Nanotechnology for water purification: electrospun nanofibrous membrane in water and wastewater treatment

The need for beneficial innovations in filtration expertise has lead to little consideration of cuttingedge materials, such as nanofiber membranes for water distillation. The presence of organic matter and traces of organics accumulation in wastewater poses a major problem and current technologies such as coagulation/flocculation and chlorine technology are unable to yield satisfying results. The extra volume of sludge generated by these technologies needs further processing and disposal. Nanotechnology has outstanding potential for filtration applications due to its capability to create precise structural controlled materials for such requirements. Electrospun nanofibrous membranes (ENMs) are cutting edge membrane technology that offer substantial high flux and high rejection rates compared to conventional membranes. ENMs present a revolution in water and sewage purification by offering a lightweight, cost-effective, and lower energy consumption process compared with conventional membranes. ENMs possess high porosity, generally approximately 80%, while conventional membranes have 5–35% porosity. Nano-engineered membranes have great potential in water treatment due to their exotic properties. In this connection, electrospinning membranes are emerging as a versatile technique with promising features for water treatment. This work highlights the application of ENM in wastewater treatment and surface modification of nanomembranes in order to address fouling issues and wastewater treatment from Tabuk Sewage Treatment Plant, Saudi Arabia. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2019.057 om https://iwaponline.com/jwrd/article-pdf/9/3/232/599000/jwrd0090232.pdf 2020 I. Tlili (corresponding author) Department of Mechanical and Industrial Engineering, College of Engineering, Majmaah University, Majmaah 11952, Saudi Arabia E-mail: l.tlili@mu.edu.sa Tawfeeq Abdullah Alkanhal Department of Mechatronics and System Engineering, College of Engineering, Majmaah University, Majmaah 11952, Saudi Arabia


INTRODUCTION
The world is facing numerous problems due to a lack of clean and fresh water: approximately 1.2 billion people have inadequate access to fresh potable water, 2.6 billion have very little or no hygiene, and millions of people die every year from sicknesses communicated through hazardous water or human excreta (Montgomery & Elimelech ; Shannon et al. ). With the human population and associated environmental degradation continuing to increase, the scarcity of a clean and portable water supply constitutes a major concern considering the present state of the world's water resources. Water crises are expected to worsen in the coming years, with water scarcity occurring globally due to droughts, population growth, and urbanization. Addressing these prevalent problems requires significant investigation into novel approaches to water treatment with economic benefits and minimum energy expenditure, as well as reducing the use of chemicals and influence on the environment. Intestinal parasitic infections and diarrheal diseases caused by waterborne bacteria and enteric viruses have become a major concern regarding malnutrition due to poor digestion of food by people sickened by water (Lima et al. ; Shannon et al. ). In developing and industrialized countries, large numbers of contaminants are entering municipal water supply systems through human activities, thereby increasing public health and environmental concerns. More effective, low-cost, technologically superior and robust methods to disinfect and decontaminate waters from the source to point-of-use are urgent requirements, without harming the environment or endangering human health during treatment (Shannon et al. ). The growth in population, urbanization, and drastic changes in life styles are the fundamental drivers for energy and water demand along with the excessive amount of wastewater generation due to human activities (Ramakrishna & Shirazi ). According to some estimations, the world population will be around nine billion by 2050 and approximately 75% will face fresh water shortages by 2075 (Tlili et al. ; Kargari & Shirazi ).
The necessity of beneficial breakthrough filtration technology has led to attention being focused on advanced materials, such as nanofiber membranes for filtering devices.
Given the importance of fresh water supply to people in both industrialized and developing countries, and considering the current supply scenario of meeting the increased demand for water, there is an obvious need for innovative technologies to address the water crisis.
Numerous technologies such as distillation, treatment with chemical disinfectants, sand filtration, reverse osmosis, and membrane filtration have been used in the past to purify water. Among these technologies, membrane filtration is a relatively new method, having some advantages such as scalability, low power consumption, free from chemicals, and low operational temperature (Tlili et al. ). A membrane is a semipermeable medium that allows only certain molecules and compounds to pass through and hinders the passage of others. Membrane filtration system can be further improved by incorporating nanofibrous media. Nanofibers possess high porosities and well-connected pore structures, good permeability and, therefore, they are ideal candidates for water purification (Timoumi et al. a, b). Electrospinning is a new and versatile technique to fabricate nanofibers. Electrospun nanofibers with high filtration efficiency, small pore size, high permeability, and low cost are the material of choice for filtration application. The structure of electrospun nanofibers is very promising in terms of permeability, selectivity, and low fouling. Consequently, the demand for technological innovation to allow desalination and water treatment cannot be overstated (Ahmadi et al. ).

Nanotechnology for water treatment
The impact of nanotechnology in the advancement of methods and techniques for water treatment will be more promising in the years to come. With the fast depletion of fresh water resources, it is expected that engineered nanomaterials will play an important role in more efficient seawater desalination, water recycling, and water remediation. Nanotechnology has been recognized as a technology that could play a significant role in addressing many problems associated with water purification (Timoumi et al. , a, b; Tlili et al. , Sa'ed & Tlili ; Khan et al. ). Nanotechnology entails the creation and utilization of particles and materials, systems, and devices at atomic and molecular level (nanoscale), in cutting-edge fields such as engineering, industry, physics, materials science, biology, and chemistry (Afridi et al. a, b).
Nanotechnologies refer to technologies at nano level associated with the conception of materials and particles termed nanomaterials and nanoparticles; they are characterized by unfamiliar and interesting proprieties lacking in other conventional substances and materials. Nanotechnology is concerned with structures or processes that can deliver benefits developed from substances at the nanoscale, i.e., 10 À9 m (Almutairi et al. ). In the last three decades, many researchers and engineers have focused on water management and desalination, which has been of great interest to decision makers. Recently, many developments in nanomaterials investigation for wastewater treatment, based on nanofiltration for biologically treated sewage from deprivation of organic colorants tissue and the paper industry by means of manganese-doped ZnO nanoparticles, have been creating promising results. Nanotechnology has the potential to contribute towards long-term water quality, availability, and viability. The deployment of numerous kinds of membranes such as nanofiltration (NF), reverse osmosis (RO), microfiltration (MF), and ultrafiltration (UF) has been required for water purification (Theron et al. ; Khan et al. a, b, c, d, e). RO has the potential to provide the highest water purity; however, nanofiltration membrane has been exploited as a new technique these days for purification of water. The macro size molecules and colloids are eliminated by ultrafiltration (UF) membranes if water is allowed to flow between them.
The pore dimension of ultrafiltration membranes varies between 2 and 100 nanometers. For several years, the elimination of micron-range particles or biological entities has been carried out by microfiltration (MF), which is a low pressure separation process capable of separating particles with diameters extending from 0.1 to 10 μm. That is why MF remains the most applied technology in the purification of ultra-pure and potable water by separating colloids, particulates, fat, and bacteria, while allowing low molecular weight molecules to pass through the membrane. Nanofiltration membranes are very economical compared to other types of filtration. Moreover, different salts, minerals, pathogens (fungus, molds, virus, and bacteria), monovalent and multivalent, cations, anions, and other suspended nanoparticles existing in surface and groundwater can be rapidly eliminated by NF membranes as well as total dissolved solids (TDS) (Agaie et al. ; Asif et al. ). The NF membrane has varied industrial and engineering applications such as in oil, textiles, beverages, food, chemicals, and many others. It is well known that the hole dimension of the nanofiltration membrane is commonly very low, around 1 nm, which helps in removing larger molecules from smaller molecules and also helps in removing bacteria (Khalid et al. ). Image processing software has been applied to assess contact angle through the tangent manner; moreover, the wettability can be performed as expressed by some researchers (Kruss EASY DROP, Hamburg, Germany). The contact angle is used to assess the membrane for modified cellulose and PH wettability through water and mineral oil. The entire membranes wetted by oil in air are in-air superoleophilic with oil CA values of approximately zero. Irregular wettability is one important method for suspension separation due to its directional liquid transport property. Chemical composition and the geometrical architecture affect surface energy and roughness, respectively, to control the wettability of materials. Nanofiltration has very high effectiveness in removing protozoa (for example, Cryptosporidium, Giardia).
Similarly, nanofiltration can remove bacteria (for example, Campylobacter, Salmonella, Shigella) effectively. Nanofiltration can also remove viruses (for example, enteric viruses, hepatitis A, norovirus, and rotavirus) effectively. However, nanofiltration has moderate effectiveness in removing chemicals. Membrane filtration has the potential to replace conventional filtration processes, since conventional filtration processes have limitations and are unable to remove several impurities consisting of activated carbon, sedimentation, flocculation, and coagulation (Alharbi et al. ). Several investigators have studied the application of nonreactive membranes from metal nanoparticles and nanostructured membranes from nanomaterials like nanoparticles, dendrimers, and carbon nanotubes (Kim & Van der Bruggen ). Absorption is an extensively performed technology due to its efficiency, usefulness, and relatively low process charges for water purification. Removal of diverse pollutants from contaminated water is achieved by operative absorbents such as activated carbon, modified clays, zeolites, silica, and layered double hydroxides. Nanotechnology offers a pioneering solution for sustainable water purification, distribution, and security. Membrane filtration produces high quality water. Actually, the enhancement of polymeric and ceramic membrane is well recognized and greatly influences the application of a membrane in water purification. Bae et al. () fabricated PES membrane with NMP solvent for water treatment. It was found that low roughness and strengthened fibers have a great effect on electrospun nanofibrous membranes (ENMs) manufactured with NMP solvents, which will have flux recovery capability and high rejection compared to ENMs with DMF solvents. Furthermore, it is important to note that the flux performance was found to be eight times greater than commercial membranes. The only problem with membrane filtration is the fouling. With the development of membrane at nano or molecular scale, the fouling issue can be addressed, according to some studies (Asmatulu et al. a, b). Recently, the utilization of nanomembrane infiltration has been involved in numerous technical concerns, especially elimination of biological and organic toxins and impurities. Furthermore, some contaminants and toxins such as binding metal ions and 4-nitrophenol in water solution can be decomposed by nanomembranes manufactured by nonreactive materials (Dolez et al. ). It has been noted that to reach higher operative and effective material to eliminate viruses, polysulfonate ultrafiltration membrane should be ingrained with silver nanoparticles (Asmatulu et al. a, b).

Electrospun nanofiber membrane
Membrane filtration is playing a vital role in water purification, since conventional water treatment processes such as flocculation, sedimentation, coagulation, and activated carbon are unable to remove organic pollutants to meet the necessary specifications (Taylor ). Electrospinning is a straightforward and novel process for fabricating nanofiber membranes, based on creating fibers from micro to nano size depending on electrostatic repulsive forces. This process is characterized by the lower cost of exploitation and small duration. Since the 1930s, the electrospinning technique has been identified and recognized, but lately this procedure has been given prodigious consideration since it has great potential to manufacture nanofibers with exceptional and specific characteristics, for example, higher permeability and surface with volume ratio and lesser diameter (Grafe & Graham ). Although there are a number of processes available for fabricating nanomembrane, electrospinning has a leading advantage over all of them, since electrospinning can easily control the morphology and orientation of fibers due to its relatively low initial investment (Balamurugan et al. ). There are various other methods to fabricate nanofibers, including drawing, template synthesis, phase separation, and self-assembly. These processes are timeconsuming and generally require large investment.
Electrospinning is a straightforward, simple, and easy process with minimum investment. In electrospinning, the morphology of fibers can be controlled by process par-

Electrospinning
The electrospinning procedure of polymer nanofibers is shown in Figure 1. Applying a higher voltage to melt or to polymeric solution leads to the creation of a higher electrostatic field, which leads to creation of nanofibers. At the extremity of a capillary tube, the solution or the polymer melts arise under its superficial tension. Furthermore, the electric field can be attributed to a substantial charge into the liquid generated forces which are seen to be enhanced due to common charge repulsion owing to the reduction of tension in the surface. It is worthwhile noting that the summit of the capillary tube in the semicircular surface of the solution will be extended by applying an electric field which, in turn, leads to the generation of a new structure identified as the Taylor cone (Taylor ).
It is well known that enhancing the electric field leads to increasing the repulsive electrostatic force which, in turn, The bubble-point method is used to determine the pore size of the ENM. The process involves the measurement of pressure needed to blow air through a liquid-filled membrane (Yoon et al. a, b). The membrane is placed in the supporting cell of distilled water and connected to a bubble-flow meter. Pressure is applied to the membrane base and at each pressure, the corresponding bubble flow rate is measured. The Young-Laplace equation, which relates pore size with the corresponding pressure, is used here: where R is the radius of the pore, Δp is the differential pressure, γ is the surface tension, and θ is the contact angle.
Recently, a number of journal articles have been pub-

SURFACE MODIFICATION OF ENMS
The ENMs produced by polymeric solution are an effective means of purifying water. However, they suffer from fouling during the filtration process. Surface modification is one way to alleviate membrane fouling, since it helps to maintain high levels of water productivity. Fouling is the unwanted accumulation of solutes on the membrane surface or within the pores, thereby increasing the resistance to mass-transfer and decreasing membrane productivity Superhydrophobicity is a physical characteristic of a solid surface where water contact is higher than 150 . A droplet of water can easily bounce on such a surface and can roll off with a lower than 10 receding angle. Due to the surface chemistry and irregularity, hydrophobic surfaces have extremely high water-repelling characteristics, which make it very hard for the surface to get wet. Such surfaces are also called self-cleaning. Therefore, fungi, bacteria, algae, and other microorganisms are not able to develop on superhydrophobic surfaces. Hydrophobic or water hating surfaces have very little or no tendency to absorb water, therefore water droplets tend to be ejected on such surfaces. When water is placed on such a surface, many beads form on the surface. Hydrophobic surfaces possess low surface tension and lack active groups in their surface to form 'hydrogen bonds' with water. On superhydrophilic surfaces, the water contact angle decreases to less than 5 in 0.5 seconds.
Superhydrophilic surfaces possess numerous benefits, like antibacterial and antifogging. A contact angle below 90 designates a good wetting, while a contact angle above 90 designates poor wetting. Additionally, when the water contact angle is higher than 90 , water is not thermodynamically stable on the surface, and wetting is generally prohibited. Figure 2 shows schematic views of different superhydrophobic, hydrophobic, hydrophilic, and superhydrophilic surfaces (right to left, respectively). The contact angle on a superhydrophobic surface is larger than 150 and the contact angle on a superhydrophilic surface is less than 5 . Hydrophilicity is a significant indicator as far as the  Table 2 (Ahmadi et al. ).
The primary stage of a wastewater controlling system is wastewater generation. Usually, sewage is categorized into three classes: first, black water (water containing feces); second, yellow water (water containing urine); and third, grey water (sink water, water from baths, laundry apparatuses, etc.). All these three components are different in Wastewater includes colony making units of virus particles, protozoan cysts, fecal streptococci, and coliform organisms. Every characteristic associated with water contamination must be taken into account during municipal wastewater treatment since purified water has to be free of toxic chemicals, effectively disinfected, reliable and fit for human consumption and the environment. Water treatment is based on the composition of wastewater. Figure 3 presents the wastewater treatment procedures.
Conventional wastewater treatment is based on the following steps: 1. The preliminary treatment: eliminating inorganic materials and large particles whose size is greater than 0.2 mm.

Membrane fouling
Municipal wastewater is generally the most abundant source of water for purification since its volume remains the same almost throughout the year. The reuse of such water requires treatment to an acceptable quality level that satisfies  A typical flux-time curve for ultrafiltration is depicted in Figure 4. Stage I shows a rapid initial drop of the permeate flux, followed by a gradual decline in flux (stage II), and stage III shows a steady-state flux. Flux decline in filtration is due to the resistance by membrane pore blockage and  formation of a layer on the membrane surface. Pore blockage and the formation of a layer on the membrane surface are considered two essential mechanisms for membrane fouling.
Electrospun nanofibrous membranes produced by polymeric solution are an effective means of purifying wastewater. However, they suffer from fouling during the filtration process. Fouling is more severe in NF due to the small size of pore and pore distribution. Surface modification is a way to alleviate membrane fouling, since it helps to maintain high levels of water productivity. Surface modification is essential to combine the attributes of a suitable surface chemistry and good mechanical stability.
Microfiltration membranes are fouled by colloidal matter and natural organic matter, as well. The pretreatment of wastewater reduces fouling to a significant extent. Natural organic matter plays a vital role in microfiltration. The contribution of natural organic matter to membrane fouling depends on many factors, such as membrane material, type of pretreatment, and type of natural organic matter.
Coagulation improves natural organic matter removal rate and reduces membrane fouling.

CONCLUSIONS
The application of nanotechnology for water/wastewater treatment is gaining tremendous momentum all over the