Process analysis of aqueous ciprofloxacin adsorption by biochar–magnetite nanocomposites

Abstract

The widespread presence of antibiotics like ciprofloxacin in hospital wastewater poses serious environmental and health risks due to antibiotic resistance. This study presents the synthesis and application of a magnetically separable biochar–magnetite nano-composites for efficient adsorptive removal of ciprofloxacin. Biochar based adsorbents are sustainable, environmentally friendly, low cost and scalable with tuneable surface chemistry and regeneration capacity. In this study, orange peel biochar (CB) was doped with magnetite (Fe₃O₄) to prepare three different magnetic biochar-magnetite nano-composites CBC-2, CBC-5, CBC-10 which were examined for maximum removal efficiency of ciprofloxacin which is a popular broad-spectrum synthetic fluoroquinolone antibiotic. It is poorly metabolized, frequently excreted from body in unmetabolized and hence reported as a major pollutant in hospital waste water. The best adsorption efficiency was found at pH 8.0 and 298 K temperature by CBC-10 composite with the highest adsorption capacity (~ 90%), 1.25 gm.L⁻¹ adsorbent dose, and the fastest equilibrium rate (60 min contact time) for the lowest ciprofloxacin concentration of 30 ppm. These results suggested that the biochar magnetite composites can serve as a promising adsorbent for the removal of antibiotic traces in the hospital and pharma industry effluents.

Article Highlights

Orange peel waste was turned into a low-cost, eco-friendly material to clean antibiotic pollution from water.

Magnetic biochar composites showed high efficiency, removing nearly 90% of ciprofloxacin in just 1 h.

This approach offers a sustainable and scalable way for hospital wastewater treatment.

Introduction

Water is crucial for public health, economic growth, and worldwide sustainable development. Ensuring safe water quality is essential, but the emergence of various pollutants generates a serious threat to the environment [1, 2–3]. The advancement in Pharmaceuticals have significantly advanced science and technology, contributing in improved health and longevity. These pharmaceutically active compounds like antibiotics, antipyretics, antihistamines, antiallergics, antifungals, analgesics, antivirals, and many others, have been detected globally in biological systems over the past 30 years [1, 4, 5–6]. In India, the use of antibiotics surged significantly, with a 47.4% increase in antibiotic consumption reported between 2010 and 2020, according to the Washington-based CDDEP (Centre for Disease Dynamics, Economics & Policy) [7]. Antibiotics are a popular class of pharmaceuticals which are widely used for diagnosis and prevention or disease treatment in humans, aquaculture, and livestock, but after ingestion they are not fully absorbed by organisms [5, 6, 8, 9]. Indiscriminate antibiotic use leads to 30–90% of the drugs remaining unmetabolized, being excreted and ultimately contaminating water bodies [10]. Their excretion leading into water and soil as parent compound or metabolite form pose risks to human health and aquatic life and overall disrupting ecosystems. Their effects on human health have been documented in various medical studies [11]. A fluoroquinolone antibiotic, Ciprofloxacin (CIP) [12, 13] is most prescribed drug to treat pneumonia, bronchitis, sinusitis, and other seasonal diseases. It is frequently detected in wastewater due to its strong accumulation ability, high persistence nature, and hard decomposability [14, 15]. Its long-term use increases antibiotic resistance. According to a report, Ciprofloxacin (CIP) concentrations near pharmaceutical plants reaches up to 50 mg.L⁻¹ [14, 15–16]. The development of economically feasible, sustainable, and efficient removal material is need of the hour. Various methods, including ultrafiltration [17], adsorption [15, 18], redox, biodegradation, advance oxidation [19], reverse osmosis [20, 21] and photocatalytic oxidation [22] are used for CIP removal. Among them, adsorption, a non-destructive physical method is preferred due to its high efficiency, low cost, safety, and broad applicability. Biosorbents like Biochar derived from abundant natural resources like agricultural, household, and industrial waste, offer a sustainable alternative for wastewater treatment, efficiently removing pollutants while being widely available and eco-friendly [14] Due to abundance of surface functional groups (i.e., -OH, -COOH, -NH₂ etc.), it is widely reported as an effective adsorbent for removal of various heavy metals and organic pollutants via the mechanisms like H-bonding and π-π stacking [14, 23] Thermochemical methods like gasification, combustion, pyrolysis and torrefaction are effective for rapidly conversion of lignocellulosic biomass into bioenergy [24, 25] Among them, pyrolysis stands out as an environmentally and economically favourable option, capable of transforming various wastes like agricultural residues, plastics, sludge and municipal solid waste etc., into chemical products and valuable energy [25] The production of biochar via microwave pyrolysis stores the carbon in a stable and useful form, reducing CO₂ emissions and that represents its long-term viability [24] The orange fruit, specifically known as Citrus sinensis (a member of citrus family), was used to formulate biochar [26, 27, 28, 29–30] As a low cost and renewable biomass resource, orange peel (OP) holds great potential as a sustainable precursor for production of high-quality activated carbon, making it a promising material in the reduction of aquatic pollutants and wastewater treatment applications in environmental technologies [26, 29, 30]. In a study, the adsorption of fluoroquinolones onto Fe/Zn + H₃PO₄–modified biochar is strongly supporting the incorporation of Fe enhances the surface complexation and magnetic separability of the material. The process influenced by pH due to changes in surface charge and adsorbate speciation [31] In this article, Iron-enhanced composites in different proportions have been ball milled to improve the adsorptive properties of these carbon-based adsorbents, that allowing for more efficient adsorption performance and separation [32]. Magnetic biochar adsorbents have promising potential because they can be rapidly separated from solution using a simple portable magnet, eliminating the need for complex centrifugation, filtration or other lengthy processes. Their applications are expanding such as, magnetically modified sludge biochar for antibiotics removal [31], Fe₃O₄loaded corn straw biochar for Cr(IV) adsorption [33], Bisphenol adsorption by magnetic biochar [34] Activated biochar-supported magnetite composite for adsorption of polychlorinated phenols from aqueous solutions [35] etc. In this study we introduce a sustainable and facile approach to synthesize orange peel–derived biochar–Fe₃O₄ nanocomposites through ball milling, easy and energy-efficient process that enhances surface homogeneity and good dispersion of magnetic nanoparticles. Ball milling is an efficient and eco-friendly method to produce nano-materials with improved surface properties and homogeneity. Studies have shown that ball-milled biochar exhibits enhanced adsorption capacity for organic pollutants and metals due to its reduced particle size and increased surface functional groups. However, the small particle size may pose challenges in recovering the spent adsorbent [36, 37]. Nano powder Fe₃O₄ metal oxide is widely used to induce non-toxic, reusable, magnetic properties in biosorbents. Fe-containing biochar adsorbent has great potential due to its ability to be rapidly separated using an external magnet, eliminating the need for complex filtration or centrifugation [36, 38]. This method not only enables waste valorization of citrus residues but also imparts magnetic recoverability in material. In CBC-10 (1:10) composite represents the low loading of Fe₃O₄ nano powder where pore structure and Active biochar sites are highly preserved as we increase the loading (CBC-5 and CBC-2) the pore blocking and oxide aggregation can begin to reduce surface area (on the basis of analysis results). Well suitable ratios for the study of comparative study, examination of which optimum mixing give maximum and favourable adsorption results [39, 40–41]. This is the first comparative evaluation of waste orange peel derived magnetic biochar nanocomposites by using ball milling for the removal of ciprofloxacin. Which is scalable, sustainable and magnetically recoverable biochar adsorbent for hospital waste water treatment Table 1.

Table 1. A comparison table of CIP adsorption on different adsorbents with their % removal efficiencies reported in previous literatures

S.No.	Materials	Preparation method	pH level	Kinetics and isotherm model	Time period	Adsorption capacity (Qe) mg.g−1	% Removal efficiency	Refs.
1	Chitosan modified Fe pretreated biochar (CS-FBC)	600 1 C for two hours at a rate of 5 1 C min−1 + FeCl3	pH = 7	Pseudo-second-order kinetic model and Langmuir isotherm model	150 min	76.72	80%	[42]
2	Activated biochar derived from millet husks and water hyacinth	Slow pyrolysis at 350 °C and 500 °C, and KOH activation	pH = 8	Pseudo-second-order model Freundlich isotherm and intraparticle diffusion models	24 h		83–88%	[43]
3	Natural clays	Crushing, washing and drying	pH = 7	Elovich and pseudo-second-order model and	7–8 h	150–194	70–80%	[44]
4	Corn straw biochar nanoparticles	At 450 °C pyrolysis and ball milling	pH = 5.5	pseudo-second-order model	24 h	-	~ 80%	[45]
5	Water hyacinth (Eichhornia crassipes) derived biochar	Slow-pyrolysis at 350 °C	pH = 7	Pseudo-second-order kinetic and Freundlich and Langmuir isotherm model	50 min	-	~ 60%	[46]
6	ZVI/biochar (Fe@C)	Alkali lignin biochar (AL) microwave tube furnace pyrolysis at 400℃ for 15 min	pH = 4	-------	4 h	357	76.52%	[47]

Experimental design

Materials

Orange peel waste used as raw material for biochar preparation was collected from a local fruit juice shop. The chemicals sulphuric acid (H₂SO₄), sodium bicarbonate (NaHCO₃), sodium hydroxide (NaOH), and hydrochloride (HCl) were purchased from Sigma-Aldrich. Ciprofloxacin hydrochloride salt, which was used as an adsorbate, was purchased from Cipla Ltd. Magnetite powder was purchased from Adnano Tech. All used chemicals were of analytical grades and used directly without any further purification. DI water (18.2 M.Ω.cm⁻¹ at 25 °C) was used for the whole experimental work. Concentration of CIP was examined by using a UV-spectrometer (LAMBDA 750, Perkin Elmer UV-Vis NIR spectrophotometer instrument). Nylon syringe filters (13 mm membrane diameter, 0.22 μm pore size) were utilized to filter the adsorbents from the treated solution. After treatment, an external magnetic field was applied to separate Ac and CBCs from the solution.

Preparation methods

Preparation of microwave assisted porous functionalised Biochar and activation

Activated carbon (AC) was formulated from orange peel waste. Briefly, orange peels (Ops) were washed to remove surface impurity and adherent contaminants, chopped and air dried for 5 days in direct sunlight. Then dried OPs were ground in an electrical grinder and sieved to get even-sized particle powder ranging between 150 and 300 μm. OP powder was then placed in a microwave for pyrolysis (at 720 W for 30–40 min and in the absence of air) without any pretreatment for carbonization. This carbonized material (biochar) is further processed for the activation process to enhance the overall performance of adsorption. It was pretreated with 200 ml of 2% H₂SO₄, neutralized with base treatment (100 ml of 1% NaHCO₃ solution). Further, dried in a hot air vacuum oven to eliminate moisture at 110 °C for 24 h under constant N₂ flux and stored in an airtight container for use.

Citrus Peel Biochar (CBC) nanocomposite Preparation

The AC and magnetite nano powder were weighed in a mass ratio of different proportions. CBC (Citrus peel biochar) nanocomposites were constructed through a facile ball milling process [48, 49, 50, 51–52]. The ball-milled Ac was prepared by ball milling in 500 ml jar containing 25 agate balls (each diameter 6 mm, weight 180gms) within a planetary ball milling machine [38]. The CBC were synthesized by mixing Fe₃O₄ nano powder and AC in different weight ratios of 1:10, 1:5 and 1:2. Fe₃O₄ nano powder, AC and agate balls mixed in ball milling jar and conditions systemized at 400 rpm for 12 h with rotation alteration after 1 h. we systematically compare different biochar-to-Fe₃O₄ ratios (10:1, 5:1, and 2:1) to identify an optimal balance between surface area preservation and magnetic functionality for ciprofloxacin adsorption. Higher Fe₃O₄ incorporation can enhance magnetic separation but may also partially block adsorption sites or reduce surface area, thereby lowering adsorption capacity. Our results showed that CBC-10 offered a good balance, maintaining high adsorption efficiency while imparting sufficient magnetization for easy separation in a laboratory magnetic field. During this investigation, direct magnetic characterization (e.g., saturation magnetization, coercivity, or separation efficiency) were not performed as we were focussed only on the recoverability of Fe₃O₄-modified CBCs, which was qualitatively confirmed by rapid separation using an external magnet [39, 40]. These magnetic nanocomposites labelled as CBC-10, CBC-5 and CBC-2 (Table 2).

Table 2. Preparation of Biochar nanocomposites with different mixing rations

S. No.	Types	Ratio Biochar: Fe3O4
1	CBC-10	10:1
2	CBC-5	5:1
3	CBC-2	2:1

Characterization methods

Scanning electron microscopy (SEM, NOVA Nano FE-SEM 450 (FEI)) was used to investigate the surface morphological analysis and microstructure of all samples. SEM operated at 30 kV working voltage and was equipped with EDS analyzer to identify and map the element distribution on the surface. Phases in the adsorbents were identified by X-ray diffraction (XRD, Panalytical Xpert Pro) with a scan range of 2θ recorded from 10° to 80° at a 2 degree per minute scan speed and out-coated with a CuKα radiation (λ = 1.54060 Å) and ran at 45 kV voltage and 40 mA current. The surface functional groups present on the samples were characterized using Fourier transform infrared spectroscopy (FTIR, FT-IR Spectrum 2, PerkinElmer). The spectra were collected at every 2 cm⁻¹ using KBr pallets and spectral ranges from 4000 cm⁻¹ to 400 cm⁻¹. The zeta potentials on the surface of the samples were measured using a Zetasizer Nano-ZSP (ZEN-5600) instrument. Brunauer–Emmett–Teller theory, an N₂ desorption test was used to analyze the specific surface area (SSA) and pore size structure of the samples (BET, Nova Touch LX2 gas sorption analyzer Quantachrome Instruments) [53].

Adsorption capacity experimental analysis

To better determine the optimum adsorption capacity and mechanism of AC and CBC nanocomposites, it was necessary to investigate the most appropriate conditions for samples working. The stock solution of CIP was prepared by mixing of 30 mg.L⁻¹ in DI water. Batch adsorption studies were carried out by varying parameters of pH (3 to 11), contact time (0 to 120 min), adsorbent dosage (0.030 to 0.070 g), and initial CIP concentration (20 to 100 mg.L⁻¹) to evaluate the optimum adsorption conditions for CIP. The adsorption thermodynamics was examined at the 298 K, 303 K, 313 K and 323 K temperature range with other parameters held constant. The pH of the solution was adjusted by adding 0.01 M HCL and 0.01 M NaOH. All experiments were performed in a 100 ml conical flask assembled with an electrical stirrer with a speed fixed at 150 rpm containing 40 ml CIP solution. After adsorption followed the conditions; solutions were filtered through a 0.22 μm nylon syringe filter, and the filtrate was analyzed for residual CIP concentration. The residual concentrations were measured by using a UV-visible spectrometer [54], and the maximum wavelength (λ_max) of absorption was 278 nm. A blank test was conducted to assess drug degradation from the adsorbent and adsorption by glass containers. Blank runs were also conducted containing only CIP solution without any adsorbent simultaneously with similar variable conditions.

The following equations were used to evaluate % removal efficiency (RE) and adsorption capacity Q_e (mg/g):

1

2

where C₀ = initial concentration of CIP, C_e = CIP concentration at equilibrium (mg.L⁻¹), V = volume of solution (L), W = weight of adsorbent (g) [55].

Results and discussion

Sample characterisation

SEM-EDS mapping

As an effective analytical technique, Scanning Electron Microscopy (SEM) can clearly represent the surface morphology of the material, and EDS can also provide information on the loading of Fe on CBCs. As shown in Fig. 1a and d, the AC sample primarily exhibits a smooth, flat surface structure along with a porous architecture featuring an uneven texture. The distinct biomass skeleton is visible due to volatization and transfiguration of organic substituents. The formulated nanocomposites CBC-2, CBC-5, CBC-10 further reveal a greater abundance of pore structures, with an irregular surface morphology. EDX analysis demonstrated the presence of Fe, O, and C in all the composites. The Fe and O signals correspond to Fe₃O₄ nanoparticles, while the C signal originates from orange peel biochar. This indicates a successful deposition of Fe₃O₄ particles on the biochar surface, as further supported by elemental mapping of all the composites. EDX layer images depict the inclusion of unevenly agglomerated Fe on surface of bio adsorbent.

[See PDF for image]

Fig. 1

a SEM-Elemental mapping of AC and CBCs nanocomposites at 30 μm magnification, EDS Images of b CBC-2, c CBC-5, and d CBC-10

FTIR and XRD spectrum

The Fourier Transform Infrared Spectroscopy (FTIR) analysis was used to identify the presence of functional groups on the external surface of the prepared AC and CBCs [56]. As shown in Fig. 2, FTIR spectra of AC, CBC-10, CBC-5, CBC-2 were recorded in the 400–4000 cm⁻¹ spectral range. Studies suggest that ball milling generates mechanical energy and heat, promoting solid-phase reactions that create new chemical bonds. Additionally, an increase in OH, C = O, phenolic-OH, and aromatic C = C bonds indicates that ball milling exposes unsaturated bonds and introduces more oxygen-containing groups to CBCs. This aligns with previous findings that biochar can undergo oxidation during ball milling in the presence of air. All samples exhibited a broad peak in the 3400–3700 cm⁻¹ region, attributed to O-H stretching vibrations. The peaks at 2250 cm⁻¹ and 2070 cm⁻¹ are associated with the appearance of the alkyne groups and R-N = C = S (alkyl isocyanates). Meanwhile, spectrum B has characteristic peak ranges between at 2428 cm⁻¹, 510–673 cm⁻¹ and 824–982 cm⁻¹, corresponding to the stretching oscillation of the Fe–O bonds in the lattice of Fe₃O₄. The spectral lines at 774–620 cm⁻¹ and 1600 cm⁻¹ were assigned to the Ar-CH group and conjugated C = C stretching vibrations of the aromatic basic structure of lignin, polysaccharides, and other carbohydrate units. The spectra of CBCs displayed distinct absorption bands associated with key functional groups involved in contaminant adsorption. These include peaks at 1400 cm⁻¹ (O = C-O, carbonyl group) and 1151 cm⁻¹ (C-O-C), all of which play an important role in CIP adsorption which is similar to the other carbonaceous adsorbents demonstrated ciprofloxacin uptake via electrostatic interaction, hydrogen bonding, and π–π stacking [42, 57, 58].

[See PDF for image]

Fig. 2

Comparison of the FTIR spectrum of AC with CBCs nanocomposites

The shape of the XRD diffraction peak helps determine the crystallinity of biochar. By the comparison of AC and CBCs, these diffraction patterns clearly assigned for the presence of Fe₃O₄ (Fig. 3). Amorphous biochar exhibits a broad steamed bread-like peak, whereas the crystalline part of biochar displays sharp peaks. A smaller grain size and higher crystallinity result in sharper XRD peaks with a reduced half-width. The crystal plane diffraction peaks of CBC-2 at 29.97° (220), 35.4° (311), 43.1° (400), 53.6° (422), 57.13° (511), 62.8° (440), 71.1° (620) and 74.3° (622) align with the JCPDS standard reference card (01-088-0866), confirming that the listed d-values are consistent. This indicates the successful loading of Fe₃O₄ onto the biochar surface, with the material exhibiting the cubic crystal form of Fe₃O₄. Additionally, the absence of impurity peaks in the CBCs spectrum suggests high crystal purity. The weakening of diffraction peak intensity due to decrement in ratio Fe₃O₄ in CBC-5 and CBC-10 as compare to CBC-2, indicates an increase in surface defects and porosity within the composites. This leads to a variation in specific surface area while simultaneously reducing crystallinity.

[See PDF for image]

Fig. 3

XRD phase spectra of AC and its corresponding CBC nanocomposites (Ref. JCPDS card no. 01-088-0866 for Fe₃O₄)

ZETA potential

The zeta potential provides the extent of mutual repulsion between nanoparticles and used to analyse the stability of particles. The higher the repulsive forces between nanoparticles, the greater would be the stability of the sample. Zeta potential values of all of the adsorbents are negative around 30 mV which indicates the good stability of AC and CBCs. Low conductivity (less than 1 mS.cm⁻¹) leads a sample towards stability. Further reduction of the zeta potential of CBCs can be attributed to ball milling, which can bring O-containing groups onto the surface of nanocomposites (Table 3).

Table 3. Zeta potential and deviation, mean diameter, conductivity and elemental composition comparisons between AC and CBC nanocomposites

Samples	Mean zeta potential (in mV)	Mean zeta deviation	Conductivity (in mS.cm−1)
AC	− 36.3	+ 6.706	0.0385
CBC-10	− 37.0	+ 5.94	0.0709
CBC-5	− 28.9	+ 4.67	0.0408
CBC-2	− 30.3	+ 5.35	0.102

Specific surface area

BET analysis was conducted to study the surface area, porosity, total pore volume (VT), and average pore size (DA) of the adsorbent using nitrogen adsorption/desorption isotherms, which exhibited a Type IV isotherm. As per IUPAC classification, the adsorption-desorption isotherm curves of all prepared samples correspond to a most common Type IV isotherm with an H3 hysteresis loop [59]. Isotherm calculations indicating micro and mesoporous structure with slit-shaped pores and showing multilayer adsorption. As pressure increases the inflection point is typically occurred near the completion of first monolayer adsorption.

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Fig. 4

a. N₂ adsorption–desorption isotherms of AC, CBC-2, CBC-5, CBC-10; b. Plot representing the comparative pore size distribution curve for AC and CBCs composites

Figure 4a and b illustrates the N₂ adsorption-desorption isotherms for all samples. The incorporation of Fe₃O₄ nanoparticles as a new phase on the external AC surface introduced greater roughness and heterogeneity, leading to a reduction in surface area. As a result, the specific surface areas of CBCs are from 21.9140 m².g⁻¹ for AC to 27.6017 m².g⁻¹, 7.9699 m².g⁻¹ and 3.1396 m².g⁻¹ for CBCs, respectively (Table 4). Additionally, the total pore volume observed was 0.0524847 cc.g⁻¹, 0.0682819 cc.g⁻¹, 0.0182384 cc.g⁻¹ and 0.00708616 cc.g⁻¹ for BJH adsorption analysis. The pore volumes observed from BJH desorption are 0.04691 cc.g⁻¹, 0.06874 cc.g⁻¹, 0.01865 cc.g⁻¹ and 0.006907 cc.g⁻¹ of AC, CBC-10, CBC-5, and CBC-2. The pore size distribution analysis shows that the pores present on the surface of biochar are micro-mesoporous (1–5 nm) in size, respectively contributing to the enhancement of the availability of adsorption sites for CIP removal. As the loading of Fe₃O₄ increases the specific surface area is decreases (124.976–41.668 m²g⁻¹) gradually, the lower Fe₃O₄ mixed composite CBC-10 retained higher adsorption capacity as compare to CBC-5 and CBC-2 [60, 61]. Excess loading reduce the active surface area because of oxide aggregation and pore blocking.

Table 4. Textural properties of nanocomposites. Pore diameter, total pore volume (V), total surface area (S_BET), and microporous surface area (Sµ)

Composites	Pore diameter (in nm)	Total pore volume (in cc g−1)	Total surface area (in m².g−1)	Specific surface area (in m².g−1)	microporous surface area (Sµ)
AC	1.92143	0.0469	21.914	124.976	0.184
CBC-10	1.91319	0.0687	27.602	41.668	3.230
CBC-5	1.71089	0.0186	7.970	12.161	1.273
CBC-2	1.53461	0.0069	3.140	3.970	0.528

Parameter optimisation for adsorption performance

The study of different parameters is necessary to better understand the process of adsorption and what actually happened on the adsorbent surface. A 30 ppm CIP stock solution was employed in this experimental work. The 40 mL of CIP solution was taken in each beaker, and a 50 mg dose of nanocomposites was added to it. The impact of altered conditions, including pH, adsorbent concentration, temperature, and contact time on the adsorption capacity of all manufactured (AC, CBC-10, CBC-5 and CBC-2) nanocomposites was studied by using ciprofloxacin as the adsorbate. The pH was adjusted between 3 and 11 using 0.1 M NaOH and 0.1 M HCl and the adsorbent concentration was varied from 30 to 70 mg. The contact time duration ranged from 0 to 120 min with continuous stirring at 200 rpm on an electrical stirrer. The adsorbent was separated after the designated time with the help of an external power magnet, filtered with a 0.22 μm syringe filter and then the remaining drug concentration was measured at 278 nm by using a UV-vis spectrophotometer. This optimization study explores the influence of pH, contact time, temperature, and adsorbent dose, which is illustrated in Fig. 5a-d to clarify the optimum values and to better understand the involved mechanism.

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Fig. 5

Effect of a. Adsorbent dosage, b. Initial pH, c. Solution temperature and d. contact time on % removal efficiency of CIP adsorption onto AC and CBC nanocomposites. ([CIP] = 30 mg.L⁻¹ stock solution, T = 298 K, adsorbent dose 50mg.L⁻¹, time 120 min) in (5a); [except adsorbent dosage 30-70mg.L⁻¹] in (5b); [except pH 3–11] in (5c); [except T 303–333 K] in (5d); [except Time 20–120 min]

Effect of solution pH and contact time

The initial pH of the solution plays a significant role in the adsorption by influencing the adsorbent surface charge, altering the ionization state of binding groups, and determining the protonated or deprotonated forms of the adsorbate. The effect of pH on CIP adsorption by AC and CBCs was studied by varying pH from 3 to 11 while keeping the other parameters- initial CIP concentration (30 mg.L⁻¹), temperature (25 °C), and contact time (120 min)-constant. Since CIP has amphoteric properties, its charge varies with pH: cationic (CIP⁺) below pH 6.09, anionic (CIP⁻) above pH 8.64, and zwitterionic (CIP⁰) between pH 6.09 and 8.64. The removal efficiency increased from pH value 3 to 8, peaking at pH 6.0–8.0, where CIP exists as a zwitterion that maximizes electrostatic attraction between functional groups on biochar and groups in CIP. At pH < 6.0, electrostatic repulsion between the positively charged adsorbent surface and CIP⁺ reduced adsorption. Above pH 8.0, repulsion between CIP⁻ and the negatively charged adsorbents led to a slight decline in removal efficiency. Therefore, pH 8.0 was selected for further adsorption experiments Fig. 5b shows the effect of altering pH in the adsorption process.

Evaluating contact time is essential to understanding adsorption kinetics and equilibrium. The impact of a 120-minutes contact period on adsorption efficiency was studied with keeping other parameters constant (Fig. 5d). There are three phases observed during time-variable examination. In initial fast phase, rapid increment observed in adsorption efficiency ranged between Q_e = 0.008437, 0.009732, 0.010735, 0.013968 from 0.010461, 0.017143, 0.016582, 0.018826 for 20 to 40 min for AC, CBC-2, CBC-5, CBC-10 respectively. This occurs because numerous active adsorption sites, vacant pores, and higher CIP concentration are available in the early stages, which leads to a high removal rate. In second phase, the adsorption efficiency increased gradually which was optimised around 60 min (adsorption efficiency Qe = 0.012591, 0.01851, 0.020884, 0.021703) that could be possible due to progressive saturation of active binding sites of CBCs and mesopore sites. CIP adsorption was rapid but decreased as equilibrium was reached after 60 min. It was due to the complete saturation of active binding sites and mesopores and increased competition raises repulsion between CIP molecules and the adsorbent surface. The aggregation and busy surface occupancies made it difficult for CIP molecules to diffuse into higher energy sites, which could be another factor limiting further adsorption.

Effect of adsorbent dose

Adsorbent dose is also a vital parameter to examine the removal efficiency in terms of overall costs. To test the impact of adsorbent quantity on adsorption of CIP, several 50 ml beakers containing a varied amount of adsorbent (0.30, 0.40, 0.50, 0.60, 0.70 mg) and 40 ml solution of 30 ppm CIP were shaken at 25 °C for 2 h. As shown in Fig. 5a, this experimental work represents that the removal rate initially increased gradually as the adsorbent dose was increasing, but during progression, the removal rate followed a reduction drift. This trend occurs because, while an increased adsorbent dose provides more surface area and active binding sites, the available CIP molecules remain limited. As a result, the utilization of active sites decreases, leading to a continuous reduction in Q_e per unit mass of the adsorbent.

Effect of temperature

The establishment of isotherms reflects that temperature is one key factor influencing the adsorption mechanism. As shown in Fig. 5c, adsorption experiments were performed at 4 different temperatures (303 K, 313 K, 323 K, and 333 K) while maintaining the adsorbent dosage (50 mg), the pH (8), and the contact time (2 h). Temperature increase allows additional active sites for binding with CIP molecules. Although adsorption is typically an exothermic process, the adsorption of ciprofloxacin (CIP) on CBCs exhibited an endothermic behaviour. Instead of decreasing, the adsorption capacity increased with increase in the solution temperature. In the plot 5c we can see that % removal efficiency slightly increases with a rise in the temperature as adsorption process may involve a mixed mechanism: predominantly physisorption with minor chemisorption contribution. This is also evident from the small positive enthalpy change (ΔH°) indicating weakly endothermic process; hence removal improves slightly with higher temperature but not drastically. This suggests that CIP molecules require additional energy to move across the biochar surface and penetrate its pores, with temperature increase enhancing the overall adsorption efficiency.

Adsorption thermodynamics

Thermodynamic analysis was conducted to assess the effect of temperature on CIP adsorption [62], as shown in Fig. 6a. The Van’t Hoff equation was used to calculate thermodynamic parameters such as Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°), with the results presented in Table 4.

3

The distribution coefficient (K_T) was calculated using the ratio of Q_e to C_e. The slope and intercept of the ln(K_T) versus 1/T plot (Fig. 6a) were used to determine ΔH° and ΔS°, while ΔG° was calculated using the Gibbs free energy equation:

4

As shown in Table 4, the negative ΔG° values indicate spontaneous CIP adsorption. Additionally, the decrease in ΔG° with increasing temperature suggests enhanced adsorption efficiency at higher temperatures. The positive ΔH° values indicate the endothermic nature of the process, while positive ΔS° values indicate increased randomness at the solid-liquid interface during adsorption (Table 5).

Table 5. Thermodynamic parameters for CIP adsorption onto nanocomposites at variable temperature-

S. No.	Sample	Temperature (in Kelvin)	ΔG° (in kJ mol−1)	ΔH° (in kJ mol−1)	ΔS° (in J mol−1 K−1)
1	AC	303	− 0.625	2.644	10.79
		313	− 0.733
		323	− 0.841
		333	− 0.949
2	CBC-2	303	− 2.645	5.0935	25.54
		313	− 2.901
		323	− 3.156
		333	− 3.411
3	CBC-5	303	− 4.238	7.7867	39.71
		313	− 4.643
		323	− 5.039
		333	− 5.429
4	CBC-10	303	− 4.915	7.7275	41.92
		313	− 5.326
		323	− 5.746
		333	− 6.165

The % Removal efficiency only slightly increases with a increase in the temperature (303–333 K) as adsorption process may involve a mixed mechanism: predominantly physisorption with minor chemisorption contribution. This is also evident from the small positive enthalpy change (ΔH°) indicating weakly endothermic process; hence removal improves slightly with higher temperature but not drastically.

Adsorption kinetics

The adsorption kinetics of ciprofloxacin onto CBCs are analyzed using the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models [62, 63, 64–65]. The PFO model, which was originally proposed by Lagergren [63]and the most used form of the PFO model is expressed in its linearized equation as:

5

where Q_e represents the adsorption capacity at equilibrium (mg.g⁻¹), Q_t is the adsorption capacity at time t (mg/g), and K₁ is the rate constant of the PFO model. The K₁ values corresponding to different time intervals were obtained from the linear plots of log(Q_e– Q_t) versus t (Fig. 6b) and the calculated parameters summarized in Table 6.

The detailed descriptions of the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models are provided in Table 6. Table 6 summarizes the fitting results for ciprofloxacin adsorption on biochar. As illustrated in Table 6, the pseudo-second-order model [63] yields the highest R² values among the kinetic models, suggesting it best fits the adsorption kinetics. This analysis indicates that the adsorption rate of ciprofloxacin is primarily governed by the availability of adsorption sites on the biochar surface.

Table 6. Kinetic parameters profile for the adsorption of CIP onto AC and CBCs at variable contact time

Kinetic models	Parameters	Adsorbents
		AC	CBC-2	CBC-5	CBC-10
Pseudo first order	R2	0.0000027	0.02526	0.00356	0.13676
	K1 (min−1)	0.0001037	0.0005135	0.0033623	0.0146931
	Qe exp (mg.g−1)	0.012598	0.0187332	0.020911	0.021798
	Qe calculated (mg/g)	0.0008135	0.0026869	0.0021975	0.0033106
Pseudo second order	R2	0.98371	0.96693	0.96574	0.99039
	K2 (g mg−1 min−1)	29.8176	7.111432	6.193206	5.490016
	Qe exp (mg.g−1)	0.012598	0.0187332	0.020911	0.021798
	Qe calculated	0.01140	0.017965	0.0196820	0.022489
Intraparticle Diffusion	R12	0.99	0.91	0.99	0.99
	Kipd(1)	0.00126	0.00275	0.0031	0.00238
	Kipd(2)	0.00048	0.00054	0.00097	0.00032
Film-Diffusion	R2	0.27	0.30	0.37	0.51
	KFD	0.001085	0.00339	0.00373	0.00312

The linear form of the PSO model is given by Eq. (6) [64]:

6

where Q_e and Q_t (mg/g) are the amounts of adsorbed CIP on the adsorbent at equilibrium and at the selected time (t (min)), respectively. K₂ (gmg⁻¹.min⁻¹) is the adsorption rate, the constant of the pseudo-second order equation, and is evaluated from the linear relationship between t/qt and t. The excellent fit of the pseudo-second-order model (Qe = 0.01140, 0.017965, 0.0196820, 0.022489) indicates that chemisorption is the dominant mechanism in CIP removal. The adsorbent dose and the initial adsorbate concentration are strongly affecting the adsorption capacity. The adsorption capacity Qe values are below 1 because adsorption capacity is the amount of adsorbate adsorbed by each gram of adsorbent and adsorbent dose used in this study are relatively high [66, 67]. Conversely % Removal efficiency depends on the relative decrease in pollutant concentration, which can appear very high when the initial concentration of the contaminant is low [66, 67]. The calculated Qe values from the pseudo-second-order model closely match with the experimental Qe values, confirming the model’s suitability. Table 5 summarizes both calculated and experimental values Qe, the first-order (K₁) and second-order (K₂) rate constants, along with the correlation coefficients (R²). The results show that the pseudo-first-order model yields lower R² values (≤ 0, 0.0000027, 0.02526, 0.00356, 0.13676), while the pseudo-second-order model demonstrates an excellent fit with R² = 0.98371, 0.96693, 0.96574, and 0.99039, indicating it better describes the adsorption process (Fig. 6c).

The intraparticle diffusion model [65] was applied to study the diffusion behaviour of ciprofloxacin (CIP) during adsorption. The intraparticle diffusion rate constant (K_diff) and boundary layer thickness (L) were determined using Eq. (7).

7

Where I = thickness of layer and K_diff = intra particle diffusion constant (mg.g⁻¹. (min^0.5)^–1).

As shown in Fig. 6d, the multi-linear plots of IPD indicate that the adsorption process occurs in multiple stages. For all the adsorbents, two distinct stages were observed. The first stage involves the transfer of CIP ions from the solution to the outer surface. During the second stage, adsorbate molecules are transferred from the outer surface of the adsorbent materials to the pores of the inner core of the adsorbent, followed by diffusion into the inner pores that reach equilibrium. The deviation of the plots from the origin suggests that intraparticle diffusion is not the sole rate-controlling step and that adsorption likely involves other complexation mechanisms such as film diffusion or surface reactions.

The liquid-film diffusion model assumes that the main resistance to mass transfer lies in the thin boundary layer (film) surrounding the adsorbent particle. A straight line indicates that the adsorption is controlled by film diffusion model [68].

The model is formulated as:

8

Where K_fd = film diffusion rate constant (min⁻¹).

In Fig. 6e, all the adsorbents are giving very low and weak fit R² value 0.27, 0.30, 0.37, 0.51. which indicating that the adsorption process is not purely film diffusion controlled. At initial stage of adsorption, steeper slopes suggest the presence of film diffusion but as the process progresses it governed by surface adsorption and IPD model. Among all of the adsorbent CBC-10 composite is more influenced by film- diffusion.

[See PDF for image]

Fig. 6

a. Thermodynamic plots between Ln (K_T) vs. 1/T, b. Linear fitted Pseudo-first-order kinetic curves of AC and CBC nanocomposites, c. Linear fitted pseudo-second-order kinetic curves of AC and CBC nanocomposites, d. Intra-particle diffusion plot with multilinear fit, e. Film diffusion model graph with R² values of linear fitting analysis

Adsorption isotherms

The application of adsorption isotherm describes the relationship between the amount of adsorbate retained per unit mass of adsorbent at equilibrium and the equilibrium concentration of the adsorbate in the liquid phase. Four types of isotherm models-Langmuir, Freundlich, Temkin, Dubinin-Radushkevich -are used to evaluate the suitable model for describing the process of adsorption.

The Langmuir model states that adsorption takes place at distinct and homogeneous adsorption sites on the surface, assuming monolayer adsorption occurring on a limited number of identical adsorption sites, where each site can hold only one adsorbate molecule, and there is no interaction or migration between adsorbed species. The equilibrium data for CIP adsorption onto CBCs suggest that the process likely conforms to the Langmuir model.

9

where Q_e = amount of adsorbed species per specified amount of adsorbent (mg.g⁻¹), C_e = equilibrium concentration (mg.L⁻¹), and K_L = Langmuir equilibrium. The Langmuir isotherm defines Q_max as the maximum amount of adsorbate needed to form a complete monolayer on the adsorbent surface. A plot of C_e/Q_e versus C_e produces a straight line, where the slope equals 1/Q_max and the intercept equals 1/(Q_maxK_L).

In contrast, the Freundlich model assumes that the adsorbent surface possesses heterogeneous energy levels, meaning the adsorption sites are not uniform. Unlike the Langmuir model, the Freundlich equation accounts for variations in adsorption capacity based on surface coverage. This model is mathematically expressed as Eq. (9):

10

where K_F and n are constants related to adsorption capacity and adsorption intensity, respectively. Where C_e is the equilibrium concentration of the adsorbate (mg.L⁻¹) and Q_e is the amount of adsorbate adsorbed per unit mass of adsorbent (mg.g⁻¹). The affinity constant K_F (L.mg⁻¹) represents the adsorption capacity of the adsorbent, while n indicates the favourability and intensity of the adsorption process. Accordingly, plotting Ln Q_e versus LnC_e yields a straight line with a slope of 1/n and an intercept of LnK_F, reflecting the applicability of the Freundlich model.

The Temkin isotherm model characterizes the interaction between the adsorbate and adsorbent, provides information on whether the adsorption process during the experiment favors physisorption or chemisorption, and is mathematically represented by the following equation:

11

Where, b = Temkin model constant which is related to the heat of adsorption and k_T =Temkin isotherm constant [69, 70].

Dubinin–Radushkevich isotherm:

12

In linear form equation :

13

Where K_DR= D–R constant (J/mol)⁻².

Table 7. Adsorption isotherm model parameters for CIP adsorption on AC and CBCs composites

Isotherms	Parameters	AC	CBC-2	CBC-5	CBC-10
Langmuir	R2	0.97	0.98	0.96	0.99
	Qmax (mg g −1)	0.0044	0.0072	0.0089	0.0124
	KL (L mg−1)	0.0097	0.0050	0.0034	0.0016
	RL	0.78	0.87	0.91	0.95
Freundlich	R2	0.99	0.99	0.97	0.97
	1/nF	1.07	0.39	0.25	0.18
	KF (L mg−1)	117.40	370.93	192.63	128.25
Temkin	R2	0.98	0.87	0.86	0.91
	b(Jmol−1)	3.05*105	1.95*105	2.09*105	2.15*105
	KT(Lg−1)	343.779	362.811	225.486	367.258
D-R model	R2	0.98	0.66	0.95	0.93
	KDR	6.295	2.653	3.796	4.567
	E (kJmol−1)	0.0.2819	0.4339	0.3628	0.3308

The calculated parameters are summarized in Table 7 Based on these results, the Freundlich model demonstrates a better fit to the experimental data compared to the Langmuir model, as evidenced by a higher R² value (0.99, 0.99, 0.97, 0.97) for Freundlich vs. (0.97, 0.98, 0.96, 0.99) for Langmuir, respectively (Fig. 7). The Freundlich model suggests a multilayer and heterogeneous adsorption surface with energetically equivalent adsorption sites. The adsorbent surface was identified as heterogenous surface and the CIP adsorption was occurring at the available active sites across a multilayered surface of bio adsorbent. The Langmuir isotherm model demonstrated a maximum adsorption capacity (Q_max) of 0.0044, 0.0072, 0.0089, 0.00124 mg.g⁻¹ and a Langmuir constant (K_L) of 0.0097, 0.005, 0.0034, 0.0016 g.L⁻¹ respectively for AC, CBC-2, CBC-5, CBC-10, indicating an effective adsorption process. The Freundlich intensity constant (1/n_F) of CBC-2, CBC-5, CBC-10 are 0.39, 0.25, 0.18 means n_F>1 which is further confirming the favourability of the adsorption process because higher values of 1/n_F representing the higher surface heterogenicity. In a favourable Freundlich isotherm model (n_F >1), the adsorption initially occurs at sites with higher binding energy and then progressing to sites with lower affinity as increase in surface heterogenicity. The study also suggested a relatively high Freundlich adsorption capacity (K_F) as compared to Langmuir constant (K_L), implying a lower free energy requirement for the adsorption process. Both the Langmuir and Freundlich isotherm models are fitting well. Overall, the isotherm analysis confirmed that the adsorption of CIP onto Citrus biochar (CBCs) is favourable and characterized by multilayer adsorption on a heterogeneous surface. For more detailed information about the nature of adsorption (physisorption or chemisorption) other models described here.

The Temkin isotherm model linear fitting plots revealed that the calculated values of heat of adsorption values for all four adsorbents were considerably high, approximately ~ 300 J·mol⁻¹. Since typical physisorption energies are relatively low (~ 1 J·mol⁻¹ up to about 40 J·mol⁻¹), while chemisorption usually falls within tens to several hundred kJ·mol⁻¹, calculated values are clearly indicates that the adsorption process is dominated by chemical interactions. Among the others CBC-10 have maximum b value so it has strongest adsorption affinity. According to linear fitting AC and CBC-10 displayed the strongest correlations with R² values of 0.98 and 0.91, respectively, making their Temkin parameters more reliable. In contrast, CBC-5 and particularly CBC-2 showed relatively lower R² values (0.86 and 0.87).

The D-R isotherm model describes the adsorption of ciprofloxacin on adsorbents. Which can be explained by graph fitting between lnQ_e vs. ɛ². The R² values are 0.98, 0.66, 0.95, 0.93 respectively for all the adsorbents AC, CBC-2, CBC-5, CBC-10. The calculated energies E are 0.2819, 0.4339, 0.3628, 0.3308 kJ.mol⁻¹ which is less than 8 kJ.mol⁻¹. which means that adsorption energies of 0.2819, 0.4339, 0.3628, 0.3308 kJ.mol⁻¹ is needed to adsorb one mole of CIP onto the adsorbent surface. Physisorption is implied by the values of E (< 8kJmol⁻¹).

[See PDF for image]

Fig. 7

a-d Freundlich model fitting, e-h Langmuir model fitting, i Temkin model and j D–R isotherm for adsorption of CIP on AC and CBCs

Proposed adsorption mechanism

The high adsorption capacity of CBC nanocomposites for CIP molecules is attributed to their unique structure, providing a large surface area, abundant active sites, and numerous π-electrons. The specific affinity between the adsorbent and adsorbate drives the adsorption process. Additionally, the negative zeta potential reflects strong electrostatic attraction between the adsorbent and CIP molecules. Key factors influencing adsorption efficiency include the presence of active surface groups and the small particle size of the adsorbent. To better understand the adsorption mechanism of CIP onto CBCs, the possible interactions between them were thoroughly investigated based on the analysis (Figure.8). Various isotherm and kinetic models were applied, which indicated that chemisorption is involved [71, 72]. XRD analysis revealed the semi-crystalline nature of CBCs, showing the presence of a Fe₃O₄ nano matrix on the surface, which facilitates physical and chemical adsorption by filling pores and cavities. Additionally, the XRD results confirmed the presence of Fe₃O₄ in CBCs. The oxygen atoms in CBCs may contribute to electrostatic interactions and hydrogen bonding with hydrogen atoms in the CIP molecules. The oxygen-containing functional groups on the CBCs surface serve as π-electron acceptors, facilitating π-π conjugation with the fluorine-substituted benzene ring and piperazine moiety of CIP [73]. Additionally, these functional groups can bind CIP molecules via surface complexation mechanisms [74]. The rich presence of such functional groups on citrus biochar makes it highly effective for CIP removal.

[See PDF for image]

Fig. 8

Surface Interaction of CIP molecules with the Biochar Nanocomposite: A Pictorial Depiction

FTIR analysis indicates the presence of key functional groups, such as -NH/-OH vibrations and carboxylate groups, which facilitate hydrogen bonding and electrostatic interactions with pollutants. Analysis revealed the presence of oxygenated functional groups at 3640 cm⁻¹ (O–H stretching vibrations) in CBCs, capable of forming strong surface complexes through hydrogen bonding with the N–H group of CIP. Additionally, peaks between 1150 and 1400 cm⁻¹ correspond to O = C-O and C-O-C- functional group vibrations; peaks in the range of 774–620 cm⁻¹ assigned to aromatic groups indicate the possibility of π–π interactions between CIP and CBCs. The efficiency of the nanocomposites as adsorbents is largely due to their reduced particle size and active surface area, as confirmed by BET analysis. N-doped bio adsorbent, rich in heterocyclic nitrogen-containing rings, provides active sites through various nitrogen functional groups, enhancing adsorption via chemical interactions. This biochar remains reusable with strong adsorption performance over multiple cycles.

The AC adsorption process follows only pure physical sorption like pore filling and van-der walls attraction. AC’s high SSA (124 m²g⁻¹) is mostly trapped in micropores too small for fast CIP diffusion. CBC-10’s moderate SSA (~ 41 m²g⁻¹) but high accessible external surface gives the best performance. On increasing Fe₃O₄ content, both SSA and total surface area drop sharply because pores are being partially blocked, but the surface becomes more exposed from particle outsides than from internal pores and total surface area (external + mesoporous) is more relevant for CIP adsorption than just SSA, because CIP is a relatively bulky molecule (~ 0.8 nm). Therefore CBC-10’s has moderate SSA but high accessible external surface gives the maximum adsorption performance.

The incorporation of metal oxides like Fe₃O₄ into the carbonaceous matrix improves surface and catalytic properties, further aiding adsorption. Notably, pH plays a critical role, with maximum CIP adsorption observed at pH 8, where hydrogen bonding and electrostatic attraction between CIP and CBCs are prominent.

The ball-milled magnetic biochar (CBCs) demonstrates greater efficiency and a significantly lower cost compared to commercial activated carbon (AC), making it a promising alternative for various applications. Overall, the synergistic combination of metal oxides and biochar enhances the adsorption mechanism through multiple interactions. Additionally, its strong magnetic properties and excellent reusability enhance its potential as an effective adsorbent for removing PACs and other organic pollutants in wastewater treatment.

Reusability examination of adsorbents

[See PDF for image]

Fig. 9

Reusability tests in 5 cycles with prepared composites (AC and CBC-10)

The reusability of a material refers to an adsorbent’s capability to be regenerated and applied repeatedly without significant loss of efficiency. The adsorbent maintained its structural integrity and adsorption efficiency over repeated cycles, indicating good stability [75, 76]. The adsorption–reusability performance of biochar nanocomposites is illustrated in Fig. 9. The 200ppm CIP solution was treated with CBC-10 and Ac at fixed optimum conditions. The CIP removal efficiency declined from ~ 90% in the irst cycle to 73.95% for CBC10 and 55.97% to 38.06% for AC by the fifth cycle, likely due to a gradual reduction in available active sites as adsorption sites became saturated over multiple uses. CIP molecules adsorbed during the initial cycle can remain bound and occupy sites in subsequent runs and participate at the next adsorption cycle in result reducing the overall uptake. As a result, CBC-10 is offered as a practical, cost-effective, and reusable adsorbent for CIP removal from aqueous solutions. The % Removal efficiency of Ciprofloxacin at equilibrium was calculated from the formula:

14

A multi pharmaceutical adsorption study (Effect of presence of other pollutants on the adsorption performance of AC and CBC-10 composites)

[See PDF for image]

Fig. 10

Laboratory made multi-pharmaceutical system study at different initial concentration of mixing 40,50 and 60ppm

The adsorption performance of activated carbon (AC) and CBC-10 magnetic modified biochar toward ciprofloxacin (CIP), paracetamol (PARA), levofloxacin (LEVO), and azithromycin (AZI) in a mixed pharmaceutical solution (a lab prepared solution) at initial concentration variation (40-60ppm) is presented in Fig. 10. CBC-10 consistently demonstrated higher removal efficiencies range between 70 and 90% compared with Ac which is 70–45%, at an adsorbent dosage of 50 mg/50 ml, pH 8, and 303 K. Different pharmaceutical molecules interact simultaneously with the same available binding sites of adsorbent so a competitive adsorption occurs in such type of multi-drug systems, that can lead to a reduction in the individual removal efficiency compared with one component system. The % removal efficiencies followed the order PARA (90.92%) > CIP (88.03%) > AZI (82.42%) > LEVO (79.19%), whereas activated carbon showed comparatively low uptake in the range of ~ 70 − 45% at initial concentration 40ppm and % removal efficiencies gradually decreases on increasing the initial concentration of adsorbates because of competition. The present results confirm that CBC-10 is not only marked as improved adsorbent over activated carbon but also underscoring its potential for treating wastewater burdened with huge variety of contaminants.

Conclusion

The study evaluated that acid pretreated CBC nanocomposites effectively function as a low-cost, mesoporous, renewable adsorbent for removal of antibiotic traces from hospital wastewater. This study demonstrated the waste orange peel derived biochar Fe₃O₄ nanocomposites as a highly potential, scalable and magnetically recoverable adsorbent for waste water treatment. Presence of magnetite (Fe₃O₄) in the composite significantly improved adsorption capacity due to the amplified structural and morphological properties such as surface area, porosity, and increased surface functional groups. Various adsorption parameters, including pH, contact time, adsorbent dose, and temperature, were examined to determine optimal conditions. The best adsorption efficiency was found at pH 8.0 and 298 K temperature by CBC-10 composite with the highest adsorption capacity (~ 90%), 1.25 gm.L⁻¹ adsorbent dose, and the fastest equilibrium rate (60 min contact time) for the lowest ciprofloxacin concentration of 30 ppm. Kinetic analysis reveals the process is energetically favoured with pseudo-second-order model, which best describes the adsorption process, while equilibrium data fit in the Freundlich model. This indicates that the heterogenous multilayer adsorption of ciprofloxacin onto the magnetic biochar surface is primarily governed by hydrogen bonding, electrostatic interactions and π-π stacking (physisorption and partial chemisorption). Thermodynamic evaluations reveal an endothermic and entropy-driven adsorption mechanism, confirming the spontaneous nature of the process. Overall, CBCs demonstrates high efficiency as an adsorbent for ciprofloxacin removal. The low cost and magnetic separability make CBC promising material for real hospital waste water treatment. it can be promoted as a superior value-added resource, contributing significantly to the circular economy.

Author contributions

M.S. performed investigations, formal analysis, data curation and wrote manuscript. Kh. S. did data validation. D. K. and F. S. G. reviewed and edited manuscript draft. T. I. provided facilities. A. K. and K.S. were involved in conceptualization and methodology while K.S. also did supervision.

Funding

No funding has been received for this work.

Data availability

Data cannot be shared openly but are available on request from authors.

Third party material

All the material is owned by the authors and no permissions are required.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not Applicable.

Dual publication

The results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration.

Competing interests

The authors declare no competing interests.

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