25 is attributed to the three-dimensional imprinted cavities that are able to bind DIC with a higher affinity. In the following investigations of photocatalysis, 60 min was chosen as the incubation time in the dark-stage experiment.

NIP25 particles displayed a large variation of particle size. In addition, according to the TEM analysis (Supplementary Fig. S4), the imprinted composite MIP25 has a rougher surface in comparison with P25, suggesting the formation of the imprinted polymer layer atop of the inorganic cores. The material containing imprinted cavities (MIP25) showed a rougher structure and agglomerated particles, which is consistent with the high PDI value found in the DLS analysis (Table 1). We should note that dynamic light scattering cannot precisely resolve polydisperse particle samples, because this analytical method measures the intensity of scattered light that is proportional to the sixth power of the particle diameter [59]. As a consequence, the light scattered by agglomerates or

3.3. Comparison of different photocatalysts The photocatalytic activities for degradation of DIC of the studied photocatalysts and the photolysis are shown in Fig. 5. After UV light Fig. 2. SEM images of NIP (a), NIP25 (b) and MIP25 (c).

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Fig. 5. Degradation of DIC catalyzed by different materials.

Fig. 3. Thermogramms of MIP25 and MICuP25.

photolysis. Concerning the imprinted composite MIP25, the Rphoto values were found to be 2.62, 3.95 and 6.1 for PARA, FLU and DIC, respectively. The highest value of Rphoto for DIC degradation can be explained as a result of the selective binding of DIC to the imprinted polymer, which enriched the target pollutant to the catalyst to enable more efficient photodegradation. The photocatalytic activities of NIP25 and P25 for degradation of DIC, FLU and PARA are shown in Fig. 6b and c, respectively. It can be seen that the degradation of the target molecule (DIC) is intermediate in the case of using NIP25, and is the lowest in the case of using P25. Actually, the degradation of FLU using P25 is quite similar to the degradation of DIC, while PARA was the most degradable molecule when P25 was used as the catalyst. This result may be attributed to the fact that PARA is a simpler molecule than DIC and FLU, and may be more easily removed using TiO2 as catalyst. Interestingly, degradation of PARA was the slowest obtained for the non-imprinted catalyst, NIP25. When NIP25 was used as the photocatalyst, FLU was the mostly adsorbed molecule and therefore was degraded the fastest. In order to further evaluate the selectivity of MIP25, the coefficient of selectivity (κselectivity) was calculated separately for the adsorption and the photodegradation stage, and the results are shown in Tables 3 and 4. During the adsorption stage (Table 3), the κselectivity of MIP25 relative to NIP25 and P25 was higher than 1.0 for both FLU and PARA. One exception is that there was no appreciable selectivity when adsorption of DIC and PARA on MIP 25 and NIP25 was compared. As it can be seen, although the selectivity of DIC binding relative to PARA on MIP25 is high (κimprinted = 6.59), NIP25 also showed high binding to DIC than PARA, making the calculated κselectivity to become insignificant. Despite this result, all the other values of κselectivity ranged from 4.03 to 329, confirming that in general the imprinted photocatalyst had a high binding selectivity for the target pollutant. For photodegradation stage, MIP25 relative to NIP25 and P25 was higher than 1.0 compared to both FLU and PARA. Similar to the adsorption stage, the exception was for the degradation of PARA using NIP25. On the other hand, the selectivity of DIC degradation using MIP25 relative to PARA degradation using P25 was also high. Although the κselectivity values measured in the degradation stage are lower than that measured at the adsorption stage, the ability of MIP25 to recognize the template molecule to facilitate photodegradation still persisted. The values of kimprinted suggest that MIP25 possesses better ability to bind FLU than PARA, which agrees to the previous discussion that FLU is more similar to the template molecule (DIC) than PARA. Compared with some recent studies [13,14,64] where the degradation of DIC was reported to vary from 46 to 55% using a loading of TiO2 from 200 to 250 mg/L (for a similar drug concentration herein investigated) and a reaction time typically of 2 h under UV light, the imprinted catalyst MIP25 developed in this work exhibited a similar

Fig. 4. Adsorption isotherms of DIC on NIP25 and MIP25.

irradiation for 300 min, the degradation of DIC with MIP25 reached 62.5%, which is much higher than that achieved with NIP25 and with NIP. In addition, MIP25 showed a photodegradation of DIC 7-fold higher than its uncatalyzed photolysis. As no imprinted cavities exist in NIP25 and NIP, this higher photo degradation result obtained with MIP25 suggests that the specific binding sites in MIP25 were able to enrich the target pollutant to the vicinity of the embedded photo catalyst to be more efficiently degraded. 3.4. Selectivity of photocatalysis with non-doped materials Fig. 6 shows the photocatalytic activity of MIP25, NIP25 and P25 for degradation of DIC, FLU and PARA. As shown in Fig. 6a (photodegradation with MIP25), after 120 min of reaction under UV irradiation, the degradation of DIC was higher than those of PARA and FLU. Compared to FLU, faster degradation of DIC was achieved in the first 5 min. Also, after an extended reaction time DIC was found to be the mostly degraded compound. These results suggest that using DIC as a template molecule it is possible to create well defined imprinted cavities in composite catalyst to enable more efficient photocatalytic degradation of DIC. The reason that FLU was degraded faster than PARA in the presence of MIP25 can be explained as the following: FLU is more similar to DIC than PARA and therefore easier to bind to the DIC-imprinted cavities to be more efficiently degraded. It is interesting to compare the ratio of decomposition obtained from photocatalytic degradation with that obtained from photolysis after 2 h of reaction (herein labelled as Rphoto). A low value of Rphoto means that the photocatalytic degradation is comparable to the degradation by 734

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Table 4 Selectivity parameters for photodegradation stage (UV-light) for different non-doped systems after 2 h of reaction. System

Degradation (%)

κimprinted

κcomparasion

κselectivity

MIP25 (DIC) MIP25 (FLU) MIP25 (PARA) NIP25 (DIC) NIP25 (FLU) NIP25 (PARA) P25 (DIC) P25 (FLU) P25 (PARA)

48.72 31.40 17.13 35.20 48.0 9.45 31.49 34.51 78.3

– 1.55 2.84 – – – – – –

– – – – 0.73 3.72 – 0.91 0.40

– – – – 2.12 0.76 – 1.70 7.10

photocatalytic activity. Previous studies have shown [13,38,65] that, although the photocatalytic efficiency can be increased by increasing the amount of TiO2, high loadings of TiO2 (800–900 mg/L) however resulted in no positive effect on the degradation, probably due to the scattering of light. In this sense, the imprinting technique could be utilized to enhance not only selectivity but also the degradation performance of the photo catalysts allowing a low concentration of TiO2 to be used. From the TGA results it is possible to estimate that the current MIP25 loading corresponds to a TiO2 amount of ca. 44 mg/L, which is at least 4.5-fold lower than that reported in the literature. Therefore, the degradation of DIC using MIP25 was much more efficient than the results reported in the previous literature.

3.5. Selectivity of Cu-doped photocatalysts The selectivity of MICuP25, both for the adsorption and the degradation stages, is shown in Tables 5 and 6. In the adsorption stage (Table 5), the κselectivity of MICuP25 for DIC binding relative to NICuP25 and CuP25 was higher than unity, ranging from 1.45 to 157.8. As a general trend (except for the κselectivity relative to NICuP25 for PARA), the κselectivity values measured on MICuP25 were lower than that measured on MIP25. The selectivity of the photodegradation stage is shown in Table 6. Comparing the κimprinted values measured on the Cu2O-doped and nondoped catalysts (Tables 4 and 6), it is possible to note that there is no difference in the case of FLU, and a decrease is observed in the case of PARA. Nevertheless, in both cases the measured κimprinted values are higher than 1. This result indicates that the presence of copper did not compromise the ability of the imprinted catalyst to recognize the target molecule. The coefficient of selectivity of DIC binding on MICuP25 relative to FLU and PARA binding on NICuP25 were 3.97 and 2.95, respectively. These data indicate that κselectivity mainly depended on the structure of MICuP25 that possessed the three-dimensional imprinted cavities. Comparing with CuP25 and P25 (Tables 4 and 6), the value of κcomparision is higher than that calculated against NIPCuP25 (i.e., 3.6 and

Fig. 6. Photocatalytic activity of MIP25 (a), NIP25 (b) and P25 (c) for degradation of DIC, FLU and PARA.

Table 3 Selectivity parameters measured for adsorption stage (dark-stage) for different non-doped systems after 1 h of mixture.

Table 5 Selectivity parameters for adsorption stage (dark-stage) for different doped systems after 1 h of mixture.

System

Adsorption (%)

κimprinted

κcomparasion

κselectivity

System

Adsorption (%)

κimprinted

κcomparasion

κselectivity

MIP25 (DIC) MIP25 (FLU) MIP25 (PARA) NIP25 (DIC) NIP25 (FLU) NIP25 (PARA) P25 (DIC) P25 (FLU) P25 (PARA)

32.58 27.82 4.94 22.20 74.8 3.46 0.73 6.3 28.4

– 1.17 6.59 – – – – – –

– – – – 0.29 6.41 – 0.11 0.02

– – – – 4.03 1.02 – 10.60 329.50

MICuP25 (DIC) MICuP25 (FLU) MICu25 (PARA) NICuP25 (DIC) NICuP25 (FLU) NICuP25 (PARA) CuP25 (DIC) CuP25 (FLU) CuP25 (PARA)

70.07 56.72 8.88 31.10 37.94 32.11 0.33 0.80 6.60

– 1.23 7.989 – – – – – –

– – – – 0.82 0.96 – 0.41 0.05

– – – – 1.45 8.21 – 3.00 157.8

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Table 6 Selectivity parameters for photodegradation stage (UV-light) for different doped systems after 2 h of reaction. System

Degradation (%)

κimprinted

κcomparasion

κselectivity

MICuP25 (DIC) MICuP25 (FLU) MICuP25 (PARA) NICuP25 (DIC) NICuP25 (FLU) NICuP25 (PARA) CuP25 (DIC) CuP25 (FLU) CuP25 (PARA)

45.72 29.38 23.04 10.92 27.97 16.1 17.62 5.37 17.60

– 1.55 1.98 – – – – – –

– – – – 0.39 0.67 – 3.28 1.00

– – – – 3.97 2.95 – 0.47 1.98

2.5-fold, for FLU and PARA, respectively). The higher κcomparision values suggest that the addition of copper influenced the preferential degradation of DIC. In the case of FLU, due to its being more similar to the template molecule, it caused a lower κselectivity value of 0.47. On the other hand, PARA did not fit the imprinted sites, thus a higher selectivity for DIC is preserved. In contrast to the non-selective commercial sample of TiO2 (P25 and CuP25 from Tables 4 and 6), the average value of selectivity of the imprinted catalysts for photocatalytic degradation of DIC was estimated to be 2.8, which suggests that the specific binding sites created by the molecular imprinting are essential for gaining high catalytic selectivity and efficiency.

Fig. 8. Imprinting factor of MIP25 after several cycles of photocatalytic applications.

materials after 6 cycles of use were nearly identical to that of the fresh sample (Supplementary Fig. S5). The IR spectra for both the MIP and NIP have shown some typical peaks found in the polymers based on MAA and/or TRIM monomers [17,66], for example the bands at 1257 and 1389 cm−1 from the stretching vibration of CeO and the vibration of −Oe, respectively. No new band appeared after the polymers went through 6 cycles of applications. In addition, the morphologies of the samples have no obvious alteration after 6 cycles of use (Supplementary Fig. S6). These results suggest that the morphology and the structure of the polymers, and presumably the imprinted sites, were not destroyed over several cycles of reaction under the action of UV light.

3.6. Photochemical stability of MIP25 The photochemical stability of MIP25 was evaluated by measuring the efficiency of re-cycled MIP25 to degrade DIC. As shown in Fig. 7, after 6 cycles of use, MIP25 maintained almost the same photocatalytic efficiency to degrade DIC despite a noticeable loss of adsorption. The retention of selectivity in recycled MIP25 was further confirmed by comparing the DIC binding and photocatalytic degradation by MIP25 versus NIP25 (Fig. 8), where the results are presented in terms of imprinting factor (IF). As a general trend, when the IF for adsorption increased, the IF for degradation also increased (and vice-versa). The increase in IF up to cycle 4 was caused by that the non-imprinted catalyst (NIP25) showed lower adsorption and degradation after the repeated uses. More importantly, the IF is higher than unity for all the cycles, indicating that the imprinted sites in MIP25 were reserved and capable of molecular recognition after each regeneration step. Considering that the samples consisted of polymer network, one question arises about the possibility of degradation of the polymer itself after several cycles of 2 h of UV irridation. The FT-IR spectra of the

4. Conclusions In this work, molecularly imprinted photocatalysts containing a low loading of TiO2 and Cu2O-doped TiO2 were prepared using a precipitation polymerization method. The imprinted photocatalysts exhibited target-specific molecular binding and degradation for a model water pollutant, diclofenac. Compared to the commercially available TiO2, the molecularly imprinted photocatalysts displayed significantly higher selectivity and efficiency for degradation of the target pollutant due to the presence of specific binding sites in the composite materials. After 6 cycles of use under UV- irradiation, the imprinted photocatalysts maintained their imprinted factor and structural integrity. The findings from this work suggest that combining molecular imprinting with photocatalytic degradation is an effective strategy to design new systems for environmental remediation. Acknowledgements This project was partially funded by the Brazilian National Council for Scientific and Technological (CNPq) and the Swedish Research Council FORMAS (contract no. 212-2013-1305). C. Escobar is grateful for the grant provided by the Improvement of Higher Education Personnel (CAPES). The authors wish to thank the LNLS (Project D11ASAXS1-8691) for the SAXS beamline measurements, and Tripta Kamra for carrying out the SEM analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2017.11.044. References [1] V.V. Ranade, V.M. Bhandari, Industrial Wastewater Treatment, Recycling and Reuse, in: V.V. Ranade, V.M. Bhandari (Eds.), Industrial Wastewater Treatment,

Fig. 7. DIC adsorption and degradation obtained by MIP25 after different cycles of photocatalytic applications.

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