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Abstract

A new magnetic P. harmala biochar/Mn-ZnFe₂O₄ material was prepared by the one-step pyrolysis approach and characterized by FT-IR, SEM, XRD, and BET techniques. The efficacy of removing rhodamine B (RhB) via adsorption and catalytic degradation was examined. Adsorption removal attained 82% of 10 mg·L^-1 RhB at pH 4.0, 90 min of agitation, and 3.2 g·L^-1 of adsorbent. The adsorption kinetics indicated a diffusion-limited mechanism and matched the second-order model. The equilibrium data are better suited by the Langmuir isotherm, and the maximum monolayer adsorption capacity ( $q_{\max}$ ) was 34.5 mg·g^-1. The D-R and Tempkin isotherms both showed an endothermic chemical adsorption process and negligible lateral repulsive forces, respectively. The catalytic removal by Fenton-like degradation was 40 and 99%, respectively, for the biochar/H₂O₂ and biochar/Mn-ZnFe₂O₄/H₂O₂. The prospective use of P. harmala biochar/Mn-ZnFe₂O₄ as an alternative material for RhB decontamination of water was validated by the successful removal of RhB from industrial wastewater of greater than 77% by adsorption and of 95% by the catalytic degradation.

1. Introduction

The reuse of water after pollutant removal is one of the most serious environmental concerns. When a toxic substance enters the water, its quality deteriorates. Dyes are a major source of pollution, particularly in water treatment. Several synthetic organic dyes are widely used and essential in a variety of industries. Dye-contaminated effluent disposal can seriously pollute surface and groundwater [1]. This is because most dyes are toxic and endanger human health. The level of contamination by a specific dye distinguishes its true application. Rhodamine B (RhB) dye, for example, is widely used in the textile, food, cosmetics, and pharmaceutical industries [2, 3]. Huang et al. [4] disclosed the high toxicity in a body of water as a result of RhB disposal. Congenital diseases and cancer are among the health risks posed by RhB in water [5, 6]. The removal of RhB dye from water can be accomplished through catalytic degradation and/or adsorption. Several materials have been developed for the catalytic and adsorption removal of RhB. Among these catalysts are Ti/Ru_0.3Ti_0.7O₂ [3], iron-doped mesoporous silica [7], nanozero valent iron (nZVI)@biochar [8], and TiO₂-AuNP/polydimethylsiloxane sponge [9], which were reported. RhB adsorbent materials include zinc ferrite-polyaniline [1], activated pine cone [10], stalk corn-activated carbon [11], Argemone mexicana [12], bentonite-titanium dioxide composites [13], graphene oxide/silicalite-1 composites [14], carbon xerogels [15], and modified alpha-alumina [16]. When biomass is heated to a point of thermal breakdown without oxygen, a material called biochar is created that is rich in carbon and has fine grains. The solid biochar itself, the liquid biooil, and the syngas are the three byproducts of pyrolysis. Temperature, heating rate, and residence duration all affect the biochar’s characteristics [17]. A variety of biochars were made for RhB adsorption using a variety of materials, including macroalgae [18], olive waste biomass [19], goat manure [20], and cocoa pod husk [21]. Due to their high surface-to-volume ratio, superparamagnetic characteristics, and nanoscale size, several ferrite nanoparticles have been applied to the filtration of water. However, one of the fundamental issues with these nanomaterials’ actual usage as adsorbents is their tendency to aggregate. As a result, they are frequently combined with polymers to create nanocomposites that can be utilized as adsorbents. The spinel ferrites have the chemical formula MeFe₂O₄, where Me stands for many metals that can be included in the ferrite lattice, including Mg, Ni, Cu, Mn, Co, and Zn. These metal-based ferrites are frequently employed for adsorption research because of their strong magnetic permeability and chemical stability [22]. In environmental purification processes, magnetic biochar has the potential to be used as an adsorbent, catalyst, or soil remediation agent. This is due to the fact that its magnetic separation capabilities make it possible to easily recover pollutants and/or adsorbents from water, rendering it an environmentally safe material. Additionally, the combination of magnetic nanomaterial and biochar results in high adsorption efficiency. Many magnetic biochars have been described as adsorbents, including banana peel biochar/Fe₃O₄ [23], ferric and biological sludge biochar [24], and Eichhornia crassipes/Fe₃O₄ [25].

In this study, we sought to prepare a new magnetic biochar/Mn-ZnFe₂O₄ of the Peganum harmala plant for the adsorption removal and catalytic degradation of RhB dye in water. The adsorption performance and removal efficiency of RhB dye from aqueous solutions will be evaluated. Also, the application of the new adsorbent in real water will be studied.

2. Experimental

2.1. Instrumentation

The spectrophotometric measurements of RhB dye were made using a Cintra 1010 double-beam UV-Vis spectrophotometer and Cintra 2.4 software (GBC Scientific Equipment, Braeside, Australia) at a fixed wavelength of 554 nm. For the pyrolysis process, a Nabertherm vacuum muffle furnace model B180 (Lilienthal, Germany) with a protective gas connection and a programmed furnace controller with a temperature range of 30-3000°C was used. The FT-IR spectra of the P. harmala biochar/Mn-ZnFe₂O₄ were recorded in the wavenumber range of 400–4000 cm^-1 using a Fourier transform infrared spectrometer (Perkin Elmer 1750, MA, USA) using the KBr discs technique.

On the Bruker D8 Advance diffractometer (Berlin, Germany), an X-ray diffraction (XRD) pattern was captured using Cu-K1 anode radiation at 1.54058 and a secondary monochromator. To determine the phase composition and average crystallite size of the magnetic nanoparticles, the scanning speed was set at 1° min^-1. The surface area and porosity were determined using the fully automated Micromeritics® TriStar II Plus analyzer model 2390t V2.03 (GA, USA). At 77 K and a relative pressure (P/Po) in the range of 0.0586–0.996, N₂ was adsorbed and then desorption. Before measurement, the samples were degassed at 120°C for two hours. The specific surface area and total pore size were determined using the Brunauer-Emmett-Teller (BET) and the Barrett-Joyner-Halenda (BJH) models. The SEM micrograph of the surface morphology of the prepared magnetic biochar was performed on JEOL scanning electron microscope (MA, USA). A Jenway bench pH meter model 4510 (Keison International Ltd., Chelmsford, UK) was used to test the pH of the samples, and it was calibrated using buffer solutions of pH 4 and 9. Stuart orbital mechanical shaker model S1500 (London, UK), with a speed of 30-300 rpm, was used to agitate the samples. Hamilton water purification system (Hamilton Glass Ltd., Margate, UK) provided double-distilled water for the duration of the study. For all filtering purposes, qualitative filter paper No. 101 (Dorsan Filtration SL Co., Barcelona, Spain) was used. Glass bottles with Teflon caps were used for the adsorption study.

2.2. Reagents

All reagents used were of analytical reagent grade, and the standard solutions were prepared in DDW. The standard RhB solution was prepared using rhodamine B dye purchased from Suvachem Co. (Maharashtra, India). Manganese chloride (MnCl₂), zinc chloride (ZnCl₂), and ferric chloride nonahydrate FeCl₃·9H₂O, from Alpha Chemika™ (Mumbai, India), and sodium hydroxide (NaOH), from Sigma-Aldrich, were used to prepare the magnetic nanoparticles. By taking the appropriate amount of the reagent, a stock solution of 1000 mg·L^-1 RhB was prepared. The working RhB concentration of 10 mg·L^-1 was obtained by diluting the stock solution with double-distilled water.

2.3. Preparation of P. harmala Biochar/Mn-ZnFe₂O₄

It was decided to employ Mn and Zn in the magnetic biochar’s ferrite since Mn-ZnFe₂O₄ is one of the most prevalent soft ferrites. These ferrites are favored because of their good stability, high saturation magnetization, and high permeability. This establishes them as well-known and popular materials for catalysis and wastewater treatment. The one-step impregnation pyrolysis method was applied to prepare the P. harmala biochar/Mn-ZnFe₂O₄ adsorbent. According to the modified method [26], the P. harmala powder was dried at 105°C for 24 h and sieved to 0.1 mm. A 100 mL chemical solution of the nanoparticle precursor was prepared by mixing 30 mL of 0.25 mol·L^-1 MnCl₂, 20 mL of 0.25 mol·L^-1 ZnCl₂, and 50 mL of 0.5 mol·L^-1 FeCl₃ solution to get molar ratios of 0.6 : 0.4 : 2.0 from Mn²⁺, Zn²⁺, and Fe³⁺, respectively. Then, 10 g of the plant powder was added to the above solution and mechanically stirred for 2 h. Thereafter, 50 mL of 10 mol·L^-1 sodium hydroxide was added dropwise, and the slurry was heated to 90°C for one hour till a viscous gelatinous solution was formed. The resulting gel material was placed in a tightly sealed and cleaned porcelain crucible and pyrolyzed under an N₂ atmosphere in the NABER/B180 furnace for one hour at a setting temperature of 600°C and a heating rate of 10°C·min^-1 from 100 to 600°C. Further, the magnetic biochar was cooled at room temperature, immersed in 0.1 mol·L^-1 HCl solution under stirring for one hour, washed with double-distilled water to effluent that was barely neutral, and then dried, milled, and stored for further use. A ferrite with the chemical formula Mn_0.6Zn_0.4Fe₂O₄ is produced using these molar ratios of the metal ions, which is referred to as Mn-ZnFe₂O₄ throughout this study. A schematic diagram of the synthesis protocol and adsorption mechanisms is shown in Figure 1. Similarly, the P. harmala biochar was produced by direct pyrolysis of plant powder at a temperature of 600°C for 1 hour.

Figure 1

One-step impregnation synthesis protocol of P. harmala biochar/Mn-ZnFe₂O₄.

The point of zero-electric charge ( $p H_{PZC}$ ) of P. harmala biochar/Mn-ZnFe₂O₄ was determined using the potassium chloride solid addition method [27]. For this purpose, a total of 50 mL of 0.1 mol·L^-1 KCl portions was adjusted to pH 2, 4, 6, 8, and 10. Each solution was fed with 100 mg of the magnetic biochar, and the pH was measured both immediately and after 24 h. By plotting the pH change ( $Δ pH$ ) versus the initial pH, the intersection point of the line at $Δ pH = 0$ is taken as the value of $p H_{PZC}$ .

2.4. Adsorption Procedure

The removal of RhB dye was investigated using a 10 mL solution containing 10 mg·L^-1 and 50 mg adsorbent and shaking for time intervals ranging from 2 to 100 minutes at 250 rpm at room temperature. The effect of pH on dye removal was studied from 1.0 to 9.0 using 0.1 mol·L^-1 solution of HCl or NaOH for adjustment. The adsorbent dose was investigated from 0.2 to 5.0 g·L^-1. The initial concentration of RhB was examined from 5.0 to 100 mg·L^-1. After the adsorption process, the adsorbent was collected by the magnet, and the remaining dye was measured. For dye quantification, a calibration curve was created by plotting the absorbance (A) of RhB standard solutions at concentrations ( $C$ ) ranging from 0.1 to 10.0 mg·L^-1 and measured at $λ_{\max}$ 554 nm. The resulting regression equation was $A = 0.068 C + 0.002 (R^{2} = 0.992)$ .

With $C_{0}$ and $C$ representing the initial and remaining dye concentrations (mg·L^-1), $V$ for the sample volume (L), and $m$ for the adsorbent weight (g), the RhB removal (%) was calculated using the following equation: $\begin{matrix} (1) & Removal (%) = [\frac{C_{0} - C}{C_{0}}] \times 100 . \end{matrix}$

Also, the adsorbent RhB amount at equilibrium ( $q_{e}$ , mg·g^-1) could be calculated by using $\begin{matrix} (2) & q_{e} = \frac{[(C_{0} - C) \times V]}{m} . \end{matrix}$

2.5. Catalytic Degradation Procedure

For the catalytic degradation of RhB, the heterogeneous Fenton-like degradation approach was examined. Four trials on RhB removal were conducted in accordance with the reported method [23] using the following materials: P. harmala biochar alone, P. harmala biochar and H₂O₂, biochar/Mn-ZnFe₂O₄ alone, and biochar/Mn-ZnFe₂O₄ and H₂O₂. When utilizing the biochar alone or the biochar/Mn-ZnFe₂O₄, 50 mg of the substance was added to 20 mL of a 10 mg·L^-1 RhB solution. The removal efficiency was measured following a pH 4.0 adjustment and 90 min of shaking. The mixture for the P. harmala biochar/H₂O₂ or P. harmala biochar/Mn-ZnFe₂O₄/H₂O₂ combinations was made by combining 50 mg of the material with 20 mL of 10 mg·L^-1 RhB solution, followed by the addition of a 0.5 mL aliquot of 1.2% ( $v / v$ ) H₂O₂, yielding a final H₂O₂ concentration of 0.03% ( $v / v$ ). The resulting mixture was similarly adjusted to pH 4.0 and agitated for 90 min before the remaining dye concentration was measured using the suggested technique.

3. Results and Discussion

3.1. Characterization of Biochar/Mn-ZnFe₂O₄

Figure 2 depicts the obtained FT-IR spectra of the biochar alone and the biochar/Mn-ZnFe₂O₄. A characteristic absorption band at 551 cm^-1 corresponded to the interaction between metal and oxygen ions that occurred in the tetrahedral and octahedral positions in the spinel crystal lattice of ferrite [28]. A weak absorption peak at 1702, 2334, and 2941 cm^-1 were assigned for the C=O, C-N, and C-H stretching vibrations of the aromatic compounds in the biochar. Also, the broad absorption band ranged from 3200 to 3640 cm^-1 corresponding to the O-H stretching vibration of metal-OH. As a result, this confirmed that both characteristics of the P. harmala biochar and the impregnated Mn-ZnFe₂O₄ nanoparticles are depicted in the FT-IR spectrum. Characteristic bands for the negatively charged metal oxide groups of Fe-O, Zn-O, and Mn-O appeared at 543 and 591 cm^-1.

Figure 2

FT-IR spectra of untreated the P. harmala biochar (a) and biochar/Mn-ZnFe₂O₄ (b).

The morphological microstructure of the prepared P. harmala biochar/Mn-ZnFe₂O₄ was investigated using SEM analysis. The SEM analysis for determining the morphological surface structure is shown in Figure 3. The prepared magnetic biochar has an amorphous microstructure, and the stuffing Mn-ZnFe₂O₄ nanoparticles were randomly dispersed over the carbon of the biochar surface as white spots. The rough and irregular microstructure of the surface also indicates surface heterogeneity.

Figure 3

SEM micrographs for P. harmala biochar alone (a), biochar/Mn-ZnFe₂O₄ at 500x (b), and biochar/Mn-ZnFe₂O₄ at 10000x (c).

(b)

(c)

Figure 4 shows the XRD pattern of the developed P. harmala biochar/Mn-ZnFe₂O₄. The profile of the material contains a broad peak around 10-30° corresponding to the cellulose crystal. Also, a small peak was observed at 43° which is typical of graphite [29]. Moreover, the characteristic peaks for Fe₃O₄ nanoparticles were clearly distinguished by peaks at 2Ɵ of 29.71°, 34.99°, 36.60°, 42.51°, 52.73°, 56.20°, 61.70°, and 72.94°, which correspond to the (220), (311), (222), (400), (422), (511), (440), and (533) planes. This indicates that the prepared magnetic biochar has the same crystal planes as both ZnFe₂O₄ and MnFe₂O₄ according to the XRD JCPDS (Joint Committee on Powder Diffraction Standards) data files No. 22-1012 and No. 74-2403, respectively [30].

Figure 4

XRD pattern of the prepared P. harmala biochar/Mn-ZnFe₂O₄.

The surface area and porosity of magnetic biochars derived from a plant that is open to interaction with RhB are determined by the plant’s initial composition and structure, the activation method used, and the agents used. The isotherm profile of N₂ adsorption-desorption on the developed adsorbent is shown in Figure 5. The multipoint BET model at a relative pressure (P/Po) from 0.058 to 0.274 was used to determine the surface area of the prepared biochar/Mn-ZnFe₂O₄. The specific surface area ( $S_{BET}$ ) before adsorption was 3.605 m²·g^-1. The profile exhibited type III adsorption-desorption isotherm, indicating that the adsorbent has a wide range of pore sizes [31]. This confirmed the simultaneous formation of the monolayer, bilayer, trilayer, and other layers with an exponential rising in the adsorbed dye amount. The pore size distribution presented in Figure 6 showed that the obtained material was mesoporous and macroporous. The cumulative pore volume and average pore size ( $4 V_{total} / S_{BET}$ ) calculated by the BJH method were 0.0178 cm³·g^-1 and 28.58 nm, respectively.

Figure 5

The N₂ adsorption-desorption isotherm of P. harmala biochar/Mn-ZnFe₂O₄ (a) and multipoint BET isotherm model (b).

Figure 6

Variation of pore volume and incremental pore volume with pore width (a). The BJH model of pore area (b) and pore volume (c) with pore width of P. harmala biochar/Mn-ZnFe₂O₄.

3.2. Adsorption Removal

3.2.1. Effect of pH

The effect of pH on the adsorption of RhB dye is a significant parameter since it can alter the ionic form of the dye as well as the surface charge of the adsorbent. As can be seen in Figure 7(a), the removal was higher than 82% at pH 2-5 and then rapidly decreased from 80 to 15% at pH 6-10. As a result, pH 4 was chosen as the appropriate value. As shown in Figure 7(b), the estimated value of $p H_{PZC}$ was found to be pH 6.3. This means that the surface is positively charged at a pH less than $p H_{PZC}$ . RhB has a pKa of 3.7, indicating that the cationic form predominates at pH below 3.7 and the zwitterionic form predominates above pH 3.7 [32].

Figure 7

Effect of pH on RhB removal by P. harmala biochar/Mn-ZnFe₂O₄ (a) and determination of the point of zero-electric charge (b): RhB (10 mg·L^-1), sample volume of 20 mL, adsorbent weight 100 mg, and shaking time of 100 min.

(b)

Interpretation of higher removal at lower pH values can be due to the adsorbent surface being positively charged and the dye existing in the cationic and the zwitterions forms. At pH less than 6.3, electrostatic attraction and the hydrophobic-hydrophobic ( $π ‐ π$ ) adsorption processes are the two basic adsorption mechanisms that might take place. In this situation, the hydrophobic aromatic portion of the dye forms a $π ‐ π$ bond with the carbon in the biochar as the negative anionic COO^- groups in RhB bind to the positive surface [33]. This can be divided into two stages. At pH less than 3.7, the hydrophobic-hydrophobic interaction between the hydrophobic carbon skeleton in the biochar and the aromatic moiety of RhB would be efficient when both the dye and the adsorbent are positively charged. The hydrophobic interaction mechanism currently has the upper hand. The electrostatic attraction takes over in the pH range of 3.7 to 6.3 as the negatively charged carboxylic group (COO^-) in the dye’s zwitterionic form strongly attracts the positively charged protonated OH groups in the ferrite. Notably, the removal remained higher than 80% even at a pH of 6.0, showing that the dye’s conversion to its zwitterionic form at pH 3.7 involved an electrostatic interaction that contributed to adsorption and the pH’s ability to counteract the loss of hydrophobic-hydrophobic attraction. As a result, it is anticipated that electrostatic interaction will rule at pH values between 3 and 6.0. The removal decreased quickly at $pH > 6.0$ , possibly because the ferrite OH groups had changed into negatively charged oxygenates [34]. This caused the adsorbent surface to become negatively charged, repelling the dye’s negatively charged carboxylic groups. Furthermore, in moderately acidic to neutral media where the OH groups of the ferrite are not protonated or ionized, hydrogen bonding can contribute to dye adsorption. It is important to note that the dye appears in monomeric form in acidic conditions, which is smaller and more easily absorbed. Controversially, in an alkaline solution, RhB monomers’ intermolecular electrostatic attraction to negatively charged COO^- and xanthene groups results in the creation of larger dimer aggregates that are difficult to diffuse into adsorbent pores and, as a result, to adsorb. The quick decline in removal in an alkaline media may be caused by the hydrogen bonding interaction to polar surface groups. But the massive dye aggregates could not be adsorbed by this force. Finally, pH 4.0 was chosen as the ideal value for the adsorption investigation to be lower than $p H_{PZC}$ , which is advised for cationic removals, such as RhB dye in this instance.

3.2.2. Adsorbent Dose

The impact of adsorbent dosage on removal effectiveness and adsorbed amount ( $q_{e}$ ) was investigated due to the financial implications. The adsorbent dosage was examined from 0.2 to 5.0 g·L^-1 at a dye concentration of 10 mg·L^-1. Figure 8 shows the results obtained. Steadily, the removal (%) increased as the dosage increased from 0.2 to 4.6 g·L^-1 and reached a maximum removal of 86% afterward. No substantial change in the removal efficiency was observed beyond 4.6 g·L^-1. This could be because the higher adsorbent amount increases the number of active sites accessible for adsorption, leading to efficient dye removal [35]. In contrast, the adsorbed amount ( $q_{e}$ ) decreased by increasing the adsorbent dose which can be due to the incomplete saturation of the adsorbent surface with the necessary amount of dye as a higher adsorbent amount can prevent the dye molecules from accessing the available active sites on the adsorbent surface. According to this profile, even if the dose is raised after the ideal adsorption condition is obtained at a particular dosage, the leftover RhB dye concentration remains constant. The intersection of the two curves was discovered at 3.2 g·L^-1, which is regarded as the ideal adsorbent dose when comparing removal effectiveness and price.

Figure 8

Effect of P. harmala biochar/Mn-ZnFe₂O₄ dosage on the adsorption of RhB (10 mg·L^-1), pH 4, sample volume of 20 mL, and shaking time of 100 min.

3.2.3. Initial RhB Concentration

The influence of initial dye concentration on removal efficiency and the adsorbed amount was examined at dye concentrations ranging from 5 to 100 mg·L^-1 to compute the experimental adsorption capacity ( $q_{\exp}$ ) and apply isotherm models. The removal (%) was strongly dependent on the initial concentration and gradually declined by increasing the dye concentration. The gradual decrease in removal could be caused by raising the dye concentration above the adsorbent capacity, which causes the amount of the remaining dye to gradually increase. As the initial concentration rose, the amount adsorbed rose quickly until it roughly reached its maximum at 80 mg·L^-1, which matched an experimental adsorption capacity ( $q_{\exp}$ ) of 27.7 mg RhB·g^-1.

3.2.4. Adsorption Removal Kinetics

The time effect on removal justifies the use of this magnetic biochar in terms of the rate at which the dye can transfer from the aqueous phase to the solid adsorbent. The agitation period was examined over a while of 5 to 110 min. Figure 9 depicts the results obtained for the variation of the adsorbed amount $q_{t}$ (mg·g^-1) at any time $t$ . The uptake was rapid within the first 40 min, with more than 50% of the adsorbed amount achieved. After this, adsorption increased at a slower rate, reaching equilibration and maximum adsorption after 90 min. The early quick adsorption shows how easily RhB may be removed using the adsorbent material. This verifies the strong binding force between the dye ions and the adsorbent substance, indicating that it is important for the removal process. Finally, the shaking time in subsequent experiments was set at 90 min to ensure equilibration and quantitative adsorption.

Figure 9

Effect of shaking time on the P. harmala biochar/Mn-ZnFe₂O₄ adsorption of RhB (10 mg·L^-1), pH 4, sample volume of 20 mL, and adsorbent weight 50 mg.

By comparing the fit of experimental results to the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, the mechanism that controls adsorption kinetics was investigated. Given that $q_{e}$ and $q_{t}$ represent the amount of RhB adsorbed (mg·g^-1) at equilibrium and at any time $t$ (min), respectively, the pseudo-first-order rate model of Lagergren and Kungliga [36] is shown as follows: $\begin{matrix} (3) & \log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1} t}{2.303} . \end{matrix}$

Plotting $\log (q_{e} - q_{t})$ versus $t$ could be used to test the validity of the pseudo-first-order model.

The pseudo-second-order suggested by Ho et al. [37] is expressed by $\begin{matrix} (4) & \frac{t}{q_{t}} = \frac{1}{K_{2} . q_{e}^{2}} + \frac{1}{q_{e}} t, \end{matrix}$ where $k_{2}$ is the pseudo-second-order rate constant (μg·g^-1·min^-1).

Figures 10(a)–10(c) display the plotting of these models. Additionally, Table 1 includes the collected kinetic data. A higher linear correlation coefficient ( $R^{2}$ ) is found in the regression analysis results of the pseudo-second-order model than in the pseudo-first-order one. Additionally, compared to the pseudo-first-order model, the computed $q_{e}$ value for the pseudo-second-order model was closer to the experimental value. Therefore, the pseudo-second-order model is more suitable to describe the experimental results. This shows that the characteristics of the dye and the adsorbent surface groups both affect the adsorption process. The Morris-Weber model [38] of the intraparticle diffusion behavior is given by Equation (5). The intraparticle diffusion rate constant $k_{id}$ (mg·g^-1·min^-1/2) was calculated using the slope of the plot of $q_{t}$ values versus the square root of time $t$ . $\begin{matrix} (5) & q_{t} = k_{id} t^{1 / 2} \end{matrix}$

Figure 10

Pseudo-first-order (a), pseudo-second-order kinetic (b), and intraparticle diffusion (c) kinetic models.

(b)

(c)

Table 1

Adsorption kinetic data for RhB dye adsorption onto P. harmala biochar/Mn-ZnFe₂O₄.

Pseudo-first-order model				Pseudo-second-order model			Intraparticle diffusion
$q_{e}$ , $q_{\exp}$ (mg·g^-1)	$q_{e}$ (mg g^-1)	$K_{1}$ (min^-1)	$R^{2}$	$q_{e}$ (mg·g^-1)	$K_{2}$ (g·mg⁻¹·min⁻¹)	$R^{2}$	$k_{id}$ (mg·g^-1·min^1/2)	$R^{2}$
2.77	5.09	0.044	0.924	4.71	0.003	0.978	0.346	0.995

A multistep adsorption mechanism was implied by a linear relationship with a steeper slope at beginning intervals, followed by a second linear range with a lower slope, as depicted in Figure 10(c). It was also proposed that a film diffusion phase occurred during the initial adsorption stages, which were indicated by straight lines that did not pass through the origin. Thus, intraparticle diffusion contributes to the rate-limiting step. With an $R^{2}$ value of 0.996, a strong connection was found at the first stage, demonstrating the stronger dependency of adsorption on the diffusion process. A higher mass transfer of the dye to the adsorbent surface during the initial stages of adsorption was confirmed by the $k_{id}$ value of 0.346 mg·g^-1·min^-1/2. The adsorption amount was smaller in line with diffusion in the second stage, with an $R^{2}$ of 0.857 and a lower rate constant of 0.036 mg·g^-1·min^-1/2. This may be because the large-sized dye molecules are traveling slowly and possibly being affected by different repellent forces inside the adsorbent micropores. As a consequence, the intraparticle diffusion mechanism will approach equilibrium slowly in the sorbent’s microporous regions, governing bulk adsorption. Based on the aforementioned, the RhB adsorption kinetics onto the biochar/Mn-ZnFe₂O₄ is now being governed by pseudo-second-order and diffusion-controlled models.

3.2.5. Adsorption Equilibrium

The equilibrium study allows for the evaluation of the developed biochar’s adsorption capacity as well as the dye-adsorbent binding strength. Equilibrium experiments were carried out at pH 4.0 with a shaking time of 90 minutes to reach equilibrium, and the initial concentration of RhB ranged from 5 to 100 mg·L^-1. A comparison of the experimental data and fitting to the Langmuir and Freundlich isotherms is presented in Figure 11. The experimental data followed a profile very close to the Langmuir isotherms.

Figure 11

Experimental equilibrium data and the fitting to the Langmuir isotherm and Freundlich isotherm models: RhB concentration 5-100 mg·L^-1, pH 4, sample volume of 20 mL, P. harmala biochar/Mn-ZnFe₂O₄ adsorbent weight 50 mg, and shaking time 90 min.

To obtain the isotherm parameters, the linear forms of the Langmuir, Freundlich, Tempkin, and Dubinin-Radushkevich (D-R) isotherm models were tested to fit the experimental results. The Langmuir isotherm [39] is expressed by $\begin{matrix} (6) & \frac{C_{e}}{q_{e}} = \frac{C_{e}}{q_{\max}} + \frac{1}{q_{\max} K_{L}}, \end{matrix}$ where $q_{e}$ is the adsorbed amount of RhB (mg·g^-1) at equilibrium, $q_{\max}$ is the maximum adsorbed amount of RhB (mg·g^-1) to form a complete monolayer, and $C_{e}$ is the equilibrium RhB concentration (mg·L^-1). $K_{L}$ stands for the Langmuir constant, which is related to the RhB affinity of binding sites. If this model matches the experimental results, it assumes a monolayer chemical adsorption mechanism. This model could also be used to calculate the maximum monolayer adsorption capacity ( $q_{\max}$ , mg·g^-1). The dimensionless equilibrium constant or separation factor $R_{L}$ is used to predict favorable adsorption. It is given in Equation (7). Based on the value of $R_{L}$ , the adsorption isotherm can be classified into linear ( $R_{L} = 1$ ), favorable ( $0 < R_{L} < 1$ ), or unfavorable ( $R_{L} > 1$ ), or irreversible ( $R_{L} = 0$ ). $\begin{matrix} (7) & R_{L} = \frac{1}{1 + K_{L} C_{e}} . \end{matrix}$

The Freundlich isotherm [40], which is an empirical model of heterogeneous surface adsorption, is given by $\begin{matrix} (8) & \ln q_{e} = (\frac{1}{n}) \ln C_{e} + \ln K_{F}, \end{matrix}$ where $n$ and $K_{F}$ are the Freundlich constants that relate, respectively, to adsorption intensity and capacity.

The adsorbent-adsorbate interaction and heat of adsorption could be evaluated using the Tempkin isotherm model [41] as described by $\begin{matrix} (9) & q_{e} = B \ln A_{T} + B \ln C_{e}, \end{matrix}$ where $A_{T}$ is the Tempkin isotherm equilibrium binding constant (L·g^-1) and $B$ is the slope. The Tempkin isotherm constant ( $b_{T}$ ) correlated to the heat of adsorption (J·mol^-1) is given by $\begin{matrix} (10) & B = \frac{R T}{b T} . \end{matrix}$

The homogeneous surface or a constant adsorption potential is not assumptions made by the Dubinin-Radushkevich (D-R) isotherm model [42] as represented by $\begin{matrix} (11) & \ln q_{e} = \ln X_{m} - β ε^{2} . \end{matrix}$

The slope ( $β$ ) and intercept ( $\ln X_{m}$ ) values can be determined by displaying $\ln q_{e}$ versus $ε^{2}$ .

$X_{m}$ (mol·g^-1) denotes the maximum amount of RhB that may be adsorbed onto one gram of the magnetic biochar. The adsorption energy is represented by the constant $β$ (mol²·J^-2). The Polanyi potential ( $ε$ ) can be calculated by using the following equation: $\begin{matrix} (12) & ε = R T \ln (1 + \frac{1}{C_{e}}), \end{matrix}$ where $R$ is the universal gas constant (8.314 J/mol K) and $T$ is the absolute temperature of 298 Kelvin.

Adsorption free energy ( $E$ , KJ·mol^-1) is designated as the free energy change when one mole of RhB is transported from the solution infinite to the surface of the adsorbent. Equation (13) can be used to get the value of $E$ . $\begin{matrix} (13) & E = \frac{1}{\sqrt{- 2 β}} . \end{matrix}$

The tested isotherm models are plotted in Figures 12(a)–12(d). Also, the calculated isotherm parameters are shown in Table 2. The RhB adsorption was best fitted to the Langmuir model ( $R^{2} = 0.981$ ) than the Freundlich with a lower $R^{2} = 0.903$ . The calculated maximum capacity ( $q_{\max}$ ) from the Langmuir model was found to be 34.5 mg·g^-1 which is higher than many other adsorbents such as fly ash [43], Fe-N-co-modified biochar [44], modified zeolite [45], municipal waste biochar [46], and ZnFe₂O₄-nanocomposite [47]. The high value of $q_{\max}$ (>> 1.0) indicated strong interaction between RhB and the magnetic Biochar, and the removal process proceeded via a monolayer chemical adsorption. Furthermore, the values calculated of separation $R_{L}$ varied from 0.02 to 0.48 (between $0 > R_{L} < 1$ ), supporting the favorable adsorption of RhB onto the magnetic biochar/Mn-ZnFe₂O₄. The Freundlich constant $K_{F}$ was 3.42 L·g^-1 which shows a stronger affinity of the adsorbent towards RhB. The $n$ constant value was 1.75 ( $n > 1$ ), indicating that the adsorbent has a higher affinity for RhB dye and that cooperative adsorption onto a heterogeneous surface is advantageous. The values of both constants indicated the strong adsorption of RhB by the developed magnetic biochar. This is consistent with the presence of macroporous and mesoporous structures in the SEM micrographs of the magnetic biochar adsorbent. The adsorption process may begin with the RhB diffusion from the bulk of the solution to the sorbent surface and then to the inner surface to combine with active groups for ultimate accumulation in a monolayer pattern.

Figure 12

The linear plots of the Langmuir (a), Freundlich (b), Tempkin (c), and Dubinin-Radushkevich (d) isotherm models.

(b)

(c)

(d)

Table 2

Adsorption isotherm parameters for RhB dye adsorption onto P. harmala biochar/Mn-ZnFe2O4.

Isotherm model	Parameter
Experimental	$q_{\max}$ (mg·g^-1)	27.7

Langmuir model	$q_{\max}$ (mg·g^-1)	34.5
	$K_{L}$ (L·mg^-1)	0.81
	$R_{L}$	0.02-0.48
	$R^{2}$	0.981

Freundlich model	$n$	1.75
	$K_{F}$	3.42
	$R^{2}$	0.903

Tempkin model	$B$ (J·mol^-1)	6.80
	$A_{T}$ (L·g^-1)	1.11
	$b_{T}$ (KJ·mol^-1)	0.37
	$R^{2}$	0.992

Dubinin-Radushkevich model	$X_{m}$ (mg·g^-1)	21.12
	$β$ (mol²·J^-2)	$- 1.127 \times 10^{- 6}$
	$E$ (KJ·mol^-1)	0.67
	$R^{2}$	0.910

Figure 12(c) shows the linearized Tempkin adsorption isotherm applied to the experimental results. Good correlation ( $R^{2} = 0.992$ ) was higher than that obtained for other isotherms confirming that this isotherm could provide a reasonable model for adsorption. The binding constant $A_{T}$ was 1.11 L·g^-1 indicating strong binding between RhB and the adsorbent surface. The Tempkin heat of the adsorption constant, $b_{T}$ , was equal to 0.37 KJ·mol^-1, and constant $B$ had a positive value of 6.80. This demonstrated that the adsorption is an endothermic process, with a linear increase in adsorption heat as the dye layer thickness on the adsorbent gradually increases. This could be the outcome of stronger dye-biochar/Mn-ZnFe₂O₄ interaction. The $b_{T}$ value was rather higher than other adsorbents used for removing RhB such as halloysites [32]. Furthermore, according to this concept, the active sites are distributed uniformly across the magnetic biochar, and there is little evidence of lateral interaction between adsorbed RhB molecules.

A high correlation coefficient ( $R^{2} = 0.910$ ) was seen in the D-R model (Figure 12(d)). A heterogeneous microporous structure was visible on the biochar/Mn-ZnFe₂O₄ surface because the porosity factor ( $β$ ) was less than 1.0. The maximum adsorption capacity ( $X_{m}$ ) estimated is 21.1 mg·g^-1, which is less than the maximum capacity of the Langmuir monolayer ( $q_{\max}$ ). The free energy of adsorption ( $E$ ) had a positive value of 0.67 kJ·mol^-1 which emphasized that adsorption is an endothermic chemisorption process. These findings are following the results obtained from the pseudo-second-order kinetic and Langmuir isotherm models.

3.3. Catalytic Removal of RhB

Using the Fenton-like method and hydrogen peroxide (H₂O₂) as a catalyst for degradation, the degradation of RhB was investigated. The results were compared to the adsorption removal using the adsorbent biochar/Mn-ZnFe₂O₄ in the absence of H₂O₂. The results obtained are depicted in Figure 13.

Figure 13

Catalytic degradation and adsorption removal of RhB using P. harmala biochar and biochar/Mn-ZnFe₂O₄: 20 mL of 10 mg·L^-1 RhB solution, pH 4.0, shaking time 10-100 min, adsorbent weight 50 mg, and H₂O₂ of 0.03% ( $v / v$ ).

In the absence of H₂O₂, the results indicated no degradation effect on RhB when using the biochar and magnetic biochar/Fe₃O₄. This was evidenced by measuring the absorbance of the eluted RhB after adsorption. The removal was ≥33 and 84% for biochar and biochar/Mn-ZnFe₂O₄, respectively.

In the presence of H₂O₂, the removal reached 40 and 99%, with an increase in the removal by 7 and 15%, respectively. Further, by testing the adsorption in light-exposed and light-protected samples, there was a significant difference in the adsorbed amount $q_{e}$ which emphasizes the pronouncing effect of catalytic degradation. Generally, either for biochar or biochar/Mn-ZnFe₂O₄, the removal profile after the addition of H₂O₂ is higher than before its addition. Also, the maximal value for removal was reached after shaking of 80 and 70 min for the biochar and biochar/Mn-ZnFe₂O₄, respectively. There was a 20 min shorter interval than that obtained for biochar/Mn-ZnFe₂O₄ without the addition of H₂O₂; it was 90 min. Interpretation of the higher degradation efficiency of the magnetic biochar/Mn-ZnFe₂O₄ than that of the biochar alone can be due to the electron-rich surface of the magnetic biochar by the existing quinone groups as well as the incorporated ferrite nanoparticles that facilitate the electronic exchange in the oxidation reaction of RhB by the generated hydroxide radicals from H₂O₂ [48].

In biochar alone, the proposed degradation reaction is a homogeneous catalytic degradation as can be represented by Equations (14)-(16). $\begin{matrix} (14) & H_{2} O_{2} + {hv}_{sunlight} = = = = = = 2 {HO}^{•} \end{matrix}$

The formed hydroxyl radicals are strong oxidizing agents which can easily oxidize RhB dye to produce an intermediate RhB^• which decolorizes the parent dye. $\begin{matrix} (15) & {HO}^{• .} + RhB = = = = = = = = {RhB}^{•} dye degradation \end{matrix}$

Reactions (14) and (15) can occur with biochar in the absence of ferrite nanoparticles and lead to a slight enhancement for dye removal.

The physicochemical properties of the prepared biochar are primarily dependent on the pyrolysis temperature, heating rate, retention time, and chemical pretreatment. The pyrolysis of agricultural waste at 600°C leads to the thermal degradation of hemicellulose, cellulose, and lignin into biosolid, syngas, and biooil. The main superficial functional groups on the surface of biochar (biosolid) are COOH, C=O, OH, and C-O which affect both the adsorption and catalytic degradation of RhB.

When using the biochar/Mn-ZnFe₂O₄ heterogeneous Fenton degradation method, the biochar support is deemed to increase the reactivity of the loaded ferrite nanoparticles. The ferrite nanoparticles had the intrinsic peroxidase-like activity that enables Mn-ZnFe₂O₄ to react with H₂O₂ to afford the hydroxyl radicals (HO^•) [49]. The sunlight illumination of the ferrite portion in Mn-ZnFe₂O₄ permits the UV radiation to react with nanoparticles and producing of electron- ( $e_{CB}$ -) hole ( $h_{VB}$ ) pairs as shown in $\begin{matrix} (16) & Mn - {ZnFe}_{2} O_{4} + {hv}_{sunlight} = = = = = = = = = = = Mn - {ZnFe}_{2} O_{4} (e_{CB} + h_{VB}) \end{matrix}$

The generated $e_{CB}$ can be captured by H₂O₂ to produce OH^•. Also, the photoinduced $h_{VB}$ can react with water or OH^- to furnish hydroxyl radicals: $\begin{matrix} (17) & h_{VB} + H_{2} O = = = = = = = = {OH}^{•} + H^{+} \\ h_{VB} + {OH}^{-} = = = = = = = = {OH}^{•} \end{matrix}$

Moreover, the Fe³⁺ ions on the surface of Mn-ZnFe₃O₄ nanoparticles can react with H₂O₂ to initiate the Fenton reaction to produce additional HO^• radicals as shown by $\begin{matrix} (18) & {Fe}^{3 +} + e^{-} = = = = = = = = = = {Fe}^{2 +} \\ {Fe}^{3 +} + H_{2} O_{2} = = = = = = = = = = {HOO}^{•} + H^{+} \\ {Fe}^{2 +} + H_{2} O_{2} = = = = = = = = = = {Fe}^{3 +} + {HO}^{•} + {OH}^{-} \end{matrix}$

The enormous amount of HO^• produced in the case of biochar/Mn-ZnFe₂O₄ heterogeneous Fenton degradation is expected to reveal a higher degree of dye degradation and thereby efficient and fast removal than adsorption removal as compared to the biochar alone which had limited sources of radical generation. Worth mentioning, as can be seen in Figure 13, the rate of dye removal was linear in the case of the biochar and biochar/Mn-ZnFe₂O₄ which indicates that the concentration of H₂O₂ was suitable for RhB decoloration and the self-quenching of the hydroxyl radicals (HO^•) was negligible.

3.4. Removal of RhB from Tap and Industrial Wastewater

The developed magnetic biochar was applied to the removal of RhB from industrial wastewater using 50 mg adsorbent, 20 mL spiked wastewater of RhB at concentrations of 5 and 10 mg·L^-1, and 0.03% ( $v / v$ ) H₂O₂. The results obtained are presented in Table 3. The removal ranged from 77 to 83% and from 95 to 98% for biochar/Mn-ZnFe₂O₄ and biochar/Mn-ZnFe₂O₄/H₂O₂, respectively. The precision represented as relative standard deviation (RSD, $n = 4$ ) at RhB concentration of 5.0 and 10 mg·L^-1 was 2.4 and 5.7% for biochar/Mn-ZnFe₂O₄ and 7.6 and 5.7% for biochar/Mn-ZnFe₂O₄/H₂O₂, respectively. The average removal was increased after adding H₂O₂ by a value of 16.5%. This confirmed that adsorption removal is significant and feasible, but the heterogeneous catalytic degradation was a powerful mechanism. Therefore, a combination of the two mechanisms can be preferred in samples which may counteract the removal by applying a single approach.

Table 3

Removal data of RhB dye from industrial wastewater using by adsorption onto P. harmala biochar/Mn-ZnFe₂O₄ ( $n = 4$ ) and by heterogeneous Fenton catalytic degradation by P. harmala biochar/Mn-ZnFe₂O₄/H₂O₂.

RhB added (mg·L^-1)	Adsorption removal biochar/Mn-ZnFe₂O₄			Heterogeneous Fenton degradation biochar/Mn-ZnFe₂O₄/H₂O₂
RhB added (mg·L^-1)	Remained concentration ( $m e a n \pm S D$ ) (mg·L^-1)	Removal (%)	RSD (%)	Remained concentration ( $m e a n \pm S D$ ) (mg·L^-1)	Removal (%)	RSD (%)
5.0	$0.85 \pm 0.02$	83	2.4	$0.091 \pm 0.007$	98	7.6
10.0	$2.28 \pm 0.13$	77	5.7	$0.53 \pm 0.03$	95	5.7

4. Comparison to Other RhB Removal Adsorbents

The adsorption capacity of P. harmala biochar/Mn-ZnFe2O4 was compared to other reported adsorbents in Table 4. The developed magnetic biochar had a higher capacity than other reported biochars [44, 46, 50] and other adsorbents [43, 45, 47] for the removal of RhB. Despite this, some other plant biomass such as Gmelina arborea leaf powder [51] showed extremely higher RhB adsorption capacity than the present material. The obtained capacity is primarily dependent on the method of preparation, pyrolysis temperature, heating rate, and retention time, so it can vary to some extent at other preparation conditions. Despite this, the capacity obtained is sufficient for RhB removal from natural waters. Table 5 compares the developed magnetic biochar’s catalytic degradation removal effectiveness to that of other catalysts. The removal was superior to Fe-bentonite/H₂O₂ [52] in terms of heterogeneous Fenton catalytic degradation and comparable other listed materials. Thus, the developed magnetic biochar proved to be an alternative and suitable for the adsorption and catalytic removal of RhB from a water sample.

Table 4

Adsorption capacity of RhB using different various reported materials.

Adsorbent	pH	Maximum capacity (mg·g^-1)	Ref.
Biochar/Mn-ZnFe₂O₄	4.0	34.5	This work
Bamboo biochar	—	85.8	[29]
Fly ash	—	2.33	[43]
Fe-N-co-biochar	—	12.41	[44]
Surfactant-zeolite	8.0	2.03	[45]
Municipal waste biochar	8.7<	20.30	[46]
ZnFe₂O₄ nanocomposite	4.4	6.02	[47]
Musa biochar	4.0	6.87-7.00	[50]
Gmelina arborea leaf	3.0	1000	[51]

Table 5

Catalytic degradation removal of RhB using P. harmala biochar/Mn-ZnFe₂O₄ compared to other materials.

Catalyst	pH	Time (min)	RhB (mg·L^-1)	Removal (%)	Ref.
P. harmala biochar/Mn-ZnFe₂O₄/H₂O₂ (0.03% $v / v$ )	4.0	60	100	99	This work
Black liquor-sludge biochar/H₂O₂ (2 mM)	3.0	10	50	100	[8]
Fe-bentonite/H₂O₂ (12 mM)	4.2	180	80	93	[52]
Fe/MCM-41/H₂O₂ (20 mmol)	4.0	30	100	100	[53]
Tin oxide/montmorillonite	—	120	50	100	[54]
Snake fruit peel biochar/H₂O₂ (0.5%)	7.0	—	—	99.9	[55]

5. Conclusions

The magnetic biochar was prepared by a one-step impregnation pyrolysis under nitrogen at 600°C of P. harmala plant powder pretreated with MnCl₂, ZnCl₂, and FeCl₃. The FT-IR analysis showed that the characteristic bands for metal oxide appeared at 543 and 591 cm^-1. The XRD pattern showed the crystalline structure of Mn-ZnFe₂O₄ embedded in the biochar matrix. The microstructure of the prepared magnetic biochar had higher porosity. The adsorption isotherm was best fitted with the Langmuir model ( $R^{2} = 0.981$ ) and Tempkin ( $R^{2} = 0.992$ ), and the separation factor $R_{L}$ ranged from 0.02 to 0.48 confirming a favorable chemical adsorption process. The monolayer adsorption capacity of 34.5 mg·g^-1 was obtained at pH 4.0, a shaking time of 90 min, and an adsorbent dosage of 3.2 g·L^-1. The adsorption kinetics adopted pseudo-second-order kinetic and diffusion-controlled models confirming that diffusion is a rate-limiting step. Heterogeneous Fenton degradation using 0.03% ( $v / v$ ) H₂O₂ showed enhancement of recovery from 33 to 40% and from 84 to 99% for biochar alone and biochar/Mn-ZnFe₂O₄, respectively. The wastewater application showed RhB removal ranged from 77 to 88% and 95 to 98% for biochar/Mn-ZnFe₂O₄ and biochar/Mn-ZnFe₂O₄/H₂O₂ methods, respectively.

Footnotes

Data Availability

The authors can provide the available data on request after the publication of this article, which will be considered by the corresponding author.

Conflicts of Interest

The authors declare no potential conflict of interest.

Authors’ Contributions

Abdulrahman F. Alharbi was responsible for the material characterization and reviewed and edited the manuscript. Abdullah A. Alotaibi was responsible for the conceptualization, resources, and supervision and reviewed and edited the manuscript. Hassan E. M. Gomaa was responsible for the data curation and resources and reviewed and edited the manuscript. Abdulaziz A. M. Abahussain reviewed and edited the manuscript. Sami M. Abdel Azeem was responsible for the conceptualization, data curation, project administration, and funding acquisition and wrote, reviewed, and edited the manuscript.

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at Shaqra University for funding this research work through the project number SU-ANN-202222.

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Magnetic Biochar by One-Step Impregnation Pyrolysis of Peganum harmala L. for Removal of Rhodamine B

Abstract

1. Introduction

2. Experimental

2.1. Instrumentation

2.2. Reagents

2.3. Preparation of P. harmala Biochar/Mn-ZnFe2O4

2.4. Adsorption Procedure

2.5. Catalytic Degradation Procedure

3. Results and Discussion

3.1. Characterization of Biochar/Mn-ZnFe2O4

3.2. Adsorption Removal

3.2.1. Effect of pH

3.2.2. Adsorbent Dose

3.2.3. Initial RhB Concentration

3.2.4. Adsorption Removal Kinetics

3.2.5. Adsorption Equilibrium

3.3. Catalytic Removal of RhB

3.4. Removal of RhB from Tap and Industrial Wastewater

4. Comparison to Other RhB Removal Adsorbents

5. Conclusions

Footnotes

Data Availability

Conflicts of Interest

Authors’ Contributions

Acknowledgments

References

2.3. Preparation of P. harmala Biochar/Mn-ZnFe₂O₄

3.1. Characterization of Biochar/Mn-ZnFe₂O₄