Enhanced Adsorption of Aqueous Ciprofloxacin Hydrochloride by a Manganese-Modified Magnetic Dual-Sludge Biochar

Abstract

In this study, an effective composite material, manganese-modified magnetic dual-sludge biochar (Mn@MDSBC), was developed for the adsorption of ciprofloxacin hydrochloride (CIP). This composite was prepared by means of a simple one-pot method, which involved the pyrolysis of iron-based waterworks sludge (IBWS) and paper mill sludge (PMS) loaded with manganese (Mn) under controlled conditions in a nitrogen atmosphere. The synthesized Mn@MDSBC was subjected to a comprehensive suite of characterization approaches, which included N₂ adsorption–desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Subsequently, static adsorption tests were conducted to investigate how different factors, including the initial solution pH, reaction time and temperature, CIP concentration, and ionic strength influence the adsorption of CIP by Mn@MDSBC. Mn@MDSBC had the maximum CIP adsorption capacity of 75.86 mg/g at pH 5, among the pH values ranging from 3 to 9. The pseudo-second order model provided the best description of the adsorption process, while the experimental data aligned more closely with the Langmuir equation than with the Freundlich model, indicating monolayer adsorption. The adsorption process was found to be non-spontaneous and exothermic according to thermodynamic analysis. The presence of Cl⁻ and SO₄²⁻ enhanced CIP adsorption, while PO₄³⁻ weakened it. After five cycles of reuse, Mn@MDSBC experienced a 17.17% loss in CIP adsorption capacity. The primary mechanisms for CIP removal by Mn@MDSBC were identified as physical and chemical adsorption, hydrogen bonding, and π-π stacking interactions. In summary, the study underscores the high efficiency of Mn@MDSBC as a composite material for CIP adsorption, highlighting its potential for application in wastewater treatment processes.

1. Introduction

Ciprofloxacin hydrochloride (CIP), a third-generation fluoroquinolone antibiotic, is ubiquitous in the medical and veterinary fields because of its wide-ranging antibacterial efficacy, low cost, and high therapeutic efficacy [1,2]. A significant portion of ingested CIP is excreted through feces and urine, and the extensive discharge of medical wastewater and pharmaceutical manufacturing wastewater leads to the widespread distribution of CIP in the aquatic environment [3]. The concentration of CIP detected in different types of wastewater ranges from less than 1 μg to tens of mg, while in the aquatic environment, the concentration of CIP is generally below 1 μg [4]. CIP exhibits resistance to microbiological degradation, primarily attributed to its highly stable chemical structure. The presence of CIP in water can trigger the development of antibiotic resistance in microbial communities, thereby posing a significant threat to the aquatic ecosystems. Moreover, CIP exhibits chronic toxicity to aquatic organisms, which can result in bioaccumulation within their tissues. This bioaccumulation can potentially propagate through the food chain, ultimately posing risks to human health via dietary intake [5,6,7].

To address the issue of CIP contamination in wastewater, various treatment techniques have been explored, including adsorption [8,9], advanced oxidation processes [10,11], membrane separation, and biological treatment [12]. Among these methods, adsorption stands out due to its simplicity, ease of operation, high efficiency, and low cost [13]. To date, a wide variety of materials, including metallic materials, non-metallic materials, and metal–non-metal hybrid composites, have been utilized for the adsorptive removal of CIP from water [14,15,16]. Among these materials, the application of biochar for CIP removal has emerged as a prominent research focus in recent years [13,17,18,19,20].

Biochar (BC), known for its rich reactive units and vast specific surface area [21,22], has become a successful and eco-safe adsorbent for removing various pollutants, including antibiotics [23,24,25]. Paper mill sludge (PMS), a globally abundant and cheap industrial waste [26,27], contains rich carbon precursors such as cellulose, hemicellulose, and lignin, making it an ideal feedstock for BC production to remove a wide range of contaminants from water [22,28,29]. To increase the adsorptive capability and specificity of BC, it is often modified chemically [11,30,31]. The magnetization of BC can further improve its separation performance and reusability. For example, Yu et al. employed a magnetic S/N co-doped BC to remove tetracycline from aqueous solutions. The results demonstrated that the composite exhibited a magnetization of 17.80 emu/g, enabling it to be easily separated from water. After four cycles of reuse, the composite retained 88.3% of its initial adsorption capacity, with only an 11.7% loss compared to the fresh composite under conditions of 4 g/L adsorbent dosage, 298 K reaction temperature, a solution pH value of 7, and a tetracycline concentration of 100 mg/L [32].

Iron-based waterworks sludge (IBWS), a solid waste generated in drinking water treatment plants that use iron-based chemicals as coagulants, has been shown to be effective in producing magnetic BC when reacted with other carbon-containing feedstocks at high temperatures in a nitrogen atmosphere [33]. Manganese oxide, known for its active hydroxyls and large specific surface area, can significantly enhance adsorptive removal of aqueous pollutants [34,35]. For instance, Shao et al. developed a rice straw biochar modified with KMnO₄ and NaOH, and evaluated its performance in removing CIP from water. The study revealed that manganese oxide particles (MnO_x) roughened the surface of the biochar, thereby significantly increasing its specific surface area, which enhanced the biochar’s adsorption efficiency for CIP. The biochar achieved a maximum CIP adsorption capacity of 32.25 mg/g under conditions of 0.3 g/L adsorbent dosage, 298 K reaction temperature, and a solution pH value of 3 [34].

To our knowledge, no studies have yet explored the adsorptive removal of CIP using manganese-modified magnetic BC derived from PMS and IBWS. Therefore, in this study, PMS, IBWS, and manganese were used as raw materials to fabricate manganese-modified magnetic dual-sludge BC (Mn@MDSBC). This composite was then employed as an adsorbent in static tests to investigate its CIP adsorption performance and mechanism.

2. Materials and Methods

2.1. Chemicals and Materials

MnCl₂ (>99%, Shanghai McLean Reagent Co., Ltd., Shanghai, China), NaOH (AR, Tianjin Kermel Chemical Reagent Co., Ltd., Tianjin, China), HCl (Guangzhou Chemical Reagent Factory, Guangzhou, China), CIP (>99%, Baikesaisi Biotechnology Co., Ltd., Dingzhou, China). The PMS and IBWS used in this study were obtained from a paper mill in Xinmi in Henan Province and from a drinking water treatment plant in Huludao in Liaoning Province, respectively.

2.2. Mn@MDSBC Synthesis

To prepare the BC, PMS was first rinsed with purified water until it reached neutrality. Both PMS and IBWS were then rinsed and dehydrated at 105 °C for 2 h, followed by pulverization to pass through an 80-mesh sieve. The two raw materials were blended at a mass ratio of IBWS to PMS of 1:2.5, a proportion previously determined in our study to be optimal for obtaining a 105 g mixture. A 3 g mixture was calcined in a tube furnace at 500 °C for 3 h, under a constant N₂ flow of 300 mL/min. The cooled black powder was smashed and sieved through an 80-mesh screen, and labeled as MDSBC.

MnCl₂ was combined with the mixture of IBWS and PMS (IBWS+PMS) at various mass ratios of MnCl₂ to (IBWS+PMS) = 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 to prepare Mn@MDSBC. These mixtures were added to 100 mL of 0.1 M NaOH solution, sonicated for 30 min, mixed for 6 h, and then filtered and dried to pass through an 80-mesh sieve again. The resulting combinations were treated under the same conditions as described previously. The final composites were identified as Mn@MDSBC.

2.3. Artificial CIP-Bearing Solution

A stock solution was prepared by dissolving 1 g of CIP in 1 L of distilled water, which was subsequently diluted to achieve the desired concentrations for each group of static adsorption tests. Then, 1 M HCl and NaOH solutions were employed to adjust the solution pH from 3 to 9.

2.4. Static CIP Adsorption Test

Static CIP adsorption tests were conducted in triplicate using Erlenmeyer flasks, with each holding 20 mL (80–360 mg/L) of artificial CIP-bearing wastewater and 0.05 g of adsorbent. The mixtures were agitated in a shaker (120 rpm) for various time intervals and temperatures. The residual CIP in the filtered solutions after passing through 0.45 μm membranes were quantified using an ultraviolet spectrophotometer at 277 nm [36].

2.5. Regeneration and Reuse of Adsorbent

The adsorption experiment was conducted using the pristine Mn@MDSBC under the following conditions: adsorbent dosage of 0.05 g per 20 mL of solution, temperature at 25 °C, duration of 12 h, pH value of 5, and initial concentration (C₀) of 80 mg/L. Upon completion of the experiment, the used adsorbent was collected and immersed in 50 mL of distilled water, followed by stirring at 150 rpm for 5 min. Subsequently, it was dried in vacuum at 80 °C for 12 h and calcined at 500 °C for 3 h. All other conditions were maintained as described in Section 2.2. The regenerated adsorbent was sieved through an 80-mesh screen. The adsorption experiment was conducted using the regenerated Mn@MDSBC as the adsorbent, under the identical conditions specified previously. The regeneration process of the composite and the subsequent CIP adsorption experiments using the regenerated material were each conducted 5 times.

2.6. Characterization of Adsorbent

N₂ adsorption–desorption of Mn@MDSBC was conducted using a nitrogen sorption analyzer (BSD-PS2, Beishide, Beijing, China). The surface morphology of Mn@MDSBC was imaged using SEM (Zeiss Genimi500, Oberkochen, Germany), and the surface elements were analyzed using SEM-EDS (Ultim Max 100, Oxford Instruments, Oxfordshire, British). The crystalline structure of Mn@MDSBC was analyzed by XRD (Ultima IV, Akishima, Japan). Surface functional groups were identified using FTIR (Nicolet iS50, Thermo Fisher, Waltham, MA, USA). The zeta potential of Mn@MDSBC was measured using a zeta potential analyzer (Zeta-check, Particle metrix, Bavaria, Germany). The valence states of Fe, Mn, C, F, N, and O were detected using XPS (Thermo Scientific Escalab 250Xi+, Waltham, MA, USA).

3. Results and Discussion

3.1. Exploration of Mn@MDSBCs Preparation

3.1.1. Effect of MnCl₂ to (IBWS+PMS) Mass Ratio on CIP Adsorption

Figure S1a illustrates the CIP adsorption capacities of MDSBC and eight distinct Mn@MDSBCs synthesized at varying mass ratios of MnCl₂ to (IBWS+PMS). MDSBC had a CIP adsorption capacity of 51.13 mg/g, which was significantly lower than those of all eight types of Mn@MDSBCs, confirming that manganese modification enhanced CIP adsorption. As the mass ratio of MnCl₂ to (IBWS+PMS) increased from 2:1 to 1:2, the CIP adsorption capacity increased from 76.58 mg/g to 80.39 mg/g. However, as the mass ratio continued to decrease from 1:2 to 1:3, 1:4, 1:5, 1:6, and 1:7, the adsorption capacity gradually decreased to 79.72 mg/g, 79.39 mg/g, 79.28 mg/g, 76.14 mg/g, and 74.67 mg/g, respectively. Statistical analysis revealed that as the mass ratio increased from 1:2 to 1:5, no significant differences (p > 0.05) in the adjacent adsorption capacity were detected among the groups. In contrast, when the mass ratio was further increased to 1:6, significant differences (p < 0.05) in adsorption capacity emerged. Meanwhile, the saturation magnetic induction values of the seven Mn@MDSBCs synthesized at different ratios were 6.43 emu/g, 7.54 emu/g, 7.75 emu/g, 7.82 emu/g, 7.97 emu/g, 8.11 emu/g, and 8.57 emu/g, respectively (Figure S1b). The results indicate that the magnetism of the synthesized material increased as the mass ratio of the two components decreased from 2:1 to 1:7. This enhancement was attributed to the increased iron content in the raw materials as the mass ratio was reduced. As a result, more magnetic substances were formed during the high-temperature calcination process under anaerobic conditions. A lower mass ratio implies the use of less Mn and more PMS and IBWS in the composite synthesis, which was advantageous as it minimized chemical usage and promoted waste recycling. Considering both CIP adsorption and magnetic properties, a mass ratio of MnCl₂ to (IBWS+PMS) = 1:5 was selected for composite synthesis.

3.1.2. Influence of Pyrolysis Temperature on CIP Adsorption

CIP adsorption by Mn@MDSBC increased from 75.46 mg/g to 79.28 mg/g as the pyrolysis temperature rose from 400 °C to 500 °C (Figure S1c), but decreased to 36.14 mg/g as the temperature increased to 700 °C. As a result, 500 °C was determined as the optimal temperature for Mn@MDSBC preparation.

3.2. Adsorbent Characterization

3.2.1. Specific Surface Area and Pore Analysis

Figure S2 presents the N₂ adsorption–desorption isotherms of Mn@MDSBC within the relative pressure range from 0 to 1, which exhibited the typical type IV isotherms accompanied by a H3 hysteresis loop, suggesting the existence of a mesoporous structure [37]. The specific surface area (S_BET), total pore volume (V_tot), and average pore diameter of Mn@MDSBC were 48.42 m²/g, 0.18 cm³/g, and 14.54 nm, respectively (Table S1). These values confirm that Mn@MDSBC was a typical composite with both micropores and mesopores, which contributed to its high adsorption capacity [38,39]. Figure S2 also reveals that the pore size distribution of Mn@MDSBC was characterized by a distinct bimodal pattern. The two peaks in the distribution corresponded to pore sizes ranging from 3.6 nm to 4.1 nm and from 11.51 nm to 12.93 nm, respectively [40]. The smaller pores (3.6 nm to 4.1 nm) originated from the pyrolysis of PMS in the raw materials, whereas the larger pores (11.51 nm to 12.93 nm) were formed through the pyrolysis of IBWS in the raw materials [41].

3.2.2. SEM Images and SEM-EDS Results

As shown in Figure 1a,b, the pristine Mn@MDSBC consisted of flake-like structures and small particles, with a relatively rough surface appearance. After being used and regenerated five times, the number of small particles on the surface decreased, and the flake-like structures became more pronounced (Figure 1c,d). After the fifth repeated use, due to the adsorption of CIP on the surface, the number of small particles on the surface increased again, making the surface rough once more (Figure 1e,f).

Figure 1g,h illustrates the surface chemical composition of pristine Mn@MDSBC as determined by EDS analysis. Initially, the contents of C, O, Fe, and Mn were 48.9%, 39.2%, 6.7%, and 5.3%, respectively, with no detectable N. After undergoing five regeneration cycles (Figure 1i,j), the elemental proportions shifted to 55.6% for C, 28.0% for O, 11.7% for Fe, and 3.7% for Mn, while N emerged with a content of 1.0%. This change could be attributed to the composite’s final preparation step involving magnesium modification, which positioned magnesium on the outermost layer. Following five regenerations, the surface magnesium was depleted, exposing the underlying C, O, and Fe and thereby increasing their measured contents. The newly detected N was likely sourced from adsorbed CIP.

3.2.3. XRD Analysis

Figure S3 presents the XRD spectrum of Mn@MDSBC, which closely matched the standard diffraction patterns of carbon (C) (PDF#04-007-2136), Fe₃O₄ (PDF#04-001-7822), and MnO₂ (PDF#04-007-3892). The peaks at 2θ = 29.50°, 42.21°, 52.33°, 61.22°, and 69.40° corresponded to the (110), (200), (211), (220), and (013) carbon crystallographic planes, respectively. The peaks at 2θ = 35.45°, 43.08°, 47.17°, 56.97°, and 62.56° corresponded to the (110), (200), (211), (220), and (311) crystal planes of Fe₃O₄, respectively [42]. The peaks at 2θ = 28.85°, 43.03°, 56.35°, 57.76°, 59.76°, 66.82°, and 68.57° corresponded to the (110), (111), (211), (121), (220), (310), and (130) crystal planes of MnO₂, respectively, confirming that manganese was effectively incorporated onto the surface of MDSBC [43]. The lower peak intensities of Fe₃O₄ and MnO₂ were ascribed to the dominant intensity of the carbon peaks, which overshadowed the weaker signals of these compounds.

3.3. Static Adsorption

3.3.1. Influence of Starting Solution pH

The pH value is a critical determinant in the adsorption procedure as it influences both the form of adsorbate and surface properties of the adsorbent. CIP existed in its positively charged cationic state (CIP⁺) when the solution pH was less than 5.9 due to the fact that its amine group was protonated [44]. It adopts a zwitterionic form within the pH range of 5.9 to 8.9. When the pH exceeds 8.9, the carboxyl group ionizes, resulting in an anionic form of CIP⁻ [44].

As depicted in Figure 2, Mn@MDSBC has an isoelectric point at pH 3.28, indicating that it carries a positive charge when the pH was below 3.28 and a negative charge when the pH is above 3.28. At pH 3, the positively charged Mn@MDSBC repelled CIP⁺, leading to a low CIP uptake of 56.48 mg/g. When the pH increased to 5, the CIP adsorption capacity rose to 75.86 mg/g. This increase was attributed to the charge-related interaction between Mn@MDSBC surface carrying a negative charges and the cationic form of CIP (CIP⁺), which was prevalent at pH values below 5.9. When the pH reached 9, the anionic form of CIP (CIP⁻) reduced CIP adsorption through electrostatic repulsion, causing the adsorption capacity to drop to 56.46 mg/g. Similar observations have been made when magnetic-activated carbon derived from eucalyptus sawdust and oil sludge were used for CIP adsorption [45]. Raheem et al. observed that the adsorption of CIP by MILDH increased progressively as the pH of the solution rose from 3 to 7, reaching a peak at pH 7. Conversely, when the pH was further elevated to 10, the adsorption capacity of the material for CIP declined [46].

3.3.2. Adsorption Kinetics

The adsorption of CIP by Mn@MDSBC exhibited a rapid increase within the initial 30 min, regardless of the two distinct initial CIP concentrations. This sharp rise was ascribed to the abundant reactive sites on the Mn@MDSBC surface. Subsequently, as these active sites became progressively occupied, the rate of CIP adsorption decelerated, and both reactions reached equilibrium at 960 min (Figure 3a).

To further elucidate the adsorption process, three kinetic models were employed. The first two models are the pseudo-first order (PFO) model (Equation (1)) and the pseudo-second order (PSO) model (Equation (2)). These models are designed to describe physical adsorption and chemical adsorption, respectively [47]. The third model is the intra-particle diffusion model (Equation (3)), which is typically used to depict the inner or pore interaction processes [48].

$$\mathrm{l}\mathrm{n}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathrm{l}\mathrm{n}{\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t}$$

(1)

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

(2)

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{b}}{\mathrm{t}}^{0.5}+\mathrm{I}$$

(3)

where q_e (mg/g) and q_t (mg/g) represent the CIP adsorption capacities at equilibrium and at time t, respectively. k₁ (min⁻¹) was the PFO rate constant, k₂ (g/mg·min) was the PSO rate constant. k_b (mg/(g·h^0.5)), and I (mg/g) were the rate constant and the intercept constant, respectively.

The nonlinear fitting results are depicted in Figure 3a, and the fitting parameters are summarized in

Table 1. Parameters obtained from the three kinetic models.

Kinetics	Parameters	Initial Concentration (C0) (mg/L)
		100	200
Pseudo-first order	q e,exp(mg/g)	36.572	76.233
	q e,cal(mg/g)	35.112	72.779
	k1 (min−1)	0.02337	0.02669
	R2	0.989	0.983
Pseudo-second order	k2 [g/(mg·min)]	38.368	79.026
	q e,cal(mg/g)	0.000912	0.000517
	R2	0.999	0.999
Intra-particle diffusion	ki1 [mg/(g·min0.5)]	2.126	3.753
	C1	8.339	24.726
	R12	0.955	0.875
	ki2 [mg/(g·min0.5)]	0.821	1.716
	C2	22.318	45.807
	R22	0.980	0.999
	ki3 [mg/(g·min0.5)]	0.149	0.334
	C3	32.550	67.657
	R32	0.990	0.867

. The coefficients of determination (R²) for the PSO model exceeded those for the PFO and Elovich models. Moreover, the experimental q_e values approached the values obtained from the PSO model, indicating that the PSO model was appropriate for describing the kinetic process and that chemisorption was the predominant mechanism [49].

The intra-particle diffusion model was employed to further explore the rate-limiting steps during the process of CIP adsorption by Mn@MDSBC. As shown in Figure 3b, each fitting curve consisted of three linear segments, corresponding surface diffusion, intra-particle diffusion, and equilibrium adsorption, respectively [50]. Multiple adsorption mechanisms, rather than a single rate-limiting step, were involved into the adsorption process due to the fact that none of the linear segments passed through the coordinate origin [51]. Furthermore, for each of the two initial CIP concentration, the intra-particle diffusion rate constants followed the order kip₁ > kip₂ > kip₃, suggesting that surface diffusion played a more dominant role in the initial stages of the adsorption process compared to intra-particle diffusion [50].

3.3.3. Adsorption Isotherm and Thermodynamics Analysis

Figure 4a illustrates the relationship between q_e and C_e It is evident that the CIP adsorption capability of Mn@MDSBC declined as the reaction temperature increased. To analyze the adsorption data, the Langmuir and Freundlich models (Equations (4) and (5)) were employed:

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

(4)

$$\mathrm{l}\mathrm{n}{\mathrm{q}}_{\mathrm{e}}=\mathrm{l}\mathrm{n}{\mathrm{k}}_{\mathrm{f}}+{\displaystyle \frac{1}{\mathrm{n}}}·\mathrm{l}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$$

(5)

where C_e (mg/L) was the equilibrium CIP concentration, q_m (mg/g) was the theoretical CIP uptake, K_L (L/mg) was the Langmuir constant, k_f (Lⁿ·mg^1–n/g) and n are the Freundlich constants.

Figure 4a depicts the nonlinear fitting results for the Langmuir and Freundlich models, respectively, and the fitting parameters are summarized in

Table 2. Parameters obtained from the two isotherm equations and thermodynamic exploration.

T/(K)	Langmuir Parameters			Freundlich Parameters			Thermodynamic Parameters
	Qm (mg/g)	kL (L/mg)	R2	Kf (Ln·mg1–n/g)	n	R2	∆G0 (kJ/mol)	∆H0 (kJ/mol)	∆S0 (kJ/mol·K)
288	145.985	0.0955	0.999	26.941	2.513	0.972	5.558	−28.842	−0.119
298	141.643	0.0693	0.999	21.946	2.404	0.958	6.753
308	137.931	0.0436	0.999	16.253	2.258	0.959	7.947

. Compared to the R² values for the Freundlich model, the R² values for the Langmuir model were closer to one in all three tests, indicating that the Langmuir model provided a better description of CIP adsorption by Mn@MDSBC. This suggests that the adsorption process likely involved the formation of a single layer of CIP on the homogeneous surface of Mn@MDSBC [49].

Table 3. The CIP adsorption capacities of different adsorbents determined by Langmuir model.

No.	Adsorbents	qm (mg/g)	Reaction Conditions	Reference
1	MMT	416.66	-, -	[52]
2	FMB	357	298 K, pH 5	[47]
3	Mn@MDSBC	145.985	288 K, pH 5	This work
		141.643	298 K, pH 5
		137.931	308 K, pH 5
4	SBC	13.5	-, -	[53]
	Zn-SBC	77.3	-, -
	Fe/Zn-SBC	74.2	-, -
5	Dopa-CoF NPs	16.5	298 K, pH 7	[54]
	Gluta-CoFN	14.0	298 K, pH 7
	PsMela-CoF NPs	7.18	298 K, pH 7
6	CoFe-LDH-modified sludge biochar	19	298 K, -	[55]
		16	303 K, -
		12	308 K, -

Note: “-” in the table means the parameter did not show in the study.

demonstrates that the adsorption capacity of Mn@MDSBC for CIP ranges between 137.931 and 145.985 mg/g. Among the ten materials compared in the table, Mn@MDSBC ranks third. This highlights that the material developed in this study exhibits a robust CIP adsorption ability from aqueous solutions.

Three Equations (6)–(8) were employed to determine the thermodynamic parameters, thereby providing further insight into the characteristics of CIP adsorption by Mn@MDSBC.

$${\mathsf{\Delta }\mathrm{G}}^0=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{L}}$$

(6)

$$\mathsf{\Delta }{\mathrm{G}}^0=\mathsf{\Delta }{\mathrm{H}}^0-\mathrm{T}·\mathsf{\Delta }{\mathrm{S}}^0$$

(7)

$$\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{L}}={\displaystyle \frac{-\mathsf{\Delta }{\mathrm{G}}^0}{\mathrm{R}\mathrm{T}}}={\displaystyle \frac{\mathsf{\Delta }{\mathrm{S}}^0}{\mathrm{R}}}-{\displaystyle \frac{\mathsf{\Delta }{\mathrm{H}}^0}{\mathrm{R}\mathrm{T}}}$$

(8)

where ΔG⁰ (kJ/mol) was the Gibbs free energy change, ΔS⁰ (kJ/mol·K) was the standard entropy change, and ΔH⁰ (kJ/mol) was the standard enthalpy change. The values of ΔS⁰ and ΔH⁰ were derived from Figure 4b in accordance with Equation (8). K_L (L/g) was the Langmuir constant, R (8.314 J/mol·K) was the ideal gas constant, and T was the reaction temperature (K).

As indicated in Table 2, ΔG⁰ values, ranging from 5.558 to 7.947 kJ/mol, are all positive, confirming that CIP adsorption by Mn@MDSBC was a non-spontaneous process [56]. Moreover, the increase in ΔG⁰ values with rising reaction temperature suggests that lower temperatures were more conducive to CIP adsorption [57]. The negative ΔH⁰ value indicates that the adsorption process was exothermic. Furthermore, the negative ΔS⁰ value suggests a decrease in randomness at the interface between Mn@MDSBC and the liquid phase throughout the CIP adsorption [57,58].

3.3.4. Effect of Coexisting Anions

The impact of three common anions including Cl⁻, SO₄²⁻, and PO₄³⁻ on the adsorption of CIP by Mn@MDSBC was investigated. As depicted in Figure 5, both Cl⁻ and SO₄²⁻ enhanced CIP adsorption in a concentration-dependent manner, with SO₄²⁻ having a more pronounced effect than Cl⁻. The salting-out effect of Cl^- and SO₄²⁻ reduced the solubility of CIP, thereby promoting its diffusion towards the surface of Mn@MDSBC and improving adsorption. Lu et al. discovered that the addition of 3% sodium chloride to the solution significantly enhanced the adsorption capacities of ciprofloxacin (CIP) by resins XAD-4 and MN-202, with respective increases of 93.48% and 53.94% [59]. Another studies have demonstrated that the presence of salts in the solution could diminish the adsorption of CIP. For instance, Seibert et al. observed that the addition of KCl and Na₂SO₄ to the solution notably reduced the adsorption capacity of cork for CIP. This reduction is primarily due to the competitive interaction between the added ions and the ionic form of CIP for the limited active adsorption sites on the adsorbent’s surface [60]. Conversely, CIP adsorption significantly decreased as the increase in PO₄³⁻ concentration rose from 100 to 300 mg/L. This reduction is attributed to the hydrolysis of PO₄³⁻, which generates OH⁻, raising the solution’s pH and creating competition between OH⁻ and CIP⁻. The same phenomena was observed by Afzal et al. when they used chitosan/biochar hydrogel beads as adsorbents to remove CIP from water [61].

3.3.5. Reusability of the Adsorbent

As shown in Figure 6, the pristine Mn@MDSBC exhibits a CIP adsorption capacity of 28.74 mg/g. Upon repeated use, the adsorption capacity gradually decreased, reaching 23.80 mg/g by the fifth reuse. At this point, the CIP adsorption capacity was 82.83% of that of the pristine Mn@MDSBC, indicating that the composite maintains good reusability. This characteristic suggests that Mn@MDSBC could potentially reduce operational costs in practical applications. As detailed in Section 3.2.2, following five regeneration cycles, the surface concentration of Mn diminished and residues of CIP were identified. These alterations were presumably responsible for the observed decline in the composite’s adsorption capacity.

Table 4. Reusability of CIP adsorbents.

No.	Adsorbents	Cycles of Use	Adsorption Capacity Loss	Reference
1	Fe3O4@SiO2/ι-CRG/GPTMS	4	19%	[62]
2	ACAF/Fe3O4/ZnO	6	7.9%	[63]
3	MBC	5	17.01%	[64]
4	MNPC-700-0.4	4	17%	[65]
5	Mn@MDSBC	6	17.17%	This study

presents the reusability five different CIP adsorbents. Given that the number of uses varied among these materials, the analysis focuses on the loss rate per individual use to ensure a fair comparison. The average loss rate across the five materials is 3.3%. Among them, Fe₃O₄@SiO₂/l-CRG/GPTMS exhibits the highest loss rate at 4.8%, while ACAF/Fe₃O₄/ZnO has the lowest at 1.3%. The material developed in this study demonstrates a loss rate of 2.9%, which is below the average. This suggests that the material possessed excellent reusability and maintained a relatively stable adsorption capacity over multiple cycles.

4. CIP Removal Mechanism

4.1. FTIR

As shown in Figure S4, the peak at 1429.0 cm⁻¹ corresponded to the aromatic C=C bonds in the pristine biochar [66]. After CIP adsorption, this peak shifted to 1427.1 cm⁻¹ and broadened, indicating its involvement in the CIP adsorption process. New peaks appeared at 2847.0 cm⁻¹, 1625.2 cm⁻¹, 1580.3 cm⁻¹, 1310.8 cm⁻¹, and 1251.1 cm⁻¹, which were assigned to the CH₂ stretching, quinolones, aromatic C=C stretching, CN stretching, and O-H bending, respectively [67,68,69,70]. These new peaks originated from the CIP molecules, confirming the successful adsorption of CIP by Mn@MDSBC.

4.2. XPS

Figure S5a presents the complete XPS spectrum of Mn@MDSBC pre- and post-CIP adsorption. The presence of the Mn peak confirms that Mn was successfully loaded onto Mn@MDSBC. The appearance of the F peak and the significant enhancement of the N peak after CIP adsorption indicate that CIP was successfully adsorbed onto the surface of Mn@MDSBC [45].

The C 1s XPS spectrum before CIP adsorption exhibits three peaks at 284.8 eV, 286.5 eV, and 289.4 eV, which were attributed to C=C, C-N, and C=O, respectively (Figure S5b) [71,72,73]. After CIP adsorption (Figure S5c), the C=C peak shifted to a binding energy of 284.4 eV [74], suggesting π-π stacking interactions between the C=C bond in Mn@MDSBC and the benzene ring in the CIP structure [75]. The C-N peak disappears, and a new peak corresponding to C-OH appears, indicating that the material binds to the -OH groups contained in CIP [76]. The peak corresponding to C-F at 289.4 eV shifts to 289.0 eV after CIP adsorption [77], further confirming the successful adsorption of CIP by Mn@MDSBC.

As depicted in Figure S5d, the peaks at 686.5 and 687.2 eV belonged to C-F bonds after CIP adsorption [76,77]. Research has indicated that peaks in the range of 688–689 eV are associated with organic fluorine, suggesting that Mn@MDSBC has successfully adsorbed CIP [78,79].

Figure S5e,f illustrate that the peak at 399.4 eV was attributed to C-N bonds [80]. The peak area of C-N decreased from 100% to 88.62%, while a new peak of -NH₂ appeared at 400.5 eV after CIP adsorption [81]. This indicates that nitrogenous groups underwent protonation and participated in the CIP adsorption.

The O1s XPS spectrum (Figure S5g) shows peaks at 529.5 eV and 531.3 eV, which belonged to the functional groups of MnO₂ and C=O, respectively [82,83]. As shown in Figure S5h, after CIP adsorption, the peak of MnO₂ disappears, and the peak at 531.1 eV likely corresponded to C=O and N-C=O, suggesting that MnO₂ was involved in the reaction and that CIP was successfully adsorbed.

Figure S5i displays the Fe 2p high-resolution XPS spectra before adsorption, with peaks at 711.0 eV and 724.1 eV due to the presence of Fe (II), and peaks at 714.0 eV and 727.3 eV, corresponding to Fe (III) [84]. After CIP adsorption (Figure S5j), the peaks of both Fe (II) and Fe (III) shift. Additionally, the peak regions for Fe (III) increased from 28.66% to 69.17%, while the peak areas of Fe (II) declined to 24.50%. This indicates that part of Fe (II) formed π-π bonding with the benzene ring in CIP, which enhanced the adsorption of CIP [85,86].

As shown in Figure S5k, the peaks at 641.8 eV and 653.6 eV corresponded to MnO₂ [87], indicating that Mn has been successfully loaded onto Mn@MDSBC. The peak at 644.8 eV was the satellite peak of Mn²⁺ [88]. After CIP adsorption, the peaks at 641.3 eV and 653.1 eV belonged to Mn³⁺ [89,90], suggesting that Mn³⁺ has strong oxidizing properties and adsorbs CIP through oxidation reactions (Figure S5l).

The -NH and -OH groups in CIP acted as electron donors, combining with the oxygen-bearing moieties (e.g., C=O) on Mn@MDSBC surface to form hydrogen bonds, which facilitated CIP removal from its aqueous solution [53,91].

Based on the comprehensive analysis of adsorption kinetics, isotherms, thermodynamics, FTIR, and XPS, CIP adsorption by Mn@MDSBC can be attributed to electrostatic adsorption, chemisorption, hydrogen bond, and π-π stacking.

5. Conclusions

IBWS, PMS, and manganese were used as raw materials to synthesize the Mn@MDSBC composite for the adsorption of CIP from aqueous solutions. The optimal conditions for composite preparation were determined to be a mass ratio of MnCl₂ to (IBWS+PMS) of 1:5 and a pyrolysis temperature of 500 °C. The composite was then prepared under these conditions in a controlled nitrogen atmosphere using a tube furnace.

The characteristics of the composite was explored by using a series of approaches, including N₂ adsorption–desorption, XRD, SEM, FTIR, and XPS. The XRD results revealed the presence of Mn, indicating that Mn was successfully loaded onto the surface of the material, thereby enhancing the material’s adsorption capacity for CIP. Meanwhile, the detection of Fe₃O₄ suggested that the material has been magnetized, allowing for its rapid separation from the solution under an external magnetic field after the reaction.

The composite was evaluated for its CIP adsorption performance and mechanism by using 0.05 g of the adsorbent in a 20 mL solution containing CIP. The results indicated that CIP adsorption by Mn@MDSBC was a pH-dependent process, with the highest CIP adsorption capacity of 75.86 mg/g at pH 5. The PSO model and Langmuir model were found to be suitable for describing the adsorption process, which was spontaneous and exothermic in nature. Among the three tested anions, Cl⁻ and SO₄²⁻ enhanced CIP adsorption, while PO₄³⁻ reduced it. Mn@MDSBC exhibited good durability, losing only 17.17% of its adsorption capability after being reused five times. Based on the material characterization and static adsorption test results, it can be inferred that the adsorption of CIP by the composite of Mn@MDSBC was primarily achieved through electrostatic adsorption, chemical adsorption, hydrogen bonding, and π-π stacking interactions.

The synthesis and application of the composite provide new ideas for the resource utilization of PMS and IBWS, as well as for the treatment of antibiotic-contaminated wastewater.