Removal of Ibuprofen from Aqueous Solutions by Using Graphene Oxide@MgO

Abstract

In this study, a new composite adsorbent, namely magnesium oxide modified graphene oxide (hereafter abbreviated GO@MgO), was prepared for the removal of Ibuprofen (IBU), a non-steroidal anti-inflammatory drug (NSAID) compound. Graphene oxide was modified with MgO to improve its properties. Several factors important for the evolution of the adsorption process were investigated, such as the dose of the adsorbent, the pH, and the initial IBU content, as well as the duration of the procedure and temperature. According to the results obtained, it was found that at pH 3.0 ± 0.1, by applying 0.5 g/L GO@MgO to 100 mg/L IBU, more than 80% was removed, reaching 96.3% with the addition of 1.5 g/L adsorbent in 24 h. After 30 min, the equilibrium was reached (77% removal) by adding 0.5 g/L of GO@MgO. This study proves that GO@MgO is capable of economical and efficient adsorption. The IBU kinetic data followed the pseudo-second-order kinetic model. Langmuir and Freundlich isotherm models were used to interpret the adsorption, but the Freundlich model described the adsorption method more accurately. The positive values of ΔH⁰ (14.465 kJ/mol) confirm the endothermic nature of the adsorption. Due to the increase of ΔG⁰ values with temperature, the adsorption of IBU on GO@MgO is considered to be spontaneous.

1. Introduction

Water pollution is a growing environmental concern [1]. Wastewater, produced either by industry or by human activity, is one of the main sources of toxic and harmful compounds for the aquatic environment, with emerging pollutants being a particular challenge. The complex interactions that take place in the dissolved form allow these chemicals to persevere for a longer period. Pharmaceuticals, pesticides, and several personal care products are recognized as emerging pollutants that are continuously detected, even in drinking water [2]. These products can enter the water receivers through domestic, industrial, and hospital wastewater, resulting in impacts on water quality and the aquatic ecosystem in general [3]. Therefore, they can seriously threaten both human health and the environment [4].

Ibuprofen (IBU) (is 2-[4-(2-methylpropyl) phenyl] propanoic acid) (Figure 1), a widely consumed non-steroidal medicinal product used to reduce pain and fever and has an anti-inflammatory effect, is an antacid and is classified as a non-steroidal anti-inflammatory drug (NSAID). It is a well-known fact that the COVID-19 pandemic has led to overconsumption of NSAIDs, among them IBU, to reduce the various symptoms of the virus. Primarily, IBU is used to treat a variety of diseases and musculoskeletal pain for all age groups. IBU is easily solvable in organic solvents and is slightly soluble in water; however, it has high mobility in aqueous environments [2,5]. It has been detected in sewage and rivers in many countries and in increasing concentrations in sewage treatment plants [3].

Regarding the management and reduction of IBU, advanced oxidation processes (such as photocatalysis [6], Fenton reaction [7], ozonation [8], etc.), advanced treatments (such as nanofiltration [9], membrane bioreactor [10], etc.) and hybrid technologies are effective, but also costly [11]. Nanofiltration is an effective option for pharmaceutical contaminants with low energy requirements. However, membranes are disposed to fouling [12]. Advanced oxidation processes are remarkably effective on highly recalcitrant pollutants. However, the high investment, operating and energy costs of operation, and the formation of toxic oxidizing radicals (hydroxyl or sulfate) during the treatment are significant disadvantages [13]. Catalytic membrane reactors were also found to be efficient for the degradation of emerging organic pharmaceutical pollutants [14]. However, the high cost of electrodes limits large-scale application [15,16].

Adsorption is an efficient, low-cost, and simple-to-operate technology. It can deal with various pollutant types that follow a fast kinetic process [3]. Another advantage of this technology concerns adsorbents since wastes that have no, or if they do have very little, potential economic value can be used, as well as low-cost, renewable, and sustainable materials. Therefore, the application of adsorption to mitigate IBU gains a significant advantage compared to other treatment techniques [16]. Several advanced adsorbents are used to remove IBU from aquatic environments. Carbon-based materials, like activated carbon [17,18] and graphene-based materials [2,19], are highly effective. Metal–organic frameworks (MOFs) [20,21] are promising because of their high surface areas and properties. Also, natural or modified clays [22,23] and polymers [24,25] can be used for IBU adsorption. Last but not least, biosorbents [26] are a special category, including materials resulting from natural sources, such as agricultural waste.

Graphene-based materials, such as graphene oxide (GO), have garnered significant attention as an adsorbent for the management of emergent water pollutants due to their notable properties and versatility [27]. GO has a large surface area, which provides abundant active sites for chemical reactions [28]. The presence of various oxygen-containing functional groups (e.g., hydroxyl, epoxy, and carboxyl groups) enhances its ability to interact with different pollutants [29]. One of its key advantages is its ability to effectively remove a wide range of pollutants, among them heavy metals, organic compounds, and emerging contaminants such as pharmaceuticals [30]. It is a stable material with excellent mechanical strength, making it durable (e.g., in harsh conditions, high pressures, and temperatures) and reusable in multiple adsorption cycles [31]. Another advantage of using graphene oxide is the fact that it can also be produced from abundant cheap starting materials (thereby reducing costs), as well as being easily scaled up for industrial use [32,33]. However, in addition to the benefits of applying graphene oxide in water treatment, some disadvantages should be mentioned, such as the possible release of graphene oxide into water and the high production cost, as well as the large-scale fabrication of GO nanomaterials that have not yet been established. Therefore, further research is needed to improve or make the use of graphene oxide in water treatment feasible.

To improve its properties in addition to adsorption capacity and selectivity, GO can be modified to target specific pollutants [34]. Modification of GO with metals can further improve its adsorption properties. Metal nanoparticles can increase the adsorption capacity of GO by providing additional active sites. Also, both its chemical and thermal stability and its catalytic properties can be improved. Therefore, the application of graphene oxide for the remediation of emerging pollutants is a promising, cost-effective, and efficient solution [32]. The application of nanosized metal oxides has been widely explored in processes such as adsorption and catalysis. Magnesium oxide (MgO) is a cheap, stable, non-toxic material that is easily prepared [35]. A major drawback of self-contained nanoparticles is their tendency to combine due to high surface energy, resulting in reduced adsorption efficiency. However, by choosing a suitable substrate, such as GO, aggregation can be effectively inhibited. The GO–metal oxides combination has been found to be effective for water purification, energy storage, and antibacterial applications [34]. The modification of GO with MgO can significantly improve its adsorbent properties by creating a material that takes advantage of the large surface area and functional groups of GO, and the high adsorption capacity of MgO. For example, MgO/GO was found to be particularly effective in removing pollutants such as Congo red from water due to its fast adsorption kinetics and high adsorption capacity [35,36]. In addition, one of the advantages of using MgO nanoparticles is their strong antibacterial activity against pathogens, which can consequently minimize the emergence of antibiotic-resistant strains, thereby enhancing both the removal of environmental pollution and the improvement of human health [37]. Therefore, it can be used as a good antibacterial agent as well as an adsorbent for the removal of pharmaceuticals in wastewater treatment plants and environmental remediation. Thus, considering the synergistic advantage of MgO and GO, their combination may present a potential adsorbent for IBU removal [4].

This study describes the synthesis and application of a new composite material, magnesium-modified graphene oxide (abbreviated hereafter GO@MgO), for the removal of IBU, one of the most common NSAIDs found generally in pharmaceutical wastewaters. Graphene oxide was synthesized in the laboratory, and it was subsequently modified with MgO. GO@MgO is a promising innovative material, which, after research of the literature, has not been applied to the adsorption of pharmaceuticals. Several parameters were examined for the evaluation of GO@MgO, such as the adsorbent dosage, the pH of the solution tested, the initial IBU concentration, the contact time, and the temperature. Characterization was performed by SEM images and FTIR spectroscopy. This study acquires additional potential given the impressive reusability of the prepared graphene oxide composites. In addition, classical applied kinetic and isotherm models were applied to estimate the adsorption process.

2. Materials and Methods

2.1. Materials

For the preparation of the IBU stock solution, 95% denatured ethanol was added to IBU, which was purchased from Sigma-Aldrich, Merck KGaA, Darmstadt, Germany. It should be noted that all aqueous solutions were prepared with deionized water. MgO (PMS2 pure magnesia, 96%, MW = 40.30 g/mol) was used for the modification of graphene oxide and for pH adjustment several appropriate concentrations (0.01–0.1 M) of HCl (37% HCl (Panreac, AppliChem, Barcelona, Spain)) and NaOH solutions (≥97.0% ACS NaOH pellets (Sigma-Aldrich, Merck KGaA, Darmstadt, Germany)) were used. Finally, graphite flakes, purchased from Sigma Aldrich, Merck KGaA, Darmstadt, Germany, were used for the preparation of graphene oxide.

2.2. Synthesis of GO@MgO

The GO used in this research has been synthesized and used in earlier research by the authors [38], conferring to the modified Hummer’s method [39], as enhanced by Debnath et al. (2014) [40]. For the coating of GO with MgO a simplified route was followed [4,41]. Briefly, the GO@MgO composite was obtained after adding 1 g GO and 1 g MgO (1:1) in a beaker. They were stirred in 500 mL of deionized water for 2 h at 298 K and sonicated. Then, they were subsequently filtered and washed using distilled water. Finally, the produced adsorbent was cooled to room temperature to be ready for use in the following experiments.

2.3. Analytical Determinations

The concentration of IBU was calculated by using a UV–Vis spectrophotometer (WTW Spectroflex 6100, Weilheim, Germany) 222 nm [42] by relating the absorbance to the IBU standard calibration curve.

2.4. Adsorption Experiments

Several tests were conducted to investigate the effectiveness of GO@MgO for IBU adsorption. First, 10 mL of IBU solution was added to falcon tubes with the appropriate amount of adsorbent at different initial concentrations with constant temperature. They were then placed on a Trayster overhead shaker (IKA-Werke GmbH & Co. KG/ Staufen im Breisgau, Germany) and Loopster rotator (LLG/Germany) at 80 rpm. During the experimental procedure, various experimental parameters were operated independently, keeping others constant, such as pH levels (2 to 10 ± 0.1), initial IBU concentrations (20 to 500 mg/L), adsorbent’s dose (0.2 to 1.5 g/L) and contact time (2 to 180 min for kinetics and up to 24 h when aiming to reach equilibrium). Finally, the samples were collected and filtered with a 0.45 µm nylon filter (Membrane Solutions, LLC, Auburn, WA, USA) for the following determinations. The results are the average of three experimental trials, and the rate of IBU removal, expressed as R, %, is given by the following equation (Equation (1)):

$$R \left(\%\right)=\left({\displaystyle \frac{C_0-C_f}{C_0}}\right)\times 100\mathrm{\%}$$

(1)

where C₀ = initial IBU concentration (mg/L), C_f = final IBU concentration after the experiment (mg/L).

For adsorption capacity determination of GO@MgO, Q_e (mg/g), the following Equation (2) was used:

$$Q_e={\displaystyle \frac{(C_0-C_e)\times V}{m}}$$

(2)

where C_e = IBU concentration (mg/L) at equilibrium, V = volume (L), and m = adsorbent’s mass (g).

2.4.1. Equilibrium Experiments

To comprehend the adsorption mechanism and the relationship between the concentrations of IBU and the adsorption capacity of the adsorbent, the adsorption isotherms are carried out. The Langmuir and Freundlich models were applied in this study. The Langmuir model describes monolayer adsorption as the number of available adsorption sites on the adsorbent surface being specific and the energy of adsorption being constant. This means that each position can be occupied by only one molecule without interaction between the adsorbed molecules [43,44]. Essentially, the adsorbent has a finite adsorption capacity (Q_m), i.e., the maximum amount that the adsorbent surface can adsorb under equilibrium conditions.

A fixed amount of GO@MgO adsorbent (g) was added to 15 mL falcon tubes, followed by 10 mL of IBU solution (20 to 500 mg/L), to perform equilibrium experiments on the isotherms. Langmuir and Freundlich isotherm models were applied to evaluate the results. The following Equation (3) is related to the Langmuir model [45]:

$$Q_e={\displaystyle \frac{Q_m{K}_L{C}_e}{1+K_L{C}_e}}$$

(3)

where Q_e = concentration of the adsorbate in the solid phase to the concentration in the liquid phase at equilibrium (mg/g), Q_m = maximum adsorption capacity, which is the theoretical monolayer capacity, (mg/g), and K_L = Langmuir adsorption equilibrium constant regarding the relative energy for IBU adsorption (L/mg).

The Freundlich model [46] describes both monolayer and multilayer adsorption on heterogeneous surfaces. It defines the relationship between the equilibrium IBU concentration (mg/L) and the relative adsorption capacity of the adsorbent, Q_e (mg/g), expressed as Equation (4).

$$Q_e=K_F{C}_e^{1/n}$$

(4)

where K_F = Freundlich adsorption constant that refers to the adsorption capacity (mg/g)(L/mg)^1/n, 1/n is a constant that pertains to the adsorption ability or the degree of surface heterogeneity.

2.4.2. Kinetics Experiments

In order to investigate the mechanism and the effect of contact time on the adsorption process, the pseudo-first-order (PFO) (Equation (5)) and pseudo-second-order (PSO) (Equation (6)) kinetic models were used. The resulting values contribute to a better understanding of the IBU adsorption process.

$$Q_t=Q_e(1-e^{-k_{1}t})$$

(5)

$$Q_t={\displaystyle \frac{k_2{Q}_e^{2}t}{1+k_2{Q}_{e}t}}$$

(6)

where the variables Q_t and Q_e show the quantity of IBU adsorbed (mg/g) for time t (min) and for equilibrium, individually. The rate constants k₁ (1/min) and k₂ (g/mg min) denote the rate of adsorption for the PFO and PSO models. t is for contact time measured in min.

2.5. Thermodynamics

The calculation of three thermodynamic factors, namely the Gibbs free energy change (ΔG⁰, kJ/mol), the entropy change (ΔS⁰, kJ/mol·K) and the enthalpy change (ΔH⁰, kJ/mol), at various temperatures (298, 308, 318 and 338 K) is required for the thermodynamic analysis and for a better understanding of the adsorption process. These factors are calculated with Equations (7)–(9) [47].

$${{\mathsf{\Delta }G}^0=\mathsf{\Delta }H}^0-T{\mathsf{\Delta }S}^0$$

(7)

$${\mathsf{\Delta }G}^0=-RT\mathrm{l}\mathrm{n}\left(K_c\right)$$

(8)

$$K_c={\displaystyle \frac{C_s}{C_e}}$$

(9)

For ΔG⁰ values, Equation (8) was used, and ΔH⁰ and ΔS⁰ were specified by analyzing the slope and intercept taken from the resulting graph by plotting ln(K_c) versus 1/T, according to Equation (10).

$${\mathrm{l}\mathrm{n}(K}_c)=\left(-{\displaystyle \frac{{\mathsf{\Delta }H}^0}{R}}\right)+{\displaystyle \frac{{\mathsf{\Delta }S}^0}{R}}$$

(10)

where R is the universal gas constant (8.314 J/(mol K)).

2.6. Characterization Techniques

Scanning Electron Microscopy (SEM) (Jeol JSM-6390 LV, Japan scanning electron microscope, JEOL Ltd., Akishima, Tokyo, Japan) and Fourier Transform Infrared Spectroscopy (FT-IR, Perkin Elmer, New York, NY, USA) were used for the characterization of GO@MgO surface.

2.7. Regeneration Study

Desorption of GO@MgO was performed by using 0.1 M NaOH as the regenerant [48,49]. The IBU-saturated GO@MgO was suspended in 40 mL of 0.1 M NaOH solution and shaken at 80 rpm for a specific time and pH, resulting from the following experiments, for desorption of IBU by GO@MgO. GO@MgO was then separated from the NaOH solution by using a membrane filter. The filtered GO@MgO was then heated in an oven at 80 °C for 3 h to be reused for the adsorption of IBU. After each run, the IBU concentration was determined. Following the above procedure, the recycled GO@MgO was reused for up to six cycles.

2.8. Stability Study

To evaluate pH stability, GO@MgO was immersed in 40 mL of aqueous solutions on falcon tubes, with varying pH between 2–9 ± 0.1 for 24 h. The residual mass was determined after drying and weighted.

3. Results and Discussion

3.1. Characterization of GO@MgO

According to SEM images, as illustrated in Figure 2a, the resulting surface of GO@MgO presents a smooth surface with wrinkles, which is attributed to the GO structure. MgO nanoparticles appear to be embedded in the GO matrix [50]. Figure 2b presents the surface of the GO@MgO after adsorption. It appears to retain its integrity, having no cracks, suggesting its mechanical stability. Figure 2c shows the FTIR spectra for GO, GO@MgO, and GO@MgO-IBU. For GO, the standard peak at 3300–3500 cm⁻¹ corresponds to the stretching vibrations of O–H, whereas the peak at around 1630 cm⁻¹ shows the existence of C=C groups [51]. Finally, the peaks at 1320 cm⁻¹ and 1020 cm⁻¹ correspond to the C–OH and C–O–C stretching vibrations of the sp² carbon skeleton, respectively [52]. No further peaks were established when MgO was introduced to the GO surface [4]. However, the strength of these peaks reduces, which might be attributed to the interaction between MgO and GO. Furthermore, the O–H peak appears to have been decreased the most, implying that MgO connects to the hydroxyl groups on the surface of GO [53]. After IBU adsorption on GO@MgO, no new peaks appear; just shifting of existing peaks. In general, this suggests that no new chemical bonds are formed during the adsorption process. Instead, peak position alterations indicate interactions such as hydrogen bonding, van der Waals forces, or electrostatic interactions with the adsorbate.

In

Table 1. Elemental composition of GO@MgO according to SEM/EDS analysis.

% w/w	GO@MgO
Au 1	39.85
C	34.44
O	13.61
Mg	6.40
Re	2.60
Zn	1.04
Cu	2.06

¹ Au is used for sample spattering.

, the SEM-EDS analysis of GO@MgO is displayed. The detection of oxygen (O) and magnesium (Mg) provides evidence of MgO deposition on the GO surface. GO nanosheets were assigned the carbon (C) peak. The EDS analysis’s percentage (w/w) values show that magnesium is present on the composite’s surface, with Mg making up 6.40% (w/w) of the GO@MgO structure.

3.2. Effect of Initial pH Solution

The effect of pH was examined in the pH range 2.0–9.0 ± 0.1 with a constant adsorbent dosage of 0.5 g/L, an initial IBU concentration of 100 mg/L, T = 298 K, and 24 h. As shown in Figure 3, acidic conditions favor the removal of IBU. Specifically, the highest percentage of removal was observed at pH = 3.0 ± 0.1 (82.7%) by using GO@MgO adsorbent, and pH = 3.0 ± 0.1 is selected as the optimal pH for further experiments. However, at higher pH values, the removal rate decreased dramatically when GO@MgO was applied, but when net GO was used, the procedure was less pH dependent. In addition, it can be seen that the modification of GO with MgO enhanced the removal of IBU at pH ≤ 3 ± 0.1.

The point of zero charge (pH_pzc) of GO@MgO, and of GO for comparison reasons, which is the point at which its surface charge converts neutral, appeared by measuring it within a pH range of 2.0–10.0 ± 0.1. The pH_pzc of GO@MgO was determined by plotting a relative curve of ΔpH against the pH_initial using the pH shift method (Figure 4) over a pH range of 2.0–10.0 ± 0.1. For pH values lower than the point of zero charge (for GO@MgO was measured pH_pzc = 6.52), i.e., at the optimum pH 3.0 ± 0.1, the adsorbent surface is positively charged due to protonation. On the contrary, for higher pH values, the adsorbent turns negatively charged due to the detachment of the functional groups. The pKa of IBU is 4.91, which means that at pH = 3.0 ± 0.1, it is at cation form [54], and the surface of the adsorbent is positively charged. This indicates that there are repulsive forces between GO@MgO and IBU, and the adsorption mechanism involves weaker forces. These include H-bonding between the oxygenated groups of GO@MgO and the carboxyl group of IBU π–π stacking between IBU’s aromatic ring and the sp²-hybridized carbon network of GO@MgO, n→π* interactions between hydroxyl and carbonyl groups of GO@MgO and aromatic ring of IBU and Van der Waals forces. The latter agrees with the FTIR analysis.

3.3. Effect of Adsorbent’s Dosage

Considering that the dosage of the adsorbent is one of the key parameters affecting the adsorption processes, the dosage was studied in batch experiments to specify the ability of synthesized material to remove IBU. The initial concentration of IBU was 100 mg/L, while for GO@MgO and GO, different concentrations from 0.5–1.5 g/L (0.5 g/L as the initial value of the range was chosen from preliminary experiments not seen in this study) were used in acidic pH conditions, i.e., pH 3.0 ± 0.1, T = 298 K and 24 h. As shown in Figure 5, the IBU removal rate improved by increasing the adsorbent dosage from 82.7% (using 0.5 g/L) to 96.3% (using 1.5 g/L) of GO@MgO. The relative rates for using GO were 69.4 and 78.4%, for 0.5 and 1.5 g/L as a dose, respectively. Thus, the addition of MgO to the structure of GO seems to enhance its adsorption ability. Further, in Figure 5b, the variation of adsorption capacity with respect to adsorbent dose is shown. As can be seen, by increasing the adsorbent dose, the adsorption capacity does not increase proportionally to the % removal, and this is expected due to the lack of adsorbed substance [56,57]. Furthermore, as the contaminant concentration is held constant, increasing the dosage offers higher adsorption sites that cannot be occupied as there are no more IBU molecules. This occurs because the adsorptive capacity of the available adsorbent was not fully utilized at a higher adsorbent dosage than at a lower adsorbent dosage. Therefore, it might be possible that adsorption capacity decreases as adsorbent dosage increases, i.e., from 165 mg/g by adding 0.5 g/L of GO@MgO to 64 mg/g when 1.5 g/L was added. To conclude, 1.5 g/L GO@MgO may be the optimal dosage for almost total IBU removal, but 0.5 g/L (~83% removal) is selected as the dosage used in the following experiments in order to evaluate the adsorption capacity.

3.4. Effect of Contact Time

The influence of contact time on the adsorption of IBU on GO@MgO was investigated in comparison to GO, in the range of 2–1440 min, while the further parameters remaining constant (IBU C₀ 100 mg/L, dose 0.5 g/L, pH 3.0 ± 0.1, T = 298 K). Figure 6 shows that the removal percentage increased rapidly in the first 10 min, then it was slower. Specifically, more than 70% of the IBU was removed in the first 10 min when GO@MgO was used, probably due to the high number of existing adsorption sites at the beginning of the process until they became saturated [4]. After 30 min, the equilibrium was reached (77% removal), and that time was chosen as the optimal contact time for the adsorption of IBU on GO@MgO. On the other hand, when GO was used, the removal rate in the first 10 min was only 45%, and after 30 min, it reached 56%, again confirming the superiority of GO@MgO in the removal of IBU. For this reason, only the optimal GO@MgO material is selected for further study in the study of the kinetic and isotherm models.

3.5. Adsorption Kinetics

In this research, pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were examined to evaluate the kinetics of adsorption. As shown in Figure 7, the pseudo-second-order kinetic model was best fitted to the adsorption of IBU on GO@MgO with an initial IBU concentration of 100 mg/L at pH = 3 ± 0.1 by applying 0.5 g/L of adsorbent.

Furthermore,

Table 2. Constants of pseudo-first and pseudo-second-order kinetic parameters model for the adsorption of IBU onto GO@MgO (0.5 g/L), pH 3.0 ± 0.1, T = 298 K.

	Pseudo-First-Order Model (PFO)			Pseudo-Second-Order Model (PSO)
Qe.exp (mg/g)	k1 (1/min)	Qe.cal (mg/g)	R2	k2 (L/mg∙min)	Qe.cal (mg/g)	R2
165.3	0.2251	156.9	0.9963	0.004551	160.5	0.9998

gives the pseudo-second-order model constants calculated from Equation (6). The experimental adsorption capacity (Q_e.exp) was measured to be 165.3 mg/g, while the calculated adsorption capacity (Q_e.cal) for the PSO model was recorded as 160.5 mg/g and for the PFO model as 156.9 mg/g. The fit of the PSO model to the experimental data is verified by the correlation of these values, as well as by the high coefficient parameter determined (R² = 0.9998), as the majority of researched studies use R² as a validation method [58]. The PSO model has been applied successfully to determine chemisorption in several sorption systems [59]. In a study that prepared a MgO/graphene oxide (MgO/GO) nanocomposite for the removal of aqueous Congo red (CR) the PSO model yielded an R² of 0.9999, while the adsorption capacity was calculated to be 191.20 mg/g, good agreement with the experimental value of 190.96 mg/g [35] These confirm the excellent fit of adsorption in the PSO model, which implies that adsorption was mainly based on chemical interactions and that chemisorption was the rate-controlling step [35].

3.6. Adsorption Isotherms

Two isotherm models, Freundlich and Langmuir, which are the most applied in literature, were used for the IBU adsorption on GO@MgO. The relative graph is shown in Figure 8, where the corresponding initial concentrations are shown on the diagram. The isotherm parameters are calculated according to Equations (3) and (4) and presented in

Table 3. Langmuir and Freundlich isotherm models parameters for the adsorption of IBU onto GO@MgO (0.5 g/L), pH 3.0 ± 0.1, T = 298 K.

Langmuir Isotherm Model
Qm (mg/g)	KL (L/mg)	R2
1075	0.0052	0.9843
Freundlich isotherm model
1/n	KF (mg/g)(L/mg)1/n	R2
0.7246	12.9	0.9913

. Both the Langmuir and Freundlich isotherm models fitted the experimental data well, as indicated by the high R² values (0.9843 for Langmuir and 0.9913 for Freundlich). However, the Freundlich model has a slightly higher R² value, suggesting that it may describe the adsorption process more accurately. The maximum adsorption capacity (Q_m) for the Langmuir model is notably high, i.e., 1075 mg/g, indicating that GO@MgO has a high IBU adsorption capacity. The Freundlich constant (K_F) is 12.9 (mg/g)(L/mg)^1/n, indicating an efficient adsorption process. The value of 1/n is 0.7246, which is less than 1, indicating favorable adsorption conditions [60]. While both models are suitable, the higher R² value of the Freundlich model and the nature of its parameters suggest that it may be better suited to describe the adsorption behavior of IBU on GO@MgO under the given conditions. The Freundlich model assumes a heterogeneous surface with a non-uniform distribution over the surface. This suggests that the adsorption sites on GO@MgO have varying affinities for IBU molecules and that the adsorption energy decreases exponentially with the integration of the adsorption centers of GO@MgO adsorbent [61].

3.7. Thermodynamics Results

The values of ΔH⁰ and ΔS⁰ were obtained by analyzing the plot of ln(K_c) versus 1/T (with an R² value of 0.947, but the relative data is not displayed) [47]. Typical thermodynamic parameters during the adsorption process were evaluated. As ΔG⁰ is negative in all temperatures tested, i.e., −3.481, −4.094, and −4.706 kJ/mol for 293, 303, and 313 K, respectively, it is concluded that the adsorption of IBU by GO@MgO was spontaneous. The negative Gibbs free energy showed that the adsorption decreased with increasing temperature, suggesting that the spontaneous behavior of IBU adsorption changed inversely with temperature. Positive ΔS⁰ value (0.0612 kJ/mol∙K) indicated increasing disorder and chaos at the adsorbent/IBU interface [62]. The ΔH⁰ value for IBU was found to be 14.465 kJ/mol, indicating that the adsorption process was endothermic. As ΔH⁰ was positive and the adsorption of IBU was an endothermic reaction, it is possible that an increase in temperature promoted the adsorption reaction [36], as also seen in Figure 9 regarding the effect of temperature. Particularly, at 293 K, the removal rate was ~77%, and at 313 K, it reached ~83% in 30 min.

3.8. Regeneration Study Results

Regeneration is an important aspect that determines the reusability and efficiency of adsorbents [63]. The reusability of GO@MgO for the adsorption of IBU was examined through cycling experiments, with an initial concentration of 100 mg/L IBU and 0.5 g/L adsorbent at pH 3.0 ± 0.1. After the first cycle, 0.1 M NaOH solution was added to the used GO@MgO for 30 min (adsorption pH = 3 ± 0.1, desorption pH = 10 ± 0.1). It was then washed with distilled water to remove any residual base. The process was repeated for the following six cycles. The relevant results are illustrated in Figure 10a. The desorbed GO@MgO could be reused for IBU removal for six cycles after fruitful regeneration by 0.1 M NaOH treatment. In the first cycle, the IBU removal rate was about 77.1, while it reached about 43.2% after the sixth cycle. Therefore, the reusability of GO@MgO as an adsorbent for six cycles for the removal of IBU is confirmed in this study. Moreover, in up to four cycles of regeneration, the material shows only a 15% reduction in its effectiveness. After the fifth cycle, the adsorbent pores may become blocked and there may be a loss of active sites [64]. In this study, six adsorption–desorption cycles were examined, and after this cycle, a decrease in adsorbent capacity was observed. According to Figure 10b, the adsorbent had minimal weight loss for the first four cycles, which increased significantly after six regeneration cycles. As a result, the decrease in adsorption performance could be attributed to adsorbent loss during the processes, and this could be a limitation for the use of GO@MgO.

3.9. Stability

Stability studies were performed to investigate the adsorbent’s structural stability in various pH environments. Adsorption and reusability processes rely heavily on pH stability. To evaluate pH stability, GO@MgO was immersed in aqueous solutions with varying pH (2–9 ± 0.1) for 24 h, and residual mass was determined after drying. Figure 11 shows that the adsorbent is highly resilient at neutral pH (5–7 ± 0.1). At severe pH levels, it maintains its stability, with weight losses of just 4.3% and 1.7% at pH = 2 ± 0.1 and pH = 9 ± 0.1, respectively. As a result, it is obvious that solution pH levels have no effect on adsorbent stability.

3.10. Comparison with Other MgO/Graphene Oxide Nanocomposite

Table 4. A comparison of GO@MgO with other modified MgO graphene oxide materials was found in the literature.

Composite	Pollutant	pH	R%	Qm (mg/g)	Ref.
MgO/GO	Congo red (CR)	2.0	97.8	684	[35]
Flower-like MgO-GO microspheres	Congo red (CR)	6.0–7.0	n/a*	237	[66]
MGC	Indigo carmine (IC)	4.0	99.9	252	[65]
MGC	Orange G (OG)	4.0	86.0	24	[65]
GO/MgO NCs	Methylene Blue (MB)	11.0	99.9	833	[25]
GOMO	Lead Pb(II)	6.5	n/a*	190	[67]
MgO NPs	Diclofenac (DCF)	7.0	85.0	66	[64]
GO@MgO	Ibuprofen (IBU)	3.0	96.3	1075	This study

* n/a = not available data.

provides a comparison between GO@MgO, presented in this study, and other MgO/graphene oxide nanocomposites found in the recent literature for removing several pollutants from aqueous solutions. As can be seen, modified with MgO graphene oxide materials are mainly used effectively for dye removal [4,35,65,66]. On the contrary, there are only a few cases where such composite materials were applied for the removal of metal ions [67] in the recent literature or rarely for the removal of pharmaceuticals. For the removal of diclofenac (DCF), another common NSAID, activated MgO nanoparticles, were applied, but the removal rate was lower than the carbon-based materials. Therefore, the proposed material of this research, GO@MgO, besides being effective, seems to be innovative as it was not found to be used in the literature for the removal of pharmaceuticals, i.e., IBU, from aqueous solutions.

4. Conclusions

In this study, graphene oxide modified with magnesium oxide (GO@MgO) was investigated for the adsorption of IBU. The results showed that the modification of GO with MgO enhanced the removal of IBU. At pH 3.0 ± 0.1, a removal rate of 82.7% was accomplished by adding 0.5 g/L GO@MgO, while quantitative removal was achieved with the addition of 1.5 g/L after 24 h. After 30 min, the equilibrium was reached (77% removal by adding 0.5 g/L of GO@MgO). For pH values < pH_pzc (6.52), the GO@MgO surface becomes positively charged, and thus, there are repulsive forces between the positively charged surface of adsorbents and the protonated functional groups of IBU. The Freundlich isotherm and PSO kinetic models were found to best fit the adsorption process, with R² values of 0.9913 and 0.9998, respectively, demonstrating that the adsorption of IBU on GO@MgO matches the mechanism of chemisorption. According to the kinetics, the equilibrium was reached at 30 min. Thermodynamics showed that the adsorption was endothermic in nature (ΔH⁰ = 14,465 kJ/mol) and spontaneous (ΔG⁰ < 0) at all temperatures. The positive value of ΔS⁰ (0.0612 kJ/mol∙K) confirmed the increase in the random interaction between the solid–liquid interface. In conclusion, GO@MgO as an adsorbent was effectively regenerated and reused for six cycles to remove IBU from pharmaceutical wastewater.