Abstract
Nano-enhanced membrane technology and deep eutectic solvents (DESs) have demonstrated effectiveness in addressing emerging environmental pollutants. This research centers on purifying water by removing heavy metals employing membranes enhanced with mesoporous silica and DES. Various DESs, including hexanoic acid, octanoic acid, and decanoic acid, were synthesized using tetrabutylammonium bromide (TBABr) as a base. The study combined a polysulfone-based membrane with mesoporous silica, aiming for efficient indigenous crafting to remove heavy metals. Mesoporous silica was blended with the synthesized DES solution, creating diverse membranes for heavy metal separation. The study characterized these membranes using various techniques such as scanning electron microscopy, atomic force microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and contact angle measurements. Surface mapping confirmed the integration of silicon-based DES, reducing the membrane surface roughness from 4 to 1.4 nm. By adjusting the carboxylic acid chain length with TBABr and adding mesoporous silica, leaching ratios were reduced from 4.2 to 2.3%. Silica-grafted DES-based membranes exhibited a progressive increase in flux from 2.6 to 26.67 L/m2 h. The synthesized silicon-based membranes demonstrated outstanding performance, achieving rejection rates exceeding 80% for chromium and arsenic, maintaining an impressive 90% flux recovery ratio even at high flux rates. This study will envision the potential of nano-enhanced membrane technology utilizing DES for sustainable water purification and wastewater treatment applications to achieve the sustainable development goal (SDG) of clean water and sanitation (SDG-6).
Graphical abstract
1 Introduction
Water is undoubtedly one of the most precious resources on earth, essential for all forms of life. However, the increasing industrialization and urbanization of the world has led to a concerning rise in water pollution, particularly the contamination of water sources with heavy metals, such as arsenic and chromium. Chromium and arsenic, specifically in inorganic forms, such as arsenite and hexavalent (vi) chromium, are known to be highly carcinogenic and pose health risks to humans, leading to extensive research on removal techniques [1]. Existing methods to treat water/wastewater, such as coagulation–flocculation [2], ion exchange [3], electrocatalysts for oxygen evolution reaction [4] and chemical precipitation [5], electrochemical [6], and advanced oxidation processes [7,8,9], often suffer from drawbacks, such as secondary waste generation, high operational costs, and limited selectivity. Furthermore, the environmental impact of conventional treatment processes is a growing concern in today’s world, necessitating the exploration of greener alternatives [10,11,12,13].
In recent years, significant progress has been made in the field of water and wastewater treatment, with various green technologies and materials being investigated for their potential to remove pollutants from aqueous solutions [14,15]. Among these, membrane-based separation processes have emerged as a promising approach due to their selectivity, efficiency, and ease of operation [16,17,18]. Polysulfone (PSF) membranes have gained attention for their excellent chemical resistance and mechanical properties, making them suitable for industrial applications. However, the inherent limitations of traditional PSF membranes, such as their poor fouling resistance and low selectivity for heavy metals, have prompted researchers to explore innovative approaches to enhance their performance [19]. These alternatives include surface modifications, nanocomposite membranes, hybrid membrane systems [20], and advanced manufacturing techniques. However, nanocomposite membranes offer advantages such as enhanced mechanical strength and stability, improved selectivity and efficiency, anti-fouling properties, increased flux, tailored properties, and potential for multi-functionality [21,22]. Therefore, mixed matrix membranes (MMMs), which combine the advantages of a polymer matrix with the superior properties of nanomaterials, can be the solution to the above challenges [23,24]. The guanylyl functionalized graphene/PSF used in the membrane was achieved through a two-step grafting process, resulting in high dispersibility in the casting solution and good compatibility with the polymer matrix [25]. In another study, the bio-AgNPs, which exhibited the most optimal properties, were incorporated into PSF membranes, enhancing the water separation performance [26]. Novel Zn:Al2O3 nanoparticles were synthesized and used to fabricate MMMs with improved selectivity and productivity [27]. These membranes exhibited excellent anti-fouling performance and showed potential for enhanced removal of heavy metals. However, the complexity of fabrication, cost, long-term stability, and environmental impact are the major challenges in composites [21]. These challenges were addressed by deep eutectic solvents (DES), which can be prepared easily at a low cost and show biodegradability [28]. The researchers introduced a natural DES (NDES) called l-menthol/10-camphorsulfonic acid (L-M/CSA) as a modifier for polyether sulfone (PES) nanofiltration membranes [29]. The modified membranes with LM/CSA NDES showed improved water flux, pore-forming capability, pharmaceutical removal efficiency, and anti-fouling properties, making them promising for membrane separation applications in pharmaceutical wastewater treatment. However, there are some challenges associated with polymeric membranes, such as fouling [30].
Silicon dioxide can be engineered to have different levels of porosity, enabling it to act as a molecular sieve or adsorbent material. This property finds applications in catalyst supports, chromatography columns, and gas separation processes [31]. The integration of silica with DESs enhances membrane synthesis by providing improved selectivity, tunable properties, stability, and eco-friendliness, making it a valuable approach in various separation and materials science applications. However, their implication has not been fully understood.
The current study focuses on the synthesis of an innovative membrane by incorporating a DES to assist in grafting silica. This membrane is designed for the effective removal of heavy metals, with a special emphasis on arsenic(vi) and chromium(vi), along with other contaminants present in water. The DES was synthesized using tetrabutylammonium bromide (TBABr) as a base, along with hexanoic acid, octanoic acid, and decanoic acid (DA). The DES is grafted with silica and added into a PSF-based membrane as a filler. Silica MCM-41 was blended with the synthesized DES solution, varying the compositions, and combined with PSF to produce MMMs for the removal of chromium (Cr VI) and arsenic (As VI).
2 Materials and methods
2.1 Materials
PSF with an approximate molecular weight of 22,000, mesoporous silica, and TBABr were acquired from Sigma-Aldrich (USA). Silica MCM-41 was acquired from Sigma Aldrich (USA). All the chemicals and solvents employed in this study were of analytical grade, which included hexanoic acid, octanoic acid, and decanoic acid (DA), and used as such. Pure water, obtained using a water purification unit (Adrona SIA-B30, Latvia), was utilized for washing purposes.
2.2 Preparation of DES
The synthesis procedure of TBABr was used as HBD, and hexanoic acid, octanoic acid, and DAs act as HBA species, as given in the literature [28,32]. A uniform mixture of TBABr and carboxylic acids was blended at 1:2 mass ratios for the synthesis of DES for 1–2 h at 400 rpm at room temperature. In this way, three different types of DES solutions were synthesized based on their mixing compositions, i.e., TBABr–hexanoic acid, TBABr–octanoic acid, and TBABr–decanoic acid. Table 1 shows the different formulation ratios of three DES.
Formulation ratios of three different types of DESs
| Base (HBD) | Acids (HBA) | Molar ratio |
|---|---|---|
| Tetra butyl ammonium bromide | Hexanoic acid | 1:2 |
| Octanoic acid | 1:2 | |
| Decanoic acid | 1:2 |
2.3 Functionalization of silica with DES
The functionalization method is reported elsewhere [33,34,35]. For the functionalization of DES with mesoporous silica, 2 g of mesoporous silica with 2 g of each DES (H-TBABr, O-TBABr, and D-TBABr) were mixed, and 15 mL of acetone was added separately. Each mixture was stirred for 3.5–4 h at room temperature and was placed in a fuming hood overnight at 37°C for the evaporation process until it turned into powder. The scheme of the functionalization of silica with DES is shown in Figure 1. The sample was collected and named “SiO2-DES.” The specific weight ratio for the “SiO2-DES” synthesis is listed in Table 2.
Structures of the synthesized DESs and functionalization of silica with DES.
Formulation of casting membranes in terms of weight percentage
| Sr. | Membrane IDs | Polymer | Solvent | Fillers | ||
|---|---|---|---|---|---|---|
| PSF | 1,4-dioxane | THF | DES | SiO2 | ||
| 1 | M | 4 | 12 | 4 | — | 0 |
| 2 | M-H-5-DES | 4 | 12 | 4 | 1 | 0 |
| 3 | M-O-5-DES | 4 | 12 | 4 | 1 | 0 |
| 4 | M-D-5-DES | 4 | 12 | 4 | 1 | 0 |
| 5 | M-SH-5-DES | 4 | 12 | 4 | 1 | 0.2 |
| 6 | M-SO-5-DES | 4 | 12 | 4 | 1 | 0.2 |
| 7 | M-SD-5-DES | 4 | 12 | 4 | 1 | 0.2 |
2.4 Synthesis of silica-grafted membrane
Using the solution-casting process, membranes of various compositions were fabricated, and precise formulation is shown in Table 2. Three percentages of the novel addition were introduced slowly and stirred for the following 24 h after PSF had been thoroughly dissolved in the solvents with a moderate speed stirring at 37°C for 24 h.
2.5 Casting process of silica-grafted membranes
The membrane was casted using the phase inversion method, which is a meticulously controlled procedure through which a polymer undergoes the transformation from a liquid state to a solid state. Initially, the membrane solution was gently poured onto a glass plate, and with the aid of a slider, it was spread evenly across the surface of the glass tray. This process resulted in the creation of a uniform membrane. Subsequently, the glass tray bearing the newly formed membrane was immersed in water for 15 min, allowing for the crucial phase inversion process to take place.
2.6 Characterization
2.6.1 Fourier-transform infrared spectroscopy (FTIR)
The chemical analysis of the membranes was conducted using FTIR (Nicolet 6700 instrument, USA). Each sample underwent 128 scans at a resolution of 8 cm−1 within a scan range spanning from 500 to 4,000 cm−1.
2.6.2 Morphological analysis
Field emission scanning electron microscopy (FE-SEM) was utilized to examine the surface morphology of the casted membranes. The FE-SEM technique was employed to visualize the microstructure of the membranes. It is essential to conduct FE-SEM in a high vacuum because molecules can influence the electron beam, producing secondary and backscattered electrons used for imaging. Before the morphological observation, all cast membranes were cut into smaller sizes of approximately 555 mm.
2.6.3 Energy-dispersive X-ray spectroscopy (EDX)
EDX was employed, specifically using a JEOL/EO Model JSM-6610, to map the surface of the membrane and determine the elemental compositions of the selected regions, which encompassed carbon, oxygen, sulfur, and silicon.
2.6.4 Contact angle and water absorption analysis
The surface hydrophilicity of the prepared membranes was evaluated using an optical contact angle measurement device that employs a dynamic, sessile drop method (CAM 101 optical Contact Angle Meter, KSV Instruments). Prior to testing, the membrane samples (5 cm × 5 cm) were dried in an oven at 60°C for 2 h to perform the water adsorption experiment. These dried samples were then weighed (M dry). Subsequently, the pre-weighed samples were immersed in deionized (DI) water at room temperature for 48 h. Afterward, any remaining water on the surface was removed with tissue paper, and the wetted membrane (M wet) was weighed. The membrane’s water uptake was calculated using equation (1) [36,37]:
2.6.5 Surface roughness
Atomic force microscope (AFM) is a powerful tool for examining the texture and morphology of various surfaces, even at atomic dimensions for flat surfaces like polymeric membranes. AFM enables the evaluation of membranes, providing valuable information about surface morphology in terms of 3D images and quantitative surface roughness parameters. To analyze three-dimensional micrographs and obtain quantitative measurements of line statistical parameters, AFM, specifically the Park XE-100 model, was employed. A standard software program, XEI-AFM, was used for this purpose. All roughness parameters of the membrane were determined from AFM scanned images measuring 2.5 µm × 2.5 µm, which were obtained in contact mode during the scanning process. The parameters representing the line’s minimum and maximum heights, along with the average between them, are denoted as R p, R v, and R t in the table of line statistics. On the other hand, R q, R a, R z, R sk, and R ku represent the root-mean-squared roughness, roughness average, ten-point average roughness, skewness, and kurtosis of the line, respectively.
2.7 Leaching test
For the leaching test, membrane samples measuring 5 cm × 5 cm (n = 3) were first dried at 60°C for 2 h before testing. The initial weight of the dried samples (M a) was measured to establish a baseline and determine the initial amount of leachable metal content in the membrane matrix. Subsequently, the membrane samples were submerged in distilled water for 1 week, with daily water changes. This step was crucial to simulate prolonged exposure to an aqueous environment and assess the leaching of metals from the membrane. After a week-long immersion period, the membrane samples were removed from water, dried in an oven, and weighed again (M b). This final weight measurement (M b) was taken to determine the remaining metal content in the membrane matrix after leaching.
Using the leaching ratio provided by equation (2), the leached percentage of metals was calculated as
2.8 Membrane flux
Membrane flux is a critical parameter that characterizes the efficiency and performance of membrane filtration systems. To assess the consistency and resistance of the manufactured membranes to adsorption and contamination by a dynamic metal salt solution (K2Cr2O7), a dead-end filtration brand name, Sterlitech, with a 14.6 cm² area, was utilized. Three samples of each membrane, identified as W 1, W 2, and W 3, were placed in the cell. The evaluation involved measuring pure water flux (J w1) continuously for 1 h after replacing the feed water with the metal solution (1,000 ppm). In the subsequent hour, the metal flux (J m) was determined. After cleaning the membrane surface with distilled water, the water flux (J w2) was re-measured using the same membranes and settings. equations (3) and (4) were employed to calculate the fluxes of metal solutions and water and to determine the flux recovery ratio (R FR) for the manufactured membranes. These assessments provided insights into the membrane resistance to adsorption and contamination and their ability to recover filtration performance after cleaning, which is crucial for assessing their reliability in practical filtration applications.
where V (L), A (m2), and t (h) represent the volume permeating the area of the membrane and the time of the experiment, respectively.
2.9 Resistance parameters
The resistance parameter of a membrane refers to a quantitative measure of the membrane’s hindrance or opposition to the flow of substances, such as solvents, particles, ions, or molecules, through its porous structure. This parameter is crucial in membrane filtration and separation processes as it helps characterize and understand the behavior of the membrane when subjected to different feed solutions and operating conditions. Metal fouling, both reversible and irreversible, was induced by the passage of metal solution across the membrane surface. The membrane surface changed because of irreversible fouling. The total resistance (R t), the reversible resistance (R r), and the irreversible resistance (R ir) were calculated using equations (5)–(7) [38,39]:
2.10 Heavy metal quantification
In this study, an atomic absorption spectrophotometer (Contraa 800-D model, Analytic Jena, Germany) was employed for the quantification of specific heavy metals. For the preparation of the heavy metal solutions, salts of chromium(vi) and arsenic (K2Cr2O7 and Na2HAsO4) were used for metal removal. These salt solutions were employed to prepare 1,000 ppm (parts per million) solutions of chromium and arsenic.
3 Results and discussion
3.1 Morphological and structural characterization
TBAB and DES were utilized for the structural analysis of hexanoic acid, octanoic acid, and DAs. The FTIR spectra of the three distinct DES are shown in Figure 2. In Figure 2 (a(a), b(a), c(a)), characteristic peaks were identified within the range of 2,959–2,870 cm−1. These peaks were attributed to the stretching vibration of the CH2 and CH3 groups present in the alkyl chain of TBAB. Furthermore, specific peaks at 1,498 and 1,377 cm−1 were associated with the deformation vibrations of CH2 and CH3 groups, respectively. Additionally, the peaks observed at 1,170 and 1,414 cm−1 were attributed to the stretching vibration of the C–N bond and tetra butyl groups of quaternary ammoniums, respectively [18].
FTIR spectra of TBABr, carboxylic acids and different synthesized DESs: a(a), b(a), c(a) for TBABr, a(b) HA, b(b) OA, c(b) DA, a(c) HA-TBABr, b(c) OA-TBABr and c(c) DA-TBABr.
In Figure 2 (a(b), b(b), c(b)), the observed peaks within the range of 2,959–2,855 cm−1 can be attributed to the symmetric and asymmetric vibrational states of C–H bonds in the alkyl groups of carboxylic acids. Notably, in Figure 2, HA, OA, and DA denote hexanoic acid, octanoic acid, and DA, respectively. It is pertinent to acknowledge that the findings of this study diverge from the literature reported [17]. Within the hexanoic acid spectrum (Figure 2, a(b)), distinct peaks at 1,702, 3,143, 1,290, and 1,415 cm−1 signify the stretching vibration of C…O, O–H, C–O bonds, and C–OH group scissoring, respectively. Furthermore, the substantial wide signal at 930 cm−1 can be attributed to the wagging of the OH group and the oxygen atom of the carboxylic group, a phenomenon consistent with the existing literature [18]. Similarly, in the octanoic acid spectrum (Figure 2, b(b)), the peaks at 3,473, 1,707, and 1,287 cm−1 represent the stretching vibrations of O–H, C…O and C–O, respectively. Additional peaks at 1,463 and 1,414 are attributed to the bending vibration of the C–H bonds (–CH3 and –CH2), with a broad peak at 930 cm−1 resulting from the wagging of the oxygen atom in the carboxylic group of OA [40]. In a similar manner, peaks observed at 3,410, 1,514 and 1,450 cm−1 in the DA spectrum (Figure 2, c(b)) are associated with the stretching vibration of the OH and COO- groups of DA [18]. The confirmation of the formation of three distinct DESs, hexanoic acid–TBAB, octanoic acid–TBAB, and DA–TBAB, referred to as DES(TBAB:HA), DES (OA: TBAB), DES (DA:TBAB), is substantiated by the shift in the stretching vibration of C…O (at 1,720, 1,721 and 1,717 cm−1) of carboxylic acids, as illustrated in Figure 2 (a(c), b(c), c(c)). This shift suggests the presence of an intermolecular hydrogen bond in DES synthesized from TBABr and carboxylic acids [41]. The FTIR results presented in Figure 3 for membranes with DES and silica grafting reveal distinctive signals within specific frequency ranges. Peaks appearing at 3,435–3,456 and 1,632–1,635 cm−1 are associated with the stretching and bending modes of the absorbed water molecules.
FTIR analysis of casted membranes: (a) PSF; (b) PSF + DES-HA; (c) PSF + DES-HA + SiO2; (d) PSF + DES-OA + SiO2; and (e) PSF + DES-DA + SiO2.
In the FTIR spectrum, noticeable peaks within specific wavenumber ranges are indicative of distinct vibrational modes in the membrane composition. Specifically, peaks observed in the range 1,079–1,088 cm−1 denote asymmetric vibrations in the (Si–O–Si) bonds, while those observed around 952–953 cm−1 correspond to asymmetric vibrations in the (Si–OH) bonds. Furthermore, the peaks at 804–805 cm−1 are attributed to symmetric stretching vibrations in the Si–O–Si bonds, and peaks in the range of 459–463 cm−1 are linked to the bending vibrations of the Si–O–Si bonds. Notably, the FTIR graph provides conclusive evidence confirming the presence of SiO2 within the membranes.
Material accumulation in membranes arises from the choice of a solvent with low dispersing power or surfactants characterized by low viscosity. The chain length-based carboxylic acid utilized in this study significantly impacted the separation efficiency of the membrane and its internal components. It is essential to note that the total amount of TBABr and PSF was deliberately maintained at a constant level to ensure uniformity within the doped solution. The introduction of additives not only altered the thermodynamic state of the doped solution but also influenced the polymer’s conformation and dynamics, consequently affecting the kinetics of phase separation [42].
Figure 4 provides a comprehensive surface analysis of formulated membranes, delineating two distinct sets of casted membranes. The first set comprises PES/DES formulations, denoted as H5(M-hexanoic-5-DES), O5(M-octanoic-5-DES), and D5(M-decanoic-5-DES), incorporating TBABr with carboxylic acids of varying chain lengths (refer to Table 2). The second set involves PES/DES-SiO2 membranes, incorporating silicon dioxide in addition to TBABr-carboxylic acids, with surface images labeled as SH5 (M-SiO2-hexanoic-5-DES), SO5 (M-SiO2-octanoic-5 DES), and SD5(M-SiO2-decanoic-5-DES).
SEM images of the synthesized membranes.
The addition of silicon dioxide-based DES results in the appearance of white spots on the membrane surfaces, which are absent in membranes without silicon dioxide (Figure 4). These white spots are attributed to silicon dioxide agglomeration, positively impacting the filtration properties of heavy metal solutions [34]. The presence of these white spots decreases as the carboxylic chain length increases from hexanoic acid to DA. The images of DES-SiO2-based membranes clearly depict the dispersion of SiO2 within the membrane, with membrane S-D exhibiting superior dispersion properties compared to S–H and S–O membranes. Similar morphological results were documented in other studies, where polymer molecules adhere to the large graphene oxide sheets, causing them to swell [20].
In evaluating the silica dispersion within the membrane, surface mapping techniques were employed, and the results are depicted in Figure 5. The images corresponding to H5, O5, and D5 exhibit a uniform dispersion of carbon, oxygen, and sulfur elements. The introduction of SiO2 into DES moderately amplifies the oxygen content, leading to an augmented density of dots per unit area, as prominently illustrated in the images. Furthermore, a subtle aggregation of SiO2 is discernible in the SH5, SO5, and SD5 samples. These findings imply that the elongation of the carboxylic chain positively influences the dispersion characteristics of DES-Si when paired with the hydrophobic PSF polymer. Notably, these outcomes align with the conclusions reported in other scholarly studies [43].
Surface mapping of different synthesized membranes.
Figure 6 presents the 3D images of all formulated membranes obtained by AFM. It is evident that the incorporation of DES induces alterations in the topography of the membrane surface. Various surface images depicting light brown hills and valleys illustrate distinct surface features. The aggregation or homogeneity behavior of DES/PSF and DES-SiO2/PSF is elucidated by the 3D AFM images. Membrane “M” displays a higher density of hills and valleys, which is mitigated by the addition of DES. While H5 and D5 exhibit similar 3D images, the inclusion of silicon-based DES introduces a significant divergence in their topographies. SD5 exhibits a smoother surface compared to all other membranes, indicating that the higher chain length of DA and silicon-based DES promotes better dispersion, resulting in smoother 3D images. This underscores the compatibility of the chemical composition within the membrane matrix.
Surface morphology of different synthesized membranes.
Quantitative analysis of surface roughness (Table 3) reveals that membranes H5 and SH5 have the highest peak values among the fabricated membranes, potentially indicating some aggregation of DES. Additionally, the surface roughness of H5 (Ra = 9.85) and SH5 (Ra = 17.6) surpasses that of all other casted membranes, suggesting that the increased surface roughness may be attributed to elevated hills and values or uneven distribution of relevant DES-based additives. Furthermore, membrane SD5 exhibits the lowest results for all three surface roughness parameters (R a = 1.6 nm, R z = 7.65 nm, and R q = 1.91 nm), supporting the notion that DES based on long-chain carboxylic acid (DA) facilitates good dispersion with the PSF polymer, ultimately resulting in a smoother surface than all other membranes. These findings align with the surface images in Figure 4 and the surface mapping in Figure 5.
Surface roughness data of different casted membranes
| Membrane | R p | R V | R t | R z | R a | R q | R sk | R ku |
|---|---|---|---|---|---|---|---|---|
| M | 12.4 | 18.2 | 30.4 | 30.5 | 4.58 | 5.8 | −0.27 | 2.98 |
| H5 | 18.7 | 38.4 | 57.2 | 57.2 | 9.85 | 13.6 | −1.43 | 4.55 |
| O5 | 7.44 | 13 | 20.4 | 20.4 | 2.7 | 3.61 | −0.36 | 4.63 |
| D5 | 11 | 22.1 | 33.1 | 33.1 | 6.66 | 8 | −0.73 | 2.75 |
| SH5 | 10.2 | 12.2 | 22.5 | 22.5 | 17.6 | 5.41 | 0.16 | 2.45 |
| SO5 | 33.6 | 55.6 | 89.2 | 89.2 | 4.45 | 21.2 | 0.5 | 2.76 |
| SD5 | 3.16 | 5.5 | 7.85 | 7.65 | 1.6 | 1.91 | 0.04 | 2.16 |
| nm | nm | nm | nm | nm | nm |
Note: R q, R a, R z, R sk, and R ku denote root-mean-squared roughness, roughness average, ten-point average roughness, skewness, and kurtosis of the line, respectively.
The roughness of a membrane surface can have a significant impact on its performance and interactions with other materials or substances. Table 3 presents quantitative measurements of the surface roughness of synthesized membranes, showing that membranes H5 and SH5 have higher peak values than other fabricated membranes due to the presence of DES. On the other hand, SD5 shows the lowest surface roughness results, supporting the idea that long-chain carboxylic acid (DA)-based DES leads to good dispersion and a smoother surface.
Figure 7 represents the EDX results of different membranes in which only C, O, S, and Si were targeted to find their compositional quantity and presence in the formulated membranes. The blending of DES with PSF provided almost similar results, and there was a slight increment of the “C” amount (77–9.5%) when moved from hexanoic-based DES to decanoic-based DES. EDX results proved that long-chain decanoic-based DES strongly entangled with PSF. Furthermore, the addition of silicon dioxide-based DES in the membrane compositions was also visible in the EDX results. Although a similar amount of DES-Si was used in all membranes (SH5, SO5, and SD5), membrane SD5 exhibited the highest percentage (1.4%) of “Si” in their composition. This higher value of Si in the membrane composition might be due to the strong hydrophobic–hydrophobic interactions between long-chain decanoic-Si with PSF polymer. Elemental analysis results (Figure 7) also support the FTIR results (Figure 3) that confirmed the presence of C, O, S, and Si in the membrane compositions.
EDX analysis of the synthesized membranes.
3.2 Hydrophilic behavior of the synthesized membranes
Contact angle and water absorbance analyses were used to assess the hydrophilicity or hydrophobicity of the membrane. Figures 8 and 9 depict the shape of water droplets on the membrane surface, the contact angle, and water absorption, respectively. The pure PSF membrane (M) exhibits the highest contact angle, indicating a highly hydrophobic surface (102.6). The addition of carboxylic acid and DES reduces the contact angle and increases water absorption, suggesting a reduction in the hydrophobic nature of PSF.
The initial drop-shaped images of synthesized membranes during contact angle measurements.
Analysis of (a) water absorption and (b) contact angle.
It is observed that an increase in the carbon chain length from 6 to 10 carbon acids in the silicon-based DES (SH5, SO5, and SD5) slightly increases the contact angle and decreases water absorption. However, this trend is not observed in DES/PSF membranes (H5, O5, and D5), where no silicon dioxide is used. Therefore, these results suggest that the addition of silicon dioxide in DES leads to improved dispersion properties of the membrane surface, resulting in a consistent surface trend. It is also noted that the addition of silicon dioxide in the DES increases the contact angle from 68° to 79° due to the non-polar behavior of silicon dioxide, which cancels out the dipole moments of its oxygen atoms, making it a non-polar molecule.
3.3 Leaching ratio of the synthesized silica-grafted DES membranes
Figure 10 shows various membrane formulation outcomes for the leaching ratio. It was discovered that membrane “M” had a leaching ratio of 0.61%, which may have been caused by the loss of some solvent from the membrane matrix. Compared to other casted membranes, the H5 membrane has the highest leaching ratio (4.2%). The leaching ratios of membranes were reduced by increasing the length of the carboxylic acid chain in combination with TBABr. Furthermore, the addition of SiO2 in the formulation also reduced the leaching ratio from 4.2% to 2.3%. The presence of the hydrophobic carbon chain length of carboxylic acid in combination with TBABr, SiO2, and hydrophobic polymer (PSF) developed strong hydrophobic–hydrophobic interactions that developed a good mixing ability that led to strong physical bonding [41,44,45].
Elusion ratio of the synthesized membranes.
3.4 Membrane flux and resistance analysis of the synthesized membranes
The time-dependent flux rate and flux recovery ratio are shown in Figure 11, where three test samples of each membrane were tested with pure water followed by metal solution and then again by pure water solution. As shown in Figure 11, the incorporation of DES and DES-SiO2 into the PSF membrane matrix could enhance the flux rate from 2.5 L/m2 h in “M” to 26.67 L/m2 h in SD5 membranes. Membranes from H5 to D5 showed a progressive increment of flux (7.4–13.10 L/m2 h), and the same trend was present in silicon-based DES membranes (19.8–26.6 L/m2 h). However, there was very little difference in the flux of octanoic- and hexanoic acid-based DES membranes. The pure PSF membranes were highly hydrophobic and exhibited a contact angle value of 102°, which was reduced by the blending of DES to 68° (H5) and by the addition of DES-SiO2 to 76.56°. This reduction of contact angle and hydrophobic character might be the reason for higher flux rates in DES-based membranes. The flux recovery ratios of all DES membranes were also very impressive, and more than a 90% flux rate was re-obtained after fouling the same membrane with a metal solution.
Flux recovery ratios of the synthesized membranes for arsenic-polluted wastewater.
Figure 12 presents the water flow profile of the membranes in the dead-end cell before and after they were contaminated with a metal solution. A small decrease in the flux rate (J w1 and J m) of the metal solution was seen in all membranes when the metal solution was used in place of pure water (feed solution). This deterioration may be brought about by the accumulation and adsorption of metal molecules that lead to fouling on the membrane surface and within the membrane pores [46]. The findings for R FR, R t, R r, and R ir are shown in Figure 13 for comparative evaluation of anti-fouling. As shown in Figure 13, the SO5 membrane exhibits 91% flux recovery and 12.22% Rt against metal fouling, of which 3% are reversible. The D5 membrane yield findings are comparable to the SO5 membrane yield findings; however, there is a modest decrease (1–2%) in flux recovery and R t.
Membrane flux behavior using pure water and metal solution.
Membrane fouling mechanisms against heavy metal solutions.
The higher metal fouling of the membrane “M” could be attributed to its higher surface roughness (Table 3) and high contact angle value (102°). All membranes, H5 to D5, showed an increment of flux recovery (88–90%), whereas, in silicon-based membranes, there was no continuation in the trend when SO5 (Fr = 91%) and SD5 (Fr = 86%) membranes were analyzed. The slightly higher flux recovery of the SO5 membrane made it suitable for further usage than the rest of the fabricated membranes.
3.5 Removal efficiency of heavy metals
The removal of heavy metals was analyzed at elevated pressures (9–10 bar). Nevertheless, the removal results of the heavy metals (Figure 14) demonstrated that the membrane had effectively removed the chosen metals (arsenic, chromium). Membrane “M” rejected 26% Cr and 21% Ar. In contrast, the removal efficiency was increased to 50–52% by using DES-based membranes (D5), and it was further enhanced up to 73–80% by silicon-based DES membranes. Among the fabricated membranes, SO5 showed the highest clearance rate of metal solution, followed by SD5 membranes (67–70%) with both metals (Cr and As). Thus, it was firmly revealed that a greater rejection rate was attained with a high flux and a good flux recovery ratio due to ionic interaction and electrostatic repulsion of the membrane surfaces and the pollutant ions in water. Similar results were reported in the literature [21,47,48]. The metals are generally present in the form of ions in water, and the presence of amphoteric natured chemical composition of membrane consisting of the amide group of TBABr with carboxylic acid (carbonyl carbon and non-polar carbon chain) and silicon exerted the hydrostatic repulsive effect on the incoming metal ions that might be the reason of good removal of metal ions from water solution.
The removal efficiency of arsenic and chromium using synthesized membranes.
3.6 Conclusions
This study implicated the synthesis of novel DES-assisted silica-grafted PSF MMMs for the removal of heavy metals, especially arsenic and chromium, from aqueous solutions. FTIR analysis has unequivocally validated the existence of hydrogen bonding within the DES and the subsequent casted membranes. Meanwhile, compelling surface imagery, surface mapping, and elemental analysis revealed the consistent and uniform distribution of TBABr and silicon across all synthesized membranes. A noteworthy improvement in dispersion properties with the elongation of the carboxylic acid chain length within DES was also observed. Compared to pristine PSF membranes, DES-assisted silica-grafted PSF MMMs (PSF/DES/SiO2) membranes demonstrated superior flux and enhanced heavy metal removal efficiency. The inclusion of SiO2 emerged as a pivotal factor, contributing significantly to heightened metal removal efficiency.
Moreover across all membranes, the membrane with mesoporous silica-grafted DES based on octanoic acid (SO5) emerged as the top-performing contender, showcasing an impressive 80% metal removal rate, accompanied by an exceptional 91% flux recovery. Furthermore, it exhibited commendable surface resistance against metal solutions, with only an impermanent 8% surface resistance. This study will enlighten us to explore further optimization strategies, potentially incorporating advanced materials or modifications to enhance membrane performance, selectivity, and durability. Additionally, investigating the scalability and practical application of these membranes in real-world scenarios will be the focus of future research.
Acknowledgments
We acknowledge Mr. Ameer Hamza for his assistance in research proceeding at the Interdisciplinary Research Centre in Biomedical Materials (IRCBM).
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Funding information: This work was funded by the Researchers Supporting Project Number (RSPD2023R584), King Saud University, Riyadh, Saudi Arabia.
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Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Conflict of interest: The authors state no conflict of interest.
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