UiO-67 Metal–Organic Framework as Advanced Adsorbent for Antiviral Drugs from Water Environment

Abstract

Metal–organic frameworks (MOFs) have attained significant usage as adsorbents for antiviral medicines in contemporary times. This study focused on synthesizing a UiO-67 metal–organic framework using the hydrothermal method. The synthesized framework was then characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analyses (TGA), and zeta potential measurements. The UiO-67 was then employed for the purpose of assessing the efficiency of various adsorption factors in the removal of antiviral medicines from aqueous solutions, including drugs such as ritonavir (RTV) and lopinavir (LPV), which were extensively used for the treatment of coronavirus (COVID-19) during the pandemic. The variables examined were the quantity of adsorbent used, different pH of the solution, temperature, and contact duration. The experimental findings indicate that the highest level of RTV elimination was 91.2% and of experimental adsorption capacity (q_e,exp) was 9.7 mg/g and for LPV this was 85.9%, and (q_e,exp) 8.9 mg/g, using 50 mg of UiO-67 at a pH 8, temperature of 298 K, and for 120 min. The impact of contact duration and temperature on the kinetics of adsorption was examined by employing pseudo-first-order and pseudo-second-order kinetic models. The experimental results showed a good match with the pseudo-second-order kinetic model with value of R² 0.99 and the q_e,calc was 9.7 RTV and 8.9 mg/g LPV, which is a good match with q_e,exp. Also, based on diffusion kinetic studies, the adsorption was confirmed to be catalytic in nature on the surface of the UiO-67 MOFs. A thermodynamic analysis of adsorption was conducted, whereby calculations for the Gibbs free energy change (∆G), enthalpy change (∆H), and entropy change (∆S) were performed. The positive ∆H values confirm the endothermic nature of the adsorption of RTV and LPV by UiO-67. The ΔG values exhibited negativity across all temperatures, suggesting the spontaneous nature of the adsorption process of RTV and LPV by UiO-67 from an aqueous solution. UiO-67 was shown to be highly effective in extracting RTV and LPV from real environmental samples.

1. Introduction

Over the past few decades, there has been a growing recognition of pharmaceutical compounds and their metabolites as a major source of water pollution, exerting a considerable influence on the worldwide environment [1,2]. The substantial utilization of some pharmaceutical compounds in recent years has resulted in an escalation of their levels in urban wastewater [3,4]. Among the widely used antivirals are ritonavir (RTV) are lopinavir (LPV) [5,6]. These pharmaceuticals have gained attention as emerging organic contaminants (EOCs) because of their recent utilization in the treatment of COVID-19 [7,8,9]. Also, LPV and RTV were detected in the wastewater effluent (41.0 ng/L and 4.8 ng/L), downstream of municipal wastewater treatment plants (31.5 ng/L and 5.8 ng/L), and surface water in Wuhan, China (4.7 ng/L and 4.2 ng/L) [10], and South Africa sewage treatment plants (maximum mean concentration 2.5 µg/L and 3.2 µg/L, respectively [11]. The existence of EOCs in water has been found to have adverse effects on the reproductive, neurological, and immunological systems of both aquatic organisms and human beings [12], especially RTV, and LPV [13,14]. The buildup of these medications in aquatic creatures presents a significant environmental concern, as indicated by previous studies [15,16]. Hence, the elimination of pharmaceutical compounds from water sources presents an imperative task, necessitating the exploration and implementation of alternative methodologies. There are many technologies used for the elimination of antiviral drugs such as RTV and LPV from aquatic environment, such as adsorption by sludge [17], photocatalysis [18], photolosis (UV) [19], membrane bioreactor and ozonation [20], electrochemical advanced oxidation process [21], and activated sludge [22]. The efficiency of adsorption technology in the removal of pharmaceutical pollutants from discarded water has been recognized, along with its economic advantages stemming from its efficient target absorption, high removal rate, and ease of operation [23,24]. Many adsorptive materials have been extensively studied for the purpose of pharmaceutical removal. These materials encompass silica-based materials [25,26], resins [27], metal oxides [28], activated carbon [29], and graphene oxide [30]. In recent times, there has been a growing interest in metal–organic frameworks (MOFs) as very promising adsorbents for liquid-phase adsorption [31,32,33,34]. MOFs are a highly sophisticated category of porous crystalline materials that exhibit intriguing structures and features. These include the presence of several aromatic organic ligands and Lewis’s acid sites, a large surface area, the ability for surface functionalization, and a diverse range of shapes and topologies [35,36,37].

In 1995, the first MOF with a three-dimensional open skeleton structure was developed by Yaghi et al. [38]. Since then, various excellent MOFs have been constructed in large quantities and there are various reports about water remediation by MOFs. Karimi and Namazi et al. (2024) utilized hydrogel beads of a magnetic graphene oxide@MIL-88 metal–organic framework (GO@Fe₃O₄@MIL-88@Alg) for the absorption of treatments used for COVID-19 such as doxycycline (DOX), hydroxychloroquine (HCQ), naproxen (NAP), and dipyrone (DIP) from aquatic environment [39]. Pharmaceutical drugs and personal care products (PPCPs) were treated by the manufacture of a Pd@ MIL-100 (Fe) nanocomposite [40]. The widespread use of MOFs is mostly related to the stability of the water in environmental remediation [41,42]. The University of Oslo (UiO) series, the Material of Institute Lavoisier (MIL) series, and the Zeolitic Imidazolate Framework (ZIF) series materials are common water-stable MOFs [38]. The MOFs of UiO-67 are a member of the UiO Zr-MOF family, which is characterized by its microporous nature [43,44,45,46]. This type of MOF has wide applications due to its thermal and chemical composition and mechanical stability [47,48,49,50].

The main objective of the current study is aquatic environmental sustainability through the remediation of water polluted with organic pollutants such as antiviral drugs. The novelty of the current study involved the synthesis of UiO-67 metal–organic frameworks using the hydrothermal technique and their application, for the first time, to remove RTV and LPV as antivirals drugs from aqueous solutions.

The morphological and structural characteristics of the synthesized UiO-67 will be investigated. The adsorption process will be studied with different parameters such as pH of the solution, different temperatures, varied UiO-67 mass, and contact duration. The adsorption process will be studied kinetically and thermodynamically. The treatment and removal of RTV and LPV from real water samples will also be explored by UiO-67 via an adsorption procedure.

2. Results and Discussion

2.1. Characterization of UiO-67

Solvothermal preparation was used to create the UiO-67. The powder XRD pattern in Figure 1A confirms that the synthesized materials display a typical cubic close-packed structure that is consistent with the work that has been previously reported [51,52]. A thermal gravimetric analysis (TGA) was conducted on a sample of UiO-67. As depicted in Figure 1B, the thermal gravimetric analysis (TGA) of the specimen exhibits three distinct phases: initial evaporation of the solvent at around 100 °C, subsequent removal of the compensatory agent at around 450 °C, and eventual disintegration of the framework within the temperature range of 450 °C to 600 °C. Considering the structural characteristics of the sample UiO-67, this study examines the percentage of mass loss (% ZrO₂) in the temperature range of 450 °C to 600 °C, which was around 65%. The same observations were reported previously for UiO-67 [53], UiO-66 MOFs [54], and zeolitic tetrazolate imidazole frameworks (ZTIFs) [55]. The UiO-67 material had an average zeta potential value of +20.8 mV, indicating a satisfactory level of stability Figure 1C in accordance with previous reported work [56]. A higher magnitude of zeta potential signifies enhanced physical colloidal stability as a result of electrostatic repulsion among the constituent particles. In the N₂ adsorption–desorption isotherms shown in Figure 1D, UiO-67 demonstrates a type-I isotherm, indicating that it is a characteristic microporous material [57], with a BET specific surface area of 1415 m²/g, whereas the average pore width and the pore volume of the UiO-67 channels were 2.06 nm and 0.801 cm³/g, respectively, which agreed well with previous reported value for different MOFs such as UiO-67 [56], MIL-53(Al) MOFs modified with rice husk [58], MIL-53(Al) modified by biomass hydrochar [59], and sulfonic-functionalized MIL-100-Fe MOF [60]. In the FTIR spectra presented in Figure 2A, the absorption bands at 3382 cm⁻¹ are ascribed to hydrogen bonding. The peaks detected at 1600 and 1581 cm⁻¹ are assigned to the coordinated carboxylate groups (COO-). The strong peak detected at 1418 cm⁻¹ is assigned to the skeletal vibrations of the benzene ring (C=C). The morphology of UiO-67 depicted by SEM in Figure 2B,C, where the octahedral structure of Zr-MOF crystals is visible with an average crystal dimension of 450 nm, is in accordance with a previous reported study [61].

2.2. Adsorption Parameter

The parameters encompassed in this study are contact time, UiO-67 mass, and solution conditions including pH and temperature. Therefore, the effect of different factors on the interaction between UiO-67 and RTV and LPV was explored and studied. Figure 3A shows the effect of a different UiO-67 mass on the removal of RTV and LPV. The mass added started with 5 mg of UiO-67; the percentage removal was 7.9% of RTV and 4.6% of LPV after 60 min. The removal percentage of RTV and LPV gradually increased with the increasing amount of UiO-67 added: 50 mg resulted in 92.3% of the RTV and 86.5% of LPV. These percentages of RTV and LPV removed increased by adding 70 mg UiO-67 to the solution until they reached 94.2% and 97.6% removal of RTV and LPV, respectively. A further increase in the amount of UiO-67 to 100.0 mg increased the % removal of RTV to almost 99.2%, and of LPV to 94.9%, after 60 min. It is clear from the findings that there is an increase in the % removal of RTV and LPV with the increase in UiO-67 mass.

The influence of solution pH on the removal process is a significant physico-chemical component that affects the adsorption mechanism at the interface between the solid and solution. Hence, an investigation was conducted to examine the impact of solution pH on the elimination of RTV and LPV by the utilization of UiO-67. The pH levels ranged from 2.0 to 12.0, and the corresponding results in Figure 3B show that the maximum elimination was seen at pH 8 as 95.6% for RTV and 79.9% for LPV, which is the optimal pH value. Figure 3C shows the impact of solution temperature on the adsorption process for RTV and LPV. Additionally, as the temperature of the solution increased, there was a progressive increase in the removal % of RTV and LPV.

Figure 4 shows the dependence of the % removed of RTV and LPV by UiO-67 on the contact time at different temperatures, 296 K, 308 K, and 322 K. In general, Figure 4 shows that the percentage of adsorption of the RTV and LPV by UiO-67 after 5.0 min at 296 K was 91.2% for RTV and 85.9% for LPV, and at 308 K the removal percent was increased to 91.7% for RTV and 86.9% for LPV. The results showed that by increasing the temperature to 322 K, the removal percentage also increased to 92.7% for RTV and 89.3% for LPV. Meanwhile, increasing the contact time by more than 10.0 min slowly increased the removal percentage until it reached 97.4% and 89.46% at 296 K, 96.9% and 94.4% at 308 K, and 99.5% and 95.3% at 322 K for RTV and LPV, respectively. Figure 4 shows the effect of contact time on the removal of RTV and LPV by the UiO-67 as a function of amount removed. The amount removed after 5.0 min by UiO-67 was 9.2, 9.0, and 9.3 mg/g of RTV at 296 K, 302 K, and 322 K, respectively. The amount removed for LPV after 5 min was 8.6, 8.7, and 8.9 mg/g LPV by UiO-67 at 296 K, 302 K, and 323 K. Increasing the contact time was associated with a slight increase in the amount removed until it became constant after 60 min with amount removed of RTV being 9.6, 9.7, and 9.9 mg/g at 296 K, 302 K, and 323 K, respectively, and the amount removed of LPV being 8.9, 9.4, and 9.5 mg/g at 296 K, 308 K, and 322 K, respectively. Overall, the data shown in the results indicate that the process of adsorption exhibited endothermic behavior.

2.3. Adsorption Kinetics Models

To obtain a comprehensive understanding of the adsorption and remediation process, it is imperative to conduct a thorough investigation of the kinetic adsorption process. This entails closely monitoring the experimental variables that may influence the rate of adsorption, thereby facilitating the achievement of equilibrium within a suitable period. These investigations provide insights into the potential adsorption process and the transitional phases involved in the production of the ultimate adsorbate–adsorbent complex. Once the reaction rates and their corresponding dependent parameters have been definitively determined, this knowledge might apply to the development of adsorbent materials for commercial purposes. Additionally, it will contribute to a deeper understanding of the intricate dynamics involved in the process of adsorption.

The adsorption of RTV and LPV into UiO-67 was applied to pseudo-first-order model Equation (8) by plotting ln (q_e − q_t) vs. t; RTV and LPV at different temperatures did not converge well and did not give straight lines with an R² value of 0.66 as is clear from Figure 5, with unacceptable regression coefficients, as is presented in

Table 1. Different kinetic models for the adsorption of ritonavir on UiO-67 at different temperatures.

Pseudo-First-Order Kinetic Model
Temperature	qe,exp (mg/g)				qe,calc (mg/g)		$k_1 \left({\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}\right)$				$R^2$
294 K	9.707				1.014		0.024				0.620
308 K	9.718				0.611		0.047				0.662
322 K	9.952				0.921		0.046				0.826
Pseudo-Second-Order Kinetic Model
Temperature	qe,exp (mg/g)			qe,calc (mg/g)		$k_2$ (g/mg·min)			h		$R^2$
294 K	9.707			9.71		0.274			25.859		0.999
308 K	9.718			9.80		0.539			51.897		0.999
322 K	9.952			9.95		0.372			36.882		0.999
Liquid Film Diffusion Model
Temperature			kfd (${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$)					$R^2$
294 K			0.011					0.863
308 K			0.014					0.540
322 K			0.023					0.847
Intra-Particle Diffusion Model
Temperature		$k_{id}$ (mg/g·${\mathrm{m}\mathrm{i}\mathrm{n}}^{1/2}$)				C (mg/g)				$R^2$
294 K		0.357				7.038				0.224
308 K		0.384				6.918				0.225
322 K		0.367				7.250				0.223

and

Table 2. Different kinetic models for the adsorption of lopinavir on UiO-67at different temperatures.

Pseudo-First-Order Kinetic Model
Temperature	qe,exp (mg/g)			qe,calc (mg/g)		$K_1 \left({\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}\right)$			$R^2$
294 K	9.0			0.581		0.026			0.495
308 K	9.5			0.866		0.029			0.594
322 K	9.6			0.739		0.029			0.563
Pseudo-Second-Order Kinetic Model
Temperature	qe,exp (mg/g)		qe,calc (mg/g)		$K_2$ (g/mg·min)		h			$R^2$
294 K	9.0		8.949		0.719		57.581			0.999
308 K	9.5		9.454		0.398		35.531			0.999
322 K	9.6		9.554		0.493		44.972			0.999
Liquid Film Diffusion Model
Temperature			kfd (${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$)				$R^2$
294 K			0.003				0.405
308 K			0.008				0.613
322 K			0.008				0.613
Intra-Particle Diffusion Model
Temperature		$k_{id}$ (mg/g·${\mathrm{m}\mathrm{i}\mathrm{n}}^{1/2}$)			C (mg/g)			$R^2$
294 K		0.329			6.569			0.219
308 K		0.367			6.743			0.249
322 K		0.356			6.744			0.227

. Also, the estimated values of the amount adsorbed at equilibrium (q_e,calc) were not close to the experimental values (q_e,exp). This indicated that the adsorption of the RTV and LPV by UiO-67 could not described by the pseudo-first-order Lagergren equation.

The linearized form of pseudo-second-order rate Equation (9) for the adsorption of RTV and LPV onto UiO-67 by plotting t/q_t vs. t is presented in Figure 5A; good converging was obtained to the experimental data and the straight lines obtained gave excellent regression coefficients (R² > 0.99), as is presented in Table 1 and Table 2.

These findings confirm the suitability of the pseudo-second-order rate equation for the description of RTV and LPV adsorption by UiO-67 from aqueous solution. Also, it is clear from the results that the amount adsorbed per unit mass of UiO-67 at equilibrium (q_e,calc) calculated from the slope of the pseudo-second-order plot was in good agreement with the experimental values (q_e,exp), as is presented in Table 1 and Table 2. These findings agreed well with previous studies and showed the suitability of the pseudo-second-order kinetic model to describe the removal process of different pharmaceuticals, personal care products, dyes, and endocrine-disrupting chemicals (EDCs) in aqueous solution, such as graphene nanoplatelets for the removal of aspirin, acetaminophen, and caffeine [62], Multiwalled Carbon Nanotubes for the decolorization of methylene blue cationic dye [63], the removal of Ketoprofen and Ibuprofen by an Al/Ni-layered double-hydroxide onto polyaniline-wrapped sisal fibers [64], the removal of hydroxychloroquine sulfate by cocoa shell activated carbon [65], the adsorption of metronidazole antibiotic from water with a stable Zr(IV)-MOFs [54], adsorption of the antiviral drugs (oseltamivir and ritonavir) by zeolitic tetrazolate imidazole frameworks (ZTIFs) [55], adsorption of glyphosate by a MIL-53(Al)@rice husk hybrid [58], MIL-53(Al) modified by biomass hydrochar for the removal of ketorolac and naproxen [59], and the removal of diclofenac by the sulfonic-functionalized MIL-100-Fe MOF from aqueous solution [60].

2.4. Adsorption Rate-Controlling Mechanism

The adsorption process usually occurs through a series of consecutive steps. Initially, the adsorbate molecules are transported from the bulk aqueous phase to the surface of the stagnant-boundary liquid film surrounding the solid adsorbent. Subsequently, diffusion takes place through the boundary layer, allowing the adsorbate molecules to reach the external surface of the solid adsorbent. At this point, adsorption takes place at an active site on the surface of the solid adsorbent. Finally, intra-particle diffusion and adsorption occur through the pores and aggregates of the solid adsorbent. This section describes an investigation that was conducted to determine the rate-controlling mechanism for the adsorption of RTV and LPV by UiO-67 from an aqueous solution. Two distinct models, namely the liquid film diffusion model and the intra-particle diffusion model, were employed for this purpose. The liquid film diffusion model is another kinetic model that assumes the flow of adsorbed molecules through a liquid film surrounding the solid adsorbent, as in the following Equation (1):

$$\mathit{ln}(1-\mathrm{F})=-k_{fd}\times t$$

(1)

where F is the fractional attainment of equilibrium (q_t/q_e) and k_fd (min⁻¹) is the film diffusion rate coefficient. A linear plot of −ln(1 − F) vs. t with zero intercept suggests that the kinetics of the adsorption process is controlled by liquid film. The rate of diffusion within particles is expressed by the following Equation (2):

$$q_e=k_{id}{t}^{1l2}+C_i$$

(2)

where k_id (mg/g·min) is the intra-particle diffusion constant. As shown in Figure 6A,B, the experimental data for absorption of RTV and LPV by UiO-67 were analyzed using an intra-particle diffusion model.

The use of the liquid film diffusion model to analyze the adsorption data of RTV and LPV by UiO-67 exhibited moderate convergence and yielded moderate linear regression coefficients, as depicted in Figure 6C,D and Table 1 and Table 2. However, the use of the intra-particle diffusion model to analyze the experimental data did not provide straight lines passing through the origin, resulting in unsatisfactory linear regression coefficients, as shown in Table 1 and Table 2. The convergence and the linear regression of the liquid film diffusion model are much better than the intra-particle diffusion model, which may indicate that the adsorption processes of RTV and LPV by UiO-67 occur mainly on the surface of the UiO-67 active sites, and not inside the pores of the UiO-67. These results suggested that the adsorption was catalytic in nature on the surface of the UiO-67 MOFs due to the interaction of the RTV and LPV with the active sites present on the UiO-67 surface. It is well known in heterogeneous catalysis reactions that reactants diffuse from the bulk fluid phase to adsorb on the catalyst surface, which in this case is the solid adsorbent UiO-67. So, the adsorbate molecules (RTV and LPV), must migrate across the surface to an active site of the UiO-67, where RTV and LPV molecules are adsorbed and immobilized physically to form physical bonds with the UiO-67 (product) by following a more energetically facile path through catalytic intermediates, as the adsorption takes place in different intermediate steps, and the adsorption rate is affected by different factors including solution temperature and mass transfer. After the desorption process, the active site on the MOF adsorbent is available for another cycle of adsorption [49].

2.5. Thermodynamic Studies

The adsorption of RTV and LPV by UiO-67 was studied thermodynamically to explore the spontaneity of the process. Different thermodynamic parameters were calculated, Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) from the variation in the thermodynamic distribution coefficient D with a change in temperature according to Equation (3), and the ΔH and ΔS were calculated according to the Equation (4):

$$D={\displaystyle \frac{q_e}{C_e}}$$

(3)

$$lnD={\displaystyle \frac{{\mathsf{\Delta }S}^{°}}{R}}-{\displaystyle \frac{{\mathsf{\Delta }H}^{°} }{2.303 RT}}$$

(4)

where R is the gas constant (8.314 J/(mol K)), T is the absolute temperature (K), and D is the distribution coefficient of the adsorbate.

When plotting lnD vs. 1/T for the adsorption of RTV and LPV by UiO-67, straight lines were obtained, as presented in Figure 7. The ΔH and ΔS values were calculated from the slope and the intercept of the straight line. The ∆H values were positive, +23.5 kJ/mole and +24.4 kJ/mole for the adsorption of RTV and LPV, respectively, by UiO-67 from aqueous solution. The positive ∆H values confirm the endothermic nature of the adsorption of RTV and LPV by UiO-67, which explains the increase in adsorption with the rising solution temperature. Also, the values of ∆H suggests a strong affinity between RTV and LPV for UiO-67, and the physical nature of the adsorption. The positive values of ∆S, +36.2 KJ/mole for RTV and +40.3 KJ/mole for LPV, suggested an increase in the degree of randomness at the UiO-67 solution interface during the adsorption and immobilization of RTV and LPV. The ΔG values were calculated for 296 K using the following Equation (5):

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

(5)

The adsorption of RTV and LPV by UiO-67 was spontaneous, as indicated by the negative values of the calculated free energy change, ∆G −6.9 kJ/mole and −7.2 kJ/mole for the adsorption of RTV and LPV, respectively, by UiO-67 from aqueous solution. Generally, the negative values of ∆G positive values of ∆H and ∆S suggested that the adsorption of RTV and LPV by UiO-67 process is an entropy-driven process.

3. Environmental Applications

The present study aimed to assess the effectiveness of UiO-67 as a solid adsorbent for the removal of RTV and LPV from real water samples. For this investigation, a selection of five samples was chosen, including sea water, wastewater, well water, tap water, and bottled water. The amounts of RTV and LPV were measured first, and it was found that they were below the detection limit of the HPLC method. This suggests that the selected water samples were free from these pollutants. Consequently, the real water samples were spiked with RTV and LPV, with a resultant concentration of 50.0 mg/L for both RTV and LPV, and then the real samples were equilibrated overnight.

Following this, an amount of 50.0 mg of UiO-67 was added to the spiked samples and left for 60 min. The amount adsorbed was calculated by comparing the concentrations of RTV and LPV before and after the adsorption, and the results are presented in Figure 8. The data shown in Figure 8 demonstrate that around 90% of RTV and LPV was removed from the spiked real water samples, confirming the applicability of UiO-67 for environmental remediation, especially in the case of pollution with antiviral drugs such as RTV and LPV.

4. Materials and Methods

4.1. Materials

The chemical reagents employed in the production of UiO-67 include Zirconium tetrachloride (ZrCl₄) 98%, which was purchased from Energy Chemical (Shanghai, China), 4,4′-biphenyldicarboxylic acid (BPDC) 98%, obtained from Tianjin Hewns Biochem LLC. (Tianjin, China), acetic acid, hydrochloric acid (HCl) at a concentration of 37%, dimethylformamide (DMF) 99%, and acetone 96%, which were obtained from Shenzhen Biocomma Biotech (Shenzhen, China). The pharmaceutical drugs, ritonavir (RTV) and lopinavir (LPV), the chemical composition is shown in Figure 9, were purchased from Sigma–Aldrich (Darmstadt, Germany) at an analytical purity of 98%. All solutions were prepared using deionized water.

4.2. Preparation of UiO-67

The UiO-67 metal–organic framework (MOF) was synthesized as shown in Scheme 1 using a mixture of ZrCl₄ (233 mg, 1 mmol), 4,4′-biphenyldicarboxylic acid (BPDC, 242 mg, 1 mmol), acetic acid (0.6 g, 10 mmol), and HCl (37%, 12 M; 0.16 mL, 2 mmol) mixed in 30 mL of N, N-dimethylformamide (DMF) using the process of sonication. The mixture was placed in a 100 mL Teflon-lined stainless-steel autoclave and heated at 120 °C for 48 h. Subsequently, the UiO-67 white product was obtained by the process of centrifugation and subjected to three rounds of washing with DMF. The UiO-67 material, in its original form, was immersed in DMF at ambient temperature for a duration of 6 h in order to eliminate any residual BPDC that was not bound. Subsequently, the particles that were acquired underwent several washes with acetone to facilitate the displacement of the entrapped DMF, followed by a drying process at a temperature of 100 °C for a duration of 24 h, as shown in Scheme 1.

4.3. Characterization of UiO-67

Powder XRD (X-ray diffraction) patterns were obtained using a D8 ADVANCE ECO (Bruker, Billerica, MA, USA) with Cu Kα radiation (40 kV, 40 mA) at a rate of 6° min⁻¹ over a range of 4–40° (2θ). The thermogravimetric analyses (TGA) were carried out using a TG/SDTQ600 (USA) instrument in atmosphere within a temperature range of 0.0–900 K at a heating rate of 10 °C ${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$ under a nitrogen flow of 100 mL ${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$ with open alumina pans. The morphology of the samples was examined by using a scanning electron microscope (SEM, JSM-6701F, Tokyo, Japan). The FTIR spectra were recorded using a FTIR spectrophotometer (Spectrum 100, Perkin Elmer, Shelton, CT, USA). The zeta potential was obtained through measurement of the electrophoretic mobility of the particles using a Zeta sizer (Malvern Instruments GmbH, Malvern, WR14 1XZ, UK). Nitrogen adsorption–desorption isotherms were measured by using a N₂ porosimeter (Tristar 3020, Micromeritics, Norcross, GA, USA) at 77 K; the samples were degassed at 150 °C for 12 h under vacuum before this analysis.

4.4. Analytical Method

The concentrations of RTV and LPV were determined using high-performance liquid chromatography (HPLC) with a Hewlett Packard 1100 series liquid chromatograph (Avondale, CA, USA). The high-performance liquid chromatography (HPLC) system was comprised of two pumps and a UV detector. The separations were conducted using an analytical reversed-phase C18 column with dimensions of 4.6 mm × 150 mm and a particle size of 5 μm. The mobile phase flow rate was set at 1.5 mL/min, and gradient conditions were employed. The mobile phase consisted of a combination of deionized water with 1% formic acid and acetonitrile, with a volume ratio of 40:60. A volume of 10 μL was utilized for the injected sample size, while the detection wavelength for UV analysis was set at 254 nm. The chromatogram results are shown in Figure 10.

4.5. Adsorption Studies

Adsorption tests were conducted in the study to investigate the impact of several adsorption factors, including adsorbent mass, solution pH, solution temperature, and adsorption time. A set of 10.0 mL solutions, each containing 50.0 mg/L of lopinavir and ritonavir, were created in 25.0 mL glass bottles and maintained at a certain temperature. A test for the effect of UiO-67 mass was performed by using different masses (0, 5, 10, 15, 30, 50, 70, and 100 mg) in 10 mL of diluted RTV and LPV drug mixture and 60 min contact time at 296 K. A test for the effect of solution pH was performed by using different pH values (2, 4, 6, 8, and 11) that were adjusted using HCl solution and NaOH solution and using 50 mg of UiO-67 in 10 mL of diluted drug mixture and 60 min contact time at 296 K. A test for the effect of temperature was performed by measuring the amount adsorbed at different temperatures (280, 294, 302, 308, 313, 318, and 322 K) using 10 mg of UiO-67 in 10 mL of diluted drug mixture and 60 min contact time. A test for the effect of adsorption time was performed by measuring the amount adsorbed at different times (1, 2, 3, 5, 7, 10, 15, 20, 30, 45, 60, 90, and 120 min) using 10 mg of UiO-67 in 10 mL of diluted drug mixture and 60 min contact time at 296 K. Subsequently, the solution was promptly filtered using a filter paper (specifically, Whatman^® quantitative filter paper, ashless, Grade 42, 2.5 μm) to collect the supernatant. The concentrations of lopinavir and ritonavir in the aqueous solution were subsequently analyzed using high-performance liquid chromatography (HPLC). Prior to injection, the solution was filtered through a 0.45 μm syringe filter made of cellulose acetate. This filtration step was implemented to ensure that the injected solution did not contain any UiO-67, which could potentially obstruct the HPLC column and lead to inaccurate findings. The percentage of removal in the solution was determined by utilizing Equation (6) [35]:

$$Removal \%={\displaystyle \frac{C_i-C_e}{C_i}}\times 100$$

(6)

where C_i is the initial concentration and C_e is the equilibrium concentration of RTV and LPV in solution (mg/L).

The adsorption capacity at any time was calculated according to the following Equation (7):

$$q_e={\displaystyle \frac{C_i-C_e}{m}}\times V$$

(7)

where q_e is the amount of RTV and LPV adsorbed by the UiO-67 (mg/g) at equilibrium, V is the initial solution volume (L), and m is the mass of UiO-67 (g). It is noteworthy to mention that each experiment was performed in triplicate and the reported values are the average of three measurements.

The adsorption process of RTV and LPV by solid adsorbents such as UiO-67 was applied to the most used kinetic models: Lagergren pseudo-first-order model Equation (8) and the pseudo-second-order Equation (9) [50,51]:

$$\mathit{ln}\left(q_e-q_t\right)=\log q_e-{\displaystyle \frac{k_1}{2.303}}t$$

(8)

$${\displaystyle \frac{\mathrm{t}}{q_t}}={\displaystyle \frac{1}{k_2{q}_{e}2}}+{\displaystyle \frac{1}{q_e}}t$$

(9)

where k₁ (min⁻¹) is the pseudo-first-order rate coefficient, k₂ (g/(mg·min)) is the pseudo-second-order rate coefficient, and q_e and q_t are the values of the amount absorbed per unit mass mg/g at equilibrium and at any time t, respectively.

4.6. Real Water Samples

Five environmental water samples were used to evaluate the efficiency of UiO-67 in the removal of lopinavir and ritonavir. The water samples include seawater, sewage water, well water, tap water, and bottled water. The samples underwent filtration using a 0.45 L Millipore filter paper and after that were kept in Teflon bottles at a temperature of 5 °C in the dark.

5. Conclusions

This study investigated the adsorption of antiviral drugs such as RTV and LPV compounds from aqueous solutions onto the metal–organic framework UiO-67. The adsorption experimental findings show that the highest level of adsorption capacity for RTV and LPV was (q_e,exp) 9.9 mg/g, which was attained with an efficiency of 91.2% for RTV and 85.9% for LPV using 50 mg of UiO-67 at a temperature of 298 K, pH 8, and duration time 120 min. The adsorption of RTV and LPV on the UiO-67 MOFs followed the second-order-model with R² 0.99, and the data depict that the adsorption capacity at equilibrium was 9.9 mg/g (q_e,calc), which is in good agreement with the experimental values (q_e,exp). Moreover, the rate-determining step of adsorption was mainly due to the liquid film diffusion, which suggested the catalytic nature of the adsorption mechanism at the surface of the UiO-67 MOFs due to the interaction of the RTV and LPV molecules with the active sites present on the UiO-67 surface. The adsorption was spontaneous, as indicated by the negative values of the calculated free energy change, ∆G −6.9 kJ/mole. Generally, the negative values of ∆G and the positive values of ∆H +24.2 kJ/mole endothermic nature and the ∆S +39.5 KJ/mole suggested that the adsorption of RTV and LPV by UiO-67 process is one of random disorder. The utilization of UiO-67 as an adsorbent has shown significant efficacy in the removal of RTV and LPV from real environmental samples. Therefore, it is evident that UiO-67 has high efficiency and adsorption capabilities as a composite material for the removal of RTV and LPV from aqueous solutions. In future studies, UiO-67’s exhibited potential as a promising adsorbent, capable of effectively facilitating the removal of diverse components through the utilization of adsorption techniques, will be further studied.