Preparation and Characterization of Unactivated, Activated, and γ-Fe₂O₃ Nanoparticle-Functionalized Biochar from Rice Husk via Pyrolysis for Dyes Removal in Aqueous Samples: Comparison, Performance, and Mechanism

Abstract

This study aimed at preparing three types of biochar derived from rice husk via pyrolysis, including unactivated biochar, biochar chemically activated after with H₃PO₄, and biochar impregnated with γ-Fe₂O₃ nanoparticles. These materials were subsequently characterized using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analysis, which revealed favorable textural properties, such as an increased surface area and porosity, as well as the presence of functional groups that facilitate the adsorption of methylene blue and malachite green in aqueous solutions. Several factors that affect the adsorption capacity, including the type of material, pH effect, and adsorbent dosage, were evaluated and optimized. The adsorption behavior was analyzed using isotherm and kinetic models to better understand the mechanisms involved. Under optimal conditions, biochar@γ-Fe₂O₃ NPs emerged as the most effective material due to its high surface area, functionalized surface, and magnetic properties, allowing easy water recovery without the need for complex instrumentation. Among the kinetic models evaluated, the pseudo-second-order model exhibited the highest linear regression coefficient (R² = 0.99), supporting a chemisorption process driven by strong interactions and stable chemical bond formation between the adsorbate and the adsorbent, while equilibrium data fit well with the Sips isotherm model, indicating a combination of monolayer and multilayer adsorption mechanisms. This magnetic biochar achieved removal efficiencies of 97% for methylene blue and 95% for malachite green, demonstrating a high performance and reusability over four cycles. Moreover, a possible adsorption mechanism of MB on the magnetic biochar was proposed to explain the interaction between the dye and the adsorbent surface. Thus, this work demonstrates that magnetic biochar is a sustainable and cost-effective adsorbent for wastewater treatment, integrating circular economy principles by transforming rice husk into a high-value material. The incorporation of γ-Fe₂O₃ nanoparticles enhances adsorption while enabling magnetic recovery, providing an eco-friendly and scalable solution for dye removal.

1. Introduction

Reactive, acid, and basic dyes are commonly used in the industry for various applications such as dyeing textiles, paper products, cosmetics, as well as in the food industry. In this way, the textile industry consumes large amounts of water, and a major source of water pollution is colored wastewater [1]. These dyes are soluble in both oil and water, and contain chromophores responsible for vibrant and fluorescent colors [2]. However, approximately 10–15% of these dyes remain in wastewater after treatment, contaminating other water sources due to their resistance to degradation. In addition, dyes reduce light penetration in water, hindering photosynthesis and negatively impacting the water quality and aquatic life. Among dyes, methylene blue (MB) is a water-soluble synthetic dye that can lead to side effects such as gastrointestinal discomfort and rare allergic reactions [3]. Moreover, malachite green (MG), another cationic dye commonly used in aquaculture, has been linked to reproductive, immune, genotoxic, and carcinogenic effects [4]. In this way, prolonged exposure to or ingestion of these dyes can have negative effects on the ecosystem [5].

Dyed wastewater treatment involves several stages, including pretreatment to remove large solids, followed by primary treatment using physical methods such as sedimentation and flotation, and secondary treatment employing chemical and biological processes. Advanced technologies such as adsorption, reverse osmosis, photocatalytic degradation, and membrane filtration are employed for further purification, followed by disinfection that ensures compliance with environmental regulations [6,7]. Among these techniques, surface adsorption is particularly effective for dye removal due to its cost efficiency and reusability [8]. The use of natural adsorbents derived from a lignocellulosic biomass, such as agricultural waste, represents a sustainable strategy aligned with circular economy principles and significantly contributing to the mitigation of water pollution [9]. In this way, the abundance of lignocellulosic wastes, such as sugarcane bagasse, corn stover, rice straw, and husks, along with less commonly utilized residues such as wheat straw and post-harvest byproducts [10], as well as non-lignocellulosic wastes, poses significant environmental and economic issues due to their low valorization [11]. Although some are used for energy generation, many are burned or stored, releasing pollutants. Their low energy density and ash formation hinder their use as solid fuels, causing corrosion in boilers, which highlights the need to convert these wastes into valuable materials, such as adsorbents. Garba et al. [12] used cow dung and rice husk ash, a byproduct of rice husks generated through combustion in cyclone furnaces, commonly used as a fuel source in rice drying operations, for the removal of glyphosate and aminomethylphosphonic acid (AMPA) from aqueous samples. The results showed that AMPA can be effectively removed using both bioadsorbents, providing an alternative method for recycling and waste management.

The conversion of lignocellulosic biomass waste into valuable products such as biochar is crucial to effectively adsorbs dyes due to its large surface area and functional groups [13]. In this context, the adsorption efficiency of biochar depends on pyrolysis conditions. Higher temperatures (300–700 °C) enhance the surface area, porosity, and aromatic carbon content, while lower temperatures (<300 °C) favor aliphatic structures suitable for pollutant removal [14]. Lower heating rates preserve functional groups, whereas higher rates promote bio-oil and syngas production, reducing available adsorption sites. Furthermore, feedstock composition plays a key role in the adsorption performance [15]. Therefore, it is important to study these conditions under specific parameters and compare the results with previously reported studies on the removal of the target analytes.

Biochar refers to any organic residue derived from biological materials, produced through gasification or pyrolysis between 300 and 800 °C in the absence of oxygen [16]. Key factors for efficient and sustainable biochar production include raw material selection, thermal process choice, optimization, and its effectiveness in water contaminant removal [17]. A significant byproduct of rice production is rice husk, along with rice straw and rice bran. Rice husk biochar represents 20% of the total weight of rice, which can be recycled in rice fields as a soil amendment or used in environmental remediation, providing an effective solution for rice waste management while minimizing greenhouse gas emissions [18]. In this way, biochar obtained through pyrolysis emerges as a sustainable alternative for the removal of organic and inorganic pollutants, overcoming limitations associated with conventional adsorbents such as MOFs, activated carbon, and zeolites [19]. Unlike these materials, pyrolysis-derived biochar is produced from waste without releasing toxic substances during the adsorption process, which is a notable drawback of other adsorbents. Likewise, its adsorption capacity can be tailored by adjusting pyrolysis parameters such as the temperature (300–800 °C) and residence time [20]. This versatility, coupled with its feasibility for large-scale production, positions pyrolysis biochar as a practical solution for water remediation applications [21].

However, a main limitation of biochar is the difficulty of separating it after contaminant removal, often requiring costly methods such as centrifugation or filtration. Studies reported in the literature indicate that the incorporation of magnetic materials into biochar can enhance biochar recovery and regeneration [22]. Thus, this approach not only enhances the operational efficiency of biochar in wastewater treatment, but also offers a more sustainable and cost-effective solution for its reuse [1]. In this way, Yi et al. [23] developed magnetic biochar from rice straw, which demonstrated a high removal efficiency of 97% for crystal violet. Kinetic analysis, conducted using both pseudo-first-order and pseudo-second-order models, as well as, material characterization like XPS, Raman spectroscopy, and FTIR, revealed that the main mechanisms of sorption were hydrogen bonding and electrostatic interactions. In addition, the adsorption efficiency of the biochar remained at 71.91% after three adsorption–regeneration cycles. In addition, Osman et al. [24] used a magnetic composite biochar, produced from biomass waste (olive pomace leaves) and plastic waste (PET bottles). The extraction capacity was 256.41 mg/g for removing crystal violet dye from aqueous solutions, and the high regeneration of the biochar was demonstrated with 92.4% recyclability after five adsorption–desorption cycles.

On the other hand, the chemical activation of biochar can potentially improve its physicochemical properties, including the porosity, specific surface area, and adsorption capacity [25]. This activation can be performed either before (pre-pyrolysis) or after pyrolysis (post-pyrolysis), often employing acids and alkalis as activating agents [26]. Among these, phosphoric acid is considered a promising option due to its potential to minimize environmental impacts and facilitate efficient recovery, offering a viable pathway for the preparation of high-performance activated biochar [27]. Lee et al. [28] pretreated palm kernel shells and coconut shells with H₃PO₄ to prepare microporous and mesoporous bioadsorbents, resulting in biochar with a significantly higher yield, larger surface area, high thermal stability, and more functional groups for iodine adsorption compared to untreated biochar. In any case, the efficiency of sorption depends on factors such as the chemical composition of the sorbent, its porous structure, and interactions between the sorbent and the dye, including hydrogen bonding, electrostatic forces, van der Waals interactions, and hydrophobic effects [29].

The rice husk used in this study was chosen because it is a widely available agricultural residue produced during rice milling. As rice is one of the most consumed staple foods worldwide, its processing generates large amounts of husk, which represents approximately 20% of the rice grain weight [11]. Composed mainly of 50% cellulose, 25–30% lignin, 15–20% silica, and 10–15% moisture, rice husk is often burned or discarded, contributing to environmental pollution [30]. Converting it into biochar offers a sustainable alternative that reduces waste and enhances pollutant removal.

Thus, the main objective of this work is to develop three distinct types of biochar from rice husk through pyrolysis: a non-activated biochar, one activated with H₃PO₄, and another impregnated with γ-Fe₂O₃ nanoparticles, for the removal of MB and MG dyes. Moreover, the immobilization of maghemite nanoparticles on biochar offers advantages over other materials, such as iron oxides [31], including superparamagnetic properties, high stability, a large surface area for adsorption, efficient magnetic separation, recyclability, and the combination of biochar and nanoparticles properties. Likewise, this work offers a comprehensive comparison of the three sorbents under identical pyrolysis operating conditions, enabling a deeper understanding of the efficient mechanism for removing dyes from water samples. Compared to conventional adsorbents such as activated carbon, which require energy-intensive production processes, biochar derived from pyrolysis offers a low-cost and environmentally friendly alternative that minimizes waste generation and maximizes resource utilization. This study contributes to sustainability by transforming rice husk into a reusable and efficient adsorbent for wastewater treatment, while the incorporation of γ-Fe₂O₃ nanoparticles enhances adsorption and allows magnetic recovery, promoting effective water management and resource conservation.

2. Materials and Methods

2.1. Production of Biochar from Rice Husk

The rice husk used was obtained from local rice mills in Riobamba, Ecuador, ensuring consistency in the feedstock source. The biomass was randomly sampled, collecting 0.5 kg per day from different sites across the city over one month to ensure a representative selection. Since its composition varies slightly due to cultivation and milling conditions, the biomass was used in its natural form, only washed to remove impurities, and then ground into a fine powder before pyrolysis, following previous studies for biochar production [30].

Subsequently, the powdered material was pyrolyzed in a GSH-5.0 reactor (Weihai Global Chemical Machinery, Weihai, China) at 500 °C with a heating rate of 10 °C min⁻¹. The process was conducted in triplicate, and the resulting biochar, initially obtained as large particles, was milled to reduce its size. Finally, a sieving step ensured a uniform particle size for further experiments.

2.2. Preparation of Activated Biochar from Rice Husk

The chemical activation process with phosphoric acid (H₃PO₄) was conducted as follows: 15 g of biochar was mixed with 105 mL of a 20% H₃PO₄ solution, maintaining a 1:7 biochar-to-activator ratio, and allowed to interact for 16 h to ensure proper impregnation. The mixture was subsequently heated at 110 °C for 8 h to enhance the activation process. Following this, the biochar was extensively washed with distilled water and neutralized using a 0.1 mol/L NaOH solution. Finally, the activated biochar was dried at 90 °C overnight to remove any residual moisture.

2.3. Obtaining Biochar@γ-Fe₂O₃ Nanoparticles

The γ-Fe₂O₃ nanoparticles used in this study were sourced from Luoyang Tongrun (Luoyang, China) and provided with a reported purity of 99.5% and a particle size of 30 nm. The impregnation of biochar with γ-Fe₂O₃ NPs was performed by adapting procedures described in previous reports [32]. A total of 0.5 g of γ-Fe₂O₃ NPs was added to 100 mL of deionized water, followed by the addition of 2 g of biochar, which was diluted in 100 mL of water. The mixture was then combined with the previous dispersion and 80 mL of a 3% glutaraldehyde solution. The resulting dispersion was sonicated for 15 min and subsequently stirred for 24 h at 25 °C. The magnetic biochar was separated from the solution using an external magnet, as shown in Figure 1.

2.4. Characterization of the Prepared Biochars

The surface characteristics and elemental composition of the biochar samples were examined with a TESCAN MIRA 03 scanning electron microscope (SEM) (TESCAN model MIRA 03, Warrendale, PA, USA) paired with an energy-dispersive spectrometer (EDS)—specifically, a Bruker X-Flash 6-30 offering 123 eV resolution. For SEM analysis, the samples were fixed onto stubs using conductive, double-sided carbon tape. Functional groups present in the biochars, along with any chemical surface modifications and nanoparticle impregnation, were identified via Fourier-transform infrared (FTIR) spectroscopy using a JASCO FT/IR-4100 spectrometer (PerkinElmer, Waltham, MA, USA). Additionally, nitrogen adsorption–desorption isotherms were recorded at 77 K with a Micromeritics GEMINI VII gas adsorption analyzer after the samples had been out-gassed overnight at 473 K. The isotherm data were subsequently analyzed using the Brunauer–Emmett–Teller (BET) model to determine the specific surface area, while the two-dimensional non-local density functional theory (2D-NLDFT) model was applied to assess the pore volume and pore size distribution.

2.5. Batch Experiments

The adsorption capacity of the biochar for the selected dyes was evaluated in aqueous solutions under batch conditions at 25 °C. Typically, 3 mg of each biochar was added per mL of water into a volumetric flask containing a dye solution under continuous stirring. The concentration of the dye remaining in the supernatant after extraction was determined using UV–Vis spectrophotometry. Moreover, adsorption tests were performed at varying pH values of the dye solution (pH = 3, 5, 7, and 10), ensuring that the biochar remained in contact with the solution until equilibrium conditions. The removal capacity (%R) and equilibrium adsorption capacity Qe (mg/g) were calculated using the following equations:

$$\%\mathrm{R}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{i}}}}·100$$

(1)

$${\mathrm{Q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{m}}}·\mathrm{V}$$

(2)

where C_i (mg/L) is the initial dye concentration, C_e (mg/L) is the concentration remaining at equilibrium, V represents the solution volume in L, and m denotes the mass of the biochar used in g. All tests were conducted three times, and the average result was recorded.

2.6. Adsorption Kinetics and Isotherm

For the kinetic experiments, the concentration of the remaining dye in the solution was monitored at specific time intervals under batch conditions, with a standard dye solution of 20 mg/L, maintained at pH 7 and room temperature. The adsorption data were analyzed using non-linear pseudo-first-order (PFO) and pseudo-second-order (PSO) models, whose equations are presented below. Additionally, the adsorption process was evaluated considering the classical intraparticle diffusion model to gain further insight into the adsorption mechanism.

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{Q}}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}}\right)$$

(3)

$${\mathrm{Q}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{k}}_2{\mathrm{Q}}_{\mathrm{e}}2\mathrm{t}}{1+{\mathrm{k}}_2{\mathrm{Q}}_{\mathrm{e}}\mathrm{t}}}$$

(4)

Q_t (mg/g) is the adsorption capacity at different time intervals, t (min) is the adsorption time, and k₁ (min⁻¹) and k₂ (g/mg min) are the rate constants of the PFO and PSO models, respectively.

To evaluate the adsorption behavior at equilibrium, adsorption isotherms were determined using different concentrations of MB (5–50 mg/L) over 24 h at 25 °C and pH 7. The experimental data were fitted to the Langmuir, Freundlich, Temkin, and Sips models, as described by the following equations [33]:

$${\mathrm{Q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{C}}_{\mathrm{e}}}}$$

(5)

$${\mathrm{Q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{1/{\mathrm{n}}_{\mathrm{F}}}$$

(6)

$${\mathrm{Q}}_{\mathrm{e}}={\displaystyle \frac{\mathrm{R}\mathrm{T}}{\mathrm{B}}}\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{t}}{\mathrm{C}}_{\mathrm{e}}$$

(7)

$${\mathrm{Q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{s}}{{\mathrm{K}}_{\mathrm{s}}{\mathrm{C}}_{\mathrm{e}}}^{{\mathrm{n}}_{\mathrm{s}}}}{1+{{\mathrm{K}}_{\mathrm{s}}{\mathrm{C}}_{\mathrm{e}}}^{{\mathrm{n}}_{\mathrm{s}}}}}$$

(8)

where Q_m and Q_s (mg/g) represent the maximum adsorption capacities derived from the Langmuir and Sips models, respectively. K_L (L/mg), K_F (mg^1−1/n L^1/n/g), K_t (L/mg), and Ks (L/mg) are the constants for the Langmuir, Freundlich, Temkin, and Sips isotherm models, respectively. B is the Temkin constant, which is related to the heat of adsorption (J/mol), and T denotes the absolute temperature (K). n_F and n_s are the Freundlich and Sips model exponents, respectively, which describe the degree of non-linearity in the adsorption process.

3. Results

3.1. Biochar Yield (%)

Figure 2 shows that increasing the pyrolysis temperatures significantly reduces the biochar yield due to the enhanced decomposition of cellulose, hemicellulose, and lignin, as moisture and volatiles compounds are released. This process produces a stable carbonaceous material with ideal textural properties for pollutant adsorption [34]. Furthermore, compared to activated carbon, biochar offers lower production costs and relies on widely available resources, making it a sustainable alternative for environmental applications. As can be noted, the slightly different yields at 400 °C, 500 °C, and 700 °C can be explained by the stabilization of the decomposition process, where most easily degradable components have already been volatilized, resulting in minimal further mass loss. This trend is consistent with previous studies [11,35], which report that at temperatures above 400 °C, more solid material is converted into biogas and bio-oil, leading to a reduction in the biochar yield. Thus, biochar produced at 500 °C was selected as the precursor material due to its optimal yield (39%) and lower energy consumption compared to the biochar obtained at 700 °C, which had a yield of 35%.

3.2. Characterization of Unactivated Biochar, Acid-Activated Biochar, and Biochar@γ-Fe₂O₃ NPs

The pyrolysis conditions significantly affect the structural and physicochemical properties of biochar, including its elemental composition, functional groups, particle size, and surface area [36]. Thus, an analysis of the chemical composition of the unactivated biochar was conducted to evaluate its potential influence on adsorption properties, as these nutrients can affect the surface charge, porosity, and overall efficiency of the biochar as an adsorbent.

Table 1. Chemical composition of rice husk biochar.

pH	C (%)	N (%)	P (%)	S (%)	K2O (%)	CaO (%)	MgO (%)
9.0	29.6	0.2	2.9	2.5	2.6	4.0	0.6

shows that rice husk biochar is rich in carbon, with an average content above 30% through the thermal decomposition of biomass. Nitrogen was almost undetectable in the ash, but a small amount remained in the biochar, which is consistent with previous studies that suggest that high temperatures increased the surface area, carbon content, and pH, while reducing nitrogen [37]. Low concentrations of oxide compounds were also found under the applied pyrolysis conditions.

Figure 3A,B shows that the unactivated biochar obtained after the pyrolysis process exhibits a cylindrical morphology with the presence of surface pores, a characteristic attributed to the formation of a porous structure. This is due to the degradation of lignin and cellulose during pyrolysis, driven by thermal decomposition, structural changes, gas release, and chemical transformations, which contribute to the generation of a material significantly more porous than the original biomass, enhancing its specific surface area and effectiveness as a sorbent for pollutants removal [34]. Following chemical activation (Figure 3C), the biochar exhibits a rough, fibrous, cylindrical surface with a more defined and organized structure, along with a higher number of pores on its surface. Figure 3D,E shows the successful impregnation of γ-Fe₂O₃ nanoparticles on the surface of the biochar. Moreover, the deposition of the nanomaterial was confirmed by EDS analysis (Figure 3F), where peaks corresponding to iron were observed.

As shown in

Table 2. Textural properties of samples.

Sample	BET Surface Area (m2/g)	Total Pore Volume (cm3/g)	Average Pore Size DBJH (nm)
RHB300	17.54	0.012	4.31
RHB500	88.21	0.029	5.04
Activated RHB	207. 44	0.047	6.84

, the BET surface area of the rice husk biochar (RHB) increased with the pyrolysis temperature, from 17.54 m² g⁻¹ at 300 °C to 88.21 m² g⁻¹ at 500 °C. Furthermore, the mean pore size and micropore volume of RHB also increased with increasing the pyrolysis temperature, while acid activation further enhanced both the surface area and porosity. These findings align with similar studies, highlighting the potential of RHB for environmental remediation [38].

The materials, i.e., unactivated biochar, activated biochar with H₃PO₄, and biochar@γ-Fe₂O₃ NPs, were characterized by FTIR spectroscopy to examine the functional groups and identify structural changes in the biochar under the studied pyrolysis conditions (Figure 4). In this way, all biochars exhibited notable peaks between 1100 and 1000 cm⁻¹, attributed to Si–O–Si asymmetric stretching, and at 788 cm⁻¹ (Si–H), which correspond to the functional groups of silica. Silica is a major component in the chemical structure of rice-derived materials and remains present even at high temperatures [39], which match well with silica detected in the EDS spectrum. In addition, the bands in the range of 1030–1110 cm⁻¹ can also be attributed to the C–O–C stretching of cellulose and hemicellulose [40,41]. Chemically activated biochar showed significant changes compared to the original, with new absorption bands in the 1450–1600 cm⁻¹ range corresponding to aromatic C=C and C=O stretching, suggesting an increase in the presence of phenolic and carboxylic compounds derived from lignin, as well as aromatic C–H bending at 870–880 cm⁻¹. Furthermore, other absorption bands appeared around 1750 cm⁻¹ (ester groups), which could be attributed to the interaction between cellulose and lignin [42,43]. For the biochar@γ-Fe₂O₃ NPs, intense absorption bands at 650 and 800 cm⁻¹ were observed, attributed to Fe–O bond stretching vibrations, which corroborates the successful deposition of the nanomaterial onto the biochar. These positive observations align with previously reported works in the literature [32,44]. Moreover, significant peaks were identified at 1580 cm⁻¹ (C=O) and in the 3320–3250 cm⁻¹ range, corresponding to –OH and –NH groups. Likewise, the presence of a C–O stretching peak between 1300 and 1000 cm⁻¹ could be attributed to the covalent bonding of ester or ether groups with γ-Fe₂O₃ NPs. In any case, the -OH, NH, carbonyl, and carboxyl groups observed are viable sorption sites for the interaction between the dyes and biochar.

3.3. Optimization of Extraction Parameters

3.3.1. Selection of the Biochar for Removal Dyes

Unactivated biochar, activated biochar, and biochar@γ-Fe₂O₃ NPs were investigated to identify the most suitable adsorbent for dye removal (Figure 5). MB was chosen as a model contaminant for adsorption experiments under batch conditions. A standard solution containing 10 mg L⁻¹ of MB at pH 7 was used to evaluate the extraction efficiency for 30 min. The experiments were performed in triplicate and analyzed using a spectrophotometer after adsorption at a wavelength of 664 nm. As expected, biochar@γ-Fe₂O₃ NPs exhibited a higher extraction efficiency for MB, which can be attributed to the enhanced interactions between the dye from the aqueous solution with the nanoparticle-impregnated adsorbent. Mittal et al. [45] report that Fe₃O₄ nanoparticles’ incorporation increased the surface area, pore volume, and binding sites, enhancing the dye adsorption capacity, which supports the results obtained. On the other hand, activated RHB shows higher adsorption compared to unactivated biochar, probably due to the larger number of oxygen-containing functional groups on its surface, providing more adsorption sites for MB through interactions such as van der Waals forces, which benefit the adsorption reactions [46]. Thus, biochar@γ-Fe₂O₃ NPs were selected for further studies, also due to their easy magnetic separation from the solution using an external magnet without the need for complex instrumentation, providing a cost-effective solution for its reuse.

3.3.2. Effect of pH and Biochar@γ-Fe₂O₃ NPs’ Dosage on Dye Adsorption Capacity

Taking into account the impact of pH on the adsorption capacity, which can alter the surface charge of the material during the removal of MB and MG and the chemical form of ionizable organic compounds. The adsorption capacity was evaluated at pH levels of 3, 5, 7, and 10. Figure 6A shows relatively low adsorption capacities in strongly acidic environments for MB. At pH 3, the MB sorption efficiency was 82.92%, which gradually increased at higher pH values. Between pH 5 and 10, the extraction efficiency remained unchanged (97%), which is consistent with similar previous studies [47]. MB is a cationic dye, and its adsorption is favored at higher pH levels. This is due to the increased concentration of OH⁻ ions at an elevated pH, which enhances the negative charge on the biochar surface, which facilitates strong electrostatic interactions with the positively charged MB molecules, promoting efficient adsorption. Therefore, this adsorbent shows potential for the removal of MB from real water samples, typically found within a pH range of 6.5–7.5. On the other hand, for MG, a higher extraction efficiency (95%) was achieved at pH 5, consistent with results from similar studies [48], where the maximum dye removal capacity was observed in the pH range of 4–6. MG, being a cationic dye, is influenced by the repulsion from the positive charge on the adsorbent surface. As the pH increases, the carboxyl and hydroxyl groups on the surface of the biochar become deprotonated, creating a negatively charged environment that attracts the positively charged dye molecules. However, at even higher pH levels, a decrease in the adsorption was observed, which can be attributed to the reduction in the ionization potential of the functional groups on the biochar at a higher pH, leading to a lower capacity for dye removal.

Furthermore, the removal efficiencies of biochar@γ-Fe₂O₃ NPs for MB and MG were investigated using an initial dye concentration of 10 mg L⁻¹ at dosages ranging from 3 to 5 mg per mL, as shown in Figure 6B. Consequently, 3 mg of biochar was determined to be the optimal adsorbent dosage, which provided more surface sites for adsorption as further increases did not significantly improve the adsorption performance for either dye. Similar results were found in a study using RHB [49] and pineapple crown leaf-derived biochar with magnetite (Fe₃O₄) [50] for cationic dye removal, with an optimal dosage of 6 mg/mL.

3.4. Adsorption Kinetics

The adsorption rate is a critical parameter for dye removal, which can be evaluated by fitting it to kinetic models. In this way, three models were used: (i) the pseudo-first-order model, which describes adsorption based on the concentration of adsorbate on the adsorbent surface; (ii) the pseudo-second-order model, which suggests a chemical adsorption process; and (iii) the intraparticle diffusion model, which focuses on the diffusion of the adsorbate from the solution into the adsorbent pores. Thus, kinetic studies were performed by measuring the remaining concentration of the 20 mg/L MB solution in contact with biochar@γ-Fe₂O₃ NPs at set time intervals (0–140 min). The results showed that the adsorption data fit well to a pseudo-second-order kinetic model (

Table 3. Kinetics models used for an MB concentration = 20 mg/L at 25 °C and pH = 7.

Parameter	Pseudo-First-Order			Pseudo-Second-Order			Intra-Particle Diffusion
	Qe (mg/g)	K1	R2	Qe (mg/g)	K2	R2	Kipd	C	R2
Biochar@γ-Fe2O3 NPs	6.01	0.091	0.96	6.67	0.018	0.99	0.51	1.39	0.82

), with a correlation coefficient of 0.99 for MB, suggesting that the rate-limiting step may involve a chemical adsorption process.

3.5. Adsorption Isotherm Modeling

Adsorption data at different concentrations were fitted to several models: (i) the Langmuir isotherm, which describes monolayer adsorption on homogeneous sites without interactions between molecules; (ii) the Freundlich model, which explains heterogeneous adsorption on surfaces with varying site affinities; (iii) the Temkin model, which considers a uniform distribution of adsorption heat, with energy decreasing as the surface coverage increases; and (iv) the Sips isotherm, which combines features of the Langmuir and Freundlich models, accounting for adsorption on both homogeneous and heterogeneous sites.

Table 4. Isotherm models used for biochar@γ-Fe₂O₃ NPs at 120 min and 298 K.

Adsorption Isotherm	Parameters	Values
Langmuir isotherm	R2	0.93
	KL	4.00
	Qm	11.45
Freundlich isotherm	R2	0.40
	KF	7.10
	nF	6.02
Temkin isotherm	R2	0.78
	B	1.52
	Kt	139.06
Sips isotherm	R2	0.99
	Qs	10.37
	Ks	19.62
	n	2.46

shows that the adsorption data fit well with the Sips model (R² = 0.99) for MB adsorption using biochar γ-Fe₂O₃ NPs, which results from the coexistence of multilayer adsorption on a heterogeneous surface at low sorbate concentrations, as described by the Freundlich model, and monolayer adsorption on a more uniform surface at higher concentrations, following the Langmuir model. This behavior is consistent with the material properties, where variations in the surface composition and functional groups create sites with different adsorption affinities [34,51].

3.6. Adsorption Mechanism

The adsorption of MB and MG by magnetic biochar relies of both physical adsorption and chemisorption processes, including electrostatic interactions, hydrogen bonding, π–π interactions, n–π interactions, cation exchange, surface adsorption, pore filling, and interactions with functional groups [38,52]. FTIR analysis revealed functional groups like –OH, C=O, and –COOH on the material, facilitating multiple simultaneous interactions. Consequently, a mechanism for the adsorption of MB and MG onto rice husk-derived magnetic biochar was proposed, as shown in Figure 7.

At a neutral or alkaline pH, several oxygenated functional groups like –OH and –COOH ionize and acquire a negative charge, favoring electrostatic interactions with the cationic dyes. These interactions are further enhanced by the presence of phosphates in the biochar. Moreover, these oxygenated functional groups can form hydrogen bonds with nitrogen atoms in the dye molecules [52]. A cation exchange also contributes significantly to the adsorption, attributed to the presence of inorganic minerals such as Ca²⁺, Mg²⁺, and K⁺ identified in the biochar composition [53]. The presence of surface deposits, observed in SEM micrographs, suggests additional interactions through surface adsorption and pore filling. On the other hand, π–π interactions were also observed, attributed to the π–electron system of the biochar and the aromatic groups of the dye molecules. The presence of Si–O–Si groups in the biochar enables n–π interactions with the aromatic structure of the dyes, while oxygen-containing functional groups on the biochar act as electron donors, interacting with the electron-accepting aromatic rings of the dyes [54]. Finally, the Fe-O structures present in the biochar serve as active adsorption sites, further enhancing the adsorption capacity and overall performance [55].

3.7. Reusability of the Adsorbent

The ability to reuse adsorbents is essential for their economic and environmental sustainability. To better evaluate the efficiency of the obtained biochar@γ-Fe₂O₃ NPs for the extraction of MB, recyclability studies were carried out over five consecutive cycles. Between each cycle, the adsorbent was washed with methanol (5 mL) and water (5 mL) to facilitate regeneration, then dried and reused for subsequent adsorption experiments. Figure 8 shows that regenerated biochar@γ-Fe₂O₃ NPs can be reused for at least four adsorption–desorption cycles while maintaining an effectiveness above 90%, demonstrating high stability and an excellent performance from the combination of biochar with nanoparticles. However, the decline in the adsorption efficiency observed after the fifth cycle can be attributed to the accumulation of MB molecules on the biochar surface and the incomplete recovery of active sites during desorption.

3.8. Comparison with Other Metholodogies

Table 5. Comparison of the performance of biomass-derived biochar adsorbents on the removal of methylene blue and malachite green.

Adsorbent	Pyrolysis Conditions 1	Dye	Removal Efficiency (%)	Reusability	Reference
Biochar from oak forests infused with Fe2O3 NPs and humic acid	T = 300–600 °C, RT: 3 h	Methylene blue	97	—	[56]
Biochar from leaf and stem of Lantana camara L.	T = 600 °C	Methylene blue	69.1	—	[57]
Untreated biochar from palm oil empty fruit branches and biochar activated with KOH	T = 450 °C RT = 90 min	Methylene blue	<90	4 cycles	[58]
Nanobiochar from rice husk	T = 600 °C RT = 2 h	Malachite green	87.5	—	[48]
Sludge biochar modified with ZnCl2	T = 700–900 °C, HR = 5 °C/min, RT: 1–2 h	Malachite green	99.1	2 cycles	[59]
Mg-Al/biochar from rice husk	-	Malachite green	66.7	1 cycle	[60]
Rice husk-derived biochar with γ-Fe2O3 NPs	T = 500 °C, HR = 10 °C/min.	Malachite green and methylene blue	95–97	4 cycles	This work

¹ T: Temperature, RT: Residence Time, and HR: Heating Rate.

compares the performance of the adsorbent developed in this work with other adsorbents derived from biomass pyrolysis reported in the literature for the removal of cationic dyes. It should be noted that the proposed magnetic biochar shows satisfactory removal efficiencies, attributed to its high surface area, reactivity, adsorption sites, and magnetic properties. This magnetic biochar is a low-cost, sustainable material that can be easily separated from solutions using an external magnet, even with low-density biochar particles. Furthermore, it maintains a high removal capacity over four reuse cycles compared to other studies. In contrast, activated biochar requires additional steps, adding complexity and limiting its practical application. Moreover, the magnetic biochar does not require acid activation, a process that not only involves hazardous chemicals, but also complicates large-scale production. As result, this material is contaminant-free and can potentially treat large volumes of polluted water in a short time, making it a more efficient and scalable alternative for wastewater treatment [21].

4. Conclusions

In this work, biochar@γ-Fe₂O₃ nanoparticles derived from rice husk were prepared by a simple pyrolysis process followed by impregnation, characterized, and tested as a sorbent for cationic dye removal. This material demonstrated a superior adsorption performance compared to unactivated biochar and chemically activated biochar, owing to its enhanced surface area, functional groups, and easy separation through the application of an external magnetic field without the need for complex equipment. Furthermore, transforming rice husk into biochar is a sustainable solution for dye removal while reducing agricultural waste that is often burned or discarded, causing environmental pollution. This process provides a low-cost, energy-efficient alternative to conventional adsorbents like activated carbon. Finally, its high regeneration efficiency ensures long-term applicability, retaining over 90% of its adsorption capacity after four cycles, which optimizes costs and reinforces biochar as a viable, sustainable solution for wastewater treatment.