Adsorption of Pharmaceutical Compounds from Water on Chitosan/Glutaraldehyde Hydrogels: Theoretical and Experimental Analysis

Abstract

Chitosan-based hydrogels are used in the adsorption of pharmaceutical compounds from water. The adsorption process of diclofenac and naproxen on chitosan hydrogels cross-linked with glutaraldehyde has been studied theoretically and experimentally. According to the thermodynamic properties, the adsorption processes were spontaneous and endothermic, due to the negative values of Gibbs free energy, and the enthalpies of formation were positive. Furthermore, the different systems were studied by electrostatic potential maps, where the functional groups (amino and hydroxyl) represented the active sites of the hydrogel. The maximum adsorption capacity obtained for diclofenac and naproxen was 108.85 and 97.22 mg/g, respectively, at a temperature of 308.15 K. On the other hand, the adsorbent was characterized by FTIR (Fourier Transform Infrared Spectroscopy) and XRD (X-ray Diffraction) before and after the adsorption of the drugs to confirm the binding of the adsorbates on the surface of the material.

1. Introduction

Water represents an important factor for human, animal, and plant life. However, factors such as industrial, agricultural, livestock, urban development, and climate change generate greater pollution in rivers, lakes, lagoons, or oceans. Around 80% of wastewater from industry and municipalities cause damage to the environment and human life [1,2,3]. There are different techniques for removing contaminants from water, but adsorption is a widely used physical method for monitoring and separating environmental contaminants, where the contaminant is adsorbed onto the porous surface of an adsorbent material [4,5]. The adsorption processes depend on factors such as the structures of the hydrogels and the nature of the contaminants [6,7]. The adsorption process allows the analysis of various contaminants such as drugs, nitrates, phosphates, plastics, and fertilizers [4,5].

Hydrogels are three-dimensional cross-linked structures insoluble in polar solvents such as water, with properties of biodegradability, biocompatibility, and responsiveness. There are two types of hydrogels: (a) physical, which are characterized by hydrogen bonding, ion exchange, and hydrophobic interactions, and (b) chemical, with covalent bonds. In addition, the functional groups present in hydrogels increase the swelling capacity up to 400 times their original weight [8]. Hydrogel synthesis uses synthetic or natural polymers. However, hydrogels based on natural polymers such as chitosan and dextran have better biological and chemical properties [9,10].

Chitosan is a natural biopolymer—cationic, biodegradable, and biocompatible [11,12]. Due to its properties, chitosan is widely used in the adsorption processes of heavy metals, pesticides, pharmaceuticals, chemical compounds, dyes, etc. [13,14] Chitosan has many functional groups that can interact with pharmaceutical compounds in an adsorption process. Therefore, its adsorption capacities are significantly higher compared to other polysaccharides such as cellulose and starch, because these two polysaccharides have fewer reactive functional groups and lower mechanical strength, resulting in lower adsorption capacities [15,16,17,18].

On the other hand, pharmaceutical compounds are structures with specific functionalities and physicochemical and biological characteristics; they are polar and have molecular weights between 200 and 1000 Da [19,20]. Pharmaceutical compounds are found in the water due to various situations. Pharmaceutical compounds are observed in wastewater because treatment plants do not have the capacity to remove them from effluents [21,22]. On the other hand, rainwater, farms, yard waste, and septic tanks contribute to the presence of pharmaceutical compounds (see

Table 1. Pharmaceutical compounds are found in wastewater.

Pharmaceutical	Water Source	Concentration (ng/L)	Reference
Ampicillin	Industrial effluents	5.8 × 103	[25]
Caffeine	Urban effluents	23–776	[26]
	Surface water	2.9–194	[26]
	WWTP influents	2448–4865	[27]
Diclofenac	Urban effluents	8.8–127	[26]
	Surface water	1.1–6.8	[26]
	WWTP influents	9–13	[27]
	River	21–98	[28]
Ibuprofen	Urban effluents	10–137	[26]
	Surface water	11–38	[26]
	River	35–270	[28]
Ketoprofen	Hospital effluents	1034.5	[29]
	WWTP influents	24–177	[29]
	River	17–620	[30]
Naproxen	Urban effluents	20–483	[26]
	Surface water	1.8–18	[26]
	River	81–360	[28]
Salicylic acid	WWTP influents	433–8036	[27]
Tetracycline	Industrial effluents	1.5 × 103	[25]

) [23,24].

In recent years, different technologies related to the removal of pharmaceutical contaminants have been evaluated, including biological methods [31,32], advanced oxidation [33,34], photodegradation [35,36], ozonation [37], adsorption [38,39], and others.

Currently, the adsorption process is of greater importance in the removal of water contaminants because it is an economical process, allows on-line operation, produces no sludge, and enables reusability [40,41]. The use of various adsorbents derived from biopolymers has received great attention [42,43]. Chitosan has been used for the adsorption of pharmaceutical products (Figure 1, Figure S1 and S2) [44,45,46,47].

Computational modeling involves the use of computers to analyze molecules or macromolecules using mathematics, physics, chemistry, and computer science to obtain different energies, thermodynamic and QSAR properties, HOMO/LUMO orbitals, FTIR, and electronic distribution, using semi-empirical methods such as AM1 to obtain new materials [15]. QSAR properties are based on molecular and mathematical descriptors to predict the relationship between biological activity and chemical structure of the molecule or macromolecule. In addition, hybrid methods of molecular mechanics and quantum mechanics determine the binding affinity and capacity of a molecule [16,48].

As can be seen, recent studies have demonstrated the efficacy of chitosan-based hydrogels for removing pharmaceutical pollutants from water. However, most prior work has been limited to single-compound adsorption experiments and conventional isotherm/kinetic analyses, providing little insight into molecular-level interaction mechanisms or multi-contaminant scenarios. The study of multicomponent systems is crucial to subsequently compare with realistic wastewater systems. On the other hand, computational approaches (e.g., QSAR modeling or molecular simulations) are seldom integrated into these adsorption studies, leaving a knowledge gap in understanding how pharmaceutical molecules bind to the adsorbent and how adsorbent structure influences performance. In this context, the present study offers a novel integrated experimental–theoretical approach for diclofenac and naproxen uptake on a glutaraldehyde-cross-linked chitosan hydrogel.

This work combines advanced computational chemistry tools—including geometry optimizations, QSAR property analysis, electrostatic potential mapping (EPM), and Monte Carlo simulations—with traditional batch adsorption experiments and sample characterization. This methodology provides a novel insight into the adsorption mechanism.

2. Materials and Methods

2.1. Computational Section

2.1.1. Geometry Optimization

Using HyperChem Professional 8.0v software, each molecular structure was drawn. Initially, the atoms of each molecule were drawn, and in the “Build” menu, the “Add H and Model Build” option was selected to complete the hydrogen atoms and spatially organize each molecule. In this study, a hybrid method between molecular mechanics (AMBER) and quantum mechanics (AM1) was used to geometrically optimize the analyzed molecular structures (naproxen and diclofenac showed in Figure 2) and to describe the potential energy of each system. The main objective of using this hybridization was to reduce the computational time of each molecule to calculate its minimum energy. For the AMBER method, the Polak–Ribiere algorithm was used with 450 iterations for each drug. For chitosan, the iterations were 3015. Then, the calculation was performed with the semi-empirical AM1 method without restrictions, using the conjugate gradient method and 450 iterations for the drugs and 3015 for chitosan.

2.1.2. Determination of QSAR and Thermodynamic Properties

In the “Compute” menu, the “QSAR Properties” option was selected, and the partition coefficient (LogP) was determined. Thermodynamic properties were obtained from the “Properties” option in the “Compute” tool, which was performed for each molecule.

2.1.3. Electrostatic Potential Map (EPM)

To determine the EPM, the option “Plot Molecular Graphs” was selected in the “Compute” menu.

2.1.4. Adsorption Process

Hyperchem Professional 8.0 was used to analyze the interactions between each nonsteroidal anti-inflammatory compound and glutaraldehyde cross-linked chitosan (Figure 3). Then, in the “Build” menu, the “Add H and Model Build” option was used to complete the hydrogen atoms.

2.2. Experimental Studies

2.2.1. Reagents

Chitosan (CAS-No: 9012-76-4) from shrimp shells, the pharmaceuticals naproxen and diclofenac with a purity of 98%, and the glutaraldehyde (25%) were obtained by Sigma-Aldrich (Sigma-Aldrich Química S. de RL. de C.V, Toluca, 50200, Mexico). Acetic acid (99.7%) from J. T. Baker and deionized water (DI water) were supplied by MERCK (MERCK Mexico, Naucalpan de Juárez, 53500, Mexico).

2.2.2. Hydrogel Synthesis

A total of 1.5 g of chitosan was dissolved in a 3% acetic acid solution. The chitosan colloid was magnetically stirred at 80 rpm for 180 min at a temperature of 50 °C.

Subsequently, a 10% glutaraldehyde solution was prepared and slowly added to the chitosan solution, this was mixed for another 2 h. A viscous solution with a pale-yellow color was obtained; this was rapidly centrifuged at 6000 rpm for 6 min to sediment the particles that remained suspended in the solution, and finally it was placed on Petri dishes to drain at 25 °C for 72 h to obtain chitosan films cross-linked with glutaraldehyde.

2.3. Characterization of Hydrogel

2.3.1. FTIR

The synthesized hydrogel was analyzed by Fourier transform infrared spectroscopy (FTIR) using a Nicolet iS10 infrared spectrophotometer from Thermo Scientific (Waltham, MA, USA). For the analysis, 32 scans were performed, and the spectra were collected in a wavenumber range of 4000–400 cm⁻¹.

2.3.2. XRD

XRD analysis was carried out using an Empyrean diffractometer from Malvern-Panalytical and PIXel ID detectors (Malvern Panalytical, 66269, N.L, Mexico). The analysis was performed at 45 kV and 40 mA.

2.4. Adsorption Studies of Pharmaceuticals

A 500 mL of solution was prepared with 250 mg/L of each pharmaceutical (diclofenac and naproxen) at pH 6. The adsorbate concentration varied from 20 to 250 mg/L and the hydrogel mass remained fixed at 0.02 g. The experiments were performed in batches using a volume of 10 mL at temperatures of 298.15K and 308.15 K, with stirring at 120 rpm for 24 h. After 24 h, the adsorbent was separated from the pharmaceutical solutions using filters, and samples were subsequently quantified by HPLC. To obtain the adsorption capacities, Equation (1) was used.

$$q_e={\displaystyle \frac{\left(Co-Ce\right)V}{m}}$$

(1)

where “$q_e$” is the adsorption capacity, “Co” and “Ce” are the initial and equilibrium concentrations of the pharmaceutical solutions, respectively, “V” is the volume used, and “m” is the mass of the adsorbent.

3. Results

3.1. Theoretical Studies

The energetic properties of diclofenac, naproxen, and the adsorption systems yielded negative results for the hydrogel–drug systems (see

Table 2. Energetic properties and QSAR at 298.15 K.

Energetic Properties	Diclofenac	Naproxen	Hydrogel–Diclofenac	Hydrogel–Naproxen
Gibbs free energy (kcal/mol)	−81,614	−67,954	−586,105	−572,472
Enthalpy of formation (kcal/mol)	−48	−100	1683	1721
Dipolar moment (Debyes)	1.79	2.21	9.72	16.38
QSAR Properties	Diclofenac	Naproxen	Hydrogel–Diclofenac	Hydrogel–Naproxen
Volume (Å3)	768	710	4018	3969
Surface area (Å2)	460	438	2027	2006
Log P	−0.21	−0.56	−14.34	−12.86
Polarizability (Å3)	29.69	25.32	160.68	156.31

), that is, the adsorption systems are spontaneous and endothermic.

The dipole moment is a measure of the separation of opposite electrical charges, so it is possible to find the nature (polar, nonpolar, or ionic) of any bond. In this case, glutaraldehyde is considered a polar molecule, meaning that the charge distribution in both pharmaceuticals and adsorption systems is asymmetric. Therefore, the electronic distribution will be oriented toward the most electronegative atom. This is because the value of this parameter was greater than 0 in all cases. The dipole moment is very important since it predicts an improvement in polarity and affinity in the union of the bonds of a molecule. A higher dipole moment reveals a greater hydrogen bond interaction and higher binding affinity [49].

The partition coefficient (Log P) is a parameter that determines whether a molecule has affinity with water (hydrophilic) or, conversely, repels water (hydrophobic). In addition, it indicates whether the studied molecule can be dissolved in polar solvents. All the studied systems presented negative Log P partition coefficient values (−0.21, −0.56, −14.34, and −12.86 for diclofenac, naproxen, hydrogel–diclofenac, and hydrogel–naproxen, respectively) (see Table 2), indicating a hydrophilic character. This negative value of the partition coefficient also shows that diclofenac and naproxen can be dissolved in polar solvents.

For diclofenac, the EPM obtained ranged from −0.085 to +0.328, with a negative electron-rich region marked in red (see Figure 4a), where the oxygens are located, which has the possibility of receiving an electrophilic attack due to its high electronegativity and the sp² hybridization in one of the oxygens of the carboxyl group. The green color showed a region of medium reactivity where the chlorines are attached to their respective carbon. The hydrogens are found in the positive region represented by blue, indicating the possibility of a nucleophilic attack due to the lower presence of electrons.

The electrostatic potential map (EPM) allows determination of the reactive zones that may exhibit electrophilic and nucleophilic charge in the calculated geometry. For the naproxen molecule, the EPM has a range from −0.138 to +0.472 (see Figure 4b). The regions with negative partial charge (red color) are located on the oxygen atoms of the carboxyl and ether groups, these are areas that can receive electrophilic attacks, while the region of positive partial charge was located on the hydrogen atom of the carboxyl group. Some hydrogen atoms from the methyl and aromatic groups were observed for nucleophilic attacks.

The electrostatic potential map of the interaction between the hydrogel and diclofenac had a range from −0.064 to +1.041 (see Figure 5). The amino and hydroxyl groups of the hydrogel are negatively charged (red areas) and can be exposed to electrophilic attacks. The neutral region (green area) was in the hydrogen atoms bonded to carbon along the chain. Finally, positive regions (blue and translucent areas) were observed in the carbon–hydrogen and nitrogen–hydrogen bonds, where nucleophilic attacks can occur. The diclofenac molecule presented a low electron density prone to electrophilic attacks (red areas) in the carbons and hydrogens of the aromatic group and the oxygens of the carboxyl group. Furthermore, neutral charge (green area) was observed on chlorine atoms, and a positive charge (blue areas) was found on hydrogen–carbon bonds, which are prone to nucleophilic attacks. The electrostatic potential map determined the possible reaction site for adsorption, where the carboxyl group of the diclofenac form hydrogen bonds with an amino group of the hydrogel.

Figure 6 shows the potential map of the interaction between hydrogel and naproxen. A high electron density was generally observed in the hydrogel in the oxygens and nitrogen of the amines (red regions); these areas are susceptible to electrophilic attacks. Likewise, a slightly positive zone was shown in the external hydrogens (blue region) with the possibility of receiving nucleophilic attacks. For naproxen, there is a negatively charged zone (red region) along its carbons and oxygens prone to receiving electrophilic attacks. Positive charges were found in its hydrogens (blue regions), capable of receiving nucleophilic attacks.

A hydrogen bond can be formed with the sp²-hybridizied oxygen of the carboxyl group of naproxen and a hydrogen bonded to an oxygen atom (OH) of the hydrogel.

3.2. Monte Carlo Simulation at 308.15 K

3.2.1. Hydrogel–Diclofenac

Table 3. Energetic properties and QSAR of hydrogel–diclofenac adsorption at several temperatures.

Energetic Properties	298.15 K	308.15 K
Gibbs free energy (kcal/mol)	−585,943	−585,950
Dipolar moment (Debyes)	11.89	12.3
QSAR Properties	298.15 K	308.15 K
$\mathrm{Volume}\left({\AA }^3\right)$	3989	3987
$\mathrm{Surface}\mathrm{area}\left({\AA }^2\right)$	1922	1928
Log P	−14.34	−14.34

shows the energetics and QSAR properties obtained by the Monte Carlo method. The Gibbs free energy was constant with the change in temperature, which means that the adsorption occurred stably and spontaneously. Moreover, the increase in temperature favors the dipole moment and promotes adsorption by increasing the intermolecular forces. The dipole moment is a measure of the distribution of electrical charges in a molecule and is related to polarity. The increase in the dipole moment can be related to the influence of temperature on the distribution of electrical charges. At higher temperatures, molecules tend to have greater thermal agitation, which can lead to changes in conformations and charge distribution. That is, at 308.15 K, a conformational change in the molecule or a redistribution of charges occurred, resulting in this notable increase in the dipole moment. This change was influenced by interactions between the different parts of the molecules, favoring their attraction, as well as by the intermolecular forces present in the environment.

Volume is a physical property related to the space occupied by a molecule, and surface area is related to the amount of space a molecule occupies on a surface. Both properties are affected by several factors, such as intermolecular interactions and temperature. The observed changes in the system’s volume and surface area could be due to interactions between diclofenac and hydrogel. On the other hand, OH stretching was observed between 3637 and 3382 cm⁻¹ (see

Table 4. FTIR vibrations of hydrogel–diclofenac at different temperatures.

Functional Group	298.15 K	308.15 K	Vibrational Mode
OH *	3637, 3577, 3375	3634, 3535, 3382	Stretching
C=O *	2057	2042	Stretching
C=O Δ	1979	1976	Stretching
C=N *	1961	2042	Stretching
NH2 *	1709	1727	Scissoring

* hydrogel, ^Δ diclofenac.

).

In addition, the carboxyl (diclofenac) and the carbonyl group (hydrogel) presented close stretching vibrations at the analyzed temperatures, suggesting the possibility that these groups are favorable sites for obtaining hydrogen bonds during the adsorption process. However, a shift was observed in the vibrations of the carbonyl group of the hydrogel due to the interaction of the adsorbed compounds and the hydrogel, which increased with temperature.

3.2.2. Hydrogel–Naproxen

The energetic characteristics at several temperatures for the removal of naproxen by the hydrogel (see

Table 5. Energetic properties and QSAR of hydrogel–naproxen adsorption at different temperatures.

Energetic Properties	298.15 K	308.15 K
Gibbs free energy (kcal/mol)	−572,290	−572,251
Dipolar moment (Debyes)	16.35	16.02
QSAR Properties	298.15 K	308.15 K
$\mathrm{Volume}\left({\AA }^3\right)$	3970	3978
$\mathrm{Surface}\mathrm{area}\left({\AA }^2\right)$	2002	2015
Log P	−12.86	−12.86

) showed stability in Gibbs free energy, indicating a spontaneous and stable process that can occur in nature. Naproxen presented higher dipole moment values compared to diclofenac, due to its interaction and affinity with the hydrogel during the adsorption process.

Table 6. FTIR vibrations of hydrogel–naproxen.

Functional Group	298.15 K	308.15 K	Vibrational Mode
OH *	3489, 3418, 3292	3470, 3407, 3288	Stretching
C=O Δ	2077	2164	Stretching
C=O *	1996, 1978	1964	Stretching
C=N *	1971	2036, 1876	Stretching
NH2 *	1713	1731	Scissoring
C-O *	1583	1593	Stretching

* hydrogel, ^Δ naproxen.

shows the main FTIR signals of the hydrogel/naproxen adsorption at 308.15 K, where peaks between 3489 and 3288 cm⁻¹ corresponded to the OH stretching. Carboxyl and carbonyl groups were observed between 2164 and 2077 cm⁻¹ and between 1593 and 1583 cm⁻¹, which allowed physical adsorption by hydrogen bonding. The shift in the carboxyl group bands at 308.15 K was attributed to the charge distribution of the molecule, which generated higher energy and increased its vibration.

3.3. Experimental Analysis

Prior to adsorption experiments, glutaraldehyde cross-linked chitosan hydrogel was analyzed by FTIR and XRD. Figure 7 shows the IR of hydrogel before the drug adsorption experiments. The broad band located between 3450 and 3200 cm⁻¹ was assigned to the O-H bond; the low intensity peaks located at 2950 and 2850 correspond to C-H (chitosan chain), and the peak at 1550 cm⁻¹ corresponded to the amino groups (NH). The band at 1400 cm⁻¹ is attributed to C-N bond of the hydrogel, and that at 1100–1000 cm⁻¹ corresponded to the glycosidic zones (chitosan) [50,51].

The XRD analysis before the adsorption experiments showed the following. The hydrogel presents two important zones, at 10°, which were assigned to the amorphous zone of the adsorbent, and at 20°, which corresponded to the crystalline zone, assigned to the inter- and intramolecular of the hydroxyl and amine bonds of the polymer chain (see Figure 8).

3.3.1. Adsorption Isotherms in a Single System

The isotherms of the individual experiments (Figure 9) indicated a maximum adsorption of 108.85 and 94.37 mg/g for diclofenac, and 97.22 and 82.90 mg/g for naproxen at 308.15 and 298.15 K, respectively. The results determined an endothermic process, that is, the removal of naproxen and diclofenac depends on temperature [52,53].

3.3.2. Adsorption Isotherms of Naproxen and Diclofenac in a Binary System

The isotherms indicated that the maximum adsorption was 96.29 and 83.78 mg/g for diclofenac and naproxen, respectively, at 308.15 K. On the other hand, the maximum capacities obtained at 298.15 K were 78.78 and 72.11 mg/g for diclofenac and naproxen, respectively (Figure 10). The pharmaceutical removal followed the trend diclofenac > naproxen under the indicated operating conditions. The adsorption capacities decreased slightly with respect to the single systems, which was attributed to competition for the amine and carboxyl groups.

3.3.3. FTIR After Adsorption Experiments

The spectra of the adsorbent after adsorption are observed in Figure 11b–d for naproxen, diclofenac, and the combination of both, respectively. Specifically, the area of 3450–3200 cm⁻¹ was assigned to the addition of naproxen/diclofenac to the hydrogel surface. Similarly, the peaks at 2950–2850 cm⁻¹ increased in intensity due to the conjugated double bonds of both pharmaceuticals. The bands between 1650 and 1750 cm⁻¹ corresponded to both diclofenac and naproxen, respectively [54].

3.3.4. XRD After Adsorption Experiments

Figure 12 shows a decrease in the signals of both regions of the hydrogel, which may be associated with the ionic interaction between the hydrogel groups and the COOH groups of the drugs [54].

4. Conclusions

Based on the results obtained, the hydrogel represents an excellent option for the adsorption of drugs. The adsorption capacities are significant, considering that in a real effluent, the concentration of contaminants can be lower, which ensures the complete removal of drugs from water. The characterization results confirm the presence of adsorbates on the hydrogel. In addition, different functional groups, mainly hydroxyl (OH) and amine (NH), contribute to the removal of drugs. The competition of drugs over the hydrogel in binary solutions is mainly due to the similarity between diclofenac and naproxen molecules, since they both contain a carboxyl group (COOH) and conjugated double-bond rings, which also interact with the hydrogel. Finally, these results demonstrate the potential of this hydrogel not only for drug removal but also in the removal of other types of contaminants.