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. 2026 Sep 29;11(40):62040–62049. doi: 10.1021/acsomega.6c08278

Adsorption of Levofloxacin onto Chitosan- and Layered Double Hydroxide-Functionalized Montmorillonite

Preyangkar Kundu †,*, Masahito Yoshimura ‡, Tomohito Kameda †,*
PMCID: PMC13670248  PMID: 42889809

Abstract

Antibiotic contamination of water resources is a major environmental concern. Beyond their direct toxicity to nontarget aquatic organisms, antibiotic-contaminated waters can serve as reservoirs for the development of antimicrobial resistance (AMR). Levofloxacin (LVX), a third-generation fluoroquinolone antibiotic, has frequently been detected in various environmental media. In this study, the LVX adsorption performance of chitosan/montmorillonite (CTS/MMT) and CO3-type MgAl layered double hydroxide/montmorillonite (LDH/MMT) composites was evaluated in batch experiments. MMT, a naturally occurring phyllosilicate clay, was functionalized with either the polysaccharide CTS or the anionic clay LDH. The synthesized CTS/MMT composite maintained a negative surface charge across the whole pH range from 2.0 to 12.0, whereas LDH/MMT exhibited an isoelectric point (IEP) at pH 5.1. Key operational parameters, including solution pH, contact time, temperature, and initial LVX concentration, were then optimized. Both adsorbents showed maximum adsorption at pH 3 in a 30 mg L–1 LVX solution after 60 min, with removal efficiencies of 77.7% and 99.1% for 0.01 g of LDH/MMT and CTS/MMT, respectively. Temperature variation data showed that LVX adsorption efficiency for LDH/MMT remained largely unaffected, ∼78.0% for a 0.01 g dosage, ∼90.0% for 0.05 g, and ∼87.0% for 0.1 g throughout the temperature range 15–35 °C. In contrast, CTS/MMT consistently achieved near-complete LVX removal (∼100.0%) across all tested temperatures regardless of dosage. For 0.01 g of adsorbent dosage, CTS/MMT adsorbed >99.0% LVX within 60 min, whereas adsorption onto LDH/MMT achieved equilibrium within 10 min with 77.1% removal efficiency. The Langmuir model fitted the CTS/MMT adsorption data well, with a maximum adsorption capacity of 107.53 mg g–1, whereas the Freundlich model better described the adsorption behavior of LDH/MMT. For CTS/MMT, at pH < 5.59, surface adsorption through electrostatic attraction between cationic LVX and the negatively charged composite surface, coupled with cation exchange into the MMT interlayer, may be the principal adsorption phenomenon. On the other hand, heterogeneous intercalation of cationic LVX molecules into the MMT interlayer was the principal mechanism for LVX adsorption onto LDH/MMT.

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1. Introduction

Antibiotic contamination of aquatic environments has become a major global environmental issue. For many decades, antibiotics have been extensively used to treat microbial infections in humans and animals. They have also been used as growth promoters in livestock and aquaculture, contributing to their widespread occurrence in the environment. , Worldwide antibiotic consumption increased by 65.0% from 2000 to 2015 and is projected to increase by up to 200.0% by 2030. Antibiotics can enter water bodies through multiple pathways, including improper disposal and dumping of unused medications, hospital effluents, untreated or poorly treated wastewater discharges, effluents from wastewater treatment plants, pharmaceutical manufacturing, agricultural activities, livestock farming, and urban surface runoff. , These contaminated water resources serve as potential reservoirs for the development and spread of antibiotic-resistant bacteria, thereby contributing to antimicrobial resistance (AMR). In addition, low antibiotic concentrations may exert toxic effects on nontarget aquatic biota, including freshwater algae, microphytes, macrophytes, zooplankton, and fish.

Several classes of antibiotics have been detected in rivers, lakes, groundwater, and even drinking water supplies at concentrations typically ranging from below 1 ng L–1 to as high as 100 μg L–1. Among these antibiotics, fluoroquinolones, including levofloxacin (LVX), ciprofloxacin, and norfloxacin, are frequently detected in hospital wastewater treatment plants. LVX, a third-generation fluoroquinolone antibiotic, is widely used in human and veterinary medicine, and is listed by the World Health Organization as an essential medicine. Recently, LVX residues have frequently been detected in various environmental media. For example, the maximum LVX concentration in wastewater treatment plants across several regions of Japan was 2431 ng L–1, whereas 710 ng L–1 LVX was detected in the surface water of the Buriganga River in Bangladesh. The highest reported LVX concentration in the Chaohu Lake watershed in Southeast China was 89.86 ng L–1. Yasojima et al. detected 552 ng L–1 LVX in a sewage treatment plant in Japan. Notably, during the COVID-19 pandemic, extreme concentrations up to 2000 μg L–1 were recorded in hospital effluents. These figures significantly exceed the predicted no-effect concentrations (PNECs) recommended by the AMR Industry Alliance for LVX (minimum inhibitory concentration-based PNEC for AMR (PNEC-MIC, 0.25 μg L–1) and the environmental PNEC based on ecotoxicity (PNEC-env, 1.52 μg L–1)), underscoring an urgent imperative to develop effective and target-driven remediation strategies.

Various treatment strategies, including adsorption, ozonation, biological treatment, and membrane separation, have been investigated for removing antibiotics and pharmaceuticals from drinking water and wastewater. However, adsorption is one of the most practical and cost-effective methods for advanced water treatment. This is mainly attributed to its high selectivity, ease of operation, lack of toxic byproduct formation, and high efficiency in capturing pollutants present at low concentrations, such as antibiotics, where biological breakdown is inefficient. For instance, Hamad et al. demonstrated high adsorption efficiencies of 89.0%–91.0% for levofloxacin at low initial concentrations of 1000 μg L–1 using nanozero-valent iron and copper oxide nanoparticles, yet the high operational cost and potential secondary toxicity of synthetic nanomaterials necessitate the development of eco-friendly and low-cost alternatives.

Clay minerals have attracted considerable attention as adsorbents for antibiotic removal from aqueous media because of their low cost, wide availability, high surface area, ion-exchange properties, strong uptake capacity, and selective affinity. Montmorillonite (MMT), a naturally occurring phyllosilicate clay, consists of negatively charged octahedral sheets of aluminum or magnesium oxides interleaved with tetrahedral silicon layers. Interlayer cations provide an overall charge balance. This clay material exhibits high adsorption potential because of its high cation exchange capacity. However, pristine MMT particles are difficult to retain in aqueous media and challenging to recover or regenerate after adsorption, which constrains their practical application in industrial-level wastewater treatment, but its layered structure enables various modifications, facilitating the development of a wide range of composites to overcome these drawbacks. , Besides, effective surface modification of MMT improves its adsorption capacity.

Chitosan (CTS), a polyaminosaccharide, which is mainly composed of unbranched β-(1 → 4)-linked 2-acetamido-2-deoxy-d-glucose, is an effective organic intercalant capable of expanding MMT interlayers, enhancing its thermal stability and introducing versatile binding sites to enhance adsorption performance. Conversely, layered double hydroxides (LDHs) are anionic clays with the general formula [M1–x 2+M x 3+(OH)2] x+[A x/n n–]·mH2O, which consist of positively charged octahedral host layers of divalent metal hydroxides, M­(OH)2. The cations occupy octahedral sites in the hydroxide layers, and partial substitution of M2+ by M3+ produces a positive layer charge. A guest layer of exchangeable anions and water molecules is intercalated to neutralize the host-layer charge. LDHs are widely used as adsorbents for different types of pollutants due to their high specific surface area and ability to intercalate diverse functional species. − Incorporating LDHs onto MMT creates an inorganic/inorganic heterostructure with high surface area, variable surface charges, and synergistic anion–cation binding properties.

Although organic-modified clays (CTS/MMT) and inorganic composite clays (LDH/MMT) have been individually explored for various environmental applications, a direct, systematic comparative study evaluating the performance and mechanism of an organic/inorganic composite (CTS/MMT) versus an inorganic/inorganic composite CO3·MgAl LDH/MMT for LVX removal remains unaddressed in the existing literature. To bridge this gap, this study presents a comprehensive comparative evaluation of CTS/MMT and CO3·MgAl LDH/MMT composites for LVX remediation from aqueous systems under systematically controlled conditions. Batch adsorption experiments were conducted to determine the effects of key operational parameters, including pH, adsorbent dosage, initial LVX concentration, and temperature, on LVX removal efficiency.

2. Experimental Section

2.1. Synthesis of CTS/MMT

MMT, with the structural formula Na0.85Si8(Al3.15Mg0.85)­O20(OH)4 and a cation exchange capacity (CEC) of 115 cmol kg–1, was supplied by Kunimine Industries Co., Ltd., Japan. The clay contained Na+ as the sole interlayer cation, and cation substitution (Al → Mg) occurred at the octahedral layer. The CTS/MMT composite was synthesized according to the method reported by Kameda et al. Briefly, 0.5 g of CTS was dissolved in 50 mL of 10.0% (v/v) acetic acid solution to protonate the native amino groups (−NH3 +) into the CTS structure. The mixture was stirred for 30 min at 700 rpm and 60 °C. Afterward, the solution was cooled to 30 °C. Subsequently, 5.0 g of MMT was added to achieve a CTS:MMT mass ratio of 1:10, and the mixture was agitated for 24 h. The suspension was collected by centrifugation at 10,000 rpm for 5 min and washed three times with 10.0% (v/v) acetic acid solution, followed by Milli-Q water. The solid residue was vacuum-dried at 40 °C for 24 h and then pulverized.

2.2. Synthesis of CO3·MgAl-LDH/MMT

The CO3·MgAl-LDH/MMT composite was synthesized using the codeposition method described by Guan et al., with minor modifications. To achieve a 3:1 Mg2+/Al3+ molar ratio in the LDH structure and a mass loading based on 5.0 g of MMT, 250 mL of a solution containing 0.375 mol L–1 magnesium nitrate and 0.125 mol L–1 aluminum nitrate was added to a mixture of 5 g of MMT and 250 mL of 0.125 mol L–1 Na2CO3 at a rate of 10 mL min–1. The reaction pH was maintained at 10.5 using 1.5 mol L–1 NaOH, and the mixture was gently agitated at 300 rpm. The reaction was conducted at 30 °C, after which the mixture was allowed to settle for 1 h. The suspension was collected by filtration and repeatedly washed with deionized water until the pH of the washing solution was between 7 and 8.

2.3. Adsorption of LVX onto CTS/MMT and CO3·MgAl-LDH/MMT

LVX adsorption onto the synthesized composites was examined using a series of batch experiments. A specified amount of adsorbent (0.01, 0.05, or 0.1 g) was added to 20 mL of 30 mg L–1 LVX solution. After the solution pH was adjusted to 3, the mixture was shaken at 150 rpm and 25 °C. After a specified contact time, the adsorbent was separated by filtration using a 0.2-μm membrane filter, and the filtrate was analyzed to determine the residual LVX concentration. For the isotherm study, adsorption experiments were performed at initial LVX concentrations of 30, 90, 150, 210, and 300 mg L–1. To optimize other adsorption conditions, experiments were carried out at different pH values (3, 5, 7, 9, and 11), contact times (10, 20, 30, and 60 min), and temperatures (15, 25, and 35 °C).

2.4. Characterization Methods

X-ray diffraction (XRD; Rigaku MiniFlex600) was used to examine the adsorbents before and after LVX adsorption. The zeta potential of the composites was evaluated across a pH range of 2.0 to 12.0 to determine surface charge behavior. After each adsorption experiment, the residual LVX concentration was determined by high-performance liquid chromatography (HPLC; Agilent 1290 Infinity II) with a detection limit of 0.15 mg L–1. The analytical method uses a Waters XBridge BEH C18 column; chromatographic separation was performed with CH3CN/20 mM H3PO4 in H2O (10:90, v/v) as the mobile phase at a flow rate of 1.0 mL min–1, maintaining column temperature of 40 °C, UV detection at 294 nm, and an injection volume of 10 μL. LVX concentrations below the detection limit were treated as 0 mg L–1 for subsequent calculations.

3. Results and Discussion

3.1. Synthesis and Characterization of CTS/MMT and CO3·MgAl-LDH/MMT

The XRD patterns of MMT, CTS-modified MMT, and CO3·MgAl-LDH-modified MMT are shown in Figure a. The characteristic diffraction peaks of MMT were retained in the XRD patterns of the composites, although they shifted to lower 2θ values, indicating that the layered structure of MMT was preserved after modification. The basal spacing (d 001), which represents the interlayer distance, was 1.23 nm for MMT. After modification with CTS, this value increased to 1.51 nm, corresponding to an expansion of approximately 0.28 nm. This increase supports the successful intercalation of CTS molecules into the MMT interlayer. Similar changes in interlayer spacing have been reported after MMT modification with CTS. , In the XRD pattern of the CO3·MgAl-LDH-modified MMT composite, the characteristic diffraction peaks associated with the LDH structure confirmed the presence of the LDH phase in the composite. Similar XRD features have been reported for MMT/MgAl-LDH nanocomposites, in which the simultaneous presence of MMT basal reflections and characteristic LDH peaks confirms the formation of an LDH/MMT composite. In addition, the increase in basal spacing (d 001) from 1.23 to 1.46 nm suggests intercalation of LDH layers into the MMT interlayers, along with LDH deposition on the external surfaces.

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(a) XRD patterns and (b) Zeta potential as a function of solution pH for the CTS/MMT and CO3·MgAl-LDH/MMT composites.

Both CTS/MMT and LDH/MMT composites exhibit distinct surface charge characteristics depending on the solution pH (Figure b). For CTS/MMT, the surface charge remains negative throughout the whole tested pH range (2.0 to 12.0), whereas LDH/MMT exhibits a positive zeta potential in acidic conditions, reaching a maximum value (+15.0 mV) at pH 4.0. The observed decrease in zeta potential at pH 2.0 (+7.5 mV) compared to pH 4.0 (+15.0 mV) may be due to the partial dissolution of the LDH hydroxide layers under strongly acidic conditions, combined with electrical double layer (EDL) compression caused by elevated ionic strength from dissolved species. As the pH rises beyond 4.0, the zeta potential values decrease rapidly, crossing the neutral charge axis, yielding an isoelectric point (IEP) at pH ∼ 5.1; above this pH, the LDH/MMT surface carries a net negative charge.

3.2. Adsorption of LVX

3.2.1. Effect of pH

The effect of pH on LVX adsorption onto (a) CTS/MMT and (b) CO3·MgAl-LDH/MMT composites is shown in Figure . For CTS/MMT (Figure a), nearly complete LVX removal (approximately 99.0%–100.0%) was achieved at pH 3–11 using 0.1 g of adsorbent, at pH 3–9 using 0.05 g, and at pH 3 using 0.01 g. These results indicate that, within these pH ranges, CTS/MMT provides sufficient adsorption sites and strong affinity for LVX molecules. In contrast, 0.01 g of CTS/MMT showed comparatively lower adsorption efficiency, particularly at higher pH, because of the limited number of active sites available for interaction with LVX molecules. At low pH, LVX adsorption may be favored by enhanced electrostatic attraction and favorable interlayer intercalation because LVX exists predominantly in a cationic form at pH < 5.59, as a zwitterion at 5.59 < pH < 7.94, and exists in an anionic form at pH > 7.94.

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Effect of pH on LVX adsorption onto (a) CTS/MMT and (b) CO3·MgAl-LDH/MMT in 20 mL of 30 mg L–1 LVX solution at 25 °C for 60 min.

For CO3·MgAl-LDH/MMT (Figure b), the maximum adsorption efficiencies were observed at pH 3, with removal values of approximately 78.0%, 90.0%, and 87.0% using 0.01, 0.05, and 0.1 g of adsorbent, respectively. At pH 3, adsorption was slightly higher with 0.05 g than with 0.1 g of adsorbent, which may be attributed to particle aggregation that reduced the availability of active sites at the higher adsorbent dose under strongly acidic conditions. LVX adsorption efficiency decreased gradually with increasing solution pH, likely because the transition of LVX from cationic to zwitterionic and finally anionic forms reduced its intercalation into the MMT interlayer.

3.2.2. Effect of Temperature

The temperature variation data for CO3·MgAl-LDH/MMT (Figure b) revealed no significant thermal dependence on LVX removal efficiency, suggesting that the adsorption process is not highly sensitive to temperature. A 0.05 g adsorbent dosage showed higher adsorption efficiency (approximately 90.0%) than 0.01 g (approximately 78.0%) and 0.1 g (approximately 87.0%) under the same temperature conditions. The superior efficiency of the 0.05 g dosage may result from an optimal balance between active-site availability and adsorbent dispersion in solution. At the lower dosage (0.01 g), the number of adsorption sites was insufficient, whereas at the higher dosage (0.1 g), partial aggregation of adsorbent particles may have limited the accessibility of some active sites, thereby reducing adsorption capacity.

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Effect of temperature on LVX adsorption onto (a) CTS/MMT and (b) CO3·MgAl-LDH/MMT in 20 mL of 30 mg L–1 LVX solution at pH 3 for 60 min.

In contrast, CTS/MMT exhibited near-complete LVX removal (∼100.0%) across all temperatures and dosages (Figure a). Rather than indicating temperature independence, this flat profile is attributed to the exceptionally high binding affinity of CTS/MMT, which causes rapid solute depletion and masks thermal variations under these conditions.

3.2.3. Effect of Contact Time

The adsorption efficiency of LVX onto CTS/MMT and CO3·MgAl-LDH/MMT composites at different contact times is shown in Figure . For CTS/MMT (Figure a), the adsorption efficiency reached 99.1% after 60 min using 0.01 g of adsorbent, whereas complete removal was achieved within 10 min using 0.05 and 0.1 g of adsorbent. In contrast, for CO3·MgAl-LDH/MMT (Figure b), equilibrium was reached within 20 min using 0.05 and 0.1 g of adsorbent. The progressive adsorption observed during the initial stage may be attributed to the high LVX concentration gradient and the availability of adsorption sites. As adsorption proceeded, these active sites were gradually occupied, resulting in saturation of the adsorbent surface.

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Effect of contact time on LVX adsorption onto (a) CTS/MMT and (b) CO3·MgAl-LDH/MMT in 20 mL of 30 mg L–1 LVX solution at pH 3 and 25 °C.

3.2.4. Effects of Initial Concentration and Adsorption Isotherms

To evaluate the effect of initial LVX concentration, removal efficiencies were examined by varying the LVX concentration from 30 to 300 mg L–1 (Figure S1). For CTS/MMT, the LVX removal efficiency decreased with increasing initial concentration when 0.05 g of adsorbent was used, whereas 100.0% removal was achieved at all initial concentrations using 0.1 g of adsorbent. This result indicates that 0.1 g of CTS/MMT provided sufficient active sites for LVX adsorption from 30 to 300 mg L–1 LVX solutions, whereas the decreasing trend observed at 0.05 g may be attributed to LVX overloading relative to the available adsorption sites. The CO3·MgAl-LDH/MMT composite also showed a decreasing trend in LVX removal efficiency with increasing initial concentration.

Figure presents the equilibrium adsorption isotherms of LVX onto CTS/MMT and CO3·MgAl-LDH/MMT composites. For CTS/MMT (0.05 g), the equilibrium adsorption capacity, Q e, increased from 12 to 104.12 mg g–1 as the initial LVX concentration increased from 30 to 300 mg L–1. For CO3·MgAl-LDH/MMT, Q e increased from 10.91 to 56.02 mg g–1 using 0.05 g of adsorbent and from 5.25 to 34.83 mg g–1 using 0.1 g of adsorbent as the LVX concentration increased from 30 to 300 mg L–1. The proportional increase in Q e with equilibrium concentration, C e, suggests enhanced interaction between LVX molecules and the available active sites on the adsorbent surface. This finding also indicates a stronger mass-transfer driving force at higher concentrations, which may improve adsorption performance. Similar trends have been reported in previous studies on LVX adsorption, in which higher adsorption capacities were attributed to enhanced interactions between the adsorbate and the adsorbent surface.

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Adsorption isotherms for LVX adsorption onto (a) CTS/MMT (0.05 g) and (b) CO3·MgAl-LDH/MMT (0.05 and 0.1 g) in 20 mL of LVX solution at pH 3 and 25 °C for 60 min.

For isotherm analysis, the LVX adsorption data for CTS/MMT and CO3·MgAl-LDH/MMT were fitted using the Langmuir and Freundlich equations, as shown in Figures S2–S4.

The linearized Langmuir equation is expressed as follows

CeQe=1QmKL+CeQm 1

where Q m is the maximum adsorption capacity (mg g–1) and K L is the Langmuir constant. The Langmuir model assumes monolayer adsorption on a homogeneous adsorbent surface.

The linear form of the Freundlich equation is expressed as follows

log⁡Qe=log⁡KF+1n⁡log⁡Ce 2

where K F is the Freundlich constant and 1/n represents adsorption intensity or surface heterogeneity. When 0 < 1/n < 1, the adsorption process is favorable; when 1/n > 1, the adsorption process is unfavorable; and when 1/n = 1, adsorption is linear. The Freundlich model describes adsorbent surface heterogeneity and the exponential distribution of active sites and active-site energies.

For CTS/MMT (0.05 g), the Langmuir model effectively described the adsorption data, with R 2 and Q m values of 0.99 and 107.53 mg g–1, respectively. For CO3·MgAl-LDH/MMT, the Freundlich model effectively described the adsorption data at both 0.05 and 0.1 g adsorbent doses, with R 2 values of 0.95 and 0.99 and 1/n values of 0.38 and 0.53, respectively.

3.3. Comparison of Adsorption Performance with Other Adsorbents

In this study, compared to LDH/MMT, CTS/MMT demonstrated higher adsorption capacity with a maximum adsorption capacity (Q m) value of 107.53 mg g–1. This value significantly exceeds those of pure chitosan (9.9 mg g–1), Cu–Fe LDH (45.47 mg g–1), and Zn–Al LDH 11.87 mg g–1. Besides, some other studies reported Q m value of 51.5 mg g–1 for graphene oxide/chitosan, 48.61 mg g–1 for iron-pillared montmorillonite, 390.0 mg g–1 for Cu-Cyanoguanidine–Zn–Co-Fe LDH, and 7.43 mg g–1 for chitosan/walnut shells composite.

3.4. Adsorption Mechanism

The solution pH plays a vital role in the adsorption phenomena by determining both the ionization state of the LVX molecules and the surface charge of the adsorbent. LVX exists predominantly in a cationic form at pH < 5.59, as a zwitterion at 5.59 < pH < 7.94, and exists in an anionic form at pH > 7.94. Besides, zeta potential measurements demonstrated that the synthesized CTS/MMT composite carries a negative surface charge from pH 2.0 to pH 12.0. At pH < 5.59, the electrostatic attraction between cationic LVX molecules and the negatively charged CTS/MMT surface is the primary driving force for adsorption. At the same time, cation exchange occurs between LVX+ and CTS polymer chains within the MMT interlayer. This adsorption phenomenon reconciles the observations of Wu et al., who identified cation exchange as the primary force for adsorption onto montmorillonite clays, and Li et al., who emphasized surface electrostatic interactions for the adsorption process onto MMT. Also, fitting to the Langmuir isotherm model confirms homogeneous monolayer adsorption onto a finite number of adsorption sites (Figure ). As the pH value shifted to the zwitterionic range, adsorption proceeds through localized electrostatic interactions and cation exchange involving the protonated amine group (−NH+) despite the net neutrality of LVX molecules, coupled with secondary nonelectrostatic forces. These phenomena align with the study by Wu et al., who demonstrated that despite the net neutrality of fluoroquinolone zwitterions, the −NH+ group participates in cation exchange. At alkaline condition (pH > 7.94), both LVX and CTS/MMT surfaces have negative charges, introducing electrostatic repulsion between them. Consequently, adsorption in this range is predominantly governed by nonelectrostatic interactions (e.g., hydrogen bonding, surface complexation, etc.). These phenomena were further supported by XRD data. Under varying contact times (Figure a) and temperatures (Figure S5a), the d001 value decreased from 1.51 nm to 1.41–1.47 nm, confirming the exchange of expanded polymer chitosan chains by LVX molecules into the MMT interlayer. When the initial LVX concentration was varied (Figure S7a), the interlayer spacing decreased further from 1.51 to 1.35 nm as the initial LVX concentration increased from 30 to 300 mg L–1, which indicates that interlayer cation exchange becomes even more pronounced at higher concentrations. Similarly, in the pH variation (Figure S6a) data, the d 001 value at acidic pH decreases, while at alkaline pH, it increases slightly to 1.57 nm at pH 11 because of deprotonation of the amino group in chitosan, lowering the electrostatic cross-linking within the MMT gallery, resulting in the layers swelling slightly.

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Possible adsorption phenomena of LVX onto the CTS/MMT composite.

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XRD patterns of (a) CTS/MMT and (b) CO3·MgAl-LDH/MMT after LVX adsorption in 20 mL of 30 mg L–1 LVX solution at pH 3 and 25 °C.

The LDH/MMT exhibits a positive surface charge at pH < IEP = 5.1, and LVX exists as a cation at pH < 5.59. Extending the concepts of CTS/MMT systems, despite electrostatic repulsion between the positively charged LDH/MMT surface and cationic LVX, significant adsorption occurs within this pH range through intercalation of cationic LVX into the MMT interlayer (Figure ). The adsorption equilibrium data at pH 3 fitted well to the Freundlich isotherm model, demonstrating the formation of a heterogeneous multilayer. As the pH increases and exceeds 5.1, the composite surface develops a net negative charge. Besides, at 7.94 > pH > 5.59, LVX exists in its zwitterionic form; then, the adsorption is governed by the localized electrostatic interactions between the protonated amine groups of LVX and the negatively charged adsorbent surface, coupled with cation exchange in the MMT interlayer. At pH > 7.94, both the composite surface and LVX molecules carry negative charge and repel each other. Consequently, nonelectrostatic mechanisms, including hydrogen bonding and surface complexation, play the primary role in the adsorption process. These phenomena may be further confirmed from XRD data. As shown in Figures b and S5b–S7b, the MMT-associated d 001 peak varied slightly from 1.39 to 1.53 nm relative to its original value of 1.46 nm, confirming the intercalation of LVX cations into the MMT interlayer. In contrast, the LDH-associated d 003 peak remained nearly unchanged at approximately 0.77 nm under all experimental conditions, demonstrating that the LDH interlayer interaction was totally absent.

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Possible adsorption phenomena of LVX onto the CO3·MgAl-LDH/MMT composite.

3.5. Environmental Relevance and Study Limitations

The antibiotic (LVX) concentrations used in this study (mg L–1) are higher than those typically detected in natural aquatic receiving environments (ng L–1 to μg L–1). These high concentrations were selected to saturate the adsorbent surface completely, allowing appropriate determination of the maximum theoretical adsorption capacity (Q m) and mechanistic pathways. However, these higher concentration levels simulate concentrated industrial wastewater point sources, e.g., raw pharmaceutical effluents, and hospital wastewater streams. While the HPLC detection limit was suitable for the elevated concentration used in this study, determination of the material’s practical world compliance with rigorous regulatory threshold limits will require future investigations with continuous-flow column testing coupled with trace analytical methods like LC/MS.

4. Conclusion

In this study, MMT was functionalized with CTS and LDH for LVX adsorption from aqueous solutions. The CTS/MMT composite exhibited excellent performance, with 0.1 g of adsorbent sufficient to completely remove LVX from a 30 mg L–1 aqueous solution across the tested pH range (pH 3–11). For CO3·MgAl-LDH/MMT, the maximum removal efficiencies were observed at pH 3, with values of approximately 78.0%, 90.0%, and 87.0% using 0.01, 0.05, and 0.1 g of adsorbent, respectively. Adsorption equilibrium was achieved within 20 min for CO3·MgAl-LDH/MMT, with removal efficiencies of approximately 90.0% and 87.0% using 0.05 and 0.1 g, respectively, whereas 0.05 and 0.1 g of CTS/MMT achieved complete LVX removal within 10 min. Temperature had no discernible effect on the overall adsorption efficiencies of LDH/MMT. The maximum LVX adsorption capacity of CTS/MMT, calculated from the Langmuir model, was 107.53 mg g–1. In contrast, the Freundlich model better described the adsorption data for CO3·MgAl-LDH/MMT. For pH < 5.59, surface adsorption through electrostatic attraction coupled with partial cation exchange into the MMT interlayer was the principal mechanism for LVX adsorption onto CTS/MMT, while cation exchange into the MMT interlayer was the vital phenomenon in the case of LDH/MMT. The high adsorption capacity of CTS/MMT (107.53 mg g–1) and its effective performance across a wide pH range (pH 3–11 at 0.1 g dosage) support its potential as a low-cost adsorbent for LVX-contaminated water treatment. However, from this study, it could be suggested that further studies regarding testing real wastewater matrices, investigating adsorbent regeneration and multicycle reusability, and exploring synthesis scalability should be carried out.

Supplementary Material

ao6c08278_si_001.pdf (490.8KB, pdf)

Acknowledgments

The authors profoundly appreciate the Technical Division, School of Engineering, Tohoku University, for providing technical support. Specifically, their expertise in analysis and the utilization of HPLC analysis instrument significantly contributed to this research.

The data that support the findings of this study are available upon request from the corresponding author.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c08278.

  • Effect of initial concentration on LVX adsorption onto CTS/MMT and CO3·MgAlLDH/MMT (Figure S1); Langmuir and Freundlich adsorption isotherm models for LVX adsorption onto CTS/MMT (0.05 g) (Figure S2); Langmuir and Freundlich adsorption isotherm models for LVX adsorption onto CO3·MgAlLDH/MMT (0.05 g) (Figure S3); Langmuir and Freundlich adsorption isotherm models for LVX adsorption onto CO3·MgAlLDH/MMT (0.1 g) (Figure S4); XRD patterns of CTS/MMT and CO3·MgAlLDH/MMT after LVX adsorption across different temperatures (Figure S5); XRD patterns of CTS/MMT and CO3·MgAlLDH/MMT after LVX adsorption across different pH levels (Figure S6); and XRD patterns of CTS/MMT and CO3·MgAlLDH/MMT after LVX adsorption across different initial concentration levels (Figure S7) (PDF)

P.K.: Investigation, data curation, formal analysis, writingoriginal draft; M.Y.: Visualization, writingreview and editing; T.K.: Conceptualization, supervision, writingreview and editing

The authors declare no competing financial interest.

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Associated Data

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Supplementary Materials

ao6c08278_si_001.pdf (490.8KB, pdf)

Data Availability Statement

The data that support the findings of this study are available upon request from the corresponding author.

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