Layered Double Hydroxides Modified with Carbon Quantum Dots as Promising Materials for Pharmaceutical Removal

Abstract

Pharmaceutical contaminants such as ibuprofen are increasingly detected in water sources due to widespread use and insufficient removal by conventional treatment processes. Given its persistence and adverse effects on human health and aquatic ecosystems, efficient removal technologies are needed. This study reports the synthesis of a Mg/Al-layered double hydroxide (LDH) hybridized with carbon quantum dots (CQDs) via in situ co-precipitation to enhance adsorptive performance. The hybrid (LDH-CQD) was characterized by FTIR, XRD, DSC, TGA-DTG, SEM-EDS, BET, and pH in the point of zero charge (pH_PZC) analysis. Results indicated a marked increase in surface area (2.89 to 66.9 m²/g), a shift in surface charge behavior (pHpzc from 8.57 to 6.21), and improved porosity. Adsorption experiments using ibuprofen as a model contaminant revealed superior performance of the hybrid compared to pristine Mg/Al-LDH, with a maximum capacity of 22.13 mg·g⁻¹ (% Removal = 88.53%) at 25 ppm, and in lower concentrations (5 and 10 ppm), the hybrid showed 100% removal. Kinetic modeling followed a pseudo-second-order mechanism, and the isotherm was the SIPS model (maximum adsorption capacity = 24.150 mg.g⁻¹). These findings highlight the potential of LDH-CQD hybrid as efficient and tunable adsorbents for removing emerging pharmaceutical pollutants from aqueous media.

1. Introduction

Ibuprofen (IBU) is an extensively used non-steroidal anti-inflammatory drug (NSAID), indicated for the treatment of pain, fever, and inflammation in both human and veterinary medicine. Its broad therapeutic use, coupled with over-the-counter availability, has led to its frequent detection in surface waters and wastewater effluents. In addition to its pharmacological relevance, IBU is associated with adverse effects in humans, including gastrointestinal disturbances, renal dysfunction, and cardiovascular risks. Moreover, it poses ecotoxicological threats to aquatic organisms even at low concentrations. The low removal efficiency of IBU in conventional wastewater treatment plants underscores the urgent need for more effective remediation strategies.

Among the various approaches explored, adsorption stands out as a simple, cost-effective, and efficient technique for water treatment, particularly when advanced adsorbent materials are employed. In this context, layered double hydroxides (LDHs), known as hydrotalcite-like compounds or anionic clays, have attracted significant attention due to their high anion exchange capacity, structural tunability, and moderate thermal stability. LDHs, also known as anionic clays, are composed of positively charged brucite-like layers intercalated with charge-balancing anions and water molecules. They follow the general formula [M²⁺_1−xM³⁺_x(OH)₂]^x+(Aⁿ⁻)^x/ⁿ·mH₂O. However, despite LDHs having high anion exchange capacity and adjustable lamellar structure, their practical applications as adsorbents are often hindered by limitations such as low surface area, poor stability in acidic environments and weak interactions with neutral or weakly polar organic molecules, such as pharmaceuticals [1]. To overcome these challenges, the combination of LDH with other compounds such as LDH-modified g-C₃N₄, palygorskite-LDH-Mg-Al and La-doped FeCo-LDH/Carbon Felt [2,3,4] to form hybrid materials has emerged as a promising strategy to enhance performance in the environmental remediation. Thus, the development of hybrid materials has proven to be a promising strategy to overcome the inherent limitations of layered double hydroxides (LDHs) in adsorptive processes [5,6]. Such hybrid systems can offer synergistic effects that significantly enhance adsorption capacity, selectivity, and regeneration potential [7,8]. These advances underscore the growing scientific interest in hybrid adsorbents as a frontier in water purification technologies.

In this sense, the development of hybrid materials that combine the structural characteristics of layered double hydroxides (LDHs) with the high surface reactivity of carbon quantum dots (CQDs) emerges as a promising strategy to overcome the intrinsic limitations of LDH-based adsorbents. CQDs are zero-dimensional carbon-based nanoparticles (<10 nm), composed of sp² and sp³ hybridized carbon atoms, and are characterized by high specific surface area, excellent biocompatibility, and the presence of abundant oxygenated and nitrogenated functional groups (e.g., –OH, –COOH, –NH₂) [9]. These features confer exceptional surface reactivity and ease of functionalization, enabling CQDs to act as effective surface modifiers when incorporated into LDH matrices [10]. Their inclusion leads to an increase in surface area, the introduction of additional active sites for adsorption, modulation of surface charge, and enhanced interactions with organic pollutants [11]. Furthermore, the functional groups on CQDs can favor specific interactions, such as hydrogen bonding, π–π stacking, and electrostatic attractions, which significantly improve the efficiency of adsorption processes [12]. Therefore, the synthesis of LDH/CQD hybrid materials represents an innovative and synergistic approach for developing high-performance adsorbents, particularly relevant for the remediation of emerging contaminants like pharmaceuticals in aquatic environments [13].

Therefore, this work aims to develop and evaluate a novel hybrid adsorbent material based on Mg/Al-layered double hydroxide (LDH) functionalized with carbon quantum dots (CQDs), synthesized via in situ coprecipitation of the LDH counterpart in the presence of CQDs. The main objective is to investigate how the incorporation of CQDs enhances surface properties of the LDH matrix, improving its adsorption performance toward IBU, a widely used pharmaceutical and relevant environmental contaminant. Through comprehensive characterization and adsorption studies, this research seeks to elucidate the synergistic effects between LDHs and CQDs, contributing to the advancement of efficient materials for the remediation of pharmaceutical pollutants in aqueous media.

2. Materials and Methods

Magnesium chloride hexahydrate (MgCl₂·6H₂O, 99%), aluminum chloride nonahydrate AlCl₃·9H₂O, 99%), and sodium hydroxide pearls (NaOH, 99%) were obtained by Êxodo (São Paulo, Brazil). The IBU sodium salt (≥98%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Deionized water (18.0 µS·cm⁻¹) was produced with a Millipore System (Merck Millipore, Burlington, MA, USA).

2.1. Synthesis of CQD

CQD were synthesized via a solvothermal method, adapted from Qu et al. [14]. Firstly, 1 g of citric acid and 2 g of urea were dissolved in 10 mL of N,N-dimethylformamide (DMF, Sigma Aldrich, St. Louis, MO, USA.) under magnetic stirring. After, the solution was transferred to a 15 mL Teflon-lined stainless-steel autoclave. The system was heated at 160 °C for 6 h, then cooled to room temperature. The reaction mixture was treated with 20 mL of NaOH solution (50 g·L⁻¹), stirred briefly, and centrifuged at 15,000 rpm for 30 min. The precipitate was washed twice with distilled water and re-centrifuged under the same conditions. After the washing step, the nanomaterial was redispersed in a small volume of water, frozen, and lyophilized at −35 °C for 72 h. The final product was stored in a desiccator for further use.

2.2. Synthesis of Pristine Mg/Al-LDH and Hybrid Material LDH-CQD

The materials were synthesized via the coprecipitation method under an inert atmosphere. The system included a three-neck round-bottom flask equipped with a magnetic stirrer, a pH meter, and a nitrogen gas inlet. Initially, a metal solution containing MgCl₂·6H₂O and AlCl₃·9H₂O (2:1 in molar ratio) was prepared and transferred to a separatory funnel. This solution was added dropwise to 125 mL of bidistilled water, which had been previously purged with nitrogen gas for 15 min to mitigate interference from carbonate anions. For the hybrid system, 50 mg of CQDs were dispersed in the reaction medium before salt addition. The pH was continuously monitored and maintained between 9.0 and 10.0 by controlled addition of 1 mol·L⁻¹ NaOH solution from a burette, while the suspension was magnetically stirred throughout the process under a nitrogen atmosphere. After complete addition, the reaction mixture was aged under stirring for 24 h to promote crystal growth. The solids were recovered by centrifugation (4000 rpm for LDH; 15,000 rpm for LDH-CQD), washed with decarbonated bidistilled water to remove residual alkali, and dried in an oven at 60 °C overnight.

2.3. Characterization of Materials

The physicochemical properties of the synthesized materials were thoroughly investigated using complementary techniques. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer, equipped with a Cu Kα radiation source (λ = 1.5406 Å), operating at 40 kV and 35 mA, over a 2θ range of 5–70°, with a step size of 0.02° and an acquisition time of 1.5 s per step. Raman experiment was recorded on Horiba Jobin Yvon Lab RAM HR Raman microscope with a He-Ne 632.8 nm laser line. The Raman spectra were deconvoluted by Lorentzian fitting to obtain the ratio I_D/I_G. Fourier-transform infrared (FTIR) spectra were obtained using a Shimadzu IR-Prestige-21 spectrometer (Tokyo, Japan) in the range of 4000–400 cm⁻¹, employing KBr pellets, with 4 cm⁻¹ resolution and 40 scans. Pure KBr was used as the background. Thermal stability was assessed by differential scanning calorimetry (DSC) on a Shimadzu DSC-60H system and thermogravimetric analysis (TGA/DTG) on a TA Waters SDT 650 analyzer, both under nitrogen atmosphere (100 mL·min⁻¹), using a heating rate of 10 °C·min⁻¹. The temperature ranges were set from 30 to 500 °C for DSC and up to 800 °C for TGA/DTG. Morphological and elemental analyses were carried out by scanning electron microscopy (SEM) using JEOL JSM-IT700HR instrument (Akishima, Tokyo, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (Oxford, UK), and transmission electron microscopy (TEM) with a JEOL JEM-2100F (Akishima, Tokyo, Japan). Samples were mounted on carbon tape without sputter coating. Finally, textural parameters were evaluated via nitrogen adsorption–desorption isotherms at −196 °C using a Quantachrome analyzer (Boynton Beach, FL, USA). Samples were previously degassed at 100 °C for 4 h under vacuum. The Brunauer–Emmett–Teller (BET) method was applied for surface area determination, while the Barrett–Joyner–Halenda (BJH) method was used for pore size distribution.

2.4. Point of Zero Charge

The pH_PZC was determined by adding 5 mg of either LDH or LDH-CQD to 25 mL of 0.1 mol·L⁻¹ KCl solution with initial pH adjusted from 2 to 12. All KCl solutions were purged with nitrogen for 15 min before the adsorbent addition and maintained in closed containers to avoid CO₂ interference. The suspensions were stirred at room temperature for 24 h, and the final pH was measured using a Hanna HI-2002 pH meter (Woonsocket, RI, USA). The pH_PZC was estimated graphically from the ΔpH (pHi − pHf) vs. pHi plot, where ΔpH = 0 [15].

2.5. Adsorption Study

The adsorption capacity (q_t, mg·g⁻¹) and removal efficiency (% Removal) of Mg/Al-LDH and LDH-CQD for the pharmaceutical compound ibuprofen (IBU) were determined through batch experiments conducted under controlled conditions. For each experiment, 0.04 g of the adsorbent was added to 0.04 L of the IBU solution, under constant stirring at 300 rpm, at room temperature, and protected from light. Initially, a preliminary study of influence of contact time and initial concentration on adsorption effect was carried out using IBU solutions at concentrations of 5, 10, 15, 20, and 25 mg·L⁻¹, and the time required to reach adsorption equilibrium. Aliquots of 3 mL were collected at predetermined time intervals (0, 5, 10, 20, 40, 60, 90, and 120 min), centrifuged at 4000 rpm to separate the adsorbent from the supernatant, which was then analyzed by UV-Vis spectrophotometry at 221 nm. Subsequently, in the optimal time, the effect of pH on IBU adsorption was investigated by varying the pH to 3, 5, 7, and 9, adjusted using 0.1 mol·L⁻¹ NaOH or HCl solutions. Residual IBU concentrations were quantified using a calibration curve derived from Beer-Lambert’s law. The adsorption capacity and removal efficiency were calculated according to Equations (1) and (2), based on the initial concentration (C_o), the equilibrium concentration (C_e), the solution volume (V), and the adsorbent mass (m). After establishing the ideal conditions in terms of concentration, pH, and contact time, kinetic modeling, adsorption isotherm studies, and recycling assessments were performed exclusively with the LDH-CQD hybrid material, in accordance with the main objective of this study.

(1)$q_t={\displaystyle \frac{C_o-C_e}{m}} \times V$

(2)$R\left(\%\right)=100 \times {\displaystyle \frac{C_o-C_e}{C_o}}$

2.5.1. Adsorption Kinetics

The adsorption kinetics of IBU using LDH-CQD were investigated at room temperature and pH 7, with an initial concentration of 25 ppm. After the designated contact period, 3 mL aliquots were obtained at predetermined time intervals, followed by centrifugation to eliminate unadsorbed particles. These aliquots were then analyzed utilizing UV-Vis spectrophotometry, employing a pre-established calibration curve. The kinetic adsorption models applied were the pseudo-first-order model [16], the pseudo-second-order model [15], the intraparticle diffusion model [17], and Elovich model [8]. These models were used to describe the mechanism underlying the adsorption process. The linearized forms of Equations (3)–(6), were employed for the analysis of the experimental data.

(3)$ln\left(q_t-q_e\right)=ln{q}_e-k_{1}t$

(4)${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{\left(q_e\right)}^2}}+ {\displaystyle \frac{1}{q_e}}t$

(5)$q_t=k_{id} t^{{\displaystyle \frac{1}{2}}}+ C$

(6)$q_t={\displaystyle \frac{1}{\beta }}ln\left(t\right)+ {\displaystyle \frac{1}{\beta }}ln\left(\alpha \beta \right)$

In the given equations, q_e and $q_t$ represent the amounts of IBU that have been adsorbed per gram of adsorbent (mg.L⁻¹) at equilibrium and at time t, respectively. The adsorption rate constant associated with the pseudo-first-order model is denoted by k₁ (min⁻¹), and the rate constant of the pseudo-second-order model is denoted by k₂ (g.mg⁻¹ min⁻¹). Furthermore, C (mg.g⁻¹) is a constant parameter associated with the thickness of the boundary layer, while k_id (mg.g⁻¹.min^−0.5) is a constant parameter associated with the intraparticle diffusion rate. α is the initial adsorption rate (mg.g⁻¹.min⁻¹) and β is the desorption constant (g.mg⁻¹).

2.5.2. Adsorption Isotherms

For the study of adsorption isotherms, 0.04 g of LDH-CQD hybrid material was utilized, with a drug solution volume of 0.04 L, maintained at pH 7 and room temperature, for a contact time of 40 min. The initial concentrations of the IBU solutions were adjusted to 5, 10, 15, 20 and 25 (mg·L⁻¹) for the adjustment of the adsorption isotherms. The experimental data were subsequently adjusted to the Langmuir [18], Freundlich [19] Dubinin-Radushkevich (D-R) [20] and Sips [21] isothermal models to determine the relevant adsorption parameters associated with each model. The equations for both models are presented in Equations (7), (8), (9) and (10), respectively:

(7)${\displaystyle \frac{C_e}{q_t}}=\left({\displaystyle \frac{1}{q_{max}}}\right)C_e+{\displaystyle \frac{1}{K_L q_{max}}} Langmuir$

(8)$lnln{q}_e=lnln{K}_F+{\displaystyle \frac{1}{n}}lnln{C}_e Freudlich$

(9)$\ln q_e=\ln q_{max}-\mathsf{\beta }. {\epsilon }^2 \mathrm{D}\mathrm{u}\mathrm{b}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{n}\text{-}\mathrm{R}\mathrm{a}\mathrm{d}\mathrm{u}\mathrm{s}\mathrm{h}\mathrm{k}\mathrm{e}\mathrm{v}\mathrm{i}\mathrm{c}\mathrm{h} \left(\mathrm{D}\text{-}\mathrm{R}\right)$

(10)$q_e={\displaystyle \frac{q_{max·}(Ks·C_e)n}{1+(Ks·C_e)n}}\hspace{1em}\mathrm{Sips}$

The equilibrium adsorption capacity (q_e) is expressed in mg·g⁻¹, while q_max represents the maximum adsorption capacity, also in mg·g⁻¹. C_e refers to the equilibrium concentration of IBU in solution (mg·L⁻¹). The Langmuir constant (K_L) is associated with the free energy of adsorption and the affinity of the adsorption sites. In the Freundlich model, K_F and n represent the adsorption capacity and the intensity or heterogeneity of the adsorption process, respectively. The values of K_L and q_max can be obtained from the linear plot of C_e/q_e versus C_e, where K_L corresponds to the slope and _max to the intercept. For adsorption to be considered favorable, the Freundlich constant (K_F) typically ranges between 1 and 10, and the parameters K_F and n are derived from the linear relationship between ln(q_e) and ln(C_e). The Dubinin–Radushkevich (D-R) model is employed to estimate the adsorption energy and assist in differentiating between physical and chemical adsorption. The D-R equation relates q_e to the Polanyi potential (ε), allowing the calculation of the mean free energy of adsorption (E, kJ·mol⁻¹). If E < 8 kJ·mol⁻¹, the mechanism suggests physisorption, while values between 8 and 16 kJ·mol⁻¹ suggest chemisorption. Additionally, the Sips model, also known as the Langmuir-Freundlich model, is used to describe adsorption on heterogeneous surfaces. This model combines features of the Langmuir and Freundlich isotherms and introduces the heterogeneity factor (n_s). When n_s approaches 1, the Sips model simplifies to the Langmuir isotherm, indicating a homogeneous adsorption surface. The Sips equation provides the maximum adsorption capacity (q_max) and the affinity constant (K_s), offering a more flexible description of adsorption systems that do not fit purely Langmuir or Freundlich behavior.

3. Results and Discussion

3.1. PXRD Measurements

Powder X-ray diffraction (PXRD) was employed to investigate the crystalline structure of the synthesized materials (Figure 1). The diffraction pattern of the pristine Mg/Al-layered double hydroxide (LDH) exhibits sharp and well-defined peaks at 2θ ≈ 11.3°, 22.9°, 34.7°, 38.7°, 45.4°, 60.7°, and 61.9°, which are indexed to the (003), (006), (012), (015), (018), (110), and (113) planes, respectively [22]. These reflections are characteristic of hydrotalcite-like compounds, as described in JCPDS card number 22-0700. The prominent basal reflections (003) and (006) confirm the formation of a layered crystalline structure with long-range order. The interlayer spacing (d₀₀₃) calculated from the (003) reflection corresponds to 0.76 nm. Considering a brucite-like layer thickness of approximately 0.48 nm, this value agrees with typical LDH structures containing chloride anions in the interlayer region [22,23].

The XRD pattern of CQD shows two main reflection planes at 2θ = 5.07° and 26.97°, corresponding to the d-spacing of 1.74 nm and 0.33 nm, respectively (Figure 1). The first peak can be associated with the (001) plane diffraction of graphite oxide, which occurs at 2θ = 10.6° (JCPDS no 00-041-1487) [24]. This peak shifted to lower angles, which are related to functional groups such as hydroxyl, carbonyl, carboxylate, and amino groups formed at the edges of the CQD sheets, increasing the interlayer spacing and introducing disorder. The second peak is in accordance with the 002 plane (2θ = 26.5°; d = 0.34 nm) of pure graphite (JCPDS 96-901-2231) [25], however, a slight reduction in d-spacing is observed, indicating the formation of a non-ideally arranged graphite-like structure in CQD [26]. The broad nature and low intensity of these peaks might be indicative of partial graphitization and the presence of disordered carbon [27]. This result indicates that the CQD obtained possesses a layered structure with a certain degree of crystallinity and the presence of disordered carbon, which is consistent with other reports [24,28]. In addition, Raman analysis was conducted to gain information about CQD structure. The Raman spectrum of CQD (Figure S1) exhibits four bands at 1102.4 cm⁻¹, 1336.7 cm⁻¹, 1524.7 cm^−1, and 1616.1 cm⁻¹, which correspond to D*, D, G, and D′ bands, respectively. The G band is ascribed to the stretching modes of carbon with sp² hybridization (ordered carbon), while the D band arises from defects and disorders in the carbon lattice, including defects induced by heteroatom doping and oxygen-containing functional groups [29]. The D* and D’ bands are also defect-related, but the presence of the first one can be associated with vibrations of carbon atoms that are bonded to oxygen-containing groups [30,31]. In addition, the intensity ratio of D and G bands (I_D/I_G) allows evaluating the extension of the disorder and defects in carbon materials. In this regard, the Raman spectrum was deconvoluted by Lorentzian fitting (R² = 0.975) to obtain I_D/I_G. For the CQD synthesized in this work, the value was found to be 1.42, further affirming the partial graphitization of the carbonaceous structure with defect-rich regions, which is expected for CQD.

In the LDH-CQD hybrid material, the Mg/Al-LDH basal reflections were preserved, suggesting that the lamellar structure was preserved after hybrid formation. The structural modifications in the LDH and the hybrid material were observed by the Full Width at Half Maximum (FWHM) of the most intense plane (003) and by the Scherrer equation. For pristine Mg/Al-LDH, the FWHM was calculated as 0.55 and the crystallite size of 14.7 nm, and for the hybrid LDH-CQD, the FWHM was calculated as 0.669 and the crystallite size of 12.1 nm. The slight broadening and reduction in reflection intensity suggest decreased crystallinity or reduced crystallite size, likely due to the surface modification by CQDs.

However, no distinct reflections from the CQDs in this diffractogram were detected. This absence can be associated with two main factors: (i) The CQDs exhibit broad and low-intensity diffraction peaks due to their small size and disordered graphitic domains, which limit their detectability when embedded within more crystalline matrices [26,32]; and (ii) the relatively low mass fraction of CQDs used in the synthesis—designed to act as surface modifiers rather than structural constituents—likely results in a dilution effect, whereby their signal is masked by the dominant LDH diffraction. However, a subtle shift of the most intense plane (003) to a lower value of 2θ was observed, since the peak position of Mg/Al-LDH is 11.51°, while the peak position of LDH-CQD is 11.35°, indicating that there was an expansion in the LDH lattice after interacting with the CQD. This subtle expansion in the Mg/Al-LDH structure was confirmed by Rietveld refinement, as a change in the lattice parameters and the cell volume was observed (Figure S2 and Table S1). These subtle structural changes, therefore, are consistent with the hybrid material in which CQDs interact with LDH matrix predominantly via surface association, were not intercalated into the interlayer space, and this interaction does not disrupt its long-range order. Although there is no evidence of CQD intercalation into the interlayer gallery, the interaction of CQDs with the outer surfaces of the LDH particles appears to induce local strain or distortions in the layered structure. Such effects are likely due to hydrogen bonding, electrostatic forces, and other interfacial interactions between the functional groups of the CQDs and the hydroxyl-rich surface of the LDH. These interactions may weaken the interlamellar cohesion or alter stacking regularity causing lattice changes/interplanar distances in the XRD pattern.

3.2. FTIR Analysis

FTIR spectroscopy was employed to identify functional groups and evaluate possible interactions between the LDH and CQD structures (Figure 2). The spectrum of the pristine Mg/Al-LDH displayed characteristic vibrational bands at 3453 cm⁻¹ and 1639 cm⁻¹, corresponding to O–H stretching and H–O–H bending, respectively, indicating the presence of surface hydroxyl groups and interlayer water molecules [32]. The band near 1390 cm⁻¹ was assigned to asymmetric stretching of carbonate ions (CO₃²⁻), commonly present due to atmospheric CO₂ uptake during synthesis [33]. In the low-frequency region (below 1000 cm⁻¹), bands associated with metal–oxygen (M–O) vibrations were observed, confirming the presence of Mg-O and Al-O bonds and thus supporting the integrity of the brucite-like layered structure [34].

For the CQD structure, several bands characteristic of oxygenated and nitrogen-containing functional groups are revealed. Broad O–H stretching vibrations were detected in the range of 3549 to 3415 cm⁻¹, consistent with abundant surface hydroxyls [35]. A prominent band at 1637 cm⁻¹ was ascribed to C=O stretching vibrations, while the asymmetric and symmetric COO⁻ stretching bands at 1611 and 1351 cm⁻¹, respectively, indicated the presence of surface carboxylate groups [36]. The band at 1025 cm⁻¹ was assigned to C–O stretching vibration. Additionally, the C=C stretching at 1637 cm⁻¹ and an aromatic–H out-of-plane bending vibration at 613 cm⁻¹ confirmed the partial aromatic character of the carbon framework [37]. Signals at 3234 cm⁻¹ and 1351 cm⁻¹ were associated with N-H and C–N stretching, respectively, indicating successful nitrogen doping of the CQDs [38].

In the FTIR spectrum of the LDH-CQD hybrid, the main vibrational modes of the LDH were preserved, indicating that the lamellar structure remained intact after the functionalization. Although the characteristic spectral bands of the CQDs appeared with lower intensity—likely due to their low mass ratio—a slight redshift (toward lower wavenumbers) in the O–H stretching band was observed, suggesting the establishment of hydrogen bonds between LDH hydroxyl groups and CQD surface functionalities. Moreover, the appearance of weak bands in the 1600–1700 cm⁻¹ region supports the existence of specific interactions—likely hydrogen bonding—between CQDs and surface hydroxyls [39]. These spectral features confirm the successful surface modification of the LDH by CQDs without compromising the structural framework, potentially improving the material’s interfacial properties for adsorption applications.

3.3. Thermal Behavior and Stability (DSC and TGA-DTG)

Thermal properties of the synthesized materials were evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA/DTG) (Figure 3). As shown in the DSC curves (Figure 3a), all samples exhibited endothermic events below 150 °C, which correspond to the evaporation of physically adsorbed and interlayer water molecules. For the pristine Mg/Al-LDH, a prominent endothermic peak was observed near 400 °C, attributed to the decomposition of intercalated carbonate and chloride species [40]. In contrast, the DSC profile of the CQDs revealed a characteristic exothermic event around 512 °C, associated with the thermal degradation of surface functional groups, such as hydroxyl, carbonyl, carboxylate, and amine moieties [41]. In the case of the LDH-CQD hybrid, a slight shift in thermal transitions was observed compared to its components, with new events at 385 °C and 492 °C, suggesting interfacial interactions between the CQDs and the LDH matrix, which corroborates the FTIR data (Figure 2).

The TG-DTG results supported the DSC findings and provided additional insights into the mass loss behavior. For the LDH (Figure 3b), two main stages of weight loss events were detected: the first, below 200 °C (with the maximum at ~92 °C), corresponding to the release of interlayer and surface water; and the second, between 300–500 °C (DTG maximum at ~422 °C), attributed to the dehydroxylation of the brucite-like layers and decomposition of the intercalated anions, leading to structural collapse and formation of metal oxides such as MgO and MgAl₂O₄. The total weight loss for the LDH was 44.5%.

For the CQD, a three-step mass loss pattern was observed (Figure 3c). The first weight loss is related to removal of physisorbed water and occurs up to 150 °C (maximum at ~61 °C). The second and third mass loss occur sequentially with the degradation of oxygen- and nitrogen-containing functional groups in the ranges of 200–550 °C, with DTG peaks at approximately 280 °C and 470 °C. The total weight loss for the CQD was 64%.

The thermal profile of the LDH-CQD hybrid (Figure 3d) exhibited combined features of both components. The first mass loss event occurred below 150 °C (with a maximum at 65 °C), consistent with moisture removal. The second and third step observed at ~200 °C and ~379 °C in the DTG curve can be attributed to the decomposition of CQD surface groups and dehydroxylation of the LDH, respectively. The total mass loss observed for the hybrid material (47.6%) was slightly higher than that of the pristine LDH (44.5%), reflecting the presence of thermally labile organic moieties from the CQDs. Importantly, the hybrid material maintained thermal stability up to 150 ° C, highlighting its potential applicability under moderate thermal conditions, such as in environmental remediation or biomedical systems.

3.4. Morphological and Compositional Analysis (SEM–EDS and TEM)

Surface morphology and elemental composition of the materials were revealed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). As shown in Figure 4a, the pristine Mg/Al-LDH displayed the typical agglomerated lamellar morphology characteristic of hydrotalcite-like compounds. Due to the resolution limitations of SEM, individual lamellae could not be clearly distinguished. In the LDH-CQD hybrid (Figure 4b), a finely dispersed morphology is observed, with reduced agglomeration. In the TEM images of the pure LDH (Figure 4c,d), thin and partially overlapping lamellar sheets are present, typical of brucite-like materials. In contrast, the TEM images of LDH-CQD hybrid material revealed well-defined planes associated with the layered domains corresponding to the LDH structure (Figure 4e,f), while the presence of CQDs may be suggested by less organized regions, which contrast with the classical lamellar ordering. In general, these materials seem to exhibit reduced large-scale agglomeration and a more uniform distribution of lamellar domains. This morphological change may be attributed to the ability of carbon quantum dots to improve the dispersion of LDH particles during co-precipitation, likely by modifying surface interactions and reducing particle aggregation.

EDS analysis of the pristine LDH confirmed a Mg/Al atomic ratio of approximately 1.92:1, which is consistent with the expected synthesis stoichiometry. In this sample, it was possible to detect the presence of NaCl crystals, resulting from the reaction of Na⁺ and Cl⁻ ions from the hydroxide and chloride salts used in the synthesis. On the other hand, in the LDH-CQD hybrid, the Mg/Al atomic ratio remained close to the theoretical value (2.01:1), while the chlorine content decreased significantly and sodium was no longer detected, indicating improved washing and purification. Furthermore, a noticeable increase in oxygen content was observed, likely due to the incorporation of oxygen-rich functional groups from the CQDs. These compositional changes support the successful integration of CQDs on the LDH matrix and the effectiveness of the post-synthesis purification process.

3.5. N₂ Physisorption Analysis

These structural and compositional observations confirm the successful formation of the LDH-CQD hybrid, with carbonaceous domains anchored to the LDH surface and minimal interference with the lamellar organization. To further elucidate the impact of CQD incorporation on the textural characteristics of the materials, nitrogen adsorption–desorption isotherms were measured (Figure 5). According to the IUPAC classification, both pristine LDH and LDH-CQD exhibited Type IV isotherms, which are indicative of mesoporous solids [42]. These profiles reveal multilayer adsorption followed by capillary condensation within the mesopores. In both materials, an H3-type hysteresis loop was observed, typically associated with slit-shaped pores or aggregates of plate-like particles, which is expected for hydrotalcite-like compounds [43].

For the pristine LDH, the isotherm showed relatively low nitrogen uptake and an open hysteresis loop, with a specific surface area of 2.89 m²/g and an average pore diameter of 6.80 nm. These values are consistent with the restricted accessibility of mesopores often found in unmodified layered double hydroxides [44,45]. The low total adsorbed volume (~1 cm³/g) may also reflect instrumental limitations in detecting subtle porosity in materials with inherently low surface area. In contrast, the LDH-CQD hybrid exhibited a significantly enhanced textural profile. The isotherm displayed a broader hysteresis loop and a steep increase in nitrogen uptake near P/P₀ ≈ 1.0, suggesting the presence of interparticle voids or increased textural irregularities resulting from CQD incorporation. The specific surface area rose markedly to 66.9 m²/g, i.e., over 23 times greater than that of the pristine LDH, while the average pore diameter increased slightly to 7.67 nm.

This notable improvement confirms that the integration of CQDs promotes the development of new mesoporous domains and increases the density of surface-active sites. Collectively, these results demonstrate that the functionalization of LDH with carbon quantum dots substantially alters the textural architecture of the material, thereby enhancing its suitability for adsorption-related applications.

3.6. Point of Zero Charge

The pHpzc is a fundamental parameter for understanding the electrostatic behavior of adsorbent surfaces and their interaction with ionic species under varying pH conditions. As shown in Figure 6, the pHpzc of the pristine LDH was determined to be 8.57, consistent with literature values for LDHs containing chloride as the interlayer anion [46]. Below this pH, the LDH surface becomes positively charged (M–OH₂⁺), favoring electrostatic attraction towards anionic species. Conversely, at pH values above the pHpzc, the surface is negatively charged (M–O⁻), decreasing its affinity for anions. In the case of the LDH-CQD hybrid, the pHpzc shifted significantly to 6.21, indicating a pronounced alteration in surface charge behavior following the CQD incorporation. This shift can be attributed to the introduction of oxygenated and nitrogenated functional groups, such as carboxyl, hydroxyl, and amine moieties onto the LDH surface via CQD functionalization, as supported by the FTIR spectrum (Figure 2). These groups increase the density of acidic sites, enhancing surface protonation in acidic media environments and thus modifying the overall electrostatic profile of the material.

Considering that the pKa of sodium IBU is approximately 4.85, it is evident that under typical environmental pH values (5–7), the drug exists predominantly in its desprotonated or anionic form. Consequently, under such conditions, the positively charged surface of pristine LDH (pH < pHpzc) promotes adsorption via electrostatic interactions, enhancing its affinity for IBU. In the case of LDH-CQD hybrid, the surface assumes a negative charge at lower pH values compared to unmodified LDH, potentially improving the material’s capacity to adsorb positively charged or protonated species in mildly acidic environments. These results collectively demonstrate that CQD incorporation not only alters the surface charge but also enriches the chemical environment of LDHs, expanding their applicability and enhancing their performance as versatile adsorbents for pharmaceutical contaminants in aqueous media.

3.7. Adsorption Experiments

3.7.1. Influence of Contact Time and Initial Concentration on Adsorption Effect

The effects of contact time and initial concentration of the adsorbate are key parameters influencing the adsorption performance. The results are presented in Figure 7, comparing the adsorption capacities of pristine Mg/Al-LDH and CQD-LDH hybrid, considering initial IBU concentration (5, 10, 15, 20, and 25 ppm), under a fixed contact time of 120 min. For hybrid material, in low concentrations (5 and 10 ppm), the amount of adsorbed IBU increased progressively with the initial concentration, ranging from 5.00 mg.g⁻¹ (R(%) = 100%) at 5 ppm to 10 mg.g⁻¹ (R(%) = 100%) at 10 ppm. In comparison, the pristine Mg/Al-LDH exhibits a lower adsorption performance, with values increasing from only 0.44 mg.g⁻¹ (R(%) = 8.8%) to 4.45 mg.g⁻¹ (R(%) = 44.53%) over the same concentration range and time. The hybrid material showed a high adsorption capacity at low concentrations of 5 and 10 ppm, thus indicating that the material has a high adsorption efficiency of the contaminant at real concentrations. These findings indicate that the LDH-CQD hybrid outperforms the unmodified LDH, particularly at lower concentrations, where the pristine material demonstrates limited uptake.

For high concentrations (15 and 25 ppm), the concentration of adsorbed IBU on the CQD-LDH was 10.88 mg.g⁻¹ (R(%) = 72.53%) at 15 ppm to 17.78 (R(%) = 71.12%) at 25 ppm, and the Mg/Al-LDH increasing from 10.71 mg.g⁻¹ (R(%) = 71.41%) to 13.62 mg.g⁻¹ (R(%) = 54.47%). It can be observed that at 120 min, overall, the hybrid material demonstrated superior adsorption performance compared to the unmodified Mg/Al-LDH, since the pure LDH does not maintain the adsorption capability for extended periods. Notably, the pristine Mg/Al-LDH exhibited a profile of desorption after approximately 20 min, suggesting limited stability during the adsorption process. Meanwhile, the CQD-LDH hybrid seems to show a more consistent adsorption behavior over time, indicating enhanced stability and interaction with target pollutants. The observed trend for the hybrid sample may result from a driving force created due to the high concentration gradient obtained by increasing the initial adsorbate concentration [47]. Moreover, higher IBU concentrations in solution promote a greater local density of molecules near the active adsorption sites on the hybrid material’s surface, thereby increasing the probability of molecular interactions between the IBU molecules and LDH-CQD surface, leading to a higher amount of drug adsorbed [48].

In addition, it can be noted that the hybrid material presents a high adsorption capacity in the time of 40 min, as the initial concentration of IBU increases from 5 ppm to 25 ppm, since the adsorption capacity was 5 mg.g⁻¹ (R(%) = 100%) to 22.13 mg.g⁻¹ (R(%) = 88.53%), which is notably higher than the pristine Mg/Al-LDH with an adsorption capacity of 2.52 mg.g⁻¹ (R(%) = 50.50%) to 18.12 mg.g⁻¹ (R(%) = 72.50%). Although the material did not show the total removal of the drug in the concentration of 25 ppm, the high efficiency of the hybrid can still be noted. Furthermore, a rapid uptake of IBU by the LDH-CQD hybrid was observed within the first 5 min, followed by a gradual approach towards equilibrium, suggesting the progressive saturation of active adsorption sites. In contrast, pristine Mg/Al-LDH showed rapid adsorption in the first 5 min and did not reach equilibrium, since after 20 min, there was a reduction in removal efficiency. Thus, the best behavior of the hybrid material may be associated with increased surface area, a stronger interaction with IBU molecules, and the abundance of accessible active sites. The overall adsorption process can be governed by specific interactions, such as hydrogen bonding between the IBU and the active sites on the LDH-CQD surface [49,50].

Given this, we can consider that for a real application, the contact time of 40 min is ideal for high adsorption efficiency of the pollutant, since in longer times it can cause a reduction in the adsorption capacity. Moreover, the hybrid system maintains more stable adsorption behavior, while the pristine LDH appears to struggle with equilibrium retention, which may be attributed to weaker interactions or desorption effects.

3.7.2. Influence of the IBU Solution pH on Adsorption Effect

The influence of pH on the adsorption performance of IBU by the LDH-CQD hybrid material was evaluated at pH values of 3, 5, 7, and 9, at the optimal time of 40-min time frame, during which the hybrid material exhibited a high adsorption capacity. As illustrated in Figure 8, the highest removal efficiency by LDH-CQD hybrid was achieved at pH 7 (88.53%), followed by comparable performances at pH 3 and pH 9 (both 85.23%). It can be evidenced that a pronounced decline in adsorption efficiency was observed at pH 5, with removal dropping to 47.37%. On the other hand, superior performance at pH 7 is particularly noteworthy, as it reflects typical environmental conditions. At this pH, the drug exists predominantly in its anionic form (–COO⁻), while the LDH-CQD surface carries a net negative charge, as indicated by the pH at the zero point of zero charge result (pHpzc = 6.21). Despite the expected electrostatic repulsion between the negatively charged species, the high removal efficiency suggests that the non-electrostatic mechanisms predominate, including hydrogen bonding and π–π stacking interactions, which are enhanced by the presence of functional groups introduced by CQDs (e.g., –OH, –COOH, –NH₂) [51,52]. For the pristine Mg/Al-LDH (72.5%), the adsorption efficiency declines due to weakened electrostatic interactions, as the pH approaches its pHpzc.

Although IBU is fully deprotonated (–COO⁻) and both adsorbents are negatively charged, which should result in electrostatic repulsion at pH 9, high adsorption was sustained by both materials, especially for the Mg/Al-LDH (90.06%), suggesting that other mechanisms such as hydrogen bonding and ligand exchange contribute substantially to the retention of IBU at alkaline medium. For the hybrid LDH-CQD, although performance slightly decreased, the preservation of adsorption capacity suggests that non-electrostatic interactions continue to dominate. On the other hand, at pH 3, IBU exists in its neutral (–COOH) form, since the pH is lower than the pKa (~4.85) of the drug, while the Mg/Al-LDH (78.57%) surface is positively charged and the LDH-CQD surface is weakly positive or close to neutral pHpzc. Therefore, in this environment there is no electrostatic interaction. Thus, for the hybrid material, the favorable removal at this pH may be attributed to the higher availability of functional groups of CQD on the hybrid surface (e.g., –OH, –COOH, –NH₂), confirmed by FTIR data, favoring hydrogen bonding and may also facilitate additional interactions with IBU. Nevertheless, the significant reduction in adsorption efficiency at pH 5 can be explained by the proximity of this value to the hybrid’s pHpzc (6.21), where the surface tends to be electrically neutral, thus reducing electrostatic contributions to adsorption. Moreover, this pH is also near the pKa of IBU (~4.85), where the drug exists as a mixture of neutral and anionic species. This coexistence likely leads to less consistent and weaker interactions with the adsorbent surface, impairing general adsorption performance. In contrast, pristine Mg/Al-LDH at this pH (85.84%) still maintains a positive surface charge at this pH, which favors electrostatic interaction with the partially anionic form of the drug, explaining its better performance.

Overall, the results revealed distinct adsorption site behaviors influenced by interaction between the surface of the materials and the speciation of IBU. While electrostatic interactions play an important role under certain conditions, the superior performance of the LDH-CQD hybrid, particularly at acidic and neutral pH, can be attributed to specific interactions facilitated by CQD functionalization and enhanced textural properties, even under electrostatically unfavorable conditions.

The reusability of the LDH-CQD hybrid material was conducted over four consecutive adsorption–desorption cycles under favorable experimental conditions (25 ppm of IBU, pH 7, contact time at 40 min). As shown in Figure S4, the removal efficiency in the first cycle was 88.53%, followed by a progressive decrease in subsequent cycles to 68.18%, 13.9%, and 8.96%, respectively. However, this progressive reduction in adsorption performance is primarily attributed to significant mass loss of the adsorbent during the separation steps. Given the nanostructured nature of the LDH-CQD composite (12.1 nm), as indicated by XRD analysis, and its high dispersibility, effective separation by centrifugation was compromised due to insufficient sedimentation, limiting its regeneration between the cycles.

These findings emphasize that although the LDH-CQD exhibits promising adsorptive performance, its low recovery efficiency highlights the challenges associated with the reuse of nanoscale adsorbents. To overcome this limitation, future studies should explore strategies, such as integration into polymer matrices, to improve recyclability and enable practical applicability of such hybrid systems in water treatment processes.

3.7.3. Adsorption Kinetics

Kinetic models provide information about the factors influencing the rate of adsorbate removal. In this study, the adsorption of IBU onto the LDH-CQD was evaluated using pseudo-first-order, pseudo-second-order, Weber–Morris intraparticle diffusion, and Elovich kinetic models (Figure 9). The kinetic experiments of IBU adsorption on the LDH-CQD were carried out using an aqueous IBU solution with an initial concentration of 25 mg·L⁻¹ at pH 7.0 and room temperature. The pseudo-first-order model, based on the assumption of physical adsorption and weak van der Waals interactions (Figure 9a), did not adequately describe the experimental data. As shown in Table 1, the determination coefficient (R² = 0.4599) was low, and the estimated value of equilibrium adsorption capacity (q_e = 6.7855 mg·g⁻¹) was significantly lower than the experimental value of 22.1331 mg·g⁻¹. The pseudo-second-order model provided the best fit to the experimental data (Figure 9b) with a high coefficient of determination (R² = 0.9978) and a q_e value of 22.2717 mg.g⁻¹ (Table 1), in close agreement with the experimental value. The kinetic constant (K₂) was 0.0576 g.mg⁻¹.min⁻¹, indicating that the occupation of active sites occurs rapidly during the initial adsorption stage. Although the pseudo-second-order kinetic model is often associated with chemisorption processes, it primarily describes the kinetic behavior and does not alone confirm the adsorption mechanism [53].

To further investigate the diffusion mechanism, the Weber-Morris intraparticle diffusion model was applied [17,54] (Figure 9c). The data revealed a three-stage adsorption process in this system: (i) Initial diffusion of IBU molecules over the outer surface of LDH-IBU, leading to subsequent adsorption; (ii) intraparticle diffusion and gradual adsorption of adsorbate; and (iii) an equilibrium stage where adsorption slows due to the saturation of available sites. The intercept value of C = 7.6683 mg.g⁻¹ represents the boundary layer thickness, while the intraparticle diffusion rate constant was K_dif = 2.9362 mg.g⁻¹.min^−0.5 (Table 1). The multilinear behavior suggests that intraparticle diffusion is involved, but not the sole rate-controlling mechanism.

The Elovich kinetic model, which is often used to describe chemisorption processes on heterogeneous solid surfaces, did not adequately fit the experimental data (Figure 9d). The parameters α= 20.457 and β= 0.2029 (Table 1) indicate poor agreement with experimental values, suggesting that the adsorption process does not conform to the Elovich model in this system. Taken together, the kinetic analysis indicates that the pseudo-second-order model best describes the adsorption behavior of IBU onto LDH-CQD composite (R² = 0.9978). As mentioned above, although the pseudo-second-order kinetic model is often associated with the chemisorption process, adsorption mechanism cannot be directly assigned based on the kinetic fitting model [55]. Therefore, possible physical interactions during the adsorption process in this study should not be ruled out, such as hydrogen bonding between the hydroxyl groups from the LDH surface and the carboxylic group of IBU, and electrostatic interactions between the negatively charged IBU⁻ ions and the positively charged sites of the Mg/Al lamellae, indicating a hybrid adsorption mechanism.

To validate the model fit, a residual analysis was performed for all kinetic models evaluated. Residuals were calculated as the difference between experimental and theoretical adsorption capacities at each time point analyzed. It is known that theevaluation of the distribution of residuals is essential to confirm the reliability of a kinetic model, going beyond R² values. A well-fitted model should have randomly distributed residuals around zero, with no discernible patterns. The residual plots (Figure S3) showed that the pseudo-second-order model exhibited the most random and homogeneous distribution of residuals around zero, reinforcing its suitability for describing IBU adsorption on LDH-CQD. In contrast, the pseudo-first-order, intraparticle diffusion, and Elovich models showed residuals with systematic patterns and greater dispersion, indicating a less accurate description of the adsorption kinetics. In addition, the pseudo-second-order model and the other models were adjusted in their nonlinear form (Supplementary Material—Figure S3), and the residual analysis corroborated the results obtained from the linearized form, confirming the robustness and validity of the pseudo-second-order model for this system.

3.7.4. Adsorption Isotherms

The adsorption behavior of ibuprofen (IBU) onto the LDH-CQD hybrid material was evaluated using the classical isotherm models of Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Sips, in order to elucidate the predominant mechanisms and the surface characteristics of the adsorbent. The experimental isotherm was obtained by varying the equilibrium concentration (Ce) of ibuprofen, while maintaining constant contact time and other parameters (25 °C, pH 7). The adsorption data were fitted to the classical Langmuir and Freundlich equations. The Langmuir model assumes monolayer adsorption on a surface with energetically equivalent and homogenous active sites, without interaction between adsorbed molecules [56]. In contrast, the Freundlich model is empirical and applies to heterogeneous surfaces, assuming a decrease in adsorption energy with increasing surface coverage [57]. As shown in Figure 10 the experimental isotherm resembles an L4-type curve, according to the Giles classification [58,59], characterized by a slight inflection and a tendency toward a plateau [53]. This profile suggests initial monolayer adsorption followed by occupation of secondary sites, likely formed by interactions between previously adsorbed IBU molecules and new ones arriving at the surface.

Table 2 summarizes the parameters derived from fitting the experimental data to the selected isotherm models. Among them, the Sips model provided the best fit, with R² = 0.993 and Sum of Squared Errors (SSE) = 24.25, along with a maximum adsorption capacity of q_max = 24.150 mg·g⁻¹, in excellent agreement with the experimental data. As a hybrid model, Sips incorporates the adsorption characteristics of Langmuir and the surface heterogeneity considerations of Freundlich, reflecting both monolayer and multilayer adsorption behavior of the material, making it particularly suitable for systems exhibiting cooperative adsorption behavior as observed here. The Freundlich model, although yielding a calculated q_e close to the experimental values and indicating favorable adsorption (n = 1.531, R² = 0.843), provided a slightly lower quality of fit (SSE = 24.82) and failed to adequately describe the sigmoidal nature of the isotherm, as it does not account for cooperative effects. This suggests that while the surface may exhibit some heterogeneity, the adsorption mechanism is not fully captured by this empirical model. The Langmuir model, which assumes a homogeneous surface and monolayer adsorption, produced an R² of 0.565 and SSE = 28.97, and substantially overestimated the maximum adsorption capacity (q_max = 34.361 mg·g⁻¹). Although it adequately describes the initial phase of the isotherm, it does not consider multilayer adsorption or the cooperative phenomena evident in the experimental data. Finally, the Dubinin–Radushkevich (D–R) model was the least suitable, with R² = 0.089 and SSE ≥1000, and it significantly underestimated the adsorption capacity (q_m = 42.991 mg·g⁻¹), indicating that the adsorption process is not governed by micropore filling with uniform energy. Altogether, the combination of strong statistical fit, physicochemical plausibility, and the Sips model’s ability to capture sigmoidal behavior confirms it as the most appropriate isotherm to represent the IBU adsorption process onto the LDH-CQD hybrid. The observed sigmoidal isotherm strongly supports the occurrence of cooperative interactions among IBU molecules, facilitated by a heterogeneous and functionalized surface enriched by the presence of CQDs within the LDH matrix.

To better contextualize the performance of the LDH-CQD hybrid, Table 3 presents a comparative analysis of IBU adsorption capacities reported for other adsorbent materials in the literature. As can be observed, the hybrid developed in this study outperforms the most previously reported adsorbents, including conventional clays, carbon-based materials, and even some modified layered double hydroxides. This superior performance can be attributed to the synergistic effect between the LDH layered structure and the surface of the doped CQDs, which introduces a high density of active sites, enhances specific interactions, and contributes to a more uniform and energetically favorable surface for IBU adsorption [60].

In general, these results not only confirm the effectiveness of the LDH-CQD system but also highlight its promising potential as an adsorbent platform for the removal of emerging pharmaceutical contaminants in aqueous media. Given its high adsorption capacity, ease of synthesis, and functional tunability, this hybrid material can represent an alternative for water remediation, with potential relevance in both environmental and industrial settings.

4. Conclusions

In this study, a hybrid material based on Mg/Al-LDH functionalized with CQDs was successfully synthesized via the in situ co-precipitation method. Structural analysis by XRD confirmed the good crystallization of the LDH phase in the presence of CQD particles, while the presence of functional groups associated with the carbonaceous moiety in the LDH-CQD hybrid was evidenced by FTIR measurements. DSC and TGA/DTG techniques suggested thermal stability consistent with effective hybrid formation, whereas N₂ adsorption-desorption isotherms indicated significant improvements in surface area and porosity, confirming enhanced textural properties in the hybrid material. Morphological and compositional analyses revealed a more dispersed lamellar morphology, higher purity, and oxygen enrichment in the LDH-CQD hybrid, which can be associated with surface functionalization with CQDs. Adsorption experiments using IBU as a pharmaceutical contaminant model demonstrated the promising performance of the LDH-CQD system under varying conditions of contact time, concentration of contaminant, and pH of the solution. The highest removal efficiency (100%) was observed at near-neutral pH in 40 min of contact with a lower concentration of 5 and 10 ppm solution, that is, in concentrations that approach the environmental occurrence of IBU in water bodies. Moreover, in the higher concentration (25 ppm), the hybrid material shows 22.13 mg.g⁻¹ (R(%) = 88.53%) in the optimal time of 40 min. Through the adsorption study, it was observed that the pseudo-second order kinetics and pH studies indicated that the adsorption of the IBU molecule on the hybrid material occurs through chemical and physical interactions. The sips model provided the best fit to the experimental data (R² = 0.2415; SEE = 24.25), with a maximum adsorption capacity of 24.150 mg.g⁻¹. By combining features of the Langmuir and Freundlich models, the sips model was suitable for describing the observed adsorption behavior, characterized by surface heterogeneity and cooperative effects, reflecting both monolayer and multilayer adsorption behavior of the material. The superior adsorption capacity of the hybrid material is attributed to the synergistic interaction between the LDH layered structure and the functional groups introduced by CQDs, which enables multiple adsorption mechanisms, including electrostatic interactions, hydrogen bonding, π–π stacking, and hydrophobic effects. Altogether, these findings highlight the LDH-CQD hybrid as a promising adsorbent for the removal of pharmaceutical contaminants, such as IBU, in water treatment applications.

Author Contributions

Conceptualization, F.G.C., V.N.S.C., M.M.T. and A.C.S.A.; methodology, V.N.S.C., M.M.T. and A.C.S.A.; validation, F.G.C., R.J.P.A., E.S.M.C., V.N.S.C. and M.M.T.; formal analysis, F.G.C., R.J.P.A., E.S.M.C. and M.d.S.C.S.; investigation, F.G.C., V.N.S.C., M.d.S.C.S. and. A.C.S.A.; resources, A.R., M.A.S.G. and A.C.S.A.; data curation, F.G.C., V.N.S.C., R.J.P.A., M.M.T. and A.C.S.A.; writing—original draft preparation, F.G.C., V.N.S.C. and M.M.T.; writing—review and editing, F.G.C., V.N.S.C., R.J.P.A., E.S.M.C., M.M.T., A.R., M.A.S.G. and A.C.S.A.; visualization, M.M.T. and A.C.S.A.; supervision, M.A.S.G. and A.C.S.A.; project administration, A.C.S.A. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is available on request from the authors due to privacy.

Acknowledgments

F.G.C.: V.N.C.C and M.M.T. thank Capes for the Master, PhD and Post-doc scholarships, respectively. R.J.P.A. and E.S.M.C. thank CNPq and Fapema for the PhD scholarships, respectively.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Footnotes

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Figures and Tables

Figure 1 XRD patterns of LDH, CQD, and LDH-CQD hybrid material.

Figure 2 FTIR spectra for LDH, CQD, and LDH-CQD hybrid material.

Figure 3 (a) DSC curves of the synthesized materials, and TG-DTG curves of (b) LDH, (c) CQD and (d) LDH-CQD samples.

Figure 4 SEM images of (a) LDH and (b) LDH-CQD materials, and TEM images of pristine LDH (c,d) and LDH-CQD hybrid (e,f).

Figure 5 N₂ adsorption–desorption behavior of LDH and LDH-CQD materials.

Figure 6 pH_PZC analysis of the LDH and LDH-CQD materials.

Figure 7 Influence of contact time and initial concentration (a) 5 ppm, (b) 10 ppm, (c) 15 ppm, (d) 20 ppm, and (e) 25 ppm on the adsorption capacity of IBU by Mg/Al-LDH and LDH-CQD hybrid material.

Figure 8 Effect of pH on IBU Removal Efficiency by Mg/Al-LDH and LDH-CQD hybrid material.

Figure 9 The adsorption kinetics of IBU on the LDH-CQD using the (a) pseudo-first-order model, (b) pseudo-second-order model, (c) Weber–Morris intraparticle diffusion kinetic model, and (d) Elovich model.

Figure 10 IBU adsorption isotherm on LDH-CQD: experimental points represented by experimental q_e (black) and curves fitted to theoretical adsorption models: Langmuir (red), Freundlich (blue), D-R (pink), and Sips (green).

Table 1

Parameters and coefficients derived from curve fitting of the kinetic models for the adsorption of IBU on the LDH-CQD adsorbent at pH 7.0 and room temperature.

Parameter/Model	Pseudo-First Order	Pseudo-Second Order	Intraparticle Diffusion	Elovich
qe experimental (mg.g−1)	22.1331
qe calculated (mg.g−1)	6.7865	22.2717	-	-
Constant k1 (min−1)	0.585	-	-	-
Constant k2 (g.mg−1.min−1)	-	0.0576	-	-
Coefficient of determination R2	0.4599	0.9978	0.5447	0.6084
Kdif (mg.g−1. min−0.5)	-	-	2.9362	-
C (mg.g−1)	-	-	7.6683	-
α (mg·g−1·min−1)	-	-	-	20.457
β (g·mg−1)	-	-	-	0.2029

Table 2

Comparison of the parameters of the models for IBU adsorption onto the LDH-CQD.

Model	Parameter	Values
Freundlich	KF [(mg.g−1)(L.mg−1)1/n)]	9.475
	n	1.531
	R$2$	0.843
	SSE	24.82
Langmuir	qmax (mg.g−1)	34.361
	KL (L.mg−1)	0.332
	R$2$	0.565
	SSE	28.97
Dubinin-Radushkevich	qmax (mg.g−1)	42.991
	β (mol2.J−2)	3779 × 10−8
	R$2$	0.089
	SSE	≥1000
Sips	qmax (mg.g−1)	24.150
	Ks (L.mg−1)	1.020
	n	1.740
	R$2$	0.993
	SSE	24.25

Table 3

Comparison of the maximum adsorptive capacity of different materials for IBU removal.

Adsorbent	qmax (mg.g−1)	RemovalEfficiency (%)	Contact Time(min)	Initial Conc.(ppm)	Reference
Green synthesized iron oxide (Fe2O3)	19.84	90.2	30	40	[61]
Graphene oxide nanoplatelets (GONPs)	3.72	98.2	180	6	[62]
Activated carbon impregnated with TiO2	16.68	92.0	240	25	[63]
MOF aerogel	5.96	99.3	720	3	[64]
LDH-CQD	27.03	88.53	40	25	This study

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15090899/s1, Figure S1. Raman spectrum of CQD; Figure S2. Rietveld refinement for the Mg/Al-LDH and LDH-CQD samples; Table S1. Lattice parameters, cell volume, and Rietveld refinement R-factors Rwp and goodness of fit (GOF); Figure S3. Residual scatter plot for the data generated from the (a) pseudo-first-order model, (b) pseudo-second-order model, (c) Weber–Morris intraparticle diffusion kinetic model, and (d) Elovich model; Figure S4. Regeneration and reuse efficiency of LDH-CQD hybrid material for IBU adsorption over four cycles.

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