1.1.Fresh water scarcity:-
Water is one of Earth’s most abundant natural resources. Incredibly, less than 2% is available to us as fresh water in rivers, lakes and ground water aquifers, as shown in Fig (1.1). A further 1.74% is locked up in the polar ice caps. The remaining 96.5% is the salty water of the world’s oceans (Babkin et al., 2003). Water is the backbone of the global economy, with sustainable high quality supplies being vital for agriculture, industry, recreation, energy production, and domestic consumption (Matin et al., 2011).
The production of potable water has become a worldwide concern. The demand of clean drinking water has increased because of population growth. Approximately 41% of the world population suffers lack of water and over 1 billion people are completely without clean drinking water (Service, 2006). Traditional fresh water sources such as rivers, lakes and, groundwater are decreasing or becoming saline because of misuse or overuse of fresh water (Greenlee et al., 2009). Seawater desalination seems to be one of the methods to alleviate the situation (Yang et al., 2018).
Fig. (1.1): Relative amounts of Earth’s water resources
Desalination refers to the process by which pure water is recovered from saline water using different forms of energy. Saline water is classified as either brackish water or seawater depending on the salinity and water source. Desalination produces two streams – freshwater and a more concentrated stream (brine) (Kangwen, 2012).
Seawater desalination technologies can be divided into two types which are separation methods and thermal methods. Separation methods, such as reverse osmosis (RO) and electro dialysis (ED) method, remove salt from water using electrical or mechanical forces. Thermal methods separate salt through phase-change process such as distillation and freezing method (Mahdavi et al., 2011; Wang et al., 2014).
Thermal processes, except freezing, mimic the natural process of producing rain. Saline water is heated producing water vapor that in turn condenses to form distilled water as shown in Fig (1.2). These processes include multistage flash (MSF), multiple-effect distillation (MED) and vapor compression (VC). In all these processes, condensing steam is used to supply the latent heat needed to vapourize the water. Owing to their high-energy requirements, thermal processes are normally used for seawater desalination. Thermal processes are capable of producing high purity water and suited for industrial process applications. Thermal processes account for 55% of the total production and their unit capacities are higher compared to membrane processes. The technologies used in the industry are described below.
Fig.(1.2):- Desalination processes using thermal processes (www.Roplant.com).
Multi-stage flash distillation:-
The multi-stage flash (MSF) distillation process is based on the principle of flash evaporation. In the MSF process seawater is evaporated by reducing the pressure as opposed to raising the temperature as shown in Fig. (1.3).
Fig. (1.3): Schematic Diagram of Multi-Stage Flash (MSF) Process.
The multiple-effect distillation (MED) process is the oldest desalination method (Al-Shammiri and Safar, 1999) and is very efficient thermodynamically (Ophir and Lokiee, 2005). The MED process takes place in a series of evaporators called effects, and uses the principle of reducing the ambient pressure in the various effects. This process permits the seawater feed to undergo multiple boiling without supplying additional heat after the first effect. The seawater enters the first effect and is raised to the boiling point after being preheated in tubes. The seawater is sprayed onto the surface of evaporator tubes to promote rapid evaporation. The tubes are heated by externally supplied steam from a normally dual purpose power plant. The steam is condensed on the opposite side of the tubes, and the steam condensate is recycled to the power plant for its boiler feed water as shown in Fig.(1.4).
Fig.(1.4):Schematic Diagram of Multi-Effect Distillation (MED) Process.
Co-generation Using Nuclear Energy:-
Co-location of desalination and power plants has the benefit of sharing the resources such as common intake of sea water/ outfall and other infrastructural facilities. Dual purpose (power ; water) plants have inherent design strategies for better thermodynamic efficiency besides economic optimization. The production of potable water from seawater in a facility in which nuclear reactor is used as the source of energy for the desalination process is termed as nuclear desalination. Electrical and/or thermal energy is used in desalination process on the same site as shown in Fig.(1.5).In the ESCWA region, Egypt has explored several options for nuclear desalination is exploring nuclear power options. The renewed interest in nuclear power comes along with new standardized plant designs that could theoretically reduce the cost of nuclear power (Oyoh et al., 2016).
Fig.(1.5):Hybrid Desalination System Integrated with Nuclear Power Reactor.
Membrane technology has developed rapidly in recent years and is playing an increasing role in clean water production, Membrane separation is one of the most important tools for water desalination, as it offers high efficiency, stability, easy operation, and low energy requirements (Zhao et al., 2015).A range of membrane processes, such as microfiltration (MF) , ultrafiltration (UF) , nanofiltration (NF) , reverse osmosis (RO) , forward osmosis and membrane distillation (MD), have been applied to clean water production (Xu et al.,2017).
Reverse Osmosis (RO) Technology:-
A membrane is defined as a thin sheet, film, or layer, which works as a selective barrier between two phases that can be liquid, gas, or vapor (Ulbricht, 2006). RO desalination is a typical pressure-driven process, in which an external hydraulic pressure is applied as driving force and solutes are excluded by a semipermeable membrane called RO membrane as Fig.(1.6) (Malaeb and Ayoub,2011). Reverse osmosis (RO) as today’s leading process for producing fresh water from seawater and brackish water has replaced the conventional desalination process as shown in Fig. (1.7).
Fig. 1.6: Schematic diagram of Reverse Osmosis process.
Fig. (1.7): Reverse osmosis desalination process (www.Roplant.com)
This technology has successfully been utilized to solve desalination problems due to its ability to produce superior and stable quality of water in a relatively little energy demand (Istirokhatun et al., 2018). It consumes about a half of the energy used in thermal processes. Other advantages are low investment cost at low capacities, ease of operation, flexibility in capacity expansion, operation at ambient temperature, and short construction periods. Despite the advantages, thermal desalination can deal more saline waters and deliver higher permeate quality than RO (Howe, 1974).
Nowadays RO is the most important desalination technology (Lee et al., 2011).RO membranes can reject monovalent ions such as chloride and sodium, and salt rejections of seawater can be greater than 99% (Greenlee et al., 2009).Despite of good performance of RO membrane processes, the main disadvantage is fouling.
Membrane distillation (MD) is an emerging membrane technology for the production of fresh water (Tijing et al., 2014). MD is a thermally-driven transport of water molecules (in vapor phase) through porous and hydrophobic membranes. One side of the porous membrane is a hot feed with high salinity and the other side is a cold permeate. The temperature gradient between the two sides creates a vapor pressure difference that drives the vapor to pass through the membrane and collected or condensed to pure water in the other side (Curcio et al., 2010; Eykens et al., 2017).
Fouling of RO Membranes:-
Membrane fouling is one of the main challenges in the long-term operation of reverse osmosis (RO) desalination systems (Tow et al., 2018). Fouling is the accumulation of undesired deposits on the membrane surface or inside the membrane pores, causing decrease of permeation flux and salt rejection (Malaeb and Ayoub, 2011). Fouling can be divided into two different types: external and internal. External membrane fouling is the result of the accumulation of rejected particles or foulants on the external surface of the membrane. Internal membrane fouling is the result of the deposition or adsorption of tiny particles or macromolecules within the internal pore structure of the membrane (Saleh, T.A and Gupta, V.K, 2016). Lower RO membrane fouling allows higher water productivity, less cleaning, longer membrane life, and reduced capital and operational costs (Le and Nunes, 2016).. According to the characteristics of foulants, fouling in RO membranes can be classified into four major groups :- (a) particulate and colloidal matter deposition on membrane surface; (b) organic fouling; (c) scaling and inorganic fouling; and (d) biofouling due to adhesion and bacterial growth on the surface of the membrane generating a layer of gel (García et al.,2017). The complexity of membrane fouling predetermines the exploiting of a variety of approaches to control this adverse process. These approaches, which are used to minimize the RO membrane fouling, are categorized under three main topics: (1) pretreatment of feed, (2) membranes cleaning, and (3) membrane surface modification (Hilal et al., 2005).
Membrane surface modification techniques:-
In order to overcome membrane fouling and improve the performance of the synthetic RO membranes, it is important to perform surface modification for the membranes by changes in the material chemical properties or by changes of pore size. Membrane modification offers means to improve several membrane properties (e.g., hydrophilicity, biocompatibility, antifouling, surface roughness, antibacterial, conductivity, among others). To date, many approaches to membrane modification have been tried, including coating, plasma treatment, chemical treatment, graft polymerizations, and UV irradiation (Khulbe et al. 2010; Wang.2010).
The coating is commonly applied to modify RO membranes by physical adsorption with fouling-resistant and highly water-permeable polymers or surfactants. Surface coating or deposition is a simple but effective method for membrane surface modification, wherein the coating material forms a thin layer that noncovalently adheres to the substrate. (Visakh and Olga Nazarenko, 2017).
Blending is a process in which two (or more) polymers are physically mixed to obtain the required properties. Although compatible polymers have been identified, and membranes prepared from them, in general it has to be mentioned that in depth investigation and optimization of the membrane formation process is needed, since it will differ considerably from the formation process for the basic polymer. Further, also other properties such as the mechanical strength have to be evaluated since these are also expected to differ from the original.
A composite is a material made from two or more materials with different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure.
Plasma surface treatment is a convenient technique for the surface modification of polymer materials for improving the surface properties, such as adhesion and wettability. These properties result from introducing functional groups or making crosslinking in the molecule’s surface. (Wu et al., 1997)
For chemical modification, the membrane material is treated with modifying agents to introduce various functional groups on the membrane surface. The main challenge for modification by chemical treatment of commercial membranes is that the modification agent may partly block the pores of the membranes. Even if the modified membranes are less prone to fouling, the total flux after modification is generally smaller than before modification. In some cases, chemical modification during membrane formation is preferred, since it seems to compromise the flux loss (Nabe et al., 1997).
Grafting is a method where in monomers are covalently bonded onto the membrane. The techniques to initiate grafting are: (i) chemical, (ii) photochemical and/or via high-energy radiation, (iii) the use of a plasma, and (iv) enzymatic. The choice for a specific grafting technique depends on the chemical structure of the membrane and the desired characteristics after surface modification.
Classification of Membranes:-
A number of different materials are used to prepare membranes for use in water treatment. These materials can be broadly classified as either organic (For example polymer membranes) or inorganic (ceramic membranes). At present, polymer membranes are used predominantly because it is possible to select a polymer suitable for the specific separation problem from the existing huge number. Moreover, compared to other materials, polymer membranes are often cheaper. For separation of contaminants, the structure characteristics of polymers used, like thermal, chemical and mechanical stability, and the permeability are very much important (Praneeth and Tech, 2014).
1.5.1. Cellulose acetate polymer (CA):-
Cellulosic polymers are polysaccharides with molecular weights up to 1,500,000 g/mole. They can be formed of esters, such as cellulose acetate and cellulose nitrate. Cellulose is distributed throughout nature in plants, animals, algae, fungi, and minerals. However, the major source of cellulose is plant fiber (Rojas, 2016) such as wood pulp, rice and cotton, and can therefore be considered a renewable resource. Cellulose is a polymer raw material used for general purposes (Klemm et al.,2005).
Cellulose acetate is produced by chemical modification of cellulose. Each glucose unit in the cellulose backbone contains three hydroxyl groups that can undergo acetyl substitution as shown in Fig. (1.8). Thus, the hydrophilic structure makes them good membranes for reverse osmosis water desalination (Kabsch-Korbutowicz and Majewska-Nowak, 2011).
Fig. (1.8): Chemical structure of the cellulose repeat unit.
CA results when cellulose reacts with acetic anhydride to form acylated cellulose and acetic acid as shown in Fig. (1.9). The term cellulose acetate actually describes a variety of acetylated cellulose polymers, including cellulose diacetate and cellulose triacetate (Carlmark and Malmstrom, 2003).
Fig. (1.9): Formation of cellulose acetate from cellulose and acetic anhydride.
The ratio of acetyl to hydroxyl groups determines the physical characteristics of the polymer. For example, acetyl groups are more hydrophobic than hydroxyl groups, and therefore, the degree of acetylation determines the hydrophobicity of CA membranes. Also the degree of acetylation is inversely proportional to the permeability of the membrane to water and salt. In other words, a high degree of acetylation leads to high salt rejection and low flux, a low degree of acetylation leads to low salt rejection and high flux.
220.127.116.11. Advantages of CA:-
Although other membranes have been developed, CAMs are still widely used for desalination and other RO applications (McCray et al., 1991), such as the removal of viruses from wastewater (Okey and Stavenger, 1966).
Cellulose acetate (CA) is used as membrane in commercial reverse osmosis (RO) water desalination plants. These membranes are asymmetric. It is hydrophilic abundant polymer, has higher strength, relatively low coast, easy to manufacture and improved resistance to solvent and chlorine. CA is also, used in other RO processes, including food and beverages, chemical and gas separations (Saljoughi and Mohammadi, 2009). CA is a good candidate, because it is non-toxic, biodegradable, and renewable (Ding et al.,2004).It contains a number of hydroxyl, ether, and carboxyl groups in the main backbone chains, making it highly ionic in nature (Lee et al.,2006).
18.104.22.168. Disadvantages of cellulose acetate
Although CAMs exhibits high water flux and salt rejection, they suffer from several disadvantages. CA is susceptible to hydrolysis under acidic and alkaline pH conditions, limiting the operating pH of CAMs to between 4 and 8. The rate of hydrolysis also increases with temperature (Sagle and Freeman. 2004), and therefore CA is limited to an operating temperature below 30 °C. CA can also be degraded by oxidation due to chlorine and other oxidizing agents in the feed water. Furthermore, CAMs are known to undergo a period of flux decline caused by membrane compaction under high operating pressures (Goossens and Haute, 1976).
1.5.2. Thin-film composite membrane (TFC):-
These membranes are made by forming a thin, dense, solute-rejecting surface film on top of a porous substructure. TFC membranes were prepared by the interfacial polymerization reaction of m-phenylene-diamine MPD and trimesoyl chloride TMC on a PSF membrane and the process is shown in Fig.(1.14) (Kadhoma and Deng, 2018). PA-TFC membranes are composed of an outer ultra-thin skin polymer layer (; 0.2 ?m), a porous middle polysulfone (PSf) support, and a non-woven polyester (PET) fabric base (Kong et al., 2011).
Fig.( 1.14 ): Schematic illustration for the synthesis of active PA layer on the supporting layer using TMC and MPD as monomers by interfacial polymerization (IP) ( Li and Wang, 2010).
Polyamide thin-film composites, like polyamide asymmetric membranes, are highly susceptible to degradation by oxidants, such as free chlorine.
1.8. Membrane fabrication methods:-
The selection of a technique for polymer membrane fabrication depends on a choice of polymer and desired structure of the membrane. The most commonly used techniques for preparation of polymeric membranes include phase inversion, interfacial polymerization, stretching, track-etching and electrospinning.
1.8.1. Polymer membranes by phase separation (Phase inversion):-
The method is often called phase inversion, but it should be described as a phase-separation process: a one-phase solution containing the membrane polymer is transformed by a precipitation/solidification process into two separate phases (a polymer-rich solid and a polymer-lean liquid phase). Before the solidification, usually a transition of the homogeneous liquid into two liquids (liquid–liquid demixing) occurs (Drioli and Giorno, 2009). This transformation can be accomplished in several ways, namely:
Immersion precipitation:- The polymer solution is immersed in a non-solvent coagulation bath (typically water). Demixing and precipitation occur due to the exchange of solvent (from polymer solution) and non-solvent (from coagulation bath), that is, the solvent and non-solvent must be miscible.
Thermally induced phase separation (TIPS):- This method is based on the phenomenon that the solvent quality usually decreases when the temperature is decreased. After demixing is induced, the solvent is removed by extraction, evaporation or freeze drying.
Evaporation-induced phase separation: – The polymer solution is made in a solvent or in a mixture of a volatile non-solvent, and the solvent is allowed to evaporate, leading to precipitation or demixing/precipitation. This technique is also known as a solution casting method.
Vapor-induced phase separation: – The polymer solution is exposed to an atmosphere containing a non-solvent (typically water); absorption of non-solvent causes demixing /precipitation.
However, among these techniques, immersion precipitation and thermally induced phase separation are the most commonly used method in the fabrication of polymeric membranes with various morphologies.
1.7. Types of Nanoparticles:-
Nanoparticles are classified into major types, namely, organic nanoparticles which include carbon nanoparticles. While, some of the inorganic nanoparticles include magnetic nanoparticles, noble metal nanoparticles (like gold and silver) and semi-conductor nanoparticles (like titanium oxide and zinc oxide). There is a growing interest in inorganic nanoparticles i.e. of noble metal nanoparticles as they provide superior material properties with functional versatility (Xu et al., 2006).
1.9. Synthesis of Nanoparticles:-
There are three methods to synthesize nanoparticles. The physical methods include spark discharging, pyrolysis, etc. The chemical methods are electrochemical reduction, solution irradiation, cryochemical synthesis, etc. The major process involved in chemical synthesis is the reduction of metal ions to nanoparticles and preventing the aggregation of metallic nanoparticles. The former is done with the help of various reducing agents like sodium borohydride or sodium citrate as reducing agents (Kim et al., 2007).
The major disadvantage in the physical method is the low yield, and in the chemical method is the use of toxic solvents and also the generation of hazardous by-products (Mallicket al., 2004) lead to the presence of some toxic chemicals absorbed on the surface that may have adverse effects in applications, so there is a growing need to develop environmentally benign nanoparticles (Asmathunisha and Kathiresan ,2013).
1.9.1. Biosynthesis of Nanoparticles (Green synthesis):-
An alternative method for the green synthesis of nanoparticles using a biological source has been discovered recently. Green synthesis includes synthesis through plants, bacteria, fungi, algae etc. This approach is an environment-friendly, cost-effective, biocompatible, and safe. The major implication of this biological approach is its relative simplicity in the synthesis of nanoparticles, and it is less time-consuming. In addition to this, the high yield, low toxicity, low cost (Kalimuthu et al., 2010). Another advantage is that the size of the nanoparticles synthesized can also be controlled easily by various parameters like pH and temperature (Gurunathanet al., 2009). The use of stabilizers to prevent aggregation is not required as the proteins in the system act as stabilizers (Kalishwaralalet al., 2010). Nanoparticles with smaller radius of curvature have higher catalytic activity; hence, angular shapes are preferable due to their smaller radii of curvature compared to spherical particles of the same volume.
22.214.171.124. Silver Nanoparticles:-
Silver is historically known as a strong antibiotic and has wide range of industrial applications in healthcare and external medicine Huh and Kwon, (2011). The progress of reliable and eco-friendly method for the silver nanoparticles synthesis is a vital aspect in nanotechnology research.
Silver nanoparticles are of interest because of the unique properties (e.g., size and shape depending optical, electrical, and magnetic properties) which can be incorporated into antimicrobial applications, biosensor materials and composite fibers. Several physical and chemical methods have been used for synthesizing and stabilizing silver nanoparticles (Klaus et al., 1999; Senapati, 2005). The formation of colloidal solutions for the reduction of silver salts involves two stages: (1) nucleation and (2) subsequent growth, which determines the size and the shape of the nanoparticles and these stages can be controlled by adjusting the parameters, such as reaction temperature, pH value, the type of the reduction, and stabilizing agents (Korbekandi and Iravani, 2012). Recently, nanoparticle synthesis is among the most interesting scientific areas of inquiry, and there is growing attention to produce nanoparticles using environmentally friendly methods (green chemistry). Moreover, we discuss the applications of silver nanoparticles and their incorporation into other membrane material, the mechanistic aspects of the antimicrobial effects of silver nanoparticles.
1.10. Applications of Silver Nanoparticles and Their Incorporation
The developments in nanotechnology and membrane science have provided the fabrication of nanocomposite polymeric membranes in which nanoparticles are generally blended into a polymeric matrix (Yuksel et al.,2014) due to their extremely small size and large surface to volume ratio, which lead to both chemical and physical differences in their properties compared to bulk of the same chemical composition, such as mechanical, biological and sterical properties, catalytic activity, thermal and electrical conductivity, optical absorption and melting point (Daniel et al., 2004). Polymer nanocomposite, or nanofilled polymer composite, is a polymeric system that is enhanced by addition of materials at nanometric scale (Liu et al., 2007; Zhao et al., 2010; Hamouda and Elkader,2012).The nanocomposite membrane in which the nanofillers have been entrenched within the polymer matrix have improved the separation performance of the membrane and increased the permeability, selectivity and stability of the membrane which are the key factors for water purification applications (Kendouli et al., 2014).New membranes using polymer nanocomposite material can be efficient in order to develop membranes resistant to fouling and biofouling or to improve their performances in terms of permeate flux and salt rejection. Among these additives or fillers the silver nanoparticles (AgNPs) have received a great deal of attention. Silver nanoparticles have been used extensively as antimicrobial agents in health industry, and a number of environmental applications. Ag NPs can prevent the bacterial adhesion onto membrane surface and fouling resistance (Cao et al., 2010).
Algae are group of plants which are known since ancient civilizations. The term alga was first introduced by Linnaeus in 1753 and it was A. L. de Jussieu (1789) who classified the plants and delimited the algae from rest of the plant world to its present status (Fritsch, 1945). Algae cover a range of organisms from different phylogenetic groups with approximately thirty thousand species described. In general, these can be categorized as multicellular macroalgae and unicellular microalgae (microscopic algae). Macroalgae, usually found in the coastal areas, are represented by three major classes: green algae (Chlorophyceae), brown algae (Phaeophyceae), and red algae (Rhodophyceae) (Wang et al., 2015).An alga was used as natural resource because their isolated metabolites have shown biological activities and potential to provide health benefits (Ariede et al.,2017). Macroalgae, commonly named seaweeds (Sanghvi et al., 2010). Marine macroalgae are recognized as effective due to their opulence in minerals, fatty acids, and vitamins, and also many substances like proteins, saccharides, and phenols which can be used as medicinal agents against cancer and antibacterial (Namvar et al.,2012).
The Aim of the Work
The aims of this thesis can be summarized as:-
Biosynthesis silver nanoparticles using different solvent from different marine algae types.
Characterization of silver nanoparticles using UV-vis spectroscopy, FTIR, particle size analyzer, XRD, EDX, SEM and TEM measurements.
To test the antibacterial activity of the biosynthesized silver nanoparticles.
The use of nanoparticles in preparing and modifying polymeric membranes to evaluate the performance of existing desalination membranes in RO and to enhance flux and reduce fouling.
Study the properties of the prepared membranes, such as thermal stability using TGA and to clarify the functional groups using FT-IR, X-Ray diffraction, contact angle (CA) and testing of mechanical properties, and some surface morphology.
To investigate the nature of membrane fouling in RO and compare it to the modified membrane fouling in RO.
Using the prepared membranes in desalination of seawater sample.
The following charts in Fig. (1.15): Schematic chart represent the main objective in this thesis.
2. Literature Review
2.1. A review on biosynthesis of nanoparticles by marine algae
In recent times, study and preparation of inorganic nanoparticles have received considerable interest from scientists among fundamental and applied research. There are many routes available for the synthesis of NPs, but an increasing attention is needed to develop high yield, low-cost, non-toxic and eco-friendly procedures. Metal NPs produced by using plant-based materials are more stable and the rate of synthesis is also faster. Marine environmental conditions are extremely diverse from terrestrial and are excellent source of various types of bioactive compounds (Mayer et al., 2001). The marine plants have antibacterial, antiplasmodial, antiviral, antioxidant, anticancer activities (Ravikumar et al.,2011; Boopathy and Kathiresan,2010) and also proved to have high content of secondary metabolites such as polyphenols, flavonoids, alkaloids and tannins.
There is a little literature supporting the use of marine algae in nanoparticle synthesis. Vivek et al., (2011) reported the red seaweed Gelidiella acerosa to have the potential of synthesizing antifungal silver nanoparticles. Recently, Rajesh et al., (2012) have reported the synthesis of silver nanoparticles using Ulva.
Kumar et al.,(2012) focused on the synthesis of silver nanoparticles from the extract of Sargassum tenerrimum. The authors characterized synthesized silver nanoparticles by UV-Visible Spectroscopy, FTIR, TEM and Dynamic Light Scattering. They found that spherical shaped nanoparticles of size 20 nm in TEM analysis and showed effective anti-bacterial activity against standard reference strains. Altogether, extracts from seaweed were screened for phytochemicals followed by FT-IR prediction to reveal chemical functional groups present. The results showed that the anti-bacterial activity of silver nanoparticles was comparably higher than the phytochemicals present.
Castro et al.,(2013) synthesized gold and silver nanoparticles by a simple method using different algae as reducing agent. They explored the application of dead algae in an eco-friendly procedure. The nanoparticle formation was followed by UV–vis absorption spectroscopy and transmission electron microscopy. The functional groups involved in the bioreduction were studied by FTIR.
El-Rafiea et al., (2013) synthesized silver nanoparticles (AgNPs) using water soluble polysaccharides extracted from four marine macro-algae, namely, Pterocladia capillacae (Pc), Jania rubins (Jr), Ulva faciata (Uf), and Colpmenia sinusa (Cs) as reducing agents for silver ions as well as stabilizing agents for the synthesized AgNPs. They confirmed the formation Ag-NPs have been by UV–Vis spectroscopy, FTIR analysis and TEM. They also applied the resultant Ag-NPs colloidal solutions were to cotton fabrics in presence and absence of citric acid or a binder. The authors evaluated the antimicrobial activity of the treated fabrics. The results revealed that the antimicrobial activity depends on type of the fabric treatment, size of the synthesized Ag-NPs and the algal species used for polysaccharides extraction.
Shiny et al., (2013) used seaweeds as the bioreductant for the reduction of the silver salt to form the nanoparticles. They confirmed the formation of the silver nanoparticles with the dark brown color development and characterized it with UV-Visible spectroscopy. TEM indicate the size of the nanoparticles to be in the range of 25-40 nm and the nanoparticles were spherical in shape. The authors tested the activity of the nanoparticles to inhibit the growth of bacteria against a Gram positive Bacillus cereus and a Gram negative Escherichia coli. The silver nanoparticles exhibit good antimicrobial activity and hence their potential application in medicine.
Yousefzadi et al., (2014) explored the novel approaches for the biosynthesis of silver nanoparticles; the seaweed Enteromorpha flexuosa (wulfen) J.Agardh extract was mixed with silver nitrate to synthesize silver nanoparticles. The authors characterized the reduced silver nanoparticles by UV-vis spectrophotometer, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The in vitro antimicrobial activity of synthesized nanoparticles of E. flexuosa exhibited high antibacterial activity against Gram-positive bacteria and low activity against the Gram-negative organisms.
Dhas et al., 2014 used an aqueous extract of Sargassum plagiophyllum for the synthesis of silver chloride nanoparticles (Ag Cl NPs). They also used UV–vis spectroscopy, FT-IR, TEM, SEM to characterize the formation of AgCl NPs. (XRD) patterns clearly illustrate the presence of AgCl NPs. They tested the synthesized AgCl NPs for its antibacterial activity and found to cause considerable amount of deterioration to bacterial cells, when examined using electron microscope and cell viability analysis.
Verma and Shrivastava, (2014) collected Spirulina platensis (blue green algae) from two different locations as Jalmahal and Ramgarh from Jaipur for biosynthesis of nanoparticles. The authors characterized Nanoparticles using Uv-Vis absorption spectroscopy, FTIR, XRD, SEM and TEM.
Parveen and Lakshmi, (2015) used the red marine macroalgae, Amphiroa fragilissima aqueous extract was used as a reducing agent for the synthesis of nanostructure silver particles (Ag-NPs). The authors characterized Structural, morphological and optical properties of the synthesized nanoparticles using FTIR, XRD and UV-Vis spectroscopy. The formation of Ag-NPs was confirmed through the presence of an intense absorption peak at 420 nm using UV–visible spectrophotometer. The nanoparticles were crystalline in nature. This was confirmed by the XRD pattern. From the FTIR results, it can be seen that the reduction has mostly been carried out by peptides. They investigated antibacterial activity against Escherichia coli, Bacillus subtilis, Klebsiella pneumonia, Staphylococcus aureus and Pseudomonas aeruginosa. They found the synthesized Ag NPs to possess discrete antibacterial activity from 20 to 100 ?L.
Salari et al., (2016) synthesized silver nanoparticles through bio-reduction of silver ions using the Spirogyra varians in this research. The authors characterized the structure and morphology of AgNPs by UV-visible spectroscopy, (XRD) (SEM) and (FTIR). These nanoparticles indicated an absorption peak at 430 nm in the UV–visible spectrum. The crystallite average size was estimated about 17.6 nm and SEM image confirmed synthesis of relatively uniform nanoparticles. They also tested the antibacterial effect of AgNPs on several microorganisms by measuring the inhibition zone, MIC and MBC. The results confirmed that AgNPs can act as a powerful antibacterial agent against various pathogenic bacteria.
Abdel-Raouf et al., (2018) synthesized silver nanoparticles by the reduction of aqueous solutions of silver nitrate (AgNO3) with powder and solvent extracts of Padina pavonia (brown algae). The authors measured the obtained nanoparticles exhibited high stability, rapid formation of the biogenic process (2 min-3h), small size (49.58–86.37 nm) (the diameter of formed nanoparticles by TEM and DLS and variable shapes (spherical, triangular, rectangle, polyhedral and hexagonal). They also monitored preliminary characterization of nanoparticles by using UV–visible spectroscopy (UV–Vis), (TEM), (DLS) and finally by (FTIR). They recorded the ratios of converted Ag NPs as 88.5; 86.2 and 90.5% in case of P. pavonia powder extract and chloroform extract, respectively.
2.2. Review of synthesis and properties of reverse osmosis membranes:-
CA membranes were first investigated for their salt rejecting properties by Reid and Breton,(1959) but the observed water fluxes were too low to be practical for desalination. In 1960, Loeb and Sourirajan developed the first high flux, asymmetric, CA membrane with good salt rejection properties. Their membrane showed up to 100 times higher flux than any symmetric membranes known at the time (Fritzmann et al., 2007). Current cellulose acetate membranes are made from a blend of cellulose diacetate and triacetate polymers. Peterson et al., (1982) introduced the second membrane material, aromatic polyamide, in the early 1980s.
When Loeb and Souririjan made the first successful, integrally skinned desalination membrane they cold-cast onto glass from a quaternary formulation of 22.2 wt % cellulose acetate, 66.7 wt % acetone, 10.0 wt% water and 1.1 wt% magnesium perchlorate (Loeb and Sourirajan,1960). It was the addition of the magnesium perchlorate, which acted as a pore former/swelling agent that gave the vast improvements in water flux compared to previous methods. Several subsequent techniques have used other inorganic electrolytes similar to magnesium perchlorate to for CAMs (Kesting and Menefee, 1969; Manjikian et al.,1965) then made an improvement to the method by substituting both the water and the magnesium perchlorate with formamide, and replacing the cold cast procedure with a room temperature casting procedure to form membranes. They used a solution of 25 wt % CA, 30 wt % formamide, and 45 wt% acetone in their ternary formulation. Later, Kesting and Menefee,1969) studied the effects of varying the concentration of formamide in acetone solutions of CA. They found that more formamide lead to more swelling and therefore thicker membranes. The minimum amount of formamide to achieve permeability was approximately 20 wt%. Other variables affecting membrane casting include evaporation time between casting and submersion, and temperature of the annealing bath (Pinnau,2000).
Ahmed et al (2010) prepared cellulose membranes by phase inversion process from different blends of polymers/solvents/additives. The casting solutions comprising polymer concentration range from 15 to 25 wt%, and acetone, tetrachloroethane and N,N dimethyl formamide as solvents. The authors prepared different samples of membranes and tested using polymer with different acetyl contents. They characterized the prepared membranes using (SEM). Further, also they investigated performance indicators comprising: flux, operating time, permeability and selectivity according to casting solution constituents and membrane matrix morphology. They applied operating pressures up to 50 bars and found the results indicate that the appropriate polymeric content to be between 20% and 22%. They also observed enhanced performance in the presence of both polymethylhydrosiloxan (PMHS) and dibutyl phthalate (DBP) denoting better salt rejection. Almost all prepared membranes, could tolerate operating pressures up to 50 bars.
Morsy et al., (2014) produced cellulosic polymers namely cellulose, di-and tri cellulose acetate from cotton and agriculture wastes, rice hulls. In this work, the authors confirmed the presence of these cellulose derivatives by a solubility test in acetone and chloroform and nuclear magnetic resonance (NMR) analysis. They prepared cellulose acetate membranes (CA) for reverse osmosis (RO) salt water desalination using phase inversion technique. They also characterized the annealing CA-RO membranes by (FTIR) to confirm the structure, contact angle measurements to determine the degree of membrane hydrophilicity and (SEM) to observe the morphology. They also studied the effect of annealing on the water flux and salt rejection for 1000 ppm NaCl using cross flow RO testing unit. They found that salt rejection of the annealing resulted RO CA membranes was equal to 94.6%.
Ebrahim et al., (2015) extracted cellulose diacetate (CDA) and cellulose triacetate (CTA) from Egyptian rice straw. They prepared reverse osmosis (RO) membranes from this CDA using phase inversion technique. They characterized the structural, crystalline, morphological and hydrophilic properties of the prepared membranes by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance (1HNMR), (XRD), (SEM), and contact angle measurements, respectively. The NMR spectra revealed a degree of Substitution of 2.8 for CTA and 1.75 for CDA. The values of water flux and salt rejection for CA-RO membrane without annealing, tested in 10,000 ppm NaCl, were 7.1 L/m2 h and 87.4%, respectively, while the water flux of 4.76 L/m2 h and a salt rejection of 93.3% were obtained for the annealed CA-RO membrane at 14 bar. The annealed CA-RO membranes showed an asymmetric structure with ridge-and-valley on the top layer and macrovoid structures in the support layer as revealed by SEM. The CA-RO membranes grafted with 15 wt% of 2-acrylamidopropane-2-methyl sulfonic acid produced a salt rejection of 93.5% and a water flux of 8.3 L/m2 h. It was concluded that both the annealing and grafting processes enhanced the performance of the CA-RO membranes.
2.2.1. Polymeric membranes incorporated with metal/metal oxide nanoparticles: A comprehensive review:-
In the 1970s, Goosens et al used several types of mineral fillers (silicum and aluminium oxides and montmorrilonites) incorporated into CA casting solutions in order to improve compaction resistance. Recently, there has been a trend toward nanoparticle fillers, forming membranes known as nanocomposites. The advantage of using nanoparticles is their high surface area leading to more interactions and efficient interfacial stress transfer therefore very low loadings are needed (typically