Preparation and Characterization of Magnetic Banana Peels Biochar for Fenton Degradation of Methylene Blue

Co-precipitation method was used for the synthesis of biochar/Fe 3 O 4 to hete-rogeneously degrade methylene blue (MB) in an aqueous medium. This catalyst was characterized by different techniques such as Fourier Transform Infrared (FTIR) Spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and Raman Microscopy. The analysis highlighted the presence of iron oxides on the surface of the biochar in the form of magnetite (Fe 3 O 4 ). Catalytic tests performed on this composite showed significant degradation and simple magnetic separation in the solution for reuse. Maximum degradation was carried out after stirring it for 90 minutes in an MB aqueous solution at different concentrations. The per-centages of degradation were 99% and 98.6% 93.3% and 91% for concentrations of MB 40 mg/L and 60 mg/L, 80 mg/L and 120 mg/L respectively. The reactions followed a second-order kinetics with correlation coefficients r 2 = 0.9598, 0.9247, 0.9548 and 0.9614 for the same concentrations of MB at pH = 2, 0.2 mL/L H 2 O 2 and 15 mg of biochar/Fe 3 O 4 . This work provides a simple and an effective method for the preparation of biochar/Fe 3 O 4 and its use for the oxidation of MB by means of heterogeneous Fenton.


Introduction
Water pollution by dyes has become a concern for the world's population for decades. These dyes are listed in more than 100,000 types with an annual pollution Materials Sciences and Applications of 7 × 10 5 tons. The textile industries are considered as major consumers with around 36,000 tons per year. The World Bank estimates that around 20% of dye pollution comes from dyeing and textile processing [1]. According to studies 50 L to 100 L of water are consumed to tint 1 kg of cotton [2]. Insufficient treatment of textile effluents can lead to their accumulation in the water cycle, which can affect people living either through direct consumption of polluted water or through the food chain.
Physical techniques including adsorption are used for the treatment of industrial wastewater, but they are limited to a simple transfer of the pollutant from the effluent to the adsorbent, without any degradation occurs. The Advanced Oxidation process (AOP) is the burgeoning method of the chemical treatment of organic contaminants, considered to be bio-recalcitrant and/or for the disinfection of emerging pathogens [3]. It is based on the formation of highly reactive oxidative species (free radicals ˚OH) that can be induced by catalytic, sonochemical, biological, electrochemical and/or photochemical activations [4]. The high reactivity of the hydroxide radical with an oxidation potential of +2.80 V (ESH), has the power to oxidize many organic and inorganic molecules leading to their mineralization [5] [6]. The mechanism of generation of ˚OH by the Fenton reactions has been taken up by Xuang and Kim (2018) Unlike the homogeneous Fenton method using iron ion and hydrogen peroxide, the heterogeneous Fe 3 O 4 method is increasingly used because it is easy to recover in a solvent using a magnetic field and can be regenerated for multiple uses [7] [8] [9]. To increase the catalytic activity of Fe 3 O 4 , it is more and more immobilized in porous supports. Biochar [9] [10]; clay [11]; activated carbon [12] [13] [14], carbon microspheres [5], graphenes [15] [16], multi-walled carbon nanotube [17] are the most used because of their small sizes, the hydrophilic groups on their surfaces, their thermal stability as well as their ease in being dispersed in water. Biochar is used as an excellent platform to support various catalytic nanoparticles due to its unique surface properties, easily adjustable functional groups, chemical stability and electrical conductivity [18]. It is considered a reservoir of electrons, and the quinone groups on the surface facilitate electronic exchanges during catalysis [19] [20]. Banana peel is used for the preparation of biochar by its abundance. In 2016, Cameroon was considered the first banana producer in Africa. Therefore, it has been proved banana peels equivalent to 40% of the total weight of fresh ba-nana, are generated as a wasted product in industries producing banana products [21]. These peels are not being used for any other purposes and or mostly dumped as solid waste at large expense; hence the need to transform it into biochar and use it as a catalytic support. We now count several methods of immobilization of Fe 3 O 4 of magnetism on a biochar support, namely the co-precipitation technique, the hydrothermal method, ball mill method; the sol gel method [22] [23] [24]. The co-precipitation method is the one most used because it is easy to implement that and it takes place at low temperature. The coprecipitation method has the advantage of directly obtaining homogeneous nanomaterials with small size and size distribution through various chemical reactions in the solution. The main advantage is that a large quantity of nanoparticles material can be produced. The coprecipitation technique is probably the simplest and most convenient chemical pathway to synthesize magnetic nanoparticles [25].
In this work, the biochar from the dry banana peels was prepared by simple pyrolysis under nitrogen atmosphere and the co-precipitation method was used for the immobilization of magnetite used as precursors FeCl 2 ·4H 2 O and FeCl 3 ·6H 2 O. The catalyst prepared was characterized by Fourier transform infrared (FTIR), Raman spectroscopy, X-ray diffraction (XRD), Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX). The ability of the particles to facilitate the Fenton oxidation of methylene blue (MB) has been studied under conditions of pH, pollutant concentration, agitation time, catalyst mass and very precise H 2 O 2 concentration.

Preparation of Biochar/Fe3O4
Raw materials (ripe banana peels), collected in municipal garbage cans were washed, dried in the sun for 8 hours and then at 80˚C in an oven for 24 hours. Later on, the samples were scrambled into small particles followed by the introduction of 15 g of obtained samples into the carbolite brand turbolace and their carbonization at 500˚C under nitrogen N 2 atmosphere (0.15 mL/min) with a temperature change of 10˚C/min and a residence time of 2 h. The oven was allowed to cool down to room temperature. The biochar was then recovered, dried in an oven during 24 hours and then stored.
The biochar/Fe 3 O 4 was prepared by co-precipitation method as described by  (10 -12). The suspension was stirred for about 1 hour until the color changed from the brown to black color. The whole left at room temperature was filtered and the precipitate was washed several times with distilled water and ethanol to neutral pH. The obtained sample was dried at 80˚C and stored for physico-chemical characterizations. Pure magnetite was prepared by the same procedure in the absence of biochar.

Characterization of the Catalyst
The pH of zero charge was determined as follows: 50 mL of an aqueous solution of NaCl (0.01M) were introduced into six pH bottles; the pH was adjusted to 2, 4, 6, 8 and 10. These different bottles were bubbled with nitrogen to stabilize the pH. 0.15 g of Biochar/Fe 3 O 4 was introduced into these different flasks. The mixtures, stirred during 48 hours were filtered and the final pH of the filtrate was measured using a pH meter (HI 2209 pH meter). The encounter with the first bisector of the pH curve (final) = f (initial pH) indicates the pH of charge zero charge [26]. Similarly, the pH of the material was carried out using 0.15 g Biochar/Fe 3 O 4 in 50 mL of distilled water, stirred for 48 h and measuring the pH. The functional groups present in Biochar/Fe 3 O 4 were ascertained by Fourier transform Infrared spectroscopy (FTIR, Vertex 70 de BRUKER) over the region 400 -4000 cm −1 in pellet form the powder samples of 1 mg mixed with spectroscopic grade KBr (Merck) of 9 mg with a resolution of 4 cm −1 (32 scans). Spectra X-ray diffraction on XRD powder (RigakuGeigerflex, Cu Kα, λ = 1.5406A) produced at 30 kV and 25 mA scanned the diffraction angles (2θ) between 10˚ and 80˚ with the step size of 0.002˚ 2θ per second. Elemental EDX analysis performed using EDAX TEAM, 125.9 ev of resolution, to know the composition of the elements present in the material as well as SEM (VEGA3 TESCAN) to know the surface morphology. Raman spectroscopy to determine the structural and electronic properties of materials performed with a Nano brand SP (Confotec MR-SOL instrument) with the 570 nm wavelength laser. All these analyzes were carried out at the "Centre d'Analyseet de Caractérisation" Semlalia-Marrakech Faculty of Sciences of Cadi Ayyad University (Morocco).

Experimental Procedure
To evaluate the catalytic activity of the material, an aqueous solution of methy-  The kinetics of the zero order, is given by Equation (6) The first order is given by Equation (7) [ with [MB] i and [MB] t the concentrations t = 0 and t = t respectively, k is the speed constant (min −1 ) and t the time (min).
The second order is given by Equation (8) [

Characterization
Fourier transform infrared spectrum (FTIR) is done to determine the structural characterization of the dry banana peels, biochar and biochar/Fe 3 O 4 . Figure 2 shows the FTIR spectra of the banana peels, biochar and biochar/Fe 3 O 4 in wave number range of 4000 -400 cm −1 . Some characteristic bands from dry banana peels disappeared for the benefit of others bands. The wide band around 3500 -3250 cm −1 attributable to the −OH stretching vibrations. This band reappears intensely when magnetite is introduced; due to the fact that the impregnation reaction is carried out in an aqueous medium (co-precipitation). The bands around 1583.4 -1635 cm −1 attributed to the elongation vibrations of C=O, C=C functions remained unchanged and having almost the same intensities on the dried banana peel as on the calcined and magnetized peel. Just a decrease in the intensity of the peaks (on banana peel calcined at 500˚C) due to pyrolysis. The presence of O-H deformation bond is observed at 1300 cm −1 . We observe the C-O stretching vibration band at 1053 cm −1 with a higher intensity on the black curve. Two bands, one very intense (650 -567 cm −1 ) and the other less intense (444 cm −1 ) are observed and respectively corresponded to iron oxides (Fe-O) and oxides iron and silica (Fe-O-Si) [28] [29] revealed that the band between 450 -740 cm −1 belonged to the Fe-O vibrations of the nanoparticles of iron oxides. We have in this case the chemical shifts towards the high wavelengths (hypochromic effect) of the probably biochar which thanks to its surface rich in electrons, has the capacity to reduce the gap energy of the semiconductors thus causing an increase its chemical shift [30].
Raman spectroscopy is a non-destructive method used to characterize the structural and electronic properties of materials. Figure 3 is the Raman analysis curves for our biochar and biochar/Fe 3 O 4 samples. The Raman spectra of the biochar from the banana peels (Figure 3(a)) have several bands, three of which are larger and have corresponding chemical shifts. The band at 1567 cm −1 corresponding to the band G (G = graphite) of the E 2 g mode of hexagonal graphite. It is related to the vibration involving sp 2 of hybrid carbon atoms that includes graphene sheet [31] [32]. This position of the peak G indicates the degree of charge transfer. Due to the stiffness of G-peak related links, the phonon mode energy increases [33]. On the other hand, when we mix the biochar with the iron oxides, the Raman spectrum (Figure 3(b)) indicates the very weak G band (I D /I G = 2.4). This low intensity as well as the I D /I G ratio shows that iron oxides create a lot of disorder in the biochar structure.
Band D (D = disorder) at about 1372.2 cm −1 (Figure 3(a)) is known as the disorder or defect band and represents a carbon ring breathing pattern sp2, although to be active the ring must be adjacent to a graphene edge or defect. Its intensity is much greater in our material (Figure 3(a)) and the ratio I D /I G = 0.98 thus confirms the defect in the carbon structure. The presence of defects improves the performance of carbon materials because of the strong anisotropy, mechanical strength, or electrical conductivity between the plane and out-of-plane direction [34]. On the spectrum in Figure 3   where: θ is the diffraction angle and the area under the peak is proportional to the diffracted intensity; d: distance between the crystalline plane; n: diffraction order.
The position of the peaks in the diagram corresponds to the angle 2θ.
The X-ray diffraction spectra of our two materials are shown in Figure 5.

Catalytic Activities
The degradation of MB was studied by various processes namely homogeneous

pH Effects
Studies on the pH of the medium showed a slight degradation of MB at pH 4 ( Figure 7). This is due to the precipitation of Biochar/Fe 3 O 4 in the MB solution.
At pH = 2 the degradation is maximal for 15 mg of the catalyst, 0.2 mL/L H 2 O 2 , 90 min. The increase in the oxidizing power of the MB at low pH (generally between 2 and 4) is attributed to the increase of the oxidizing potential of hydroxide radicals (HO˚) and to a strong dissolution of iron in solution in MB [40].

Effects of the Masses
The That is a degradation ranging from 70% to 99% for masses 5 mg to 15 mg of the catalyst. This is explained by the increase of active sites on the surface of the catalyst which is accompanied by the generation of a large amount of iron particles with production of OH˚ radicals.

Concentration Effect of the Pollutant
The degradation of MB was also studied according to its concentration. We studied here the concentrations 40 mg/L, 60 mg/L and 80 mg/L and 120 mg/L. The results of Figure 9, shows a decrease in the degraded amount of MB when increasing the concentration of the pollutant. The maximum elimination is observed at 90 min with a mass of 15 mg of the catalyst leading to a degradation of 99%; 98.6%; 93.3% and 91.4% for MB concentrations 40 mg/L, 60 m/L, 80 mg/L and 120 mg/L. This slight decrease in degradation as a function of concentration is probably due to an increase in the number of MB molecules in the solution for the same amount of hydroxyl radicals formed (responsible for the Fenton reaction). Nevertheless, more than 60% of elimination is observed for the highest concentration of 120 mg/L after 90 min for a small mass of 5 mg of biochar/Fe 3 O 4 . What is encouraging because the concentrations found in textile wastewater is between 10 mg/L and 250 mg/L [14]. Regarding Figure 9, the highest degradation percentage was obtained with the highest mass.

Effect of Stirring Time
The influence of contact time has been studied for different catalyst masses (5 mg, 10 mg and 15 mg) of 15 to 90 min ( Figure 10). The results of Figure show an increase in degradation as a function of mass and stirring time and a decrease when increasing the concentration of MB. Indeed the reaction is slow during the first 15 minutes with degradation less than 20%, and becomes fast as from 30 min. It reaches the maximum at 90 min with a degradation percentage greater than 75% depending on the masses and the concentration of the dye. This is due to the permanent production of the electrons by the biochar as well as the radicals HO˚ by the hydrogen peroxide (H 2 O 2 ) in the medium as a function of the time, which increases the rate of the reaction of catalysis. The longer the stirring time, the more electrons and radicals HO˚ are produced and the greater the degradation.

Stability and Reusability of the Catalyst
The stability and reusability of the material is an important factor in catalysis.

Conclusion
Biochar/Fe 3 O 4 was prepared by a simple method (Co-precipitation) using a biochar based on banana peel and iron chlorides as precursors. The analysis made on these materials showed a better dispersion of the magnetite particles on the surface of the biochar with a yield of 3.54% iron. Biochar/Fe 3 O 4 was found to be very useful for the degradation of methylene blue in aqueous media by heterogeneous Fenton. The use of H 2 O 2 (0.2 ml/L) as an oxidizing agent greatly favored the process, from a degradation of less than 5% (without H 2 O 2 ) to more than 90% (in the presence of H 2 O 2 ) during 90 min of stirring, 15 mg of the catalyst and at pH 2. The recovery of the catalyst by magnetization allowed the reusability without prior treatment in the Fenton process.