Effective removal of cefixime using VCu-layered double hydroxide encapsulated with chitosan-carboxymethyl cellulose nanocomposite: Adsorption models and optimization by box-behnken design

2.1. Resources and instruments

Tables S1 and S2 delineate the various substances and devices employed in the characterization process, offering a comprehensive overview of the materials involved.

2.2. Synthesis of the adsorbent

2.2.2. VCu-LDH/CS-CMC hydrogel bead synthesis

The VCu-LDH was successfully encapsulated within a CS-CMC matrix that was crosslinked using epichlorohydrin (ECH) through a multi-step aqueous-phase approach. The process began with dissolving 1.0 g of CS in 100 mL of 1% (v/v) acetic acid while continuously stirring for 4 to 6 h, resulting in a homogeneous viscous solution. Concurrently, 1.0 g of CMC was dissolved in 100 mL of distilled water until fully solubilized, facilitated by stirring. A specific quantity (for example, 0.5 g) of VCu-LDH powder was then introduced into the CMC solution, ensuring uniform distribution through mild sonication and stirring. This resulting VCu-LDH/CMC dispersion was gradually combined with the CS solution while stirring vigorously to achieve a consistent polymer–inorganic hybrid mixture. To initiate the crosslinking process, ECH was added dropwise to the mixture (commonly between 0.5 and 1.0 mL of ECH per g of total polymer), while the pH was adjusted to approximately 9-10 using 0.1 M NaOH, which facilitated the crosslinking reaction. The mixture was maintained at a temperature of 50-60°C for 3 to 4 h, which promoted the creation of covalent bonds between ECH and the hydroxyl groups of CMC as well as the amine groups of CS. This resulted in a stable crosslinked network that encapsulated the VCu-LDH. The hydrogel composite obtained was collected via centrifugation, thoroughly washed with distilled water to eliminate any residual reagents or unreacted ECH, and then dried either under vacuum at temperatures ranging from 40 to 50°C or through lyophilization. This process yielded a structurally stable and reusable VCu-LDH@CS–CMC composite, which is well-suited for various adsorption applications. (Figure 1) [13].

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Figure 1.

Synthesis of VCu-LDH/CS-CMC hydrogel beads.

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2.3. Removal of the VCu-LDH/CS-CMC hydrogel beads and batch analysis

An analysis of critical variables influencing optimal conditions has been conducted. This study encompasses pH levels that vary between 2 and 8, the amount of adsorbent utilized, which ranges from 0.8 to 10 g/L, initial concentration levels that encompass a spectrum from 53 to 620 mg/L, contact time lasting from 5 to 100 min, and temperatures that fluctuate between 20 and 45°C. Numerous experiments were conducted to ascertain the ideal dosage, and in similar and earlier studies, the appropriate dosage fell within the specified range. Experiments involved the use of 25 mL of CFX solutions, with the subsequent addition of adsorbents to the solutions and centrifugation at 10000 revolutions per minute for a duration of 10 min [14,15].

The measurement of the adsorption capacity of CFX, expressed as q_e (mg/g), alongside the removal efficiency indicated as %R, was conducted, using Eqs. (1) and (2) independently.

At different initial CFX concentrations and adsorption temperatures, the study examined the thermodynamics and adsorption isotherms. Centrifugation was used to separate the supernatant sample after the adsorption process, and a UV-Vis spectrophotometer was utilized for measurement the amount of CFX that remained in the supernatant. For each experimental set, the adsorption capability was calculated using the mean values from the three replications. The trials were conducted multiple times (up to six) by exposing the adsorbent to a fresh CFX solution in perfect conditions to further explore its reusability. The chemical was separated from the solution by centrifugation at 10,000 rpm for 5 min after each reaction cycle. Before being employed in the ensuing absorption recycling process, it was cleaned with ethanol and dried in an oven set at 65°C [16].

2.4. Experimental design

Equations that produce several solutions frequently facilitate the development of models or statistical assessments via a technique known as RSM. The core principle underlying RSM is to define an effective association between the response variable and the factors under examination. Moreover, it facilitates the enhancement of methodologies. RSM leverages a series of specialized tests, tailored to the characteristics of the procedure, to classify the most effective solution for the overall process. The prevalent approach for refining process parameters is referred to as CCD. The experiments were conducted utilizing Design Expert Software (Version 6.0, Minneapolis, Stat-Ease, USA), adhering to recognized principles of statistical design. The selection of process variables, specifically pH level, the amount of adsorbent applied per liter, reaction time in minutes, and the specific type of adsorbent utilized, was driven by their detrimental effects on the adsorption capacity [17].

The Design Expert Software’s outputs for the parameter combinations are listed in detail in Table S3. This table offers an extensive analysis of the highest and lowest values associated with each limit, incorporating a substantial amount of center runs, “2×m” axial runs, and 2^m factorial runs. Eq. (3) can be employed to assess the number of investigational trials required, taking into account the number of factors that contribute to the analysis.

In this analysis, “m” denotes the number of procedural variables that affect the outcome, and “Np” indicates the critical minimum number of experimental iterations required. In the current study, “m” is given a value of 3. The CCD encompasses three principal phases: establishing the numbers for the model, executing the experimental outline, and forecasting the model’s presentation while evaluating the results. Through these processes, an empirical model has been established that quantifies the function’s performance based on the input parameters and their interactions. Consequently, a quadratic regression model is generated, as demonstrated in Eq. (4):

In this setting, the variables “I” and “j” signify the coefficients associated with quadratic and linear terms, respectively. The coefficients labeled as β₀, β_i, β_ii, and β_ij are associated with specific terms in a model: β₀ signifies the constant term, β_i indicates the linear constant, β_ii corresponds to the quadratic constant, and β_ij is identified as the interaction coefficient, listed sequentially according to their respective roles. The efficacy of the planned polynomial model calculation was evaluated over the application of R², adjusted R², and predicted R² metrics. An elevated R² value specifies that the equation displays a greater degree of accuracy in fitting the experimental data [18].

3.1. Characterization of VCu-LDH/CS-CMC

3.1.1. X-ray diffraction (XRD)

Figure 2(a) illustrates the XRD patterns associated with the VCu-LDH/CS-CMC hydrogel beads. The distinct basal reflection patterns observed in the samples indicate a crystalline structure, highlighting the material’s ordered arrangement at the molecular level [19]. Utilizing the Foolproof and Check Cell tools, an analysis of the crystal structure for the adsorbent combination of vanadium (V) and copper (Cu) revealed a tetragonal geometry, categorizing it within the P4 space group. The dimensions of the crystal were derived from the established constraints: a = 3.013 Å, b = 3.013 Å, c = 4.257 Å, with angles α = 90°, β = 90°, and γ = 90°. Table S4 presents the interplanar spacing (d _hkl) along with the conforming Miller indices (hkl) for the VCu-LDH/CS structure. The VCu-LDH/CS-CMC nanospheres, as illustrated in Figure 2(a), demonstrate the remarkable resilience of the crystalline structure. Notably, these nanospheres retained their diffraction peaks even after the adsorption process (Table S4).

Table S4

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Figure 2.

(a) XRD pattern of VCu-LDH/CS-CMC, (b) N₂-adsorption/desorption of VCu-LDH/CS-CMC and CFX@VCu-LDH/CS-CMC, (c) FT-IR of VCu-LDH/CS-CMC, and (d) EDX of VCu-LDH/CS-CMC.

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3.1.5. SEM analysis

The SEM-EDS elemental mapping of the VCu-LDH/CS-CMC composite demonstrates a uniform distribution of both organic and inorganic constituents, thereby validating the successful production of the hybrid material [23]. The primary SEM image reveals a textured, porous surface morphology characterized by aggregated particles that measure between approximately 228 and 598 nm, suggesting a high surface area advantageous for adsorption purposes. Elemental mapping in proximity to the SEM image indicates the presence of carbon (15%) and nitrogen (8%) derived from the CS and CMC biopolymers, which constitute the organic matrix and supply numerous functional groups such as –OH, –COOH, and –NH₂ that facilitate binding and interactions with pollutants. The oxygen content (12%) is uniformly spread throughout, signifying the existence of oxygen-rich functional groups from both the LDH structure and the biopolymers. Importantly, vanadium (38%) and copper (27%) are present in notable amounts and show uniform dispersion, indicating their prevalent incorporation into the LDH framework. Vanadium enhances redox activity and adsorption capacity, while copper contributes to the composite’s structural stability and surface reactivity. Overall, the mapping illustrates a well-integrated and uniformly structured composite, demonstrating significant potential for use in environmental applications, particularly in the removal of heavy metals or pharmaceutical pollutants (Figure 3).

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Figure 3.

SEM image of VCu-LDH/CS-CMC.

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3.1.6. XPS

The high-resolution XPS C1s spectra of the VCu-LDH/CS-CMC composite, both before and after CFX adsorption, demonstrate notable alterations in the surface chemical environment, thereby affirming the interaction between the composite and the drug molecules. Before CFX adsorption, the C1s spectrum of VCu-LDH/CS-CMC presents three primary peaks: C–C/C–H at 585.28 eV (27.96%), which indicates the aliphatic backbone of CS and CMC; C–O/C–N at 586.85 eV (23.14%), corresponding to hydroxyl and amine groups; and O–C=O/C=O at 589.94 eV (18.9%), which signifies the presence of carboxyl and carbonyl functionalities. Following CFX adsorption, the C1s spectrum reveals distinct shifts and variations in intensity: the C–C/C–H peak decreases to 584.58 eV (15.16%), suggesting a partial modification or shielding of hydrophobic regions, while the C–O/C–N peak significantly increases to 585.51 eV (43.16%) and the O–C=O/C=O peak rises to 586.72 eV (41.68%) [24]. These alterations reflect strong interactions between the functional groups of CFX and the surface of the composite. Such changes indicate the formation of hydrogen bonds, electrostatic attractions, and potential coordination between CFX and surface –OH, –NH₂, and –COOH groups, demonstrating that the adsorption process is influenced by chemisorption through multiple pathways of interaction (Figure 4).

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Figure 4.

XPS of VCu-LDH/CS-CMC and CFX@VCu-LDH/CS-CMC.

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High-resolution XPS N1s analysis of the VCu-LDH/CS-CMC composite, conducted both before and following CFX adsorption, illustrates marked alterations in the nitrogen chemical environment, indicating a robust interaction between the composite and the antibiotic. Initially, the N1s spectrum of the unmodified composite reveals two distinct peaks: one at 400.22 eV (32.21%) associated with –NH₂ and =NH groups derived from CS, and another at 401.51 eV (32.21%) linked to –CN functionalities, which may originate from protonated amines or interactions with the LDH framework. Post-adsorption, the spectrum demonstrates a significant increase in nitrogen signal intensity, with peak positions shifting to 400.61 eV (49.27%) and 401.12 eV (50.73%). This shift suggests the incorporation of nitrogen-containing groups from CFX, including amines and heterocyclic nitrogen. The increase in peak area, along with slight adjustments in binding energy, signifies potent surface interactions that likely include hydrogen bonding, electrostatic interactions, and potential coordination between CFX and the amine-rich composite. Such findings underscore the critical function of nitrogen-containing groups in the adsorption mechanism and highlight the substantial chemical modifications that the composite surface experiences following CFX adsorption [25].

The high-resolution XPS O1s spectra of VCu-LDH/CS-CMC, examined before and after the adsorption of CFX, demonstrate notable alterations in surface oxygen functionalities, which suggest substantial interactions between the composite material and the antibiotic. Initially, the O1s spectrum presents three distinct deconvoluted peaks: 530.26 eV (4.3%), linked to carboxylate groups (–COO⁻) originating from CMC, 532.03 eV (89.37%), associated with hydroxyl groups (–OH) and lattice oxygen (O²⁻) within the LDH structure, and 533.79 eV (6.33%), indicative of adsorbed water or CO₂. These oxygen-dense groups enhance the composite’s hydrophilic and chemically active properties, promoting interactions with polar molecules. Following the adsorption of CFX, two principal peaks are noted at 530.01 eV (25.65%), which indicate an increased presence of carboxyl and carbonyl groups from CFX, and at 531.57 eV (74.35%), pertaining to hydroxyl and ether-type oxygen functionalities that participate in hydrogen bonding. The observed shifts in peak positions and relative intensities affirm that CFX engages in strong interactions with the composite, primarily through hydrogen bonding and electrostatic forces, and potentially via coordination with oxygen-containing functional groups. This underscores a chemisorption-based mechanism of adsorption and demonstrates the composite’s efficacy in the removal of pharmaceutical pollutants [26].

The high-resolution XPS V2p spectra of VCu-LDH/CS-CMC, both before and after the adsorption of CFX, provide valuable insights into the chemical states of vanadium and its role in the adsorption process. In the unmodified composite, the V2p spectrum displays two distinct spin-orbit peaks: V2p₃/₂ and V2p₁/₂, which are further deconvoluted into multiple components [27]. This indicates that vanadium exists in a mixture of oxidation states, specifically V⁴⁺ and V⁵⁺. These oxidation states are indicative of vanadium being present in LDH structures, which are crucial for redox activity and electron transfer processes. Following the adsorption of CFX, both V2p₃/₂ and V2p₁/₂ peaks show minor shifts in binding energy along with alterations in peak symmetry and intensity. These changes suggest that CFX interacts with vanadium via coordination or electron-donating methods. The observed broadening and reallocation of the peaks point to a modification in the electronic environment surrounding the vanadium atoms because of the adsorption, likely involving charge transfer between CFX’s functional groups and the vanadium centers. These spectral alterations highlight the active participation of vanadium in the chemisorption of CFX, which enhances the composite’s ability to effectively remove pharmaceuticals.

The high-resolution XPS Cu2p spectra of VCu-LDH/CS-CMC, both prior to and following the adsorption of CFX, offer significant information regarding the oxidation states and surface interactions of copper within the composite material. In the original VCu-LDH/CS-CMC, the Cu2p spectrum displays two separate spin–orbit doublets, Cu2p_3/2 and Cu2p_1/2, along with notable shake-up satellite peaks. This indicates the presence of both Cu²⁺ and Cu⁺ species. The pronounced satellite features linked to the Cu2p_3/2 region affirm the prevalence of Cu²⁺, which is typically found in LDH structures and plays a critical role in surface coordination reactivity. Following the adsorption of CFX, both Cu2p_3/2 and Cu2p_1/2 peaks remain visible, but they exhibit significant shifts in binding energy and a decrease in peak intensity. This indicates that the electronic environment of copper has been modified through its interaction with CFX molecules [28]. These alterations suggest that CFX establishes coordination bonds or electrostatic interactions with the surface copper ions, potentially involving its carboxyl or amino functional groups. The preservation of Cu⁺/Cu²⁺ redox states after adsorption confirms that copper retains its chemical activity and is directly involved in the adsorption mechanism. This further supports a chemisorption-driven process that enhances the composite’s capability to adsorb pharmaceutical contaminants (Figure 4).

The XPS survey spectra of VCu-LDH/CS-CMC, both prior to and following the adsorption of CFX (CFX), provide an in-depth analysis of the elemental composition and validate the effective interaction between the composite and the drug. The spectrum of the unmodified composite reveals significant peaks corresponding to C 1s (∼285 eV), N 1s (∼400 eV), O 1s (∼531 eV), V 2p (∼517–525 eV), and Cu 2p (∼933–955 eV). These peaks are associated with the organic components (CS and CMC) and the metallic elements present in the LDH (VCu-LDH). Following the CFX adsorption, these characteristic peaks remain evident, indicating that the composite maintains its structural integrity. Importantly, there is a notable increase in the intensity of the N 1s signal, reflecting the successful incorporation of nitrogen-rich CFX molecules [28]. Additionally, minor reductions and shifts in the Cu 2p and O 1s peaks indicate alterations in the surface chemical environment, which may result from coordination or hydrogen bonding between CFX and the metal or oxygen-containing functional groups on the composite surface. In summary, the survey spectra support the conclusion that CFX adsorption is associated with surface modifications consistent with chemisorption, thereby confirming the composite’s strong affinity and potential for interacting with pharmaceutical contaminants Figure 4.

3.1.7. The zero point of charge (ZPC)

In the examination of sorption CFX, it was observed that pH shows a critical role, influencing both the types of ions within the adsorbent solutions and the surface loading characteristics of the adsorbent. The assessment of the surface charge of the VCu-LDH/CS-CMC was conducted by measuring the ZPC, which is clear as the specific pH (pH_ZPC) at which there is an equilibrium between surface charges that are positive and negative. The determined pH_ZPC for the VCu-LDH/CS-CMC was found to be 6.08, as illustrated in Figure 5(a). The hydrogel composed of VCu-LDH/CS-CMC demonstrates a negative charge when the pH exceeds the pH_ZPC and exhibits a positive charge when the pH is less than this threshold, a phenomenon attributed to the protonation of its functional groups. Consequently, it is more likely that CFX will adsorb at pH stages larger than the pH_ZPC, resulting in a negatively charged surface [29].

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Figure 5.

(a) Determination of pH_zpc; (b) The pH’s impact on the capacity for adsorption (adsorbent dose: 0.8 g/L, pH: 2 to 8, C_i: 500 mg/L, contact time 100 min., Temp.: 25^oC); (c) The result of adsorbent dose (adsorbent dose: 0.8 to 10 g/L, pH: 2 to 8, C_i: 500 mg/L, contact time 100 min., Temp.: 25^oC); (d) The result of starting concentration upon the capacity for adsorption (adsorbent dose: 0.8 g/L, pH: 4, C_i: 50 to 620 mg/L, contact time 100 min., Temp.: 25^oC); (e) The result of connection duration on the capacity for adsorption (adsorbent dose: 0.8 g/L, pH: 4, C_i: 50 to 500 mg/L, contact time 5 to 100 min., Temp.: 25^oC) (f) Temperature’s result on the adsorption capability, and (g) The result of temperature on the adsorption capacity at different contact time intervals (adsorbent dose: 0.8 g/L, pH: 4, C_i: 50 to 500 mg/L, contact time 100 min., Temp.: 25 to 45^oC).

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3.2. Batch experimentations

3.3. Adsorption isotherm

The Langmuir isotherm model aids in calculating the maximum adsorption capacity and surface similarity [32], while the Freundlich model facilitates the determination of adsorption intensity and capacity, especially for heterogeneous surfaces [33]. The model of Dubinin-Radushkevich makes adsorption energy more understandable by distinguishing between chemical and physical adsorption. In the meantime, the model of Temkin provides information on the heat of adsorption while considering the influence of indirect interactions between adsorbates [34]. The Jossens model is cooperative in figuring out the energy distribution on heterogeneous surfaces and can accommodate multi-layer adsorption [34]. The models provide a thorough sympathetic of the adsorption mechanism in the study of CFX adsorption onto VCu-LDH/CS-CMC, which helps to enhance the adsorbent’s design for optimal efficacy (Table S5).

The Langmuir adsorption isotherm model provides important analytical perspectives regarding the adsorption behavior of CFX on VCu-LDH/CS-CMC. It delivers an accurate and measurable framework for sympathetic the dynamics of the adsorption procedure. The Langmuir model is particularly helpful in explaining a homogenous adsorption surface, where every site binds a single adsorbate molecule [32]. This makes understanding the adsorption behavior easier. The model indicates that VCu-LDH/CS-CMC has a significant capacity for CFX absorption, demonstrating an exceptional adsorption capability (q_m) of 630.32 mg/g, highlighting their highly effective adsorbent properties. Additionally, the hydrogel beads demonstrate a significant relationship with CFX, as evidenced by the Langmuir constant (K_L) of 0.053 L/mg, a crucial factor for effective adsorption. Furthermore, the model exhibits a value of 0.61 for the equilibrium parameter (RL), often referred to as the dimensionless separation factor. This value is situated within the favorable range of 0 to 1, implying that the adsorption procedure is advantageous. It also indicates that the efficiency of the adsorption process is potentially high under the experimental conditions evaluated (see Table S6) [5].

The utilization of the model of the Freundlich adsorption isotherm to represent the adsorption of CFX onto VCu-LDH/CS-CMC provides several advantages. This is attributed to the model’s ability to effectively characterize adsorption processes on heterogeneous surfaces and its relevance in non-ideal adsorption situations, particularly those that involve the formation of multilayer structures [33]. With a determined value of 0.37, the exponent of the model, 1/n, indicates a robust and advantageous adsorption process for CFX, meaning that the adsorption effectiveness stays high even when the pollutant concentration rises. The observation is further substantiated by the Freundlich constant (K_F) of 116.2 (mg/g) (L/mg)^1/n, indicating that the hydrogel beads exhibit a considerable adsorption capacity. This suggests their viability as effective adsorbents for CFX [33]. The comprehensive use of the Freundlich isotherm suggests a certain level of adaptability and effectiveness in removing CFX from water solutions, which is crucial for practical water treatment purposes. It also makes it possible to better consider the hydrogel beads’ adsorption properties under different circumstances (Figure 6a).

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Figure 6.

(a) Models of the adsorption isotherms, (b) Models of the adsorption kinetics, (c) IPD model, and (d) Mechanism of diffusion of CFX onto VCu-LDH/CS-CMC hydrogel beads.

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The utilization of the model of Dubinin-Radushkevich (D-R) isotherm in evaluating the adsorption of CFX on VCu-LDH/CS-CMC provides a significant analytical perspective on the underlying adsorption mechanism while also emphasizing the operational benefits associated with the use of this particular adsorbent. It is especially useful because the D-R model may distinguish between physisorption and chemisorption depending on the energy expended during the adsorption process. A strong capability for CFX adsorption is shown by the predicted adsorption capability (Q_DR) of 538.4 mg/g, which is suggestive of a beneficial porous structure within the hydrogel beads that enables a high degree of pollutant uptake. The constant value of K_DR, 1.88E-5 (mol²kJ⁻²), suggests that weaker interactions associated with a chemisorption process may be the primary driver of adsorption. The lower energy bonds that arise from these kinds of interactions may make it easier for the adsorbent to regenerate. With a mean free energy of adsorption (E_a) of 28.72 kJ/mol, over the 8 kJ/mol threshold, chemisorption is the most common route for CFX adsorption onto the hydrogel beads. Therefore, a thorough assessment that confirms the effectiveness, potential cost-effectiveness, and sustainability of employing these hydrogel beads to extract CFX from aqueous solutions can be obtained by utilizing the model of D-R isotherm.

Because the model of Temkin adsorption isotherm considers the communication between the adsorbate and adsorbent, it is useful for demonstrating the adsorption of CFX onto VCu-LDH/CS-CMC. In order to portray a more realistic adsorption scenario that takes into consideration the variety in binding energies as the surface site saturation increases, this model admits that the heat of adsorption declines linearly rather than being constant [34]. It is possible to assess the adsorption efficiency of the beads and their potential for regeneration and reuse by using the Temkin constant, b_T, which stands for moderate adsorbate-adsorbent interaction strength at 17.26 kJ/mol. This designation proposes that the adsorption procedure is most likely a dual mechanism involving both chemisorption and physisorption [34]. Furthermore, the high equilibrium-binding constant (_AT value of 0.74 L/g) suggests that the hydrogel beads and the CFX have a positive affinity. This means that the beads would be very effective at adsorbing CFX, especially at higher concentrations until they reach their maximum capacity (Figure 6a).

Using the model of Jossens adsorption isotherm to describe the adsorption of CFX onto VCu-LDH/CS-CMC has the advantage of considering the complexity of the interaction because of the varied adsorbent surface and the possible communication between adsorbed molecules. The model shows a substantial affinity between the CFX molecules and the hydrogel beads, with a Jossens equilibrium constant (K) of 368. It suggests that the beads are quite successful in adsorbing CFX. Additionally, the J value of 0.04 indicates a relatively low level of adsorbed molecule interaction or surface heterogeneity, indicating a steady and homogeneous adsorption throughout the adsorbent and, eventually, predictable performance. The utilization of the Jossens model offers an enhanced comprehension of the adsorption mechanism, potentially leading to improved predictability, efficiency, and applicability of hydrogel beads in water treatment contexts [34].

3.4. Adsorption kinetics

The models of adsorption kinetics are a useful tool for accurately forecasting how chemicals would behave during the adsorption process [35]. For instance, these models deliver precise estimates of the adsorption of CFX onto VCu-LDH/CS-CMC [36]. They aid in the identification of critical rate constants that control the rate of adsorption, which is essential for process optimization. Additionally, by revealing whether a material adheres to a chemical or physical adsorption pathway and whether pore diffusion plays a key role, these models provide insight into the adsorption mechanisms (Table S5). Furthermore, by determining ideal parameters like contact time and beginning concentration, kinetic models help maximize the effectiveness of pollutant removal [37]. These models provide consistency and predictability in process performance when extending from the lab to the industrial setting. Thus, a better comprehension of adsorption kinetics results in more economical treatments, lower adsorbent consumption, and assurances that pollutants are effectively eliminated in accordance with environmental standards [38,39]. To put it briefly, these models serve to both facilitate better scientific communication and understanding as well as to stimulate additional study and teamwork within the field of adsorption science (Figure 6b) [40].

CFX (CFX) has undergone adsorption onto VCu-LDH/CS-CMC, which is effectively defined by a pseudo-first-order kinetic model characterized by a rate constant (K₁) of 0.024 (min⁻¹)×10⁻² [35]. This methodology presents numerous advantages pertinent to both the design of processes and the assessment of efficiency, as detailed in Table S7 [35]. This model makes the complicated adsorption process easier to understand and more predictable by demonstrating a clear correlation between the amount of empty space on the adsorbent and the rate of adsorption. In the early stages of the process, this behavior usually persists, particularly when the concentration of adsorbate is high. This reduction makes it simpler to forecast behaviors and analyses experimental data, which are crucial for configuring and optimizing operating conditions. Instances of such behaviors consist of the duration of the adsorption procedure and the duration required to attain adsorption equilibrium [41,42]. Furthermore, following the model can verify whether the VCu-LDH/CS-CMC is working efficiently in the early phases, offering prompt input for process modifications. A kinetic model of this nature offers a more profound understanding of the underlying adsorption mechanisms and provides insight into whether the process is predominantly influenced by chemical or physical interactions. In summary, the pseudo-first-order model offers a comprehensive framework that is helpful for both effectively optimizing the removal of pollutants and scaling up adsorption systems from laboratory to industrial applications, ensuring that affordability solutions meet environmental regulations.

Employing a model of pseudo-second-order kinetics to analyze the adsorption behavior of CFX onto VCu-LDH/CS-CMC results in a rate constant, K₂, of 1.958E-5 ((g.mg⁻¹min⁻¹)x10⁻²) along with an equilibrium adsorption capability (q_e) of 631.39 (mg/g). This approach offers a comprehensive understanding of the intricate mechanisms underlying the adsorption procedure, as thorough in Table S7 [36]. Earlier, the model accurately matched the experimental data across the entire range of adsorption. This implies that the primary mechanism controlling the absorption is probably chemisorption, which is the chemical binding of CFX molecules to active spots on the adsorbent beads. Plotting t/q_t versus time yields a linear relationship that makes kinetic parameters easily determined. Furthermore, the high q_e value highlights the effectiveness and capability of the VCu-LDH/CS-CMC to filter water by containing a significant amount of the antibiotic, demonstrating the practicality of its use on a broader, commercial scale. Using a kinetic model of this kind helps water treatment plants better scale up operations while maintaining environmental standards with carefully considered, extremely powerful adsorbent materials [43].

The study of CFX adsorption on VCu-LDH/CS-CMC entailed the utilization of multiple adsorption kinetic frameworks, with a particular emphasis on the Intraparticle diffusion model. The findings from the model indicated a diffusion rate coefficient of 66.12 mg.g⁻¹min^1/2, alongside a boundary layer effect value of 23.14 mg/g [37]. These results offer significant information that could enhance the efficiency of the adsorption mechanisms involved. The K_i value serves as an indicator of the velocity at which CFX traverses the pores within hydrogel beads. This metric is crucial in optimizing the pore architecture of the adsorbent, thereby improving the overall efficacy of pollutant extraction. A higher K_i suggests more rapid diffusion, which is advantageous for the swift adsorption of pollutants, as illustrated in Figure 6(c). However, the X value quantifies how the width of the border layer affects the rate of adsorption, which aids in modifying flow rates or agitation to lower mass transfer resistance at the bead surface for more effective adsorption [37]. Intraparticle diffusion, a kind of sophisticated kinetic modelling, is a valuable tool for optimizing adsorbent materials and purification systems designed to target particular contaminants like CFX (Figure 6d).

Understanding of the adsorption kinetics of CFX onto VCu-LDH/CS-CMC is made easier by employing the model of Elovich adsorption kinetics. The model’s β value of 0.056 (g/mg) and α value of 126.44 (mg.g⁻¹min⁻¹) provide a deep insight into the adsorption process and rate. The α value indicates a quick uptake of CFX at the beginning, indicating a high initial adsorption rate that is ideal for effective pollutant removal from water [40]. The complex chemisorption process indicated by the β parameter highlights the intricate interactions between the hydrogel matrix and the pollutant, which may involve electron exchange or sharing during the adsorption phase. The Elovich values demonstrate this chemisorptive behavior, which is crucial since it suggests a strong, potentially irreversible link between CFX and the adsorbent, resulting in efficient and long-lasting pollutant removal. Precise forecasts of the system’s efficacy over time are also made possible by the model’s ability to precisely represent the kinetics throughout the whole adsorption process, which is essential for continuous water treatment applications. As a result, the Elovich model guarantees sustained performance, which is in line with the realistic requirements of water purification systems, in addition to supporting the original design and efficiency optimization of the adsorbents [40].

3.5. Diffusion mechanism

The graphical representation illustrates the three stages of the diffusion process of CFX onto VCu-LDH/CS-CMC. The graph shows a linear relationship between the square root of time and the amount of CFX absorbed. Initially, there is a gradual, less pronounced phase indicative of intraparticle or pore diffusion, which transpires shortly after CFX navigates the resistive microenvironment of the beads’ pores. Prior to this phase, the CFX undergoes initial rapid external surface adsorption, which is shown by a sharp upward slope caused by the high concentration gradient. The end of the procedure, when the rate of adsorption remains constant and CFX molecules occupy all of the available adsorption sites, is indicated by a plateau that denotes a state of equilibrium. The entire diffusion journey of CFX within the hydrogel beads is depicted by the graph’s distinct slope changes, which show the different rates of adsorption across the surface adsorption and diffusion phases, as well as the plateau, which signifies the point of saturation where no more adsorption occurs [37].

3.6. Adsorption thermodynamics

The association between temperature and the adsorption efficiency of CFX on VCu-LDH/CS-CMC was analyzed at four specific temperatures, spanning from 293 to 318 K. Figure 7(a) illustrates how varying temperatures influence the effectiveness of CFX removal. Thermodynamic equations were utilized to compute the thermodynamic limits (∆H°, ∆S°, and ∆G°), facilitating the prediction of the adsorption process characteristics (refer to Table S8) [44]. The positive value of ∆H° at 97.4 kJ.mol⁻¹ specifies that the adsorption procedure is endothermic. This suggests that an increase in the system’s temperature facilitates the adsorption process, as illustrated in Figure 7(b). The observed change in entropy (∆S°) is quantified at 343.3 J.mol⁻¹K⁻¹, indicating an increase in disorder at the interface among the solid and liquid phases upon the adsorption of CFX onto VCu-LDH/CS-CMC. The rise in negative Gibbs free energy (∆G°) values specifies that the adsorption procedure is becoming spontaneous as temperatures increase. Empirical evidence indicates a correlation between temperature elevation and a decrease in negativity, accompanied by an increase in ∆G° values. This relationship suggests that higher temperatures enhance the spontaneity of the procedure. It is plausible that this phenomenon is associated with the endothermic characteristics of the VCu-LDH/CS-CMC reaction [45].

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Figure 7.

(a) The impact of temperature on the adsorption of CFX (Van’t Hoff model), (b) The impact of temperature on ΔG.

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3.7. Mechanism of interaction

The VCu-LDH/CS-CMC exhibits a layered architecture characterized by the presence of vanadium (V) and copper (Cu) centers that are enclosed by hydroxide (OH⁻) groups, as depicted in Figure 8. The point of CS-CMC hydrogel beads functions as a polymeric matrix interspersed within the layers of the LDH components. The rapid interaction observed between CFX and the octahedrons can be attributed to the diffusion process, which facilitates adsorption that appears to be driven by diffusion, as demonstrated by the results of the kinetic investigation. Multiple interaction mechanisms may arise when CFX molecules approach designated active sites, as depicted in Figure 8.

• The prevalent notion of electrostatic attraction primarily focuses on its ability to facilitate interactions among surfaces that exhibit differing electrical charges. This phenomenon is significantly influenced by the magnitude of the charge carried by the contaminant being analyzed, in addition to the acidic or basic nature of the surrounding solution. The VCu-LDH/CS-CMC hydrogel beads exhibit a negative charge owing to the deprotonation of surface reactive groups in an alkaline setting, as evidenced by the compound’s zero point of charge of 6.08. This indicates a potential for electrostatic connections among the negatively charged sites and the CFX molecules present on the surface of the VCu-LDH/CS-CMC.

• The interaction between binary atoms that function as hydrogen donors and acceptors leads to the formation of a dipole-dipole interaction, commonly referred to as a hydrogen bond (H-bond). This interaction is crucial for the effective immobilization of various organic compounds onto VCu-LDH/CS-CMC. In this context, the hydrogen acceptors primarily consist of nitrogen and oxygen atoms, which are derived from the CFX. Conversely, the hydrogen donor is anticipated to originate from the ‒OH groups present in the components associated with the VCu-LDH/CS-CMC hydrogel beads. Moreover, there exists a significant correlation between the proportions of nitrogen and/or oxygen detected in the CFX and the point of hydrogen bonding observed in the VCu-LDH/CS-CMC, highlighting the interdependence of these factors.

• The uptake of CFX by VCu-LDH/CS-CMC can be attributed to the phenomenon of pore filling, as evidenced by the notable reduction in surface area, pore size, and volume observed before and after the adsorption process. This proposes that the adsorbent’s pores have effectively accommodated a portion of the CFX.

• Coordination bonds between positive cations (V and Cu) and CFX atoms with single electron pairs could be one way.

• The FTIR spectra analysis of the VCu-LDH/CS-CMC composite and CFX@VCu-LDH/CS-CMC demonstrates significant shifts in their distinctive peaks, which serve as evidence for the effective adsorption of CFX onto the composite material. Specifically, the broad O–H/N–H stretching band exhibits a shift from approximately 3430 cm⁻¹ to around 3400 cm⁻¹. This variation suggests the presence of hydrogen bonding interactions between CFX and the hydroxyl or amine functionalities present in CS and CMC. The C–H stretching peak, positioned at approximately 2925 cm⁻¹, undergoes a minor displacement to around 2920 cm⁻¹. This variation implies the presence of hydrophobic interactions with the aromatic structures of CFX. Furthermore, a notable transition in the C=O stretching (amide I) from roughly 1648 to 1628 cm⁻¹, coupled with a change in the N–H bending (amide II) from about 1540 to 1525 cm⁻¹, underscores the participation of both carbonyl and amine functionalities in their interactions with CFX. Moreover, the COO⁻ symmetric stretching band experiences a shift from about 1412 to 1385 cm⁻¹, which reflects electrostatic interactions between the carboxylate groups of CFX and the positively charged layers of LDH. The observed transition of the C–O–C and C–OH stretching frequencies from approximately 1055 to around 1035 cm⁻¹ indicates a likely interaction between CFX (CFX) and the polysaccharide backbone of CMC. Furthermore, the M–O vibrational band, which is initially positioned in the range of approximately 550 to 520 cm⁻¹, exhibits a slight shift towards 540 to 510 cm⁻¹. This shift signifies minor alterations within the LDH structure due to the incorporation of the drug, while still preserving the framework’s overall integrity. Collectively, these spectral variations provide strong evidence that CFX interacts with the composite through mechanisms such as hydrogen bonding, electrostatic interactions, and potentially coordination, thereby supporting the notion of its effective encapsulation (Figure S1).

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Figure 8.

Interaction mechanism between VCu-LDH/CS-CMC and CFX.

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An analysis of the manufactured composite to other resources indicates that its peak adsorption capacity at equilibrium reached 630.32 mg/g. This positions the composite as one of the most proficient materials for the adsorption of CFX. Furthermore, it is highly economical, as it eliminates the need for expensive additives such as zeolite imidazole frameworks or activated carbon.

3.8. Influence of salinity

The influence of various salts like Na₂SO₄, MgSO₄, K₂SO₄, and CaCl₂ on the adsorption effectiveness of VCu-LDH/CS-CMC for CFX was inspected and is depicted in Figure 9. Observable alterations in the CFX uptake process onto VCu-LDH/CS-CMC due to the presence of coexisting ions such as Na⁺, Mg²⁺, K⁺, and Ca²⁺ were noted. It was observed that the removal percentages for CFX were 78.4, 70.2, 84.3, and 82.4%, respectively. Instead of adhering to the surface of VCu-LDH/CS-CMC, the carboxylic groups in CFX can help sodium and potassium ions create their respective salts, which tend to increase their solubility in aqueous solution. On the other hand, calcium and magnesium ions can attach to the practical groups on the active surface, lowering the number of sites that are accessible and, as a result, the removal percentage of CFX. It is possible to relate the variable adsorption behavior of antibiotics by VCu-LDH/CS-CMC in the presence of numerous dissolved salts to changes in ionic charges, ion radius, and the kind of functional groups involved in the adsorption procedure (Figure 9).

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Figure 9.

Salinity’s effect on CFX adsorption on VCu-LDH/CS-CMC.

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3.9. Reusability

One important consideration in assessing the viability of the suggested system is the adsorbent’s capacity for recycling. Prior to insertion into a room-temperature methanol solution for desorption, VCu-LDH/CS-CMC hydrogel beads loaded with CFX were washed briefly with double-distilled water two or three times. The specimens were cleaned again with deionized water and allowed to dry in an oven for twelve hours. This allowed for six repeated series of adsorption and desorption by reevaluating the materials gathered during the desorption phase for adsorption. According to adsorption-desorption investigations, VCu-LDH/CS-CMC samples can be recycled and reused for up to eight cycles without substantially losing their ability to remove CFX (Figure 10a). The removal efficacy was found to have almost completely decreased after the sixth cycle because there were fewer open pores and fewer active sites. Adsorbent materials’ industrial and financial sustainability also depends on their structural stability. To do this, XRD was utilized to observe the regenerated VCu-LDH/CS-CMC particles. The observation that there were no alterations in the distinguishing XRD peaks of the elements following six series of adsorption and desorption suggests that the crystalline structure of the VCu-LDH/CS-CMC particles remained intact. This is evidenced by the data presented in Figure 10(b).

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Figure 10.

(a) Adsorption efficiency of CFX onto VCu-LDH/CS-CMC, and (b) VCu-LDH/CS-CMC and regenerated XRD pattern.

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3.10. Relative to alternative adsorbents

Table S9 offers a comparative analysis of the highest adsorption capabilities of different adsorbents for CFX, arranged by increasing complexity. The data presented in Table S9 indicates that the VCu-LDH/CS-CMC utilized in this research demonstrates significant possibility as an effective adsorbent for the elimination of CFX from solutions.

3.11. Analysis of response surfaces and design of experiments modeling

3.11.3. Diagnostic plots

The fundamental diagnostic diagrams illustrated in Figure 11(a) substantiate the statistical theory suggesting that, given appropriate approximations, a model can effectively replicate the behavior of a real system. In particular, the juxtaposition of the empirical data and the predicted graphs in Figure 11(a) serves as a means to evaluate the model’s adequacy. Overestimation is indicated by points above or below the diagonal line in Figure 11(b). The correspondence between the predicted values from the regression models and the observed data from the concrete tests along the diagonal signifies a general consistency between the two sets of information. This implies that potential errors arising from specific experimental inaccuracies may remain undetected. Enhancing the models for CFX adsorption can yield benefits by deploying empirical models that have been confirmed as effective predictors via response surface methodologies. Figure 11(c) illustrates the use of standard probability plots to evaluate whether the standard deviations of the observed and expected data points adequately reflect their usual distributions. The reliability of an experimental model can be assessed by analyzing the occurrence of noise within the data points or the existence of outliers. A decrease in the number of outliers enhances the overall credibility of the model. The internal studentized residuals demonstrate a distribution that aligns perfectly with the standard model, as revealed in Figure 11(c). Furthermore, the arrangement of data points reveals a linear dispersion, lacking identifiable patterns or tendencies. Consequently, this model is consistent with experimental findings and demonstrates a high degree of accuracy. The significant variations in the number of runs illustrated in Figure 11(c) provide additional insight into the patterns of data, both regular and irregular, that may arise in potential situations [49].

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Figure 11.

(a) Scheme Prediction versus Actual, (b) Typical residue scheme, (c) Run scheme versus residual, (d) Power converts using the Box-Cox technique, (e) Scheme for rate response agitation (for A: pH, B: Dose and C: time), and (f) Graphical optimization is used to maximize adsorption capacity.

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Consequently, the illustration in Figure 11(c) exhibited an erratic variation instead of exhibiting a scattered configuration, lacking any recognizable form that could be compared to a megaphone. Nonetheless, this result was consistent with the diagnostic visual representations introduced previously. The Box-Cox methodologies depicted in Figure 11(d) serve as a tool to improve the normality of residuals. The application of power transformations, as outlined in the Box-Cox framework, is crucial for normalizing distributions that do not conform to normality. The evidence presented in a probability plot, which identified deviations among the data points, further supports this necessity. Through this analysis, the most appropriate lambda parameter for rectifying the CFX was resolved to be 3.968. The relationship between the measured and estimated CFX removal efficiency exhibits a noteworthy correlation, as illustrated in the graph in Figure 11(e), which presents both the actual and predicted CFX removal performance. In conclusion, the optimal parameters for the absorption of CFX onto VCu-LDH/CS-CMC have been identified as a pH of 4, a connection time of 100 min, and a dosage of 0.8 g. These specific details are illustrated in Figure 11(f). When these conditions are applied, the anticipated absorption capacity is calculated to be 631.1 mg/g [50].

3.11.4. Model adequacy checking

Three-dimensional response surface plots were created to thoroughly assess the impact of the variables under investigation on the elimination of CFX. This method made it possible to find important connections between the several elements at play. Figure 12(a) illustrates the relationship between the pH levels and the dosage of VCu-LDH/CS-CMC and their effect on the adsorption efficiency of CFX, considering a fixed modification duration of 100 min. The data accessible in Figure 12(a) specifies that a rise in pH from 2 to 4 correlates with an enhanced capacity for CFX adsorption. Conversely, a rise in adsorption capacity is correlated with a reduction in pH, indicating that the ideal circumstances are reached at pH 4 with an adsorbent mass of 0.8 g for a volume of 25 mL. Figure 12(b) depicts how the dosage and duration of VCu-LDH/CS-CMC hydrogel beads affect the affinity of CFX for various substances. Furthermore, it is apparent that the capacity for adsorption demonstrated an increasing tendency with extended contact duration. Particularly, the most effective adsorption occurred at a reduced adsorbent dosage of 0.8 g, accompanied by an extended contact time of 100 min. Figure 12(c) illustrates the influence of pH and various sizes of VCu-LDH/CS-CMC on the adsorption capabilities of CFX. The adsorption capability decreased as the pH increased from 2 to 4 and then declined further as the pH continued to rise, particularly at neutral levels. Additionally, the adsorption capacity increased through the length of interaction [51].

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Figure 12.

(a-c) Both 2D and 3D response surfaces.

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3.11.5. The validation of the model

In Figure 13(a), it is shown that at a pH of 4 and a contact period of 100 min, the most effective adsorbent dosage is identified as 0.02. This combination resulted in a significant adsorption capacity measured at 2.46 mmol/g. The resolution of this review was to evaluate the predicted adsorption quantity against the observed value by employing one or more defined criteria. The effectiveness of the RSM model is underscored by the experimental data, which did not show any statistically significant changes (p > 0.05). This indicates that the response model is sufficiently robust to accurately reflect the expected adsorption capability, as demonstrated by the strong correlation between the expected and observed data points. The significance of the desirability function is illustrated in Figure 13(b). The specified boundaries for each experimental variable are delineated as follows: C₀ is set between 53 and 620 mg.L⁻¹, while the temperature ranges from 20 to 45°C, and the contact time spans from 5 to 100 min. The experimental outcomes exhibit a strong correlation with the expected responses. The findings validated the appropriateness and effectiveness of the model illustrated in Figure 13(c). A pair of compliance assessments was conducted utilizing the optimized input variables. This analysis substantiated the performance and accuracy of the BBD alongside the intended characteristics for determining the optimal adsorption parameters aimed at maximizing output [52]. Following this, the isothermal, kinetic, and thermodynamic studies were executed under these optimal conditions, as depicted in Figure 13(d).

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Figure 13.

(a) Growing attention in the numerically ideal solutions, (b) Desirability of each response, (c) Distinct desirability displayed as a bar graph, (d) contour of desirability for every response interaction.

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3.12. Molecular orbital

Assessing the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CFX is important for understanding its adsorption behavior on VCu-LDH/CS-CMC. The respective energy levels of the HOMO and LUMO are -0.2115 eV and -0.105 eV, which together indicate a bandgap of 0.106 eV. This bandgap suggests that CFX has a favorable potential for reactivity, enabling it to form chemical bonds with the VCu-LDH/CS-CMC material. The CFX exhibits a balanced degree of reactivity and softness, as evidenced by its electronegativity value of 0.158 eV and a chemical hardness measurement of 0.053 eV. An ionization potential of -0.158 eV indicates that CFX possesses a propensity for easy electron donation to the VCu-LDH/CS-CMC [53]. Additionally, the data supporting this claim includes an electrophilicity index of 0.235 eV⁻¹, a chemical softness index of 9.41 eV⁻¹, and an absolute softness figure of 18.8 eV⁻¹. Collectively, these parameters indicate that CFX is not only reactive but also capable of establishing robust chemical bonds, as summarized in Table 4.

The parameter of electronic responsibility, designated as ΔN_max, plays a critical role in assessing the adsorption properties of CFX on the VCu-LDH/CS-CMC composite. A ΔN_max value of 2.97 suggests a considerable capacity for adsorption, as it implies that CFX is capable of transferring a significant quantity of electrons to the surface of the adsorbent. The identification of CFX’s HOMO and LUMO is crucial for understanding its adsorption appearances on the VCu-LDH/CS-CMC hydrogel. This analysis may facilitate the establishment of optimized adsorption parameters, thereby enhancing the overall adsorption performance. The properties of CFX, which include a narrow bandgap, considerable electrical charge, and a degree of reactivity, suggest that it has significant potential as an actual adsorbent within the context of VCu-LDH/CS-CMC systems (refer to Table 4) [54].

3.13. Active sites

Establishing the optimized configuration of a magnetic VCu-LDH/CS-CMC is essential for understanding the adsorption dynamics of CFX on its surface (refer to Figure 14a). The specifics of bond lengths, along with the angles and overall molecular framework of the optimized structure, can significantly influence both the adsorption properties and reactivity associated with CFX. The HOMO and LUMO are integral components of the frontier molecular orbital distribution, which offers a detailed empathetic of the electronic outline and reactive potential of CFX, as illustrated in Figure 14(b). The spatial arrangement of CFX’s HOMO and LUMO plays a crucial role in its electron-donating and electron-accepting abilities. This distribution significantly influences CFX’s potential to establish chemical bonds with the metal-organic framework, as illustrated in Figure 14(c). The comprehensive electron density, molecular electrostatic potential (MEP) illustrated in Figure 14(d), alpha density represented in Figure 14(e), and beta density further elucidate the electronic shape and reactivity of CFX, as evidenced by the data presented in Figure 14(f). Contour diagrams, alongside the Laplacian of the overall CFX density, provide further insights into the electronic structure and reactivity of CFX, as depicted in Figure 14(g). Contour plots serve as effective tools for illustrating the electron density distribution in a molecule. However, a deeper understanding of the regions of higher density concentration and areas of depletion can be obtained through the analysis of the CFX density Laplacian. This additional insight indicates that the efficacy of CFX in the context of adsorption onto VCu-LDH/CS-CMC can be significantly augmented, thereby optimizing the conditions for adsorption [54].

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Figure 14.

(a) Enhanced CFX structure, (b) Frontier molecule molecular orbital density distribution optimization structures, (c) Total electron density, (d) MEP, (e) Alpha density, (f) Beta density, (g) Shape for CFX.

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