Efficient adsorption of naproxen and ibuprofen by gelatin/zirconium-based metal–organic framework/sepiolite aerogels via synergistic mechanisms

Highlights

• 3D gelatin-based aerogels were fabricated via direct mixing and freeze-drying.

• Gel-1.0MOF-Sep aerogels exhibited great adsorption capacity and recyclability.

• Maximum adsorption capacities for NPX and IBP are 8.515 and 10.23 mg/g, respectively.

• Adsorption was due to the hydrogen bonding, electrostatic, and π-π interactions.

Abstract

The removal of non-steroidal anti-inflammatory drugs (NSAIDs) from effluent wastewater is critical because of their adverse impacts on human health and the ecosystem. In this study, we successfully fabricated a novel biopolymer-based aerogel composite by incorporating a zirconium-based metal–organic framework, UiO-66 (MOF), and sepiolite (Sep) into gelatin (Gel) to efficiently remove naproxen (NPX) and ibuprofen (IBP). The physicochemical properties of the prepared adsorbents were comprehensively characterized, as well as batch experimental studies were carried out to probe the effect of contact time, solution pH, temperature, and coexisting ions on the adsorption process. The adsorption by the 3D mesoporous aerogel (Gel-1.0MOF-Sep) followed the pseudo-second order and the Langmuir isotherm models with maximum adsorption capacities of 8.515 and 10.23 mg/g for NPX and IBP, respectively (at 20 °C and pH 7). Furthermore, central composite design (CCD) in response surface methodology (RSM) was used to assess the simultaneous interactions of independent variables, results of which suggested that the initial concentration and pH were the dominant parameters in the adsorption process. Moreover, a thermodynamic study showed that the adsorption process was exothermic (ΔH° < 0) and thermodynamically spontaneous (ΔG° < 0). Reusability studies demonstrated that the composite aerogel exhibited superior adsorption efficiencies after five successive runs, indicating its potential use in practical applications. Furthermore, the adsorption mechanisms for the pollutants were ascribed to electrostatic interactions, π–π interactions, and hydrogen bonding. The insights show that the Gel-1.0MOF-Sep aerogels are promising alternative adsorbents for the removal of NSAIDs.

Introduction

Non-steroidal anti-inflammatory (NSAIDs) drugs, such as naproxen (NPX) and ibuprofen (IBP), are commonly used for their anti-inflammatory analgesic and antipyretic properties to treat pain and inflammation in human and veterinary therapy [1]. Widespread use and incorrect disposal lead to pollution of various environmental matrices, such as surface water, underground water, wastewater, and drinking water, with detection ranging from ng/L to μg/L [2], [3], [4]. Numerous acute and chronic toxicity caused by hazardous pharmaceuticals, their metabolites, and conjugates to the ecosystem is of great concern. Therefore, there is a need to develop efficient materials and methods for their removal from aqueous media [5], [6].

Various methods and technologies have been employed to remove NSAIDs, such as advanced oxidation processes, membrane separation, adsorption, and biological treatment [7], [8], [9], [10], [11], [12], [13]. Adsorption has been widely adopted owing to ease of operation, low cost, environmental friendliness, high removal efficiency, and simplicity. Numerous adsorbent materials have been synthesized to remove NSAIDs from water and wastewater. Metal-organic frameworks (MOFs) have received tremendous attentions as efficient adsorbents for water treatment applications. MOFs comprise a metal–ligand complex and organic linkers to produce crystalline materials with high porosity, large pore volumes, surface functionalization, and a high specific surface area [14]. This makes them ideal candidates for contaminant adsorption in wastewater treatment. In particular, UiO-66 (MOF), which consists of Zr₆O₄(OH)₄ clusters and benzene-1,4-dicarboxylate (BDC) organic linkers has been used as an adsorbent owing to its inherent properties, such as large specific surface area; excellent structural, thermal, and chemical stability; easy modification; tunable surfaces; and excellent adsorption capacity [15], [16]. Furthermore, recent studies have shown that UiO-66 possesses selective size and ion separation properties, which is ideal for the removal of pollutants in an aqueous environment [17]. However, in real applications, conventional powder adsorbents face challenges, such as poor separation and recoverability, particle agglomeration, and poor recyclability, which hinders their large-scale application. In contrast, aerogels are three-dimensional (3D) materials with low density, low cost, large surface area, easy processability, and efficient recyclability; thus, they have attracted attention for the adsorption of various contaminants [18], [19], [20]. Compounding metal–organic frameworks (MOFs) on supporting substrates or structural materials such as biopolymers to form 3D structures (such as aerogels) enhances their performance, resulting in synergistic effects and complementary functionalities [21].

Natural polysaccharides have been extensively used in wastewater treatment because they are biodegradable, biocompatible, inexpensive, environmentally friendly, and non-toxic. Various bio-based adsorbents have been synthesized from natural polysaccharides, such as chitosan [22], cellulose [23], alginate [24], xanthan gum [25], starch [26], cyclodextrin [27], and gelatin [28]. Gelatin (Gel) is an amphoteric biopolymer derived from the partial hydrolysis of collagen and abundant hydroxyl (–OH), amino (–NH₂), and carboxyl (–COOH) functional groups, which makes gelation and grafting functionalization easy [29]. Sepiolite (Sep), [Mg₈·[Si₁₂O₃₀](OH)₄·(OH₂)₄]·8H₂O, is a natural clay mineral that has been extensively studied and used as an adsorbent because of its low cost, swelling resistance, light weight, environmental friendliness, abundance, and chemical and thermal stability. In neutral aqueous medium, the surface charge of sepiolite is negative [30], [31]. It comprises alternating blocks and channels that extend in the fiber direction [32], [33]. The structural block consists of continuous tetrahedral silica sheets and discontinuous octahedral magnesium sheets. Moreover, it has a fibrous morphology, a high specific surface area, and various functional groups, which play a key role in the adsorption process. Abundant Si-OH clusters on the surface allow interactions through hydrogen bonding and attachment to other materials [34].

Different bio-based adsorbents have been fabricated and applied for the removal of various pollutants. Peng et al. [35] synthesized a gelatin-ZIF-L aerogel for the adsorption of tetracycline. The 3D porous aerogels exhibited a maximum adsorption capacity of 387.6 mg/g with the adsorption mechanism attributed to pore filling, hydrogen bonding, electrostatic, and π-π interactions. Wu et al. [36] reported the synthesis of a chitosan–gelatin-graphene adsorbent for the removal of orange II. The synthesized composite beads showed ease of separation and great potential for the removal of the organic dye with a 72.2 mg/g adsorption capacity. Jiang et al. [29] prepared a gelatin-based aerogel to remove anionic and cationic dyes. The synthesized composite aerogel was used for oil and water separation. Furthermore, the aerogel exhibited excellent adsorption capacity and reusability. Nevertheless, to the best of our knowledge, the combination of gel, MOF, and Sep has not been previously studied for the removal of pharmaceuticals from water.

This novel study provides insight into a simple approach for the synthesis of a lightweight biopolymer-based aerogel composite that can be used for the adsorption of NSAIDs. The physicochemical properties of the prepared adsorbents were investigated. In addition, batch experiments were performed to explore the effects of various key parameters, including contact time, solution pH, temperature, and coexisting ions, on the adsorption process. The synergistic effects of the constituent elements were thoroughly studied, and their respective optimal mass ratios were identified. Response surface methodology (RSM) was used to examine the simultaneous interaction between independent parameters and the optimum combination of process variables for the removal of organic pollutants. The current study provides new insights into the potential use of environmentally friendly bio-sorbents for the adsorption of organic pollutants.