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Article

Aspects of Polymeric-Based Membranes in the Water Treatment Field: An Interim Structural Analysis

1
Department of Mechanical Engineering, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
2
Department of Biomedical Technology, College of Applied Medical Sciences, King Saud University, Riyadh 11451, Saudi Arabia
*
Author to whom correspondence should be addressed.
Water 2023, 15(6), 1114; https://doi.org/10.3390/w15061114
Submission received: 21 February 2023 / Revised: 6 March 2023 / Accepted: 12 March 2023 / Published: 14 March 2023
(This article belongs to the Special Issue Research in Application of Advanced Water Treatment Technology)

Abstract

:
Solar-driven interfacial evaporation (SDIE) is considered a sustainable and environmentally friendly technology for using solar energy to produce fresh water, which is a crucial resource for the existence of human life. Porous membranes are widely used in SDIE owing to their porous structure, which is highly suitable for this kind of photothermal material and allows an efficient supply of water and escape of vapor during the evaporation process. Electrospinning is perhaps the most versatile technique to produce highly porous structures of nanofiber membranes with a large surface-to-volume ratio, high porosity, low density, and many advantages. Nevertheless, acquiring a stronger background on the initial research questions in this enticing field of research needs further investigation. Typically, for the enhancement of process control, the impact of flow rate on the morphology of the prepared membrane is quite important. This research article has two-fold objectives: firstly, it discusses the fundamental description of the photothermal conversion mechanism of polymer-based photothermal materials for water treatment. A systematic investigation supported by previous studies revealing the working mechanism and the design of solar-driven interfacial evaporation has been provided. On the other hand, our interim experimental results elaborate on the influence of process conditions such as electrospinning parameters on the structural morphology and diameter of fabricated electrospun nanofibers produced by using the coaxial electrospinning setup in our lab. The scanning electron microscope (SEM) was used to examine the morphology of the electrospun nanofibers. Our introductory results provide a useful insight into tuning the necessary process parameters to fabricate electrospun polyacrylonitrile (PAN) nanofiber membranes by electrospinning technique. From our preliminary results after the three processing experiments, it is revealed that a polymer concentration of 10% wt., an applied voltage of 20 kV, a tip-to-collector distance of 18 cm, and a flow rate of 0.8 mL/h produce the optimum nanofiber membranes with a uniform structure and a diameter in the range 304–394 nm. The variation in the diameter of nanofibers in the three processing conditions is endowed by the regulation of the initiating droplet extruded from the tip of the metallic needle (syringe jet) to the collector using the electrospinning setup.

1. Introduction

The scarcity of fresh water is recognized as a major global problem and indeed is critical for human life [1]. Due to population growth, climatic changes, and escalating pollution, many people have been experiencing a freshwater crisis in recent years [2,3]. Although 70% of the Earth’s surface is covered by water, most of the oceans and seas are too salty to drink. Water is one of the most prevalent chemical components on Earth. For this reason, many efficient technologies for water treatment and desalination are urgently needed. The water treatment process includes a variety of techniques, such as microfiltration (MF) [4], ultrafiltration (UF) [5], nanofiltration (NF) [6], and reverse osmosis (RO) [7], that have been investigated to produce fresh water. These techniques are difficult to use, especially in off-grid villages or remote regions, due to high energy consumption, costly infrastructure, and installations. For this reason, people tend to use solar energy to generate fresh water by using solar evaporation systems.
In general, in the solar evaporation system that is designed without the solar stills, they are conventionally used for heat concentration, and the corresponding process of heat generation occurs at the receiver surface; however, the process of vapor generation occurs in another part of the system [8]. Nonetheless, this separation of vapor production from heat generation zones causes unwanted temperature dips at the surfaces where the heat is being evaporated, which eventually results in unavoidable heat losses and leads to lower evaporation efficiencies for solar stills’, which range from 30–45% [8]. To mitigate this issue, solar absorption techniques utilizing optical nanofluids have been described by shifting the location of heat generation to the inside of the fluid to minimize heat losses on the surface [9,10]. Owing to its inadequacy for evaporation applications, where a high surface temperature is required, this kind of heating approach has generated only moderate improvements in the overall evaporation efficiency. Furthermore, it is still not sustainable to pump and disperse nanofluids under prolonged, intense radiation from sunlight. Quite recently, a methodology related to an interfacial evaporation system has been proposed for the confinement of heat at the liquid surface. Interestingly, this method of water treatment, which selectively warms the evaporative section of water rather than the entire body of water, has led to an evaporation efficiency of about 90% under lower optical concentrations and is known as solar-driven interfacial evaporation (SDIE) [11,12]. Membrane technology could be used in SDIE to produce a porous polymeric membrane with a porous structure. The porous polymeric membranes can incorporate photothermal materials and allow efficient water transport and escape of vapor during the evaporation process [13]. Porous membranes can be made in a variety of ways depending on the polymeric material and the purpose. Phase inversion is one of these methods that has previously been thoroughly developed and disseminated. Others, such as electrospinning, are more inventive but are still in their infancy and need further research. Recently, the electrospinning technique has been gaining popularity for the production of polymeric nanofiber membranes, which are appreciated for their undoubted advantages such as a high level of large material selection, versatility, and one-step preparation [14]. This study is aimed at examining the existing literature in order to provide sufficient background information for this field of research. Simultaneously, an introductory framework for investigating the effect of solution parameters such as flow rate (mL/h) and processing parameter (applied potential voltage) is explored as well. From our experimental results, it is summarized that a moderate flow rate of 0.8 mL/h mitigates the collection of solvent and improves solvent evaporation, which in turn has a positive effect on the lesser formation of electrospinning defects such as blobs and flattened web-like structures [15,16].

2. Principles of a Solar-Driven Interfacial Evaporation (SDIE) System

2.1. Key Components in Design and Their Main Attributes

SDIE is recognized as a novel route of distillation mechanism that is also considered to be a sustainable and environment-friendly technology for consuming solar energy to produce fresh water. Thus, in SDIE systems, the utilization of photothermal material is decreased as the entire liquid is not heated, and hence there are more efficient methods to tune the evaporation performance with this kind of water treatment process [16]. Further benefits of SDIE include low requirements for raw seawater’s water quality, the absence of conventional energy use, the absence of secondary contamination, and high freshwater purity.
The SDIE system and design has three major components with specific attributes:
I.
A solar absorber that can take in solar energy, convert it to heat, and still allow vapor to pass through the front face.
II.
A floating evaporation structure that may increase the evaporation rate and, in tandem, deliver water to the heated region.
III.
A thermal insulator that can effectively reduce the loss of the converted solar thermal energy to the bulk water (see Figure 1 for the schematic) [17].
Many absorber materials can be used in SDIE systems, such as plasmonic metals, carbon-based materials, semiconductors, and polymers, including porous electrospun nanofiber membranes [18,19]. Porous electrospun nanofiber membranes are widely used in SDIE owing to their porous structure that can incorporate photothermal materials, allow the efficient supply of water, and escape of vapor during the evaporation process [14].

2.2. Membranes as Solar Absorbers for Solar Steam Generation

Efficient conversion into heat and the absorption of wide-spectrum solar radiation are the first critical steps to drive water evaporation. Two groups of solar-absorbing materials are called carbon-based materials [20,21,22,23] and plasmonic-based absorbers [24,25].
Naturally, carbon-based materials are black; therefore, they are potentially better at absorbing solar radiation over a much broader spectrum. To date, a variety of carbon-based absorbers have been investigated, including graphite [20], reduced graphene oxides [22], and hollow carbon spheres [21]. These kinds of carbon-based materials are good candidates due to their high absorbance, low cost, and good processability. For example, Gao et al. [25] prepared an electrospun nanofiber membrane of hydrophobic PVDF and a composition of hydrophilic carbon black/polyacrylonitrile (CB/PAN) for solar steam generation. They found that the CB/PAN composite layer with a 45% weight percentage of CB content has a higher efficiency of solar absorption (98.6%) within a broadband wavelength range of 250–2500 nm, which is much higher than that of the pure PAN electrospun nanofiber membrane layer. In addition, a double-layered evaporation structure consisting of a floating carbon foam and an exfoliated graphite photothermal converting layer on top has achieved 97% efficiency of absorption of solar radiation ranging from 250 to 2250 nm [20].
Nanoscale plasmonic absorbers have been used for solar-driven evaporation as well as localized and effective photothermal energy conversion [26,27,28]. It is due to the fact that carbon-based materials and plasmonic nanoparticles such as Au can absorb a narrow band of solar light around their resonance peak. For example, Wu et al. [28] have reported the simultaneous inclusion of Au nanocages (AuNCs) with an electrospun nanofiber membrane of PVDF for the SDIE application. The incorporation shows efficient photothermal conversion absorbance of solar radiation ranging from 400 to 1200 nm, which increased the evaporation rate. The evaporation rate efficiencies reached 67% and 79.8% for the AuNC/PVDF containing AuNCs at weight percentages of 0.05% and 0.10%, respectively [28,29].

2.3. Membranes as an Integrated Structure for Water Evaporation

Principally, to achieve high evaporation performance and high total evaporation efficiency, photothermal heat generation must be localized at the air/liquid interface. As shown in Figure 1, one straightforward method to achieve interfacial heating is to float the solar absorber at the air-to-liquid interface either via its surface hydrophobicity, low density, or the usage of porous materials such as air-laid papers [30], carbon foams [20], and nanofiber membranes [31]. An efficient water supply to the heating region of SDIE is critical to achieving high water evaporation performance and high evaporation efficiency. For porous floating evaporation systems, the capillary wicking effect of the porous structure has been used to pump water to the heated region. Porous structures with small pore sizes and high porosity amplify the capillary pumping of water toward the heating region. The pore size and porosity of nanoscale materials are more appropriate than micrometer-sized porous materials for efficient capillary pumping of water [22]. In this floating evaporation system of SDIE, closed pores also help reduce the thermal conductivity between the heated region and water, that way keeping the generated heat from the heat loss to the bulk water and surrounding environment [20,21].

2.4. Importance of Efficient Structural Design for Thermal Energy Localization

To concentrate heat at the air-to-water evaporative interface, lower heat losses between the heated area and bulk water, and increase evaporation efficiency, the engineering of an efficient thermal insulation design is crucial. It is because the downward conduction loss ( q w a t e r r) is dependent on the thermal conductivity of the insulation material (k) and can be estimated by the following Equation (1) [32]:
q w a t e r = k A d T d x
where A is the solar absorber’s surface area that faces the sun, and dT/dx is the temperature gradient between the solar absorber and the water below. The radiation loss and the upward convection might theoretically be computed using the temperature of the evaporation surface that was measured.

2.5. Evaporation Rate and SDIE Efficiency Measurements

The water evaporation rates ( v ) can be calculated using Equation (2) [33]:
v = m l o s s A t
where A is the surface area of the electrospun nanofiber membrane, m l o s s is the weight loss of pure water, and t represents the irradiation time under simulated sunlight.
In addition, the efficiency of SDIE (solar-to-vapor conversion efficiency, η S V ) can be evaluated. The efficiency of SDIE ( η S V ) is linearly dependent on the evaporation rate ( v ) and refers to the ratio of stored thermal energy in the generated vapor to the incoming solar energy. ηsv could be calculated by using Equation (3) [32]:
η S V = v h L V q s o l a r
where h L V is the change in total evaporation enthalpy that involves the sensible heat and phase-change enthalpy (2260 kJ/kg); q s o l a r is the incident solar flux.

3. Development of Electrospinning Technique for Membrane Fabrication

3.1. Methods, Materials, Components, and Mechanism

Many ways were used to fabricate polymeric fibrous membranes. Traditionally, polymer fibers are produced by wet-spinning, melt-spinning, dry spinning, and gel-state spinning; these techniques use mechanical forces to draw polymeric solution from the spinneret. The average fiber diameter from these techniques was 5–500 microns. In template synthesis, the nonporous membrane is used as a template to produce nanofibers with hollow or solid shape; most importantly, nanofibers of semiconductors, metals, and electronic conductive polymers can be produced continuously [33]. In the phase separation technique, the polymer is dissolved and extracted using different solvents, then it is frozen and dried, which results in the formation of a nonporous foam. This technique is a time-consuming process [34]. The self-assembly process is the one in which pre-existing components arrange among themselves into the required patterns. This is also a time-consuming process used as a phase separation technique in producing continuous polymers [35]. However, electrospinning technology is one of the most versatile and efficient technologies, allowing for the production of much smaller fibers when compared to the above-mentioned techniques [36].
Electrospinning is a term derived from electrostatic spinning; this technology has been known for many decades, and it is a process in which electrostatic forces are used on synthetic fibers. In the year 1934, a scientist named Formhals published a few patents related to electrospinning technology. Generally, electrospinning utilizes a high-voltage source to inject a positive charge into a polymer solution. The charged polymer coming out of a needle tip (jets) will be attracted or pulled towards a collector to which a negative charge is given and placed at a certain distance from the needle tip (as shown in Figure 2). Formhals, in his second patent, described the use of multiple jets of the same polymer simultaneously in the electrospinning process [37,38]. The work done by Formhals turned the focus of scientists towards understanding and developing the electrospinning process, and it took around 30 years to come out with a new publication related to electrospinning. During the year 1969, a scientist named Taylor published work regarding the formation of the jet during electrospinning. He examined the polymer droplet behavior at the tip of a capillary and stated that when an electric field is applied, there will be an increase in electrostatic repulsion due to like charges in the polymer solution, which leads to the formation of a cone shape at the leading edge of the solution. This cone is called the Taylor cone [39]. Soon after Taylor’s publication on fiber jet, the focus shifted from fundamental understanding to other parameters and properties, such as the effect of various parameters such as flow rate, solution viscosity, applied voltage, etc. Baumgarten et al. [40] examined the physical properties of electrospun fibers by dissolving polyacrylonitrile in dimethylformamide and passing it through a metallic capillary. By using a high-resolution camera, he determined that a monolithic fiber was being drawn from an electrically charged Taylor cone. He also revealed that the diameter of the fibers directly depends on the viscosity of the solution, i.e., the higher the viscosity, the larger the fiber diameter.

3.2. Preliminary Results for Establishing the Process Parameter for Fabricating the Membrane Material in the Present Research

To fabricate uniform nanofiber membranes, the effects of the main processing parameters on the diameters of nanofibers have been investigated, including applied voltage, tip-to-collector distance, and flow rate. Many studies have found that the diameters of nanofiber membranes increase with increasing applied voltage [40,41] and flow rate [42], while increasing the distance from the needle to the collector leads to a decrease in the diameters of the nanofibers [43].
In this manuscript, different process parameters were used to fabricate electrospun polyacrylonitrile (PAN) nanofiber membranes by the electrospinning technique. Before the electrospinning process, a PAN solution was prepared. The solution consisted of 10% PAN, which was dissolved in a N, N-dimethylformamide (DMF) solution. More details: 1 g of PAN was dissolved in 10 mL of DMF solution and stirred overnight using a magnetic stirrer at room temperature. After electrospinning, the resulting nanofiber membrane from each process condition was characterized using the SEM technique to reveal the morphology of the nanofibers and the presence of any beads or defects. Table 1 summarizes the process conditions tested that were used to fabricate PAN nanofibers and the corresponding findings revealed by SEM images (see Figure 3). In summary, among all the parameters, it is concluded that a polymer concentration of 10% wt., an applied voltage of 20 kV, a tip-to-collector distance of 18 cm, and a flow rate of 0.8 mL/h produce the best nanofiber membranes with uniform structure. Therefore, these process conditions will be applied to prospective work in SDIE applications.

3.3. Characteristics of Electrospun Nanofibrous Membranes

Electrospun nanofibrous membranes possess remarkable characteristics and some appropriate advantages, such as high surface area-to-volume ratios, high porosity, adjustable pore size, membrane thickness, and the possibility to incorporate photothermal materials, which make them attractive for SDIE. Porous electrospun nanofiber membranes are widely used in SDIE owing to their porous structure that can allow an efficient supply of water and the escape of vapor during the evaporation process. The porosity of electrospun nanofiber membranes, expressed as a percentage, represents the ratio of the pore volume of electrospun nanofiber membranes to the total volume of the electrospun nanofiber membranes. The porosity significantly influences the liquid water distribution and transport path in the electrospun nanofiber membranes, which is important to ensure the water supply to the heated region in the SDIE system [43,44]. The porosity of electrospun nanofiber membranes is normally measured by several established methods, which are summarized in Table 2.
Many absorber materials can be incorporated with electrospun nanofiber membranes to increase the efficiency of the photothermal conversion in the SDIE system, such as plasmonic metals, carbon-based materials, and semiconductors. For example, Wu et al. [29] have reported the incorporation of Au nanocages (AuNCs) with electrospun nanofiber membranes of polyvinylidene fluoride (PVDF) for SDIE application. The incorporation shows efficient photothermal conversion, which increased the evaporation rate. The evaporation rate efficiencies reached 67% and 79.8% for the AuNC/PVDF electrospun nanofiber membranes containing AuNCs at weight percentages of 0.05% and 0.10%, respectively. Gao et al. [26] prepared electrospun nanofiber membranes of hydrophobic PVDF and electrospun nanofiber membranes composed of hydrophilic carbon black/polyacrylonitrile (CB/PAN) for solar steam generation. They found that the CB/PAN composite electrospun nanofiber membrane layer with a 45% weight percentage of CB content has a higher efficiency of solar absorption (98.6%) within a broadband wavelength range of 250–2500 nm, which is much higher than that of the pure PAN electrospun nanofiber membrane layer.

4. Application of a Solar-Driven Interfacial Evaporation System

In the last few decades, many researchers have also endeavored to explore the fabrication of porous nanofibers using different technologies. Broadly speaking, the ultrathin membrane has potential in multiple platforms of nanotechnology such as tissue engineering [44,45], photovoltaic cells [46,47], membranes in separation processes [48], high-performance air filters [49], advanced composites [50], biosensors [51], etc. Predominantly, the electrospun nanofiber membranes have remarkable characteristics such as high porosity, large pores with a narrow pore size distribution, a large surface area, and are easy to incorporate absorber materials [52]. These characteristics are most desirable for SDIE applications to produce a high water vapor flux. Recently, many studies have used various functional polymer electrospun nanofiber membranes in SDIE applications. Table 3 summarizes the various types of electrospun nanofiber membranes used in SDIE applications. Some of the main findings are briefly discussed below:
-
Guo et al. [53] developed graphene oxide (GO) functionalized with polyvinyl alcohol (PVA) electrospun nanofiber membranes. The GO/PVA electrospun nanofiber membranes possess good photothermal desalination efficiency due to their extreme hydrophilicity, which could ensure continuous water supply. The light absorption efficiency reached up to 94%, and the optimal evaporation rate could achieve 1.42 kg m−2 h−1.
-
Li et al. [54] have prepared electrospun nanofiber membranes of hydrophilic polymethylmethacrylate (PMAA) with silica and GO/CNT. The top layer of GO/CNT is sprayed on a PMAA/silica nanofiber membrane. A silica electrospun nanofiber membrane with low thermal conductivity can pump water continuously; meanwhile, the GO/CNT hybrid layer can localize heat energy and generate water vapor. The evaporation rate reached 1.3 kg m−2 h−1 with 74% efficiency.
-
Xu et al. [55] fabricated electrospun nanofiber membranes of PAN and PMMA, followed by spray deposition of CB nanoparticles. The two layers are the upper nanoparticle CB coating of PMMA as a solar absorber layer and for water evaporation, and the bottom hydrophilic PAN layer for pumping water. The evaporation rate result was 1.3 kg m−2 h−1 with a conversion efficiency of 72%.
-
Another recent study by Ding et al. [33] has prepared two layers of polylactic acid (PLA) electrospun nanofiber membranes loaded with Chinese ink nanoparticles and lower modified (hydrophilic) PLA (2 wt% Chinese ink/PLA-PLA). They used modified hydrophilic PAN as a transport layer for pumping water. The result showed that the evaporation rate reached up to 1.29 kg m−2 h−1 with 81% SDIE efficiency.
-
Zhu et al. [56] designed and prepared flexible and washable CNT-embedded PAN electrospun nanofiber membranes. The CNT-PAN exhibits high hydrophilicity, which could ensure continuous water supply. The system has a photoabsorption efficiency of 90.8% with a high seawater evaporation rate of 1.44 kg m–2 h–1.
-
In addition, another study by Fan et al. [57] has prepared reduced graphene oxide (rGO) composited with PAN membrane by the electrospinning method. The rGO/PAN membrane presented good advantages in heat localization and high evaporation efficiency. The membrane converts 89.4% of the light into heat, allowing 1.46 kg m−2 h−1 of seawater to evaporate.
-
Jin et al. [58] have studied PAN, Polystyrene (PS), and nylon 6 nanofibers as matrices and inorganic CB nanoparticles inside the matrix as light-absorbing components. The photothermal membrane with an optimized carbon loading exhibits desirable underwater black properties, absorbing 94% of the solar spectrum. The result is an evaporation rate of 1.24 kg m−2 h−1 with 83% efficiency.
-
Qi et al. [59] have fabricated a silicon dioxide/carboxylated multi-walled carbon nanotube/polyacrylonitrile (SiO2/MWCNTs-COOH/PAN) nanofiber membrane. After that, the interfacial water evaporator is assembled by attaching it to a piece of filter paper with insulated (PS) foam used as a support layer and cotton yarns (CYs) used for water transportation. As a result, the composite nanofiber membrane presented an evaporation rate capacity of 1.28 kg m−2 h−1 with a photothermal conversion efficiency of 82.52%.
-
Wu et al. [60] have also prepared porous carbonaceous membranes that consist of continuous ultrafine carbon nanofibers. This membrane has hydrophilic properties and continuous channels for sufficient water supply. The prepared carbonaceous membranes can absorb 95% of the solar spectrum and have an evaporation rate capacity of 1.33 kg m−2 h−1 with 81.71% SDIE efficiency.
-
Wu et al. [29] have reported the incorporation of AuNCs into electrospun nanofibers of PVDF. The surface of PVDF became hydrophilic after being treated with oxygen plasma, so AuNC/PVDF nanofibers can easily pump and evaporate the water. As a result, the evaporation rate reached up to 1.27 kg m−2 h−1 with 79.8% efficiency.
-
Chala et al. [61] have prepared reduced tungsten oxide/polylactic acid (WO2.72/PLA) nanofiber membranes. The WO2.72/PLA nanofiber membranes are floatable on water due to their surface hydrophobicity. The water evaporation efficiency reached 81.39%.
-
In addition, Gao et al. [26] have prepared two layers of an upper (CB) coating with (a PAN) layer and a lower (PVDF) layer (CB/PAN/PVDF). They used the hydrophobic PVDF with punched holes for water transport due to the capillary effect. The hydrophilic CB/PAN composite nanofiber layer on top has a high broadband solar absorption of 98.6%. The assembled CB/PAN/PVDF has an evaporation rate capacity of 1.2 kg m−2 h−1.
-
Meng et al. [62] have studied ultra-light three-dimensional aerogels assembled by hierarchical Al2O3/TiO2 nanofibers and (rGO). The hydrophilic Al2O3/TiO2 nanofibrous channels are linked up with the graphene hot spots and bulk water for sufficient water transport and bulk water insulation. Meanwhile, the Al2O3/TiO2 layer can localize the heat energy and generate steam. The evaporation rate of introducing Al2O3/TiO2 nanofibers into rGO reached 2.19 kg m−2 h−1.
Table 3. List of various functional polymer electrospun nanofibrous membranes used in SDIE applications.
Table 3. List of various functional polymer electrospun nanofibrous membranes used in SDIE applications.
#MaterialsMorphology of Fibrous Membrane (Diameter, Porosity, and Alignment)Light Absorbance (Wavelength Range)Evaporation Rate
(kg m−2 h−1) and Efficiency (%)
Refs.
1Upper and lower layers of polymethylmethacrylate (PMMA) and polyacrylonitrile (PAN) are coated with carbon black nanoparticles (CB).
  • Evap. layer: CB/PMMA (hydrophobic)
  • Transport layer: PAN (hydrophilic)
  • Support layer: NA
  • Fiber diameter:
    Upper PMMA: 1.4 μm
    Lower PAN: 0.5 μm
  • Porosity: not reported
  • Alignment: random from image (not reported)
97%
(200–2500 nm)
1.3 with 72% efficiency[54]
2Chinese ink nanoparticles are embedded in two layers of PLA fibrous membrane, along with less modified (hydrophilic) PLA.
(2 wt% Chinese ink/PLA-PLA)
  • Evap. layer: Chinese ink/PLA (hydrophobic)
  • Transport layer: modified PAN (hydrophilic)
  • Support layer: insulation foam layer
  • Fiber diameter:
    Upper PLA: nanometer scale (not reported)
    Lower PLA: 5–10 μm
  • Porosity: not reported
  • Alignment: random from image (not reported)
~0.53
(300–2500 nm)
1.29 with 81% efficiency[32]
3Graphene oxide-functionalized PVA electrospun nanofibrous membrane
  • Evap. and transport layer: GO/PVA EFMs (hydrophilic)
  • Support layer: NA
  • Fiber diameter: 228 ± 10 nm
  • Porosity: not reported
  • Alignment: random from image (not reported)
94%
(200–2500 nm)
1.42[52]
4CNT-Embedded PAN nonwoven fabrics
  • Evap. and transport layer: 2wt% CNT-PAN (hydrophilic)
  • Support layer: PS foam (heat-insulation layer)
  • Fiber diameter: 200–300 nm
  • Porosity: 88.6%
  • Alignment: random from image (not reported)
90.8%
(350–2500 nm)
1.44 with 90% efficiency[55]
5Polyacrylonitrile (PAN), polystyrene (PS), and nylon 6 nanofibers as a matrix and inorganic carbon black (CB) nanoparticles inside the matrix as light-absorbing components
  • Evap. layer: nylon 6
  • Support layer: nylon–C cloth (hydrophilic)
  • Fiber diameter: 100–400 nm
  • Porosity: not reported
  • Alignment: random from image (not reported)
94%
(350–2000 nm)
1.24 with 83% efficiency[57]
6Membrane composed of silicon dioxide, carboxylated multi-walled carbon nanotubes, and polyacrylonitrile (SiO2/MWCNTs-COOH/PAN)
  • Evap. layer: fiber membranes
  • Transport layer: cotton yarns (CYs) (hydrophilic)
  • Support layer: filter paper on PS foam
  • Fiber diameter: 1.4–2 μm
  • Porosity: not reported
  • Alignment: random from image (not reported)
96.52%
(300–2500 nm)
1.28 with 82.52% efficiency[58]
7Porous carbonaceous membrane
  • Evap. and transport layer: carbonaceous membrane (hydrophilic)
  • Support layer: PS foam
  • Fiber diameter: 200–400 nm
  • Porosity: not reported
  • Alignment: random from image (not reported)
95%
(300–2500 nm)
1.33 with 81.71% efficiency[59]
8Reduced graphene oxide (rGO)/polyacrylonitrile (PAN) composite membrane
  • Evap. layer: rGO/PA (hydrophobic)
  • Transport layer: filter paper (hydrophilic)
  • Support layer: PS foam (heat-insulation layer)
  • Fiber diameter: 600 ± 50 nm
  • Porosity: not reported
  • Alignment: random from image (not reported)
89.4%
(350–2500 nm)
1.46[56]
9Polydimethylsiloxane/carbon nanotube/poly (vinylidene fluoride) (PDMS/CNT/PVDF) membrane
  • Evap. layer: PDMS(top)/CNT/PVDF) (hydrophobic)
  • Transport layer: PVA (hydrophilic)
  • Support layer: foam
This is used in membrane distillation which has a slightly different setup compared to SDIE.
  • Fiber diameter: 0.49 μm
  • Porosity: not reported
  • Alignment: random from image (not reported)
92%1.43[30]
10Gold nanocages (AuNCs)/Poly (vinylidene fluoride) (PVDF)
  • Evap. and transport layer: 0.10 wt% AuNC/PVDF (only the surface of PVDF became hydrophilic after being treated with oxygen plasma)
  • Support layer: NA
  • Fiber diameter: 280.63 ± 41.45 nm
  • Porosity: not reported
  • Alignment: random from image (not reported)
AuNCs range: (400–1200 nm)1.27 with 79.8% efficiency[28]
11Tungsten oxide/Polylactic acid (WO2.72/PLA) fiber membranes
  • Evap. and transport layer: 7wt%WO2.72/PLA (hydrophobic)
  • Support layer: NA
  • Fiber diameter: 8−13 μm
  • Porosity: not reported
  • Alignment: random from image (not reported)
(300–2500 nm)81.39% efficiency[60]
12Two layers of upper carbon black nanoparticles (CB) coating the polyacrylonitrile (PAN) layer and lower polyvinylidene fluoride (PVDF) layer
(CB/PAN//PVDF)
  • Evap. layer: 45wt% CB/PAN (hydrophilic)
  • Transport layer: PVDF with punched holes for water transport due to capillary effect (hydrophobic)
  • Support layer: NA
  • Fiber diameter:
  • Upper CB/PAN: Not reported
  • Lower PVDF: 500 nm
  • Porosity:
  • Upper CB/PAN: 88%
  • Lower PVDF: 86%
  • Alignment: random from image (not reported)
98.6%
(250–2500 nm)
1.2[24]
13PMAA/Silica nanofiber membrane and GO/CNT
A top layer of GO/CNT sprayed on PMAA/Silicon nanofiber membrane
  • Evap. layer: GO/CNT
  • Transport layer: PMAA/Silica (hydrophilic)
  • Support layer: PS foam
  • Fiber diameter: not reported
  • Porosity: not reported
  • Alignment: random from image (not reported)
80%
(250–2500 nm)
1.3 with 74% efficiency[53]
14Hierarchical Al2O3/TiO2 nanofibers and reduced graphene oxide (RGO)
  • Evap. and transport layer: Al2O3/TiO2/RGO (hydrophilic)
  • Support layer: NA
  • Fiber diameter: Not reported
  • Porosity: 90.4%
  • Alignment: random from image (not reported)
Not reported2.19[61]
15Rubidium tungsten bronze and recycled triacetate cellulose (RbxWO3/rTAC) porous fiber membranes
  • Evap. and transport layer: 15 wt% RbxWO3/rTAC (hydrophobic)
  • Support layer: NA
  • Fiber diameter: 80–150 nm
  • Porosity: 90%
  • Alignment: (Not reported)
(300–2200 nm)90.4 ± 2.1% efficiency[62]

5. Challenges and Future Work

Electrospun nanofiber membranes possess some appropriate advantages, including high porosity, adjustable pore size, and membrane thickness, which make them attractive for SDIE application. Yet when dealing with actual water sources (such as saltwater, river water, industrially contaminated water, groundwater, and city sewage), the long-term durability of electrospun nanofiber membranes and photothermal materials has been subject to considerable difficulty as far as sustainability is concerned. As observed by Kaur et al. [63,64,65,66], the electrospun nanofiber membranes are in a ‘cotton-like’ state because they were spun with little structural integrity and fiber cohesion. That’s why they are difficult to handle and unsuitable for many practical applications. Researchers who are interested in solar evaporation have mostly concentrated on increasing evaporation rates and efficiency, paying little consideration to the stability of the materials utilized or the device’s performance against water. For this reason, we are planning, as future work, to apply some post-treatment methods (e.g., crosslinking, solvent welding, heat treatment, and hot-pressing) on the electrospun nanofiber membranes to enhance their mechanical and thermal stability. Then, the treated electrospun nanofiber membranes will be investigated for the water evaporation performance of SDIE and overall evaporation efficiency. Nonetheless, among various foreseen challenges, environmental pollution has long been a crucial area of concern that needs immediate attention. With the rise of industrialization, the issue of eliminating pollutants from water and wastewater has also gotten worse. Owing to this new avenue of research, exploiting the potential of nanofibers in the absorption of pollutants, antibacterial properties, oil resistance, and salt resistance are also being explored in tandem in the field of water treatment and purification [67,68,69,70,71].

6. Conclusions

SDIE is recognized as a new branch of distillation that is considered a sustainable and environmentally-friendly technology for consuming solar energy to produce fresh water. Electrospinning is a cost-effective method with a large material selection for fabricating nanofiber membranes. Electrospun nanofiber membranes have characteristics such as large surface area-to-volume ratios, high porosity, adjustable pore size, high membrane thickness, easy-to-incorporate photothermal absorber materials, and various fiber morphologies and geometries. For SDIE applications that require high water vapor flow, these qualities are ideal.
The main results of this research article can be summarized as follows:
  • The optimized PAN nanofibrous membrane was successfully fabricated using the electrospinning technique.
  • The definition and components of SDIE have been discussed.
  • The process parameters of electrospinning, such as the applied voltage, the distance between the needle and collector, the flow rate, etc., significantly affect the nanofiber morphology, and by manipulating these parameters, one can get the characteristics desired for SDIE application.
  • Our experimental results from the electrospinning setup for fabricating the membrane were also validated.
  • Following SEM characterization revealed that during the electrospinning method, the flow rate of the aqueous solutions has a stronger influence on the fiber diameter and structural morphology of electrospun nanofibers.
  • At lower flow rates of PAN solutions, the electron-spun fabricated fibers showed irregular morphology with large variation in fiber diameter, whereas at the optimum flow rate of 0.8 mL/h, the electron-spun fiber exhibited very few beads and the resulting nanofibers had a diameter in the range of 304–394 nm.
  • The outlook and challenges for the various types of electrospun nanofiber membranes used in SDIE applications have also been summarized for associated future works.

Author Contributions

Conceptualization, M.F.I., H.F.A., A.Z.A. and A.K.A.; methodology, M.F.I., H.F.A., A.Z.A. and A.K.A.; software, M.F.I., H.F.A., A.Z.A. and A.K.A.; validation, M.F.I., H.F.A., A.Z.A. and A.K.A.; formal analysis, M.F.I., H.F.A., A.Z.A. and A.K.A.; investigation, M.F.I., H.F.A., A.Z.A. and A.K.A.; resources, M.F.I., H.F.A., A.Z.A. and A.K.A.; data curation, M.F.I., H.F.A., A.Z.A. and A.K.A.; writing—original draft preparation, M.F.I., H.F.A., A.Z.A. and A.K.A.; writing—review and editing, M.F.I., H.F.A., A.Z.A. and A.K.A.; visualization, M.F.I., H.F.A., A.Z.A. and A.K.A.; supervision, M.F.I., H.F.A., A.Z.A. and A.K.A.; project administration, M.F.I., H.F.A., A.Z.A. and A.K.A.; funding acquisition, H.F.A. and A.Z.A. All authors have read and agreed to the published version of the manuscript.

Funding

Researchers Supporting Project number (RSPD2023R579), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors extend their appreciation to the Researchers Supporting Project number (RSPD2023R579) at King Saud University, Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic displaying the features of the solar-driven interfacial evaporation (SDIE) system.
Figure 1. Schematic displaying the features of the solar-driven interfacial evaporation (SDIE) system.
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Figure 2. Schematic showing the features of the electrospinning setup for fabricating the membrane material in the present study.
Figure 2. Schematic showing the features of the electrospinning setup for fabricating the membrane material in the present study.
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Figure 3. Scanning electron microscopy (SEM) images of pristine PAN nanofibrous membranes of (A) Exp. 1, (B) Exp. 2, (C) Exp. 3, and (D) a high-resolution image of Exp. 3.
Figure 3. Scanning electron microscopy (SEM) images of pristine PAN nanofibrous membranes of (A) Exp. 1, (B) Exp. 2, (C) Exp. 3, and (D) a high-resolution image of Exp. 3.
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Table 1. Process conditions and the corresponding findings.
Table 1. Process conditions and the corresponding findings.
Exp.Electrospinning ParametersResults
Voltage (kV)Distance (cm)Flow Rate (mL/h)
115150.5More beads and defect; small nanofibers with a diameter and range (225–342 nm) (Figure 3A)
218181Fewer beads, nanofiber diameter and range (248–362 nm) (Figure 3B)
320180.8Well-uniform fibers, very few beads, nanofiber diameters in the range (304–394 nm), and optimum conditions (Figure 3C,D)
Table 2. Comparative analysis of several techniques to characterize the pore structure.
Table 2. Comparative analysis of several techniques to characterize the pore structure.
Analysis TechniquePrinciplePore Size on Nanofiber SurfacePore Size on a Nanofiber MembranePorosity Rate
Electron Microscopy
(SEM, TEM)
Building a 3D image of a sample by scanning its surface with an electron beam.Yes (limited)Yes, but on the top surface onlyNo
ARMA sharp probe scans a sample surface at a distance over which atomic forces act. The forces between the tip and sample are the cantilever deflection, and from this information, a map of the sample topography can create.YesNo (ability decreases with increasing pore diameter)No
Molecular
Resolution
Porosimetry
(BET Analyzers)
Undertaken by mixing a known volume of gas—typically nitrogen—with a solid substance in a sample vessel while the temperature is kept below freezing. The gas molecules will adsorb onto a solid material as a result of weak molecular attractive forces. The amount of vapor adsorbed at a pressure significantly below the equilibrium vapor pressure is used to calculate surface area. The amount of vapor condensed in pores as a function of vapor pressure is used to calculate the pore volume and diameter.YesNo (only pores up to 200 nm can be detected)Yes (only fiber porosity)
Intrusion Porosimetry (Mercury Porosimetry)By increasing the external pressure and driving liquid mercury into pores, pore information is collected. To determine the pore structures, this information is combined with data on the contact angle. No (requires through pores)YesYes
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Ijaz, M.F.; Alharbi, H.F.; Alsaggaf, A.Z.; Assaifan, A.K. Aspects of Polymeric-Based Membranes in the Water Treatment Field: An Interim Structural Analysis. Water 2023, 15, 1114. https://doi.org/10.3390/w15061114

AMA Style

Ijaz MF, Alharbi HF, Alsaggaf AZ, Assaifan AK. Aspects of Polymeric-Based Membranes in the Water Treatment Field: An Interim Structural Analysis. Water. 2023; 15(6):1114. https://doi.org/10.3390/w15061114

Chicago/Turabian Style

Ijaz, Muhammad Farzik, Hamad F. Alharbi, Ahmed Zaki Alsaggaf, and Abdulaziz K. Assaifan. 2023. "Aspects of Polymeric-Based Membranes in the Water Treatment Field: An Interim Structural Analysis" Water 15, no. 6: 1114. https://doi.org/10.3390/w15061114

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