Natural Hematite and Siderite As Heterogeneous Catalysts for an Effective Degradation of 4-Chlorophenol Via Photo-Fenton Process

chemengineering

Article Natural Hematite and Siderite as Heterogeneous Catalysts for an Effective Degradation of 4-Chlorophenol via Photo-Fenton Process

Haithem Bel Hadjltaief 1, Ali Sdiri 1 ID , María Elena Gálvez 2,* ID , Haythem Zidi 1, Patrick Da Costa 2 ID and Mourad Ben Zina 1

1 Laboratory of Water, Energy and Environment, National Engineering School, University of Sfax, P.O. Box 1173 W, 3038 Sfax, Tunisia; [email protected] (H.B.H.); [email protected] (A.S.); [email protected] (H.Z.); [email protected] (M.B.Z.) 2 CNRS, Institut Jean Le Rond d’Alembert, Sorbonne Université, 2 place de la gare de ceinture, 78210 Saint-Cyr-L’Ecole, France; [email protected] * Correspondence: [email protected]; Tel.: +33-1-30-85-48-77

Received: 18 May 2018; Accepted: 9 June 2018; Published: 21 June 2018

Abstract: This paper describes a simple and low-cost process for the degradation of 4-Chlorophenol (4-CP) from aqueous solution, using natural Tunisian Hematite (M1) and Siderite (M2). Two natural samples were collected in the outcroppings of the Djerissa mining site (Kef district, northwestern Tunisia). Both Hematite and Siderite ferrous samples were characterized using several techniques, including X-Ray Diffraction (XRD), Nitrogen Physisorption (BET), Infrared Spectroscopy (FTIR), H2-Temperature Programmed Reduction (H2-TPR), Scanning Electronic Microscopy (SEM) linked with Energy Dispersive X-ray (EDS) and High-Resolution Transmission Electron Microscopy (HRTEM). Textural, structural and chemical characterization conﬁrmed the presence of Hematite and Siderite phases with a high amount of iron on the both surface materials. Their activity was evaluated in the oxidation of 4-CP in aqueous medium under heterogeneous photo-Fenton process. Siderite exhibited higher photocatalytic oxidation activity than Hematite at pH 3. The experimental results also showed that 100% conversion of 4-CP and 54% TOC removal can be achieved using Siderite as catalyst. Negligible metal leaching and catalyst reutilization without any loss of activity point towards an excellent catalytic stability for both natural catalysts.

Keywords: heterogeneous catalysts; photo-Fenton; hematite; siderite; oxidation; 4-chlorophenol

1. Introduction The phenol-derivate 4-chlorophenol (4-CP) is a widely used both as a solvent and as a platform molecule in the synthesis of dyes, insecticides, pesticides, biocides, paints, phenolic resins, plastics and other chemicals [1–3]. Nevertheless, 4-CP is known for its carcinogenic and mutagenic effects [2]. 4-CP can result in respiratory tract irritation, in addition to its deleterious effects on eyes, skin and mucous membrane. According to the European Union standards, the threshold limit of total phenols in potable water is 0.5 mg/L [3,4]. Thus, it is fundamentally important to reduce phenol contents in both potable and wastewater. To achieve this goal, various methods have been developed, including adsorption [5], biological degradation [6], electrochemical oxidation [2], wet oxidation [4], photocatalytic degradation [1] and photo-Fenton oxidation [7–9]. A homogeneous Fenton or photo-Fenton reaction is considered one of the most promising ways to remove highly hazardous phenolic species, because it is not inﬂuenced by their toxicity [10,11]. Fenton oxidation is however not efﬁcient enough at neutral pH. Moreover, the separation and regeneration of the catalyst

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(homogeneous) and the possible secondary contamination due to the presence of 50–80 ppm iron in the treated efﬂuent are important drawbacks for its scale-up and feasible application [11,12]. To overcome these disadvantages of homogeneous system, special attention has been given to the development heterogeneous Fe-catalyst. Different support materials, such as synthetic or natural zeolites, clays, polymers, silica, carbon or resins, have shown to be good candidates for the heterogenization of Fe-containing catalysts for the oxidative degradation of organic compounds through the Fenton or photo-Fenton reaction [13–15]. In addition, the so-prepared catalysts have generally shown an excellent catalytic activity and high stability. Nevertheless, they have high preparation cost via the use of chemicals, high-energy demands and calcinations. In this context, the use of Natural clay-like heterogeneous materials such as hematite [11], pyrite and chalcopyrite [16], siderite [17], magnetite [18] and goethite [19] are alternative ecosystem friendly natural materials for the organic pollutants in water. In addition, their use as clay-like materials is gaining importance due to their high stability under irradiation, possibility of recovery and reuse while no strict control of the pH is required during the reactions [11,16–19]. Natural pyrite (FeS2) and chalcopyrite (CuFeS2) were used as heterogeneous Fenton catalyst for the degradation of phenolic compounds found in olive mill wastewater (OMW). The related results showed that high TOC removal, low leaching of iron species and spontaneous formation of a small amount of H2O2 under acid conditions [16]. In a subsequent work, the catalytic activity of Natural Tunisian clay-like material (i.e., Hematite) was also investigated in oxidation of phenol and 2-chlorophenol using heterogeneous photo-Fenton [11,20]. Since, generally, high content of iron oxides are needed, the use of environmentally friendly and more economical raw material may be a good option for the preparation of such Fe-catalysts. In this sense some natural Tunisian materials containing very important amounts of iron can be of considerable interest in photo-Fenton processes. Such materials can be directly used in heterogeneous Fenton applications, bypassing the need of employing any other Fe-source, as well as the catalyst preparation process itself. To the best of our knowledge and based on a detailed literature review, in the present work, we show for the ﬁrst time that these materials (Hematite and Siderite) can be used as catalysts for the degradation of organic pollutants by means of photo-Fenton processes. In this sense, the main objective of the present work is to evaluate the activity of two raw Tunisian materials (i.e., Hematite (M1) and Siderite (M2)) in the degradation and mineralization of 4-chlorophenol in aqueous solution, considering as well its stability on a prolonged operation time.

2. Materials and Methods

2.1. Materials and Reagents

Raw ferrous samples M1 and M2 were collected from Djerissa mines, kef district, northwestern Tunisia (Figure1). These materials were first crushed, dispersed and purified in water, and the desired particle size of less than 60 µm was extracted by simple sedimentation. This fraction was then ultrasonicated in 95% ethanol for 5 min, washed with 0.1 M HNO3, rinsed with deionized water and 95% ethanol and finally dried to 30 ◦C. Hydrogen peroxide (35%), 4-Chlorophenol (>99%) and MnO2 (>99%) were of analytical reagent grade, purchased from Sigma-Aldrich (now Merck KGaA, Darmstadt, Germany).

2.2. Physicochemical Characterization Chemical composition of the both siderite and hematite samples was determined by X-ray ﬂuorescence with an ARL® 9800 XP (Thermo Fischer Scientiﬁc, Waltham, MA, USA) spectrometer. Nitrogen adsorption–desorption isotherms were determined at −196 ◦C on a Micromeritics ASAP 2010 (Micromeritics Norcross, Norcross, GA, USA), after degassing (10−5 Pa) for 24 h at ambient temperature. Surface areas were calculated using the BET equation, whereas mean pore size, pore size distribution and pore volume were estimated using the BJH method. ChemEngineering 2018, 2, 29 3 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 3 of 14

Raw Material M1 Raw Material M2

Figure 1. OutcroppingsOutcroppings of the materials in Jrissa mining site.

2.2. PhysicochemicalX-ray diffraction Characterization (XRD) patterns were obtained in a Philips® PW 1710 PAN (PAN Analytical, Almelo,Chemical The Netherlands) composition analytical of the both Empyrean siderite diffractometers and hematite equippedsamples was with determined Cu Kα (λ = by 1.5406 X-ray Å) ◦ ◦ ◦ fluorescenceradiation source with and an 2ARLθ range® 9800 between XP (Thermo 3 and Fischer 80 , with Scientific, a step size Waltham, of 0.02 /s.MA, USA) spectrometer. NitrogenThe profilesadsorption–desorption of temperature-programmed isotherms were reduction determined (H2 -TPR)at −196 were °C on acquired a Micromeritics in a BELCAT-M ASAP 2010(BEL Japan)(Micromeritics device, equipped Norcross, with Norc a thermalross, GA, conductivity USA), after detector degassing (TCD). (10 The−5 Pa) sample-based for 24 h at catalystsambient ◦ temperature.were first degassed Surface to 100areasC were for 2 h,calculated and then reducedusing the in BET 5% (vol.) equation, H2/Ar, whereas while increasing mean pore temperature size, pore ◦ ◦ sizefrom distribution 100 to 900 andC at pore a increment volume ofwere 7.5 estimatedC/min. using the BJH method. X-rayThe morphology diffraction (XRD) was ascertained patterns were using obtained a scanning in a electronPhilips® microscopePW 1710 PAN (SEM, (PAN HitachiSU-70, Analytical, 2 Almelo,Hitachi, The Tokyo, Netherlands) Japan) equipped analytical with Empy anrean Oxford diffractometers X-Max 50 equipped mm X-ray with spectroscopy Cu Kα (λ = 1.5406 systems Å) radiationthrough dispersivesource and energy 2θ range (EDX), between which 3° and enabled 80°, with qualitative a step size evaluation of 0.02°/s. of chemical composition. HighThe resolution profiles of transmission temperature-programmed electron microscopy reduction (HRTEM) (H2-TPR) images were wereacquired also in obtained a BELCAT-M on a (BELJEOLJEM Japan) 2011 device, equipped equipped with LaBwith6 filament a thermal and co operatingnductivity at detector 200 kV. Images(TCD). wereThe collectedsample-based with × catalystsa 4008 were2672 pixelfirst degassed CCD camera to 100 (Gatan °C for Orius 2 h, and SC1000, then reduced Gatan, Pleasanton, in 5% (vol.) CA,H2/Ar, USA) while coupled increasing with temperatureDIGITAL MICROGRAPH from 100 to 900 software °C at a (GMSincrement 3). of 7.5 °C/min. The morphology was ascertained using a scanning electron microscope (SEM, HitachiSU-70, 2.3. Photo-Fenton Experiments and Analytical Procedures Hitachi, Tokyo, Japan) equipped with an Oxford X-Max 50 mm2 X-ray spectroscopy systems throughHeterogeneous dispersive energy photo-Fenton (EDX), experimentswhich enabled were qu performedalitative evaluation in an open of Pyrex-glass chemical composition. cell (250 mL Highvolume, resolution 5 cm inner transmission diameter and electron 11 cm height). micros Acopy detailed (HRTEM) description images of catalytic were also reactor obtained was reported on a JEOLJEMby Bel Hadjltaief 2011 equipped et al. [10 ,21with]. Irradiation LaB6 filament was and carried operating out using at 200 2 parallel kV. Images UV lamps were at collected 254 and 365with nm a 4008at 930/13 × 2672µW/cm pixel 2CCD. All ofcamera the experiments (Gatan Orius were SC1000, performed Gatan, keeping Pleasanton, the distance CA, betweenUSA) coupled the solution with DIGITALand the lamps MICROGRAPH constant at 15 software cm and (GMS under 3). room temperature conditions (i.e., 25 ◦C). The initial pH of the 4-Chlorophenol solution was adjusted to acidic condition (pH = 3), optimum pH for photo-Fenton 2.3.processes Photo-Fenton [22]. The Experiments initial concentration and Analytical of 4-CP Procedures in the treated solution was 20 mg/L. After stabilization of theHeterogeneous stirring (200 rpm), photo-Fenton 0.05 g of catalystexperiments was addedwere pe torformed 100 mL in of an an open aqueous Pyrex-glass 4-CP solution. cell (250 Then, mL volume,a fixed volume 5 cm inner (2 mL) diameter of a 1000 and mg/L 11 cm H 2height).O2 solution A detailed was poured description into the of 4-CPcatalytic solution. reactor H was2O2 reportedaddition wasby Bel taken Hadjltaief as the initialet al. [10,21]. time of Irradiation the reaction. was Aliquots carried ofout supernatant using 2 parallel (1 mL) UV were lamps regularly at 254 extracted from the reaction vessel with the aid of a syringe; Residual H O was immediately quenched and 365 nm at 930/13 μW/cm2. All of the experiments were performed2 keeping2 the distance between thewith solution MnO2 (99% and Merck)the lamps to preventconstant the at dark15 cm Fenton and un reactionder room occurring temperature through cond theitions possible (i.e., presence 25 °C). µ Theof leached initial pH iron. of Beforethe 4-Chlorophenol analysis, the liquidsolution was was filtered adjusted using to acidic PTFE filterscondition (0.45 (pHm). = 3), In optimum addition, pHthe liquidfor photo-Fenton solutions after processes reactions [22]. were The analyzedinitial concentration by ICP-OES of device 4-CP in to the determine treated thesolution iron content.was 20 mg/L.The choice After of stabilization these experimental of the stirring conditions (200 was rpm), based 0.05 in g the of previouscatalyst was published added papersto 100 onmL Fenton of an and photocatalysis using clay-derived catalysts. aqueous 4-CP solution. Then, a fixed volume (2 mL) of a 1000 mg/L H2O2 solution was poured into The concentration of 4-chlorophenol in the solution was analyzed using an Agilent 2025 GC the 4-CP solution. H2O2 addition was taken as the initial time of the reaction. Aliquots of supernatant (Agilent, Santa Clara, CA, USA) gas chromatography (GC), equipped with a Zebron capillary column (1 mL) were regularly extracted from the reaction vessel with the aid of a syringe; Residual H2O2 was ZB-5MSi (30 m × 0.32 mm × 0.25 µm) and a flame ionization detector (FID). The extent of the immediately quenched with MnO2 (99% Merck) to prevent the dark Fenton reaction occurring through the possible presence of leached iron. Before analysis, the liquid was filtered using PTFE filters (0.45 μm). In addition, the liquid solutions after reactions were analyzed by ICP-OES device to ChemEngineering 2018, 2, x FOR PEER REVIEW 4 of 14 determine the iron content. The choice of these experimental conditions was based in the previous published papers on Fenton and photocatalysis using clay-derived catalysts. The concentration of 4-chlorophenol in the solution was analyzed using an Agilent 2025 GC (Agilent, Santa Clara, CA, USA) gas chromatography (GC), equipped with a Zebron capillary column ZB-5MSi (30 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID). The extent of the mineralization of the 4-CP was determined based on total organic carbon measurements using a Shimadzu TOC-5000A (Shimadzu, Kyoto, Japan) analyzer. Degradation and mineralization efficiencies were calculated as follows:

ChemEngineering 2018, 2, 29 4 of 14 Degradation/mineralization efficiency = ((C0 − Ct)/C0) × 100 (1) where Ct denotes the time-dependent concentration/TOC and C0 is the initial concentration/TOC. Notemineralization here that the of thedegradation 4-CP was experiments determined were based re onpeated total three organic times carbon to check measurements the repeatability using of a theShimadzu results TOC-5000Aobtained. (Shimadzu, Kyoto, Japan) analyzer. Degradation and mineralization efficiencies were calculated as follows: 3. Results and Discussion Degradation/mineralization efficiency = ((C0 − Ct)/C0) × 100 (1) 3.1. Catalysts Characterization where Ct denotes the time-dependent concentration/TOC and C0 is the initial concentration/TOC. NoteChemical here that thecomposition degradation of the experiments raw materials were M repeated1 and M three2 was timescomparable, to check but the M repeatability1 had a distinctly of the higherresults obtained.content of iron (Table 1). High amount of iron was found used in heterogeneous photo-Fenton oxidation of 4-CP. Nitrogen adsorption–desorption isotherms of both catalysts are given3. Results in Figure and Discussion 2. The isotherm exhibited a typical International Union of Pure and Applied Chemistry (IUPAC) type IV with a tight hysteresis loop at high relative pressure (p/p0 = 0.4–1.0); this 3.1. Catalysts Characterization indicated the presence of laminar-structured materials containing slit-shaped mesopores [11]. As shownChemical in Table composition 2, the specific of surface the raw area materials (SBET), Mpore1 and volume M2 was and comparable, average pore but diameter M1 had afor distinctly M2 are higherhigher than content that of of iron M1. (Table 1). High amount of iron was found used in heterogeneous photo-Fenton oxidation of 4-CP. Nitrogen adsorption–desorption isotherms of both catalysts are given in Figure2. The isothermTable exhibited1. Chemical a composition typical International (percent oxides, Union X-ra of Purey fluorescence) and Applied of th Chemistrye two raw materials. (IUPAC) type IV with a tight hysteresis loop at high relative pressure (p/p = 0.4–1.0); this indicated the presence of Samples SiO2 Al2O3 Fe2O3 CaO MgO0 MnO K2O PF laminar-structured materials containing slit-shaped mesopores [11]. As shown in Table2, the specific M1 9.45 2.91 46.89 5.38 4.35 0.26 0.13 31.54 surface area (S ), pore volume and average pore diameter for M are higher than that of M . BET M2 7.63 0.94 57.6 2.93 2.2 0.222 0.14 28.53 1

Table 1. Chemical composition (percent oxides, X-ray ﬂuorescence) of the two raw materials. Table 2. Textural properties of the two raw materials.

SamplesSamples SurfaceSiO2 AreaAl2 (mO3 2 g−Fe1) 2O Mean3 CaO Pore Size MgO (nm) MnO Pore Volume K2O (cm PF3 g−1)

MM1 1 9.458.00 2.91 46.89 5.38 16.1 4.35 0.26 0.13 0.03 31.54 MM2 2 7.6337.5 0.94 57.6 2.93 8.00 2.2 0.22 0.14 0.07 28.53

Figure 2. XRD patterns for the raw materials M1 and M2. Figure 2. XRD patterns for the raw materials M1 and M2.

Table 2. Textural properties of the two raw materials.

Samples Surface Area (m2 g−1) Mean Pore Size (nm) Pore Volume (cm3 g−1)

M1 8.00 16.1 0.03 M2 37.5 8.00 0.07

Figure3 depicts the mineralogical analysis and phase structures of M 1 and M2 samples. The crystalline structure of the raw material M1, identiﬁed by XRD measurement, showed the main crystalline phases of hematite; the main impurity was calcite and quartz. The XRD patterns of the M1 showed ChemEngineering 2018, 2, x FOR PEER REVIEW 5 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 5 of 14

Figure 3 depicts the mineralogical analysis and phase structures of M1 and M2 samples. The ChemEngineering 2018, 2, 29 5 of 14 Figure 3 depicts the mineralogical analysis and phase structures of M1 and M2 samples. The crystalline structure of the raw material M1, identified by XRD measurement, showed the main crystalline structure of the raw material M1, identified by XRD measurement, showed the main crystalline phases of hematite; the main impurity was calcite and quartz. The XRD patterns of the M1 diffractionshowedcrystalline diffraction phases peaks nearof peaks hematite; 2θ valuesnear the2θ of valuesmain 24.3 ◦impurity ,of 33.3 24.3°,◦, 35.81 was33.3°, ◦calcite, 35.81°, 41.08 and◦ ,41.08°, 49.6 quartz.◦, 49.6°, 54.18, The 54.18, 57.7XRD◦, 57.7°,patterns 62.57 ◦62.57°,, 64.3 of the◦ 64.3°and M1 72.13showedand 72.13°◦ corresponding diffraction corresponding peaks to (012), nearto (012), (104), 2θ values (104), (110), of(110), (113), 24.3°, (113), (024), 33.3°, (024), (116), 35.81°, (116), (118), 41.08°, (118), (214), 49.6°, (214), (300) 54.18, and(300) (119)57.7°, and planes (119)62.57°, planes of 64.3° the Hematiteofand the 72.13° Hematite type, corresponding respectively type, respectively to (JCPDS (012), (JCPDScard(104), file (110), card No. 85-1575)(113),file No. (024), 85-1575) [23]. (116), Based [23]. (118), on Based XRD (214), data, on (300) XRD siderite and data, (119) is siderite the planes major is mineralogicaltheof the major Hematite mineralogical compound type, respectively compound in the structure (JCPDS in the of structurecard natural file No. material,of natural 85-1575) along material, [23]. with Based quartzalong on with XRD as minor quartz data, compound siderite as minor is (Figurecompoundthe major3). Peaksmineralogical (Figure located 3). Peaks atcompound 24.7 located◦, 32.2 inat◦ ,the24.7°, 38.4 structure◦ ,32.2°, 42.4◦ ,38.4 46.2of °,natural◦ ,42.4°, 50.8◦ material,46.2°,, 52.3◦ 50.8°,, 60.9 along◦ 52.3°,, 64.6 with ◦60.9°,and quartz 6564.6°◦ indicated as and minor 65° theindicatedcompound presence the (Figure of presence siderite, 3). Peaks of but siderite, thoselocated nearbut at those24.7°, 26.7 and near32.2°, 67 26 38.4 are.7 and°, indicative 42.4°, 67 are 46.2°, indicative of quartz50.8°, 52.3°, [of24 –quartz26 60.9°,]. FT-IR [24–26]. 64.6° spectra and FT-IR 65° is givenspectraindicated in is Figure giventhe presence4 .in It Figure showed of 4.siderite, theIt showed presence but thethose of presence OH near stretching 26 of.7 OHand bands stretching 67 are near indicative 1790bands cm near of− 1quartz, 1790 as well cm [24–26]. as−1, theas well broadFT-IR as −1 Si-Othespectra broad stretching is givenSi-O bandstretching in Figure near 10004.band It showedcm near−1 1000[27 the]. cm Vibrationpresence−1 [27]. Vibration bandsof OH atstretching 736 bands cm–1 at bands, in736 the cm near range–1, in1790 the 860–880 cm range, as cm 860–880 well–1 and as −1 –1 1390–1440cmthe–1 broad and 1390–1440Si-O cm–1 stretchingareattributed cm–1 areband attributed to near the 1000 CO to bondcm the [27].CO of the bondVibration carbonates, of the bands carbonates, confirming at 736 cm confirming the, in results the range the of XRDresults 860–880 and of cm–1 and 1390–1440 cm–1 are attributed to the CO bond of the carbonates, confirming the results of chemicalXRD and analyseschemical (Siderite: analyses FeCO(Siderite:3, Calcite: FeCO3 CaCO, Calcite:3 and CaCO Dolomite:3 and Dolomite: MgCa (CO MgCa3)2). This(CO3 latter)2). This result latter is inresultXRD agreement and is in chemical agreement with the analyses literature with (Siderite:the [literature24,25 ,FeCO28,29 [24,25,28,29].].3, TPRCalcite: analysis CaCO TPR was3 andanalysis performed Dolomite: was toperformed MgCa get detailed (CO to3) 2 information).get This detailed latter result is in agreement with the literature [24,25,28,29]. TPR analysis was performed to get detailed aboutinformation the nature about of the the nature iron oxides. of the iron The oxides. reduction The profile reduction of the profile raw Mof1 thedisplayed raw M1 twodisplayed hydrogen two information about the nature of the iron◦ oxides. The reduction profile of the raw M1 displayed two consumptionhydrogen consumption peaks in the peaks range in 200–800 the rangeC, 200–800 coinciding °C, withcoinciding the reduction with the profile reduction of Fe 2profileO3 [20 ].of As Fe2 forO3 hydrogen consumption peaks in the range 200–800 °C, coinciding with the reduction profile of Fe2O3 M[20].2 sample, As for threeM2 sample, reduction three peaks reduction were peaks observed were (Figure observed5). An (Figure increase 5). An in temperature increase in temperature caused the [20]. As for M2 sample, three reduction peaks were observed (Figure 5). An increase in temperature dissociationcaused the dissociation of iron carbonates of iron incarbonat iron oxideses in iron such oxides as FeO such and Feas 3FeOO4. and Fe3O4. caused the dissociation of iron carbonates in iron oxides such as FeO and Fe3O4.

Figure 3. FT–IR spectra for the raw materials M1 and M2. Figure 3. FT–IR spectra for the raw materials M1 and M2. Figure 3. FT–IR spectra for the raw materials M1 and M2.

Figure 4. Nitrogen adsorption–desorption isotherms for raw materials M1 and M2. Figure 4. Nitrogen adsorption–desorption isotherms for raw materials M1 and M2. Figure 4. Nitrogen adsorption–desorption isotherms for raw materials M1 and M2. ChemEngineering 2018, 2, 29 6 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 6 of 14

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FigureFigure 5. Temperature5. Temperature programmed programmed reduction reduction (H (H22-TPR)-TPR) proﬁlesprofiles for for raw raw materials materials M M1 1andand M M2. 2.

SEMSEM images Figureimages 5. are Temperatureare shown shown in inprogrammed Figure Figure6 .6. They reductionThey indicated indicated (H2-TPR) sphericalspherical profiles for to raw sub-round materials Mparticles particles1 and M2 .with with some some agglomerated platelets for the raw M1 sample. In contrast, M2 sample is predominantly particles agglomerated platelets for the raw M1 sample. In contrast, M2 sample is predominantly particles with someSEM rhombohedralimages are shown shapes. in Figure EDX 6. analysis They indicated confirmed spherical the presence to sub-round of high particles amount with of someiron on with some rhombohedral shapes. EDX analysis confirmed the presence of high amount of iron on theagglomerated surface of both platelets materials, for the with raw smallM1 sample. amount Ins contrast,of impurities, M2 sample e.g., Si,is predominantlyAl, Fe, Ca, Mg, particles and Mn, the surface of both materials, with small amounts of impurities, e.g., Si, Al, Fe, Ca, Mg, and Mn, confirmingwith some the rhombohedral results of X-ray shapes. fluorescence. EDX analysis TE confM imagesirmed the further presence confirmed of high theseamount changes of iron inon the confirmingthe surface the of results both materials, of X-ray fluorescence.with small amount TEMs of images impurities, further e.g., confirmed Si, Al, Fe, Ca, these Mg, changes and Mn, in the morphology of the M1 and M2 samples (Figure 7). Interlayer distances were measured from the Fast morphologyconfirming of the the results M1 and of X-ray M2 samples fluorescence. (Figure TE7M). images Interlayer further distances confirmed were these measured changes in from the the Fourier Transform (FFT) of the HRTEM image. For M1, the presence of (012) planes with an morphology of the M1 and M2 samples (Figure 7). Interlayer distances were measured from the Fast Fastinter-reticular Fourier Transform distance(FFT) of 3.65 of Å the characteristic HRTEM image. of Hematite For M was1, the clearly presence distinguished, of (012) planes whereas, with for an inter-reticularFourier Transform distance of(FFT) 3.65 of Å the characteristic HRTEM image. of Hematite For M1 was, the clearly presence distinguished, of (012) planes whereas, with an for M , M2, HRTEM images showed lattice fringes from which the (110) plane of siderite and (311) of 2 inter-reticular distance of 3.65 Å characteristic of Hematite was clearly distinguished, whereas, for HRTEMMagnetite images with showed the respective lattice d fringes-spacing from of 2.3 which and 2.5 the Å. (110) plane of siderite and (311) of Magnetite M2, HRTEM images showed lattice fringes from which the (110) plane of siderite and (311) of with the respective d-spacing of 2.3 and 2.5 Å. Magnetite with the respective d-spacing of 2.3 and 2.5 Å.

Raw M1 Raw M1 Raw M1 Raw M1

RawRaw M M2 2 RawRaw M M2 2

FigureFigureFigure 6. 6.SEM 6. SEM SEM images images images and andand EDX EDXEDXspectra spectra forfor the the raw raw materials materials M M1 M and1 1andand M 2M. M2. 2. ChemEngineering 2018, 2, 29 7 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 7 of 14

Raw M1

Raw M2

Figure 7. TEM images for the raw materials M1 and M2. Figure 7. TEM images for the raw materials M1 and M2. 3.2. Catalytic Performance in the Photo-Fenton Oxidation of 4-Chlorophenol 3.2. Catalytic Performance in the Photo-Fenton Oxidation of 4-Chlorophenol Preliminary experiments were carried out to assess the importance of H2O2, catalysts and Preliminary experiments were carried out to assess the importance of H2O2, catalysts and irradiation for 4-CP degradation. Insignificant degradation was observed under H2O2, 4-CP and both irradiation for 4-CP degradation. Insigniﬁcant degradation was observed under H2O2, 4-CP and catalysts alone. In the dark condition, a mixture of H2O2 with catalysts showed 4-CP conversion of both catalysts alone. In the dark condition, a mixture of H2O2 with catalysts showed 4-CP conversion 52.1% and 40.3% for M1 and M2, respectively. This may indicate that Fenton reaction took place via of 52.1% and 40.3% for M and M , respectively. This may indicate that Fenton reaction took place via the enhanced radical formation,1 2 which, in turn, was involved in the oxidation of organic the enhanced radical formation, which, in turn, was involved in the oxidation of organic compounds. compounds. When reaction was performed in the presence of the M1 or M2 catalyst, upon H2O2 addition and UV irradiation, significant degradation of 4-CP during photo-Fenton process was observed. ChemEngineering 2018, 2, 29 8 of 14

ChemEngineeringWhen reaction 2018 was, 2, performedx FOR PEER REVIEW in the presence of the M1 or M2 catalyst, upon H2O2 addition and 8 of UV 14 irradiation, significant degradation of 4-CP during photo-Fenton process was observed. As shown in Figure 8, Siderite (M2) exhibited a higher catalytic activity for 4-CP conversion than As shown in Figure8, Siderite (M 2) exhibited a higher catalytic activity for 4-CP conversion Hematite (M1) in the photo-Fenton reaction. Chlorophenol (i.e., 4-CP) removal efficiency of 100% than Hematite (M1) in the photo-Fenton reaction. Chlorophenol (i.e., 4-CP) removal efficiency of was100% achieved was achieved within within 60 min 60 minby siderite by siderite sample, sample, but butlonger longer time time of of120 120 min min was was required required to to completelycompletely degrade degrade 4-CP 4-CP with hematite. These These re resultssults correlated correlated well well with with the physicochemical properties of those materials due to higher iron content and specific surface area of M2 sample properties of those materials due to higher iron content and specific surface area of M2 sample (Tables(Tables 11 andand2 ).2). It It is is well well known known that that complete complete oxidation oxidation (i.e., (i.e., mineralization) mineralization) of of 4-CP 4-CP occurred occurred throughthrough severalseveral intermediateintermediate reactions, reactions, such such as hydroxybenzoquinoneas hydroxybenzoquinone (yellow), (yellow),o-benzoquinone o-benzoquinone (red), (red),and hydroquinone and hydroquinone (colorless) (colorless) and/or and/or maleic andmaleic other and carboxylic other carboxylic acids, some acids, of them some being of them even being more eventoxic more than 4-CPtoxic itselfthan [4-CP30]. Bel itself Hadjltaief [30]. Bel et Hadjltaief al. [10,11] etobserved al. [10,11] yellow observed color yellow during color the degradation during the degradationof phenol and of 2-Chlorophenol phenol and 2-Chlorophenol using raw and modifiedusing raw clay and as catalystsmodified by clay photo-Fenton as catalysts process. by photo-FentonBrownish color process. can be assignedBrownish to color the presence can be ofassigned hydroxybenzoquinone, to the presence consideredof hydroxybenzoquinone, an intermediate consideredin the 4-CP an oxidation intermediate process. in the 4-CP oxidation process.

100

40 Degradation efficiency (%) Degradation efficiency 20

0 0 20406080100120 Irradiation time (min)

FigureFigure 8. 4-CP degradation efficiency efﬁciency measured measured using using the photo-Fenton experiments experiments at at initial initial 4-CP 4-CP concentration: 20 mg/L, pH: 3, 0.05 g/L catalyst loading and 2 ml of H O concentration: 20 mg/L, pH: 3, 0.05 g/L catalyst loading and 2 ml of H2O22. 2.

FigureFigure 99 showsshows thethe mineralizationmineralization ofof 4-CP,4-CP, measuredmeasured byby TOCTOC analysisanalysis underunder thethe samesame experimentalexperimental conditions conditions as as those those of of Figure Figure 88.. TOC reductionreduction raterate reachedreached 34.56%34.56% forfor MM11 andand 50.05% 50.05% forfor M22 afterafter 120120 min min of of irradiation irradiation time. time. Generally, Generally, more more than than 3 h irradiation 3 h irradiation time is time needed is needed to achieve to achievea complete a complete mineralization mineralization of 4-CP. of However, 4-CP. Howe shorterver, time shorter of high time efﬁciencies of high efficiencies by M1 and by M M21 samplesand M2 sampleswere recorded, were recorded, conﬁrming confirming their good their catalytic good activity catalytic in activity photo-Fenton in photo-Fenton reaction. On reaction. the other On hand, the otherafter 120hand, min, after iron-leaching 120 min, wasiron-leaching evaluated towas 0.081 evaluated and 0.079 to ppm 0.081 for and M1 and0.079 M 2ppm, respectively for M1 and (Table M32)., respectively (Table 3). ChemEngineering 2018, 2, 29 9 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 9 of 14

Mineralization efficiency(%) 10

0 0 20 40 60 80 100 120 Irradiation time (min)

Figure 9. 4-CP mineralization efﬁciency measured using the photo-Fenton experiments at initial 4-CP concentration:Figure 9. 4-CP 20 mineralization mg/L, pH: 3, 0.05efficiency g/L catalyst measured loading using and the 2 mLphoto-Fenton of H2O2. experiments at initial

4-CP concentration: 20 mg/L, pH: 3, 0.05 g/L catalyst loading and 2 mL of H2O2. Table 3. Iron leaching in solution with different reaction time in the both raw materials system. Table 3. Iron leaching in solution with different reaction time in the both raw materials system. Samples Time, min 0 20 40 60 80 100 120 Iron concentration Samples Time, min 0 20 40 60 80 100 120 Iron concentration(ppm) M1 0 0.035 0.052 0.060 0.065 0.077 0.080 0.081 MM1 0 0 0.028 0.035 0.042 0.052 0.0490.060 0.0560.065 0.0580.077 0.0600.080 0.0790.081 (ppm) 2 M2 0 0.028 0.042 0.049 0.056 0.058 0.060 0.079 The effect of temperature on photo-Fenton degradation of 4-CP was also studied in the presence of bothThe catalysts. effect Theof temperature kinetic degradation on photo-Fenton of 4-CP was degrad performedation of at 4-CP different was temperaturesalso studied in of the 20− presence50 ◦C. A typicalof both first-ordercatalysts. The kinetic kinetic model degradation was fitted toof the4-CP experimental was performed results at asdiff expressederent temperatures in Equation of (2) 20 [−3150]. °C. A typical first-order kinetic model was fitted to the experimental results as expressed in Equation (2) [31]. C0 ln = Kapp × t (2) Ct C0 ln =×K t −1 (2) where kapp is the apparent rate constant of the first-orderapp reaction (min ), and C0 and Ct are the Ct concentrations (mg/L) of the 4-CP at time 0 and t, respectively. whereFigure kapp is 10 thea,b showsapparent the plotsrate constant of 4-CP degradation, of the first-order corresponding reactionto (min the− fitting1), and of C experimental0 and Ct are data the obtainedconcentrations at different (mg/L) temperatures of the 4-CP (20at time−50 ◦0C) and to t the, respectively. apparent-first-order kinetic model. Rate constant, k, canFigure be obtained 10a,b shows directly the from plots regression of 4-CP degradation, analysis of thecorresponding linear form ofto the plot.fitting The of experimental coefficient of 2 determinationdata obtained at (R di) forfferent this temperatures linear regression (20− ranged50 °C) to 0.9904–0.9958 the apparent-first-order for M1 and 0.9953–0.9689 kinetic model. for Rate M2, indicatingconstant, k, a highcan be degree obtained of fitting directly to the from first-order regression kinetic analysis model. of the linear form of the plot. The coefficientFor both of determination catalysts, the rate(R2) constantsfor this linear increased regression with initialranged temperature. 0.9904–0.9958 It for was M also1 and observed 0.9953– that0.9689 temperature for M2, indicating increase a enhancedhigh degree the of hydroxylation fitting to the first-order rate at the catalystkinetic model. active sites, resulting in an increasedFor both production catalysts, of the•OH rate radicals. constants Moreover, increased a higherwith initial reaction temperature. temperature It was can also provide observed more energythat temperature for the reactants increase to enhanced go beyond the the hydroxylation activation energy rate at barrier the catalyst [32]. Such active an sites, excess resulting energy in may an leadincreased to higher production collision of frequency •OH radicals. between Moreover, the •OH a higher radicals reaction and 4-CP temperature molecules can (or intermediate),provide more whichenergy possibly for the reactants results in to faster go beyond degradation the activati processon [energy11]. barrier [32]. Such an excess energy may lead to higher collision frequency between the •OH radicals and 4-CP molecules (or intermediate), which possibly results in faster degradation process [11]. ChemEngineering 2018, 2, 29 10 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 10 of 14

(a) Irradiation time (min)

0 102030405060 -0.05

-0.15 )

0 y = -0.0033x

/C R² = 0.9689 t -0.25

Ln(C 20°C y = -0.0048x 30°C R² = 0.9758 -0.35 50°C

y = -0.0066x -0.45 R² = 0.9851

(b) Irradiation time (min) 0 0 102030405060

-0.2

-0.4 ) 0

/C y = -0.0095x t -0.6 R² = 0.9958 20°C Ln(C 30°C y = -0.012x -0.8 R² = 0.9933 50°C

-1 y = -0.018x R² = 0.9904 -1.2

Figure 10. Effect of temperature on the photo-Fenton degradation of 4-CP using: (a) M1; and (b) M2. Figure 10. Effect of temperature on the photo-Fenton degradation of 4-CP using: (a)M1; and (b)M2.

The apparent activation energy (Ea) for the degradation of phenol by photo-Fenton oxidation The apparent activation energy (Ea) for the degradation of phenol by photo-Fenton oxidation can can be calculated from Arrhenius equation as follows: be calculated from Arrhenius equation as follows: = ⋅−() KappA expERT a / (3) Kapp = A · exp(−Ea/RT) (3)

The linear form ln (kapp) = f(1/T) gives a straight line, with a slope equal to Ea/R. From the data The linear form ln (k ) = f(1/T) gives a straight line, with a slope equal to E /R. From the data obtained for all the catalysts,app Ea was 32.89 and 36.41 kJ/mol for M1 and M2 samples,a respectively (Figureobtained 11). for This all theapparent catalysts, energyEa wasrepresents 32.89 andthe to 36.41tal activation kJ/mol for energy M1 and of photo-Fenton M2 samples, degradation respectively of(Figure 4-CP. 11). This apparent energy represents the total activation energy of photo-Fenton degradation of 4-CP.To test the feasibility of cyclic use of the studied raw materials, five photo-Fenton degradation cycles of 4-CP were performed under optimized conditions. At the end of the degradation process, the suspension was centrifuged; the catalyst was washed and dried at 120 °C for subsequent uses. ChemEngineering 2018, 2, x FOR PEER REVIEW 11 of 14

The obtained results are given in Table 4. They showed that 4-CP removal efficiency, during photo-Fenton degradation, decreased with the number of recycling runs. Compared to similar studies, iron leaching was somewhat lower [11,33] confirming the good stability of both supports.

ChemEngineeringThe concentration2018, 2, 29 of Fe in the liquid solutions upon the photo-Fenton reaction was found to 11be of at 14 all times lower than 20 ppm.

4 ) app 3 -Ln (K

2 M1 M2 1

0 0.315 0.32 0.325 0.33 0.335 0.34 1/T

FigureFigure 11. 11.The The plots plots of lnof (lnkapp (k)app versus) versus 1/ 1/T,(T,T (Tis is the the temperature). temperature).

Table 4. Degradation efficiency through five consecutive catalysts reuse cycles. To test the feasibility of cyclic use of the studied raw materials, five photo-Fenton degradation cycles of 4-CP were performed under optimized conditions.Run Number At the end of the degradation process, Samples the suspension was centrifuged; the catalyst1 was2 washed 3 and 4 dried at 5 120 ◦C for subsequent uses. The obtained results are given inM1Table 4100. They100 showed99.87 that98.66 4-CP 98.07 removal efficiency, during photo-Fenton degradation, decreasedM2 with 100 the100 number 100 of99.01 recycling 98.24 runs. Compared to similar studies, iron leaching was somewhat lower [11,33] confirming the good stability of both supports. The3.3. concentration Comparison of of Efficiency Fe in the of liquid Reaction solutions with Previous upon the Studies photo-Fenton reaction was found to be at all times lower than 20 ppm. Based on previous relevant studies, a comparative assay was performed to highlight the higher efficiency of the present natural ferruginous materials (i.e., hematite and siderite) in the degradation Table 4. Degradation efficiency through five consecutive catalysts reuse cycles. of 4-CP solute (Table 5). Considering different treatment approaches, such as adsorption, biological treatment, wet oxidation and photodegradation,Run photo-Fenton Number process showed more favorable application for the removalSamples of 4-CP than other methods reported in the literature. As can be seen, the 1 2 3 4 5 obtained maximum 4-CP degradation by raw ferrous samples was higher than those of other M 100 100 99.87 98.66 98.07 catalysts. The high efficiency1 of our system was related to the high iron on the surface and the nature M2 100 100 100 99.01 98.24 of catalytic support. For instance, both M1 and M2 natural ferrous samples showed full degradation efficiency (100%). This was the highest removal rate when compared to photo-Fenton degradation 3.3. Comparison of Efficiency of Reaction with Previous Studies using iron PILC clay (94%) [8], sunlight-Fenton in ice (80%) [7] and photodegradation [1]. Furthermore,Based on previous the degradation relevant studies,efficiency a comparative of 4-CP by assayM1 and was M performed2 natural samples to highlight was much the higher larger efficiencythan that of recorded the present by natural Khanikar ferruginous and Bhattacharyya materials (i.e., [4] hematitewho found and 69% siderite) degradation in the degradation efficiency by ofCu-kaolinite 4-CP solute (Tableand Cu-montmorillonite5). Considering different catalysts treatment via a approaches,wet oxidative such method. as adsorption, Both hematite biological and treatment,siderite samples wet oxidation have achieved and photodegradation, high degradation ra photo-Fentonte for 4-CP under process the showedexperimental more conditions favorable of applicationthe present for study. the removal Therefore, of 4-CPtheir use than as other cost-effec methodstive catalysts reported for in the the removal literature. of 4-chlorophenol As can be seen, in theaqueous obtained conditions maximum is a 4-CP highly degradation recommended by raw option ferrous that samplescan be developed was higher into than a viable those technique. of other catalysts. The high efficiency of our system was related to the high iron on the surface and the nature of catalytic support. For instance, both M1 and M2 natural ferrous samples showed full degradation efficiency (100%). This was the highest removal rate when compared to photo-Fenton degradation using iron PILC clay (94%) [8], sunlight-Fenton in ice (80%) [7] and photodegradation [1]. ChemEngineering 2018, 2, 29 12 of 14

Furthermore, the degradation efﬁciency of 4-CP by M1 and M2 natural samples was much larger than that recorded by Khanikar and Bhattacharyya [4] who found 69% degradation efﬁciency by Cu-kaolinite and Cu-montmorillonite catalysts via a wet oxidative method. Both hematite and siderite samples have achieved high degradation rate for 4-CP under the experimental conditions of the present study. Therefore, their use as cost-effective catalysts for the removal of 4-chlorophenol in aqueous conditions is a highly recommended option that can be developed into a viable technique.

Table 5. Comparison of degradation efﬁciency and times for different methods.

Reaction Time Initial Concentration Degradation Method Ref. (min) (mg/L) Efﬁciency (%) Biodegradation 120 10 86 [6] Adsorption (Activated carbon) 700 100 78.5 [5] Wet oxidation (Cu-Clays) 480 100 69 [4] Photodegradation (Cu@AgCl) 40 20 91 [1] Photo-Fenton 180 100 80 [7] Photo-Fenton (PILC) 120 100 94 [8]

Photo-Fenton (M1/M2) 120 20 100 this work

4. Conclusions

Mined natural Hematite (M1) and Siderite (M2) can be effectively used as heterogeneous photo-Fenton catalysts for the degradation of 4-Chlorophenol (4-CP) from aqueous solution. SEM, EDX and XRD analyses conﬁrmed the good structure of these materials as catalysts with high iron content dispersed on the surface. Photo-Fenton degradation process was successfully applied for the removal of 4-Chlorophenol in aqueous conditions. Our results showed that 4-Chlorophenol could be effectively removed from the aqueous media by a heterogeneous photo-Fenton process in the presence of both materials as the catalysts. Moreover, regeneration experiments proved that Siderite had a good stability and reusability, demonstrating the feasibility of using these Tunisian materials for this environmental application. Finally, these pioneering results suggest a further in-depth study for the application of those natural materials as catalysts for the removal of several organic compounds in aqueous conditions. The application of heterogeneous photo-Fenton process and/or Fenton-like process is also appropriate.

Author Contributions: Conceptualization, A.S. and H.B.H.; Methodology, M.E.G. and P.D.C.; Experiments: H.Z.; Formal Analysis, M.E.G., P.D.C. and H.Z.; Interpretation, all authors; Writing—Original Draft Preparation, A.S. and H.B.H.; Writing—Review and Editing, A.S. and H.B.H.; Visualization, X.X.; Supervision, P.D.C. and M.B.Z.; and all authors participated in the discussion of the results and the drafting of the paper. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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