Influence of Operational Parameters in the Heterogeneous Photo-Fenton Discoloration of Wastewaters in the Presence of an Iron-Pillared ClayHaithem 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’Ingenieurs de Sfax, Universite de Sfax,B.P1173.W.3038 Sfax, Tunis‡Institut Jean Le Rond d’Alembert, UPMC Sorbonne Universites, UMR CNRS 7190, 2 place de la gare de ceinture, 78210 Saint CyrL’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) wasprepared and used as the catalyst in the heterogeneous photo-Fenton oxidation of Red Congo and Malachite Green inaqueous solution. The catalyst Fe-PILC was characterized byXRF, XRD, BET, and FTIR methods. This physicochemicalcharacterization 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 concen-tration was evaluated. A solution pH in the range 2.5−3, theaddition of 8 mL of 200 mg/L H2O2, and a catalyst dosage of0.3 g/L appeared as the most favorable reaction conditions for achieving complete discoloration, either for Red Congo orMalachite Green, although oxidation was found to be slower and more complicated in the former case. The kinetics ofdiscoloration of both dyes followed a pseudo-first-order rate law. In general, 20 min of UV irradiation was enough to achieve100% discoloration of the aqueous solution. UV−vis and chemical oxygen demand measurements indicated, however, that longerreaction times of around 1 h were required for achieving dye mineralization. Leaching tests confirmed a very low amount ofdissolved iron and good stability of the catalyst, with almost unaltered discoloration efficiency upon three cycles. Hence, takinginto account the favorable photocatalytic properties and low leaching of iron ions, such iron-pillared clay can be considered apromising catalyst for dye wastewater treatment.
1. INTRODUCTION
Wastewaters from the textile and dye industries represent animportant source of pollution, as well as an environmentalconcern of worldwide relevance. Such industrial processesproduce large quantities of highly colored effluents generatedduring the textile dyeing/printing process, with concentrationsin the range of 10−200 mg/L.1,2 Because dyes are stable,recalcitrant, colorant, and even potentially carcinogenic andtoxic,3−5 their release into the environment poses seriousenvironmental, aesthetical, and health problems. Industrial dye-laden effluents need to be therefore effectively treated beforebeing 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 consideredfor the removal of dyes from wastewaters, those globally namedadvanced oxidation processes (AOPs), appear as one of themost effective and feasible alternatives.11−14 Generally speaking,AOPs are oxidation processes in which large amounts ofhydroxyl radicals (•OH) are generated. Because of the strongoxidizing potential of such radicals, a more effective degradation
of the organic pollutants can be attained. Photo-Fentonoxidation is a well-known example of AOPs. Furthermore, itis relatively environmentally friendly because it does not involvethe use of harmful chemical reagents; i.e., Fe2+ ions, hydrogenperoxide (H2O2), and the produced hydroxyl radicals (•OH)are nontoxic. Besides, the photo-Fenton oxidation method isvery promising for achieving high reaction yields with lowtreatment cost and has been efficiently applied to degrade manydifferent types of organic compounds.15−23 Normally, theFenton process occurs within the liquid phase; however,recently, attempts have been made to develop heterogeneoussystems such as iron supported on materials like clays17,24−26
carbon materials,27,28 silicas,24,29,30 or zeolites.24 Such aheterogeneous photo-Fenton process offers unique advantages,such as bypassing the complete recuperation of iron from thewater stream, as well as the generation of large amounts of
Received: June 9, 2013Revised: October 29, 2013Accepted: November 1, 2013
sludge in its neutralization.26,31,32 On the other hand, theadsorption of UV radiation, as well as the different transportphenomena involved, can be substantially hindere using aheterogeneous catalytic system.Clay-based catalysts, i.e., pillared clays, have been frequently
used in heterogeneous photo-Fenton applications. This is dueto the low cost and ready abundance of clays, together with thesimplicity of the pillaring process, resulting in the successfulimmobilization of iron ions on the surface of a material withrelatively high surface area. Therefore, pillared clay catalystshave been employed in the photo-Fenton degradation oforganic pollutants such phenol and of some phenolicderivates,14−16,26 organic dyes,17−21,26,27 toluene,22 tyrosol,23
and other persistent compounds.26,29 Generally, whatever theparticular application, the degradation efficiency reachedstrongly depends on the catalyst origin and features, as wellas on the operational conditions employed. As a result, it turnsout to be necessary to evaluate the influence of theseparameters 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 thediscoloration and mineralization of Red Congo and MalachiteGreen dyes in aqueous solution, determining the feasibility andoptimal operation conditions for the practical use of thiscatalyst under UV irradiation. The influence of severalimportant operation parameters in a heterogeneous photo-Fenton 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 thepillared clay were also studied.
2. EXPERIMENTAL SECTION2.1. Catalyst Preparation. A natural clay from the deposit
of Jebal Cherahil (Kairouan, Central−West of Tunisia) wasused as the starting material. The natural clay was first purifiedby dispersion in water, decantation, and extraction of thefraction with a particle size smaller than 2 μm. This fraction wasthen dispersed in a 1 M NaCl solution and stirred at roomtemperature for 12 h. The supernatant was removed aftersettling. This procedure was repeated three times. Aftercomplete exchange, sodium clay was separated by centrifuga-tion, washed with distilled water, and finally dialyzed toeliminate chloride ions in excess (confirmed by means of aAgNO3 test
33). The resulting solid was dried at 60 °C, groundto 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 ofFe(NO3)3 [Fe(NO3)3·9H2O; 97%, Merck] with stirring at 100rpm for 2 h at room temperature until the molar ratio Fe/Na2CO3 reached 1:5. The solution was then aged for 4 days at60 °C. Finally, the resulting oligomeric iron(III) solution wasadded to a 2% wt aqueous dispersion of the purified sodium-exchanged 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 finallycentrifuged at 4000 rpm for 10 min. The resulting solidmaterial was calcined at 300 °C for 24 h, and subsequentlyground to 100 mesh, to obtain a pillared clay catalyst namedFe-PILC.2.2. Raw Clay and Catalyst Physicochemical Charac-
terization. Both the raw clay and iron-pillared clay catalystwere characterized by means of (i) powder X-ray diffraction
[XRD; Philips PW 1710 diffractometer (Kα, 40 kV/40 mA, andscanning rate of 2θ per min)], (ii) IR spectroscopy (DigilabExcalibur 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-emissiongun (SEM-FEG; Hitachi SU-70).
2.3. Photocatalytic Reactor and Activity Tests. RedCongo (RC) and Malachite Green (MG) dyes were used; bothsupplied by Sigma Aldrich, their chemical characteristics areshown in Table 1S in the Supporting Information. Thestructures of this two azo dyes are presented in Figure 1.
Discoloration experiments were carried out in a 250 mL Pyrexreactor equipped with a magnetic stirrer and in the presence ofUV light at a wavelength of 365 nm (UV-A, Black-Ray B 100 WUV lamp, V-100AP series). In all runs, the distance between theaqueous dye solution and the UV source was kept constant at15 cm. All of the experiments were performed at a temperatureof 25 °C. After stabilization of the stirring speed (150 rpm) andpH, the desired amount of pillared clay was added to 100 mL ofan aqueous RC or MG solution. Then, 8 mL of the H2O2reagent (prepared from 35% H2O2, Merck; H2O2 solutionsagitated in a thermostatic water-bath shaker for 60 min at 25°C) was poured into the dye solution. H2O2 addition was takenas the initial time for the reaction. Solution aliquots wereperiodically withdrawn from the reaction vessel with the aid ofa 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−visspectrophotometer model 160A (Kyoto, Japan), at the maximaladsorption wavelengths of RC and MG, λmax = 497 nm and λmax= 617 nm, respectively. Therefore, the dye discolorationefficiency was calculated as follows:
η = − ×C C C(%) ( )/ 100t0 0 (1)
where C0 and Ct are the concentrations (mg/L) of the RC orMG dye at times 0 and t, respectively. Additionally, chemicaloxygen demand (COD) was determined using the reactordigestion method based on bichromate acidic oxidation. Amineralization efficiency can also be defined on the basis of eq1, with Ct corresponding to COD measured at time t and C0 tothe 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, theinfluence of the dye solution pH was assayed. The solution pHwas adjusted from 1.50 to 7.00 using HCl (0.1 M) and NaOH
Figure 1. Structures of (a) MG and (b) RC dyes.
Industrial & Engineering Chemistry Research Article
(0.1 M), while fixing the dye concentration at 230 mg/L foreither RC or MG and the dosages of H2O2 at 200 mg/L and ofthe Fe-PILC catalyst at 0.3 g/L The influence of the H2O2amount added to the dye solution was evaluated by varying itsdosage from 100 to 400 mg/L, at a fixed pH of 4 and dyeconcentrations of either RC or MG of 230 mg/L and Fe-PILCof 0.3 g/L. In order to study the influence of the catalystdosage, the Fe-PILC concentration was varied from 0.05 to 1g/L, for 230 mg/L of either a RC or MG solution, at pH 4 for200 mg/L H2O2. To assess the influence of the initial dyeconcentration, experiments were performed at either RC orMG concentrations of 350, 230, 175, 120, and 90 mg/L,respectively, while fixing the pH at a value of 4, the H2O2dosage 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 thecatalytic activity of Fe-PILC during successive experiments andtherefore assess the possibility of catalyst reuse. The catalystswere used in three consecutive experiments by using fresh dyesolutions at either a RC or Mg concentration of 230 mg/L, pH3, 200 mg/L H2O2, and 0.3 g/L catalyst. Between eachexperiment, the catalyst was removed by filtration, then washedwith distilled water several times, and dried at 110 °C for 12 h.The total iron ion concentration leached from the catalyst inthe solution was determined by using an atomic absorptionspectrophotometer (AAS model, Analytic Jetta).
3. RESULTS AND DISCUSSION
3.1. Raw Clay and Catalyst Characterization. Thechemical composition of the raw clay and pillared clay catalyst,Fe-PILC, determined by XRF, are reported in Table 2S in theSI. SiO2 and Al2O3 are the major constituents of the raw claywith other oxides, such as MgO, CaO, K2O, and Na2O, presentin lower amounts. The iron oxide content in the pillared claycatalyst, Fe-PILC, appears expectedly higher, 32.5%, than thatin the raw clay, 7.7%. The surface area, total pore volume, andporosity of the raw and pillared clay are shown as well in Table2. During the pillaring process, expansion in the clay structureand desegregation of the clay particles contribute to a notable
increase of the surface area and porosity in the iron-pillaredcatalyst, in agreement with what has been previously reportedin the existing literature.34−37 Moreover, the Brunauer−Emmett−Teller surface area of Fe-PILC is 143.4 m2/g, whereasthat of the initial clay amounts to 64 m2/g. The large increase inthe surface area for Fe-PILC indicates successful pillaring ofFe2O3 species into the silicate layers of the clay.XRD patterns acquired for both the raw clay and pillared Fe-
PILC catalyst are shown in Figure 2. XRD evidence that theraw clay contains smectite (montmorillonite) associated withillite 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 thesharp (101) basal reflection at 3.34 Å. The basal d-spacingvalues, d001, for the smectite component in both the raw clayand iron-pillared clay catalyst are shown in Table 1S in theSupporting Information. The d spacing increases from 14.5 Å inthe raw clay to 19.3 Å in the iron-pillared clay Fe-PILC. Thisshift is expected because of expansion of the interlayer spacingin the raw clay after pillaring treatment, indicating thathydroxyliron of the polymerization degree has been successfullyintercalated 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 andpillared material Fe-PILC. The FTIR spectrum of the raw clayexhibits two peaks at 3630 and 3440 cm−1 in the −OHstretching region. These two bands can be respectively assignedto the −OH stretching vibration of the structural hydroxylgroups in the clay and water molecules present in theinterlayer.38−40 Upon pillaring, these bands at 3630 and 3440cm−1 appear broadened because of the introduction of new−OH groups, as a consequence of the insertion of hydroxylironspecies between the clay sheets.38 Typical bands of the silicatecomponents appear between 1200 and 400 cm−1: concretelyone at 1047 cm−1, because of in-plane band stretching of theSi−O bonds, and the second one at 513 cm−1, corresponding toSi−O−Si vibrations. Bands at 472 and 533 cm−1 can beassigned to Si−O−Mg and Si−O−Al species, respectively.41,42
Figure 2. XRD spectra of natural clay and iron-pillared clay (Fe-PILC).
Industrial & Engineering Chemistry Research Article
The SEM micrographs of natural clay and iron-pillared clayare presented in Figure 4 and are quite helpful to clarify thechange in the morphological features upon a pillaring process.In fact, clearly, the surface morphology of natural clay (Figure 4a) is different from that of Fe−PILC (Figure 4 b). Whereas the
raw clay presents cornflake-like crystals on its surface with afluffy appearance, revealing its extremely fine platy structure,upon pillaring, the Fe-PILC surface becomes notably moreporous and fluffy.
3.2. Iron-Pillared Clay Photocatalytic Activity: Influ-ence of the Operational Parameters. In order to achieveefficient photo-Fenton oxidation of RC and MG, the influenceof the main operational parameters (initial pH, catalyst dosage,concentration of H2O2, and initial dye concentration) wasevaluated. The goal of the present study is thus to quantify thecatalytic efficiency of our clay catalyst in the photo-Fentondegradation of the dyes, evaluating the influence of suchdifferent operational parameters in order to obtain optimalreaction conditions. Dark Fenton experiments were prelimi-narily performed by obtaining much lower efficiencies, about50% less than the ones obtained in the presence of UVirradiation. A recent study by Xu and co-workers43 shows how,in the absence of UV light, TOC removal was only 3.5% inspite of discoloration reaching 40%, pointing to slowermineralization and leading mostly to unconverted reactionintermediates. Such low efficiencies are no longer interestingbecause total mineralization, even further from merelydiscoloration of the wastewater, is being demanded in spiteof 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, fora fixed amount of catalyst of 0.3 g/L, a H2O2 dosage of 200mg/L, and an initial dye concentration of 230 mg/L. Theresults obtained are reported in Figure 5, in terms of the
discoloration efficiency as a function of the solution pH. Notethat the plotted values of the efficiency are those measured after1 h of reaction time. The discoloration efficiency sharplyincreases with increasing solution pH between pH values of 1and 2.5. This fact can be explained by the formation of anoxonium ion (H3O
2+), which enhances the stability of H2O2and restricts generation of •OH at low pH conditions (pH <2.5).44−46 In addition, the scavenging of •OH by the excess ofH+ is another reason for the decrease of the discolorationefficiency at pH 1.5.44,47 Within the pH range 2.5−3, completediscoloration of both RC and MG dye solutions is attained. Athigher pH, the photocatalytic activity of the iron-pillared claystarts to decrease notably. Moreover, in the case of RCelimination, a decrease in the discoloration efficiency of almost70% can be observed after the pH is increased from 4 to 6.Catalyst deactivation at high pH can be assigned to theformation of ferrous/ferric hydroxide complexes during
Figure 3. FTIR spectra of (a) natural clay and (b) iron-pillared clay(Fe-PILC).
Figure 4. SEM images of the samples: (a) natural clay; (b) iron-pillared clay (Fe-PILC).
Figure 5. Effect of the initial pH on the discoloration efficiency of (●)RC and (×) MG.
Industrial & Engineering Chemistry Research Article
reaction, resulting in a lower amount of •OH radicals, whichhindered the dye oxidation process.44,45 Note that theformation of such hydroxide complexes is most probablyindependent 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 iron-active sites on the clay surface. The influence of the pH on thecatalytic activity will determine the compromise between theachievement of high degradation efficiencies and suitablecatalyst stability in terms of iron ion leaching, soluble in acidicmedia. Note as well that, under less favorable reactionconditions in terms of the pH, the oxidation process becomesmore sluggish in the case of RC because of its more complexchemical structure and therefore highest molecular weight incomparison to MG.The H2O2 dosage is a crucial factor affecting the generation
of •OH radicals and therefore conditioning of the efficiency ofthe photo-Fenton degradation process. The experiments wereperformed by variation of the H2O2 dosage from 100 to 400mg/L at a pH of 4, 0.3 g/L Fe-PILC, and 230 mg/L dyeconcentration. Figure 6 illustrates the influence of the H2O2
dosage on the photo-Fenton discoloration efficiency of RC andMG, in the presence of Fe-PILC. First of all, note that, in theabsence of H2O2, one can already observe an initialdiscoloration amounting to 41% and 66% for RC and MG,respectively. This initial conversion (41 and 66%) is due toboth adsorption and the intrinsic oxidation catalytic propertiesof the iron species in the pillared clays, even in the absence ofH2O2 for initiation of the radical formation mechanism. Amaximal degradation efficiency is achieved at a H2O2concentration of 150 mg/L, in the case of Rboth C and MG,although at lower H2O2, higher degradation efficiencies weremeasured all of the time for MG in comparison to RC. Aspreviously commented, this fact can be assigned to the morecomplex chemical structure of RC in comparison to that ofMG. For H2O2 dosages higher than 150−200 mg/L, thediscoloration efficiencies of RC and MG slightly decrease. Thisfact can be explained in terms of the favorable production ofhydroperoxyl radicals (•HO2) in the presence of a local excessof H2O2. These hydroperoxyl radicals are substantially lessreactive and do not contribute to oxidative degradation of theorganic substrate, which takes place only through reaction with•OH.48
The catalyst concentration in the treated dye solution isanother key parameter in the photo-Fenton reaction, which
influences the catalyzed decomposition of H2O2 to generate•OH radicals. The effect of the catalyst dosage was evaluated bymeans of a change in its load from 0.05 to 1 g/L in the dyesolution containing 230 mg/L of each dye, at pH 4 and 200mg/L of initial H2O2 dosage. Figure 7 shows the discoloration
efficiency measured as a function of the Fe-PILC catalyst load,for both dye solutions. First of all, note that, in the absence of acatalyst, no discoloration was measured. As can be observed inFigure 7, 12.1 and 32.1% of RC and MG, respectively, wereconverted for a Fe-PILC catalyst load of 0.05 g/L, respectively.The removal efficiency of both dyes increases with increasingFe-PILC dosage, reaching almost 100% discoloration for acatalyst concentration of 0.3 g/L. This increase in thediscoloration efficiency in the presence of an increased amountof catalyst in the solution is obviously due to the increase of theactive material, and thus iron active sites, resulting in anenhanced free hydroxyl radical generation. However, forcatalyst loads higher than 0.3 g/L, a steady state is reached interms of discoloration.The effect of the initial concentration of RC and MG dyes on
the discoloration efficiency was as well studied. As described inthe Experimental Section, the dye concentration was variedbetween 350 and 90 mg/L, while operating at a pH of 4, aH2O2 dosage of 200 mg/L, and 0.3 g/L of Fe-PILC catalyst.The results are presented in Figures 8 and 9. Plots evidencethat degradation becomes more difficult with increasing dyeconcentration, especially in the case of RC solutions. Accordingto Figure 8a, the RC discoloration efficiency decreases from100% at an initial dye concentration of 90 mg/L to around 86%when the dye concentration was increased to 350 mg/L. In thecase of MG, the oxidation rate remains almost constant,reaching 100% after 20 min of reaction time. This isindependent of the dye initial concentration (Figure 9a),except for the highest concentration of dye, 350 mg/L, forwhich a slower reaction can be observed, yet reaching 100%discoloration efficiency upon 20 min of UV irradiation. Moresignificant influence of the dye concentration in the case of RCis, first of all, due to the higher molecular weight and morecomplex molecular structure of this azo dye in comparison withthose of MG. Moreover, dye molecules need to be firstadsorbed onto the surface of the Fe-PILC catalyst, thencontacting the active iron ion sites nearby. Therefore, anincrease in the dye concentration results in a competition foractive 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 discolorationefficiency of (●) RC and (×) MG.
Figure 7. Effect of the Fe-PILC concentration on the discolorationefficiency of (●) RC and (×) MG.
Industrial & Engineering Chemistry Research Article
RC will more difficultly reach the catalytic active sites situatedin the inner porosity of the Fe-PILC material. One must note aswell that, for a higher initial dye concentration, but at a constantconcentration of •OH radicals, the relative concentration active
radicals per, i.e., dye molecule, will be lower, leading to adecrease in the degradation efficiency.50 The same can beexpected in terms of UV radiation flux.Previous studies have indicated that the kinetics of the photo-
Fenton process can be adequately described by a pseudo-first-order reaction:51,52
= −C C ktln( / )t 0 (2)
in which C0 and Ct are the concentrations (mg/L) of the RC orMG dye at time 0 and t, respectively. Figures 8b and 9brespectively show the plots for RC and MG discoloration,corresponding to the fitting of the experimental data obtainedat different dye concentrations (from 350 to 90 mg/L) to theapparent-first-order kinetic model in eq 2. The values of therate constant, k, can therefore be obtained directly fromregression analysis of the linear curves in the plot. As presentedin Table 1, the values of the correlation coefficients (R2) for this
linear regression are in the range of 0.9904−0.9941 for RC and0.9801−0.9953 for MG, respectively, indicating that the first-order kinetic model can adequately describe the experimentalobservations. Rate constants decrease with increasing initial dyeconcentration, more noticeably in the case of RC discoloration,consequently, and due to the same facts already commented onin the sight of the plots of discoloration efficiency for differentdye concentrations, as a function of the reaction time.
3.3. UV−Vis Spectral Analysis of RC and MGDegradation. The UV−vis absorption spectra of the dyesolution at different reaction times were recorded to investigatethe 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/Ldye concentration, 0.3 g/L of the Fe-PILC catalyst, and 200mg/L of H2O2. Solution samples were analyzed in timeintervals of 1, 5, and 20 min. Note thus that the correspondingdegradation efficiency curve in terms of percent efficiency (η)are the filled-square symbol plots in Figures 8a and 9a for RCand MG, respectively. Comparisons of the UV−vis spectra ofthe initial dye solutions and after different photo-Fentonoxidation reaction time intervals, recorded over the wavelengthrange of 200−800 nm, are shown in Figures 10 and 11 for RCand MG, respectively. As can be observed in Figure 9, the initialRC solution UV−vis spectra present three absorption peaks at499, 345, and 236 nm. The absorbance peak at 499 nm hasbeen assigned to the azo bonds of RC, whereas the peaks at 236and 345 nm can be attributed to benzene and naphthalene ringstructures.53 The spectrum corresponding to the initial solutionof MG (Figure 11) shows two peaks at 618 and 425 nm, whichare ascribed to the extended chromophore, comprising all
Figure 8. Effect of the initial dye concentration on the discolorationefficiency of RC: (×) 90 mg/L; (●) 120 mg/L; (△) 175 mg/L; (■)230 mg/L; (◆) 350 mg/L. (a) Discoloration efficiency versusirradiation time. (b) Rate constants as a function of the initial dyeconcentration.
Figure 9. Effect of the initial dye concentration on the discolorationefficiency of MG: (×) 90 mg/L; (●) 120 mg/L; (△) 175 mg/L; (■)230 mg/L; (◆) 350 mg/L. (a) Discoloration efficiency versusirradiation time. (b) Rate constants as a function of the initial dyeconcentration.
Table 1. Pseudo-First-Order Kinetic Parameters forDiscoloration of RC and MG, with pH 4, 200 mg/L H2O2,and 0.3 g/L Fe-PILC
conjugated aromatic rings connected through CC and CNdouble bonds, and the third absorption band is at 315 nm inthe UV region, which is due to the benzene ring structure of thedye molecule.54 The peak intensities decrease with increasingreaction time, indicating the effective degradation of both RCand MG dyes. The decrease in the intensity occurs similarly forall 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 reactiontime. Then naphthalene and benzene rings are as welldestroyed. However, after 20 min of reaction, the alreadyslightly visible absorbance peak at 236 nm points to the moredifficult degradation of the remaining benzene structures. Infact, some recent studies claim that discoloration occurs and iscompleted faster than the total mineralization, completeoxidation, of the dye in the treated water solution.48 Thesame can be observed for the degradation of MG, although, asalready commented before, its oxidation seems much easierthan that of RC because of the lower-molecular-weight, simplerstructure of the dye molecule in this case.
Degradation under such reaction conditions and in thepresence of the Fe-PILC catalyst requires longer irradiationtimes. COD was measured at different time intervals under thesame reaction conditions as those for the UV−vis experiments.A COD removal efficiency was calculated and plotted as afunction of the irradiation time; see Figure 12. As expected,
COD decreases, and therefore the COD removal efficiencyincreases, with irradiation time. After 20 min of reaction, themaximal time at which UV spectra were acquired, the CODremoval efficiency reached 86% in the MG solution, whereas aCOD removal efficiency of 60% was measured for the RCsolution. These results confirm that after 20 min of irradiationcomplete degradation was not yet been reached. A CODremoval efficiency of 100%, that is, complete degradation, isachieved after 45 min of reaction for MG and upon 1 h ofirradiation for RC, as can be observed in Figure 12.
3.4. Leaching and Stability Tests. Chemical stability is animportant property in an efficient catalyst. Leaching runs wereperformed in order to evaluate the catalytic activity and stabilityof Fe-PILC during successive experiments. Therefore, the samecatalyst was used in three consecutive experiments by usingfresh dye solutions at the following reaction conditions: RC orMG concentration of 230 mg/L, pH 3, 200 mg/L H2O2, and0.3 g/L catalyst. After each experiment, the catalyst wasremoved by filtration, carefully washed with distilled water, anddried 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 inthe three subsequent cycles. Moreover, after the threesuccessive experiments, the concentration of iron ions in thesolution was below 0.2 mg/L, conforming the EnvironmentalQuality Act 1974 standards. Note as well pH 3 in these tests,i.e., lower than those in other runs. Therefore, this result furtherproves 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 claypresented in this work, to other previously published results,considering different treatment approaches, such as adsorption,biological treatment, or photodegradation, as well as photo-Fenton processes in the presence of different catalysts, such aszeolite, carbon, and collagen fibers. Interesting reviews havebeen as well published in the last years, compiling the resultsobtained in the most recently published works on Fentoncatalysts.24,26,63 In general terms, there is a huge influence of thetype of pollutant considered, as well as of the initialconcentration. Each catalytic system behaves differently;optimal conditions for achieving maximal degradation of the
Figure 10. Evolution of UV−vis spectra of an RC aqueous solutionwith reaction time t = (a) 0, (b) 1, (c) 5, and (d) 20 min.
Figure 11. Evolution of UV−vis spectra of a MG aqueous solutionwith reaction time t = (a) 0, (b) 1, (c) 5, and (d) 20 min.
Figure 12. Mineralization in terms of the COD removal efficiency as afunction of the irradiation time for MG and RC.
Industrial & Engineering Chemistry Research Article
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 theviability and optimal reaction conditions for each catalyticsystem, prior to considering its practical application to suchphoto-Fenton treatments of wastewaters.
4. CONCLUSIONS
A local Tunisian clay was successfully pillared with iron. Itsactivity was assayed in the photo-Fenton oxidation of RC andMG in an aqueous solution, under UV-light irradiation,determining the influence of several operational parameterson its discoloration efficiency.The sesults showed that both dyes could be effectively
removed from the aqueous media by means a heterogeneousphoto-Fenton process in the presence of Fe-PILC as thecatalyst, demonstrating the feasibility of using this particularTunisian clay in the present application. The study of theinfluence of the different reaction parameters pointed tooptimal discoloration when using pH 3, for a catalyst dosage of0.3 g/L, adding a H2O2 concentration of 200 mg/L, at roomtemperature conditions. MG was always more easily eliminatedthan RC, whose degradation seemed to be more intenselyinfluenced by nonoptimal reaction conditions. This can be dueto the more complex structure and higher molecular weight ofRC in comparison to MG. Data fitting indicated that thediscoloration kinetics of RC and MG followed a first-order ratelaw. Transient evaluation of dye degradation by means of theacquisition of UV−vis absorption spectra evidenced firstdiscoloration due to destruction of the azo core of the dyestructure but more difficult and incomplete destruction of lightaromatics such as benzenes, which are more immune to thephoto-Fenton treatment, especially in the case of the RCsolution. COD measurements confirmed total mineralization atirradiation times of 45 and 60 min for MG and RC solutions,respectively. Moreover, leaching and stability tests showed thatthe catalyst is stable over the evaluated reaction time and cycles.
■ ASSOCIATED CONTENT
*S Supporting InformationTables 1S and 2S, respectively containing additionalinformation on dyes and properties of the clay, before andafter Fe-pillaring. This material is available free of charge via theInternet at http://pubs.acs.org.
■ AUTHOR INFORMATIONCorresponding Author*Tel.: +33 1 30 85 48 65. Fax: +33 1 30 85 48 99. E-mail:[email protected] ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSThe authors thank the University of Sfax and UPMC SorbonneUniversite.
■ REFERENCES(1) O’Neill, C.; Hawkes, F. R.; Hawkes, D. L.; Lourenc, N. D.;Pinheiro, H. M.; Delee, W. Color in textile effluents−sources,measurement, discharge consents and simulation: A review. J. Chem.Technol. Biotechnol. 1999, 10, 1009−1018.(2) Arslan-Alaton, I.; Gursoy, B. H.; Schmidt, J. E. Advancedoxidation of acid and reactive dyes: effect of Fenton treatment onaerobic, anoxic and anaerobic processes. Dyes Pigm. 2008, 78, 117−130.(3) Vijayaraghavan, K.; Yun, Y. S. Bacterial biosorbents andbiosorption. Biotechnol. Adv. 2008, 26, 266−291.(4) Wu, F. C.; Tseng, R. L. High adsorption capacity NaOH-activated carbon for dyes removal from aqueous solution. J. Hazard.Mater. 2008, 152, 1256−1267.(5) Forgacs, E.; Cserhati, T.; Oros, G. Decolorization of wastewaters:A review. Environ. Int. 2004, 30, 953−971.(6) Mahmoodi, N. M.; Arami, M.; Bahrami, H.; Khorramfar, S. Novelbiosorbent (Canola hull) surface characterization and dye removalability at different cationic dye concentrations. Desalination 2010, 264,134−142.(7) Erol, F.; Ozbelge, T. A. Catalytic ozonation with non-polarbonded alumina phases for treatment of aqueous dye solutions in asemi-batch reactor. Chem. Eng. J. 2008, 139, 272−283.(8) Tan, I. A. W.; Ahmad, A. L.; Hameed, B. H. Adsorption of basicdye on high-surface area activated carbon prepared from coconut husk:equilibrium, kinetic and thermodynamic studies. J. Hazard. Mater.2008, 154, 337−346.(9) Sadri Moghaddama, S.; Alavi Moghaddama, M. R.; Arami, M.Coagulation/flocculation process for dye removal using sludge fromwater treatment plant: Optimization through response surfacemethodology. J. Hazard. Mater. 2010, 175, 651−657.(10) Ciardelli, G.; Corsi, L.; Marucci, M. Membrane separationforwastewater reuse in the textile industry. Resour. Conserv. Recycl.2000, 31, 189−197.
Table 2. Comparison of Discoloration Efficiencies and Times for Different Methods and Catalysts
(11) Parsons, S. Advanced Oxidation Processes for Water andWastewater Treatment; IWA: London, 2004.(12) Pera-Titus, M.; Garcıa-Molina, V.; Baos, M. A.; Gimenez, J.;Esplugas, S. Decolorization of chlorophenols by means of advancedoxidation processes: a general review. Appl. Catal., B 2004, 47, 219−256.(13) Aleboyeh, A.; Aleboyeh, H.; Moussa, Y. Critical effect ofhydrogen peroxide in photochemical oxidative decolorization of dyes:acid Orange 8 Acid Blue 74 and Methyl Orange. Dyes Pigm. 2003, 57,67−75.(14) Zhou, S.; Gu, C.; Qian, Z.; Xu, J.; Xia, C. The activity andselectivity of catalytic peroxide oxidation of chlorophenols over Cu−Alhydrotalcite/clay composite. J. Colloid Interface Sci. 2011, 357, 447−452.(15) Catrinescu, C.; Arsene, D.; Teodosiu, C. Catalytic wet hydrogenperoxide oxidation of para-chlorophenol over Al/Fe pillared clays(AlFePILCs) prepared from different host clays. Appl. Catal., B 2011,101, 451−460.(16) Iurascu, B.; Siminiceanu, I.; Vione, D.; Vicente, M. A.; Gil, A.Phenol degradation in water through a heterogeneous photo-Fentonprocess catalyzed by Fe-treated laponite. Water Res. 2009, 43, 1313−1322.(17) Chen, Q.; Wu, P.; Dang, Z.; Zhu, N.; Li, P.; Wu, J.; Wang, X.Iron pillared vermiculite as a heterogeneous photo-Fenton catalyst forphotocatalytic degradation of azo dye reactive brilliant orange X-GN.Sep. Purif. Technol. 2010, 71, 315−323.(18) Chen, Q.; Wu, P.; Li, Y.; Zhu, N.; Dan, Z. Heterogeneousphoto-Fenton photodegradation of reactive brilliant orange X-GNover iron-pillared montmorillonite under visible irradiation. J. Hazard.Mater. 2009, 168, 901−908.(19) Maezono, T.; Tokumura, M.; Sekine, M.; Kawase, Y. Hydroxylradical concentration profile in photo-Fenton oxidation process:Generation and consumption of hydroxyl radicals during thediscoloration of azo-dye Orange II. Chemosphere 2011, 82, 1422−1443.(20) Feng, J.; Hu, X.; Yue, P. L.; Zhu, H. Y.; Lu, G. Q. Discolorationand mineralization of Reactive Red HE-3B by heterogeneous photo-Fenton reaction. Water Res. 2003, 37, 3776−3784.(21) Feng, J.; Hu, X.; Yue, P. L.; Zhu, H. Y.; Lu, G. Q. A novellaponite clay-based Fe nanocomposite and its photocatalytic activity inphoto-assisted degradation of Orange II. Chem. Eng. Sci. 2003, 58,679−685.(22) Predrag, B.; Aleksandra, M. N.; Zorica, M.; Aleksandra, R.;Zeljko, C.; Davor, L.; Dusan, J. Toluene degradation in water usingAlFe pillared clay catalysts. Chin. J. Catal. 2009, 30, 14−18.(23) Wahiba, N.; Azabou, S.; Sami, S.; Abdelhamid, G. Catalytic wetperoxide photo-oxidation of phenolic olive oil mill wastewatercontaminants Part I. Reactivity of tyrosol over (Al−Fe)PILC. Appl.Catal., B 2007, 74, 11−18.(24) Navalon, S.; Alvaro, M.; Garcia, H. Heterogeneous Fentoncatalysts based on clays, silicas and zeolites. Appl. Catal., B 2010, 99,1−26.(25) Catrinescu, C.; Arsene, D.; Apopei, P.; Teodosiu, C.Degradation of 4-chlorophenol from wastewater through heteroge-neous Fenton and photo-Fenton process, catalyzed by Al-Fe PILC.Appl. Clay Sci. 2012, 58, 96−101.(26) Herney-Ramirez, J.; Vicente, M. A.; Madeira, L. M.Heterogeneous photo-Fenton oxidation with pillared clay basedcatalysts for wastewater treatment: A review. Appl. Catal., B 2010,98, 10−26.(27) Zhang, Z. H.; Shan, Y.; Wang, J.; Ling, H.; Zang, S.; Gao, W.;Zhao, Z.; Zhang, H. Investigation on the rapid degradation of congored catalyzed by activated carbon powder under microwave irradiation.J. Hazard. Mater. 2007, 147, 325−333.(28) Yao, K.; Gong, J.; Zheng, J.; Wang, L.; Tan, H.; Zhang, G.; Lin,Y.; Na, H.; Chen, X.; Wen, X.; Tang, T. Catalytic carbonization ofchlorinated poly(vinyl chloride) microfibers into carbon microfiberswith high performance in the photodegradation of congo red. J. Phys.Chem. C 2013, 117, 17016−17023.
(29) Pariente, M. I.; Martínez, F.; Melero, J. A.; Botas, J. A.;Velegraki, T.; Xekoukoulotakis, N. P.; Mantzavinos, D. Heterogeneousphoto-Fenton oxidation of benzoic acid in water: Effect of operatingconditions, reaction by-products and coupling with biologicaltreatment. Appl. Catal., B 2008, 85, 24−32.(30) Miao, Z.; Tao, S.; Wang, Y.; Yu, Y.; Meng, C.; An, Y.Hierarchically porous silica as an efficient catalyst carrier for highperformance vis-light assisted Fenton degradation. MicroporousMesoporous Mater. 2013, 176, 178−185.(31) Walling, C. Fenton’s reagent revisited. Acc. Chem. Res. 1975, 8,125−131.(32) Sabhi, S.; Kiwi, J. Degradation of 2,4-dichlorophenol byimmobilized iron catalysts. Water Res. 2001, 35, 1994−2002.(33) Darder, M.; Colilla, M.; Ruiz-Hitzky, E. Chitosan−claynanocomposites: applica- tion as electrochemical sensors. Appl. ClaySci. 2005, 28, 199−208.(34) Li, S.; Wu, P. Characterization of sodium dodecyl sulfatemodified iron pillared montmorillonite and its application for theremoval of aqueous Cu(II) and Co(II). J. Hazard. Mater. 2010, 173,62−70.(35) Dorado, F.; de Lucas, A.; Garcia, P. B.; Romero, A.; Valverde, J.L. Copper ion-exchanged and impregnated Fe-pillared clays: Study ofthe influence of the synthesis conditions on the activity for theselective catalytic reduction of NO with C3H6. Appl. Catal., A 2006,305, 189−195.(36) Manjanna, J. Preparation of Fe(II)-montmorillonite byreduction of Fe(III) montmorillonite with ascorbic acid. Appl. ClaySci. 2008, 42, 32−38.(37) Borgnino, L.; Avena, M. J.; De Pauli, C. P. Synthesis andcharacterization of Fe(III)-montmorillonites for phosphate adsorption.Colloids Surf., A 2009, 341, 46−52.(38) Binitha, N. N.; Sugunan, S. Preparation, characterization andcatalytic activity of titania pillared montmorillonite clays. MicroporousMesoporous Mater. 2006, 93, 82−89.(39) Xu, X.; Pan, Y.; Cui, X.; Suo, Z. Catalytic Combustion ofMethane over Ti-Pillared Clay Supported Copper Catalysts. J. Nat.Gas Chem. 2004, 13, 204−208.(40) Mishra, B. G.; Ganga Rao, G. Physicochemical and catalyticproperties of Zr-pillared montmorillonite with varying pillar density.Microporous Mesoporous Mater. 2004, 70, 43−50.(41) Martínez, F.; Calleja, G.; Melero, J. A.; Molina, R. Iron speciesincorporated over different silica supports for the heterogeneousphoto-Fenton oxidation of phenol. Appl. Catal., B 2007, 70, 452−460.(42) Nogueira, F. G. E.; Lopes, J. H.; Silva, A. C.; Goncalves, M.;Anastacio, A. S.; Sapag, K.; Oliveira, L. C. A. Reactive adsorption ofmethylene blue on montmorillonite via an ESI-MS study. Appl. ClaySci. 2009, 43, 190−195.(43) Xu, T.; Liu, Y.; Ge, F.; Liu, L.; Ouyang, Y. Application ofresponse surface methodology for optimization of azocarmine Bremoval by heterogeneous photo-Fenton process using hydroxy-iron-aluminum pillared bentonite. Appl. Surf. Sci. 2013, 280, 926−932.(44) Deng, Y.; Englehardt, J. D. Treatment of landfill leachate by theFenton process. Water Res. 2006, 40, 3683−3694.(45) Benitez, F. J.; Acero, J. L.; Real, F. J.; Rubio, F. J.; Leal, A. I. Therole of hydroxyl radicals for the decomposition of p-hydroxyphenylacetic acid in aqueous solutions. Water Res. 2001, 35, 1338−1343.(46) Kwon, B. G.; Lee, D. S.; Kang, N.; Yoon, J. Characteristics of p-chlorophenol oxidation by Fenton’s reagent. Water Res. 1999, 33,2110−18.(47) Feng, J.; Hu, X.; Yue, P. L.; Zhu, H. Y.; Lu, G. Q. Degradation ofazo-dye orange II by a photoassisted Fenton reaction using a novelcomposite of iron oxide and silicate nanoparticles as a catalyst. Ind.Eng. Chem. Res. 2003, 42, 2058−2066.(48) Neamtu, M.; Catrinescu, C.; Kettrup, A. Effect of dealuminationof iron (III)-exchanged Y zeolites on oxidation of Reactive Yellow 84azo dye in the presence of hydrogen peroxide. Appl. Catal., B 2004, 51,149−157.
Industrial & Engineering Chemistry Research Article
(49) Du, W. P.; Xu, Y. M.; Wang, Y. S. Photoinduced degradation ofOrange II on different iron (hydr)oxides in aqueous suspension: rateenhancement on addition of hydrogen peroxide, silver nitrate, andsodium fluoride. Langmuir 2008, 24, 175−181.(50) Sun, S. P.; Li, C. J.; Sun, J. H.; Shi, S. H.; Fan, M. H.; Zhou, Q.Decolorization of an azo dye Orange G in aqueous solution by Fentonoxidation process: effect of system parameters and kinetic study. J.Hazard. Mater. 2009, 161, 1052−1057.(51) Feng, J. Y.; Hu, X. J.; Yue, P. L. Novel bentonite clay-based Fe-nanocomposite as a heterogeneous catalyst for photo-Fentondiscoloration and mineralization of Orange II. Environ. Sci. Technol.2004, 38, 269−275.(52) Tokumura, M.; Znad, H. T.; Kawase, Y. Decolorization of darkbrown colored coffee effluent by solar photo-Fenton reaction: effect ofsolar light dose on decolorization kinetics. Water Res. 2008, 42, 4665−4673.(53) Cao, Y.; Hu, Y.; Sun, J.; Hou, B. Explore various co-substratesfor simultaneous electricity generation and Congo red degradation inair-cathode single-chamber microbial fuel cell. Bioelectrochemistry 2010,79, 71−76.(54) Bai, C.; Xiao, W.; Feng, D.; Xian, M.; Guo, D.; Ge, Z.; Zhou, Y.Efficient decolorization of Malachite Green in the Fenton reactioncatalyzed by [Fe(III)-salen]Cl complex. Chem. Eng. J. 2013, 215−216,227−234.(55) Khattri, S. D.; Singh, M. K. Removal of malachite green fromdye wastewater using neem sawdust by adsorption. J. Hazard. Mater.2009, 167, 1089−1094.(56) Chen, C. Y.; Kuo, J. T.; Cheng, C. Y.; Huang, Y. T.; Ho, I. H.;Chien Chung, Y. Biological decolorization of dye solution containingmalachite green by Pandoraea pulmonicola YC32 using a batch andcontinuous system. J. Hazard. Mater. 2009, 172, 1439−1445.(57) Saha, S.; Wang, J. M.; Pal, A. Nano silver impregnation oncommercial TiO2 and a comparative photocatalytic account to degrademalachite green. Sep. Purif. Technol. 2012, 89, 147−159.(58) Liu, X.; Tang, R.; He, Q.; Liao, X.; Shi, B. Fe(III)-loadedcollagen fiber as a heterogeneous catalyst for the photo-assisteddecomposition of Malachite Green. J. Hazard. Mater. 2010, 174, 687−693.(59) Lian, L.; Guo, L.; Guo, C. Adsorption of Congo red fromaqueous solutions onto Ca-bentonite. J. Hazard. Mater. 2009, 161,126−131.(60) Hsueh, C. C.; Chen, B. Y. Comparative study on reactionselectivity of azo dye decolorization by Pseudomonas luteola. J.Hazard. Mater. 2007, 141, 842−849.(61) Chen, C. C.; Lu, C. S.; Chung, Y. C.; Jan, J. L. UV light inducedphotodegradation of malachite green on TiO2 nanoparticles. J. Hazard.Mater. 2007, 141, 520−528.(62) Kondru, A. K.; Kumar, P.; Chand, S. Catalytic wet peroxideoxidation of azo dye (Congo Red) using modified Y zeolite as catalyst.J. Hazad. Mater. 2009, 166, 342−347.(63) Soon, A. N.; Hameed, B. H. Heterogeneous catalytic treatmentof synthetic dyes in aqueous media using Fenton and photo-assistedFenton processes. Desalination 2011, 269, 1−16.
Industrial & Engineering Chemistry Research Article
