Introduction

In recent years, antibiotics have been extensively used to treat human diseases and promote livestock growth and aquaculture. Because most antibiotics are incompletely metabolized or absorbed, their residues and degradation products enter aquatic environments in various ways. Residual sulfonamides (SAs) and fluoroquinolones (FQs) are two classes of frequently detected antibiotics in the surface water worldwide, with concentrations reaching up to 16.2 µg/L and 4.85 µg/L, respectively (Yan et al. 2024). These antibiotics may cause ecotoxicity to bacteria, algae, and aquatic animals, e.g., induce a direct harmful effect on fish, including oxidative stress, histopathological lesions, metabolic disorders, as well as immunosuppressive and genotoxic effect (Fayaz et al. 2024). Importantly, the continuous accumulation of antibiotics even at low concentrations may result in the spread and proliferation of antibiotic-resistant bacteria (ARB) and antibiotic-resistance genes (ARGs) (Wang et al. 2022). Infections with ARB are hard to treat due to the reduced antibiotic efficacy, and lead to increased morbidity and mortality. Statistics indicated that the number of deaths from antibiotic-resistant infections was estimated at 7 × 105 per year worldwide (Shao et al. 2022). Therefore, it is highly urgent and imperative to remove antibiotics, including SAs and FQs, from aquatic systems.

Adsorption is considered as a promising approach for elimination of antibiotics from water owing to its simple operation, safety, low cost, and high efficiency (Gao et al. 2021). Among the most frequently used adsorbents, biochar derived from the thermal pyrolysis of waste biomass under oxygen-limiting conditions is an effective and low-cost adsorbent because of its porous structure, large specific surface area, and abundant surface functional groups. However, owing to the small size and low density of biochar, separating powdered particles after adsorption is difficult in practical applications, which may cause secondary pollution (Li et al. 2020b). Magnetic biochar is a biochar composite material loaded with magnetic components, which not only maintains the excellent properties of biochar, but also possesses the magnetic separation characteristics for recovery and reuse (Kang et al. 2023). In addition, the magnetization process can change the surface and porous structure of biochar and create additional active sites, thereby significantly improving its adsorption properties (Jung et al. 2017). Therefore, magnetic biochar has been extensively applied as an economic, efficient, and sustainable adsorbent for antibiotic removal.

Compared with other magnetic media, iron-based materials, including Fe3O4, Fe2O3, and Fe0, are most commonly used to synthesize magnetic biochar because of their low cost, environmental friendliness, and high activity. Many studies have been recently conducted to evaluate the adsorption performance of iron-based magnetic biochar on SAs and FQs. For example, coconut shell biochar modified by KOH activation and FeCl3 magnetization showed a good SA removal efficiency, and the adsorption capacities were five to seven times higher than those of raw biochar (Sun et al. 2022). Yang et al. (2023) synthesized bamboo-based magnetic biochar (Fe3O4) using the ZnCl2 activation and coprecipitation method, and the adsorption capacity of sulfadiazine was enhanced to 645 mg/g. Ma et al. (2022) fabricated a magnetic sludge biochar (γ-Fe2O3) under hydrothermal conditions for removing ciprofloxacin, norfloxacin, and ofloxacin with a maximum adsorption capacity of 49.9 mg/g, 55.7 mg/g, and 47.4 mg/g, respectively. Although numerous studies have reported the excellent adsorption performance of magnetic biochar for antibiotics, most have focused on the adsorptive removal of a single antibiotic or a class of antibiotics. However, the adsorption efficiency is not only related to the physiochemical properties of magnetic biochar and environmental conditions, but also to the types of antibiotics. It is not comprehensive enough to evaluate the adsorbent effectiveness based on only one type of antibiotics. To the best of our knowledge, few studies have been conducted on the adsorptive elimination of two different groups of antibiotics to assess the adsorption performance of magnetic biochar.

In this study, coconut shell, a typical abundant and cheap agricultural waste, was used as the raw material to prepare magnetic biochar using the chemical coprecipitation method. Sulfadiazine (SDZ) and norfloxacin (NOR) were selected as the target SA and FQ, respectively. The primary aims of this study were to (1) analyze the physicochemical properties of magnetic biochar and pristine biochar using various characterization techniques; (2) investigate the adsorption performance of SDZ and NOR on magnetic biochar through batch experiments; (3) evaluate the influence of initial pH, ionic strength, and temperature on the SDZ and NOR adsorption processes; and (4) elucidate the possible adsorption mechanisms of SDZ and NOR onto magnetic biochar. The findings of this study will provide a scientific basis for the application of magnetic biochar in the treatment of wastewater containing SDZ and NOR.

Materials and methods

Materials and chemicals

The coconut shell biochar used in this study was obtained from Henan Lize Environmental Protection Technology Co., Ltd. (Henan, China). The biochar powder was ground, passed through an 80-mesh sieve (0.177 mm), and sealed in a reservoir. Sulfadiazine (SDZ, > 99%) and norfloxacin (NOR, > 98%) standards were purchased from Sigma-Aldrich (USA). The structures and physicochemical properties of the two antibiotics are listed in Table S1. FeCl2·4H2O (> 99%) and FeCl3·6H2O (> 99%) were supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Other chemicals, including NaCl, HCl, NaOH, and ethanol, were of analytical grade and acquired from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China).

Preparation of magnetic biochar

The magnetic biochar was prepared using a chemical coprecipitation method (Fig. 1). Briefly, 2 g FeCl2·4H2O and 6.66 g FeCl3·6H2O were dispersed into a beaker with 200 mL of ultrapure water and magnetically agitated to obtain a mixed solution of Fe2+ and Fe3+ (molar ratio of 1:2). Afterwards, 5 g coconut shell biochar powder was immersed into this solution, and the mixture was thoroughly stirred at 150 r/min for 30 min at 40 ℃. Then 5 M NaOH solution was added dropwise under stirring at 40 ℃ until the pH of the reaction solution reached 10–11. The iron-containing precipitates were formed during this process according to Eq. (1). The suspension was then continuously stirred for an hour and stayed to settle for 12 h at room temperature. The retained solids after filtration were washed with ultrapure water until the pH of the filtrate was neutral. The residue was then rinsed with ethanol and dried at 50 ℃ to a constant weight. Finally, the magnetic biochar sample was crushed and sieved through an 80-mesh sieve and reserved for future use.

Fig. 1
figure 1

Schematic illustration of the preparation of magnetic biochar

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$$\mathrm{Fe}^\mathrm{2+}+\mathrm{2Fe}^\mathrm{3+}+\mathrm{8OH}^-\rightarrow{\mathrm{Fe}}_3{\mathrm{O}}_4+{\mathrm{4H}}_2\mathrm{O}$$
(1)

Characterization

The surface morphology of the biochars was observed using scanning electron microscopy (SEM, Regulus 8100, Hitachi) by an acceleration electron voltage at 3 kV. The possible crystalline structures were obtained using an X-ray powder diffractometer (XRD, Ultima IV, Rigaku) equipped with Cu Kα radiation over a scanning interval (2θ) of 10°–90° with a step increment of 0.05, a scan step time of 1 s, a voltage of 40 kV, and a current of 40 mA. The elemental composition and valence states were characterized using X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Scientific), and the binding energies of the spectra were calibrated with the C1s peak at 284.8 eV. The spectra were excited using monochromatic Al Kα X-rays (hv = 1486.6 eV) with a power of 150 W and a beam spot of 500 μm. The specific surface area and pore size distribution were assessed by the Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively through N2 adsorption–desorption measurements (Autosorb-iQ, Quantachrome, USA) under liquid nitrogen (− 196 ℃). The surface functional groups were identified using Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700, Thermo Scientific) with a resolution of 4 cm−1 in the wavenumber range of 4000–400 cm−1. Samples were mixed with KBr and then pressed into pellets, and the spectral resolution was 1 cm−1 and the number of scans was 60. The surface charge of the biochar was analyzed by zeta potential measurements using a Zetasizer Nano ZS90 (Malvern instruments, UK). Samples were pre-equilibrated in a 0.01 M NaCl solution at pH of 3–9 for 4 h and then shaken for 24 h at 25 ℃. The zeta potential of the biochar at each pH was measured, and the pH of the zero-point charge (pHzpc) was determined. The magnetic hysteresis loop was recorded by a vibrating sample magnetometer (Lakeshore 7404, USA) at room temperature under applied magnetic fields of ± 20 kOe.

Batch adsorption experiments

All adsorption experiments were conducted in 50 mL polypropylene centrifuge tubes containing 50 mg of biochar and 20 mL of SDZ or NOR solution. The tubes were placed in a shaking incubator (HZQ-F160A, Yiheng, Shanghai, China) at 25 ℃ and 200 rpm. For the adsorption kinetics, the initial SDZ or NOR concentration was 10 mg/L, and the pH was maintained at 4. The samples were collected at specific time intervals (5, 10, 20, 40, 60, 90, 120, 180, 240, 300, and 360 min). Adsorption isotherm experiments were conducted by adding 50 mg of biochar to 20 mL of SDZ or NOR solutions at initial concentrations of 5, 10, 15, 20, 30, 40, and 50 mg/L for 240 min. Experimental investigation of the effect of temperature on the adsorption process was performed under the same test conditions as isotherm experiments except for the temperatures at 15, 25, and 35 ℃, respectively. To examine the effect of pH on the adsorption capacity of magnetic biochar, the initial solution pH values were adjusted to a range of 3–9 with 0.1 M NaOH and HCl. The influence of ionic strength was evaluated at NaCl concentrations of 0.05–0.5 mol/L. After adsorption, each sample was centrifuged and filtered through a 0.22 μm membrane. The concentrations of SDZ and NOR were analyzed using a UV–vis spectrophotometer (UV-2550, Shimadzu, Japan) at wavelengths of 254 and 273 nm, respectively. All experiments were performed in triplicate, and the average values were reported.

Data analysis

The amount of antibiotic adsorbed by the biochar was calculated using Eq. (2):

$${\mathrm{Q}}=\frac{\mathrm{(}{\mathrm{c}}_{0}-{\mathrm{c}}_{\mathrm{e}}\mathrm{)V}}{\mathrm{m}}$$
(2)

where Q (mg/g) is the adsorption amount; C0 (mg/L) and Ce (mg/L) are the initial and equilibrium concentrations of antibiotics, respectively; m (mg) is the dosage of the biochar; and V (mL) is the solution volume.

The adsorption kinetic curves of the antibiotics onto the biochar were fitted using pseudo-first-order (Eq. (3)) and pseudo-second-order models (Eq. (4)). The models are as follows:

$${\mathrm{ln}}\mathrm{(}{\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{t}}\mathrm{)=}{\mathrm{ln}}{\mathrm{Q}}_{\mathrm{e}}-{\mathrm{k}}_{1}{\mathrm{t}}$$
(3)
$$\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{t}}}\mathrm{=}\frac{1}{{\mathrm{k}}_{2}{{\mathrm{Q}}}_{\mathrm{e}}^{2}}\mathrm{+}\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{e}}}$$
(4)

where Qt (mg/g) and Qe (mg/g) are the adsorption amount at time t (min) and equilibrium, respectively; k1 (1/min) and k2 (g/(mg min)) represent the pseudo-first-order and pseudo-second-order rate constant, respectively.

To identify a suitable kinetic model, a chi-square analysis was conducted based on the following mathematical equation:

$${\chi }^{2}=\sum \frac{{\left({\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{e},\mathrm{m}}\right)}^{2}}{{\mathrm{Q}}_{\mathrm{e},\mathrm{m}}}$$
(5)

where Qe and Qe,m are the equilibrium adsorption capacities obtained from the experimental data and the model, respectively.

The adsorption isotherm data were fitted using the Langmuir (Eq. (6) and Eq. (7)), Freundlich (Eq. (8)), and Dubinin-Radushkeich (D-R) models (Eq. (9) and Eq. (10)). The equations are as follows:

$$\frac{{\mathrm{C}}{\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{Q}}_{\mathrm{m}}{{\mathrm{K}}}_{\mathrm{L}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{m}}}$$
(6)
$$\mathrm{R}_\mathrm{L}=\frac1{1+\mathrm{K}_\mathrm{L}{\mathrm{C}}_0}$$
(7)
$${\mathrm{log}}{\mathrm{Q}}_{\mathrm{e}}={\mathrm{log}}{\mathrm{K}}_{\mathrm{F}}+ \, \frac{1}{{\mathrm{n}}}{\mathrm{log}}{\mathrm{C}}_{\mathrm{e}}$$
(8)
$$\mathrm{In} {\mathrm{Q}}_{\mathrm{e}}={\mathrm{lnQ}}_{\mathrm{m}}- \, \beta {\mathrm{(RTln(}\frac{{\mathrm{C}}_{\mathrm{e}}+{1}}{{\mathrm{C}}_{\mathrm{e}}}\mathrm{))}}^{2}$$
(9)
$${\mathrm{E}}={\mathrm{(2}\beta \mathrm{)}}^{-\mathrm{0.5}}$$
(10)

where Qm (mg/g) is the maximum adsorption capacity of antibiotics; KL (L/mg) is the Langmuir equilibrium constant; RL is dimensionless separation factor; KF (mg/g·(L/mg)1/n) is the Freundlich equilibrium constant and represents the adsorption capacity; n is the isothermal constant and represents the adsorption strength; β (mol2/J2) is the constant related to adsorption energy; R (8.314 J/mol K) is the molar gas constant; T (K) is reaction temperature; and E (KJ/mol) is the adsorption free energy.

The thermodynamic parameters, including enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG), were calculated based on the following equations:

$$\Delta {\mathrm{G}}=-{\mathrm{RTlnK}}$$
(11)
$$\Delta {\mathrm{G}}=\Delta {\mathrm{H}}-{\mathrm{T}}\Delta {\mathrm{S}}$$
(12)

where K denotes the thermodynamic equilibrium constant. ln K was calculated by plotting lnKd (Kd = Qe/Ce) versus Ce and extrapolating Ce to zero.

Results and discussion

Characterization of biochar and magnetic biochar

The microscopic morphologies of the coconut shell biochar and magnetic biochar were characterized using SEM. As shown in Fig. 2a, the surface of the coconut shell biochar was uneven and consisted of particles of different sizes. At high magnification, holes, voids, and irregular spherical protrusions were observed on the biochar surface (Fig. 2b). After coprecipitation, the magnetic biochar exhibited a rougher surface with numerous spherical particles in various sizes (Fig. 2 c and d), indicating that the iron oxide particles were successfully loaded onto the surface of the biochar.

Fig. 2
figure 2

SEM images of biochar (a, b) and magnetic biochar (c, d)

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The phase structure of the biochar was investigated using XRD. As shown in Fig. 3a, the XRD patterns showed distinct diffraction peaks for the pristine and magnetic biochars, confirming their different crystal structures. For coconut-shell biochar, two diffraction peaks at 23° and 43° were ascribed to the (002) and (100) crystal planes, respectively, indicating the presence of amorphous carbon and graphite structures (Kicinski et al. 2013). After magnetization, several characteristic peaks were detected at 30.1°, 35.5°, 43.1°, 56.9°, and 62.6°, corresponding to the (220), (311), (400), (511), and (440) planes of magnetite (Fe3O4), respectively (JCPDS No. 65–3107) (Li et al. 2019b). The strong and sharp XRD reflection peaks confirmed the successful deposition of Fe3O4 particles on the biochar surface. The reflections of pristine biochar were not observed because of the overwhelmingly strong signals of Fe3O4 (Zhang et al. 2012). In addition, the peak positions of the XRD pattern were also highly matched with the standard JCPDS No. 39–1346, namely 30.24° (220), 35.63° (311), 43.28° (400), 53.73° (422), 57.27° (511), and 62.92° (440), demonstrating that the magnetic biochar may also contain maghemite (γ-Fe2O3) (Yang et al. 2021).

Fig. 3
figure 3

XRD patterns (a), FTIR spectra (b) and Zeta potentials (c) of biochar and magnetic biochar, and magnetization curves of magnetic biochar (d)

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XPS spectra were further obtained to determine the elemental compositions and chemical states of magnetic biochar. The survey spectrum shown in Fig. 4a indicates the presence of C (38.5%), O (39.4%), and Fe (22.1%) elements in the magnetic biochar. The high resolution XPS spectra of C 1 s, O 1 s, and Fe 2p, and deconvoluted spectra are shown in Fig. 4b–d. The C 1 s spectrum was deconvoluted into four peaks. The binding energy at 284.6, 285.5, 286.7, and 288.8 eV were attributed to the C–C/C═C, C–O, C═O (carbonyl), and –COOH (carboxyl) bonds, respectively (Zhu et al. 2024). The O 1 s spectrum contained three peaks at 529.9, 531.2, and 533.1 eV, which can be ascribed to the Fe–O, Fe–OH/C═O, and C–O/O–H, respectively (Jin et al. 2023). Two peaks at 710.6 and 724.4 eV in the high-resolution Fe 2p spectrum were assigned to Fe 2p3/2 and Fe 2p1/2, respectively. The Fe 2p peaks were decomposed into six peaks. Peaks with binding energy of 718.1 and 732.0 eV belong to satellite peaks (Sat.) (Wang et al. 2023). Additionally, the peaks at 710.2, 712.0, 723.8, and 725.8 eV correspond to Fe2+ 2p3/2, Fe3+ 2p3/2, Fe2+ 2p1/2, and Fe3+ 2p1/2, respectively, suggesting the co-existence of Fe2+ and Fe3+ (Bai et al. 2021). This further proved the successful preparation of Fe3O4 and γ-Fe2O3, which agrees with the results of XRD analysis. When the NaOH solution was added dropwise into a Fe2+/Fe3+ mixed solution, Fe3O4 was formed via the chemical reaction, as expressed in Eq. (1). The generated Fe3O4 can be further converted to γ-Fe2O3+and Fe2+, according to Eq. (9) (Feng et al. 2021). Overall, these findings demonstrated that magnetic media Fe3O4 and γ-Fe2O3 were successfully loaded on the biochar surface. To further elucidate this, the magnetization hysteresis loop of the magnetic biochar was recorded at room temperature. As shown in Fig. 3d, the magnetic saturation (Ms) was 15.75 emu/g, indicating that biochar exhibited a superior magnetic response. The inset reveals that the biochar particles can be easily separated by an external magnetic field, which again proves the attachment of magnetic iron oxides to the surface of the biochar.

Fig. 4
figure 4

XPS spectra of the magnetic biochar: a survey spectrum, b C 1 s, c O 1 s, and d Fe 2p

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$${\mathrm{Fe}}_3{\mathrm{O}}_4+\mathrm{2H}^+\rightarrow\gamma-\mathrm{Fe}_2{\mathrm{O}}_3+\mathrm{Fe}^{2+}+{\mathrm{H}}_2\mathrm{O}$$
(13)

The N2 adsorption–desorption isotherms and pore-size distributions of the biochar are shown in Fig. S1; the textural parameters are listed in Table 1. BET results revealed that the specific surface areas of coconut shell biochar and magnetic biochar were 905 m2/g and 529 m2/g, respectively. The total pore volume and average pore diameter of magnetic biochar were increased compared with pristine biochar. For biochar, according to IUPAC classification, high adsorption potential energy at low relative pressure belonged to type I isotherm, suggesting a microporous-dominated structure. This result coincides with the pore size distribution characteristics of biochar, with pore diameters mainly distributed in the range of 1–2 nm in BJH plots. After magnetization, with the increase of P/P0 to 0.4–1.0, the isotherms of magnetic biochar exhibited type H4 hysteresis loops. A mixture of type I and IV isotherm curves was indicative of the existence of partial mesopores in magnetic biochar (2–50 nm). The pore size distribution profiles further confirmed the microporous and mesoporous feature of this material. The higher mesoporous structure of the magnetic biochar may be related to the merging and collapse of the micropore walls to form more mesopores during the formation and penetration of iron-oxide particles into the inner biochar structure (Li et al. 2017; Yang et al. 2019). The increased mesoporosity contributed a higher pore volume and lower surface area of magnetic biochar (Liu et al. 2023). Additionally, the decreased surface area of magnetic biochar was also associated with the aggregated iron oxide particles, which may inlay or occupy certain pore channels of the biochar (Dong et al. 2022). The increase in average pore diameter was attributed to the pore expansion effects of iron oxide particles after magnetic functionalization, which was consistent with the findings of Feng et al. (2023).

Table 1 BET surface area (SBET), total pore volume (Vp), and average pore diameter (Dp) values of biochar and magnetic biochar
Full size table

The functional groups of the biochars were determined using FTIR spectroscopy. As illustrated in Fig. 3b, the strong peak at 3427 cm−1 was attributed to –OH stretching vibration in H–O–H or –COOH. The peak at 2920 cm−1 corresponded to the stretching of the C–H bond of the methyl and methylene groups (Liu et al. 2018). The bands in the range of 1625 cm−1 and 1563 cm−1 might be due to the skeletal vibration of the benzene ring C = C or C = O stretching modes in the aromatic structure (Hao et al. 2018). The peak observed at 1052 cm−1 represented the C–O–C stretching vibration (Zhou et al. 2017b). The peak at 667 cm−1 was assigned to the C–H bending, which shifted to 628 cm−1 after magnetization (Saeed et al. 2021). These data were in accordance with the XPS spectra, which indicated the presence of abundant –OH and –COOH groups on the surface of the magnetic biochar. The strong adsorption peak appearing at 575 cm−1 in magnetic biochar was ascribed to Fe–O of iron oxides, confirming the successful loading of iron oxides onto the biochar surface (Chen et al. 2011).

The zeta potential of the biochar was measured as a function of pH (Fig. 3c). The pH at the zero-point charge (pHzpc) was determined by interpolating the pH value at a zero zeta potential. When the pH value increased from 3 to 9, the zeta potential of coconut shell biochar decreased from − 31.8 to − 53.2 mV, suggesting the negatively charged surface of the biochar within the pH range studied. Compared with the pristine biochar, the zeta potential of the magnetic biochar changed to a positive value at pH 3 and 4. The pHzpc of magnetic biochar was at ~ pH = 4.2. Thus, the magnetic process decreased the pHzpc of the biochar, which may have been caused by the surface coverage with iron oxides and the increase in surface acidic functional groups since the slightly increased –OH stretch absorbance at 3427 cm−1 in the FTIR spectra (Hao et al. 2018).

In summary, based on various characterization techniques, Fe3O4 and γ-Fe2O3 were successfully loaded on the surface of coconut shell biochar. The magnetic biochar exhibited a microporous and mesoporous structure, more abundant functional groups, a lower pHzpc value, and sufficient magnetic separability, which could offer a better adsorption capacity for antibiotics removal and an effective approach for the recovery of biochar simultaneously.

Adsorption performance

The adsorption properties and environmental safety of biochar and magnetic biochar

The removal rates and adsorption capacities of coconut shell biochar and magnetic biochar for SDZ and NOR were compared, and the results are presented in Fig. S2. Both the biochar and magnetic biochar exhibited excellent SDZ and NOR removal efficiencies. The removal rates of SDZ and NOR by biochar were 93.8% and 92.4%, respectively, which became 94.8% and 92.3% after magnetic modification. In addition, the adsorption amounts of SDZ and NOR were not significantly different between the biochar and magnetic biochar. Therefore, magnetic modification of coconut shell biochar not only maintained a high adsorption capacity but also enabled magnetic separation of the biochar. The following sections mainly focus on discussing the adsorption behavior and mechanism of magnetic biochar for SDZ and NOR.

The environmental safety of magnetic biochar is critical for its practical application. The leaching of Fe ion from magnetic biochar during adsorption process may cause secondary water pollution. Therefore, the leaching concentrations of Fe after SDZ and NOR adsorption were determined by inductively coupled plasma-mass spectrometry (Agilent 7800). Results showed that the concentrations of Fe in the SDZ and NOR solution (10 mg/L, 2.5 g/L, pH = 4, contact time 240 min) were 0.46 and 0.35 mg/L, respectively. The leaching concentrations were below 0.5 mg/L (Standards for Drinking Water Quality, GB5749-2006), which indicated that the magnetic biochar synthesized in this study exhibited a good stability and environmental security during SDZ and NOR removal.

Adsorption kinetics

Pseudo-first-order and pseudo-second-order kinetic models were fitted to investigate the adsorption kinetics of SDZ and NOR on magnetic biochar. The fitting curves are shown in Fig. 5. The parameters obtained are listed in Table 2. In the initial stage, SDZ and NOR were rapidly adsorbed, which was attributed to the large number of vacant adsorption sites on the biochar surface and the high concentration of antibiotics at the beginning of the reaction. The binding sites were gradually occupied as the reaction proceeded, and the adsorption efficiency decreased until equilibrium was reached after ~ 240 min. The maximum adsorption capacities of magnetic biochar for SDZ and NOR were 3.79 and 3.69 mg/g, respectively. The fitting results of the two kinetic models (Table 2) indicate that both models are applicable for describing the adsorption processes of SDZ and NOR because of their high correlation coefficients (R2 > 0.99).

Fig. 5
figure 5

Adsorption kinetics of SDZ and NOR on magnetic biochar

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Table 2 Kinetic model parameters for adsorption of SDZ and NOR on the magnetic biochar
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To identify a suitable kinetic model, a chi-square analysis was conducted and χ2 values were obtained. The much lower χ2 values of the pseudo-second-order model for SDZ (5.4 × 10−5) and NOR (4.7 × 10−5) revealed that this model could better explain the adsorption kinetic process. The pseudo-second-order model assumes that the adsorption rate is determined by the square of the number of unoccupied adsorption vacancies on the adsorbent surface and that the adsorption process is mainly controlled by the chemisorption mechanism (Zhang et al. 2020b). Consequently, these results imply that chemical adsorption involving the sharing or exchange of electrons or valency forces between SDZ or NOR and the functional groups of the magnetic biochar may dominate the adsorption process (Zou et al. 2022).

Adsorption isotherms

The adsorption isotherms of SDZ and NOR fitted by the Langmuir, Freundlich, and D-R models are shown in Fig. 6. The estimated parameters are listed in Table 3. With an increase in the initial concentration, the equilibrium adsorption capacities of magnetic biochar for SDZ and NOR increased accordingly. The fitting results suggested that the isothermal adsorption process better followed the Langmuir model, as reflected by the higher R2 values of the Langmuir model for SDZ (0.946) and NOR (0.954). This illustrated that SDZ and NOR adsorption occurred on the surface of the magnetic biochar, and homogeneous monolayer coverage was the main mechanism (Li et al. 2019a). The Qm values of SDZ (16.7 mg/g) and NOR (25.8 mg/g) derived from the Langmuir model were much higher than the experimental data, probably because the equilibrium concentrations were not sufficiently high (Guan et al. 2022).

Fig. 6
figure 6

Adsorption isotherms of SDZ and NOR on magnetic biochar

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Table 3 Isotherm model parameters for adsorption of SDZ and NOR on magnetic biochar
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The dimensionless separation factor (RL) estimated from the Langmuir parameters was used to predict the affinity between the adsorbate and adsorbent, with a value of RL < 1 representing favorable adsorption (Zhu et al. 2014). The calculated RL values under different initial SDZ (0.031–0.244) and NOR (0.204–0.719) concentrations were < 1, confirming the energetically favorable adsorption of SDZ and NOR onto magnetic biochar (Velusamy et al. 2021). The D-R isotherm model was used to elucidate the interactions between the antibiotics and the magnetic biochar. Although the D-R model was not the best-fit model for the SDZ (R2 = 0.778) and NOR (R2 = 0.864) isothermal adsorption, it could provide useful information about the types of adsorption processes. Values of mean adsorption energy (E) > 8 kJ/mol indicate chemical adsorption; otherwise, they indicate physical adsorption (Zhou et al. 2017a). The E values in this study were 11.8 and 12.3 kJ/mol for SDZ and NOR, respectively, implying that chemisorption was the main mechanism, which is consistent with the fitting results of the pseudo-second-order model.

Adsorption thermodynamics

The effect of temperature on the SDZ and NOR adsorption processes was determined, and adsorption experiments were conducted at 288–308 K. As shown in Fig. 7, the adsorption capacities of magnetic biochar for SDZ and NOR increased with increasing temperature, suggesting that higher temperatures were beneficial for SDZ and NOR adsorption. The enhanced adsorption may be attributed to the activation of the adsorbent surface and the reduction of the reaction barrier at higher temperatures, which is more conducive to mass transfer between SDZ or NOR and magnetic biochar (Zhao et al. 2019). The values of all the thermodynamic parameters are listed in Table 4. Negative values for ΔG at three temperatures indicated that the adsorption of SDZ and NOR onto magnetic biochar was spontaneous and thermodynamic favorable (Meng et al. 2020). Furthermore, raising the temperature decreased ΔG values, revealing that the adsorption affinity became stronger at a higher temperature (Li et al. 2020a). The spontaneous strength of SDZ adsorption onto the magnetic biochar was greater than that of NOR adsorption, confirming the superior adsorption performance of SDZ. The positive ΔH values (13.1 kJ/mol for SDZ and 11.0 kJ/mol for NOR) implied that the adsorption reaction of SDZ and NOR on magnetic biochar was an endothermic process. The positive ΔS values (0.0603 kJ/(mol K) for SDZ and 0.0495 kJ/(mol K) for NOR) reflected the increase of randomness at the solid–liquid interface, suggesting the adsorption process of SDZ and NOR was irreversible (Zhu et al. 2014).

Fig. 7
figure 7

Effect of temperature on the adsorption of SDZ and NOR on magnetic biochar

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Table 4 Thermodynamic parameters for SDZ and NOR adsorption on the magnetic biochar
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Effect of initial pH

The effect of pH on SDZ and NOR adsorption by magnetic biochar was studied within the pH range of 3–9. As illustrated in Fig. 8a, the overall trends in the influence of initial pH on SDZ and NOR adsorption were similar. With an increase in pH from 3 to 4, the adsorption capacities of SDZ and NOR on the magnetic biochar increased and then decreased as the pH increased to 9. Maximum adsorption occurred at pH 4 for both antibiotics. The solution pH can change the surface charges of biochar and the speciation of antibiotics, thereby affecting the interactions between magnetic biochar and SDZ or NOR. The pHzpc of the magnetic biochar was 4.2, and its surface charge was positive at pH < 4.2 and negative at pH > 4.2. As revealed in Fig. S3, SDZ has two pKa values: 1.57 (amino) and 6.5 (sulfonamide), corresponding to the cation (pH < 1.57), neutral (1.57 < pH < 6.5), and anion species (pH > 6.5) (Oymak and Şafak 2022). Similarly, the pKa1 (carboxylic) and pKa2 (piperazinyl) of NOR are 6.22 and 8.51, and they can exist in cationic (pH < 6.22), zwitterionic (6.22 < pH < 8.51), and anionic form (pH > 8.51) (Fig. S4) (Yan et al. 2017).

Fig. 8
figure 8

Effect of initial pH (a) and ionic strength (b) on the adsorption of SDZ and NOR on magnetic biochar

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For SDZ, the percentage of cationic species is 3.6% at pH 3 (Xu et al. 2011). The lower removal efficiency may be due to electrostatic repulsion between SDZ+ and the positively charged magnetic biochar. When the pH increased from 3 to 4, the cationic fraction declined, and neutral species (SDZ0) became the dominant form. Maximum adsorption occurred at this pH value, suggesting that SDZ0 played a vital role in the overall adsorption of SDZ onto the magnetic biochar. Electrostatic interactions were insignificant for the neutral SDZ molecules because they were not ionized. As the pH increased, the decrease in SDZ adsorption was attributed to a decrease in the proportion of the neutral form. When the pH was further increased to > 6.5, the electrostatic repulsion between SDZ− and the negatively charged magnetic biochar restrained the adsorption capacity because of the increased anionic fraction.

NOR+ was the major species at pH < 4. As the pH increased from 3 to 4, the proportion of NOR+ decreased, and the surface of the magnetic biochar gradually became less positively charged. Thus, the electrostatic repulsion between the positively charged magnetic biochar and NOR+ weakened, leading to an increase in the adsorption of NOR. The adsorption forces between magnetic biochar and NOR+ in an acidic environment are mainly cation exchange and hydrogen bonds (Wang et al. 2020). At pH 5, the surface of the magnetic biochar became negatively charged, and NOR remained cationic. Nevertheless, the decrease in adsorption amount manifested that some other interactions rather than electrostatic attraction were involved in the adsorption process, such as π-π EDA interaction and hydrogen bonds (Yang et al. 2012). With a continuous increase in pH, the electrostatic repulsion between the emerging anionic species of NOR and the negatively charged biochar reduced its adsorption efficiency.

Effect of ion strength

Various concentrations of NaCl (0.05–0.5 mol/L) were added to the solution to explore the effect of ionic strength on the adsorption of SDZ and NOR. As shown in Fig. 8b, the adsorption efficiencies of SDZ and NOR did not change with increasing NaCl concentration, suggesting that electrostatic interactions play a limited role in SDZ and NOR adsorption (Xu et al. 2018). Ionic strength exerts a two-sided effect on the adsorption process. The addition of NaCl decreases the solubility of antibiotics by salting them out, thereby promoting their diffusion to the biochar surface and improving the adsorption capacity (Peng et al. 2018). Furthermore, Na+ competes with antibiotics and partially substitutes the hydrogen ions of acidic groups on the magnetic biochar surface (i.e., the squeezing-out effect), which inhibits the formation of hydrogen bonds and restricts SDZ and NOR adsorption (Nguyen et al. 2022). The influence of ionic strength is a trade-off between the decrease in antibiotic solubility and the weakening of hydrogen bonds. The salting-out effect was considerable along with the squeezing-out effect in this study; therefore, the ionic strength had no notable effect on the adsorption behaviors of SDZ and NOR. This result also confirmed that the main adsorption forces of SDZ and NOR were not surface electrostatic effects, which is consistent with the results of the pH effect experiments. In addition, Na+ ions compete with NOR+ for adsorption sites via ion exchange, thereby reducing NOR adsorption. However, the NOR adsorption capacity was not reduced by increasing the ion strength, demonstrating the insignificant contribution of cation exchange to NOR adsorption onto the magnetic biochar.

Adsorption mechanism

The interactions between SDZ or NOR and magnetic biochar are controlled by the physicochemical properties of the biochar and antibiotics, as well as environmental conditions. Notably, the highest adsorption was obtained at pH 4, where SDZ and NOR existed in neutral and cationic forms, respectively, and the surface of the magnetic biochar was positively charged. The lower adsorption capacity for NOR compared to that of SDZ may be associated with the electrostatic repulsion between NOR+ and the magnetic biochar. Moreover, the hydrophobicity of organic compounds, characterized by the octanol–water partition coefficient (LogKow), is a crucial factor for hydrophobic adsorption. SDZ and NOR are hydrophilic compounds with LogKow values < 2; thus, the hydrophobic effect has a limited influence on their adsorption processes (Razanajatovo et al. 2018). Meanwhile, the LogKow values followed the order NOR > SDZ, which did not coincide with the magnitude of the adsorption efficiencies, implying that hydrophobic interactions could not be a primary mechanism. Based on the effects of pH and ionic strength on the adsorption performance, electrostatic attraction and ion exchange may not occur between SDZ/NOR and magnetic biochar, and other interactions are responsible for their adsorption.

To further elucidate the adsorption mechanisms, magnetic biochar after SDZ and NOR adsorption was characterized by BET, FT-IR, and XPS analysis. The surface area total pore volume and average pore diameter of magnetic biochar after antibiotic adsorption were measured, and results were shown in Table S2. The surface area, total pore volume, and average pore diameter of magnetic biochar decreased to 304 m2/g, 0.22 cm3/g, and 2.94 nm, respectively after SDZ adsorption and to 335 m2/g, 0.23 cm3/g, and 2.76 nm, respectively after NOR adsorption. These reduced values are attributed to the occupy and blockage of SDZ and NOR molecules on the surface of magnetic biochar, confirming that pore-filling effect played a dominant role in their adsorption (Li et al. 2021).

The FT-IR spectra of the magnetic biochar before and after adsorption are shown in Fig. S5. The –OH peak shifted from 3427 to 3406 cm−1 after the absorption of SDZ, indicating hydrogen bonds between the –NH2– and –NH– groups of SDZ and the O atoms in –OH of the biochar surface (Zhao et al. 2016). In addition, after SDZ adsorption, the transfer of bands around 1625–1563 cm−1 for C = C or C = O to 1541–1479 cm−1 supported the π-π EDA interaction that existed between SDZ and magnetic biochar. The SDZ molecule can be a strong π-electron acceptor owing to the amino (–NH2–) and sulfonamide (–SO2NH–) groups (Kah et al. 2017). The C = C on magnetic biochar surface can act as a π-electron donor, and therefore, the formation of π-π EDA interaction was another adsorption mechanism. Moreover, the Fe–O peak shifted from 575 to 560 cm−1 after adsorption, possibly because of the surface complexation of the magnetic biochar with SDZ (Zhang et al. 2023a). Additionally, the loan-pair electrons of the amino groups of SDZ may donate to form a complex with the protonated enriched surface functional groups of the magnetic biochar, suggesting that Lewis acid–base interactions may participate in the adsorption process (Ahmed et al. 2017). For NOR, the stretching peaks of –OH and C = C/C═O groups on the magnetic biochar shifted slightly after adsorption, implying NOR was adsorbed through hydrogen bonds and π-π interactions. Oxygen-containing functional groups, such as –OH –COOH, can interact with O, N, and F atoms on the piperazinyl ring of NOR via hydrogen bonds (Paredes-Laverde et al. 2018). The π-π EDA interaction was formed between the benzene ring of NOR and the aromatic ring of magnetic biochar (π-electron donors) since NOR can be regarded as a π-electron acceptor due to the fluorine-contained groups with a strong electron-accepting ability (Wang et al. 2017). In addition, NOR could also bind to magnetic biochar via surface complexation, as revealed by the shift in the Fe–O adsorption peak.

The XPS analysis of magnetic biochar after adsorption was used to further elaborate the possible mechanism (Fig. S6 and S7, Table S3). The peak area ratios of C–C/C═C in C 1 s after SDZ and NOR adsorption decreased from 49.59 to 45.72% and 42.85%, respectively, revealing that π-π EDA interaction played a crucial role in their adsorption (Zhang et al. 2023b). Moreover, the relative percentages of C = O in C 1 s after SDZ and NOR adsorption increased from 10.26 to 16.35% and 19.21% respectively. Meanwhile, the –COOH content was slightly decreased from 11.84 to 10.50% after SDZ adsorption and increased to 14.30% after NOR adsorption. The peak area ratios of Fe–OH/C = O and C–O/O–H in O 1 s exhibited a considerable increase after adsorption, especially for the C–O/O–H increasing from 0.15 to 26.79% and 18.13% after SDZ and NOR adsorption, respectively. These indicate that oxygen-rich functional groups, such as –OH and –COOH groups were involved in the adsorption process, which can interact with SDZ via hydrogen bonds or Lewis acid–base interaction, and with NOR through hydrogen bonds (Lv et al. 2021). In addition, the Fe–O content in O 1 s was significantly reduced from 68.58 to 40.77% and 43.35% after SDZ and NOR adsorption, respectively, suggesting surface complexation may exist for their removal (Li et al. 2022). The binding energy of Fe 2p peaks of magnetic biochar shifted to higher sites after adsorption, thus confirming that Fe participated in the adsorption (Ma et al. 2022). The decrease in Fe2+ content from 41.60 to 36.49% and 34.34% after SDZ and NOR adsorption, respectively, illustrates that Fe II have undergone electron transfers during adsorption, which was attributed to surface complexation between antibiotic and magnetic biochar (Zhang et al. 2020a). The results of XPS were in accordance with those of FTIR.

Based on the above analysis, it is therefore concluded that the adsorption processes of SDZ and NOR onto magnetic biochar could be governed by pore filling, π-π EDA interaction, hydrogen bonds, and surface complexation. Besides, Lewis acid–base interaction was also responsible for the SDZ adsorption (Fig. 9).

Fig. 9
figure 9

Schematic illustration of the adsorption mechanisms of SDZ and NOR onto magnetic biochar

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Conclusions

In this study, magnetic coconut shell biochar with a well-developed porous structure, numerous functional groups, a lower pHzpc value, sufficient magnetic separability, and good environmental security was successfully prepared using a chemical coprecipitation method. The magnetic biochar exhibited good adsorption performance for SDZ and NOR, with the removal rate of above 90%. SDZ and NOR were mainly chemisorbed onto the magnetic biochar surface as a monolayer, and the adsorption process was spontaneous and endothermic. The acidic conditions were beneficial for the adsorption of both antibiotics, and the adsorption efficiency was not affected by the interference of coexisting ions. Adsorption mechanisms were ascribed to pore filling, π-π EDA interactions, hydrogen bonds, surface complexation, and Lewis acid–base interaction for SDZ removal and pore filling, π-π EDA interactions, hydrogen bonds, surface complexation for NOR removal, respectively. Therefore, magnetic coconut shell biochar may be a promising adsorbent for SDZ and NOR removal. Further studies on the reusability and stability of magnetic biochar should be conducted to promote its practical application.