A Literature Review of Modelling and Experimental Studies of Water Treatment by Adsorption Processes on Nanomaterials

A significant growth in the future demand for water resources is expected. Hence researchers have focused on finding new technologies to develop water filtration systems by using experimental and simulation methods. These developments were mainly on membrane-based separation technology, and photocatalytic degradation of organic pollutants which play an important role in wastewater treatment by means of adsorption technology. In this work, we provide valuable critical review of the latest experimental and simulation methods on wastewater treatment by adsorption on nanomaterials for the removal of pollutants. First, we review the wastewater treatment processes that were carried out using membranes and nanoparticles. These processes are highlighted and discussed in detail according to the rate of pollutant expulsion, the adsorption capacity, and the effect of adsorption on nanoscale surfaces. Then we review the role of the adsorption process in the photocatalytic degradation of pollutants in wastewater. We summarise the comparison based on decomposition ratios and degradation efficiency of pollutants. Therefore, the present article gives an evidence-based review of the rapid development of experimental and theoretical studies on wastewater treatment by adsorption processes. Lastly, the future direction of adsorption methods on water filtration processes is indicated.


Introduction
Water is arguably the main life source on planet earth and is vital for preservation of the modern world. In many parts of the world, water exploitation in agriculture and land development has led to significant economic progress [1]. On the other hand, water pollution is the biggest challenge facing the world and causing the destruction of water resources around the world [2]. This is because of its pollution with oil, chemicals, industrialization, human consumption, leakage of sewage lines, or the absence of these lines [3]. In addition, according to the United Nations (UN) report, some countries in the Middle East region suffer from water scarcity [1]. Moreover, human activities such as overgrazing, cultivation in wet areas, and discharging waste into rivers has changed the water flow patterns in rivers in terms of quantity, quality and flow times [4]. Changing flow quantities can affect water quality significantly in ground and surface water such as rivers [5]. An example of this is in Rajasthan, India due to a very large increase in groundwater pumping [6]. This led to a decrease in water quality due to an increase in the concentration of fluoride above normal levels causing disease in bones and teeth [6]. In other cases, when the consumption of surface water sources is very high, such as rivers and lakes, the decrease in water levels leads to an increase in the concentration of minerals [7]. These problems have led to a lack of safe drinking water for more than a third of the world population [8]. In addition, it creates the need to look for high quality water by converting low quality water to pure water. Some of the latest technologies that revolutionised the field of water filtration in recent years are membrane-based methods and photocatalysis degradation of

Water Filtration
Filtration is the process of removing solid particles or gases from liquids through filter media which allow the liquid to pass through and blocking solid and gases particles [9]. In filtration processes, filtered water goes one way and other particles collected through the filter area go in a different route [10]. Filtration processes have received wide attention since the 18th century to purify polluted water to obtain freshwater sources for drinking and commercial uses [11]. Unfortunately, many systems which have been developed to meet the needs of water purification were unsuccessful due to drawbacks in each method. In the 18th century, filtration was adopted to remove undesirable particles from water such as copper and lead [12]. Sponge filter was the first water filtration device discovered around the mid-1700s, and Joseph Amy obtained a patent for that device [13]. Furthermore, water purification devices were provided for the first time for domestic use in 1750 [14]. In addition, the first known slow sand filtration system was made in 1804 by John Gibb in Scotland and was very effective in the removal of bacteria, viruses, and heavy metals to produce drinking water [15]. There were great developments in water filtration systems in the 19th and 20th centuries such as the development of the reverse osmosis (RO) method [16], membrane technology [17], and photocatalytic degradation of organic pollutants [18]. The RO method forces water to move through a semi-permeable membrane by an external pressure [19,20]. A larger volume of water subsequently passes through the membrane compared to the volume of dissolved salts or organic molecules [19]. It is capable of removing 99% of ions, particles, collides, and bacteria from the feed water [19].
Energy consumption is a key factor which affects the freshwater production cost in the RO process [21]. One of the main factors for the high energy consumption in the RO method is the high-pressure pump which consumes almost 74% of total energy consumption in RO systems [22]. It has become more efficient for water desalination with the advance of nanotechnology [23]. The level of RO energy consumption has declined in the past 40 years due to membrane technological improvements offering higher permeability [24]. Membrane technology is a general term for several different separation processes that depend on a semi-permeable membrane for gas separation and the removal of undesirable ions or molecules from liquids [25]. The semi-permeable membranes with specific pore size are fixed between two media to block particles at the molecular or ionic level by a driving force based on the pressure difference between the two sides [26,27].
Nanocomposite membranes have attracted researchers to develop better materials to achieve remarkable properties such as selectivity, mechanical properties, and enhancement of membrane performance in water desalination [28]. For instance, mechanical properties (including Young's modulus, bulk modulus, and shear modulus) and selectivity of graphene membrane have improved after the combination with titanium dioxide (TiO 2 ) as indicated in a previous study [29][30][31]. Nanofiltration is a process depending on an external pressure force in which molecules and particles in a range between 1-2 nm are rejected by the membrane which make it one of the most widely used membrane processes in desalination and wastewater treatment [32]. However, researchers have focused on developing a variety of nanoporous membranes which have open pores diameter ranging between 1 to 100 nm for water desalination such as graphene and molybdenum disulphide (MoS 2 ) [33]. These two nanoporous membranes achieved a high water permeability and salt rejection performance [34,35]. Graphene membrane has contributed to other applications in the water filtration field. In 2016, Cohen-Tanugi and colleagues synthesized a multilayer graphene membrane and it proved to be more economical than a single layer graphene membrane [36]. In addition, the multilayer graphene membrane can offer higher salt rejection than a single membrane with high permeability [36]. However, these promising results attracted researchers to synthesize other nanocomposite materials such as multilayer MoS 2 membrane, and multilayer graphene oxide (GO) membrane [37]. The multilayer membranes are expected to offer more flexibility in terms of membrane productivity, membrane lifetime, and their performance in filtration processes [38]. In addition, a lot of studies mentioned the contribution of spinel ferrites (MFe 2 O 4 ) with metal oxides, MFe 2 O 4 /carbon-based materials, MFe 2 O 4 /polymers, and MFe 2 O 4 /metal nanoparticles for the photocatalytic degradation of dyes and other inorganic pollutants as mentioned in previous studies [39][40][41]. In addition, Zeolite nanostructured membranes have been widely used for the removal of hazardous chemicals from a contaminated water solution as shown in previous studies [42][43][44].
On the other hand, nanocomposite materials such as MoS 2 showed distinctive properties in the photocatalytic degradation of organic pollutants [45]. This is because of its unique electronic [46] and optical properties [47], and its size-dependent bandgap [48]. In 2020, Yan et al. synthesized MoS 2 /TiO 2 nanotube composite for efficient water disinfection using anodic oxidation method and hydrothermal method [49]. The results showed an excellent photocatalytic disinfection under visible light irradiation for the removal of bacteria up to 98.5% with the possibility of reusing the nanocomposite and recycling it efficiently [49]. Photocatalysis is an eco-friendly technique for the removal of many different pollutants such as nitrogen oxides (NO x ), pesticides and other organic pollutants using ultraviolet (UV) light or sunlight [50,51]. TiO 2 is considered to be the most promising material for the photocatalytic removal of organic pollutants [52]. This is because of the high photocatalytic activity of TiO 2 which make it environmentally friendly [53]. Furthermore, the number of pores located in the TiO 2 surface that are generated by photons make it easy to be harvested by free electrons from outside the surface of TiO 2 [53]. In addition to wastewater treatment, photocatalysis has many other applications such as storing energy [54], air purification [55], and antifouling by preventing the nonspecific interaction between membrane surface and foulants [56]. However, photocatalytic reactions proved their effectiveness in degradation of organic pollutants in non-toxic filtration medias, without using chemicals [57]. One of the important steps in the photocatalytic process is the adsorption between the reactant substances with the catalyst surface [58]. The adsorption process affects the efficiency of the photocatalysis process because it mainly depends on the catalyst absorption of light source [59,60]. Therefore, it is one of the most important water filtration processes under development that controls filtration [61]. Moreover, there are many advantages of adsorption process such as being inexpensive [62], fast [63], and simple in operation and implementation [64]. It has many applications in wastewater treatment to remove pollutants such as bacteria and heavy metals and has received considerable attention from researchers [65].

Adsorption Techniques
In water filtration, adsorption is the process of removing organic pollutants from wastewater in which a binding energy is present between the molecules of the substance (ions or atoms) with other surfaces by chemical or physical attraction [66]. It is an important process in catalysis [67], chemical engineering [68], and material science [69]. There are two main types of adsorption process: physical adsorption (physisorption) and chemical adsorption (chemisorption) [70]. Physical adsorption occurs by adsorbing gas molecules onto a solid surface using low intensity forces called van der Waals force [71,72]. Londonvan der Waals is a dispersion force acting between microscopic non-deformable bodies such as atoms and molecules [73]. There are many applications of physisorption process such as: hydrogen storage [74], acoustic wave sensors [74], gas sorption [75] and water filtration [76]. Furthermore, physical adsorption improves membrane efficiency in terms of water flux, hydrophilicity, and antifouling [77]. For instance, Peng and colleagues modified the performance of PVDF (polyvinylidene fluoride) microfiltration membrane for water filtration by a strong physisorption of amphiphilic copolymers experimentally [78]. The results showed a high improvement in the permeability of PVDF membrane with better antifouling properties without any effect on the membrane structure [78].
On the other hand, in chemical adsorption the bonding occurs between the surface molecules of a metal with high energy and another substance (adsorbate) in contact with it, which may be a liquid or a gas [79]. The bonds formed are comparable in strength to ordinary chemical bonds and are much stronger than the van der Waals forces characteristic of physical adsorption [79]. Chemisorption has been widely used in industrial wastewater treatment for the removal of heavy metals [80][81][82]. For instance, Liu et al. investigated the chemical adsorption behaviour in the removal of arsenic by experimental and simulation techniques using microporous metal-organic framework (MIL-125(Ti)). The results showed a fast and efficient removal of arsenic with low concentrations by chemical adsorption [83]. Most studies have not focused on adsorption kinetics by either ignore it, excluded it, or by assuming the adsorption interaction approaches the equilibrium [84][85][86]. However, Luo et al. have studied the photocatalytic degradation kinetics of graphitic carbon nitrate (g-C 3 N 4 ) for contaminant removal by using a 36 W LED light (λ = 400 nm) in a dark chamber [87].
There are many factors that affect the performance of chemical and physical adsorption processes in water filtration such as: atmospheric and experimental conditions [88], contact time between the adsorbate and adsorbent [89], and particle size [90]. It was noted that atmospheric conditions significantly affect the effectiveness of physical adsorption process in terms of absorption capacity [88]. The adsorption process generally is an exothermic process where the energy is expelled in the form of heat or light [91]. Therefore, physical adsorption is more efficient at lower temperatures because when the temperature rises, the ability of the material to absorb reduces as indicated in previous studies [92][93][94]. On the other hand, chemical adsorption range increases with increasing temperature to a certain extent and then begins to decrease [95]. In addition, chemical and physical adsorption increases with increasing gas pressure to a certain extent until saturation is reached [96]. Contact time and particle size have a major effect on the efficiency of both types of adsorption process [89]. Zhang et al. noted that the adsorption of methylene blue (MB) from aqueous solution increased with contact time with the adsorbent [89]. Along the same lines, Laabd et al. studied the adsorption capacity of polyaniline (PANi) film for the removal of trimellitic and pyromellitic acids by experimental and density functional theory (DFT) methods [97]. They estimated the optimum experimental conditions for the adsorption of the acids in terms of contact time, pH, initial concentration, and temperature. Meanwhile, they investigated the results by using first principle DFT calculations and studied the physical interactions between the adsorbate and adsorbent surface molecules [97]. In addition, Bergaoui et al. studied the adsorption mechanism of methylene blue (MB) onto organo-bentonite [98]. The results showed a high MB removal with a maximum adsorption capacity up to equal to 321 mg/g. However, adsorption technique enhanced the performance of photocatalysts based membranes in wastewater treatment as reported in previous studies [99][100][101]. For instance, Zhang et al. modified Ag@BiOBr/AC/GO membrane system for efficient removal of rhodamine B (RhB) by membrane separation and high adsorption capacity [99].
So far, molecular dynamic (MD) and DFT calculations have been widely used by researchers to investigate and predict experimental results and save money, as well as acquiring faster results [102]. Adsorption process and its contribution in water and wastewater treatment has been studied experimentally and investigated by simulation in 70 publications which will be reviewed in this paper. Figure 1 shows the rapid increase in recent years in the number of publications related to water treatment by adsorption process using experimental and simulation methods. Figure 1. Number of publications in water and wastewater treatment by adsorption (experimentally validated by simulation) between 2011 and 2021 (topic keywords "adsorption", "nanomaterials" "DFT" "simulation" and "wastewater treatment" searched from ScienceDirect and Springer), data updated 16 June 2021.

Computational Methods
Computer simulation methods are compatible with experimental work in a laboratory to serve as a bridge between laboratory experiments and theoretical calculations [103]. Furthermore, simulation is an approximate emulation for an integrated complex system to analyse the behaviour and performance of the system over time [104]. There are many applications for simulation such as: manufacturing [105], economics [106], safety engineering [106], and simulation of technology for performance or optimization [107]. Simulation has many advantages such as validating the results obtained by other analytical methods, and sometimes finding unexpected phenomenon while studying the behaviour of the system [108]. MD and DFT simulation methods have been used in many studies in fabrications and modifications of membrane structure for water desalination, gas separation, electrolysis, and many other applications [35,109,110]. In addition, they have been used to investigate the experimental results of the adsorption process as indicated in previous studies [111][112][113].

Calibration and Validation
Due to the significant increase in the use of computer simulation methods in the 21st century, it was necessary to find ways to verify the data issued by simulation programs. Therefore, many models have been modified to measure the error ratio between the data obtained experimentally and data obtained by simulation such as SWAT (Soil and Water Assessment Tool) [114]. SWAT is used to calibrate and validate the data by a procedure using the shuffled complex evolution method [114]. Calibration "is a major element to this evaluation and refers to the estimation and adjustment of model parameters to improve the agreement between model output and a data set" [115]. However, validation "is a model using parameters that were determined during the calibration process and

Computational Methods
Computer simulation methods are compatible with experimental work in a laboratory to serve as a bridge between laboratory experiments and theoretical calculations [103]. Furthermore, simulation is an approximate emulation for an integrated complex system to analyse the behaviour and performance of the system over time [104]. There are many applications for simulation such as: manufacturing [105], economics [106], safety engineering [106], and simulation of technology for performance or optimization [107]. Simulation has many advantages such as validating the results obtained by other analytical methods, and sometimes finding unexpected phenomenon while studying the behaviour of the system [108]. MD and DFT simulation methods have been used in many studies in fabrications and modifications of membrane structure for water desalination, gas separation, electrolysis, and many other applications [35,109,110]. In addition, they have been used to investigate the experimental results of the adsorption process as indicated in previous studies [111][112][113].

Calibration and Validation
Due to the significant increase in the use of computer simulation methods in the 21st century, it was necessary to find ways to verify the data issued by simulation programs. Therefore, many models have been modified to measure the error ratio between the data obtained experimentally and data obtained by simulation such as SWAT (Soil and Water Assessment Tool) [114]. SWAT is used to calibrate and validate the data by a procedure using the shuffled complex evolution method [114]. Calibration "is a major element to this evaluation and refers to the estimation and adjustment of model parameters to improve the agreement between model output and a data set" [115]. However, validation "is a model using parameters that were determined during the calibration process and comparing the predictions to observed data not used in the calibration" [115]. Recently, researchers' Membranes 2022, 12, 360 6 of 38 dependence on calibration and validation of their results have been gradually increased. Therefore, the number of publications that studied adsorption process in wastewater treatment using combined experimental and simulation methods increased at the beginning of 2017. Figure 2 shows the number of publications in water treatment by adsorption using experimental and simulation methods in the last 10 years. comparing the predictions to observed data not used in the calibration" [115]. Recently, researchers' dependence on calibration and validation of their results have been gradually increased. Therefore, the number of publications that studied adsorption process in wastewater treatment using combined experimental and simulation methods increased at the beginning of 2017. Figure 2 shows the number of publications in water treatment by adsorption using experimental and simulation methods in the last 10 years. For instance, de Oliveira and colleagues studied the adsorption process of 17βestradiol in graphene oxide through methanol co-solvent experimentally and through simulation using SIESTA code (Spanish Initiative for Electronic Simulations with Thousands of Atoms) [113]. The DFT model was calibrated using the experimental findings and was in good agreement after studying the electronic density of state (DOS) and the interactions between graphene oxide with methanol molecules [113]. Along the same lines, Zhu et al. studied the degradation of bisphenol using highly efficient heterogeneous Fenton catalysts (CNTs/Fh) by experiment and simulation [116]. The main aim of this study was to accelerate Fe(III)/Fe(II) cycling by carbon nanotubes (CNTs) and they had an excellent agreement in the experimental and DFT results [116]. More detail about the fabrication and characterization of the nanocomposite materials experimentally and by simulation are given in Section 2. For instance, de Oliveira and colleagues studied the adsorption process of 17βestradiol in graphene oxide through methanol co-solvent experimentally and through simulation using SIESTA code (Spanish Initiative for Electronic Simulations with Thousands of Atoms) [113]. The DFT model was calibrated using the experimental findings and was in good agreement after studying the electronic density of state (DOS) and the interactions between graphene oxide with methanol molecules [113]. Along the same lines, Zhu et al. studied the degradation of bisphenol using highly efficient heterogeneous Fenton catalysts (CNTs/Fh) by experiment and simulation [116]. The main aim of this study was to accelerate Fe(III)/Fe(II) cycling by carbon nanotubes (CNTs) and they had an excellent agreement in the experimental and DFT results [116]. More detail about the fabrication and characterization of the nanocomposite materials experimentally and by simulation are given in Section 2.

Synthesis and Simulation of Nanomaterials
In the last decade, synthesis and simulation of nanomembranes have received vast attention and have been widely studied by researchers [117]. Figure 3 shows the most common methods used in the synthesis of nanomaterials.

Synthesis and Simulation of Nanomaterials
In the last decade, synthesis and simulation of nanomembranes have received vast attention and have been widely studied by researchers [117]. Figure 3 shows the most common methods used in the synthesis of nanomaterials.

Nanomembranes
Synthesis and characterization of nanomembranes have received wide attention since the 18th century [118]. During the 18 th century, membranes were under fabrication, functionalization, and modification at the laboratories without any commercial use [119]. Since 2004, membrane experimental designs have increased and the number of materials available for these experiments has increased [118]. One of the famous designs was by Jani and colleagues which designed nanoporous anodic aluminium oxide membranes with desired functions, parameters and properties [120]. Similarly, Mei and colleagues fabricated ultrathin AlN/GaN porous crystalline nanomembranes with different layouts including tubes, spirals, and curved sheets [121]. The structural, morphological and chemical properties of nanomembranes will be characterizing using analyses such as Xray diffraction (XRD), scanning electron microscope (SEM), thermal electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, etc. [122]. The optical and electrical properties of the nanomembranes also will be analysed using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [123]. "Nanomembranes are synthetic structures with a thickness less than 100 nm and the aspect of surface-area-volume ratio increases to at least a few orders of magnitude" [124]. Nanomembranes can be classified based on surface chemistry, bulk structure, morphology, and production method [119]. Nanomembranes have been widely used in many applications such as water and wastewater treatment [125], biomedical applications [126], and smart energy storage devices [127]. In this section, we demonstrate the most common methods used in the synthesis and simulation of nanomembranes.

Synthesis of nanomaterials
Nanocomposites Nanomembranes Improved Hummers' method

Nanomembranes
Synthesis and characterization of nanomembranes have received wide attention since the 18th century [118]. During the 18 th century, membranes were under fabrication, functionalization, and modification at the laboratories without any commercial use [119]. Since 2004, membrane experimental designs have increased and the number of materials available for these experiments has increased [118]. One of the famous designs was by Jani and colleagues which designed nanoporous anodic aluminium oxide membranes with desired functions, parameters and properties [120]. Similarly, Mei and colleagues fabricated ultrathin AlN/GaN porous crystalline nanomembranes with different layouts including tubes, spirals, and curved sheets [121]. The structural, morphological and chemical properties of nanomembranes will be characterizing using analyses such as X-ray diffraction (XRD), scanning electron microscope (SEM), thermal electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, etc. [122]. The optical and electrical properties of the nanomembranes also will be analysed using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [123]. "Nanomembranes are synthetic structures with a thickness less than 100 nm and the aspect of surface-area-volume ratio increases to at least a few orders of magnitude" [124]. Nanomembranes can be classified based on surface chemistry, bulk structure, morphology, and production method [119]. Nanomembranes have been widely used in many applications such as water and wastewater treatment [125], biomedical applications [126], and smart energy storage devices [127]. In this section, we demonstrate the most common methods used in the synthesis and simulation of nanomembranes.

Synthesis of Nanomembranes
Synthesis is a term for producing nanostructured materials including organic, inorganic, and hybrid nanomembranes [128]. It exploits the special physicochemical properties of ionic fluids to control transit and growth [129]. Many methods have been used for the synthesis of nanomembranes such as modified Hummers' method [130], solvothermal method [131], and solvothermal chemical deposition [132]. Modified Hummers' method is one of the most common methods used for the synthesis of nanomembranes such as graphene oxide (GO) [133]. It was developed in 1958 with many advantages such as being safer, faster, and a more efficient method for producing graphite oxide [134]. The chemical method can generate graphite oxide through the addition of potassium permanganate to a solution of graphite, sodium nitrate, and sulfuric acid [129]. However, for the synthesis of other nanomembranes such as molybdenum disulphide, the microwave-assisted route has been used [135]. The microwave-assisted route is "a unique and simple technique for fast and efficient processing of materials with higher reproducibility" [136]. It has drawn attention due to its homogeneous heating, fast kinetics, high phase purity, and high yield rate of products in relatively short time [136]. Table 1 shows nanomembranes synthesized by different synthesis methods. PyTTA-Dva-COF Nitrogen (N), covalent organic framework Solvent-thermal method [142] Ultrafiltration PSF/GO membrane Graphene oxide (GO), polysulfone (PSF) Phase inversion method [143] Nitrogen doped carbon (CNs) Carbon (C), nitrogen (N), titanium (Ti) Chlorination [144] Graphene oxide Graphene oxide Improved Hummers' method [145] Single-layer graphene nanosheets Graphite Solution-phase exfoliation integrating bath sonication and microwave irradiation in organic solvents [146] Carbon nanotubes (CNTs) Carbon nanotube (CNT) Nuclear magnetic resonance (1H and 13C NMR) and high resolution-mass spectrometry (HR-MS) [147] Graphene oxide Graphene oxide Modified Hummers' method [148] Graphene Solvothermal-chemical deposition [154] As shown in Table 1, 11 types of nanomembranes have been synthesized by using different materials. Graphene and graphene oxide (GO) were the most synthesized nanomembranes by using Hummers' method because of their widespread use in water and wastewater treatment. However, other nanocomposites such as nitrogen doped carbon (CNs), are synthesized by using chlorination of Ti(C 0.7 N 0.3 ) at various temperatures resulting in well-developed micro-pores and small meso-pores with uniform pore structures.

Simulation of Nanomembranes
Density functional theory (DFT) is a computational simulation method used in chemistry, physics, and materials science for the calculation of the mechanical and electronic properties of atoms and molecules [155]. There are many simulation software used for DFT calculations such as Material Studio, Vienna Ab initio Simulation Package (VASP), and GAMESS, etc. The simulation software have been used by researchers and engineers to improve the performance of materials in many applications including pharmaceuticals, catalysts, polymers and composites, metals and alloys, batteries and fuel cells [156]. They have many advantages such as developing new cost-effective materials with better performance and more efficiently than with test and experimentation alone [157]. Material studio is a three-dimensional (3D) modelling and simulation software developed and distributed by BIOVIA to allow researchers in material science and chemistry to understand the behaviour and relationships of a material's atomic and molecular structure [156]. Similarly, VASP, Gaussian 09, and GAMESS have been used for atomic scale materials modelling using DFT with different functional groups including (B3LYP) and different methods such as the projector augmented wave method (PAW), and Perdew-Burke-Ernzerhof (PBE) method [158]. PAW and PBE methods are both efficient for the electronic structure calculations of large systems [159]. Furthermore, they are used to improve the accuracy of the electrical and electronic calculations for magnetic materials, alkali and alkali earth elements [160]. Figure 4 shows simulation software used to produce nanomembranes with the number of publications using each software.  As shown in Figure 4, Gaussian 09 and VASP contributed to the simulation of 15 nanomembranes out of 22 nanomembranes in this review paper. 8 of the 15 nanomembranes were graphene or graphene oxide (GO). This is due to the high accuracy in the simulation of graphene and GO nanomembranes by these simulation software as indicated in previous studies [161,162]. In addition, molybdenum disulphide (MoS2) nanosheet has been simulated by using VASP with PAW simulation method, while other nanocomposites such as Zn-Fe LDH, and (CF/BiOBr/Ag3PO4) cloth, have been simulated As shown in Figure 4, Gaussian 09 and VASP contributed to the simulation of 15 nanomembranes out of 22 nanomembranes in this review paper. 8 of the 15 nanomembranes were graphene or graphene oxide (GO). This is due to the high accuracy in the simulation of graphene and GO nanomembranes by these simulation software as indicated in previous studies [161,162]. In addition, molybdenum disulphide (MoS 2 ) nanosheet has been simulated by using VASP with PAW simulation method, while other nanocomposites such as Zn-Fe LDH, and (CF/BiOBr/Ag 3 PO 4 ) cloth, have been simulated by using Material Studio with DMol3 and GGA-PBE codes, respectively. Table 2 shows the simulation software and methods used for simulation of nanomembranes. Exchange-Correlation functional [154] As shown in Table 2, PAW, PBE, and B3LYP are the most common methods used for the DFT calculations of nanomembranes. These calculations are performed based on the solution of Kohn-Sham equations by PAW method. On the other hand, the exchangecorrelation functional model, and the Thomas, Yoon-Nelson, and Adams-Bohart model have been solved by B3LYP and PBE method, respectively. Along the same lines, these methods (PAW, PBE, and B3LYP) have been used for the simulation of nanocomposite materials as explained later in Section 2.2.2.

Nanocomposites
Nanocomposite materials "are composed of several multiple nanomaterials entrapped within a bulk material, which may comprise a combination of a soft and a hard nanomaterial, two soft nanomaterials, or two hard nanomaterials" [169]. They are characterized by their very small size, measured in nanometres [170]. Nanocomposite materials have attractive properties resulting from the combination of inorganic or organic components at the molecular level [171,172]. There are many applications of nanocomposite materials in wastewater treatment [173], energy storage [174], drug delivery [175], and for biomedical purposes [176]. In wastewater treatment, nanocomposite materials have been widely used to treat surface water, sewage, and ground water [177]. By 2009, nano-processing technologies were documented at 44 cleaning sites around the world, most of them in the United States [178]. The synthesis of these nanocomposites received wide attention by the researchers in the last decade [179]. In this section, we demonstrate the most common methods used in the synthesis of nanocomposite materials.

Synthesis of Nanocomposites
For synthesis and characterization of these nanomaterials, many methods have been used including the hydrothermal method [180], chemical vapor deposition (CVD) [181], and one-pot synthesis [182]. The hydrothermal method is one of the most common methods used in the synthesis of nanocomposites [180]. Figure 5 shows the percentages of the number of publications reviewed in this review paper by each experimental method. As shows in Figure 5, the hydrothermal method has been used in more than 56% of the publications reviewed. In this section, we demonstrate the most common methods used in the synthesis of nanocomposite materials.

Synthesis of Nanocomposites
For synthesis and characterization of these nanomaterials, many methods have been used including the hydrothermal method [180], chemical vapor deposition (CVD) [181], and one-pot synthesis [182]. The hydrothermal method is one of the most common methods used in the synthesis of nanocomposites [180]. Figure 5 shows the percentages of the number of publications reviewed in this review paper by each experimental method. As shows in Figure 5, the hydrothermal method has been used in more than 56% of the publications reviewed.

The Hydrothermal Method
Hydrothermal synthesis is a method that uses very high temperatures ranging from room temperature to much higher temperatures to synthesize nanomaterials [183]. It was given the name "hydrothermal" because water is used as the solvent [184]. The hydrothermal method was first discovered in the 19th century [185]. It has been widely used by researchers and the first publication on this method appeared in 1813 [185]. The

Chemical Vapor Deposition
Chemical vapor deposition (CVD) is a coating process that is defined as a method to produce solids with high purity by using thermally induced chemical reactions at the surface of a heated substrate [219]. CVD has many applications in medicine [220], electronic applications [221], and chemical industries [222]. It has many advantages over other synthesis methods such as the ability to deposit a wide variety of materials with very high purity [223]. The CVD method started in the 19th century with the production of lamp filaments. Then, Van Arkel in the 20th century deposited metals from the gas phase for application in the lamp industry [224,225]. The CVD method has three different types based on the conditions of the process classified by applied pressure [226], physical properties of the vapor [227], and plasma methods [228]. It has been used in the production of several materials including monocrystalline, polycrystalline, amorphous, preparation of carbon nanotubes (CNTs) and carbon nanofibers [221,229]. In addition, CVD is famous for producing semiconductors such as the synthesis of 2D Tungsten disulphide (WS 2 ) monolayer [230]. Table 4 shows nanocomposite materials synthesized by the chemical vapor deposition (CVD) method. COOH/CNTs Carbon nanotubes (CNTs), carboxylic functionalized groups (COOH) [232] In this review paper, the CVD method contributed in 7% of the total number of publications as shown in Figure 5. Mainly, it has been used for the synthesis of carbon nanotubes (CNTs). As shown in Table 4, the CVD method has been used for the combination of Co 3 O 4 , and COOH with CNTs. At a sufficiently high temperature, carbon source (hydrocarbon gas) decomposes with the catalyst in a tubular reactor [233]. By using CVD, Yang et al. synthesized a vertically aligned carbon nanotube hybrid membrane for gas separation [164]. The results showed an excellent separation membrane with high conductivity and resistance stability after 50 cycles of tensile deformation [164]. In addition, Zhang et al. studied the adsorption of lead (Pb 2+ ) on oxidized (O-CNTs) and graphitized multi-walled carbon nanotubes (G-CNTs) synthesized by the CVD method [163]. The results showed a high stability in the adsorption mechanism of Pb 2+ [163].

One-Pot Synthesis
One-pot synthesis is a process that is used to improve the efficiency of chemical reactions and focuses on the reduction of number of steps of chemical reactions in one single reaction flask [234]. It is a hydrothermal approach based on a general phase transfer and separation mechanism which occurs at interface of water, solution, and solid phases [190]. Das et al. reported in their book that the one-pot synthesis method has many advantages such as "saving time and resources, improves the efficiency of a chemical reaction, and offers better chemical yield" [190]. An example of one-pot synthesis is the synthesis of highly stable CsPbBr 3 @SiO 2 Core-Shell Nanoparticles [182]. The reported method showed that the formation rates, determined by reaction temperature, precursor species, pH value, etc., of both CsPbBr 3 and SiO 2 are critical for the successful preparation of core-shell NPs [182]. Table 5 shows nanocomposite materials synthesized by the one-pot synthesis method. Table 5. Nanocomposite materials synthesized by the one-pot synthesis method.

S, N co-doped graphene aerogel (SN-rGO-A)
Graphene oxide (GO), sulfur (S), nitrogen (N). [165] ZIF-67 Carbocatalysts, Nitrogen-doped magnetic carbon (Co@N-C) Cobalt (Co), nitrogen (N), carbon (C) [235] Fe/Fe 3 C@PC Graphitized porous carbon (PC), Fe-based nanoparticle core (Fe/Fe 3 C) [236] As shown in Table 5, Ren and his colleagues synthesized a 3D porous sulphur and nitrogen co-doped graphene aerogel (SN-rGO-A) for the degradation of Rhodamine B (RhB) [165]. The results showed that sulphur and nitrogen co-doping could synergistically enhance the catalytic performance for activating peroxydisulfate (PDS) compared to the original and N doped graphene aerogels [165]. Along the same lines, Li and colleagues synthesized a series of catalysts by using ZIF-67 for the removal of bisphenol A (BPA) and total organic carbon (TOC) [235]. The results showed an excellent degradation efficiency for TOC and BPA.

Other Synthesis Methods
In addition to the methods presented in this section, there are other known methods for the synthesis of nanocomposite materials such as solvothermal methods. Solvothermal methods "offer a simple, direct, and low-temperature route to obtain nanometric particles with narrow size dispersions, and represent an alternative to calcinations for promoting crystallization under milder temperatures" [237]. It is mainly used for the synthesis of highly crystallized lanthanide (UCNPs) at relatively low temperature [238]. It has many advantages such as being a simple, economical and efficient method [239]. On the other hand, there are some disadvantages of this method such as the long processing time and the contraction that occurs during processing [240]. However, solvothermal method is very similar to the hydrothermal route, the only difference being that the precursor solution is usually non-aqueous [238]. Table 6 shows nanocomposite materials synthesized by other synthesis methods.

Simulation of Nanocomposites
For the simulation of nanocomposites, VASP was the most used simulation software with 27 publications as shown in Figure 6. 10 of these publications were for the simulation of carbon-based material including CNTs and g-C 3 N 4 . However, 14 publications were for the simulation of other nanocomposites such as MnFe 2 O 4 nanocubes using the PAW method, Fe 3 O 4 -HBPA-ASA using B3LYP functional group, and granular TiO 2 -La using the PBE method as shown in Table 7. Figure 6 shows simulation software used to produce nanoparticles with the number of publications by each software. For the simulation of nanocomposites, VASP was the most used simulation software with 27 publications as shown in Figure 6. 10 of these publications were for the simulation of carbon-based material including CNTs and g-C3N4. However, 14 publications were for the simulation of other nanocomposites such as MnFe2O4 nanocubes using the PAW method, Fe3O4-HBPA-ASA using B3LYP functional group, and granular TiO2-La using the PBE method as shown in Table 7. Figure 6 shows simulation software used to produce nanoparticles with the number of publications by each software.      [241] (N-rGO/BiVO 4 ) Not supplied DFT Exchange-Correlation functional [193] Cerium zirconium oxide (CexZryO 2 ) VASP DFT (PBE) Exchange-Correlation functional [195] NiO nanobelt VASP DFT (PAW) Kohn-Sham equations [204] ZnO/Al 2 O 3 VASP, COMSOL DFT (PBE) Exchange-Correlation functional [196] ZnO@C Molecular Operating Environment software (MOE, 2008.10)

DFT
Exchange-Correlation functional [194] C, N, F/TiO 2 NTs VASP DFT Exchange-Correlation functional [197] iN-Ti 3 C 2 /TiO 2 hybrid VASP DFT (PBE) Exchange-Correlation functional [198] TiO 2 nanoflowers (TNFs) VASP DFT (PBE) Exchange-Correlation functional [199] Cr-TiO 2 supported on Fe 3 O 4 Not supplied DFT (M06 L) a Langmuir-Hinshelwood model [242] Titanate nanotubes supported TiO 2 (TiO 2 /TiNTs) Gaussian 03 DFT (B3LYP) Exchange-Correlation functional [200]  Kohn-Sham equations [212] As shown in Table 7, DFT (PBE), and DFT (PAW) have been mainly used for the simulation of the nanocomposite materials. This is due to the high accuracy of these two simulation methods. The augmented plane wave (PAW), and PBE was used to describe the electron-ion interactions [256]. For instance, Maji and colleagues used DFT (PBE) simulation method through VASP software for the simulation of Fe 2 O 3 -PC nanohybrids [203]. The results showed an agreement between the experimental and simulation results [203]. Similarly, Regmi and colleagues simulated an N-doped BiVO 4 model using the DFT (PBE) method through VASP simulation software [208]. The results showed good agreement with experimental results in terms of the electronic property calculations such as the band structure and density of state (DOS) [208].

Water Filtration by Membrane Technology
Due to its important role in water purification, membrane technology is rapidly developing. The ability of rejection/adsorption may differ from one membrane to another due to the membrane pore size, surface charge, hydrophobicity/hydrophilicity, and surface morphology [257]. As shown in Figure 7, highly porous zeolitic imidazolate frameworks (ZIFs) have the highest adsorption rate for the removal of Uranium as ZIF-8 (540.4 mg/g) > Zn/Co-ZIF (527.5 mg/g) > ZIF-9 (448.6 mg/g) > ZIF-67 (368.2 mg/g). This is due to the high elimination capacity for Uranium because of (ZIF-8) large surface area and active metal ion [247]. However, porous graphitic carbon nitride (g-C 3 N 4 ) has shown a high adsorption rate of Uranium with 149.7 mg/g. The results showed a strong interaction between uranyl and g-C 3 N 4 (Ead = 156.83 kcal/mol) and the most effective sorption site was inside the holes of g-C 3 N 4 [214]. Similarly, as shown in Figure 7, molybdenum disulphide (MoS 2 ) showed a good elimination capacity of Uranium with 117.9, 45.7 and 37.1 (mg/g) [150,215]. This is due to the binding energy through U-S bond which improved by the molybdenum group [150].
In this section, we discuss the rejection rate and adsorption capacity of highly effective membranes including carbon-based membranes, metal oxides, and other nanocomposite membranes.
active metal ion [247]. However, porous graphitic carbon nitride (g-C3N4) has shown a high adsorption rate of Uranium with 149.7 mg/g. The results showed a strong interaction between uranyl and g-C3N4 (Ead = 156.83 kcal/mol) and the most effective sorption site was inside the holes of g-C3N4 [214]. Similarly, as shown in Figure 7, molybdenum disulphide (MoS2) showed a good elimination capacity of Uranium with 117.9, 45.7 and 37.1 (mg/g) [150,215]. This is due to the binding energy through U-S bond which improved by the molybdenum group [150].
In this section, we discuss the rejection rate and adsorption capacity of highly effective membranes including carbon-based membranes, metal oxides, and other nanocomposite membranes.

Carbon-Based Membranes
Carbon-based membranes including graphene, graphene oxide (GO), and carbon nanotubes (CNTs) are some of the most important nanocomposite membranes for the adsorption of pollutants from wastewater. Carbonaceous nanofiber (CNF), and graphene oxide have shown the highest adsorption rate for the removal of toxic metals of 341.2 and 288 mg/g, respectively, as shown in Figure 8. However, porous graphitic carbon nitride (g-C3N4), and carbon nanotubes (CNTs) have also shown a good adsorption rate for the removal of toxic metals such as Uranium and Indigo carmine (IC) dye of 149.7 and 136 mg/g, respectively. Figure 8 shows the rate of adsorption for toxic metals by carbon-based membranes. In addition, carbon-based membranes have been used for the removal of other toxic metals such as chloroform molecule (CHCl3), oils and organic solvents, and typical pharmaceuticals as shown in Table 8.

Carbon-Based Membranes
Carbon-based membranes including graphene, graphene oxide (GO), and carbon nanotubes (CNTs) are some of the most important nanocomposite membranes for the adsorption of pollutants from wastewater. Carbonaceous nanofiber (CNF), and graphene oxide have shown the highest adsorption rate for the removal of toxic metals of 341.2 and 288 mg/g, respectively, as shown in Figure 8. However, porous graphitic carbon nitride (g-C 3 N 4 ), and carbon nanotubes (CNTs) have also shown a good adsorption rate for the removal of toxic metals such as Uranium and Indigo carmine (IC) dye of 149.7 and 136 mg/g, respectively. Figure 8 shows the rate of adsorption for toxic metals by carbonbased membranes. In addition, carbon-based membranes have been used for the removal of other toxic metals such as chloroform molecule (CHCl 3 ), oils and organic solvents, and typical pharmaceuticals as shown in Table 8.      Graphene oxide Adsorption of 17 βestradiol on graphene oxide 17 βestradiol 169.49 mg/g [113] Graphene oxide Adsorption of As(III) on graphene oxide As(III) 288 mg/g [145] Graphene oxide

Rate of Rejection (%)/Adsorption Capacity (qe) (mg/g) Reference
Removal of Ni(II) from wastewater by adsorption on graphene oxide surface Ni(II) 197.8 mg/g [148] Graphene As shown in Table 8, carbon-based materials have been mainly used for the rejection/adsorption of toxic metals with high adsorption capacity (qe). Graphene oxide (GO) has been used for the removal of Ni(II), methylene blue (MB), arsenic (As(III)), 17 βestradiol, and nitrate with high adsorption capacity ranging between 169-288 mg/g. However, carbon nanotubes (CNTs) have been used for the removal of Pb 2+ , sulfamethoxazole (SMZ) and ketoprofen (KET), bisphenol A, 17a-ethinyl estradiol (EE2), and typical pharmaceuticals, phosphate, indigo carmine (IC) dye, phenol separation, and binary CO 2 /N 2 mixture. The rate of adsorption by carbon nanotubes (CNTs) was also high as it ranged between 0.64-136 mg/g. It can be observed that carbonaceous nanofiber/Ni-Al layered double hydroxide (CNF/LDH) showed an excellent adsorption capacity for the removal of Cu(II) and Cr(VI) with 219.6 and 341 mg/g, respectively.

Metal Oxides
Binary and ternary metal oxides have been widely studied for supercapacitors, and wastewater treatment because of its high structure stability, low cost, and high electronic conductivity [258,259]. Metal oxides including ZnO, and MnFe 2 O 4 have shown a good adsorption capacity for the removal of barium ions and uranium with 64.4 and 119.9 mg/g, respectively. The high adsorption capacity of metal oxides is due to the ionic bonding which has important consequences for the adsorption of molecules at their surfaces [260]. In addition, Oct-Cu 2 O NCs showed an excellent adsorption capacity with 1112.6 mg/g of tetracycline on octahedral Cu 2 O nanocrystals. Table 9 shows rejection/adsorption capacity of metal oxide.

Other Nanocomposite Membranes
In addition to carbon-based materials and metal oxides, there are other nanocomposite membranes that can be used in the adsorption of pollutants from water. For instance, MoS 2 has shown a high adsorption capacity for the removal of toxic metals such as lead (Pb 2+ ) with 638 mg/g as shown in Figure 9. However, MOF-545, M-ATP, and Fe 3 O 4 -HBPA-ASA showed similar values of adsorption for Pb 2+ as 73, 53.88, 88.36 mg/g, respectively. The high adsorption capacity of MoS 2 is due to the strong ionic capture and electrostatic attractions which improve Pb 2+ removal capacity under sunlight irradiation [151]. Table 10 shows rejection/adsorption capacity and the role for each nanocomposite membrane in the adsorption process.  Adsorption mechanisms for   [215] As shown in Table 10, nanocomposite materials have shown a good rejection/adsorption capacity on toxic materials. For instance, Granular TiO 2 -La shows a good adsorption capacity of Arsenic (As III), and Fluoride (F) equal to 114 and 78.4 mg/g, respectively. Similarly, zirconium-based highly porous metal-organic framework (MOF-545) showed an excellent removal of Pb 2+ with adsorption capacity up to 73 mg/g. In addition, the developed SPE-MOF-545 can be reused for up to 42 extraction cycles without a significant loss of extraction efficiency. Furthermore, Amino-modified attapulgite (M-ATP) also showed a high removal of Pb 2+ , and Cu 2+ as follows: 53.58 and 28.86 mg/g, respectively.

Photocatalytic Degradation of Organic Pollutants
Photocatalytic degradation of organic pollutants has been widely used by researchers because of its great role in removing undesirable contaminants from water and wastewater [261]. The earliest mention of photocatalysis dates back to 1911, when German chemist Alexander Ebner incorporated the concept into his research on the illumination of zinc oxide (ZnO) on the bleaching of the dark blue dye, Prussian blue [262]. Around this time, Brunner and Kosack published an article discussing the degradation of oxalic acid in the presence of uranyl salts under illumination [263], while in 1913, Landau published an article explaining the phenomenon of photocatalysis [264]. Photo-Fenton oxidation is an advanced process uses hydroxyl radicals which increase the rate of degradation of organic pollutants [265]. While heterogeneous photocatalysis is one of the most studied processes for environmental purposes such as water purification and emission cleaning [266]. In this section, we discuss the role of titanium dioxide (TiO 2 ), carbon nanomaterials, metal oxides, and other nanocomposites in the degradation of organic pollutants.

Titanium Dioxide (TiO 2 )
Titanium dioxide (TiO 2 ) contributes significantly in the water purification process through degradation of organic pollutants such as Methylene blue (MB), Methyl orange, and Bisphenol A (BPA). In this process, TiO 2 acts as a catalyst to accelerate photoreaction for the removal of organic pollutants. Table 11 shows the decomposition rate/degradation efficiency of titanium dioxide (TiO 2 ). As shown in Table 11, TiO 2 has shown a high efficiency in the degradation of organic pollutants. For instance, Wang and colleagues synthesized C, N, F/TiO 2 NTs nanocomposite material for the degradation of methyl orange under UV-light and simulated sunlight [197]. The results showed a high photocatalytic activity under UV-light for C/TiO 2 NTs with degradation efficiency up to 100% [197]. Along the same lines, De Santiago et al. synthesized Cr-TiO 2 nanocomposite supported on Fe 3 O 4 for the degradation of malachite green dye (MG), and total organic carbon (TOC) [242]. The results showed a high removal under solar radiation up to 100% of MG, and 60% of TOC [242]. Furthermore, Cheng et al. synthesized titanate nanotubes supported on TiO 2 (TiO 2 /TiNTs) for the removal of Cu(II), and phenanthrene under UV-light [200]. The results showed a high adsorption capacity of Cu(II) up to 115 mg/g, while for phenanthrene the degradation efficiency was more than 95% [200].

Carbon Nanomaterials
Carbon nanomaterials have showed an excellent contribution in the degradation of toxic materials with high efficiency for reduction of highly toxic contaminants up to 99%. As shown in

Metal Oxides
In addition to titanium dioxide (TiO 2 ) and carbon nanomaterials, metal oxides can be used in the adsorption of pollutants from water. For instance, ZnO has shown a high adsorption capacity for the removal of toxic metals such as methylene blue, methyl orange dye (MO), and TOC with high degradation efficiency for TOC up to 80.4%. However, Cerium zirconium oxide (CexZryO 2 ) also showed a high degradation efficiency for the removal of sulphonamides of 91.33% as shown in Table 13. Furthermore, Kitchamsetti and colleagues synthesized NiO nanobelt composite material for the removal of organic pollutants such as RhB, CV, MB, and MO. The results showed a high degradation efficiency up to 89%, 76.7%, 82.7%, and 79.1%, respectively [204].

Other Nanocomposites
For other nanocomposites, we demonstrate the decomposition rate/degradation efficiency for the most effective compounds for the degradation of organic pollutants such as nickel (Ni(II)), methylene blue (MB), sulfamethoxazole (SDZ), and trimethoprim (TMP). As shown in Table 14, for the degradation of MB, sodium titanate nanotubes (Na-TNT) showed a high degradation efficiency up to 99.5%, while Fe 2 O 3 -PC removed 75% of MB. For the degradation of sulfamethoxazole (SDZ), carbon dots/g-C 3 N 4 (C-CN) heterostructures showed a high degradation efficiency up to 92.8% for the 3C-CN heterostructure. However, other nanocomposites such as black phosphorus quantum dots/Tubular g-C 3 N 4 (BPQDs/TCN), CdSe-Ag-WO 3 -Ag photocatalyst, and 1D/2D W 18 O 49 /g-C 3 N 4 nanocomposites, also showed a high degradation efficiency for the degradation of organic pollutants up to 96%.

Future Direction of Nanomembrane Adsorption Processes in Wastewater Treatment
In this review, we have discussed the role of the adsorption process in water and wastewater treatment through nanomembranes experimentally and by simulation. Below we will discuss the latest expected developments of the adsorption process in the near future with some recommendations.

•
With the rapid development of simulation software in some important DFT codes such as PBE, B3LYP and PAW, it will be easier to understand the physical and chemical properties of the adsorption process to fill the scientific gaps in realizing the adsorption mechanism, isotherm, kinetics, thermodynamics and other aspects of the adsorption process.

•
Further economic feasibility studies should be conducted on adsorbents including the cost effectiveness of the choices of the materials, which is an important aspect of adsorption investigations. • A huge improvement in the synthesis of nanomaterials using simulation will become possible by linking the density functional theory (DFT) codes using software such as Material Studio and Reactive forefield (ReaxFF) with the molecular dynamic (MD) simulation which will give more realism in acquiring accurate results before starting the experimental work. This step can reduce costs of conducting trials and save time. • Evolution in the ability and durability of nanomembranes in selectivity of undesirable materials by adsorption which increase the adsorption capacity (qe). This is possible by improving the mechanical properties of the nanomembranes by creating special nanocomposites such as graphene/TiO 2 , and graphene/MoS 2 . These two nanocomposites have proven their ability to expel salt and permeate water with high efficiency, so we expect a high adsorption capacity (qe) from them.

•
More comprehensive studies should be conducted on the effect of multi-layer membranes in the adsorption process, which is expected to increase the adsorption capacity (qe) due to the increase in attractions between organic pollutants and membranes. In addition, it is possible to use different layers in the same system which can adsorb different pollutants at the same time. We recommend simulating the system using a molecular dynamic simulation software using two different layers and then testing the possibility of adsorption on different organic pollutants.

•
More studies should be conducted on the possibility of developing more effective forcefields which is highly required in some molecular dynamics simulation software. Creating and developing high effective forcefields will increase the possibility of simulating all kinds of atoms and molecules with high accuracy without errors.

Conclusions
Based on the unique properties of the adsorption technique and its excellent contribution in water and wastewater treatment, it has attracted researchers with great interest in the membrane-based separation field. In this review paper, the preparation of nanomembranes and nanocomposite materials were summarized by experimental and simulation methods. Then we focused on water treatment by membrane technology using the adsorption technique. The main characteristics related to adsorption technology are discussed including the adsorbed/rejected material, rate of rejection/adsorption capacity, and the role of the nanocomposite membrane. In synthesis of nanocomposite materials, we have discussed the latest development techniques such as the hydrothermal method, chemical vapor deposition (CVD), and one-pot synthesis. In this review, the hydrothermal method was the most common method used as it contributed in 56% of the total papers reviewed, while chemical vapor deposition and one-pot synthesis methods contributed 7% and 6%, respectively. This is because of the hydrothermal method's low-cost synthesis, being simple and easy. Graphene oxide (GO) showed promising results in water and wastewater treatment by using the adsorption technique. GO membrane has showed a high adsorption capacity up to 288 mg/g for the removal of As(III), while for the removal of Ni (II), the adsorption capacity equalled 197.8 mg/g. In addition, GO has been used for the removal of 17 βestradiol with high adsorption capacity up to 169.49 mg/g. In addition, GO contributed to the degradation of organic pollutants such as methylene blue (MB) with a degradation efficiency up to 99.3%. In addition to GO, metal oxides and other nanocomposites has also showed a high adsorption capacity for the removal of toxic metals such as lead (Pb 2+ ), barium ions and uranium. Highly porous zeolitic imidazolate frameworks (ZIFs) has the highest adsorption rate for the removal of Uranium of 540.4 mg/g. For the removal of bisphenol A, PyTTA-Dva-COF membrane had the highest adsorption capacity of 285 mg/g. For the degradation of organic pollutants, TiO 2 nanoflowers showed an excellent contribution for the degradation of Bisphenol A (BPA), Diphenyl phenol, P-tert-butyl phenol, and Resorcinol under UV-light up to 95%. Moreover, Cr-TiO 2 degraded 100% of Malachite green dye (MG), and 60% of total organic carbon (TOC). In a comparison with other nanocomposite materials, metal oxides including ZnO@C, Cerium zirconium oxide (CexZryO 2 ), ZnO/Al 2 O 3 , and NiO nanobelt have also showed good results for the degradation of MB, sulphonamides up to 91.33%, TOC, and other organic pollutants. Finally, special attention has been paid to the future direction of nanomembrane adsorption processes in water and wastewater treatment. The need for an economic feasibility study on adsorbents was mainly highlighted with the need to connect DFT with MD simulation for more realism before starting the experimental work.
Author Contributions: Q.I., writing, methodology, reviewing; L.C., supervision, editing, validation; S.G., concept, supervision, reviewing, editing and validation. All authors have read and agreed to the published version of the manuscript.