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Article

Ball-Milling-Assisted Fe3O4 Loadings of Rice Straw Biochar for Enhanced Tetracycline Adsorption in Aquatic Systems

by
Yuxin Liu
1,†,
Haizhang Yu
1,†,
Yuchen Xing
1,
Qi Zhao
1,
Rukeya Ashan
1,
Bo Feng
1,
Bo Tao
1,
Qianyi Shangguan
1,
Yucheng Liu
1,2,
Haiyan Zhang
3,* and
Guanya Ji
1,*
1
Key Laboratory of Modern Agriculture Equipment and Technology, School of Agriculture Engineering, Jiangsu University, Zhenjiang 212013, China
2
College of Biological and Agricultural Engineering, Jilin University, Changchun 130012, China
3
Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2025, 15(8), 1987; https://doi.org/10.3390/agronomy15081987
Submission received: 20 July 2025 / Revised: 31 July 2025 / Accepted: 11 August 2025 / Published: 19 August 2025
(This article belongs to the Section Agricultural Biosystem and Biological Engineering)
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Versions Notes

Abstract

Antibiotic contaminants such as tetracycline (TC) from agricultural production have become widely distributed and persistently accumulated in aquatic environments (rivers, lakes, and oceans), posing severe threats to ecological security and human health. This study developed a modified rice-straw-derived biochar through NaOH activation and ball-milling-assisted Fe3O4 loading, which simultaneously enhanced TC adsorption capacity and enabled magnetic recovery. The Box–Behnken design (BBD) response surface methodology was employed to optimize three key preparation parameters: ball-milling time (A, 39.95 min), frequency (B, 57.23 Hz), and Fe3O4/biochar mass ratio (C, 2.85:1), with TC adsorption capacity as the response value. The modified biochar was systematically characterized using SEM, BET, FTIR, XRD, and XPS, while adsorption mechanisms were elucidated through kinetic studies, isotherm analyses, and pH-dependent experiments. The results demonstrate that modification via ball-milling with Fe3O4 loading significantly enhanced the biochar’s tetracycline adsorption capacity. The maximum adsorption capacity of the modified biochar reached 102.875 mg/g, representing a 114.85% increase from the initial value of 47.882 mg/g observed for the pristine biochar. Furthermore, the modified biochar exhibited excellent stability, maintaining robust adsorption performance across a wide pH range. The primary adsorption mechanisms involved metal coordination complexation, supplemented by hydrogen bonding, π-π interactions, and pore filling.

1. Introduction

With the progression of industrialization, water quality faces increasing threats from anthropogenic activities, industrial discharges, and agricultural waste. Inadequate wastewater treatment poses significant risks to both human health and ecosystems [1,2,3,4]. Tetracycline (TC), ranking among the most extensively used antibiotics globally [5], exhibits low metabolic rates in humans and livestock. Approximately 70–90% of administered TC is excreted unchanged or as active metabolites through urine and feces [6,7,8], leading to substantial environmental contamination. Of particular concern is TC’s environmental persistence, demonstrating resistance to microbial degradation while concurrently promoting the development of antibiotic resistance genes (ARGs) in aquatic microbiota, thereby exacerbating ecological and public health threats [9]. Monitoring data reveal TC concentrations averaging 3.743 µg·L−1 in surface waters, with hospital wastewater exhibiting more pronounced variability (0.3–400 µg·L−1) [10]. These findings underscore the critical need for developing effective TC removal technologies to mitigate environmental and health risks.
Various technologies have been developed for TC elimination from water bodies, including photocatalytic degradation, chemical oxidation, bioremediation, electrochemical treatment, and adsorption [11,12,13,14,15]. Among these, adsorption has been widely adopted due to its rapid kinetics, high efficiency, operational simplicity, and cost-effectiveness [16,17,18].
Biochar, as a promising adsorbent, has attracted significant research attention owing to its abundant feedstock availability, environmental friendliness, and effective adsorption performance. Currently, agricultural and forestry residues represent the most prevalent biomass sources for biochar production [19,20,21]. Crop straw, a widely distributed carbon-rich resource, poses environmental challenges when burned in open fields, releasing CO2 and other greenhouse gases, a growing global concern. Research indicates that straw-to-biochar conversion technology is relatively mature. This process not only yields high-value biochar and syngas [22,23], contributing to carbon sequestration, but also enables sustainable straw valorization. However, pristine straw-derived biochar typically exhibits limited surface area and insufficient active sites for TC adsorption, resulting in suboptimal removal efficiency [24].
To address the limitations of pristine biochar in TC adsorption, activation, and modification are essential for performance enhancement [25]. Common modification techniques include acid/alkali treatment, metal impregnation, gas activation, and ball-milling [26,27,28,29]. Alkali treatment effectively alters biochar’s pore structure, increases specific surface area [30,31,32], and introduces oxygen-containing functional groups (e.g., hydroxyl and carboxyl groups) [33]. These enhanced functional groups facilitate hydrogen bonding and π-π interactions between biochar and TC molecules [34]. Shao et al. [35] demonstrated this through NaOH/KMnO4-modified rice straw biochar (Mn/Na-RBC), which exhibited significantly improved surface area, carbonization degree, and oxygen functional groups, leading to enhanced ciprofloxacin (CIP) adsorption capacity. Metal impregnation optimizes biochar’s pore structure and surface potential, providing additional active adsorption sites [36]. Nanoscale metal oxide–biochar composites (e.g., Fe3O4) overcome traditional metal oxide agglomeration while synergizing both materials’ advantages [37]. The incorporated metal elements (e.g., Fe-O, Mg-O groups) enhance chelation and electrostatic interactions with TC molecules. Jiang et al. [38] developed a novel magnetic photocatalyst (MZB) through ZnO/Fe3O4 co-doping on poplar-derived biochar. The material achieved 90.4% TC removal (40 mg/L, 50 mL) using only 10 mg MZB, maintaining high stability across different aqueous matrices.
Ball-milling modification, as an efficient and environmentally friendly mechanophysical treatment technology, has attracted widespread attention in recent years due to its unique modification advantages [39]. This method efficiently grinds biochar into uniform nano-sized particles through the mechanical motion of the milling media. During this process, not only is the physical refinement of particles achieved, but more importantly, the breaking of chemical bonds in biochar is induced, thereby significantly improving its physicochemical properties and ultimately greatly enhancing the material’s adsorption performance for pollutants. Researchers such as Qu et al. [40] confirmed through systematic characterization analysis that magnetic carbon materials prepared by ball-milling exhibit a larger specific surface area and a smaller particle size distribution compared to the original material, leading to a remarkable enhancement in adsorption capacity. Biochar possesses excellent ion exchange capacity, making it one of the superior metal carriers [41,42]. Therefore, combining ball-milling with the in situ loading of magnetic metal oxides onto biochar not only enhances the chemical adsorption of tetracycline (TC) through the coordination complexation between TC molecules and metal–oxo groups (Fe–O) but also endows biochar with magnetism, facilitating the recovery of the adsorbent. Additionally, the pH value represents a critical parameter governing adsorption processes, as it significantly influences the TC adsorption behavior of modified biochar by modulating the surface charge characteristics of the adsorbent, the molecular speciation of the contaminant, and the interactive forces between them [43].
In summary, to enhance the adsorptive removal of tetracycline (TC) by biochar, this study designed a composite modification strategy for rice straw biochar involving NaOH activation followed by ball-milling with Fe3O4 loading. As the influence patterns of various parameters in this composite modification approach on the biochar’s TC adsorption performance, as well as the adsorption mechanisms of modified biochar for TC, remain unclear, response surface methodology (RSM) was employed to optimize the ball-milling conditions by investigating their impact on the TC adsorption performance of the modified biochar. The physicochemical properties of the biochar before and after modification were characterized using scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Adsorption kinetics and isotherm experiments were conducted to elucidate the adsorption mechanisms of TC onto the modified biochar. Given that the adsorption capacity of the modified biochar may vary under acidic and alkaline conditions, pH effect experiments were performed to further assess its TC adsorption capability across different pH values.

2. Materials and Methods

2.1. Chemicals and Materials

The rice straw was collected from Jingkou District, Zhenjiang City, Jiangsu Province, China. The straw was crushed, washed with ultrapure water, and then dried. Tetracycline (TC) and iron(II,III) oxide (Fe3O4) were purchased from Macklin, Shanghai, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were obtained from Sinopharm, Beijing, China.

2.2. Preparation of Straw Biochar

The air-dried rice straw was pulverized using a grinder and sieved through a 200-mesh screen. The powdered straw was then pyrolyzed at 600 °C for 3 h under a N2 atmosphere with a heating rate of 5 °C/min to obtain pristine biochar (BC). Subsequently, 1.0 g of BC was mixed with 50 mL of a 5 wt% NaOH solution in a Teflon-lined autoclave and subjected to hydrothermal treatment at 200 °C for 12 h. After cooling to room temperature, the product was washed with ultrapure water until neutral pH was reached and dried, yielding NaOH-activated modified biochar (SBC). The SBC was further used for ball-milling-assisted Fe3O4 loading modification.

2.3. Response Surface Optimization of Ball-Milling-Assisted Fe3O4 Loading Conditions

To investigate the effects of different ball-milling-assisted Fe3O4 loading conditions on the adsorption performance of biochar, a Box–Behnken design (BBD) [44] was employed by considering three key factors: (A) ball-milling time, (B) ball-milling frequency, and (C) Fe3O4-to-SBC mass ratio, with respect to their impacts on TC adsorption capacity. A three-factor, three-level experimental design was adopted for adsorption parameter optimization, with the specific factors and levels presented in Table 1. Based on Table 1, 13 distinct modified biochar samples were prepared under various ball-milling conditions. A total of 17 experimental runs were designed (Table S1), consisting of 12 factorial points for parameter effect analysis and 5 center point replicates for error estimation. All experiments were conducted in triplicate to evaluate the influence of ball-milling conditions on TC adsorption by modified biochar and to determine the optimal ball-milling parameters. The resulting Fe3O4-loaded biochar was designated as Fe3O4@BM-SBC.

2.4. Characterization of Biochar Materials

The biochar samples before and after modification were characterized by scanning electron microscopy (SEM) using a Hitachi Regulus-8100 microscope (Hitachi, Tokyo, Japan). Fourier transform infrared spectroscopy (FTIR) analysis was performed on a Thermo Fisher Nicolet IS 50 spectrometer (Thermo Fisher, Waltham, MA, USA) to identify functional groups of the biochar before and after modification. The crystalline phases of biochar samples were analyzed by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, Germany). The specific surface area and pore volume were determined by nitrogen adsorption–desorption measurements using a JW-BK200B analyzer (JWGB, Beijing, China). X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Shimadzu AXIS ULTRADLD spectrometer (Shimadzu, Tokyo, Japan).

2.5. Adsorption Studies of Tetracycline onto Modified Biochar

2.5.1. Adsorption Kinetics Experiments

A 100 mL volume of TC solution at concentrations of 10, 40, and 80 mg/L was added to conical flasks, followed by the addition of 0.05 g biochar. The mixtures were shaken at 150 rpm for 12 h at 25 °C in a constant-temperature shaker. Samples were collected at 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, and 12 h time intervals.
The adsorption kinetics were analyzed using
qt = qe·[1 − exp (−k1 t)]
qt = qe2 k2 t/(1 + qe k2 t)
where qt (mg/g) and qe·(mg/g) represent the adsorption capacity of TC at time t and at equilibrium, respectively. t (h) is the adsorption time and k1 (h−1) is the rate constant of the pseudo-first-order model. k2 (g·mg−1·h−1) is the rate constant of the pseudo-second-order model.

2.5.2. Adsorption Isotherm Experiment

A series of glass vials were prepared, each containing 20 mL of TC solutions with gradient concentrations (10, 20, 40, 60, 80, 100, and 120 mg/L), and each vial was supplemented with 0.01 g of biochar. These vials were then sealed and agitated on an orbital shaker at 150 rpm for 12 h at a constant temperature of 25 °C. After the reaction period, samples were withdrawn for analysis. All experiments were conducted in triplicate.
The equilibrium data were fitted with the following models:
Langmuir Model:
qe = qmax KL ce/(1 + KL ce)
Freundlich Model:
qe = KF ce 1/n
where ce (mg/L) denotes the equilibrium concentration of tetracycline in solution; qe (mg/g) represents the equilibrium adsorption capacity of tetracycline; qmax (mg/g) is the theoretical maximum monolayer adsorption capacity; KL stands for the Langmuir adsorption constant; KF corresponds to the Freundlich adsorption constant; n is an empirical constant related to adsorption intensity.

2.5.3. pH Effect Experiments

A 20 mL aliquot of tetracycline (TC) solution (40 mg/L) was added to glass vials, and the pH was adjusted to 3, 5, 7, 9, and 11 using 0.1 M HCl or NaOH solutions. Subsequently, 0.01 g of biochar was added to each vial. These vials were then sealed and agitated on an orbital shaker at 150 rpm for 12 h at a constant temperature of 25 °C. After the reaction period, samples were withdrawn for analysis. All experiments were performed in triplicate to ensure reproducibility.

2.6. Determination of TC Concentration

After filtration through 0.22 μm membrane filters, the absorbance of TC solutions was measured at 358 nm using a UV–Vis spectrophotometer (UV-5100B, Shanghai Yuanxi Instrument Co., Ltd., Shanghai, China), with TC concentrations determined via a pre-established standard calibration curve. Standard Curve Preparation is as follows:
c = −0.090 + 29.49 × A
where c (mg/L) is the TC concentration in solution (mg/L) and A is the absorbance of TC solution (dimensionless).

2.7. Calculation of Adsorption Capacity

The adsorption capacity of modified biochar for TC can be calculated using the following equation:
q = [(c0 − ce) × V]/m
where q (mg/g) is the adsorption capacity and c0 (mg/L) is the initial TC concentration (mg/L). ce (mg/L) represents the equilibrium TC concentration and V (mL) represents the solution volume. m (mg) is the mass of adsorbent.

3. Results and Discussion

3.1. Optimization of Ball-Milling-Assisted Fe3O4 Loading Conditions

3.1.1. Results of Box–Behnken Response Surface Experiments

The experimental results of the 17 orthogonal tests designed based on Table 1 are presented in Table 2. As shown in Table 2, the maximum adsorption capacity was achieved in Test 7 (65.058 mg/g) with the following parameter combination: ball-milling time of 30 min, frequency of 50 Hz, and mass ratio of 3:1. In contrast, the minimum adsorption capacity was observed in Test 10 (33.111 mg/g), with parameters of 60 min ball-milling time, 60 Hz frequency, and 1:1 mass ratio. The significant range of 31.947 mg/g demonstrates that parameter selection profoundly affects the adsorption performance.
The experimental data were analyzed using Design-Expert 13 software for multiple linear regression and binomial fitting to establish a response surface model [45]. The quadratic regression model for TC adsorption capacity by modified biochar was developed, with the summary statistics shown in Table S2.
Analysis of variance (ANOVA) results for the regression model are presented in Table 3. The extremely significant model244 (p = 0.0003 < 0.01) [46] indicates excellent regression performance. The regression model equation is Y = 57.81 − 3.79A − 0.8245B + 5.15C − 2.18AB − 0.6143AC + 7.88BC + 0.3930A2 − 7.74B2 − 0.8875C2, with a correlation coefficient of 0.9638, demonstrating good fitting of the regression equation. The influence of three factors on TC adsorption capacity follows the order: C > A > B [47].

3.1.2. Influence of Ball-Milling-Assisted Fe3O4 Modification Conditions

The interaction effects between experimental factors and the optimal ball-milling-assisted Fe3O4 modification conditions were evaluated based on the Box–Behnken response surface plots (Figure 1). In the response surface analysis, steeper curvature of the plots indicates stronger factor influence on TC adsorption capacity and more significant interactions, while flatter curves suggest weaker effects [48]. As shown in Figure 1, the significant curvature of the surfaces for ball-milling time (A) and Fe3O4-to-SBC mass ratio (C) demonstrates their pronounced effects on TC adsorption capacity, whereas ball-milling frequency (B) shows relatively weaker influence. The strong interaction between BC indicates that the combined effect of ball-milling frequency (B) and mass ratio (C) significantly affects TC adsorption. Conversely, the weaker AB and AC interactions suggest that the combinations of ball-milling time (A) with frequency (B) and ball-milling time (A) with mass ratio (C) have limited effects on adsorption performance.
Using the maximization of TC adsorption capacity as the optimization objective [49], the optimal ball-milling conditions were determined as follows: Ball-milling time—39.95 min; ball-milling frequency—57.23 Hz; Fe3O4-to-SBC mass ratio—2.85:1.

3.2. Characterization of Biochar

3.2.1. SEM

To investigate the surface morphological characteristics of the adsorbent materials, scanning electron microscopy (SEM) characterization was performed on the pristine biochar (BC) and modified biochar (Fe3O4@BM-SBC), with the results presented in Figure 2. Comparison of Figure 2a,b reveals distinct morphological differences between the pristine biochar (BC) and the modified Fe3O4@BM-SBC. The pristine BC exhibits an intact morphology with larger particle sizes, predominantly on the order of tens of micrometers. The smallest identifiable fragment measures approximately 4.19 μm. In contrast, the Fe3O4@BM-SBC particles demonstrate a significant reduction in size accompanied by a markedly more developed porous structure. Particle sizes in the modified material range down to the nanoscale, with larger particles measuring around 2 μm.
These observations indicate that the NaOH activation and ball-milling modifications led to the erosion and fragmentation of amorphous carbon regions on the biochar surface, resulting in a loose and porous structure. Cai et al. [50] demonstrated that NaOH treatment significantly enhanced the specific surface area (SSA) of biochar. The NaOH-ethanol-modified biochar (EBC) exhibited an SSA of 91.76 m2/g, while the NaOH-modified biochar (SBC) reached 56.83 m2/g, both surpassing that of the pristine biochar. Furthermore, the ball-milling medium caused the fracture of biochar particles, thereby reducing the average particle size, increasing the specific surface area, opening previously closed pores, and forming interconnected pore channels [51]. These structural modifications are consistent with our SEM observations of surface roughening and pore structure development in Fe3O4@BM-SBC, suggesting synergistic effects between chemical activation and mechanical modification in creating hierarchically porous structures favorable for adsorption applications.
The EDS spectrum of Fe3O4@BM-SBC (Figure 2c) confirms the coexistence of Fe and C (biochar matrix) signals, with Fe and O distributions showing strong overlap, verifying successful Fe3O4 loading [37]. As observed in Figure 2b, Fe3O4 nanoparticles appear as fine white spots uniformly dispersed on biochar pore walls or carbon skeleton surfaces. Some particles are embedded within pores, while larger aggregates form localized iron-rich regions on the biochar surface. Table 4 compares the elemental composition of BC and Fe3O4@BM-SBC. The modified biochar exhibits significantly higher mass percentages of O (63.62%) and C than BC (46.08%), attributed to oxygen-containing functional groups introduced by NaOH activation and additional oxygen from Fe3O4 loading [52]. This increased oxygen content enhances the material’s surface reactivity and adsorption potential.

3.2.2. FTIR

Figure 3a shows the FTIR spectra of BC and Fe3O4@BM-SBC. The modified Fe3O4@BM-SBC exhibited higher transmittance compared to BC, indicating reduced light absorption after modification. The pristine BC displayed a broad O-H stretching vibration peak near 3400 cm−1 [53], which showed significant changes after modification, indicating that a large number of hydroxyl (-OH) groups were introduced during the activation process of NaOH. This was mainly due to the alkaline treatment, which hydrolyzed esters and anhydrides, oxidized some carbon structures, and promoted the formation of phenolic hydroxyl and carboxylic hydroxyl groups [31,32]. The observed weakening and potential shift in the carbonyl (C=O) peak at approximately 1700 cm−1 after modification suggests that NaOH activation not only introduced new groups but also potentially transformed existing carbonyl functionalities. This could involve the conversion of some carbonyl groups (e.g., in esters or ketones) into carboxylate (-COO−) salts under alkaline conditions, which might exhibit different vibrational frequencies or lower intensity [33,54]. The peak at 1635 cm−1 corresponds to the stretching vibrations of -C=C/C=O bonds [54,55]. Both samples showed strong absorption in the 1500–1600 cm−1 region, with sharper peaks appearing in the modified sample, indicating enhanced graphitization through NaOH treatment. Fe3O4@BM-SBC exhibited new characteristic peaks at 1027 cm−1 (-C-O stretching) and 556 cm−1 (-C-H stretching). The combined NaOH alkaline modification and ball-milling Fe3O4 loading significantly altered the biochar’s surface chemistry by increasing oxygen-containing functional groups, introducing magnetic Fe3O4 components, improving pore structure, and enhancing surface reactivity.

3.2.3. XRD

Figure 3b presents the XRD patterns of biochar before and after modification. The BC sample shows a broad amorphous diffraction peak at 23°, attributed to amorphous carbon (002). In contrast, Fe3O4@BM-SBC exhibits distinct diffraction peaks at 2θ = 18.45°, 30.29°, 35.62°, 43.27°, 57.19°, and 62.79°, which match perfectly with the standard pattern of magnetite (JCPDS No.46-1043). These peaks correspond to the (111), (220), (311), (400), (511), and (440) crystal planes of Fe3O4, respectively, confirming the successful loading of Fe3O4 onto the biochar [56]. The incorporation of Fe3O4 imparts excellent magnetic properties to the biochar, facilitating adsorbent separation and recovery. The graphitization peak at approximately 26° is a characteristic feature of graphitic carbon in biochar. Comparative analysis reveals significantly enhanced peak intensity at 26.0034° for the modified sample (1335 counts) versus the pristine BC (593 counts), indicating that the modification process likely promoted the formation of more graphitic structures.

3.2.4. BET

Figure 4 presents the nitrogen adsorption–desorption isotherms and pore size distribution curves of BC and Fe3O4@BM-SBC, obtained through BET characterization to investigate the specific surface area and pore structure changes after modification. Both biochars exhibit Type IV isotherms with distinct hysteresis loops at high relative pressures (Figure 4a), confirming their mesoporous characteristics. The steep capillary condensation stages indicate uniform mesopore distributions in both materials.
Table 5 summarizes the porous structure parameters. While Fe3O4@BM-SBC shows a slightly increased specific surface area, its pore volume and average pore size decrease significantly compared to BC. The pore size distribution (Figure S1) reveals that BC primarily contains pores of 20–40 nm, whereas Fe3O4@BM-SBC shows dominant pores below 15 nm, suggesting increased micro- and mesopores that compensate for the reduced pore volume to maintain surface area. This pore narrowing likely results from partial pore blockage by nano-sized Fe3O4 particles generated during ball-milling [57], which occupy pore volume and could potentially decrease surface area. Although the reduced pore volume limits physical adsorption sites, Fe3O4@BM-SBC demonstrates substantially higher TC adsorption capacity than BC, indicating that physical adsorption plays a minor role in the TC removal mechanism.

3.2.5. XPS

Figure 4 presents the XPS characterization results investigating the effects of NaOH activation and ball-milling-assisted Fe3O4 loading on biochar’s adsorption mechanism, with particular focus on the elemental valence states and electronic structure of Fe3O4@BM-SBC. The survey spectrum confirms the presence of C, O, N, and Fe elements in the modified biochar. Peaks in the C 1s spectrum were observed at 284.80 eV, 285.96 eV, and 288.82 eV, corresponding to C-C, C-O-C, and O-C=O bonds, respectively [58]. The C-C bond reflects the graphitic carbon structure of the biochar. The C-O-C and O-C=O components correlate with the C-O peak at 531.52 eV and the C=O peak at 533.04 eV in the O 1s spectrum. The presence of C-O-C (characteristic of hydroxyl groups) and O-C=O (characteristic of carboxyl groups) indicates that the composite modification process (NaOH activation coupled with ball-milling and Fe3O4 loading) successfully introduced these oxygen-containing functional groups [33]. This is attributable to the hydrolytic and oxidative effects of NaOH (discussed in Section 3.2.2) and the ball-milling process. Ball-milling inherently promotes the formation of surface functional groups. Its high-energy mechanical forces disrupt chemical bonds within the biochar structure (e.g., C-C, C-O), generating reactive sites and free radicals. These species subsequently react with atmospheric oxygen or water vapor during processing, forming oxygen-containing functional groups such as carboxyl and hydroxyl groups [25,40].
The N 1s spectrum reveals two distinct peaks at 398.57 eV (pyridinic N) and 400.30 eV (C-NH2). Pyridinic N represents a nitrogen functional group typically formed during pyrolysis of nitrogen-containing precursors, while the C-NH2 group likely originates from reactions between NaOH and nitrogenous compounds in the biochar [52]. Detailed analysis of the Fe 2p spectrum shows five deconvoluted peaks: two Fe(II) peaks at 710.85 eV and 724.05 eV, one satellite peak at 718.85 eV, and two Fe(III) peaks at 713.34 eV and 726.31 eV [59]. The coexistence of Fe-O bonds (530.19 eV in O 1s) with these Fe 2p signals confirms the successful loading of Fe3O4 on the biochar surface.

3.3. Adsorption Experiments of TC by Fe3O4@BM-SBC

3.3.1. Adsorption Kinetics

Figure 5 shows the adsorption kinetic curves of TC on Fe3O4@BM-SBC, where all curves exhibit a rapid initial increase followed by gradual stabilization. This phenomenon occurs because abundant unoccupied active sites on the Fe3O4@BM-SBC surface facilitate rapid binding with TC molecules during the initial adsorption stage, leading to a sharp increase in adsorption capacity [50]. As adsorption progresses, TC molecules must diffuse into the internal pore structures of the biochar for further adsorption, resulting in slower adsorption rates [60].
The kinetic fitting parameters for TC adsorption on Fe3O4@BM-SBC are summarized in Table 6. Both pseudo-first-order and pseudo-second-order kinetic models demonstrate good fitting performance, with comparable R2 values. However, the pseudo-second-order model generally exhibits R2 values closer to 1, indicating that the adsorption process involves both physical and chemical adsorption mechanisms, with chemical adsorption playing a more dominant role, particularly at higher TC concentrations. The chemical adsorption primarily involves coordination complexation, where Fe-O groups and adjacent C=O bonds form ternary complexes with TC molecules [37]. The NaOH activation introduces additional carboxyl groups, increasing the number of C=O bonds and thereby enhancing these coordination reactions [33].
Under the optimal fitting model, the equilibrium adsorption capacities of Fe3O4@BM-SBC in TC solutions of different concentrations were determined to be 19.450, 65.946, and 89.207 mg/g, respectively.

3.3.2. Adsorption Isotherms

Figure 6 presents the adsorption isotherm fitting curves of TC on Fe3O4@BM-SBC as described by Langmuir and Freundlich models, with the corresponding fitting parameters summarized in Table 7. The adsorption capacity of Fe3O4@BM-SBC for TC showed an increasing trend with rising TC solution concentration. Compared to the Langmuir model (R2 = 0.939), the Freundlich model exhibited superior fitting performance (R2 = 0.977), indicating that the adsorption isotherm better conforms to the Freundlich model. This suggests that the Fe3O4@BM-SBC surface possesses significant heterogeneity and demonstrates multilayer adsorption characteristics for TC. The Freundlich constant n reflects the adsorption intensity, where the magnitude of 1/n indicates the strength of TC concentration’s influence on adsorption capacity [61]. In this study, the 1/n value of 0.368 (<0.5) signifies strong affinity between Fe3O4@BM-SBC and TC molecules, further confirming the favorable adsorption of TC on the magnetic biochar surface [62]. The maximum adsorption capacity of Fe3O4@BM-SBC for TC reached 102.875 mg/g at 25 °C, significantly surpassing the pristine biochar’s 47.882 mg/g. Shan et al. [57] developed an ultrafine magnetic biochar/Fe3O4 via ball-milling, achieving a maximum adsorption capacity of 94.2 mg/g for TC. This indicates that compared with the traditional ball-mill loading metal oxide modification method, a composite modification strategy for rice straw biochar involving NaOH activation followed by ball-milling with Fe3O4 loading offers superior performance in enhancing TC adsorption capacity.

3.3.3. Effect of pH

Figure 7 shows the adsorption capacity of Fe3O4@BM-SBC for TC under different pH conditions (pH = 3, 5, 7, 9, and 11). The results demonstrate that the adsorption capacity remained relatively stable with only a slight decreasing trend as pH increased. In the acidic range (3 ≤ pH < 7), Fe3O4@BM-SBC exhibited relatively high TC adsorption capacity, which may be attributed to the protonation of surface -COOH and Fe-OH groups into -COOH2+ and Fe-OH2+. These protonated groups formed strong hydrogen-bonding networks with polar functional groups (–OH, –CONH2) of TC, thereby enhancing adsorption. Additionally, under acidic conditions, the protonation of the biochar’s aromatic structure increased its electron affinity [63], strengthening π-π interactions with the tetracyclic skeleton of TC and further improving adsorption capacity. In the alkaline range (7 < pH ≤ 9), the adsorption capacity decreased moderately, likely due to electrostatic repulsion between deprotonated –COO− groups on the adsorbent surface and anionic TC species (TCH−, TC2−) [64]. Despite these pH-dependent interactions, the overall impact on adsorption performance was limited, indicating the excellent stability of Fe3O4@BM-SBC. The material maintained high adsorption capacity for TC across both acidic and alkaline conditions, demonstrating its robustness as an adsorbent.

3.4. Adsorption Mechanisms

Based on the characterization results of Fe3O4@BM-SBC and related adsorption experiments, it can be concluded that the adsorption of TC by Fe3O4@BM-SBC is not governed by a single mechanism but rather involves multiple synergistic adsorption pathways.
BET analysis revealed that compared to BC, Fe3O4@BM-SBC showed minimal changes in specific surface area but significantly reduced pore volume. Despite this, its TC adsorption capacity was substantially higher than BC, indicating that physical adsorption does not dominate the TC removal process. Combined with kinetic studies, chemical adsorption plays a more critical role than physical adsorption in TC uptake by Fe3O4@BM-SBC. The primary chemical interaction involves coordination complexation among three components: the Fe-O bonds in Fe3O4, adjacent carboxyl C=O groups, and TC molecules. Pore filling represents the predominant physical adsorption mechanism. FTIR and XRD characterizations demonstrated significant modifications in the broad O-H stretching vibration peak near 3400 cm−1 after modification, confirming the introduction of additional -OH groups through NaOH activation. XPS analysis further verified the presence of hydroxyl (-OH) and carboxyl (-COOH) functional groups through characteristic C-O-C and O-C=O peaks, respectively, providing additional evidence for successful surface functionalization via NaOH treatment. These oxygen-containing groups significantly contribute to enhanced adsorption performance through both coordination complexation and hydrogen bonding interactions.
The presence of -OH and -COOH groups significantly enhances TC adsorption on biochar through multiple mechanisms: These functional groups can chelate Fe3+ ions, effectively preventing iron oxide aggregation and increasing the active surface area, thereby providing more chemical adsorption sites. The carboxyl groups form Fe-O-C interfaces with Fe3+, which serve as coordination centers to capture TC molecules and form ternary complexes. Additionally, both -OH and -COOH can form hydrogen bonds with amide groups in TC molecules, while -OH groups also act as electron donors to strengthen π-π interactions with the electron-accepting TC benzene rings, collectively promoting TC adsorption by Fe3O4@BM-SBC.
Through this analysis, it becomes evident that ball-milling-assisted Fe3O4 loading and the introduction of hydroxyl and carboxyl functional groups constitute the primary advantages of Fe3O4@BM-SBC for TC adsorption. While pore filling serves as the main physical adsorption mechanism and contributes to some extent, chemical adsorption dominates the TC removal process. Among the chemical interactions, metal complexation represents a stronger binding force than hydrogen bonding [59], making it the most critical mechanism in TC adsorption by Fe3O4@BM-SBC. Therefore, the adsorption mechanisms of TC on Fe3O4@BM-SBC include the following: (1) coordination complexation among Fe-O bonds, carboxyl groups, and TC molecules; (2) hydrogen bonding and π-π interactions between oxygen-containing functional groups on the adsorbent and TC molecules; (3) pore filling effects between the biochar matrix and TC molecules.

3.5. Future Considerations on Feedstock and Pyrolysis Influence

While this study demonstrates the significant enhancement of TC adsorption through NaOH activation, combined with ball-milling-assisted Fe3O4 loading on rice straw biochar pyrolyzed at 600 °C, it is acknowledged that the selection of biomass feedstock and pyrolysis conditions fundamentally shapes the physicochemical properties of the pristine biochar, which serves as the substrate for subsequent modifications [65]. The inherent components of rice straw feedstock (e.g., cellulose, lignin) and specific pyrolysis parameters (e.g., temperature, duration) critically determine the initial specific surface area, pore structure, surface functional group density, and ash content of the resultant biochar. Future studies will therefore systematically vary feedstocks and pyrolysis regimes (temperature gradients, heating rates) to elucidate how precursor characteristics condition the efficacy of subsequent activation strategies.

4. Conclusions

The ball-milling-assisted Fe3O4 modification significantly alters the pore structure of pristine rice straw biochar by creating more carbon defects, enlarging specific surface area and porosity, while introducing additional oxygen-containing functional groups (hydroxyl and carboxyl) that provide abundant chemical adsorption sites. This treatment effectively reduces the average pore size while increasing the quantity of micropores and mesopores. The incorporated Fe3O4 nanoparticles not only enhance coordination complexation with TC molecules to strengthen chemical adsorption but also impart magnetic properties that facilitate adsorbent recovery. Compared to pristine biochar, the modified Fe3O4@BM-SBC demonstrates a 115% improvement in maximum TC adsorption capacity. Fe3O4@BM-SBC exhibits multiple adsorption mechanisms combining both physical and chemical processes, with metal coordination complexation dominating the chemical adsorption, followed by hydrogen bonding, π-π interactions, and pore filling. The material maintains stable adsorption capacity across wide pH ranges, demonstrating excellent acid–base resistance. In summary, the developed Fe3O4@BM-SBC offers several advantages including simple preparation, convenient magnetic separation, robust pH stability, and superior adsorption performance, providing an effective new approach for TC removal from aqueous environments.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15081987/s1, Figure S1: (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the prepared biochar materials; Table S1: Experimental design matrix for response surface methodology (RSM) investigating modified biochar’s tetracycline (TC) adsorption performance; Table S2: Summary statistics of the response surface model for tetracycline (TC) adsorption by modified biocha.

Author Contributions

Conceptualization, Y.L. (Yuxin Liu), H.Y., and Q.Z.; funding acquisition, H.Z. and G.J.; investigation, B.T. and B.F.; methodology, Y.X. and R.A.; supervision, H.Z. and G.J.; validation, Y.L. (Yuxin Liu), H.Y., and G.J.; writing—original draft, Y.L. (Yuxin Liu) and H.Y.; writing—review and editing, Y.L. (Yucheng Liu), Q.S., and G.J. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (No. 32301723, 32401732), the Postdoctoral Science Foundation of China (No. 2020M671367), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (20KJB416009), Central Public-interest Scientific Institution Basal Research Fund (No. Y2025QC04), and Priority Academic Program Development of Jiangsu Higher Education Institutions (No. PAPD2023-87).

Data Availability Statement

The original data presented in this study are openly available within the article/Supplementary Material.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Response surface plots showing interaction effects of variables on TC adsorption capacity. Warm colors (orange) indicate higher values, while cool colors (green) indicate lower values.
Figure 1. Response surface plots showing interaction effects of variables on TC adsorption capacity. Warm colors (orange) indicate higher values, while cool colors (green) indicate lower values.
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Figure 2. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis: (a) Pristine biochar (BC), (b) Fe3O4-loaded ball-milled biochar (Fe3O4@BM-SBC), and (c) EDS spectrum of Fe3O4@BM-SBC.
Figure 2. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis: (a) Pristine biochar (BC), (b) Fe3O4-loaded ball-milled biochar (Fe3O4@BM-SBC), and (c) EDS spectrum of Fe3O4@BM-SBC.
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Figure 3. Characterization spectra of BC and Fe3O4@BM-SBC: (a) Fourier transform infrared spectroscopy (FTIR) and (b) X-ray diffraction (XRD) patterns.
Figure 3. Characterization spectra of BC and Fe3O4@BM-SBC: (a) Fourier transform infrared spectroscopy (FTIR) and (b) X-ray diffraction (XRD) patterns.
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Figure 4. X-ray photoelectron spectroscopy (XPS) analysis of Fe3O4@BM-SBC: (a) Survey spectrum, (b) C 1s spectrum, (c) O 1s spectrum, (d) N 1s spectrum, and (e) Fe 2p spectrum. Different colors represent the characteristic peaks of distinct elements in the XPS spectra.
Figure 4. X-ray photoelectron spectroscopy (XPS) analysis of Fe3O4@BM-SBC: (a) Survey spectrum, (b) C 1s spectrum, (c) O 1s spectrum, (d) N 1s spectrum, and (e) Fe 2p spectrum. Different colors represent the characteristic peaks of distinct elements in the XPS spectra.
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Figure 5. Kinetic fitting curves of TC adsorption on Fe3O4@BM-SBC.
Figure 5. Kinetic fitting curves of TC adsorption on Fe3O4@BM-SBC.
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Figure 6. Adsorption isotherm fitting curves for TC on pristine biochar (BC) and Fe3O4-loaded biochar (Fe3O4@BM-SBC).
Figure 6. Adsorption isotherm fitting curves for TC on pristine biochar (BC) and Fe3O4-loaded biochar (Fe3O4@BM-SBC).
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Figure 7. TC adsorption capacity of Fe3O4@BM-SBC under different pH conditions (3–11).
Figure 7. TC adsorption capacity of Fe3O4@BM-SBC under different pH conditions (3–11).
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Table 1. Factors and levels of experimental design.
Table 1. Factors and levels of experimental design.
Influencing FactorsCodedLevels
−101
Ball-milling time/minA306090
Ball-milling frequency/HzB405060
Fe3O4 to SBC mass ratioC1:12:13:1
Table 2. Experimental design and results of response surface methodology for tetracycline adsorption by modified biochar.
Table 2. Experimental design and results of response surface methodology for tetracycline adsorption by modified biochar.
No.Ball-Milling Time/minBall-Milling Frequency/HzFe3O4-to-Biochar Mass RatioAdsorption Capacity/(mg/g)
13040252.496
29040247.856
33060257.440
49060244.081
53050155.740
69050150.805
73050365.058
89050357.666
96040152.751
106060133.111
116040349.507
126060361.382
136050257.234
146050257.352
156050260.222
166050256.329
176050257.922
Table 3. ANOVA of response surface methodology for tetracycline adsorption by modified biochar.
Table 3. ANOVA of response surface methodology for tetracycline adsorption by modified biochar.
SourceSum of SquaresDegrees of FreedomMean SquareFpSignificance
Model860.58995.6220.690.0003**
A—Ball-milling time114.961114.9624.880.0016**
B—Ball-milling frequency5.4415.441.180.3140-
Fe3O4-to-biochar mass ratio212.241212.2445.930.0003**
AB19.01119.014.110.0822-
AC1.5111.510.32660.5856-
BC248.301248.3053.730.0002**
A20.650210.65020.14070.7187-
B2252.021252.0254.530.0002**
C23.3213.320.71770.4249-
Residual32.3574.62
Lack of Fit23.7837.933.700.1192-
Pure Error8.5742.14
Total892.9316
** Significance at p < 0.01.
Table 4. Elemental composition of BC and Fe3O4@BM-SBC.
Table 4. Elemental composition of BC and Fe3O4@BM-SBC.
SampleCNOFeO/CTotal
(wt%)(wt%)(wt%)(wt%)(%)(wt%)
BC64.215.9629.590.2446.08100
Fe3O4@BM-SBC45.552.5228.9822.9563.62100
Table 5. Pore structure parameters of biochar samples.
Table 5. Pore structure parameters of biochar samples.
SampleBET Surface Area/(m2/g)Total Pore Volume/(cm3/g)Average Pore Diameter/nm
BC74.120.3719.98
Fe3O4@BM-SBC76.370.105.37
Table 6. Kinetic fitting parameters for TC adsorption on Fe3O4@BM-SBC.
Table 6. Kinetic fitting parameters for TC adsorption on Fe3O4@BM-SBC.
ModelParameterc0 (TC)/(mg/L)
104080
Pseudo-first-orderqe/(mg/g)19.45061.15484.707
k1/h−14.3591.5692.413
R20.9990.9860.985
Pseudo-second-orderqe/(mg/g)19.76565.94689.207
k2/g·(mg·h)−10.8580.0390.053
R20.9970.9980.997
Table 7. Isotherm fitting parameters for TC adsorption on Fe3O4@BM-SBC.
Table 7. Isotherm fitting parameters for TC adsorption on Fe3O4@BM-SBC.
SampleLangmuirFreundlich
KLqmaxR2KFn−1R2
L/mgmg/g mg(1-1/n)·L1/n·g−1
BC0.04047.8820.9985.0270.4630.985
Fe3O4@BM-SBC0.176102.8750.93924.0470.3680.977
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MDPI and ACS Style

Liu, Y.; Yu, H.; Xing, Y.; Zhao, Q.; Ashan, R.; Feng, B.; Tao, B.; Shangguan, Q.; Liu, Y.; Zhang, H.; et al. Ball-Milling-Assisted Fe3O4 Loadings of Rice Straw Biochar for Enhanced Tetracycline Adsorption in Aquatic Systems. Agronomy 2025, 15, 1987. https://doi.org/10.3390/agronomy15081987

AMA Style

Liu Y, Yu H, Xing Y, Zhao Q, Ashan R, Feng B, Tao B, Shangguan Q, Liu Y, Zhang H, et al. Ball-Milling-Assisted Fe3O4 Loadings of Rice Straw Biochar for Enhanced Tetracycline Adsorption in Aquatic Systems. Agronomy. 2025; 15(8):1987. https://doi.org/10.3390/agronomy15081987

Chicago/Turabian Style

Liu, Yuxin, Haizhang Yu, Yuchen Xing, Qi Zhao, Rukeya Ashan, Bo Feng, Bo Tao, Qianyi Shangguan, Yucheng Liu, Haiyan Zhang, and et al. 2025. "Ball-Milling-Assisted Fe3O4 Loadings of Rice Straw Biochar for Enhanced Tetracycline Adsorption in Aquatic Systems" Agronomy 15, no. 8: 1987. https://doi.org/10.3390/agronomy15081987

APA Style

Liu, Y., Yu, H., Xing, Y., Zhao, Q., Ashan, R., Feng, B., Tao, B., Shangguan, Q., Liu, Y., Zhang, H., & Ji, G. (2025). Ball-Milling-Assisted Fe3O4 Loadings of Rice Straw Biochar for Enhanced Tetracycline Adsorption in Aquatic Systems. Agronomy, 15(8), 1987. https://doi.org/10.3390/agronomy15081987

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