Influence of Operational Parameters in the Heterogeneous Photo

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Influence of Operational Parameters in the Heterogeneous PhotoFenton Discoloration of Wastewaters in the Presence of an IronPillared Clay Haithem Bel Hadjltaief,† Patrick Da Costa,*,‡ M. Elena Galvez,§ and Mourad Ben Zina† †

Laboratoire Eau, Energie et Environnement (LR3E), Code AD-10-02, Ecole Nationale d’Ingénieurs de Sfax, Université de Sfax, B.P1173.W.3038 Sfax, Tunis ‡ Institut Jean Le Rond d’Alembert, UPMC Sorbonne Universités, UMR CNRS 7190, 2 place de la gare de ceinture, 78210 Saint Cyr L’Ecole, France § Institute of Energy Technology, ETH Zurich, ML J 13 Sonneggstrasse 3, CH-8092 Zurich, Switzerland S Supporting Information *

ABSTRACT: An iron-pillared Tunisian clay (Fe-PILC) was prepared and used as the catalyst in the heterogeneous photoFenton oxidation of Red Congo and Malachite Green in aqueous solution. The catalyst Fe-PILC was characterized by XRF, XRD, BET, and FTIR methods. This physicochemical characterization pointed to successful iron pillaring of the clay. The influence of several operational parameters such as the pH, H2O2 concentration, catalyst dosage, and initial dye concentration was evaluated. A solution pH in the range 2.5−3, the addition of 8 mL of 200 mg/L H2O2, and a catalyst dosage of 0.3 g/L appeared as the most favorable reaction conditions for achieving complete discoloration, either for Red Congo or Malachite Green, although oxidation was found to be slower and more complicated in the former case. The kinetics of discoloration of both dyes followed a pseudo-first-order rate law. In general, 20 min of UV irradiation was enough to achieve 100% discoloration of the aqueous solution. UV−vis and chemical oxygen demand measurements indicated, however, that longer reaction times of around 1 h were required for achieving dye mineralization. Leaching tests confirmed a very low amount of dissolved iron and good stability of the catalyst, with almost unaltered discoloration efficiency upon three cycles. Hence, taking into account the favorable photocatalytic properties and low leaching of iron ions, such iron-pillared clay can be considered a promising catalyst for dye wastewater treatment.

1. INTRODUCTION Wastewaters from the textile and dye industries represent an important source of pollution, as well as an environmental concern of worldwide relevance. Such industrial processes produce large quantities of highly colored effluents generated during the textile dyeing/printing process, with concentrations in the range of 10−200 mg/L.1,2 Because dyes are stable, recalcitrant, colorant, and even potentially carcinogenic and toxic,3−5 their release into the environment poses serious environmental, aesthetical, and health problems. Industrial dyeladen effluents need to be therefore effectively treated before being discharged into the environment. Among the different biological and combined chemical and biochemical processes,6 chemical oxidation,7 absorption,8 coagulation,9 and membrane treatments,10 recently considered for the removal of dyes from wastewaters, those globally named advanced oxidation processes (AOPs), appear as one of the most effective and feasible alternatives.11−14 Generally speaking, AOPs are oxidation processes in which large amounts of hydroxyl radicals (•OH) are generated. Because of the strong oxidizing potential of such radicals, a more effective degradation © XXXX American Chemical Society

of the organic pollutants can be attained. Photo-Fenton oxidation is a well-known example of AOPs. Furthermore, it is relatively environmentally friendly because it does not involve the use of harmful chemical reagents; i.e., Fe2+ ions, hydrogen peroxide (H2O2), and the produced hydroxyl radicals (•OH) are nontoxic. Besides, the photo-Fenton oxidation method is very promising for achieving high reaction yields with low treatment cost and has been efficiently applied to degrade many different types of organic compounds.15−23 Normally, the Fenton process occurs within the liquid phase; however, recently, attempts have been made to develop heterogeneous systems such as iron supported on materials like clays17,24−26 carbon materials,27,28 silicas,24,29,30 or zeolites.24 Such a heterogeneous photo-Fenton process offers unique advantages, such as bypassing the complete recuperation of iron from the water stream, as well as the generation of large amounts of Received: June 9, 2013 Revised: October 29, 2013 Accepted: November 1, 2013

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dx.doi.org/10.1021/ie4018258 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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sludge in its neutralization.26,31,32 On the other hand, the adsorption of UV radiation, as well as the different transport phenomena involved, can be substantially hindere using a heterogeneous catalytic system. Clay-based catalysts, i.e., pillared clays, have been frequently used in heterogeneous photo-Fenton applications. This is due to the low cost and ready abundance of clays, together with the simplicity of the pillaring process, resulting in the successful immobilization of iron ions on the surface of a material with relatively high surface area. Therefore, pillared clay catalysts have been employed in the photo-Fenton degradation of organic pollutants such phenol and of some phenolic derivates,14−16,26 organic dyes,17−21,26,27 toluene,22 tyrosol,23 and other persistent compounds.26,29 Generally, whatever the particular application, the degradation efficiency reached strongly depends on the catalyst origin and features, as well as on the operational conditions employed. As a result, it turns out to be necessary to evaluate the influence of these parameters on the efficiency of each particular catalytic system, in order to determine the viability of its practical use. In this sense, the main objective of the present work is to evaluate the activity of an iron-pillared Tunisian clay, in the discoloration and mineralization of Red Congo and Malachite Green dyes in aqueous solution, determining the feasibility and optimal operation conditions for the practical use of this catalyst under UV irradiation. The influence of several important operation parameters in a heterogeneous photoFenton process, such as the solution pH, H2O2 concentration, catalyst dosage, and initial dye concentration, was investigated. In addition, iron leaching and the stability and reusability of the pillared clay were also studied.

[XRD; Philips PW 1710 diffractometer (Kα, 40 kV/40 mA, and scanning rate of 2θ per min)], (ii) IR spectroscopy (Digilab Excalibur FTS 3000 spectrometer), (iii) X-ray fluorescence (XRF; ARL 9800 XP spectrometer), (iv) nitrogen adsorption at −196 °C (Micrometrics ASAP 2010), (v) helium pycnometry, and (vi) scanning electron microscopy with a field-emission gun (SEM-FEG; Hitachi SU-70). 2.3. Photocatalytic Reactor and Activity Tests. Red Congo (RC) and Malachite Green (MG) dyes were used; both supplied by Sigma Aldrich, their chemical characteristics are shown in Table 1S in the Supporting Information. The structures of this two azo dyes are presented in Figure 1.

Figure 1. Structures of (a) MG and (b) RC dyes.

Discoloration experiments were carried out in a 250 mL Pyrex reactor equipped with a magnetic stirrer and in the presence of UV light at a wavelength of 365 nm (UV-A, Black-Ray B 100 W UV lamp, V-100AP series). In all runs, the distance between the aqueous dye solution and the UV source was kept constant at 15 cm. All of the experiments were performed at a temperature of 25 °C. After stabilization of the stirring speed (150 rpm) and pH, the desired amount of pillared clay was added to 100 mL of an aqueous RC or MG solution. Then, 8 mL of the H2O2 reagent (prepared from 35% H2O2, Merck; H2O2 solutions agitated in a thermostatic water-bath shaker for 60 min at 25 °C) was poured into the dye solution. H2O2 addition was taken as the initial time for the reaction. Solution aliquots were periodically withdrawn from the reaction vessel with the aid of a syringe and at predetermined intervals. Upon MnO2 (99%, Merck) addition and filtration through a 0.45 μm membrane, the dye concentration was determined in a Shimadzu UV−vis spectrophotometer model 160A (Kyoto, Japan), at the maximal adsorption wavelengths of RC and MG, λmax = 497 nm and λmax = 617 nm, respectively. Therefore, the dye discoloration efficiency was calculated as follows:

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A natural clay from the deposit of Jebal Cherahil (Kairouan, Central−West of Tunisia) was used as the starting material. The natural clay was first purified by dispersion in water, decantation, and extraction of the fraction with a particle size smaller than 2 μm. This fraction was then dispersed in a 1 M NaCl solution and stirred at room temperature for 12 h. The supernatant was removed after settling. This procedure was repeated three times. After complete exchange, sodium clay was separated by centrifugation, washed with distilled water, and finally dialyzed to eliminate chloride ions in excess (confirmed by means of a AgNO3 test33). The resulting solid was dried at 60 °C, ground to 100 mesh, and kept in a sealed vessel. The pillaring solution was prepared by the slow addition of a Na2CO3 powder (97%, Merck) into a 0.2 M solution of Fe(NO3)3 [Fe(NO3)3·9H2O; 97%, Merck] with stirring at 100 rpm for 2 h at room temperature until the molar ratio Fe/ Na2CO3 reached 1:5. The solution was then aged for 4 days at 60 °C. Finally, the resulting oligomeric iron(III) solution was added to a 2% wt aqueous dispersion of the purified sodiumexchanged clay, at a ratio of 10−3 mol of Fe3+ per 1 g of clay. The dispersion was agitated at 100 rpm for 24 h, then filtered, washed with deionized water several times, and finally centrifuged at 4000 rpm for 10 min. The resulting solid material was calcined at 300 °C for 24 h, and subsequently ground to 100 mesh, to obtain a pillared clay catalyst named Fe-PILC. 2.2. Raw Clay and Catalyst Physicochemical Characterization. Both the raw clay and iron-pillared clay catalyst were characterized by means of (i) powder X-ray diffraction

η (%) = (C0 − Ct )/C0 × 100

(1)

where C0 and Ct are the concentrations (mg/L) of the RC or MG dye at times 0 and t, respectively. Additionally, chemical oxygen demand (COD) was determined using the reactor digestion method based on bichromate acidic oxidation. A mineralization efficiency can also be defined on the basis of eq 1, with Ct corresponding to COD measured at time t and C0 to the initial value of COD of the dye solution. The influence of several operational parameters on discoloration of the RC and MG dyes was studied. First, the influence of the dye solution pH was assayed. The solution pH was adjusted from 1.50 to 7.00 using HCl (0.1 M) and NaOH B



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Figure 2. XRD spectra of natural clay and iron-pillared clay (Fe-PILC).

(0.1 M), while fixing the dye concentration at 230 mg/L for either RC or MG and the dosages of H2O2 at 200 mg/L and of the Fe-PILC catalyst at 0.3 g/L The influence of the H2O2 amount added to the dye solution was evaluated by varying its dosage from 100 to 400 mg/L, at a fixed pH of 4 and dye concentrations of either RC or MG of 230 mg/L and Fe-PILC of 0.3 g/L. In order to study the influence of the catalyst dosage, the Fe-PILC concentration was varied from 0.05 to 1 g/L, for 230 mg/L of either a RC or MG solution, at pH 4 for 200 mg/L H2O2. To assess the influence of the initial dye concentration, experiments were performed at either RC or MG concentrations of 350, 230, 175, 120, and 90 mg/L, respectively, while fixing the pH at a value of 4, the H2O2 dosage at 200 mg/L, and Fe-PILC at 0.3 g/L. Finally, the stability of the pillared clay Fe-PILC catalysts was tested. Leaching runs were performed in order to evaluate the catalytic activity of Fe-PILC during successive experiments and therefore assess the possibility of catalyst reuse. The catalysts were used in three consecutive experiments by using fresh dye solutions at either a RC or Mg concentration of 230 mg/L, pH 3, 200 mg/L H2O2, and 0.3 g/L catalyst. Between each experiment, the catalyst was removed by filtration, then washed with distilled water several times, and dried at 110 °C for 12 h. The total iron ion concentration leached from the catalyst in the solution was determined by using an atomic absorption spectrophotometer (AAS model, Analytic Jetta).

increase of the surface area and porosity in the iron-pillared catalyst, in agreement with what has been previously reported in the existing literature.34−37 Moreover, the Brunauer− Emmett−Teller surface area of Fe-PILC is 143.4 m2/g, whereas that of the initial clay amounts to 64 m2/g. The large increase in the surface area for Fe-PILC indicates successful pillaring of Fe2O3 species into the silicate layers of the clay. XRD patterns acquired for both the raw clay and pillared FePILC catalyst are shown in Figure 2. XRD evidence that the raw clay contains smectite (montmorillonite) associated with illite and kaolinite. These minerals are characterized by their (001) basal reflections at 14.5, 10.1, and 7.15 Å, respectively. The main impurity in the raw clay is quartz, as indicated by the sharp (101) basal reflection at 3.34 Å. The basal d-spacing values, d001, for the smectite component in both the raw clay and iron-pillared clay catalyst are shown in Table 1S in the Supporting Information. The d spacing increases from 14.5 Å in the raw clay to 19.3 Å in the iron-pillared clay Fe-PILC. This shift is expected because of expansion of the interlayer spacing in the raw clay after pillaring treatment, indicating that hydroxyliron of the polymerization degree has been successfully intercalated into the silicate layers. Fourier transform infrared (FTIR) spectroscopy provides further evidence of the effectiveness of the pillaring process. Figure 3 shows the FTIR spectra of both the raw clay and pillared material Fe-PILC. The FTIR spectrum of the raw clay exhibits two peaks at 3630 and 3440 cm−1 in the −OH stretching region. These two bands can be respectively assigned to the −OH stretching vibration of the structural hydroxyl groups in the clay and water molecules present in the interlayer.38−40 Upon pillaring, these bands at 3630 and 3440 cm−1 appear broadened because of the introduction of new −OH groups, as a consequence of the insertion of hydroxyliron species between the clay sheets.38 Typical bands of the silicate components appear between 1200 and 400 cm−1: concretely one at 1047 cm−1, because of in-plane band stretching of the Si−O bonds, and the second one at 513 cm−1, corresponding to Si−O−Si vibrations. Bands at 472 and 533 cm−1 can be assigned to Si−O−Mg and Si−O−Al species, respectively.41,42

3. RESULTS AND DISCUSSION 3.1. Raw Clay and Catalyst Characterization. The chemical composition of the raw clay and pillared clay catalyst, Fe-PILC, determined by XRF, are reported in Table 2S in the SI. SiO2 and Al2O3 are the major constituents of the raw clay with other oxides, such as MgO, CaO, K2O, and Na2O, present in lower amounts. The iron oxide content in the pillared clay catalyst, Fe-PILC, appears expectedly higher, 32.5%, than that in the raw clay, 7.7%. The surface area, total pore volume, and porosity of the raw and pillared clay are shown as well in Table 2. During the pillaring process, expansion in the clay structure and desegregation of the clay particles contribute to a notable C



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raw clay presents cornflake-like crystals on its surface with a fluffy appearance, revealing its extremely fine platy structure, upon pillaring, the Fe-PILC surface becomes notably more porous and fluffy. 3.2. Iron-Pillared Clay Photocatalytic Activity: Influence of the Operational Parameters. In order to achieve efficient photo-Fenton oxidation of RC and MG, the influence of the main operational parameters (initial pH, catalyst dosage, concentration of H2O2, and initial dye concentration) was evaluated. The goal of the present study is thus to quantify the catalytic efficiency of our clay catalyst in the photo-Fenton degradation of the dyes, evaluating the influence of such different operational parameters in order to obtain optimal reaction conditions. Dark Fenton experiments were preliminarily performed by obtaining much lower efficiencies, about 50% less than the ones obtained in the presence of UV irradiation. A recent study by Xu and co-workers43 shows how, in the absence of UV light, TOC removal was only 3.5% in spite of discoloration reaching 40%, pointing to slower mineralization and leading mostly to unconverted reaction intermediates. Such low efficiencies are no longer interesting because total mineralization, even further from merely discoloration of the wastewater, is being demanded in spite of the expected higher cost. The effect of the initial pH on the photo-Fenton degradation of RC and MG was investigated in the pH range of 1.5−7.0, for a fixed amount of catalyst of 0.3 g/L, a H2O2 dosage of 200 mg/L, and an initial dye concentration of 230 mg/L. The results obtained are reported in Figure 5, in terms of the

Figure 3. FTIR spectra of (a) natural clay and (b) iron-pillared clay (Fe-PILC).

The SEM micrographs of natural clay and iron-pillared clay are presented in Figure 4 and are quite helpful to clarify the change in the morphological features upon a pillaring process. In fact, clearly, the surface morphology of natural clay (Figure 4 a) is different from that of Fe−PILC (Figure 4 b). Whereas the

Figure 5. Effect of the initial pH on the discoloration efficiency of (●) RC and (×) MG.

discoloration efficiency as a function of the solution pH. Note that the plotted values of the efficiency are those measured after 1 h of reaction time. The discoloration efficiency sharply increases with increasing solution pH between pH values of 1 and 2.5. This fact can be explained by the formation of an oxonium ion (H3O2+), which enhances the stability of H2O2 and restricts generation of •OH at low pH conditions (pH < 2.5).44−46 In addition, the scavenging of •OH by the excess of H+ is another reason for the decrease of the discoloration efficiency at pH 1.5.44,47 Within the pH range 2.5−3, complete discoloration of both RC and MG dye solutions is attained. At higher pH, the photocatalytic activity of the iron-pillared clay starts to decrease notably. Moreover, in the case of RC elimination, a decrease in the discoloration efficiency of almost 70% can be observed after the pH is increased from 4 to 6. Catalyst deactivation at high pH can be assigned to the formation of ferrous/ferric hydroxide complexes during

Figure 4. SEM images of the samples: (a) natural clay; (b) ironpillared clay (Fe-PILC). D



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reaction, resulting in a lower amount of •OH radicals, which hindered the dye oxidation process.44,45 Note that the formation of such hydroxide complexes is most probably independent of the amount of iron ions leached into the water, which we believe to be relatively small especially at high pH, and that they can be formed as adsorbed species on the ironactive sites on the clay surface. The influence of the pH on the catalytic activity will determine the compromise between the achievement of high degradation efficiencies and suitable catalyst stability in terms of iron ion leaching, soluble in acidic media. Note as well that, under less favorable reaction conditions in terms of the pH, the oxidation process becomes more sluggish in the case of RC because of its more complex chemical structure and therefore highest molecular weight in comparison to MG. The H2O2 dosage is a crucial factor affecting the generation of •OH radicals and therefore conditioning of the efficiency of the photo-Fenton degradation process. The experiments were performed by variation of the H2O2 dosage from 100 to 400 mg/L at a pH of 4, 0.3 g/L Fe-PILC, and 230 mg/L dye concentration. Figure 6 illustrates the influence of the H2O2

influences the catalyzed decomposition of H2O2 to generate OH radicals. The effect of the catalyst dosage was evaluated by means of a change in its load from 0.05 to 1 g/L in the dye solution containing 230 mg/L of each dye, at pH 4 and 200 mg/L of initial H2O2 dosage. Figure 7 shows the discoloration •

Figure 7. Effect of the Fe-PILC concentration on the discoloration efficiency of (●) RC and (×) MG.

efficiency measured as a function of the Fe-PILC catalyst load, for both dye solutions. First of all, note that, in the absence of a catalyst, no discoloration was measured. As can be observed in Figure 7, 12.1 and 32.1% of RC and MG, respectively, were converted for a Fe-PILC catalyst load of 0.05 g/L, respectively. The removal efficiency of both dyes increases with increasing Fe-PILC dosage, reaching almost 100% discoloration for a catalyst concentration of 0.3 g/L. This increase in the discoloration efficiency in the presence of an increased amount of catalyst in the solution is obviously due to the increase of the active material, and thus iron active sites, resulting in an enhanced free hydroxyl radical generation. However, for catalyst loads higher than 0.3 g/L, a steady state is reached in terms of discoloration. The effect of the initial concentration of RC and MG dyes on the discoloration efficiency was as well studied. As described in the Experimental Section, the dye concentration was varied between 350 and 90 mg/L, while operating at a pH of 4, a H2O2 dosage of 200 mg/L, and 0.3 g/L of Fe-PILC catalyst. The results are presented in Figures 8 and 9. Plots evidence that degradation becomes more difficult with increasing dye concentration, especially in the case of RC solutions. According to Figure 8a, the RC discoloration efficiency decreases from 100% at an initial dye concentration of 90 mg/L to around 86% when the dye concentration was increased to 350 mg/L. In the case of MG, the oxidation rate remains almost constant, reaching 100% after 20 min of reaction time. This is independent of the dye initial concentration (Figure 9a), except for the highest concentration of dye, 350 mg/L, for which a slower reaction can be observed, yet reaching 100% discoloration efficiency upon 20 min of UV irradiation. More significant influence of the dye concentration in the case of RC is, first of all, due to the higher molecular weight and more complex molecular structure of this azo dye in comparison with those of MG. Moreover, dye molecules need to be first adsorbed onto the surface of the Fe-PILC catalyst, then contacting the active iron ion sites nearby. Therefore, an increase in the dye concentration results in a competition for active sites, causing a decrease in the discoloration efficiency.49 In this sense as well, the bigger and more sluggish molecules of

Figure 6. Effect of the initial H2O2 concentration on the discoloration efficiency of (●) RC and (×) MG.

dosage on the photo-Fenton discoloration efficiency of RC and MG, in the presence of Fe-PILC. First of all, note that, in the absence of H2O 2, one can already observe an initial discoloration amounting to 41% and 66% for RC and MG, respectively. This initial conversion (41 and 66%) is due to both adsorption and the intrinsic oxidation catalytic properties of the iron species in the pillared clays, even in the absence of H2O2 for initiation of the radical formation mechanism. A maximal degradation efficiency is achieved at a H 2 O 2 concentration of 150 mg/L, in the case of Rboth C and MG, although at lower H2O2, higher degradation efficiencies were measured all of the time for MG in comparison to RC. As previously commented, this fact can be assigned to the more complex chemical structure of RC in comparison to that of MG. For H2O2 dosages higher than 150−200 mg/L, the discoloration efficiencies of RC and MG slightly decrease. This fact can be explained in terms of the favorable production of hydroperoxyl radicals (•HO2) in the presence of a local excess of H2O2. These hydroperoxyl radicals are substantially less reactive and do not contribute to oxidative degradation of the organic substrate, which takes place only through reaction with • OH.48 The catalyst concentration in the treated dye solution is another key parameter in the photo-Fenton reaction, which E



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radicals per, i.e., dye molecule, will be lower, leading to a decrease in the degradation efficiency.50 The same can be expected in terms of UV radiation flux. Previous studies have indicated that the kinetics of the photoFenton process can be adequately described by a pseudo-firstorder reaction:51,52 ln(Ct /C0) = − kt

(2)

in which C0 and Ct are the concentrations (mg/L) of the RC or MG dye at time 0 and t, respectively. Figures 8b and 9b respectively show the plots for RC and MG discoloration, corresponding to the fitting of the experimental data obtained at different dye concentrations (from 350 to 90 mg/L) to the apparent-first-order kinetic model in eq 2. The values of the rate constant, k, can therefore be obtained directly from regression analysis of the linear curves in the plot. As presented in Table 1, the values of the correlation coefficients (R2) for this Table 1. Pseudo-First-Order Kinetic Parameters for Discoloration of RC and MG, with pH 4, 200 mg/L H2O2, and 0.3 g/L Fe-PILC pseudo-first-order reaction kinetic parameters RC

Figure 8. Effect of the initial dye concentration on the discoloration efficiency of RC: (×) 90 mg/L; (●) 120 mg/L; (△) 175 mg/L; (■) 230 mg/L; (◆) 350 mg/L. (a) Discoloration efficiency versus irradiation time. (b) Rate constants as a function of the initial dye concentration.

MG

concentration (mg/L)

k (min−1)

t1/2 (min)

R2

k (min−1)

t1/2 (min)

R2

90 120 175 230 350

0.085 0.072 0.056 0.033 0.019

8.16 9.66 12.44 21.26 35.91

0.9870 0.9898 0.9941 0.9910 0.9804

0.237 0.200 0.162 0.131 0.058

2.92 3.47 4.28 5.28 11.89

0.9812 0.9801 0.9908 0.9884 0.9953

linear regression are in the range of 0.9904−0.9941 for RC and 0.9801−0.9953 for MG, respectively, indicating that the firstorder kinetic model can adequately describe the experimental observations. Rate constants decrease with increasing initial dye concentration, more noticeably in the case of RC discoloration, consequently, and due to the same facts already commented on in the sight of the plots of discoloration efficiency for different dye concentrations, as a function of the reaction time. 3.3. UV−Vis Spectral Analysis of RC and MG Degradation. The UV−vis absorption spectra of the dye solution at different reaction times were recorded to investigate the structural change of RC and MG during its discoloration/ degradation in the presence of the Fe-PILC catalyst. Degradation experiments were performed at pH 4, 230 mg/L dye concentration, 0.3 g/L of the Fe-PILC catalyst, and 200 mg/L of H2O2. Solution samples were analyzed in time intervals of 1, 5, and 20 min. Note thus that the corresponding degradation efficiency curve in terms of percent efficiency (η) are the filled-square symbol plots in Figures 8a and 9a for RC and MG, respectively. Comparisons of the UV−vis spectra of the initial dye solutions and after different photo-Fenton oxidation reaction time intervals, recorded over the wavelength range of 200−800 nm, are shown in Figures 10 and 11 for RC and MG, respectively. As can be observed in Figure 9, the initial RC solution UV−vis spectra present three absorption peaks at 499, 345, and 236 nm. The absorbance peak at 499 nm has been assigned to the azo bonds of RC, whereas the peaks at 236 and 345 nm can be attributed to benzene and naphthalene ring structures.53 The spectrum corresponding to the initial solution of MG (Figure 11) shows two peaks at 618 and 425 nm, which are ascribed to the extended chromophore, comprising all

Figure 9. Effect of the initial dye concentration on the discoloration efficiency of MG: (×) 90 mg/L; (●) 120 mg/L; (△) 175 mg/L; (■) 230 mg/L; (◆) 350 mg/L. (a) Discoloration efficiency versus irradiation time. (b) Rate constants as a function of the initial dye concentration.

RC will more difficultly reach the catalytic active sites situated in the inner porosity of the Fe-PILC material. One must note as well that, for a higher initial dye concentration, but at a constant concentration of •OH radicals, the relative concentration active F



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Degradation under such reaction conditions and in the presence of the Fe-PILC catalyst requires longer irradiation times. COD was measured at different time intervals under the same reaction conditions as those for the UV−vis experiments. A COD removal efficiency was calculated and plotted as a function of the irradiation time; see Figure 12. As expected,

Figure 12. Mineralization in terms of the COD removal efficiency as a function of the irradiation time for MG and RC.

Figure 10. Evolution of UV−vis spectra of an RC aqueous solution with reaction time t = (a) 0, (b) 1, (c) 5, and (d) 20 min.

COD decreases, and therefore the COD removal efficiency increases, with irradiation time. After 20 min of reaction, the maximal time at which UV spectra were acquired, the COD removal efficiency reached 86% in the MG solution, whereas a COD removal efficiency of 60% was measured for the RC solution. These results confirm that after 20 min of irradiation complete degradation was not yet been reached. A COD removal efficiency of 100%, that is, complete degradation, is achieved after 45 min of reaction for MG and upon 1 h of irradiation for RC, as can be observed in Figure 12. 3.4. Leaching and Stability Tests. Chemical stability is an important property in an efficient catalyst. Leaching runs were performed in order to evaluate the catalytic activity and stability of Fe-PILC during successive experiments. Therefore, the same catalyst was used in three consecutive experiments by using fresh dye solutions at the following reaction conditions: RC or MG concentration of 230 mg/L, pH 3, 200 mg/L H2O2, and 0.3 g/L catalyst. After each experiment, the catalyst was removed by filtration, carefully washed with distilled water, and dried at 110 °C for 12 h. The discoloration efficiency of the dyes RC and MG by the Fe-PILC catalyst was still higher than 90% after being used in the three subsequent cycles. Moreover, after the three successive experiments, the concentration of iron ions in the solution was below 0.2 mg/L, conforming the Environmental Quality Act 1974 standards. Note as well pH 3 in these tests, i.e., lower than those in other runs. Therefore, this result further proves that the Fe-PILC catalyst possesses an adequate stability, presenting only small decay in its discoloration efficiency. Table 2 compares the results obtained in terms of the discoloration efficiency, in the presence of the pillared clay presented in this work, to other previously published results, considering different treatment approaches, such as adsorption, biological treatment, or photodegradation, as well as photoFenton processes in the presence of different catalysts, such as zeolite, carbon, and collagen fibers. Interesting reviews have been as well published in the last years, compiling the results obtained in the most recently published works on Fenton catalysts.24,26,63 In general terms, there is a huge influence of the type of pollutant considered, as well as of the initial concentration. Each catalytic system behaves differently; optimal conditions for achieving maximal degradation of the

Figure 11. Evolution of UV−vis spectra of a MG aqueous solution with reaction time t = (a) 0, (b) 1, (c) 5, and (d) 20 min.

conjugated aromatic rings connected through CC and CN double bonds, and the third absorption band is at 315 nm in the UV region, which is due to the benzene ring structure of the dye molecule.54 The peak intensities decrease with increasing reaction time, indicating the effective degradation of both RC and MG dyes. The decrease in the intensity occurs similarly for all peaks. In the case of RC, azo bonds seem to be first broken, as indicated by the sharp decrease from 0 to 1 min of reaction time. Then naphthalene and benzene rings are as well destroyed. However, after 20 min of reaction, the already slightly visible absorbance peak at 236 nm points to the more difficult degradation of the remaining benzene structures. In fact, some recent studies claim that discoloration occurs and is completed faster than the total mineralization, complete oxidation, of the dye in the treated water solution.48 The same can be observed for the degradation of MG, although, as already commented before, its oxidation seems much easier than that of RC because of the lower-molecular-weight, simpler structure of the dye molecule in this case. G



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Table 2. Comparison of Discoloration Efficiencies and Times for Different Methods and Catalysts dye

method/catalyst

reaction time

initial dye concentration (mg/L)

discoloration efficiency (%)

ref

MG

adsorption biodegradation photodegradation (Ag-TiO2) photo-Fenton (Fe-PILC)a photo-Fenton (iron−collagen fiber)a adsorption biodegradation photodegradation photo-Fenton (Fe-PILC)a photo-Fenton (iron−carbon fibers) photo-Fenton (iron zeolite)a

20 min 3.5 h 60 min 20 min 30 min 90 min 14 h 30 min 20 min 80 min 4h

8 50 70 230 46 30 100 20 230 100 350

100 85.2 92 100 100 99.2 100 100 100 100 97

55 56 57 this work 58 59 60 61 this work 28 62

RC

a

For all studies, pH 3−4.

■

dyes cannot be easily extrapolated from one system to another. This becomes especially true for materials of natural origin, such as clays. Such diversity makes it mandatory to explore the viability and optimal reaction conditions for each catalytic system, prior to considering its practical application to such photo-Fenton treatments of wastewaters.

*Tel.: +33 1 30 85 48 65. Fax: +33 1 30 85 48 99. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

4. CONCLUSIONS

Notes

A local Tunisian clay was successfully pillared with iron. Its activity was assayed in the photo-Fenton oxidation of RC and MG in an aqueous solution, under UV-light irradiation, determining the influence of several operational parameters on its discoloration efficiency. The sesults showed that both dyes could be effectively removed from the aqueous media by means a heterogeneous photo-Fenton process in the presence of Fe-PILC as the catalyst, demonstrating the feasibility of using this particular Tunisian clay in the present application. The study of the influence of the different reaction parameters pointed to optimal discoloration when using pH 3, for a catalyst dosage of 0.3 g/L, adding a H2O2 concentration of 200 mg/L, at room temperature conditions. MG was always more easily eliminated than RC, whose degradation seemed to be more intensely influenced by nonoptimal reaction conditions. This can be due to the more complex structure and higher molecular weight of RC in comparison to MG. Data fitting indicated that the discoloration kinetics of RC and MG followed a first-order rate law. Transient evaluation of dye degradation by means of the acquisition of UV−vis absorption spectra evidenced first discoloration due to destruction of the azo core of the dye structure but more difficult and incomplete destruction of light aromatics such as benzenes, which are more immune to the photo-Fenton treatment, especially in the case of the RC solution. COD measurements confirmed total mineralization at irradiation times of 45 and 60 min for MG and RC solutions, respectively. Moreover, leaching and stability tests showed that the catalyst is stable over the evaluated reaction time and cycles.

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AUTHOR INFORMATION

Corresponding Author

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of Sfax and UPMC Sorbonne Université. REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Tables 1S and 2S, respectively containing additional information on dyes and properties of the clay, before and after Fe-pillaring. This material is available free of charge via the Internet at http://pubs.acs.org. H



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Influence of Operational Parameters in the Heterogeneous Photo

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