Cite this article as:

Vinuesa, A.J., Fermanelli, C.S., Saux, C. (2023) “Activated biochars functionalization for photo-fenton degradation of organic dyes”, Latin American Applied Research, 53(1) pp 37-42.

Abstract−−Activated biochars were obtained from peanut shells, a waste from the local agricultural industry, and used as supports for iron species. These materials were evaluated as catalysts in photo-Fenton reactions for methyl orange degradation in aqueous solutions. Supports and composites were physiochemically characterized by various techniques for better knowledge, finding a good interaction between the active phase and the support. The materials were subsequently evaluated in a batch system equipped with a Hg vapour lamp. The catalysts showed good performance in the degradation of an organic dye. Reuse was evaluated for the catalyst with the highest activity (CA-FeWI), showing a slight loss in photodegradation capacity.

Keywords−− residual biomass, pyrolysis, heterogeneous catalyst, photo-degradation, methyl orange.

I. INTRODUCTION

Of the multiple contaminants that can be found in watercourses, dyes are of anthropogenic origin and their presence is associated with effluents from the textile, paper, leather and plastic industries, among others. This type of impurity reduces the penetration of light in aquatic ecosystems and can affect the quality of water for human consumption (Fradj et al., 2020). Particularly, methyl orange (MO) (4-(((4-Dimethylamino)phenyl)azo)benzenesulfonic acid) an azo dye, is worldwide used in pharmaceutical preparations, fabric and plastic dyeing, etc. (Velusamy et al., 2022). It has been categorized as an acutely toxic compound by the GHS (Globally Harmonized System) classification. Azo-type dyes have a global production of approximately 70,000 tons/year (Mousavi et al., 2017).The removal of this family of dyes presents great difficulty using conventional methods, such as flocculation, sedimentation or adsorption, due to their complex structure and high stability (Azam and Hamid, 2006). Furthermore, these methods transfer the contaminants from one phase to another, without eliminating their potential risk, so more efficient alternatives are needed (Ghoreishian et al., 2014).

Therefore, advanced oxidation processes (AOPs) are presented as safer and more efficient treatment alternatives (Maroudas et al. 2021). Among them, heterogeneous photocatalysis emerges as a promising alternative in terms of its ability to completely degrade aqueous contaminants (Mancuso et al. 2020).

According to literature (Zhao et al. 2013), α-Fe₂O₃ nanocrystals could be designed in order to enhance their activity as photocatalysts in the Fenton reaction. Moreover, the immobilization of the catalyst on carbonaceous supports can provide an important enhancement in the photoactivity of the catalyst (Silvestri et al., 2019).

Activated biochars are very attractive supports given their stability, low cost, absence of toxicity and easy recovery after the reaction. The biochars can be obtained from a variety of forest, urban, agricultural and agro-industrial waste (Silvestri et al., 2019). In this work, activated biochars were obtained from peanut shells, which are a significant source of agro-industrial waste in the central region of Argentina.

Previous studies report on photocatalysts based on composites such as ZnO/biochar (Gonçalves et al., 2020) and MnFe₂O₄/chitosan biochar (Wang et al., 2021) for the degradation of methyl orange, CuFe₂O₄/biochar composite for the degradation of rhodamine B or Fe₂TiO₅/biochar to degrade methylene blue (Herath et al., 2022). However, studies of the α-Fe₂O₃/activated biochar composite are still incipient in the current literature.

Therefore, this work proposes a novel photocatalyst based on α-Fe₂O₃/activated biochar composite to degrade methyl orange from wastewater. In this way, materials from renewable and residual sources would be used for the treatment of aqueous contaminants.

II. METHODS

A. Synthesis and modification of activated biochars.

The biochars were obtained from peanut shells (residue from the peanut industry) provided by the company Lorenzati, Ruetsch y Cia. from the province of Córdoba, Argentina. The shells were subjected to a thermal pyrolysis process in order to obtain the biochar. This process was carried out in a fixed bed tubular glass reactor (23 mm I.D., 290 mm length) which was placed in an electric furnace at 500 °C for 10 minutes. To ensure the inertia of the reaction atmosphere, N₂ was used as carrier gas. Its flow rate (60 ml/min) was regulated by a mass flow controller El-Flow Base, Bronkhorst.

The activation of the biochars was carried out following the protocol published by Fu et al. (2019), due to its simplicity and effectiveness. Firstly, potassium hydroxide (Strem Chemicals, 85%) and biochar were mechanically mixed with a 3:1 mass ratio, respectively. The mixture was placed in the previously mentioned reactor, but varying the operating conditions. A N₂ flow of 20 ml/min was selected. The heating rate was 20 °C/min from room temperature to 550 °C and further kept for 3 hours. The obtained activated biochars were then washed with distilled water until pH = 7 and dried at 120 °C until constant weight. This material was used as support and called CA.

An acid surface functionalization process was performed in order to generate anchor sites in CA for better incorporation of hematite nanoparticles. A suspension of CA and nitric acid (Biopack 65% p.a. A.C.S.) with a mass ratio of 1:25 was prepared and placed under magnetic stirring at 80 °C for 2 hours. The acid was then removed by centrifugation and the material was washed with Milli Q water until neutral pH. Finally, it was dried in an oven at 110 °C overnight. This material was henceforth called CAN.

Fe was incorporated by two methods: hydrothermal synthesis and wet impregnation. The former consisted of synthesizing α-Fe₂O₃ nanoparticles (H), following the protocol reported by Zhao et al. (2013). This procedure allows the obtention of truncated dodecahedrons of α-Fe₂O₃. It consists in the hydrothermal synthesis of a gel with a final composition of 5.2 mM FeCl₃.7H₂O (Anhedra, 97%); 0.07 M NaOH (Cicarell, 97%i) and 0.63 M Acetic Acid (Biopack, 99.7%) to which 0.6 g of CAN were added. The gel was heated at 180°C for 8 hours in a Teflon-lined autoclave. The material obtained was washed with distilled water until neutral pH and separated by centrifugation. The precipitate was then dried at 110 °C and named CAN-FeH.

Incorporation of Fe by wet impregnation was performed using the same FeCl₃, as iron precursor, and CA as support. Theoretical iron content was 6% w/w. The FeCl₃.7H₂O was dissolved in 20 ml of Milli Q water and then CA was added and kept under stirring for 20 min. The material was placed in a vacuum rotary evaporator at 80 °C until the solvent was eliminated and then dried at 110 °C overnight. Finally, a thermal treatment was carried out in a muffle at 300 °C for 2 h with a heating rate of 20 °C/min. The obtained material was called CA-FeWI.

B. Catalysts characterization

Infrared Fourier Transform Spectroscopy (FT-IR) studies were carried out to determine the functional groups present on the catalysts surface, employing a Thermo Nicolet iS10. Samples were prepared by blending a few milligrams of the catalyst with KBr.

Specific surface area (SSA) of the synthesized materials was determined by the standard BET method using N₂absorption at 77 K in a Pulse Chemisorb Micromeritics 2700 Analyzer.

Figure 1: XRD diffraction patterns of the biochar supports (CA, CAN) and the catalysts (CA-FeWI, CAN-FeH)

Fe content (on dry and ash-free basis) of CAN-FeH and CA-FeWI samples was determined by Inductively Coupled Plasma (ICP) Atomic Emission Spectroscopy with an analyzer ICP-OPTIMA 2100 DV Perkin Elmer.

The morphological analysis of the catalysts was performed by Scanning Electron Microscopy (SEM) employing a microscope FE-SEM ∑igma. It was operated at an acceleration voltage of 5 kV.

C. Photodegradation Reactions

The reaction system consisted of a magnetically stirred Pyrex glass annular photoreactor with a 125 W high pressure mercury lamp. For this study, a 2.6 ppm solution of methyl orange (Fluka) was used. The initial reaction time was the moment in which the catalyst (200 mg) was incorporated, together with the lamp and 50 µl of H₂O₂ (Ciccarelli, 100 vol, 30%). The final volume of the solution was 400 ml. Experiments were performed along 4 h.

Samples of the solution were periodically taken, filtered to separate the catalyst, and analyzed in a Persee T7DS UV-Vis Spectrophotometer. Scanning was done between 190 and 600 nm. In addition, changes in pH and temperature were monitored.

III. RESULTS

A. Materials characterization

Figure 1 shows the diffraction patterns of the supports and the synthesized catalysts. CA and CAN show two reflections centered at around 23° and 43° and they can be assigned to the (002) and (100) planes of graphite structure (JCPDS 00-056-0159).

The intensity of the reflection at 23° in CAN is notably higher and is assigned to the stacking thickness of graphitic layers (Keppetipola et al., 2021). This observation could indicate that the acid treatment increased the proportion of graphitic structure in the material.

Both catalysts showed characteristic reflections of hexagonal hematite (JCPDS 01-085-0599). This phase was expected in CAN-FeH since the thermodynamics of the hydrothermal synthesis aids in the obtention of hematite nanoparticles. The diffraction pattern is much clearer in CA-FeWI probably due to the higher Fe loading, confirmed by ICP. This catalyst shows in addition

Figure 2: FTIR spectra of CA, CAN, CAN-FeH and CA-FeWI

reflections assigned to KCl (JCPDS 00-041-1476). These crystals may have formed due to K residue of the activation process, in which KOH is used, combined with Cl anions from the Fe precursor salt.

The FT-IR spectra obtained from the samples CA, CAN, CAN-FeH and CA-FeWI were analyzed (Fig 2). The signals observed between 3200 and 3600 cm^-1 are attributed to -OH groups vibrations. Contributions from lower energy signals in this region correspond to hydrogen bonding, probably increased by moisture absorption. The signals corresponding to higher wavenumbers can be assigned to unbound -OH groups in alcohols, phenols and carboxylic acids (Gomez-Serrano et al., 1996). The increase in these signals is notable in CAN, evidencing the creation of -OH terminals in CA after nitric acid treatment. These sites are the ones expected to serve as anchors for the nanoparticles. Their intensity decreases, consequently, after the incorporation of Fe in CAN-FeH. The incorporation of Fe by wet impregnation in CA generated -OH terminals that are evident in the spectrum of CA-FeWI around 3450 cm^-1.

Common to all four materials is the signal detected around 1700 cm^-1 which is characteristic of the C=O group in different environments (Gomez-Serrano et al., 1996; Zhou et al., 2007). The band around 1600 cm^-1 is assigned to C=C conjugated olefin bonds, which evidences the presence of a graphitic fraction within the materials. In-plane bending movements of aromatic C-H bonds give numerous signals between 1225 and 950 cm^-1 and can be evidenced in CA, with a peak centered at 1125 cm^-1. However, this band loses intensity and its floor moves slightly to 1227 cm^-1 in CAN. This new signal can be associated with aromatic ethers and aryl -O stretches created after treatment with nitric acid.

SSA measurement of the carbonaceous matrices through the BET method yielded values of 638 and 482 m²/g for CA and CAN, respectively. The decrease in CAN surface area may be due to the partial destruction of cavities in the material, as can be seen in the SEM images (Fig. 3.A). Once Fe was incorporated, SSA of the catalysts diminished considerably; namely 176 m²/g for CA-FeWI and 74 m²/g for CAN-FeH.

SEM images of CA (Fig. 3.B) show a tortuous surface with a large number of channels and pores that are responsible for the high surface area. In the image corresponding to CAN (Fig. 3.A) it could be observed a section of channels devoid of a wall. This was probably caused by the aggressiveness of the acid treatment. In the CAN-FeH images (Fig. 3.C), areas densely covered by the hematite nanoparticles and some more uncovered can be distinguished. They are mainly seen over curvatures or pores in the surface which might explain the low SSA values measured. This behavior suggests that despite the oxidation treatment of the carbon surface, the interaction of the nanoparticles with the surface is very weak, showing a more physical/mechanical anchorage than a chemical one. Instead, the crystals in CA-FeWI (Fig. 3.D) are much more uniformly distributed.

Figure 4:MO photolysis and photooxidation with H2O2 tests and MO adsorption performance of CA and CAN.

EDS elemental mapping analysis was performed on Fe-incorporated catalysts. Fe signals were detected, but in CA-FeWI a large amount of Cl was also identified, which surely comes from the precursor salt. The elemental analysis performed determined that the Fe content in CA-FeWI was 5.5% and 3.8% in CAN-FeH.

D. Catalytic activity

In order to get a better knowledge of the reaction system, photolysis tests of MO, oxidation of the dye with H2O2 plus UV radiation and adsorption capacity of the carbonaceous matrices CA and CAN were first analyzed (Fig. 4). MO stability was evaluated using the reaction setup previously described, but in peroxide and catalyst absence. The dye showed to be very stable under the reaction conditions and its concentration remained stable after 4 h.

It is well known that H2O2 can be activated with UV radiation. Thus, its effect was also tested.

A reduction in the dye concentration was observed under these reaction conditions. MO relative concentration (C/C0) decreased to around 70% after 4 h. The dye and the hydrogen peroxide can be clearly sensitized and some degradation can occur.

Also, adsorption tests (light and H2O2 absence) were performed using the MO solution and the supports along 4 h. Both materials showed considerable levels of adsorption, as it was expected. However, the functionalization done to CAN with an acid wash had a notorious impact. From Fig. 4, considerable improvement was obtained on the adsorption capacity of the activated and functionalized biochars, since final MO removal goes from 40 to 70 % in CA and CAN, respectively. Besides, CAN reached adsorption equilibrium within 150 min, while CA seemed to reach its maximum adsorption capacity at 100 min. Other researchers reported adsorption times around 60 min, employing different biomass sources (Gonçalves et al., 2020). As commented by Zhao et al. (2020), surface chemistry plays a key role in organic compounds adsorption since functional groups interact by different ways with the contaminant. According to FT-IR analysis (Fig. 2), CAN sample showed higher concentration of hydroxyl groups. Thus, MO-CAN interaction was improved despite the SSA reduction. Even if the

Figure 5: Degradation ratio C/C0 for the heterogeneous reactions with CAN-FeH, CA-FeWI, CA-FeWI at pH 7 and the homogeneous reactions with Fe(III) at pH 3.5 and 7

adsorbed molecules were retained, the matrices were unable to degrade the dye, so photo-active species were incorporated onto both matrices.

The photo-Fenton reactions results employing modified biochars are presented in Fig. 5. According to the profiles, complete removal was achieved with CA-FeWI during the first hour of reaction. Although CAN support showed a remarkable adsorption capacity, CAN-FeH’s performance was inferior compared to CA-FeWI. CAN-FeH achieved an 80% reduction in the dye’s concentration after 4 h.

It should be mentioned that the initial pH of the MO solution was around 5.7. In the reaction with CA-FeWI after 5 min, a considerable acidification was verified, quickly stabilizing itself around a pH value of 3.5. Even when MO degradation produces acids that can lower the pH, such a drop can be interpreted as a sign of metal leaching.

In order to get a better knowledge of the leached metal, ICP analysis of the reaction medium after catalyst separation was done. Fe concentration was 1.76 ppm. Homogeneous reactions with this iron content were performed. Even when leached Fe plays a very important role in the discoloration of the dye at pH 3.5 (Fig 5), its degradation presented a quite different behavior. When the reaction was performed in a homogeneous medium, the degradation seems to be partial. The absorbance signal of MO at 270 nm (aromatic rings) remained unchanged. Thus, CA-FeWI has a better photocatalytic behavior since also this signal disappears when it is employed.

Considering the importance of pH conditions, reactions were also performed at neutral pH. CA-FeWI was also effective for MO removal in this situation (Fig. 5). It showed a slight reduction in the rate of disappearance of the dye from the medium but after 4 h, around 95% of the dye was removed. A homogeneous cycle at pH 7 using the concentration of leached Fe previously measured was also performed. The results mimic those obtained for the catalytic cycles, but the reduction in the reaction rate was

Figure 6: MO 270 nm band removal ratio after 4 h of reaction with homogeneous Fe at pH 3.5 and 7 and the catalytic runs.

Figure 7: MO removal ratio after 4 h of reaction using the MO band at 465 nm.

even clearer. Nevertheless, 90% of the dye was discolored after 4 h.

As previously mentioned, the role that the leached iron plays seems to be very important. But considering the whole spectrum in the UV-Vis spectroscopic analysis, the band at 270 nm is still present in all the tests using the homogeneous catalyst. Fig 6 shows that the homogeneous reaction makes a less effective usage of H2O2 and the degradation of the dye is not complete (Wang et al., 2015).

To test the reusability of CA-FeWI a new reaction cycle was carried out using recovered catalyst from another cycle. A decrease in activity was observed (Fig. 7), with a behavior like that of CAN-FeH, whose iron content was lower. According to this observation, further improvements should be done in iron-biochar interaction in order to better retain this active species.

IV. CONCLUSIONS

Aqueous solutions of the azo dye were treated employing the photo Fenton method and iron-based materials as catalysts. In order to improve activity and reusability of the materials, activated biochars from renewable sources (peanut shell) were employed as supports. Different methods for iron incorporation were evaluated.

Two kinds of supports were selected as based materials for each iron incorporation method. CAN showed remarkable adsorption capacity for the dye, achieving 70% reduction of the initial concentration after 4 h. CAN-FeH achieved 80% reduction of the dye’s concentration. Considering CAN adsorption capacity, it is possible to determine that hematite nanoparticles incorporation was not very efficient. Further tests to improve the interactions with the support are being done.

The material obtained by iron wet impregnation showed the best results. Initial degradation rate was the highest of the evaluated set. Complete discoloring of the initial solution was achieved in a first use and almost 80 % in a second one. Nevertheless, the interactions between the active phase and the carbonaceous supports should be also improved in order to prevent active phase leaching from CA-FeWI. This composite material was also effective at neutral pH evaluations.

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Received: August 30, 2022

Sent to Subject Editor: September 3, 2022

Accepted: December 13, 2022

Recommended by Subject Editor Laura Briand