Application of Nanomaterials in Water and Westwater Treatment

Abstract

Water on Earth is a precious and finite resource, which is most important component for living beings and endlessly recycled in the water cycle. Providing clean and century. Water, whose physical, chemical, or biological properties have been altered due to the addition of contaminants such as organic/inorganic materials, pathogens, heavy metals or other toxins making it unsafe for the ecosystem, can be termed as wastewater. water shortage is prevalent even in water-rich areas as immense pressure has been created by the burgeoning human population, industrialization, civilization, environmental changes and agricultural activities.

Heavy metal ions in water affect both environment and human health. Nanotechnology holds great potential in advancing water and wastewater treatment to improve treatment efficiency as well as to increase water supply through safe use of unconventional water sources. In the field of water treatment, nanotechnology come up with great potential in improving the performance and efficiency of water decontamination as well as providing a sustainable approach to secure water supply.

The effects of various factors such as adsorbent dose, pH, contact time, arsenic concentration and co-existing anions on arsenic removal were methodically studied. Furthermore, the as-prepared adsorbent showed an excellent arsenic removal performance in real underground water. Case studies provided that, traditional technologies were directly compared with nanotechnology-based technologies for the similar pollutants. Nano titanium dioxide, zerovalent iron, zinc oxide, silver oxide, carbon nanotube, and composites have been extensively used as photocatalysts, membranes and adsorbents in water and wastewater treatment. Nanomaterials have been mainly studied for heavy metal ions and dye removals from wastewater.

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The synthesis and physiochemical properties of diverse free nanomaterials, including carbon based nanomaterial, metal and metal oxides nanoparticles as well as noble metal nanoparticles, were focused on, and their performance and mechanisms towards removal of various contaminants were consider. Adsorption is one of the most efficient techniques for removing noxious heavy metals from the solvent phase and it is widely considered as the most promising and robust method of purifying water at low cost and with high-efficiency. Solar TiO2 photocatalysis is also beneficial in treating different kinds of industrial wastewater, such as paper mill wastewater, textile wastewater, and olive mill wastewater. Solar TiO2 photocatalysis is also beneficial in treating different kinds of industrial wastewater, such as paper mill wastewater, textile wastewater, and olive mill wastewater. TiO2 is a more effective photocatalyst for the photocatalytic degradation of organic pollutants. It is shows more reactivity under UV light and around 5% of solar spectrum contains UV radiations. carbon‐based 3D architectures have received increasing attention in science and technology due to their fascinating properties, such as a large surface area, macroscopic bulky shape, and interconnected porous structures, enabling them to be one of the most promising materials for water remediation and because of the introduction of these materials, it is helpful in water treatment, such as the removal of oils/organics, ions, and dyes, are summarized.

Graphene (G) is significant attention because of its unique physical and electronic properties. Graphene production through the reduction of graphene oxide (GO) is a low-cost method. These graphene-based nanomaterials are attractive for high- performance water sensors because of their unique properties, such as high specific surface areas, high electron mobilities, and exceptionally low electronic noise. The rapid development of nanotechnology and the increasing use of nanomaterials (NMs) raise concern about their fate and potential effects in the environment, especially for those that could be used for remediation purposes. That will be intentionally released to the environment. The toxicity of environmentally-relevant forms of engineered nanomaterials (ENMs), which can transform during wastewater treatment and persist in aqueous effluents and biosolids. Various toxicity measurement methods have been adopted for biological wastewater treatment processes (WWTPs). There has been a considerable amount of research for development of sustainable water treatment techniques capable of improving the quality of water. The research and development in this area have given rise to a unique class of processes called advanced oxidation processes, particularly in the form of heterogeneous photocatalysis, which converts photon energy into chemical energy. The effects of fundamental parameters such as temperature, pH, catalyst-loading and reaction time have also been reviewed. This article briefly reviews the recent development in nanotechnology for application in wastewater treatment. Here, an overview of recent advances in nanotechnologies processes is provided, including nanobased materials, such as nanoadsorbents, nanometals, nanomembranes, and photocatalysts. The innovation, forthcoming development, and challenges of cost-effective and environmentally acceptable nanomaterials for water purification are discussed too. This review also presents the application of metal oxide/graphene composites in water treatment and their role as photocatalyst, adsorbent and disinfectant in water remediation.

Introduction

Water covers one-third of the earth’s surface and most of it is saline and unusable for human consumption, only 2.5 percent total water is fresh in the world. There is., fresh water is also very unevenly distributed, and more than half of the wetlands have disappeared from earth. The availability of clean, fresh water has become a basic mandatory of mankind .[1] Pathogenic bacteria are the main source for worldwide water-borne disease cause a big threat to public health, so there is an urgent need to develop cost-effective water treatment technologies.[2] Our environment is under continual pressure of growing industri- alization and urbanization. Out of the world’s top environmental problems, water scarcity has become the most major issue facing the human race. By 2025, it was estimated by WHO(2014). that 50% of the world’s population will be living in water-stressed areas. Upto 2015, only around 20% of global wastewater is properly treated. In developing countries, approximately 70% of industrial wastewater is released without any proper disposal. Recent benefits in the manipulation of nanomaterials have facilitated the application of nanotechnology in water and wastewater treatment. Nanomaterials are generally defined as materials that at least one dimension and is smaller than 100 nm.

Upto now, numerous studies have said that nanomaterials have vast capability and also potential in water and wastewater treatment, in the areas of adsorption, membrane process, catalytic oxidation, disinfection and sensing. As the cost for nanomaterials are decreasing, they have become high competitive for water and wastewater treatment. But, there are still inherent disadvantages for direct use of free nanoparticles in water and wastewater treatment process. The development of nanocomposite is proven to be an effective and promising approach. Nanocomposite is generally fabricated by loading desired nanoparticles on the various supporting materials, as like, polymers or membranes. [3] One of the aims of this present review is to compile the important findings of various types of nanomaterials, used in water treatment either as adsorbents, photocatalysts and/or antibacterial agents, for the removal of the important aquatic pollutants.[4] A group of leading climate impact researchers have displays that climate change possibly exacerbates the regional and global water scarcity. The advanced temperature in air and raw water can affect the drinking water hygiene in respective storage systems as well as in the drinking water pipelines, ensuing in harmful infectious illnesses, In both developing and industrialized countries. A growing number of contaminants as like micropollutants are incoming into the water bodies.

The habitation of highly advanced nanotechnology to traditional process engineering offers new opportunities for development of the complex water and wastewater technology processes. Recent advances in the manipulation of nanomaterials have facilitated the application of nanotechnology in water over and above wastewater treatment. In the past decades, water nanotechnology has received sufficient kindness as a potential supplement to the traditional treat- ment methods. At such scale, materials often exhibit unique physical or chemical properties over their massive counterparts. For example, nanomaterials usually have higher density of active sites per unit mass due to their larger specific surface area. In addition, nanomaterials exhibit greater surface free energy, resulting in enhanced surface reactivity. At correct size, some materials would show super paramagnetism, or even quantum confinement effect. To avoid or moderate the potentially adverse effect brought by the application of nanotechnology, it is desirable to develop a material or a device that could minimize the release or mobilization of the nanomaterials while maintaining their high reactivity. This review focuses on various nanomaterials used for contaminant adsorption, separation and catalytic degradation from or in water. NPs in sewage sludge will also be increased with the increase of wastewater influent nanoparticle concentrations. Since the water industry is required to produce drinking water of elevated quality, there is a clear need for the development of cost-effective and stable materials and methods to address the challenges of providing the and fresh water in adequate amounts. Nanoparticles can penetrate deeper and thus can treat water/wastewater which is generally not possible by predictable technologies.

Collection and seawater desalination are practical options for addressing water shortages, an additional possible solution is the reuse of Treated Domestic Wastewater (TDW), for twin purposes. The nutrient content in the treated effluent can also be very much disadvantageous due to the eutrophication phenomenon, the accumulation of nitrogen species in the groundwate.[8] In the aqueous environment, the factors distressing the adsorption process are high surface area, adsorption activity, chemical activity, location of atoms on surface, lack of internal diffusion resistance and high surface binding energy. Organic pollutants, for example pesticides, fertilizers, hydrocarbons, phenols, plasticizers, biphenyls, detergents, oils, and greases are associated with toxicities (Damià, 2005). Emerging contaminants include pharmaceuticals and personal care products (PPCPs). From past few decades, a variety of techniques have been developed for treating the waste water. Chemo Mechanical polishing (CMP) operations are a main source of wastewater in semiconductor manufacturing plants. According to the literature, bench-scale batch respirometry studies have showed that one mg L−1 of Ag NPs can slow down the growth rate of nitrifying bacteria by up to 80%.

Nanomaterials have been broadly studied for heavy metal ions and dye removals from wastewater. The review concludes with a summary on the role of graphene based materials in removal of pollutants from water and some future strategies for designing of highly well-organized multifunctional metal oxide/graphene composites for water remediation. The present work on LbL-assembled diamond-based composites provides new alternatives for increasing diamond hybrids as well as nanomaterials towards wastewater treatment applications. Adsorption is one of the most competent techniques for removing noxious heavy metals from the solvent phase. Adsorption technology is broadly considered as the most promising and robust method of purifying water at low cost and with high-efficiency. This review summarizes the recent development in design, preparation, and applications of carbon‐based 3D architectures derived from carbon nanotubes, graphene, biomass, or synthetic polymers for water treatment. The production of graphene throughout the reduction of graphene oxide (GO) is a low-cost method. A simple, low cost, and green method was developed for the combination of water-soluble and well- dispersed fluorescent carbon nanodots (CDs) via a one-step hydrothermal treatment of potatoes. TiO2 is a more helpful photocatalyst for the photocatalytic degradation.

Wastewater: sources, composition and treatment

 Sources and Composition of Wastewater

The sources of wastewater can be broadly classified into this two classes: residential and non-residential sources. Residential wastewater, is also known as sewage, which is discharged impure water. A major part of the drinking water sources in the world are found to be contaminated with different toxins and pathogenic microbes, mostly due to the release of untreated man-made wastes or wastewater to these sources. Recent wastewater treatment technologies are demand high capital investment, high energy requirements, operation and maintenance (O&M) cost, and large plant areas. In order to address these kind of issues, it is the challenge for research, development, and technology institutions to exits with cost-effective alternative wastewater treatment technologies with small area requirements. Nanotechnology offers the potential for the development of alternative technologies for treatment of wastewater.

Wastewater Treatment

Wastewater treatment is a process where the contaminants or pollutants are differentiated from the aqueous phase with the help of a many various physical and chemical processes, before the environmental release of the water. There are various ways for treating residential and nonresidential wastewater. Most commonly the wastewater is generally discharged to a municipal sewage treatment plant. The primary treatment involves the process of removal of large and heavy spoilage from the sewage. Furthermore, screening and grit removal steps are typically included in this stage. In the screening process large floating debris, such as rags (∼60%), paper (∼25%), and plastics (∼5%), are removed with the help of screens. The debris that is left after the screening process, called screenings, is mostly contaminated with raw feces.

Nanotechnology for wastewater treatment

In this portion, provided a brief review of few typical application ofnanotechnology in water and wastewater treatment, that is: adsorption separation, disinfection sensing catalytic Nanomaterials have unique size-dependent properties directly related with high specific surface area (fast dissolution, high reactivity, strong sorption) and discontinuous properties such as superparamagnetism, localized surface plasmon resonance. These very specific nanobased characteristics allows the development of novel high-tech materials for more efficient water and wastewater treatment processes, namely membranes, adsorption materials, nanocatalysts, functionalized surfaces, coatings, and reagents. Nanotechnology facilitates innovative solutions for water treatment. Nanomaterials are fabricated with features, such as: High aspect ratioreactivity, tunable pore volume, electrostatic, hydrophilic, and hydrophobic interactions.

These all are useful for adsorption, catalysis, sensoring, and optoelectronics. Nanotechnology allows processes for the water treatment constitute major challenges to existing methods. Nanotechnology can also be extended to the purification and utilization of unconventional water sources in an economic way. It should be required that nanomaterials for purifying drinking water must be environment-friendly and nontoxic. Unsafe particles can cause several kind of injury to vital organs upon contact with the human body. Desirable nanomaterial properties, as like high surface area for adsorption and high reactivity toward photocatalysis should also have good antimicrobial properties for disinfection and also to maintain biofouling, should have super para-magnetism for particle separation, should contain optical and electronic properties, and should have good sensing nature to measure the water quality. Usually nanoadsorbents are used to remove inorganic and organic pollutants out of water and wastewater. The application of heterogeneous photocatalysis as an alternative for inactivation of pathogenic microorganisms has attracted much attention in recent years. Nanotechnology is depends on the manipulation, control, and integration of atoms and molecules to form materials, structures, components, devices, and systems at the nanoscale. Some of the promising water treatment techniquesor tools introduced by nanotechnology are:

(i) Photocatalysis

(ii) Nanofiltration

(iii) Nanosorbents

3.1. Photocatalysis

Photocatalysis is a promising technique for water purification that uses a light active nanostructured catalyst medium for disgrace various pollutants present in the water. In a typical photocatalysis system, a semiconductor material is used as catalyst medium, which upon absorption of a light energy superior than its bandgap energy generates an electronhole (e–h) pair. The degradation of the water contaminants can also occur through shortest transfer of the photo-generated electrons or holes from the catalyst surface to the contaminant molecules. Photocatalysis is a surface phenomenon and its general mechanism is a difficult process, which involves five basic steps:

(i) Diffusion of reactants to the surface of the catalyst

(ii) Adsorption of the reactants on the surface of the catalyst

(iii) Reaction at the surface of the catalyst

(iv) (iv) Desorption of the products from the surface of the catalyst

(v) Diffusion of the products from the surface of the catalyst.

The activity of a photocatalyst is highly reliant on its ability to generate an e–h pair upon absorption of light. The application of nanostructured semiconductor materials for photocatalysis is additionally suitable compared to their bulk counterparts, since most of the photo-generated electrons and holes are available at the shell of the nano-photocatalyst due to its high surface to volume ratio. [13] Photocatalysis has been generally used for the degradation of harmful organic contaminants from water into harmless byproducts, frequently carbon dioxide and water. Various types of alcohols, carboxylic acids, phenolic derivatives and chlorinated aromatic contaminants have been fruitfully degraded by the application of the photocatalysis technique. The short introduction on the industrial wastewater and the development of wastewater treatment processes, especially advanced oxidation processes (AOPs). The application of solar TiO2 photocatalysis in treating other way of industrial wastewater, such as paper mill wastewater, textile wastewater, and olive mill wastewater. Solar TiO2 photocatalysis with other AOPs and their helpfulness, energy, and chemical consumption.They are provide the future development of TiO2 photocatalysis for different industrial wastewater.

A new filter membrane is fabricated from α-MoO3 nanowires by a facile vacuum filtration method and subsequently used for removal of organic pollutants as of a vitalizing wastewater. The MoO3 filter membrane can be used after a simple heattreatment at 350 °C to removal of adsorbed organic dyes. A new approach for the deprivation of pollutants are present in wastewater they suggested by making use of nanocomposite photocatalysts. These technique hasgood potential for the treatment of wastewater by reason of the use of doped graphene-based nanocomposite photocatalysts. The advantages of using photocatalysis as a pretreatment step for a subsequent olive mill wastewater (OMW) treatment process using by membranes. These Membrane processes appear to be suitable to purify the aqueous wastewater surge polluted by organic matter such as OMW, although suffer severe fouling. The problem is that in many case, boundary flux values are very low, making the process economically not reasonable.

The structure of the nanocatalysts was visualized by BET, FESEM, XRD, and FTIR analyses. A green synthesis of SnO2 nanoparticles was properly developed using urea by a microwave heating method. This method resulted in the formation of spherical, microcrystalline SnO2 nanoparticles with an average size of ∼4.0 nm. The sensitized nanomaterials were characterized by X- ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy and vibrating sample magnetometry. Using sensitized magnetic nanocatalysts can be a suitable pre-treatment method for complete decolorization of emission from textile dyeing and finishing processes,they are optimum operating conditions are established. Environmental pollution by heavy metal is appear as the most hazardious tasks to both water sources and atmosphere quality today. The treatment of heavy metals is of special concern due to their uprising and persistence in the environment. The expand of the heavy metals within water sources, nickel oxide nanoparticles adsorbents were synthesized and characterized with the point of removal of one of the destructive heavy elements, namely; lead ions. Chlorpyrifos is widely used to control pest insects in the place of resediential, agricultural, and commercial applications. They are general use for led to the release of chlorpyrifos into sediments, wastewater and otherwater sources. The presence of chlorpyrifos in wastewaters and water sources they cause affect ecosystem and human health as its chronicle toxicity to aquatic organisms. The nanocomposite was characterized by using transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy.

The measurement of the critical and doorstep flux for modelization, prediction and control of the fouling issues of batch membranes-in-series processes, in detail ultrafiltration (UF),nanofiltration (NF) and reverse osmosis (RO), for therecruitment of olive mill wastewater (OMW-2). The Resulting existence of doorstepflux values for both UF and NF membranes, with minimum constant fouling attained on the latter, that is 54.5% lower. The catalysts as like Fe, Bi2O3, and Fe-doped Bi2O3 were synthesized for the sonophotocatalytic treatment of synthetic dye and real textile wastewater. The resultant catalysts were characterized for its size and shape using x-ray diffract gram (XRD) and scanning electron microscopy (SEM). The higher ultraviolet light absorbance capacity of the catalysts, it was also evident using diffuse reflectance spectroscopy (DRS). Synthesis of titania (TiO2) nanophotocatalyst run by cadmium (Cd) doping to activate the photocatalyst in visible part of the light spectrum. So, the Cd– TiO2 nanophotocatalyst was synthesized in various Cd/Ti molar ratios of 0, 0.05, 0.1, and 0.15 using hydrothermal assisted method. The characterization analyses of X-ray diffraction, field emission scanning electron microscopy, brunauer-emmett-teller, spread reflectance spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric- derivative thermogravimetric, transmission electron microscopy, and energy dispersive X- ray were performed to assess the physical, chemical, and optical properties of the catalysts.

Photocatalysis is an higher oxidation process employed in the treatment of water and wastewater. This technique is depends on the oxidative elimination of micropollutants and microbial pathogens. Principally, persistent compounds, for example antibiotics or other micropollutants may be eliminated through photocatalysis during polishing. To enhance photocatalytic properties of titania, which consist of civilizing activity or red- shift for energy saving, modification techniques have been explored. If highly effective nano-TiO2 able to be activated by visible light can be developed successfully, photocatalysis will become one of the most hopeful water and wastewater treatment technologies because of its flexible and manifold implementation and easy scalability. The use of various forms of photolysis or photocatalytic degradation are effective in degrading many halogenated organic compounds. Some non-halogenated organic compounds, and heavy metals in specific situations. Photocatalysis is often not effective in the long term and is imperfect by water chemistry (e.g. hardness) and the presence of co-contaminants. Catalytic or photocatalytic oxidation is an advanced oxidation pro- cess for removal of trace contaminants and microbial pathogens from water. It is a useful method in enhancing the biodegradabiliy for hazardous and non-biodegradable contaminants. Photocatalysis can also be used as a polishing step to treat intractable organic compounds. Nanocatalysts of high surface-to-volume ratio showed significantly enhanced catalysis performance over their bulky counterparts. Addi- tionally, the band gap and crystalline structure of the nanosized semi- conductors exhibited size-dependent performance. The nano- catalysts, especially those of inorganic materials such as semiconductors and metal oxides, are gaining considerable.

Removal of Organic Contaminants

Release of dyes from textile industries into rivers is one of the most concerning issues in approximately every developing countries. In this regard, semiconductor metal oxides, such as TiO2, ZnO etc., have shown great ability to photocatalytically degrade several dyes in water. To degrade natural organic matters or humic substances, photocatalysis has also been used. Humic substances are in nature occurring yellow–brown organic materials having high molecular weight. The reduction observed in humic acid concentration was recorded in about 12 minutes below the irradiation from a mercury lamp. A variety of carbon-based adsorbents have been used for the removal of debries from raw water and several factors affect this sorption. Different types of nanomaterial like nanosorbents as like (carben neon tubes) CNTs, polymeric materials (e.g., dendrimers), and zeolites have exceptional adsorption properties and are applied for removal of organics from water/wastewater. Functionalization Functionalization of CNT membranes is often a precondition for CNT-based water purification.Functionalized CNT membranes show good water permeability, mechanical and thermal stability, fouling resistance, pollutant degradation and also self-cleaning functions. Tip functionalized CNT membranes have selective functional groups on the nanotube jaws and the core functionalized CNT have functionalities at the sidewall or interior coreCNTmembrane is a novel excellent membrane technology, but several factors are hindering their commercialization. High-density CNT membranes with uniform pore distribution can control the crowning fate of the membrane.

Removal of Inorganic Contaminants

Inorganic contaminants, as like halide ions, cyanide, thiocyanate, ammonia, nitrates and nitrites can be effectively decaying with the help of photocatalytic reaction. The photocatalytic activity of TiO2 against silver nitrate (studied by Ohtani et al) ZnO nanoparticles were used to remove toxic potassium cyanide and Cr(Vi) ions from water using visible light. Inorganic supports for nanocomposites mainly include activated carbon, CNTs, as well as natural minerals as like zeolite, biochar and clay. Activated carbon (AC) is a classic adsorbent broadly applied in water and wastewater treatment facilities. AC is a very porous adsorbent of sufficient micropores (specific surface area 100–2000 m2/g) and hydrophobic surface, which could serve as an brilliant support for nanoparticles. Inorganic membranes having nano-TiO2 or modified nanoTiO2 have been used effectively for reductive degradation of contaminants, particularly chlorinated compounds.

Removal of Heavy Metals

Heavy metal removal from wastewater is one more demanding area for treatment plants, since the amount can vary, depending upon the type of wastewater. However, because of the rare availability and high cost of some metals, recovery of the metals is mostly chosen to removal of the metals. Different heavy metals have been established to be recoverable using the photocatalysis technique. Recovery of gold(III), platinum(IV), and rhodium(III) using TiO2 dispersions have been considered by Minero and co- workers as early as in 1986. From a mixture of gold(III), platinum(IV), and rhodium(1II) chloride salts, the authors have fruitfully recovered more than 90% of gold at pH value 0 under the solar irradiation. exclusion of cadmium (Cd) from wastewater was investigated by Thurnauer and co-workers with the help of nanosized TiO2 particles. After combining the activated carbon and TiO2 nanoparticles, the scientists found a more than 70% removal rate for the metallic Hg(0) adsorbed onto the activated carbon and TiO2 surface after photoreduction, which was recovered on a silver trap through heating. The main advantage of nano-TiO2 over nanosilver is the nearly endless life time of such coatings, since TiO2 as a catalyst leftovers unchanged throughout the degradation process of organic compounds and micro- organisms. Various kind of nanomaterials have been introduced for removal of heavy metals from water/wastewater for instance nanosorbents including CNTs, zeolites, and dendrimers and they have exceptional adsorption properties. Metal based nanomaterias is proved to be better in removing heavy metals than activated carbon. TiO2 is a very common nanoparticle type to inactive microbes in drinking water, wastewater, and other sources.

Removal of Microbes

Almost all the photocatalysts also be evidence for antimicrobial effect and avoid microbial growth. The process basically involves the destruction of the cell wall of the microbes through the highly immediate radicals generated during the photocatalysis process, which eventually leads the destruction of the microbes. The different harmful bacteria and viruses, as like Streptococcus mutans, Streptococcus natuss, Streptococcus cricetus, Escherichia coli, Scaccharomyces cerevisisas, Lactobacillus acidophilus, etc., they could be removed by using heterogeneous photocatalysis. TiO2 inhibits Chlorella vulgaris (green algae), which has a thick cell wall. Equally, zinc oxide (ZnO) has also shown promising antimicrobial effect against Escherichia coli and Staphylococcus aureus.[13] TiO2 nanoparticles are among the emerging and most hopeful photocatalysts for purification of water. The limited photocatalytic capability of TiO2, that is only under UV light, has improved radially by extending its optical absorbance to the visible lightregion.[7] Attempts to decrease pathogen content have been associated with deterioration in the water quality (Herrero and Perez-Coveta, 2005; Mounzer et al., 2012). The nutrient content imposed huge problems related to the treatment level of the TDW, mainly due to the seasonality of the crops cultivated and the nutrient excesses remaining in the soil after harvesting.

Nanofiltration

Besides adsorption, membrane separation is also a key module in the polishing treatment stage, enabling water reclamation from unconven- tional water sources such as municipal wastewater. Membrane separation processes are the rapidly advancing applications for water and wastewater treatment. Membranes becomes a physical barrier for substances depending on their pore size and molecule size.[5] A variety of filtration techniques, such as sand filtration and membrane filtration (microfiltration, ultrafiltration, and reverse osmosis (RO)) are effective at removing most contaminants from waste water. Flow from the membrane often requires pressure to force the fluid across the membrane, which can require significant energy inputs. Membrane filtration often also requires that specific conditions be maintained, to prevent the fouling. Nitrocellulose filter impregnated with various AgNPs were characterized. Filtration is one of the most general and important steps in water purification and wastewater treatment, which involves a filter media or a membrane that separates the solid part from the liquid. Nanofiltration (NF) is a pressure-driven membrane separation technique and is rapidly beneficial in the area of water purification and wastewater treatment due to its unique charge-based repulsion property and high rate of permeation.Because of the lower pressure requirements (7–30 atm) compared to reverse osmosis (RO) processes (20–100 atm), NF is becoming more popular recent days, being a lower energy consumption technique.

The properties of the membranes used in NF lie between the non-porous RO membranes and porous ultrafiltration membranes, and thus the transport in NF majorly occurs through the solution diffusion mechanism; it is also because of the size exclusion property of the membranes. However, the surface charge property of the membranes allows the monovalent ions in the hard water to pass through, while retaining the multivalent ions. Application of NF in the area of wastewater treatment is relatively new and the technique unique is gaining tremendous attention from various industries, as like textiles, pharmaceuticals, the dairy industry, the petrochemical industry, and so on. However, polymeric membranes have lower chemical resistance and a high rate of fouling, and so exhibit a short lifetime. Nanofiltration is one of the membrane filtration techniques and can be defined as a pressure-driven process wherein molecules and particles less than 0.5 nm to 1 nm are rejected by the membrane. Nanofiltration membranes are characterized by a unique charge-based repulsion mechanism allowing the separation of various ions. They are mostly applied for the reduction of hardness, color, odor, and heavy metal ions from groundwater. The conversion of sea water into potable water (desalination) is another prosperous field of application since comparable desalination technologies are very cost-intensive. The immobilizationof metallic nanoparticles in membrane has also been prove defective for degradation and dechlorination of toxic contaminants.

Nanocomposite membranes

Nanocomposite membranes can be considered as a new group of filtration materials comprising mixed matrix membranes and surface- functionalized membranes. In most cases, the nanofillers are inorganic and embedded in a polymeric or inorganic oxide matrix. These nanofillers feature a larger specific surface area leading to a higher surface-tomass ratio. The incorporation of zeolites improves the hydrophilicity of membranes resulting in raised water permeability. Only a small percentage of hydrophilized ordered mesoporous carbons (adding nanomaterial) is required to raise the hydrophilicity of the membrane surface, resulting in considerably increased pure water permeability. Another method used to prevent membrane clogging might be surface functionalization with chemical substances capable of oxidizing organic contaminants and thus prevent building up of fouling layers. Nanocomposite membranes are made up of ordered mesoporous carbons as nanofillers fabricated as thin-film polymeric matrices.

Thin-Film Nanocomposite

Membranes Thin-film nanocomposite (TFN) membranes are a new category of merged membranes prepared by an interfacial polymerization process. Zeolite-polyamide nanocomposite thin films were prepared by interfacial polymerization, which results in reverse osmosis membranes with improved permeability and interfacial properties when compared to equally formed pure polyamide thin films. In a study, incorporation of appropriate amounts of nano-TiO2 into the thin-film composite active layer resulted in increase in the membrane rejection for salts while maintaining the permeability.

Nanofiber Membranes

Electrospinning is a simple, inexpensive, and efficient technique to fabricate nanofibers. These nanofibers contain soaring surface area, porosity, and form nanofiber mats with complex pore structures. The physical and chemical parameters of electrospun nanofibers can be easily manipulated for diverse applications. An electrospun nanomembrane can eliminate bacteria or viruses by size exclusion. However, the utilization of these membranes incurs difficulties because of smaller pore sizes for appropriate removal of viral agents. The bionanohybrid electrospun nanofiber membranes undertake conformational change upon wetting during filtration and increase of pH to above the isoelectric point of the protein. This leads to the emergence of the hidden functional groups, molecules (thereby protein swelling) and nanosolids to be filtered. Moreover, swollen protein makes a higher steric hindrance, facilitating the capturing of the nanosolids, such as metal ions. Membranes made of hydrophobic nanofiber materials may become very appropriate for separation of organic solvents, leading to higher flux efficiency.

Carbon Nanomaterials

Carbon nanomaterials are one of the most extensively studied nanomaterials for membrane fabrication because of their ease of preparation, high mechanical robustness, and excellent rejection ability. Membranes based on hollow, unsophisticated carbon nanotubes (CNTs) have been reported to have high solvent permeability and a high rejection rate of the contaminants. The small pore diameter of the CNTs (typically in the range from the 1 nm – 10 nm) allows only water to pass through, while blocking the chemical and biological contaminants. The advantages of membranes based on CNTs are that they are robust similar to the ceramic from contaminated water. In general, the sorption process of pollutants in water on the sorbent surface occurs in three steps:

(i)Transport of the pollutant from the water to the sorbent surface

(ii) Adsorption at the sorbent surface

(iii)Transport within the sorbent.

Nanoparticles have two important properties that make them very effective as sorbents. They possess higher specific-surface areas than bulk particles and can be functionalized easily with various chemical groups to increase their affinity toward the target contaminants. Nanosorbents can also be reused by removing the absorbed pollutants, thereby regenerating them.

Nanosorbents

For any adsorption development, an adsorbent having large surface area, pore volume, and proper functionalities is the key to success. Extensive studies have found that the adsorption capacity of CNTs depends on both the surface functional groups and the nature of the sorbate. Nanomaterial based adsorbents, i.e., nanosized metal or metal oxides, carbon nanotubes (CNTs), graphene and nanocomposites, often mark large specific area, high reactivity, fast kinetics and specific affinity to various contaminants. Adsorption is the capability of all solid substances to draw to their surfaces molecules of gases or solutions with which they are in close contact. Solids that are used to adsorb gases or dissolved substances are called adsorbents, and the adsorbed molecules are usually referred to collectively as the adsorbate. Conventional desalination methods are energy-consuming and technically demanding, whereas adsorption-based techniques are simple and easy to use for point-of- use water purification devices, yet their ability to remove salts is limited. Most adsorption methods use a carbonaceous material to catch pollutant molecules inside its pore structure. While the raw materials for activated carbon can be low-cost, the energy required to produce high quality activated carbon has been shown to be a important environmental impact if non- renewable energy sources are used. In activated carbon treatment, the carbon material finally has to be regenerated to remove the adsorbed organic compounds. Ion exchange is another form of sorption that is typically used to take away heavy metal ions and other non-metal ions from solution and replace them with less toxic ions.

The advance technology in 21st century has given human ease of life and huge problem in environmental sector. The improvement on nanomaterial has mixed up researchers in the preparation of nanocellulose adsorbents. The stability of nanocellulose as adsorbent also gives good diagnoses in upgrading scale as regenerative aspects indicates soaring performance after several adsorption-desorption cycle.[57]. The effect of reaction time on the microstructure and morphology of composite samples was systemically examined. The structure mechanism of the core–shell structure was sensibly analyzed by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Brunauer– Emmett–Teller (BET) surface area measurement.[58] Magnetic Fe3O4/MoS2 nanocomposites were invented by a easy hydrothermal route in a water–ethanol system. The nanocomposites has an excellent magnetic sensitivity, could be simply and rapidly separated from the suspension by applying an external magnetic field and established excellent performance for water treatment.[59] Many researchers have used nanoparticles as adsorbents to remove water pollutants including arsenic after adjusting the properties of nanoparticles by improving reactivity, biocompatibility, chargedensity, multi – functionalities, and dispersibility. For arsenic removal, nano adsorbents appeared as the potential substitutions to existing conventional technologies.

Carbon Nanosorbents

Carbon nanomaterials have been expansively used for the adsorption of various organic and inorganic pollutants in water. Out of these nanomaterials, activated carbon is the mainly popular carbon material because of its high adsorption capacity, high thermal stability, excellent resistance against attrition losses, and low cost.[13]

Carbon nanotubes (CNTs)

Granular activated carbon (GAC) was used for the removal of various organic contaminants in addition to the odorous pollutants from water. Adsorption of benzene and toluene from industrial wastewater on activated carbon was studied by Asenjo et al. and reported high adsorption capacity for benzene (∼400–500 mg/g) and toluene (∼700 mg/g). Activated carbon was also found to be effectual for the removal of heavy metal ions, such as Hg(II), Ni(II), Co(II), Cd(II), Cu(II), Cr(III) and Cr(VI). Though, the adsorption capacity was observed to decrease when the CNTs were annealed at high temperature in an inert atmosphere resulting in defectless CNTs with a smoother surface. Hence, the defects in CNTs and their surface bumpiness are crucial for the adsorption process. CNTs are explored as substitutes for activated carbon. CNTs are categorized as the single-walled nanotubes and multiwalled nanotubes (MWCNTs) depending on their preparation.[10] CNTs are the allotropes of carbon with a cylindrical nanostructure. Depending on their manufacturing process, CNTs are categorized as the single-walled nanotubes and multiwalled nanotubes, respectively. Besides having a high specific surface area, CNTs have highly assessable adsorption sites and an adjustable surface chemistry. Furthermore, CNTs show antimicrobial properties by causing oxidative stress in bacteria and destroying the cell membranes. Although CNTs have important advantages over activated carbon, their use on an industrial scal for large municipal water and wastewater treatment plants is not expected in the midterm because of high production costs. The production of CNTs is very costly, and additional technical devices, such as, membrane filtration plants, have to be integrated in order to make absolutely sure that no nanoparticles are discharged into the aqueous environment. A major benefit of CNTs in terms of micropollutant removal is their strong adsorption capacity for polar organic compounds due to the diverse interactions between contaminants and CNTs.[5] Grafting functional molecules/groups on the surface of CNTs is another way to improve their surface characteristics. It can be carried out via different ways such as plasma technique, chemical modification and microwave.

Graphene based nano-adsorbents

Graphene is one of allotropy of carbon having special features that make it highly favorable for quite a few environmental applications. Graphene oxide (GO) is a carbon nano-material having two-dimensional structure produced by the oxidation of graphite layer by chemical method. The hydrophilic groups was induce in GO which required special oxidation process (Gopalakrishnan et al., 2015). The presence of these hydroxyl and carboxyl groups as functional groups in GO increase the adsorption of heavy METALS. GO as adsorbent for the removal of heavy metals are getting more attention due to its high surface area, mechanical strength, light weight, flexibility and chemical stability. Research has also been focused on another allotropy of carbon, which is graphene. From the past decade, there is a huge growth in the use of graphene and graphene based materials for environmen tal remediation, due their unique properties which helps to new potential to improve the performance of numerous environ- mental processes.