Visible Light Caffeic Acid Degradation by Carbon-Doped Titanium

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Visible Light Caffeic Acid Degradation by Carbon-Doped Titanium Dioxide Francesco Venditti,† Francesca Cuomo,‡ Andrea Ceglie,‡ Pasquale Avino,§ Mario Vincenzo Russo,‡ and Francesco Lopez*,‡ †

Consorzio per lo Sviluppo Industriale della Valle del Biferno (COSIB), 86075 Termoli, Italy Department of Agricultural, Environmental and Food Sciences (DiAAA) and Center for Colloid and Surface Science (CSGI), Università degli Studi del Molise, Via De Sanctis, I-86100 Campobasso, Italy § Air Chemical Laboratory, DIPIA, INAIL Settore Ricerca, Via IV Novembre 144, 00187 Rome, Italy ‡

ABSTRACT: The removal of the phenolic compound, caffeic acid, by photodegradation has been investigated using carbon-doped titanium dioxide particles as a photocatalyst under visible light. UV−vis absorption spectroscopy and gas chromatography−ion trap mass spectrometry analyses revealed a substrate concentration dependence of the removal of caffeic acid from a water solution. The k2 and t0.5 parameters of each reaction were calculated by fitting kinetics data to a second-order kinetic adsorption model. To evaluate the photodegradation event, the effect of the adsorption process on the whole degradation was also monitored in the absence of light. Adsorption isotherm studies supported by ζ potential and scanning electron microscopy data demonstrated the pivotal role of the absorption mechanism. It was found that the whole photodegradation process is governed by a synergic mechanism in which adsorption and photodegradation are involved. This study, centered on the removal of caffeic acid from aqueous solutions, highlights the potential application of this technology for the elimination of phenolic compounds from olive mill wastewater, a fundamental goal in both the agronomical and environmental fields.

1. INTRODUCTION Phenolic compounds make up a class of molecules consisting of a hydroxyl group directly bonded to an aromatic hydrocarbon group.1 These compounds are ubiquitous in plants and essential for the human diet.2,3 All of the phenolic compounds, because of their antioxidant properties, possess several common biological and chemical properties, namely, the aptitude to inhibit nitrosation and to chelate metal ions, the capability to scavenge both active oxygen species and electrophiles, the ability to modulate certain cellular enzyme activities, and the potential for autoxidation.4−7 Among these, caffeic acid, also defined as hydroxycinnamic acid, is found in every plant because it is a key intermediate in the biosynthesis of lignin, one of the principal components of plant biomass.8 Besides all the benefits related to human, environmental, and food areas, the presence of such a huge amount of phenolic compound represents an ecological problem.9,10 For Mediterranean countries, the disposal of olive mill wastewater (OMW), a byproduct of the olive oil extraction process, represents a fundamental challenge for the agronomical and environmental points of view. OMW is, in fact, extremely rich in phenolic compounds, which are responsible for very high COD values and have very strong antimicrobial and phytotoxic properties.11,12 Consequently, these substances are resistant to biological degradation, and their disposal on agricultural soils will cause their accumulation, thus determining problems of soil fertility and groundwater contamination. In addition, the low limit values of polyphenol disposal and their widespread and © XXXX American Chemical Society

threatening presence have accelerated the search for advanced and economically attractive treatment technologies for their removal. Several methods such as adsorption and coprecipitation, coagulation, filtration and evaporation in open ponds, and reverse osmosis have been used to treat OMW. In many cases, the environmentally most compatible solution gives only partial results for the problem; in other cases, the expensiveness makes the solution not feasible.12,13 Generally, a number of significant applications in the pollutant removal area stress the importance of synthesizing hybrid materials that are self-supporting, without losing the properties of adsorbent devices that are also important to such processes.14−16 The application of titanium dioxide as a heterogeneous photocatalyst is well-established for the remediation of water and air purification.17−20 However, titanium dioxide has a bandgap of 3.2 eV, which can be activated only under UV light irradiation at wavelengths lower than 387 nm. Phenolic compounds in aqueous solutions were successfully degraded by means of photocatalysis using TiO2.21−23 The practice is based on photoactivation of titanium dioxide with UV light, which leads to a sequence of reactions resulting in the production of oxidants. The so formed compounds (hydroxyl radicals) can easily react with an organic compound on the TiO2 surface.24,25 Recently, Baransi et al. reported the Received: November 21, 2014

A

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light source and the bottom of the solution was ∼15 cm. A Finnegan Trace GC Ultra gas chromatograph equipped with an Ion Trap Mass Spectrometry detector Polaris Q (Thermo Fisher Scientic, Waltham, MA), equipped with a Programmed Temperature Vaporizer (PTV) injector, was used for the GC−IT/MS analysis. A fused silica capillary column with a chemically bonded phase (SE-54, 5% phenyl and 95% dimethylpolysiloxane) was prepared in our laboratory with the following characteristics: 30 m × 250 μm inside diameter, N (theoretical plate number) of 118000 for n-dodecane at 90 °C.35 The samples were extracted and derivatized in the presence of 4benzopyrene used as an internal standard (0.2 μg L−1) with the USVADLLME (ultrasound vortex-assisted dispersive liquid−liquid microextraction) procedure reported previously.36 Scanning electron microscopy (SEM) images were obtained with a Zeiss DSM 940 instrument. Samples were deposited onto glass plates, left to dry for 5 h at room temperature, and then sputtered with gold. ζ potential measurements were performed by laser Doppler velocimetry using a Zetasizer-Nano ZS90 Malvern UK commercial instrument operating with a 4 mW He−Ne laser (633 nm wavelength). The samples were placed in dedicated disposable capillary cells. The cells were calibrated before each set of measurements with a latex standard solution (−50 ± 5 mV). 2.4. Photodegradation. Photocatalytic degradation of caffeic acid was conducted using CDT under visible irradiation. Ten milliliters of a caffeic acid solution, at different concentrations, and 10 mg of CDT were placed in 25 mL open glass flasks to allow a constant oxygen level (under atmospheric pressure) and mechanically stirred. The temperature was kept constant at 25 °C. The samples were placed in the reactor and illuminated for various time intervals. The same procedure, except for the latter step, was applied for experiments in the dark where the samples were protected from light. At predetermined time intervals, aliquots of the sample were withdrawn, diluted 1:5, and centrifuged. The caffeic acid content was determined spectrophotometrically at 312 nm, and its decrease was determined as the difference between initial and final solution concentrations with appropriate corrections based on blanks. Changes in caffeic acid concentrations due to water evaporation were taken into account and corrected. All the experiments were performed in triplicate, and the results presented were the mean values. The percentage of caffeic acid removal was calculated as Ct/C0, where C0 is the initial concentration of caffeic acid and Ct is the concentration of caffeic acid at time t. The reusability experiments were performed as follows. After the first run of the photodegradation reaction, the suspension of CDT and the purified solution was dried overnight at 100 °C. Successively, a fresh solution of caffeic acid was added to the CDT powder.

photocatalytic degradation of two phenolic compounds, pcoumaric acid and caffeic acid, performed with a suspended mixture of titanium dioxide and powdered activated carbon.26 The authors emphasize the role of the adsorption process in the entire photodegradation process. Generally, adsorption processes are mostly related to the surface charge interactions between the adsorbent and the substrate. The entire process can become relatively intricate, depending on other parameters.27−29 For these reasons, a detailed study of the adsorption phenomena occurring on titanium particles could provide considerable insight into the understanding of the whole photodegradation reaction. Efforts have been made to explore methods to provide the photoactivation of titanium dioxide under visible light. Doping of TiO2 represents a widely used approach for developing titanium dioxide-based materials and for environmental applications.30 In this direction, different methods for the synthesis of carbon-doped TiO2 particles have been proposed to improve the photocatalytic activity.31−33 Ren and co-workers synthesized a visible light-active TiO2 photocatalyst prepared through carbon doping using glucose as a carbon source, performed by a hydrothermal method at low temperatures.34 Considering the enhanced photocatalytic activity of this type of carbon-doped TiO2 (CDT) toward substances like rhodamine B and its energy saving properties, the use of this type of particles seems to be highly appropriate for the removal of pollutants. In view of the high level of phenolic compounds in OMW, caffeic acid can be considered as a suitable model molecule for purification and remediation studies. The aim of this investigation is the study of the photodegradation process of the phenolic compound, caffeic acid, as well as the adsorption properties of a visible light-active TiO2 photocatalyst. The final goal is to provide new tools for making a contribution to the challenging waste disposal issue.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Caffeic acid (3,4-dihydroxycinnamic acid, 99%), glucose, titanium dioxide (TiO2), and titanium isopropoxide (97%) were from Sigma-Aldrich and used without further treatment. Initial concentrations of caffeic acid were in the range of 18−135 mg L−1, and the pH before irradiation was in the range of 4−4.5 depending on the concentration used. 2.2. Glucose-Doped Titanium Preparation. Carbon-doped TiO2 (CDT) was synthesized following the method reported by Ren et al.34 Briefly, the amorphous TiO2 particles were prepared by controlled hydrolysis of titanium isopropoxide in ethanol; 100 mL of ethanol was mixed with 0.4 mL of a 0.1 M potassium chloride solution, followed by addition of 2.2 mL of titanium isopropoxide while the mixture was magnetically stirred. After this step, the sample was stirred for 10 min to produce a white precipitate, and then the suspension was aged for 24 h. The suspension was collected by filtration and overdried at 60 °C to yield amorphous TiO2 particles. Carbon-doped TiO2 was synthesized as follows: 0.25 g of amorphous TiO2 and 0.018 g of glucose were added to 18 mL of deionized water in a 25 mL open glass flask and placed in a heating oven. The oven temperature was kept constant at 160 °C for 12 h, and then the sample was cooled to room temperature in air. 2.3. Instrumentation. UV−vis spectra were recorded using a double-beam thermostated spectrometer (Cary 100-Varian), in the 200−800 nm region with cells with a path length of 1 cm. The irradiation experiments were performed by putting the samples in a self-built light reactor. The photocatalytic activities of the samples were evaluated with 180 W biolux OSRAM fluorescent lamps (6500 K). The photoemission spectrum of the fluorescence lamps provides visible light in the range of 400−800 nm. The distance between the

3. RESULTS AND DISCUSSION Tests of photocatalytic activity for the degradation of caffeic acid were conducted by lighting an aqueous suspension of caffeic acid and CDT particles with visible radiations. The disappearance of caffeic acid from the suspension was followed by UV−vis absorption spectroscopy. As reported in Figure 1, caffeic acid presents two absorption bands centered at 285 and 312 nm. The decrease in band intensity upon irradiation indicated that caffeic acid was removed from the aqueous solution. As shown in Figure 1, more than 90% of caffeic acid (18 mg L−1) was removed from water within 60 min, indicating a fast deletion rate. Furthermore, in the inset of Figure 1, the spectra of caffeic acid before (t = 0) and after irradiation for 60 min in absence of CDT are reported. The two spectra are very similar, indicating the stabilization of caffeic acid under visible light irradiation and the absence a self-degradation process. This is a remarkable point emphasizing that the light source used for the irradiation (in this study 180 W fluorescent lamps, 6500 K) did not affect the stability of caffeic acid in the absence of CDT. It was previously reported that the radiation from a Hg−Xe lamp led B

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(retention time of 8.6 min) completely disappeared. This analysis ensures that the data obtained by the two techniques, UV−vis spectroscopy and GC−IT/MS, are comparable from a quantitative point of view. It is established that photocatalytic reactions in the presence of TiO2 are based on a free radical reaction initiated by light.38 As for the general photodegradation effect, titanium dioxide is considered an efficient photocatalyst that can form hydrogen and oxygen from water and active species such as hydroxyl radical (OH•). The mechanism might depend on the ability of the degraded compound to be adsorbed on the surface of the catalyst, as well as on a parameter such as pH, O 2 concentration, or substrate concentration.39 In this study, the amount of O2 was kept constant for all the samples and the initial pH values were in the range of 4−4.5 depending on the caffeic acid concentration used. Furthermore, the effect of caffeic acid concentration (range of 18−135 mg L−1) on the removal process was tested. The initial caffeic acid concentration was varied while the amount of CDT was kept constant (10 mg). In Figure 3A, the kinetics of

Figure 1. UV−vis spectra obtained for various times of irradiation of caffeic acid (18 mg L−1) in the presence of CDT particles (1 mg mL−1). The inset shows spectra of the irradiated sample without CDT after 60 min and before the irradiation (t = 0) in the presence of CDT.

to photodegradation of the caffeic acid.37 Via comparison of the spectra at 60 min in the presence and absence of CDT particles, it is clear that the decrease in the caffeic acid content occurs only in the presence of the CDT particles. The removal of caffeic acid from water in the presence of CDT was confirmed by GC−IT/MS. In Figure 2, chromatograms of caffeic acid extracts, as a function of time, are displayed. As shown, after 90 min, the caffeic acid peak

Figure 3. (A) Removal of caffeic acid from acqueous solutions at different concentrations in the presence of C-doped TiO2, under visible light irradiation (λ > 420 nm). (B) Fitting of removal profiles to eq 1.

removal of caffeic acid at four different concentrations in the presence of C-doped TiO2 under visible light irradiation are reported. The experimental data, expressed with the ratio Ct/C0 as a function of time at 25 °C, indicate that the removal of caffeic acid is quite rapid; in fact, after 60 min, 92, 75, 65, and 48% of the caffeic acid had disappeared from the bulk solution, depending on the initial caffeic concentrations (18, 36, 54, and 90 mg L−1, respectively). As shown, the kinetics become slower with time, reaching a constant value after different time intervals depending on the initial concentration of the subtrate (equilibrium time). From the kinetic data, a dependence of concentration on the caffeic acid removal process was demostrated, typical of an adsorption process, and foretells a pivotal function of the adsorpion event on the photoreaction. All the removal profiles determined at different subtrate concentrations were fit to a second-order kinetic model17 expressed by eq 1 reported below; from this equation, some kinetic parameters were obtained (Figure 3B). t 1 t = + q qe k 2qe 2 (1) where k2 (kilograms per gram per minute) is the rate constant of second-order adsorption and q and qe (grams per kilogram) are the amounts of species adsorbed on the adsorbent at time t and at equilibrium, respectively. If second-order kinetics can be

Figure 2. Chromatograms of derivatized extracts obtained from the caffeic acid-containing solution (18 mg L−1) under visible light irradiation in the presence of CDT. C

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Table 1. Values of k2, qe, and t0.5 at Different Caffeic Acid Concentrations (1.8 × 10−2 to 1.35 × 10−1 g L−1) Obtained by Fitting the Experimental Data to eqs 1 and 2a

a

sample (g/L)

1.8 × 10−2

3.6 × 10−2

k2 (kg/g min) qe (g/kg) t0.5 (min)

7.496 1.81 × 10−2 7.376

1.355 3.81 × 10−2 19.36

k2 (kg/g min) qe (g/kg) t0.5 (min)

16.37 1.21 × 10−2 5.02758

2.092 2.52 × 10−2 18.94

5.4 × 10−2 light 0.6647 5.61 × 10−2 26.83 dark 1.494 3.40 × 10−2 19.67

9.0 × 10−2

1.35 × 10−1

0.2030 8.80 × 10−2 55.97

0.09148 1.29 × 10−1 84.948

0.4974 5.48 × 10−2 36.71

0.1784 5.92 × 10−2 94.70

Light, from experiments conducted in the presence of light. Dark, from experiments conducted in the absence of light.

applied, the plot of t/q versus t should give a linear relationship, as shown in Figure 3B. From eq 2, the half-life of the process can be calculated:40 1 t0.5 = k 2qe (2)

dark, a marked difference emerged, implying that it is possible to identify the existence of the adsorption process. Actually, Figure 4, besides the identification of two distinct phenomena (adsorption and photodegradation), ascertains that the kinetic profile with respect to the experiments performed in the presence of light came from the contemporary contribution of both adsorption and photodegradation processes. In a comparison of the kinetic parameters listed in Table 1, it is evident that the experiments conducted under light and in the dark are governed by different phenomena. Taking into consideration the values of qe, i.e., the amount of caffeic acid adsorbed on the adsorbent at equilibrium, we can see that these values match the amount of caffeic acid in solution only when samples are placed under light irradiation. On the other hand, in absence of light the qe values are far from the amounts of the species in solution as the concentration of caffeic acid increases. Because the qe values are smaller in the dark, the equilibrium is reached faster under these conditions, as well indicated by the values of the kinetic constants, k2, that are higher for the experiments conducted in the dark compared to those conducted under light. In Figure 5, the correlations between the two different conditions, for the extrapolated kinetic parameters, are shown by means of parity plots. Panels A and B of Figure 5 indicate the lack of correlation for parameters k2 and qe, respectively. This evidence underlines the role played by light irradiation. If the process governing the caffeic acid elimination in the light or dark had been the same, the points would have belonged to the dotted straight line represented in Figure 5. From our study, the importance of the adsorption process is well -established and could be useful for determining the caffeic acid absorption efficiency of the CDT particles under the reported experimental conditions. Equilibrium data were analyzed using the adsorption isotherms, which are helpful in determining the adsorption capacity of CDT for caffeic acid. Adsorption kinetic data of caffeic acid at different initial concentrations were fit to the Freundlich adsorption isotherm. The adsorption capacity depends on the chemical and physical properties of the adsorbent. The curves of the related adsorption isotherm were regressed with eq 3:

The k2, qe, and t0.5 parameters for the different caffeic acid concentrations are listed in Table 1. From these data, it emerges that the values of k2 decrease as a function of the subtrate concentrations, while the qe value increases. The t0.5 values represent a suitable indication and underline a high photodegradation rate for the present reaction. As a whole, these results seem to converge on the important role played by the absorption effect. To ascertain the effective role of the adsorption on the whole photodegradation process, further experiments were conducted following the rationale based on the comparison between the decrease in the caffeic acid solution content under irradiation (light) and in the absence of light (dark). Figure 4 displays the

Figure 4. Removal of caffeic acid from acqueous solutions at different concentrations in the presence of C-doped TiO2, under visible light irradiation (λ > 420 nm) and dark conditions: (A) 18 mg L−1 caffeic acid and (B) 54 mg L−1 caffeic acid.

caffeic acid removal ability of CDT in the absence and presence of light irradiation for two initial caffeic acid concentrations (18 and 54 mg L−1). In the absence of light, the consistent contribution of the adsorption process can be appreciated. At higher caffeic acid concentrations, the difference between the adsorption process and the photodegradation starts to increase considerably because of saturation of the CDT particles (Figure 4 B). This result should not surprise because CDT is a mesopourus material and, because of its physical properties (126.5 m2 g−1),34 should be able to remove a great amount of substrate from the water solution. Via comparison of the data of the experiments conducted in the presence of light and in the

Q e = KCe1/ n

(3)

where Qe is the amount of caffeic acid taken up per gram of CDT (milligrams per gram) and Ce is the caffeic acid concentration left in solution at the equilibrium (milligrams per liter). K and n are the constant isotherm parameters, indicating the adsorption capacity and adsorption intensity, respectively. Figure 6 shows the adsorption isotherm of the D

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2.4 × 10−2 ± 5.4 × 10−3, and n = 1.32 ± 0.28), because of the simultanoeus effect of the photodegradation and adsorption processes. Anyway, this information helps to highlight the actual difference between the light and dark conditions on the caffeic acid removal through CDT particles. To demostrate the further differences between the adsorption process and the whole photodegradation process, SEM observations were made. SEM morphographs reported in Figure 7 show the aspect of the synthesized CDT particles (A) and the aspect of CDT after the reaction performed in the absence (B) and in the presence of light (C and D). It is evident that the agglomerates of CDT are completely modified after the reaction with caffeic acid. From SEM images, we find that the particles without treatment (A) are monodisperse in agreement with the findings of Ren.34 On the other hand, after the reaction under visible light, CDT particles are modified and form aggregates, indicating that the surface changed after the reaction with caffeic acid (C), whereas in the absence of light, the particles remain unmodified after the adsorption of caffeic acid (B). This is proof that the caffeic acid degradation process occurs only in the presence of light and that the CDT mesopourus material is a suitable platform for subtrate adsorption. To test the ability of CDT to be reused, three cycles of caffeic acid removal have been performed. After the first cycle of removal, the suspension, containing the CDT powder and the purified solution, was dried overnight at 100 °C, and a fresh solution of caffeic acid was added. This procedure was repeated twice, and the extent of removal is reported in Figure 8. The removal efficiency is only slighly decreased by reusing the material. From the figure, it can be seen that during the first cycle the removal percentage is 100%, with the second cycle ∼90%, and with the third cycle 85%. This result indicates that, despite the surface modification of CDT particles after the first cycle of photodegradation, the ability of this material to act as a photocatalyst is not compromised, at least after three cycles of usage. The intimate relationship between the subtrate and the CDT, as well as the substrate concentration dependence for the reaction performed in the presence of light, is appreciable even with ocular inspection. It was observed that, as soon as the solution of caffeic acid is in touch with CDT, it turns brown (the particle suspension in water appears white). Figure 9 shows the brown color of the freshly prepared CDT/caffeic suspensions (t0, left side of Figure 9), and the color of the same samples after 24 h in the presence and absence of light (right side of Figure 9). In every photograph, from the left to the right, the caffeic acid concentrations are 18, 54, and 90 mg L−1, respectively. After 24 h, the mixtures are bleached, but this event is noticeable only if the samples are under the light source. It is evident that the extent of bleaching is inversely proportional to the concentration of caffeic acid. Surprisingly, if the samples were taken in the dark, the brown color remained, as shown in the figure. This is an additional evidence that ascertains that the photodegradation of caffeic acid occurs only under light. Under our experimental conditions under a visible light source, hydroxyl radicals are produced by direct excitation of CDT, thus triggering caffeic acid photocatalysis. The initial browning observed when the cafferic acid solution is in touch with the CDT is very probably due to the formation of small products, dimers and trimers, rather than to quinoid forms with extended conjugation. This is possible because the UV−vis spectra after

Figure 5. Parity plot showing the correlation between the experiments conducted under light irradiation and in the dark. A: correlation for k2 values; B: correlation for qe values.

Figure 6. Adsorption isotherms of removal of caffeic acid by CDT at 25 °C in the absence of light [K = (9 ± 3.5 × 10−3, and n = 2.28 ± 0.55). Inset: adsorption isotherm in the presence of light (K = 2.4 × 10−2 ± 5.4 × 10−3, and n = 1.32 ± 0.28).

removal of caffeic acid by CDT at 25 °C in the absence of light obtained with the experimental data. We found for this sytem the following values: K = (9 ± 3.5) × 10−3, and n = 2.28 ± 0.55. Interestingly, the data for equilibrium obtained by means of eq 1 are very close to the experimental data [K = (8.25 ± 3.4) × 10−3, and n = 2.11 ± 0.049]. Furthermore, the value of adsorption intensity, n, for caffeic acid is higher than 1 (2.28), indicating a weak competition between the solvent water and the CDT particles for caffeic acid binding. The inset of Figure 5 reports the adsorption isotherm of removal of caffeic acid by CDT at 25 °C in the presence of light. The calculated values differ considerably (K = E

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Figure 7. SEM photograph of synthesized CDT particles (A) and CDT after the reaction performed the absence (B) and presence of light (C and D).

charge of CDT particles in aqueous solution measured by means of ζ potential was found to be ∼18 mV (pH ∼4.5). After the reaction with caffeic acid, we found a charge of approximately −30 mV. Caffeic acid under our experimantal conditions was partially dissociated in the solutions used (pH ∼4). Accordingly, the interaction between the CDT and the phenolic species can be, in part, ascribed to the electrostatic interaction. This is in agreement with previous data that showed that during photocatalytic degradation, the adsorption level of unmodified TiO2 depends on the substrate charge.41 The information given above reveals that the whole photodegradation process performed with visible light in the presence of glucose-doped titanium consists of a set of stages in which the adsorption process is a crucial step.

Figure 8. Efficiency of removal of caffeic acid with three cycles of use of CDT powder.

4. CONCLUSIONS Caffeic acid was degraded in the presence of glucose-doped titanium particles through a photocatalytic process activated by visible light. Kinetic data obtained by means of UV−vis spectroscopy and GC−IT/MS revealed a high degradation rate and a substrate concentration dependence. The adsorption isotherm study supported by ζ potential data performed in the absence of light allows us to demonstrate the pivotal role of the absorption mechanism. Furthermore, the SEM observations of the glucose-doped titanium particles after the reaction with caffeic acid have allowed us to emphasize the role of the light in the photodegradation mechanism. With this approach, it is possible to demonstrate that the whole process is governed by a synergic mechanism in which adsorption and photodegradation are involved. This investigation confirmed that the doping of TiO2 with glucose can shift its optical response to the visible light region, thus allowing cost and energy saving in the degradation process. This study provides very useful information for the application of titanium as a photocatalyst for the degradation of caffeic acid as a major constituent of OMW.

Figure 9. Color change of caffeic acid solutions (18, 54, and 90 mg L−1) in the presence of CDT immediately after the addition of caffeic acid (t0) and after reaction for 24 h in the presence and absence of light.

the browning are very similar to those of caffeic acid before the start of the reaction (see Figure 1).8 Therefore, the simple mechanism of adsorption is the only mechanism that drives the adsorpion and the removal of caffeic acid in the dark, as reported in Figure 4. The action of the light adds to the adsorption mechanism, the photodegradation of caffeic acid. The importance of such a synergic effect between photodegradation and adsorption was also recently reported in caffeic acid studies performed on a suspended mixture of TiO2 and powdered activated carbon.26 Another important aspect of the whole photodegradation process is related to the surface charge of CDT particles. The

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Corresponding Author

*Department of Agricultural, Environmental and Food Sciences and Center for Colloid and Surface Science (CSGI), Università degli Studi del Molise, via De Sanctis, I-86100 Campobasso, Italy. Phone: +39 0874404632. Fax: +39 0874404652. E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work was supported by the MIUR of Italy (PRIN 2010, 2010BJ23MN) and by Consorzio Interuniversitario per lo sviluppo dei Sistemi a Grande Interfase (CSGI-Firenze).

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DOI: 10.1021/acs.langmuir.5b00560 Langmuir XXXX, XXX, XXX−XXX

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Langmuir (41) Baran, W.; Makowski, A.; Wardas, W. The effect of UV radiation absorption of cationic and anionic dye solutions on their photocatalytic degradation in the presence TiO2. Dyes Pigm. 2008, 76 (1), 226−230.

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DOI: 10.1021/acs.langmuir.5b00560 Langmuir XXXX, XXX, XXX−XXX