Electrospun Polystyrene/LDH Fibrous Membranes for the Removal of Cd Ions

Layered double hydroxides (LDHs) have been extensively studied for a broad range of applications because of their ease of synthesis and chemical modifications. The chemical and crystal structure of LDH provides opportunities of combination with polymers forming nanocomposites. In the current study, MgAl-LDH particulates have been incorporated into microand nanofibers of polystyrene (PS) using an electrospinning processing technique of their respective homogeneous solutions. The effect of the varying proportions of LDH and PS on the structure, morphology, and thermal properties of the fabricated LDH-PS fibrous membranes has been investigated. The potential application of the optimally fabricated LDH-PS fibrous membranes in the removal of Cd ions from aqueous media has been evaluated as well. Results showed the possibility of loading the PS fibrous membranes with up to 60wt% of LDH particulates, which in turn modified the thermal stability and integrity of the produced fibrous membranes. Due to the high hydrophobicity of the PS fibrous matrix, no changes in the crystal structure of the LDH inclusions were observed. Both as-prepared LDH particulates and optimally prepared LDH-PS fibrous membranes showed a high potential for the removal of Cd ions from aqueous media. This is attributed to a cation-exchange mechanism involving the adsorption of Cd ions from a solution with the preferential leakage of Al ions from the crystal structure of LDH.


Introduction
The concept of improving the distinctive features of conventional polymers, such as mechanical strength, biodegradability, thermal stability, and hydrophobic nature, fuels the research interest in developing polymer-based composites using particulate additives of various sizes (macroscale to nanoscale) [1][2][3]. One of the approaches that has been extensively studied lately is to design nanofibrous composites that will have the advantage of a high surface area of its fibers as well as close proximity between the polymer matrix and the fillers at the nanoscale. The structure of the polymeric matrix and the filler dictates the properties and applications of the fabricated composites. Among the known inorganic fillers, the addition of layered double hydroxide (LDH) to a polymer widens the scope of the applications of the produced composites. LDHs have found a wide range of applications, such as adsorption [4], separation [5], energy storage [6], corrosion protection [7], drug delivery [8], catalysis [9], and flame retardation [10].
Understanding the functional properties of LDH within various polymer matrices will contribute to improving the biodegradability, thermal resistance, and functionality of the resulting LDH-polymer composites [11,12]. First discovered in 1842 [13], LDH is an intercalation anionic clay-like solid that is also known as hydrotalcite [14] and is represented by a general chemical formula [M II (1−x) M III x (OH) 2 ] x+ (A n-) x/n mH 2 O, where "M II " denotes divalent metal ions (Mg 2+ , Cu 2+ , Ni 2+ , etc.), "M III " denotes trivalent metal ions (Al 3+ , Cr 3+ , etc.), and "A" denotes anionic species with valency n (Cl -, SO 2 -, and CO 3 -). A schematic diagram showing the detailed structure of a typical LDH [15] is shown in Figure 1.
A typical LDH structure consists of closely grouped hydroxyl anions surrounding the central metal ion in an octahedral and hexahedral lattice arrangement. Their layered sheets are constructed by hydrogen bond interaction which can interconnect the anionic species and water molecules within the interlayer space lattice [16]. LDH has the capacity to incorporate ionic species within the interlamellar spacing. Well-crystallized LDH materials are often synthesized by coprecipitation [17], hydrothermal synthesis [18], or a combination of both methods [19], with a high degree of reproducibility in all methods. Moreover, a scalable continuous process was developed to prepare ultrafine LDH nanoparticles using continuous-flow hydrothermal reactors [20]. Other methods were also explored to prepare LDH, where a LDH-like reactant is used as a template. These methods include anion exchange, dissolution crystallization, and dissociation deposition diffusion [21].
A combination of LDH and polymer in a nanocomposite has been found to produce superior properties where the physical, thermal, electrical, magnetic, optical, and mechanical properties of the polymer are enhanced [22][23][24][25][26][27][28]. LDHpolymer nanocomposites are fabricated by blending or intercalation in solution [29] or in a molten state [30] of the polymer, through in situ polymerization, through in situ formation of LDH [31], or through double in situ polymerization [32] and LDH formation methods [33]. Apart from the molten blending, homogeneous nanocomposites can be prepared. In a homogeneously formed nanocomposite, exfoliation of the LDH layers may take place, where polymeric chains can be arranged between the layered structures [34]. Exfoliation mostly occurs if the polymeric chains are hydrophilic in nature that will interact with the ionic species within the interlamellar spacing. This process takes place during solution blending due to the lower viscosity of the solution than the melt which allows the LDH layers to be exfoliated by the polymer chains that are dispersed in solution. Hydrophobic polymers, on the other hand, do not exfoliate the LDH structure unless an organic linker, such as a surfactant, is used [35].
In the current study, a MgAl-LDH and polystyrene will be homogeneously blended in a solution and will be further processed into a fibrous composite membrane using an electrospinning technique. A number of fiber-making processing methods are known [36][37][38][39], but electrospinning is characterized by its robustness. Fibrous membranes made by electrospinning feature the flexibility of design and the interconnectivity of its porosity as well as the high surface area of the fibers by virtue of the small size (nm-μm) of the fibers. Unlike functionalized nonfibrous polymer membranes [40], the characteristics of fibrous membranes make them feasible to be used for water treatment [41]. In an LDHpolystyrene fibrous composite membrane, a polystyrene fibrous structure will provide the skeleton of the membrane and will help in the removal of sub-μm solid particulates from water [42], while the LDH solid particles will act on the removal of soluble pollutants from water.
LDH-based materials have been widely used in the cleanup of soluble anionic pollutants in water, such as nitrates [43] and phosphates [44]. This application has been attributed to the diversity and functionality of the interlayer region, in the LDH structure, that contains easily exchangeable intercalated anions and solvation molecules with hydrogen bonds. Moreover, the ability of LDH to remove soluble heavy metal ions was also explored [45]. However, regeneration of LDH and their agglomeration in solution restrict their application in water treatment [46], unless compounded with other materials in the form of functional composites that will minimize these shortcomings [47].
The removal of soluble heavy metal contaminants from water takes place through ion exchange or precipitation [48]. Ion exchange takes place through the replacement of one of the metal ions of the LDH by the metal ion pollutant. Examples include the replacement of Mg 2+ ions in a MgAl-LDH [49] or Ca 2+ ions in a CaAl-LDH [50] by Zn 2+ ion pollutants. Lead (Pb 2+ ) ions, on the other hand, were removed from water by precipitation due to the increase in the pH of the aqueous medium as a result of LDH partial dissolution [51]. In general, there are still a limited number of publications reporting the cationic exchange capabilities of LDH.
In this current study, the formation and characterization of fibrous membranes of a polystyrene (PS) polymer matrix and the inclusion of MgAl-LDH particulates in the matrix, as a function of the relative proportions of the PS and LDH components of the membranes, are presented. Optimally prepared fibrous membranes have been also preliminarily evaluated for their potential as sorbents of Cd 2+ ions from aqueous media.

Fabrication of Polystyrene LDH Fiber
Composites. LDHcontaining PS solutions were prepared by suspending LDH powders in a 5, 10, and 20 wt% PS in DMF. In order to obtain a homogeneous suspension of each of the LDH-PS formulations, a predetermined weight of LDH fine powder was first suspended in DMF followed by dissolving the PS granules at room temperature until a homogeneous suspension was obtained. Based on our preliminary studies, the following LDH-PS formulations were prepared to be electrospun to nonbeaded fibrous morphology: 5% PS: 30, 40, 50, and 60 wt% LDH 10% PS: 5, 10, 20, 30, and 40 wt% LDH 20% PS: 5 and 10 wt% LDH An electrospinning technique was used for making LDH-PS composite fibrous membranes. In a typical experiment, a volume of 10 ml of each of the abovementioned formulations was electrospun at ambient conditions, an operating voltage of 20-25 kV, a feeding rate of 1 ml/hr, and a spinning distance of 15 cm. Collected LDH-PS composite membranes were dried in air to be characterized and evaluated.

Characterization and Evaluation.
The effect of the addition of LDH on the viscosity of the LDH-PS homogeneous suspensions was studied as a function of the concentration of LDH using a Brookfield viscometer (LV DV-II+Pro EXTRA) by Spindle-S34 at 50 rpm. A powder X-ray diffraction technique (Shimadzu XRD-6100, Japan) was applied to confirm the structure and crystallinity of the prepared, pristine LDH and the LDH-PS composite membranes. An IR spectroscopy technique (Thermo-Nicolet (NEXUS-470), USA) was used to further evaluate the structure of the prepared LDH powder as well as the electrospun PS fibers and their composite membranes. IR spectra were collected using an attenuated total reflectance (ATR) mode. The thermal analysis of the pure LDH, PS, and their electrospun compos-ite membranes was further carried out using a Shimadzu thermogravimetric analysis (TGA-50) and differential scanning calorimeter (DSC-60) with the heating range set between 30 and 600°C, with the gradual heating rate of 20°C/min. The morphological characteristics of the goldcoated pure LDH powder, PS fibers, and their composite fibrous membranes were studied using a scanning electron microscope (SEM, Oxford Instruments, UK) at an operating voltage of 15 kV (Beam Voltage). The effect of the addition of LDH on the hydrophobic nature of the LDH-containing PS fibers was investigated by the measurement of their water contact angle (DM-301, Kyowa Interface Science Co.) at 25°C.
To preliminarily evaluate the potential of the prepared LDH-PS fibrous membranes for the removal of heavy metal ions from waste water, a simulated aqueous medium was used containing 60 ppm aqueous solution of Cd(NO 3 ) 2 . This concentration presumes a highly contaminated waste compared with the WHO-permitted concentration of Cd 2+ ions in drinking water of 0.003 ppm. LDH-PS fibrous membranes containing the highest [LDH] were utilized for this experiment. A 0.1 g of each of the fibrous membranes was soaked in 10 ml of the Cd 2+ ion solution and kept at 25°C with continuous stirring for up to 12 hours. Aliquots were collected after predetermined time periods, filtered using a 0.22 μm cellulose acetate microfilter, and then analyzed for the concentration of Cd 2+ ions remaining after each time point. A control experiment was also conducted where 0.1 g of pristine finely ground LDH powder was used. Analysis of [Cd 2+ ] was carried out using an inductively coupled plasmaatomic emission spectroscopy (ICP-AES) after calibration, using a Cd 2+ standard solution.

Results and Discussion
3.1. Characterization of LDH-PS Fibrous Sorbents. Figure 2(a) shows the XRD pattern of the MgAl-LDH that was synthesized in the current study. All peaks were correlated with standard known crystallinity of LDH [11]. Diffraction peaks of MgAl-LDH were confirmed by the lamellar indices (003), (006), (009), and (110), whereas peaks observed at 2θ values of 11.3°, 22.8°, 34.5°, and 55.5°determine the molecular arrangements of the metallic centers of the layered structure, Mg/Al (3 : 1). The standard basal reflections (003) and (006) determine the octahedron-and hexahedron-layered arrangement of metal hydroxides (M-OH), similar to the structure of hydrotalcite and brucite clay minerals. The calculated interlayer distance of d (003) was 7.7 Å (approx. 0.7 nm). The interlayer distance was determined by the presence of the competing carbonate and nitrate anions, as shown in the IR spectrum of the prepared LDH (Figure 2(b)). The variation in ionic radii and electron affinity towards the hydroxyl layers alters the interlamellar distance. The high intensity of the diffraction peak determines the crystalline nature of the dimetal-structured layers. The IR spectrum of the as-prepared LDH powder in Figure 2(b) shows a typical spectrum of a pure LDH phase. A broad absorption band observed at 3492.3 cm -1 is attributed to the presence of -OH (stretch) vibration, while the presence of interlayer water molecules (H-OH) was confirmed at 1648.9 cm -1 . The sharp peak at 1345.5 cm -1 denotes  [52]. Both XRD and IR results hence confirm the structural purity and crystallinity of the synthesized LDH. Figure 2(c) demonstrates the morphology of the asprepared LDH powder. Porous agglomerates of the irregular-shaped LDH particulates, with an average size of 1.5 μm, were observed. These are formed as a result of the removal of the volatile ingredients, used during the precipitation of LDH, under vacuum.
In comparison, Figure 3(a) shows the IR spectrum of the as-prepared electrospun PS fibers, where bands characteristic to PS were observed at 3024.4 cm -1 and 2922.2 cm -1 , confirming the C-H (aromatic) tension and -CH 2 (asym./sym.) tension of the polymeric chain extension. The C=C (aromatic) stretch was indicated at 1642.2 cm -1 , while the -CH (bending) vibration of the methylene group was indicated at 1490.9 cm -1 and 1450.5 cm -1 . The C-H (bending) of the monosubstituted benzene derivative, at 754.2 cm -1 and 698.2 cm -1 . The presence of these bands confirms all functional vibration modes of a standard PS composition and indicates its phase purity and stability after electrospinning. Figure 3(b) shows an SEM image of the as-electrospun PS microfibers, where fibers averaging at 5 μm, with a homogeneous fiber size distribution and interconnected porosity were observed. The insert in Figure 3 The structural morphology and the elemental analysis of the electrospun LDH-PS fibers were recorded, as shown in Figures 5-8, for LDH-PS fibrous membranes made of solutions containing 5, 10, and 20 wt% PS in DMF, respectively. Compared with a homogeneous fiber size distribution of pure PS solutions as shown in Figure 3(b), all LDH-PS composite fibers showed a heterogeneous fiber size distribution. Moreover, a pronounced increase in the surface roughness of the composite fibers was also observed. This was also accompanied by an increase in the contact angle of the composite fiber surfaces, as shown in the inserts of all micrographs. This is attributed to the presence of the LDH particulates with their hydrophobic surfaces onto and into the PS fibers, while the hydrophilic layer that is made of the hydroxyl carbonate or nitrate groups is entrapped within the LDH crystalline sheets. Due to the significantly lower viscosity of the LDH-PS fibers electrospun from solutions containing 5 wt% PS, LDH particulates were found onto the surfaces of the fibers, as shown in Figures 5 and 8(a).
Increasing the initial concentration of the 5 in these solutions resulted in the increase of their respective viscosities. This was also reflected in the location of the LDH particulates that were more permeated within the fibers than onto their surfaces. This is evident in the SEM micrographs of the LDH-PS composite fibers at a higher magnification in Figure 6. It should be also mentioned that the presence of LDH particulates into and onto the PS fibers also resulted in the deformation of the fibers, as shown in Figures 6(e) and 8(b). On the other hand, low concentration of LDH (up to 10 wt%) initially added to PS solutions containing 20 wt% of the polymer resulted in the formation of fibers with smooth surfaces and homogeneous size distribution, as shown in Figures 7 and 8(c).
The composite fibrous membranes containing the highest proportion of LDH were further analyzed for their phase composition using XRD analysis. Figure 9 shows the XRD patterns of composite fibrous membranes made of 5, 10, and 20 wt% PS containing their respective highest concentrations of LDH at 60, 40, and 10 wt%, respectively. All patterns indicated the low crystallinity of the polystyrene (PS) matrix  Journal of Nanomaterials as broad peaks averaging at 10 and 19°were observed. In addition, the presence of LDH within the fibrous membranes was indicated by its main peak at 29.5 o . This peak decreased in intensity when decreasing the proportion of LDH in the LDH-PS membranes. Other lower intensity peaks were also observed in the XRD pattern of the fibrous composite made of 5 wt% PS and 60 wt% LDH. It should be mentioned that the location of all peaks in their respective positions and the absence of a peak around 2 o denote the absence of intercalation of the LDH sheets by the effect of the polymer during the preparation of the LDH-PS suspensions. These results are in accordance with the fact that the LDH sheets are hydrophobic while the hydrophilic hydroxyl, carbonate, or nitrate groups are entrapped between the sheets. Accordingly, the LDH particulates were enclosed within the hydrophobic PS fibers and adsorbed onto their surfaces, as shown in their respective high-magnification micrographs in Figure 8, causing an increase in the roughness of the PS fibers.
The presence of the LDH particulates within the fibrous composite membrane was further confirmed by IR and thermal analysis. Figure 10 shows the IR spectra of 5PS, 10PS, and 20PS. PS containing their respective highest proportions of LDH particulates. IR spectra of the as-prepared LDH and PS fibers are also shown for comparison. The presence of the polystyrene (PS) fibrous matrix was confirmed by the presence of its peaks due to the stretching and bending absorptions of the C-H bonds, as marked on their spectra. On the other hand, the presence of LDH in all fibrous composites was confirmed by the presence of two bands at 3650 and 1400 cm -1 [52]. These are related to the -OHand traces of the -CO 3 2and/or -NO 3 -, respectively, in the LDHcontaining fibers. The intensity of these bands heightened with the increase in the proportions of LDH in the fibers. The relative intensity of the bands attributed to the interlayer anions (H 2 O at 1514 cm -1 and nitrates and/or carbonates around 1348 cm -1 ) in the spectra of the 5PS and 10PS containing 60 and 40 wt% LDH was reversed, when compared with their ratio in the spectrum of pure LDH. This pattern indicates a higher affinity of the LDH layers to incubate water molecules than the nitrate or carbonate groups, when enclosed within PS fibers. It also indicates that nitrates and carbonates preferentially dissolve in the polar DMF solvent, while the interlayer hydrate layer is more intact with the metal oxide layers. No further signs of chemical interaction between the LDH and PS fibers were observed, indicating the physical existence of the LDH within the composite fibrous membrane created.
The effect of the addition of LDH particulates to PS on the thermal characteristics of the produced fibrous membranes was studied by TGA and DSC techniques. Table 1 summarizes these characteristics. Thermal performance of the as-prepared LDH particulates takes place over three events: (i) the loss of interlayer hydrate molecules (100-120°C), (ii) the deformation of LDH lattice structure due to the cleavage of M-O-H bonds (300-340°C), and (iii) the formation of metal oxides by the complete conversion of hydrates to oxides in effect of thermal decomposition (470-520°C). The remaining char residue (~82%) represents the Mg and Al oxides. In contrast, the degradation of the aselectrospun PS fibers takes place until the burnout of the organic polymer leaving a 7% carbonaceous residue. It was previously shown that the higher thermal stability of LDH 5 Journal of Nanomaterials enhances the thermal characteristics of the LDH-PS nanocomposites with slower decomposition rates [53]. Table 1 indicates that all LDH-PS fibrous membranes showed an enhanced thermal stability with a linear increase in the char residue with the proportion of LDH in each of the fibrous membranes. Based on the fact that no chemical reaction was proven between LDH and PS during the manufacturing of their fibrous membranes, this trend is therefore attributed to the presence of the higher thermally stable LDH component on the expense of the lower thermally stable PS matrix. Accordingly, there has been no pronounced variation in the melting point of the fibrous membranes, where an average melting point of 305:5 ± 2:4°C of the fibrous membranes was calculated. Moreover, the variation in enthalpy of melting (ΔH m ) was compared with pristine PS, indicating an increase in the latent heat energy of endothermic decomposition, due to the increase in LDH concentration within the PS matrix [54,55]. The amount of heat energy that is required to shift the mobile phase of the polymer chain to a semicrystalline order is reduced due to the increase in the presence of crystalline LDH nucleation sites. The increasing order of LDH concentration within the composite mixture enhances the endothermic process by limiting the mobility of the polymeric chains to an ordered orientation, due to the crystalline nature of LDH layers. The increasing order of enthalpy for melting the LDH-PS composites concluded that the increase in the LDH ratio improves the latent heat of the composites, prior to thermal decomposition of the polymer matrices [55].

Evaluation of LDH and LDH-PS Fibrous
Sorbents. LDH-PS fibrous membranes containing the highest proportions of LDH were evaluated for their efficiency in the sorption of Cd 2+ ions from a simulated waste water medium as a function of time. These LDH-PS membranes were selected based on the fact that they contained the highest proportion of LDH per a given concentration of PS. Figure 11 On the other hand, Figure 11(b) shows the concentration of Cd 2+ ions remaining after exposure of a fixed weight of each of the LDH-PS fibrous membranes to Cd 2+ aqueous media for up to 12 hours. In contrast, the as-prepared finely ground LDH powder sample was also evaluated. All samples showed variable abilities to remove Cd 2+ ions from their respective media, where a sudden decrease in the [Cd 2+ ] was observed to take place within the first 30 minutes of contact, indicating the fast removal of Cd 2+ ions. This was          Journal of Nanomaterials exposure of the LDH solid particulates to Cd 2+ ions in solution for an exchange reaction to take place. Moreover, the hydrophobic nature of the PS fibers and the ease of its fabrication are believed to participate in the chemical stability of the PS fibrous membranes carrying the LDH particulates.

Conclusion
The current study investigated the formation of PS fibrous membranes containing variable proportions of MgAl-LDH particulates by an electrospinning technique. The fabricated membranes were characterized for their composition, morphology, and thermal behavior as a function of PS and LDH proportions. Selected fibrous membranes were further evaluated for their potential as ion-exchange sorbents to remove Cd 2+ ions from water. Fibrous membranes of all compositions appeared as nonbeaded fibers with the LDH particulates incubated into and onto the surfaces of the PS fibers. Although the addition of LDH to each of the studied PS solutions increased their viscosities, there has been no indication of a chemical reaction between LDH and PS during mixing or electrospinning. In addition, mixing and electrospinning the homogeneous LDH-PS suspensions did not cause an intercalation or exfoliation of the LDH sheet structures, as proven by the XRD investigation of the fibrous membranes. However, a preferential leakage of the nitrate and carbonate ions from the LDH was evident, leaving behind a hydrate interlayer within the LDH structure. An overall improvement in the thermal characteristics of the LDH-PS fibrous membranes was observed, when compared to those of pure PS fibrous membranes. The as-prepared LDH solid particulates were proven to exchange their ions (mainly Mg 2+ ions) with Cd 2+ ions from aqueous media. This behavior was also shown at a lower extent in the case of PS Wavenumber (cm −1 ) Figure 10: Infrared spectra of 60% LDH-5% PS, 40% LDH-10% PS, and 10% LDH-20% PS fibrous membranes and as-prepared LDH particulates and as-electrospun PS fibers. 9 Journal of Nanomaterials fibrous membrane containing LDH particulates, due to the lower proportion of the LDH sorbent material in the membranes. It should be mentioned that the PS fibrous matrix is, therefore, an inert membrane that carries the LDH ion-exchange sorbent particulates as a water purification membrane. Accordingly, the proposed composite fibrous membrane overcomes the expected water pollution in case LDH solid particulates are directly used for water filtration. The findings of this study show the possibility of manufacturing chemically and thermally stable fibrous membrane with a potential to be used as sorbents for the removal of heavy metal ions from waste water.

Data Availability
All data presented in the manuscript are based on authentic experiments performed in our laboratories.

Conflicts of Interest
The author(s) declare(s) that they have no conflicts of interest.  Figure 11: Variation of the [Cd 2+ ] remaining in solution after ion exchange with LDH in (a) as-prepared LDH particulates, (b) 60% LDH-5% PS, 40% LDH-10% PS, and 10% LDH-20% PS fibrous membranes as a function of time.