Removal of Ciprofloxacin from Pharmaceutical Wastewater Using Untreated and Treated Eggshells as Biosorbents

Abstract

Pharmaceutical wastewater contains high levels of organic matter, salts, and toxic compounds that are resistant to conventional treatment methods. Even after secondary treatment, traces of dissolved organics and suspended solids often remain, contributing to environmental concerns such as increased microbial resistance and harm to aquatic life. This study introduces a sustainable “waste-to-treat-waste” approach that utilizes discarded white chicken eggshells as a low-cost biosorbent for removing ciprofloxacin, a common antibiotic. Unlike previous eggshell-based adsorption studies that primarily targeted dyes or heavy metals, this work demonstrates the first comprehensive evaluation of both untreated and chemically/thermally modified eggshells for antibiotic removal from real pharmaceutical wastewater. Batch adsorption experiments under optimized conditions showed removal efficiencies of 85% for raw eggshells, 91% after HCl activation, and 96% after thermal conversion to CaO. Batch adsorption experiments under optimized conditions (pH 7, 25 °C, 625 µm particle size, 3 g/100 mL dose, 90 min contact time) showed maximum adsorption capacities of 23.75 mg/g for untreated ES, 4.08 mg/g after HCl activation, and 1.82 mg/g after thermal conversion to CaO, with removal efficiencies of 85%, 91%, and 96%, respectively. The simplicity of preparation, use of an abundant waste material, and high removal efficiency highlight the potential for scalable cost-effective applications in industrial wastewater treatment systems.

1. Introduction

Advancements in the pharmaceutical industry have revolutionized healthcare by enabling the development of innovative drugs and therapies that target specific diseases and conditions with greater precision and effectiveness. Because of innovations in pharmaceutical technology, many types of pharmaceutical wastewater are produced from the industry’s diverse product. Highly concentrated antibiotic wastewater generates most of the biopharmaceutical waste, which has a complex composition and is biologically toxic [1]. The presence of such medical waste belongs to the list of aspects that impact nature and public health; they are also bioaccumulative in nature and have been noted to cause endocrine disruption, antibiotic resistance, and other health issues [2]. Improper disposal of pharmaceutical residues allows them to reach rivers, lakes, and oceans, contaminating ecosystems and threatening aquatic life. These toxicants may also leach into groundwater and affect drinking water sources [3].

The large-scale disposal of pharmaceuticals demands specialized handling and wastewater treatment to minimize environmental and human risks. Traditional methods often fall short and can cause additional problems [4]. There is a clear need for more efficient pharmaceutical waste management methods. Current disposal methods include burning, landfilling, and treated water release. Incineration effectively destroys toxic substances through high heat but can release harmful emissions that damage the environment. Landfill disposal involves burying waste on-site but can cause groundwater contamination and other long-term issues [5].

Wastewater treatment uses different processes to remove pharmaceutical residues but may not completely eliminate all compounds, risking ecological damage. Some methods are expensive, overlook infrastructure needs, or do not address emerging pollutants [6]. Adsorption, a sustainable and affordable approach using specific adsorbents to target contaminants, offers a promising alternative.

Eggshells, mainly composed of porous calcium carbonate crystals, offer a high surface area and are effective biosorbents for pharmaceuticals in wastewater. Their porous nature allows for efficient contaminant retention. Being non-toxic, biodegradable, abundant, low-cost, and easily accessible as waste, eggshells are an eco-friendly alternative to synthetic adsorbents [7]. They have also been successfully used to remove heavy metals, dyes, and various organic and inorganic compounds [8]. Eggshells have proven effective in adsorbing heavy metals such as cadmium, lead, and zinc from aqueous solutions [9]. Research also shows that eggshells can remove dyes, fluoride, and certain organic pollutants from wastewater, with improved results when modified to enhance their adsorptive capacity [10,11,12]. Growing environmental concerns encourage the use of eggshells as biosorbents to promote sustainable circular waste and pollution management. Their on-site recyclability makes them a promising material for environmental and wastewater applications in the pharmaceutical industry [8,11]. Eggshells are often used as a soil amendment due to their calcium content; however, in this study, only pristine and treated eggshells were considered. The safe disposal or regeneration of spent adsorbent is an important consideration discussed later.

Although eggshells have been studied for removing metals and dyes, their use for antibiotics such as ciprofloxacin is less explored. Ciprofloxacin is widely detected in wastewater and poses risks of antimicrobial resistance, but its amphoteric structure makes adsorption challenging. Comparing untreated and treated eggshells is, therefore, essential to understand how surface properties influence ciprofloxacin removal and to identify practical scalable options. Various activation methods have been explored to enhance ES adsorption capacity. Physical activation, like pyrolysis in oxygen-limited settings, produced porous structures with higher surface area, improving the removal of heavy metals and organics [13]. Chemical activation with acids altered surface chemistry, enhancing pollutant binding [13]. These modifications highlight ES as a sustainable eco-friendly biosorbent for water and soil remediation in line with green chemistry principles [13].

Therefore, the main objective of this work is to utilize eggshells as biosorbents to remove pharmaceutical waste from wastewater. Specifically, this includes using eggshells to remove ciprofloxacin from pharmaceutical wastewater, investigating the effects of physical and chemical treatments on eggshell removal efficiency, examining how operating parameters such as pH, eggshell and ciprofloxacin concentrations, and temperature influence the adsorption process, and determining the optimal conditions for ciprofloxacin removal using eggshells as adsorbents.

While biosorption offers clear advantages, the management of spent adsorbents remains a critical aspect. Potential approaches include thermal regeneration, acid/base washing, or use as soil conditioners after the safe stabilization of adsorbed pharmaceuticals [13]. These considerations are essential to prevent merely transferring contamination from water to solid waste.

2. Materials and Methods

2.1. Preparation of Eggshells

White eggshells (ES) were collected from kitchens and trash cans, cleaned with tap water to remove debris, and rinsed with ultra-pure water to eliminate residual impurities. They were then dried at 60 °C for 48 h, ground into fine particles, and sieved to obtain sizes between 200 and 1000 µm. The choice of 1 M HCl and a 48 h soaking period was based on literature reports showing that this concentration and duration are effective in partially decalcifying eggshells, enhancing surface porosity and functional groups without compromising structural stability [5,6,7,8,9,10,11,12]. Preliminary tests in this study also confirmed that longer soaking times or higher concentrations led to excessive dissolution, while shorter durations yielded minimal improvement in adsorption capacity. The powders were stored in sterile polythene containers for adsorption experiments.

This study investigated three ES forms: untreated, chemically activated, and thermally activated. Chemical activation used hydrochloric acid to introduce functional groups, enhancing pharmaceutical adsorption [14]. Thermally activated eggshells, represented in this study by activated lime powder, were typically heated between 100 and 700 °C to evaluate adsorption efficiency.

2.2. Ciprofloxacin Solution

Ciprofloxacin was selected as the model pharmaceutical contaminant. Ciprofloxacin (≥99% purity, Sigma Aldrich, St. Louis, MO, USA) was weighed on an analytical balance (±0.1 mg) and dissolved in ultra-pure water (Milli-Q, resistivity 18.2 MΩ·cm). The solution pH was maintained at neutral during preparation to avoid premature hydrolysis. All stock and working solutions were stored in amber glass bottles at 4 °C to minimize photodegradation and chemical instability. A 100 µg/L stock solution was prepared by dissolving 5 mg of ciprofloxacin in ultra-pure water using a magnetic stirrer for two hours in the dark to prevent degradation. The solution was transferred to a 1 L volumetric flask and diluted to volume with ultra-pure water [15].

All working solutions and standards were derived from this stock and stored in amber glass bottles at room temperature to avoid photodecomposition. Standard solutions (0.1–50 mg/L) were freshly prepared via serial dilution. UV-Vis spectrophotometry (250–800 nm) was used to determine the maximum absorbance wavelength (λmax), and calibration curves were constructed at λ_max to assess linearity in two ranges: 0.1–10 and 10–50 mg/L [16].

CIP has a molecular weight of 331.34 g/mol, chemical formula C₁₇H₁₈FN₃O₃, and amphoteric behavior with pKa values of 6.1 (carboxyl group) and 8.7 (piperazinyl group) [15]. These properties influence its ionization state and interaction with eggshell surfaces.

2.3. Batch Adsorption Tests

Batch adsorption experiments were conducted to evaluate the effectiveness of raw, chemically treated, and thermally treated eggshells in removing ciprofloxacin from aqueous solutions. This study examined the impact of adsorbent dose, particle size, pH, contact time, and temperature on adsorption performance. All tests were performed under controlled laboratory conditions, with some experiments repeated on different days. The resulting standard deviations in removal efficiency (1.8–2.5%) indicate high accuracy and reproducibility. Once optimal conditions were established, tests using untreated eggshells were repeated with chemically activated (ES-C), thermally treated (ES-T), or activated lime for comparison.

All adsorption tests were conducted in duplicate to ensure reproducibility, and mean values with standard deviations are reported. A thermostatic orbital shaker (Innova 44, New Brunswick Scientific, Edison, NJ, USA) was used to maintain constant agitation and temperature. After adsorption, solutions were filtered using Whatman No. 1 filter paper (11 µm pore size). Concentrations of ciprofloxacin were measured using a UV–Vis spectrophotometer (Shimadzu UV-2600, Kyoto, Japan) at 275 nm. Control experiments without adsorbent were also carried out to account for any possible photodegradation or self-decomposition of ciprofloxacin.

2.4. Effect of Operational Parameters

2.4.1. Effect of Contact Time

To evaluate the effect of contact time, batch adsorption experiments were performed with U-ES under the conditions described in Section 2.3. At intervals (0–240 min), one flask was removed, filtered, and analyzed at 275 nm. A concentration of 10 µg/L was chosen as it falls within the range reported in pharmaceutical effluents and treated hospital wastewater (1–50 µg/L) [2]. The experiment was repeated at 300 rpm to examine the effect of higher agitation on equilibrium time.

2.4.2. Effect of Adsorbent Dose

To determine the optimal eggshell dose for ciprofloxacin removal, seven 250 mL Erlenmeyer flasks were prepared with 100 mL of 10 mg/L ciprofloxacin solution at pH 7. Increasing doses of untreated eggshell powder (U-ES), from 0.5 to 6 g, were added. The flasks were shaken at 300 rpm for 90 min at 25 °C [17]. After filtration (Whatman No. 1, Whatman, Maidstone, UK), the filtrates were analyzed at 275 nm using a UV-Vis spectrophotometer. Removal efficiency was calculated from initial and final concentrations using a calibration curve.

2.4.3. Effect of Particle Size

To assess the effect of particle size on adsorption, eggshell powders were sieved into five sizes: 325, 425, 625, 700 µm, and 1 mm. For each, 3 g of U-ES was added to 100 mL of 10 µg/mL ciprofloxacin at pH 7. Samples were shaken at 300 rpm for 90 min at 25 °C. After filtration, UV-Vis analysis at 275 nm was used to compare adsorption performance, evaluating how particle size influenced surface area, pore exposure, and ciprofloxacin removal.

2.4.4. Effect of Temperature

To evaluate the effect of temperature on ciprofloxacin adsorption, experiments were conducted at 25, 40, 50, 60, and 70 °C. Each flask contained 100 mL of 10 mg/L ciprofloxacin and 3 g of U-ES at pH 7, shaken at 300 rpm for 90 min. After filtration, samples were analyzed at 275 nm and removal percentages were calculated to interpret the temperature dependence of the adsorption process.

2.4.5. Effect of pH

Since pH affects ciprofloxacin’s ionization and adsorbent surface charge, experiments were conducted using five 250 mL flasks, each with 100 mL of 10 µg/mL ciprofloxacin and 3 g U-ES. The pH was adjusted to 3, 5, 7, 9, and 11 using 0.1 M HCl or NaOH. Flasks were shaken at 300 rpm for 1 h at ~25 °C and then filtered and analyzed at 275 nm. Percentage removal versus pH was plotted to determine the optimal adsorption pH range.

2.5. Treated and Untreated Eggshells

A comparative study was performed to assess adsorption improvements from surface modification by testing U-ES, acid-treated eggshells (HCl-soaked), and thermally activated eggshells (represented by activated lime CaO). All tests used the previously optimized conditions for adsorbent dose, contact time, agitation speed, temperature, pH, and particle size.

2.6. Adsorption Isotherms

Adsorption isotherm studies were conducted using five flasks with 100 mL ciprofloxacin solutions at concentrations of 10, 20, 30, 40, and 50 µg/mL and 3 g U-ES. The pH was set to 7, and flasks were shaken at 300 rpm and 25 °C for the previously determined equilibrium time. After filtration, equilibrium ciprofloxacin concentrations were measured at 275 nm. The adsorbed amount per gram (qₑ) was calculated as follows:

qₑ = (C₀ − Cₑ) × V/m

where C₀ and Cₑ are initial and equilibrium concentrations (mg/L), V is solution volume (L), and m is adsorbent mass (g). Data were fitted to Langmuir and Freundlich isotherm models to analyze adsorption mechanisms and capacities.

2.7. Sample Characterization

To understand the structural, morphological, and chemical properties of the eggshell-based adsorbents, various characterization techniques were applied before and after adsorption to assess surface changes, composition, and functional group interactions.

2.8. Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) was used to analyze the surface morphology and microstructural features of raw and treated eggshell adsorbents. Imaging was conducted using the Ultim^® Extreme Infinity SEM detector (backscatter and secondary electron modes) from Oxford Instruments NanoAnalysis Ltd. (Oxford, UK). Its high spatial resolution and signal-to-noise ratio enabled clear visualization of particle size, porosity, and surface textures under various treatment conditions. SEM images were captured before and after ciprofloxacin adsorption to assess changes in texture, porosity, and possible surface degradation.

2.9. X-Ray Diffraction (XRD)

X-ray diffraction (XRD) was employed to identify crystalline phases in the eggshell powders and detect structural changes after adsorption. Analyses were carried out using a Panalytical X’Pert³ Powder diffractometer (Malvern Panalytical, Almelo, The Netherlands), a dedicated system with advanced detector technology and integrated control electronics. High-quality diffraction data were collected across a 2θ range suitable for identifying calcium carbonate, calcium oxide, and any crystalline complexes formed with ciprofloxacin.

2.10. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) was used to identify functional groups on the eggshell surface and evaluate interactions with ciprofloxacin. Spectra were recorded using a Thermo Scientific Nicolet iS10 FTIR spectrometer, equipped with a deuterated triglycine sulfate (DTGS) detector and a KBr beam splitter. Measurements taken before and after adsorption allowed for the identification of changes in characteristic absorption bands, indicating the involvement of specific functional groups in the adsorption process.

2.11. Energy-Dispersive X-Ray Spectroscopy (EDX)

Energy-Dispersive X-Ray Spectroscopy (EDX) was conducted to determine the elemental composition of the eggshell adsorbents before and after ciprofloxacin adsorption. Analysis was performed using the Ultim^® Max Infinity silicon drift detector (SDD) system from Oxford Instruments NanoAnalysis Ltd. (Oxford, UK). Key elements such as calcium (Ca), carbon (C), and oxygen (O) were detected, along with nitrogen (N) and phosphorus (P), which may indicate ciprofloxacin binding. Comparative EDX spectra helped identify surface chemical changes due to adsorption.

3. Results and Discussion

3.1. Effect of Contact Time

To evaluate the effect of contact time, adsorption experiments were performed with U-ES under the optimized batch conditions described in Section 2.3. Figure 1 shows the variation in CIP removal efficiency at two different agitation speeds.

As shown in Figure 1, agitation speed significantly influenced both the rate and extent of ciprofloxacin adsorption. At 100 rpm, over 35% removal occurred within 60 min, driven by a high concentration gradient and available surface sites. [18,19]. However, the removal rate slowed between 60 and 90 min, reaching 44.32%, as active sites became progressively occupied and intra-particle diffusion began to dominate [20]. The process approached equilibrium between 90 and 120 min, with a final removal of 46.89% at 180 min.

In contrast, at 300 rpm, a markedly faster and more efficient adsorption was observed (Figure 1). Within just 30 min, 78% of ciprofloxacin was removed, and the process reached equilibrium at around 90 min with a final removal of 85%. The higher agitation speed enhanced external mass transfer by disrupting the boundary layer around the particles, leading to faster diffusion of ciprofloxacin to adsorption sites [21,22]. The minimal changes observed beyond 60 min (from 83% to 85%) suggest that adsorption sites were nearly saturated [23]. These results demonstrate that increased agitation significantly improves ciprofloxacin adsorption by accelerating kinetics, enhancing mass transfer, and reducing the time to equilibrium. The final removal efficiency at high agitation (85%) is nearly double that at low speed (~47%), emphasizing the importance of mixing in facilitating adsorption in low-porosity biosorbents such as eggshells. This finding is consistent with prior studies showing that agitation reduces film diffusion resistance, thereby increasing adsorption efficiency [24].

The data indicate that the adsorption follows a chemisorption mechanism, as evidenced by the pseudo-second-order model’s high R² value of 0.998, which is significantly greater than that of the first-order model (Figure 1B). This suggests interactions such as surface complexation or electron sharing between ciprofloxacin and calcium sites on the eggshell. The pseudo-second-order kinetics imply that chemical interactions, like ion exchange or hydrogen bonding, govern the adsorption rate. Additionally, the rapid equilibrium under high agitation supports the dominance of surface-controlled adsorption mechanisms [25].

3.2. Effect of Sorbent Dose

The adsorbent dose is a critical factor influencing removal efficiency. The impact of increasing U-ES dosage on CIP adsorption is illustrated in Figure 2. As shown, the percentage removal of CIP increased markedly with increasing adsorbent dose, particularly from 23% at 0.5 g to 84% at 3.0 g, due to the greater availability of adsorption sites and surface area. Beyond 3.0 g, only marginal improvements were observed (up to 86% at 6.0 g), indicating that adsorption equilibrium had been reached. The plateau reflects adsorbate saturation and a reduced mass transfer driving force due to the diminished concentration gradient.

Excess adsorbent beyond the optimal dose may lead to particle agglomeration and pore blockage, thereby limiting accessibility to active sites and impeding diffusion-based mechanisms. This behavior is consistent with findings from similar studies using calcium carbonate-based materials, where Langmuir-type adsorption trends and saturation typically occur around 3–4 g/100 mL. From an application perspective, 3.0 g per 100 mL is identified as the optimal dose, balancing high removal efficiency (84%) with material economy. This value serves as a practical benchmark for scaling up the use of untreated eggshells as a low-cost sustainable adsorbent for pharmaceutical contaminants like ciprofloxacin.

3.3. Effect of Eggshell Particle Size

Particle size determines the available surface area and pore accessibility of the adsorbent.

Table 1. Ciprofloxacin removal efficiency for different eggshell particle sizes.

Particle Size	% Removal
1 mm	70%
700 µm	79%
625 µm	84%
425 µm	74%
325 µm	72%

summarizes CIP removal efficiency across different U-ES particle size fractions.

Among the tested sizes, the 625 µm fraction exhibited the highest removal efficiency (84%) within 30 min (Table 1). While smaller particles (325 µm and 425 µm) offer higher surface area, they may lead to agglomeration or pore clogging due to over-packing in suspension, reducing effective surface availability. Conversely, larger particles (1 mm) exhibited lower removal (70%), likely due to a reduced surface-to-volume ratio and limited diffusion pathways. The 625 µm particles achieved a balance between external surface area and internal pore accessibility, enhancing mass transfer without structural compaction. These results align with the literature findings that intermediate particle sizes typically yield optimal performance in batch adsorption systems, minimizing diffusional resistance while ensuring uniform dispersion [26,27]. Since adsorbent saturation was not a limiting factor at 3.0 g, particle size was the primary determinant of removal efficiency.

3.4. Effect of Temperature

Temperature affects both adsorbate mobility and sorbent surface properties. The influence of temperature on CIP removal is presented in Figure 3.

As shown in Figure 3, adsorption efficiency decreased with increasing temperature, with maximum removal (83%) observed at 25 °C and minimum removal (54%) at 70 °C. This inverse relationship indicates that CIP adsorption onto eggshells is an exothermic process. At elevated temperatures, the increased kinetic energy of CIP molecules weakens their interaction with the adsorbent surface and enhances desorption [28]. The decline in adsorption may also result from the disruption of weak physical forces, such as hydrogen bonding and van der Waals interactions, that typically govern biosorption in natural materials like eggshells [29]. Additionally, higher temperatures may alter the surface morphology or collapse microporous structures, diminishing the available surface area for adsorption [30]. Similar temperature-dependent trends have been reported in studies involving calcium carbonate-based and agricultural waste adsorbents [31,32].

From a practical standpoint, room temperature (25 °C) is identified as the optimal condition for CIP removal, offering high efficiency without additional energy input. This highlights the sustainability and economic viability of using untreated eggshells for ambient-temperature water treatment applications.

3.5. Effect of pH

Since pH influences both adsorbent surface charge and CIP ionization, its effect on adsorption performance was evaluated. The results are shown in Figure 4. The results (Figure 4) show a parabolic trend, with maximum adsorption efficiency (~83%) observed at neutral pH (7). Adsorption was significantly reduced under both acidic and alkaline conditions due to shifts in charge characteristics of ciprofloxacin and the eggshell surface. At low pH (3–5), the protonation of the calcium carbonate surface may result in electrostatic repulsion with cationic ciprofloxacin species, thereby decreasing adsorption. Acidic conditions may also partially dissolve the eggshell, altering surface structure and reducing available sites. At pH 7, ciprofloxacin exists in a zwitterionic form, which promotes hydrogen bonding and surface complexation, leading to enhanced adsorption. This behavior is consistent with previous studies on biosorbent–drug interactions [33,34]. At high pH (9–11), ciprofloxacin becomes anionic, and the eggshell surface also carries a negative charge, causing electrostatic repulsion. Additionally, higher pH may reduce surface affinity and increase drug solubility, further hindering adsorption [35,36].

These findings highlight the importance of pH in adsorption system design. Operating near neutral pH not only optimizes removal efficiency, but also minimizes chemical adjustment, making the process more practical for large-scale applications.

3.6. Treated and Untreated Eggshells

A comparative study was conducted to assess adsorption efficiency among untreated, HCl-treated, and thermally activated eggshells. The results are summarized in

Table 2. Ciprofloxacin removal efficiency using different adsorbents under optimized conditions.

Adsorbent Type	% Removal
U-ES	83%
HCl-Treated Eggshells	91%
Thermally Activated Lime (CaO)	96%

.

Both acid and thermal treatments significantly improved ciprofloxacin adsorption. Thermally activated eggshells achieved the highest removal (96%), followed by HCl-treated (91%) and U-ES (83%), confirming the benefits of surface modification. HCl treatment enhanced surface porosity and functional group availability through partial decalcification, promoting electrostatic interactions and hydrogen bonding [37,38]. Thermal activation transformed CaCO₃ into reactive CaO, increasing surface reactivity and removal efficiency via chemisorption and precipitation mechanisms [39,40]. The enhanced alkalinity of CaO also promotes ciprofloxacin ionization at neutral pH, further improving adsorption. These findings are consistent with the literature on calcium-based biosorbents used for antibiotic removal [41,42].

Compared to studies using similar adsorbents, eggshell-derived materials, particularly thermally calcined calcium oxide (CaO), exhibit superior performance in ciprofloxacin removal. In this study, CaO achieved a 96% removal efficiency at neutral pH (7.0) within 90 min. This surpasses results reported for chitosan-grafted silica (95% at pH 5.5 in 30 min) [43] and granular activated carbon (92% at pH 6.5 in 60 min) [44], while offering the advantage of operating under milder, more practical conditions without the need for acidic pretreatment or stringent pH control. Likewise, HCl-treated eggshells (ES-HCl) showed a high removal efficiency of 91% at neutral pH, outperforming other modified bio-adsorbents such as magnetic Fe₃O₄₋biochar (90% at pH 6.0 in 40 min) [45] and KOH-activated rice husk biochar (88% at pH 7.0 in 45 min) [46]. Even untreated eggshells (ES-U) demonstrated notable efficacy, achieving 83% removal at pH 7.0 over the same contact period.

Practical application depends on removal efficiency as well as cost, energy input, safety, and scalability. Untreated eggshells offer simplicity, low cost, and good removal efficiency, making them ideal for decentralized or resource-limited settings. HCl-treated eggshells provide enhanced performance with manageable preparation requirements and low additional cost. Thermally activated CaO delivers the highest efficiency but at the expense of greater energy demand and complexity, making it more suitable for industrial-scale applications where performance outweighs operational cost. These considerations are summarized in

Table 3. Practical considerations of adsorbent types for ciprofloxacin removal.

Factor	U-ES	HCl-Treated Eggshells	Thermally Activated Lime (CaO)
% Removal (CIP)	83%	91%	96%
Preparation Steps	Drying, grinding, sieving	Soaking in HCl (48 h), rinsing, drying	Calcination at 600 °C (2 h), grinding
Chemicals Required	None	1 M HCl	None (except for energy input)
Energy Consumption	Low	Moderate (drying)	High (heating to 600 °C)
Cost of Materials	Very low	Low (HCl is inexpensive)	Moderate–high (due to energy)
Environmental Safety	Excellent (natural waste)	Safe with proper neutralization	Safe post-cooling, caustic before use
Reusability/Regeneration	Moderate	Moderate	High
Adsorption Mechanism	Physical (surface binding)	Electrostatic and H-bonding	Chemisorption, precipitation
Ease of Scale-Up	Very easy	Easy	Technically demanding

. In conclusion, while all three adsorbents are viable, the choice depends on the specific application context, untreated and acid-treated eggshells for low-cost decentralized use, and CaO for high-performance demands.

3.7. Adsorption Isotherms

The adsorption isotherms were analyzed to understand the equilibrium interaction between ciprofloxacin (CIP) and eggshell-based adsorbents under varying initial concentrations. The experimental data for U-ES, HCl-treated eggshells (ES-HCl), and thermally activated eggshells (CaO) were fitted to the following linearized Langmuir and Freundlich isotherm models to interpret the underlying adsorption mechanisms.

The Langmuir isotherm assumes monolayer adsorption onto a surface with a finite number of identical sites. It can be expressed in the following linearized form:

$${\displaystyle \frac{1}{q_e}}={\displaystyle \frac{1}{q_{max}}}+{\displaystyle \frac{1}{q_{max}{K}_L}} {\displaystyle \frac{1}{C_e}}$$

where qₑ is equilibrium adsorption capacity (mg/g), Cₑ is equilibrium concentration (mg/L), qₘₐₓ is the maximum adsorption capacity (mg/g), and K_L is Langmuir constant (L/mg). The values of qₘₐₓ and K_L can be obtained from the slope (1/(qₘₐₓ K_L)) and the intercept (1/qₘₐₓ) of the linear plot of 1/qₑ versus 1/Cₑ.

The Freundlich isotherm describes adsorption on heterogeneous surfaces and allows for multilayer adsorption. The Linearized form of Freundlich model can be expressed as follows (log-log):

$$log{q}_e=log{K}_F+{\displaystyle \frac{1}{n}}log{C}_e$$

where K_F is the Freundlich constant related to adsorption capacity and 1/n is related to the adsorption intensity (dimensionless). The values of K_F and 1/n can be obtained from the slope (1/n) and the intercept (logK_F) of the linear plot of logqₑ versus 1/Cₑ.

Figure 5 presents the linearized adsorption isotherms used to evaluate the fit of ciprofloxacin adsorption data to the Langmuir and Freundlich models. Plot (A) corresponds to the Langmuir isotherm (1/qₑ vs. 1/Cₑ), assuming monolayer adsorption on a homogenous surface, while plot (B) shows the Freundlich isotherm (log qₑ vs. log Cₑ), which accounts for adsorption on heterogeneous surfaces. The plots visually compare the adsorption behavior of untreated, acid-treated, and thermally treated eggshell sorbents.

Table 4. Langmuir and Freundlich model parameters and corresponding R² values for U-ES, acid-treated eggshell, and thermally treated eggshell sorbents.

Adsorbent	Langmuir Model			Freundlich Model
	qmax	KL	R2	KF	1/n	R2
U-ES	23.75	0.0228	0.9918	0.6937	0.7826	0.9988
Acid-treated	4.083	0.1072	0.8926	0.6599	0.4681	0.9563
Thermally treated	1.820	0.4049	0.6222	0.8988	0.1895	0.7566

presents the isotherm parameters derived from the linearized Langmuir and Freundlich models for ciprofloxacin adsorption onto untreated, acid-treated, and thermally treated eggshells. Among the tested adsorbents, U-ES exhibited the highest Langmuir maximum adsorption capacity (qₘₐₓ = 23.75 mg/g) and a strong fit with both models (R² = 0.9918 for Langmuir; R² = 0.9988 for Freundlich), indicating favorable adsorption on a heterogeneous surface with potential multilayer interactions.

Acid-treated eggshells showed moderate adsorption capacity (qₘₐₓ = 4.083 mg/g) but a higher Langmuir constant (K_L), suggesting stronger binding affinity due to surface modifications introduced through decalcification. These include increased porosity and functional groups such as –OH and –CO₃²⁻, which enhance adsorption via electrostatic and hydrogen bonding interactions.

Thermally treated eggshells (CaO) displayed the lowest adsorption capacity (qₘₐₓ = 1.82 mg/g) and the weakest fit to both models, particularly Langmuir (R² = 0.6222), possibly due to structural alterations during calcination that reduced accessible surface area. However, CaO exhibited the highest Freundlich K_F value and the lowest 1/n, reflecting strong interactions with a heterogeneous surface, consistent with the literature on reactive CaO surfaces [47,48,49,50].

Overall, the Freundlich model provided a better fit for all samples, indicating that adsorption is governed primarily by heterogeneous surface interactions. These results underscore the importance of surface treatment: acid modification improves binding site availability and affinity, while thermal activation enhances reactivity and surface heterogeneity, albeit at the cost of total capacity.

When benchmarked against the literature, the maximum adsorption capacity obtained in this study (23.75 mg/g for U-ES) compares favorably with previously reported values for bio-waste-derived adsorbents such as chitosan–silica (18.3 mg/g) [43], rice husk biochar (16.2 mg/g) [46], and magnetic Fe₃O₄₋biochar (19.6 mg/g) [45]. Kinetic modeling in our study confirmed pseudo-second-order chemisorption, consistent with other studies on CaCO₃-based adsorbents [18,19]. These comparisons highlight the competitive adsorption performance of eggshell-derived materials under realistic conditions.

3.8. Sample Characterization

3.8.1. XRD Analysis

X-ray diffraction (XRD) analysis was conducted within a 2θ range of 5.0–70.0°, and all peaks were indexed using reference patterns from the literature for calcium carbonate and ciprofloxacin (CIP). Figure 6 illustrates the XRD patterns for untreated eggshells and eggshells loaded with CIP. A prominent peak at 0.45° in the U-ES sample corresponds to calcite (CaCO₃), consistent with reported data [51]. After CIP loading, the peak shifts to 0.75°, indicating possible interaction or attachment of CIP molecules to active sites on the eggshell surface. Additional peaks observed at 0.75° align with reference patterns for CIP, further confirming its presence. Broad-spectrum XRD patterns also revealed peaks at approximately 30.0°, 32.6°, 37.0°, 48.5°, and 49.3° (2θ). The dominant peak at ~30.0° corresponds to calcite, matching the reference at 29.5° (2θ) [52]. Peaks near 32.6° and 37° are attributed to calcium oxide [48], while those around 48.5° and 49.3° are consistent with calcium hydroxide [52]. After CIP adsorption, these peaks remain, although most exhibit reduced intensity, except the 29.5° peak, which shows increased intensity. This suggests the preservation of the primary crystalline structure of calcite, with only minor surface-level changes due to CIP adsorption. The continued presence of calcite and calcium oxide phases indicates that the eggshell crystal structure remained largely intact.

3.8.2. EDX Analysis

Energy-Dispersive X-Ray Spectroscopy (EDX) was employed to investigate the elemental composition of the samples, revealing notable differences between pure eggshells and those loaded with ciprofloxacin (CIP). These spectra are shown in Figure 7. The pure eggshells primarily consisted of calcium (Ca), carbon (C), and oxygen (O), key constituents of calcium carbonate. Trace elements such as chlorine (Cl) and potassium (K) were identified, though their presence was minimal. Following CIP adsorption, a clear change in elemental profile was observed: the carbon content on the surface increased significantly from 3.7 to 10.9 wt%, indicating the formation of an organic layer over the inorganic CaCO₃ substrate. Additionally, a new chlorine peak (~0.3 wt%) emerged, serving as a distinct marker of CIP binding.

Simultaneously, the calcium signal dropped by approximately 8.0 wt%, likely due to both surface coverage by the CIP layer and partial calcium complexation or ion exchange. Interestingly, oxygen levels remained relatively stable (around 50.0 wt%), suggesting contributions from both the carbonate matrix and the oxygen-containing functional groups of CIP. Although a weak chlorine signal was observed, this is not a structural marker of CIP (which contains fluorine and nitrogen but not chlorine). The absence of distinct F and N peaks may be due to their low atomic percentage, overlap with background signals, or instrumental detection limits. Indirect evidence of CIP adsorption is provided by the marked increase in C/O ratio. The increase in carbon content also aligns quantitatively with gravimetric and spectrophotometric uptake measurements, reinforcing the effective adsorption of CIP onto the eggshell matrix.

3.9. SEM Analysis

Scanning Electron Microscopy (SEM) was used to examine the surface morphology of pure and CIP-loaded eggshells at a magnification of 25×, revealing distinct structural changes upon adsorption. The U-ES displayed a relatively smooth and uniform surface, characterized by the presence of typical calcite crystal formations (Figure 8). In contrast, the CIP-loaded samples exhibited a noticeably rougher and more irregular surface texture (Figure 8), likely resulting from the deposition of ciprofloxacin on the eggshell matrix. This increase in surface roughness and porosity suggests a larger surface area available for interaction, providing visual evidence of successful drug adsorption. These morphological transformations align well with the EDX findings, reinforcing the conclusion that CIP is effectively bound to the eggshell surface through adsorption mechanisms.

3.10. FTIR Analysis

Fourier Transform Infrared Spectroscopy (FTIR) was employed to investigate the chemical interactions between ciprofloxacin (CIP) and the eggshell surface. The FTIR spectra of both pure eggshells and CIP-loaded samples (Figure 9) reveal clear differences in surface chemistry, providing strong evidence of adsorption. In the spectrum of the untreated eggshells, characteristic peaks of calcium carbonate (CaCO₃), the primary mineral component, can be observed. Notably, the bands near 710 cm⁻¹ and 870 cm⁻¹ correspond to the bending vibrations of carbonate ions (CO₃²⁻), while a broad band between 1400 and 1500 cm⁻¹ reflects asymmetric stretching, a signature of the calcite form of CaCO₃. Additionally, the broad absorption in the 3400–3600 cm⁻¹ range is attributed to O–H stretching vibrations, indicating the presence of surface-bound water or hydroxyl groups.

After CIP adsorption, no new CIP-specific peaks (C=O, N–H, C–F, C–N) were observed; however, distinct changes in intensity and broadening of existing bands (710 and 870 cm⁻¹, 1400–1500 cm⁻¹, and 3400–3600 cm⁻¹) were detected. These shifts indicate the disruption of the carbonate environment and involvement of hydroxyl groups in hydrogen bonding and complexation with CIP, consistent with adsorption. The carbonate peaks at 710 and 870 cm⁻¹ remain present but become broader and less intense, suggesting disruption of the local carbonate environment due to CIP binding. Similarly, the intensity reduction in the 1400–1500 cm⁻¹ band indicates a weakening of the carbonate asymmetric stretching, likely due to the formation of surface complexes between calcium ions and CIP functional groups. Moreover, the O–H stretching region also shows diminished intensity post adsorption, implying that water molecules or hydroxyl groups on the eggshell surface are being displaced or involved in hydrogen bonding with the drug.

Overall, these spectral changes confirm that CIP interacts directly with the eggshell surface through mechanisms involving ion exchange, hydrogen bonding, and complexation, corroborating the findings from SEM and EDX analyses.

The adsorption of ciprofloxacin (CIP) onto untreated and treated eggshells can be explained by kinetic, isotherm, and characterization results. The pseudo-second-order kinetics confirm chemisorption, while the Freundlich isotherm indicates multilayer adsorption on heterogeneous surfaces. FTIR and EDX analyses reveal the involvement of carbonate and hydroxyl groups, Ca–CIP complexation, and hydrogen bonding. SEM images show surface roughening and pore filling after adsorption. Overall, CIP uptake occurs through (i) electrostatic attraction with Ca²⁺ sites, (ii) hydrogen bonding with –OH and –CO₃²⁻ groups, (iii) ion exchange/complexation with calcium, and (iv) precipitation/chemisorption in thermally treated CaO. These findings are consistent with mechanisms reported in the literature [53].

4. Conclusions

This study demonstrates that discarded white eggshells are a highly effective, low-cost, and sustainable bioadsorbent for removing ciprofloxacin from wastewater. Under optimized conditions, untreated eggshells achieved up to 83% removal, with performance improving to 91% and 96% following acid and thermal treatment, respectively. Adsorption followed a pseudo-second-order kinetic model, indicating chemisorption through mechanisms like ion exchange and hydrogen bonding. Optimal performance was observed at pH 7, 25 °C, and with a 625 µm particle size. Isotherm analysis revealed that ciprofloxacin adsorption fits best with the Freundlich model, indicating multilayer adsorption on heterogeneous surfaces. Characterization techniques (SEM, EDX, XRD, FTIR) indicated likely interaction between CIP and eggshell surfaces through hydrogen bonding, ion exchange, and electrostatic attraction, supported by FTIR, EDX, and SEM evidence. These findings not only support the reuse of biowaste for environmental cleanup, but also contribute to circular economy goals and sustainable water management. Eggshells, therefore, offer a practical scalable solution for pharmaceutical wastewater treatment.