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

: This paper describes a simple and low-cost process for the degradation of 4-Chlorophenol (4-CP) from aqueous solution, using natural Tunisian Hematite (M 1 ) and Siderite (M 2 ). 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), H 2 -Temperature Programmed Reduction (H 2 -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.


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][2][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][8][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 Raw Material M1 Raw Material M2

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.
X-ray diffraction (XRD) patterns were obtained in a Philips ® PW 1710 PAN (PAN Analytical, Almelo, The Netherlands) analytical Empyrean diffractometers equipped with Cu Kα (λ = 1.5406 Å) radiation source and 2θ range between 3° and 80°, with a step size of 0.02°/s. The profiles of temperature-programmed reduction (H2-TPR) were acquired in a BELCAT-M (BEL Japan) device, equipped with a thermal conductivity detector (TCD). The sample-based catalysts were first degassed to 100 °C for 2 h, and then reduced in 5% (vol.) H2/Ar, while increasing temperature from 100 to 900 °C at a increment of 7.5 °C/min.
The morphology was ascertained using a scanning electron microscope (SEM, HitachiSU-70, Hitachi, Tokyo, Japan) equipped with an Oxford X-Max 50 mm 2 X-ray spectroscopy systems through dispersive energy (EDX), which enabled qualitative evaluation of chemical composition. High resolution transmission electron microscopy (HRTEM) images were also obtained on a JEOLJEM 2011 equipped with LaB6 filament and operating at 200 kV. Images were collected with a 4008 × 2672 pixel CCD camera (Gatan Orius SC1000, Gatan, Pleasanton, CA, USA) coupled with DIGITAL MICROGRAPH software (GMS 3).

Photo-Fenton Experiments and Analytical Procedures
Heterogeneous photo-Fenton experiments were performed in an open Pyrex-glass cell (250 mL volume, 5 cm inner diameter and 11 cm height). A detailed description of catalytic reactor was reported by Bel Hadjltaief et al. [10,21]. Irradiation was carried out using 2 parallel UV lamps at 254 and 365 nm at 930/13 μW/cm 2 . All of the experiments were performed keeping the distance between the solution and the lamps constant at 15 cm and under 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 processes [22]. The initial concentration of 4-CP in the treated solution was 20 mg/L. After stabilization of the stirring (200 rpm), 0.05 g of catalyst was added to 100 mL of an aqueous 4-CP solution. Then, a fixed volume (2 mL) of a 1000 mg/L H2O2 solution was poured into the 4-CP solution. H2O2 addition was taken as the initial time of the reaction. Aliquots of supernatant (1 mL) were regularly extracted from the reaction vessel with the aid of a syringe; Residual H2O2 was 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 X-ray diffraction (XRD) patterns were obtained in a Philips ® PW 1710 PAN (PAN Analytical, Almelo, The Netherlands) analytical Empyrean diffractometers equipped with Cu K α (λ = 1.5406 Å) radiation source and 2θ range between 3 • and 80 • , with a step size of 0.02 • /s. The profiles of temperature-programmed reduction (H 2 -TPR) were acquired in a BELCAT-M (BEL Japan) device, equipped with a thermal conductivity detector (TCD). The sample-based catalysts were first degassed to 100 • C for 2 h, and then reduced in 5% (vol.) H 2 /Ar, while increasing temperature from 100 to 900 • C at a increment of 7.5 • C/min.
The morphology was ascertained using a scanning electron microscope (SEM, HitachiSU-70, Hitachi, Tokyo, Japan) equipped with an Oxford X-Max 50 mm 2 X-ray spectroscopy systems through dispersive energy (EDX), which enabled qualitative evaluation of chemical composition. High resolution transmission electron microscopy (HRTEM) images were also obtained on a JEOLJEM 2011 equipped with LaB 6 filament and operating at 200 kV. Images were collected with a 4008 × 2672 pixel CCD camera (Gatan Orius SC1000, Gatan, Pleasanton, CA, USA) coupled with DIGITAL MICROGRAPH software (GMS 3).

Photo-Fenton Experiments and Analytical Procedures
Heterogeneous photo-Fenton experiments were performed in an open Pyrex-glass cell (250 mL volume, 5 cm inner diameter and 11 cm height). A detailed description of catalytic reactor was reported by Bel Hadjltaief et al. [10,21]. Irradiation was carried out using 2 parallel UV lamps at 254 and 365 nm at 930/13 µW/cm 2 . All of the experiments were performed keeping the distance between the solution and the lamps constant at 15 cm and under 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 processes [22]. The initial concentration of 4-CP in the treated solution was 20 mg/L. After stabilization of the stirring (200 rpm), 0.05 g of catalyst was added to 100 mL of an aqueous 4-CP solution. Then, a fixed volume (2 mL) of a 1000 mg/L H 2 O 2 solution was poured into the 4-CP solution. H 2 O 2 addition was taken as the initial time of the reaction. Aliquots of supernatant (1 mL) were regularly extracted from the reaction vessel with the aid of a syringe; Residual H 2 O 2 was immediately quenched with MnO 2 (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 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: where C t denotes the time-dependent concentration/TOC and C 0 is the initial concentration/TOC. Note here that the degradation experiments were repeated three times to check the repeatability of the results obtained.

Catalysts Characterization
Chemical composition of the raw materials M 1 and M 2 was comparable, but M 1 had a distinctly higher 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 given in Figure 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/p 0 = 0.4-1.0); this indicated the presence of laminar-structured materials containing slit-shaped mesopores [11]. As shown in Table 2, the specific surface area (S BET ), pore volume and average pore diameter for M 2 are higher than that of M 1 . 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: Degradation/mineralization efficiency = ((C0 − Ct)/C0) × 100 (1) where Ct denotes the time-dependent concentration/TOC and C0 is the initial concentration/TOC. Note here that the degradation experiments were repeated three times to check the repeatability of the results obtained.

Catalysts Characterization
Chemical composition of the raw materials M1 and M2 was comparable, but M1 had a distinctly higher 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 given in Figure 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 indicated the presence of laminar-structured materials containing slit-shaped mesopores [11]. As shown in Table 2, the specific surface area (SBET), pore volume and average pore diameter for M2 are higher than that of M1.      [24][25][26]. FT-IR spectra is given in Figure 4. It showed the presence of OH stretching bands near 1790 cm −1 , as well as the broad Si-O stretching band near 1000 cm −1 [27]. Vibration bands at 736 cm -1 , in the range 860-880 cm -1 and 1390-1440 cm -1 are attributed to the CO bond of the carbonates, confirming the results of XRD and chemical analyses (Siderite: FeCO 3 , Calcite: CaCO 3 and Dolomite: MgCa (CO 3 ) 2 ). This latter result is in agreement with the literature [24,25,28,29]. TPR analysis was performed to get detailed information about the nature of the iron oxides. The reduction profile of the raw M 1 displayed two hydrogen consumption peaks in the range 200-800 • C, coinciding with the reduction profile of Fe 2 O 3 [20]. As for M 2 sample, three reduction peaks were observed ( Figure 5). An increase in temperature caused the dissociation of iron carbonates in iron oxides such as FeO and Fe 3 O 4 .
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 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 diffraction peaks near 2θ values of 24 [23]. Based on XRD data, siderite is the major mineralogical compound in the structure of natural material, along with quartz as minor compound (Figure 3). Peaks located at 24.7°, 32.2°, 38.4°, 42.4°, 46.2°, 50.8°, 52.3°, 60.9°, 64.6° and 65° indicated the presence of siderite, but those near 26.7 and 67 are indicative of quartz [24][25][26]. FT-IR spectra is given in Figure 4. It showed the presence of OH stretching bands near 1790 cm −1 , as well as the broad Si-O stretching band near 1000 cm −1 [27]. Vibration bands at 736 cm -1 , in the range 860-880 cm -1 and 1390-1440 cm -1 are attributed to the CO bond of the carbonates, confirming the results of XRD and chemical analyses (Siderite: FeCO3, Calcite: CaCO3 and Dolomite: MgCa (CO3)2). This latter result is in agreement with the literature [24,25,28,29]. TPR analysis was performed to get detailed information about the nature of the iron oxides. The reduction profile of the raw M1 displayed two hydrogen consumption peaks in the range 200-800 °C, coinciding with the reduction profile of Fe2O3 [20]. As for M2 sample, three reduction peaks were observed ( Figure 5). An increase in temperature caused the dissociation of iron carbonates in iron oxides such as FeO and Fe3O4.     [23]. Based on XRD data, siderite is the major mineralogical compound in the structure of natural material, along with quartz as minor compound (Figure 3). Peaks located at 24.7°, 32.2°, 38.4°, 42.4°, 46.2°, 50.8°, 52.3°, 60.9°, 64.6° and 65° indicated the presence of siderite, but those near 26.7 and 67 are indicative of quartz [24][25][26]. FT-IR spectra is given in Figure 4. It showed the presence of OH stretching bands near 1790 cm −1 , as well as the broad Si-O stretching band near 1000 cm −1 [27]. Vibration bands at 736 cm -1 , in the range 860-880 cm -1 and 1390-1440 cm -1 are attributed to the CO bond of the carbonates, confirming the results of XRD and chemical analyses (Siderite: FeCO3, Calcite: CaCO3 and Dolomite: MgCa (CO3)2). This latter result is in agreement with the literature [24,25,28,29]. TPR analysis was performed to get detailed information about the nature of the iron oxides. The reduction profile of the raw M1 displayed two hydrogen consumption peaks in the range 200-800 °C, coinciding with the reduction profile of Fe2O3 [20]. As for M2 sample, three reduction peaks were observed ( Figure 5). An increase in temperature caused the dissociation of iron carbonates in iron oxides such as FeO and Fe3O4.    SEM images are shown in Figure 6. They indicated spherical to sub-round particles with some agglomerated platelets for the raw M1 sample. In contrast, M2 sample is predominantly particles with some rhombohedral shapes. EDX analysis confirmed the presence of high amount of iron on the surface of both materials, with small amounts of impurities, e.g., Si, Al, Fe, Ca, Mg, and Mn, confirming the results of X-ray fluorescence. TEM images further confirmed these changes in the morphology of the M1 and M2 samples (Figure 7). Interlayer distances were measured from the Fast Fourier Transform (FFT) of the HRTEM image. For M1, the presence of (012) planes with an inter-reticular distance of 3.65 Å characteristic of Hematite was clearly distinguished, whereas, for M2, HRTEM images showed lattice fringes from which the (110) plane of siderite and (311) of Magnetite with the respective d-spacing of 2.3 and 2.5 Å.

Raw M1
Raw M1 Raw M2 Raw M2 Figure 6. SEM images and EDX spectra for the raw materials M1 and M2. SEM images are shown in Figure 6. They indicated spherical to sub-round particles with some agglomerated platelets for the raw M 1 sample. In contrast, M 2 sample is predominantly particles with some rhombohedral shapes. EDX analysis confirmed the presence of high amount of iron on the surface of both materials, with small amounts of impurities, e.g., Si, Al, Fe, Ca, Mg, and Mn, confirming the results of X-ray fluorescence. TEM images further confirmed these changes in the morphology of the M 1 and M 2 samples (Figure 7). Interlayer distances were measured from the Fast Fourier Transform (FFT) of the HRTEM image. For M 1 , the presence of (012) planes with an inter-reticular distance of 3.65 Å characteristic of Hematite was clearly distinguished, whereas, for M 2 , HRTEM images showed lattice fringes from which the (110)  SEM images are shown in Figure 6. They indicated spherical to sub-round particles with some agglomerated platelets for the raw M1 sample. In contrast, M2 sample is predominantly particles with some rhombohedral shapes. EDX analysis confirmed the presence of high amount of iron on the surface of both materials, with small amounts of impurities, e.g., Si, Al, Fe, Ca, Mg, and Mn, confirming the results of X-ray fluorescence. TEM images further confirmed these changes in the morphology of the M1 and M2 samples (Figure 7). Interlayer distances were measured from the Fast Fourier Transform (FFT) of the HRTEM image. For M1, the presence of (012) planes with an inter-reticular distance of 3.65 Å characteristic of Hematite was clearly distinguished, whereas, for M2, HRTEM images showed lattice fringes from which the (110) plane of siderite and (311) of Magnetite with the respective d-spacing of 2.3 and 2.5 Å.

Raw M1
Raw M1 Raw M2 Raw M2 Figure 6. SEM images and EDX spectra for the raw materials M1 and M2.

Catalytic Performance in the Photo-Fenton Oxidation of 4-Chlorophenol
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 catalysts alone. In the dark condition, a mixture of H2O2 with catalysts showed 4-CP conversion of 52.1% and 40.3% for M1 and M2, respectively. This may indicate that Fenton reaction took place via the enhanced radical formation, which, in turn, was involved in the oxidation of organic 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.

Catalytic Performance in the Photo-Fenton Oxidation of 4-Chlorophenol
Preliminary experiments were carried out to assess the importance of H 2 O 2 , catalysts and irradiation for 4-CP degradation. Insignificant degradation was observed under H 2 O 2 , 4-CP and both catalysts alone. In the dark condition, a mixture of H 2 O 2 with catalysts showed 4-CP conversion of 52.1% and 40.3% for M 1 and M 2 , respectively. This may indicate that Fenton reaction took place via the enhanced radical formation, which, in turn, was involved in the oxidation of organic compounds.
When reaction was performed in the presence of the M 1 or M 2 catalyst, upon H 2 O 2 addition and UV irradiation, significant degradation of 4-CP during photo-Fenton process was observed.
As shown in Figure 8, Siderite (M 2 ) exhibited a higher catalytic activity for 4-CP conversion than Hematite (M 1 ) in the photo-Fenton reaction. Chlorophenol (i.e., 4-CP) removal efficiency of 100% was achieved within 60 min by siderite sample, but longer time of 120 min was required to completely degrade 4-CP with hematite. These results correlated well with the physicochemical properties of those materials due to higher iron content and specific surface area of M 2 sample (Tables 1 and 2). It is well known that complete oxidation (i.e., mineralization) of 4-CP occurred through several intermediate reactions, such as hydroxybenzoquinone (yellow), o-benzoquinone (red), and hydroquinone (colorless) and/or maleic and other carboxylic acids, some of them being even more toxic than 4-CP itself [30]. Bel Hadjltaief et al. [10,11] observed yellow color during the degradation of phenol and 2-Chlorophenol using raw and modified clay as catalysts by photo-Fenton process. Brownish color can be assigned to the presence of hydroxybenzoquinone, considered an intermediate in the 4-CP oxidation process.
ChemEngineering 2018, 2, x FOR PEER REVIEW 8 of 14 As shown in Figure 8, Siderite (M2) exhibited a higher catalytic activity for 4-CP conversion than Hematite (M1) in the photo-Fenton reaction. Chlorophenol (i.e., 4-CP) removal efficiency of 100% was achieved within 60 min by siderite sample, but longer time of 120 min was required to completely degrade 4-CP with hematite. These results correlated well with the physicochemical properties of those materials due to higher iron content and specific surface area of M2 sample (Tables 1 and 2). It is well known that complete oxidation (i.e., mineralization) of 4-CP occurred through several intermediate reactions, such as hydroxybenzoquinone (yellow), o-benzoquinone (red), and hydroquinone (colorless) and/or maleic and other carboxylic acids, some of them being even more toxic than 4-CP itself [30]. Bel Hadjltaief et al. [10,11] observed yellow color during the degradation of phenol and 2-Chlorophenol using raw and modified clay as catalysts by photo-Fenton process. Brownish color can be assigned to the presence of hydroxybenzoquinone, considered an intermediate in the 4-CP oxidation process.  Figure 9 shows the mineralization of 4-CP, measured by TOC analysis under the same experimental conditions as those of Figure 8. TOC reduction rate reached 34.56% for M1 and 50.05% for M2 after 120 min of irradiation time. Generally, more than 3 h irradiation time is needed to achieve a complete mineralization of 4-CP. However, shorter time of high efficiencies by M1 and M2 samples were recorded, confirming their good catalytic activity in photo-Fenton reaction. On the other hand, after 120 min, iron-leaching was evaluated to 0.081 and 0.079 ppm for M1 and M2, respectively (Table 3).  Figure 9 shows the mineralization of 4-CP, measured by TOC analysis under the same experimental conditions as those of Figure 8. TOC reduction rate reached 34.56% for M 1 and 50.05% for M 2 after 120 min of irradiation time. Generally, more than 3 h irradiation time is needed to achieve a complete mineralization of 4-CP. However, shorter time of high efficiencies by M 1 and M 2 samples were recorded, confirming their good catalytic activity in photo-Fenton reaction. On the other hand, after 120 min, iron-leaching was evaluated to 0.081 and 0.079 ppm for M 1 and M 2 , respectively (Table 3).  The effect of temperature on photo-Fenton degradation of 4-CP was also studied in the presence of both catalysts. The kinetic degradation of 4-CP was performed at different temperatures of 20−50 °C. A typical first-order kinetic model was fitted to the experimental results as expressed in Equation (2) [31].
where kapp is the apparent rate constant of the first-order reaction (min −1 ), and C0 and Ct are the concentrations (mg/L) of the 4-CP at time 0 and t, respectively. Figure 10a,b shows the plots of 4-CP degradation, corresponding to the fitting of experimental data obtained at different temperatures (20−50 °C) to the apparent-first-order kinetic model. Rate constant, k, can be obtained directly from regression analysis of the linear form of the plot. The coefficient of determination (R 2 ) for this linear regression ranged 0.9904-0.9958 for M1 and 0.9953-0.9689 for M2, indicating a high degree of fitting to the first-order kinetic model.
For both catalysts, the rate constants increased with initial temperature. It was also observed that temperature increase enhanced the hydroxylation rate at the catalyst active sites, resulting in an increased production of •OH radicals. Moreover, a higher reaction temperature can provide more energy for the reactants to go beyond the activation energy 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].  The effect of temperature on photo-Fenton degradation of 4-CP was also studied in the presence of both catalysts. The kinetic degradation of 4-CP was performed at different temperatures of 20−50 • C. A typical first-order kinetic model was fitted to the experimental results as expressed in Equation (2) [31].
where k app is the apparent rate constant of the first-order reaction (min −1 ), and C 0 and C t are the concentrations (mg/L) of the 4-CP at time 0 and t, respectively. Figure 10a,b shows the plots of 4-CP degradation, corresponding to the fitting of experimental data obtained at different temperatures (20−50 • C) to the apparent-first-order kinetic model. Rate constant, k, can be obtained directly from regression analysis of the linear form of the plot. The coefficient of determination (R 2 ) for this linear regression ranged 0.9904-0.9958 for M 1 and 0.9953-0.9689 for M 2 , indicating a high degree of fitting to the first-order kinetic model.
For both catalysts, the rate constants increased with initial temperature. It was also observed that temperature increase enhanced the hydroxylation rate at the catalyst active sites, resulting in an increased production of •OH radicals. Moreover, a higher reaction temperature can provide more energy for the reactants to go beyond the activation energy 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]. The apparent activation energy (Ea) for the degradation of phenol by photo-Fenton oxidation can be calculated from Arrhenius equation as follows: The linear form ln (kapp) = f(1/T) gives a straight line, with a slope equal to Ea/R. From the data obtained for all the catalysts, Ea was 32.89 and 36.41 kJ/mol for M1 and M2 samples, respectively ( Figure 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. The apparent activation energy (E a ) for the degradation of phenol by photo-Fenton oxidation can be calculated from Arrhenius equation as follows: The linear form ln (k app ) = f(1/T) gives a straight line, with a slope equal to E a /R. From the data obtained for all the catalysts, E a was 32.89 and 36.41 kJ/mol for M 1 and M 2 samples, respectively ( Figure 11). This apparent energy represents the total activation energy of photo-Fenton degradation of 4-CP.
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. The concentration of Fe in the liquid solutions upon the photo-Fenton reaction was found to be at all times lower than 20 ppm.

Comparison of Efficiency of Reaction with Previous Studies
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 of 4-CP solute (Table 5). Considering different treatment approaches, such as adsorption, biological treatment, wet oxidation and photodegradation, photo-Fenton process showed more favorable application for the removal of 4-CP than other methods reported in the literature. As can be seen, the obtained maximum 4-CP degradation by raw ferrous samples was higher than those 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]. 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. 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. 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. The concentration of Fe in the liquid solutions upon the photo-Fenton reaction was found to be at all times lower than 20 ppm.

Comparison of Efficiency of Reaction with Previous Studies
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 of 4-CP solute (Table 5). Considering different treatment approaches, such as adsorption, biological treatment, wet oxidation and photodegradation, photo-Fenton process showed more favorable application for the removal of 4-CP than other methods reported in the literature. As can be seen, the obtained maximum 4-CP degradation by raw ferrous samples was higher than those 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 M 1 and M 2 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]. Furthermore, the degradation efficiency of 4-CP by M 1 and M 2 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.

Conclusions
Mined natural Hematite (M 1 ) and Siderite (M 2 ) 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.