Today, water is a vital natural resource for the progress of human societies in economic, agricultural, social, health, educational, and even global peace and stability, which guarantees the survival of the ecosystem and has become the key to the sustainable development of regions. The double pressure on existing water equipment in all countries will increase unprecedentedly and the water issue will become an environmental challenge. For this reason, managing water sources by filtration and recycling is a potential solution to compensate for water shortage. Many different techniques, including coagulation process, chemical oxidation, ion exchange, and membrane separation technologies, are used to purify water [1, 2].
Among the conventional methods of water purification, membrane technology is of interest due to its high reliability in removing pollutants. The advancement also has other features such as high-quality water production, compact modular structure, less use of chemicals, and easier operation. Today, membrane separation processes are widely advancing [3, 4].
Nanofiltration(NF) membranes with properties between ultrafiltration and reverse osmosis are more distinct than other separation methods. NF membranes have a relatively higher flux due to their porous and thin structure with small pore size, and high permeability. Thin film composite (TFC) membranes, a unique class of nanocomposite membranes, have been created and are presently being used in NF processes. They are generally utilized as a crucial step in the purification of water[5, 6].
TFC membranes are composed of a microporous polymer structure as a support layer and a very thin selective layer that is superficially placed on the underlying layer. The main advantage of these membranes compared to conventional membranes is that each of the layers can be individually optimized to improve selectivity and permeability. High pollutant rejection is increasingly used in various separation processes[7, 8].
PVA can be used as a selective layer in TFC due to its properties such as being biodegradable and having a high ability to form a film. This is because of its inherent hydrophilic property that causes greater water solubility and penetration. Chemical, suitable mechanical properties, and thermal stability are appropriate options for the selected layer on the support membrane surface [9–13]. In the studies of Chien Ng et al [14] ., the effect of adding polyvinyl alcohol/graphene oxide (PVA/GO) on the polyamide active layer formed on the polysulfone (PSF) support substrate was investigated. By adding PVA/GO to PA, the pure water flux (PWF) increased from 50.94–91.93%, also the NaCl salt rejection rate increased from 96–98%. Huang et al[15]. prepared a TFC and Thin film nanocomposite (TFN) membrane consisting of a hydrophilic thin layer of the metal-organic framework of aluminum fumarate (AlFu MOF) doped in a dense layer of PVA on a microporous polytetrafluoroethylene( PTFE) substrate. The hydrophilic thin film based on nanoporous AlFu MOF doped on PVA increased the water vapor flux to 45.1 kg/m2, while the rejection of NaCl salt reached almost 99.9%. In the research of Nathaniel B. et al [9]., TFN membranes for nanofiltration were prepared by incorporating single-walled aluminosilicate nanotubes (SWNT) in PVA matrix on a PSF support. By adding PVA-SWNT to the active layer of the PSF/PVA-SWNT membrane, by reducing the membrane's roughness and decreasing the interaction angles from 64.2 to 50.5, the membrane's hydrophilicity increased. The rejection of monovalent and divalent ions increased to 59% and 97%, respectively. TFC membranes have disadvantages such as poor chlorine resistance, low chemical stability, and clogging tendency, so the structure of TFN membranes is a solution to these disadvantages [16–19].
TFN membranes are created by combining different amounts of nanoparticles in the upper active layer of PVA. These membranes have excellent anti-fouling properties, higher permeability than TFC membranes, and more rejection rate than T FC membranes. TFN membranes consist of three layers, including a polymer support layer with a microporous structure and a dense selective polymer layer[20–23].
Among various polymer structures and human-made macromolecules, dendrimers are very popular as three-dimensional nanostructures[24, 25]. Dendrimers have many branches with completely regular structures, multifaceted, with high density and end groups. These macromolecules are radially symmetric molecules with a well-defined structure, uniform and integrated into tree-like arms or branches, which are prepared by repetitive reactions. Polyaminoamide (PAMAM) dendrimers are synthesized through a divergent method with a repetitive sequence of Michael addition reaction or alkylation with methyl acrylate and amidation reactions with ethylenediamine [26–28]. In water treatment, nanoparticles that are used as adsorbents in wastewater and effluent treatment have limitations such as low absorption capacity, PAMAM dendrimers have a lot of internal cavities due to having extensive branching in their branches, and also solubility in water that causes them to have a fast absorption rate and a high absorption property for water purification, and in addition, due to the presence of amine end groups, they are the best place for the accumulation of metal ions[29, 30].
There are almost no reports on the preparation of TFN membranes using PAMAM dendrimer nanostructures in the selective active layer by dip-coating method. Therefore, in our recent research, in the first step, polyaminoamide Generation 2 dendrimer nanostructure (PAMAM-G2) multifaceted was synthesized, initially, The primary ingredient in the synthesis of the dendritic structure is silica nanoparticles formed with 3-aminopropyltriethoxysilane (APTES). PAMAM-G2 was used through sequential addition of Michael's addition reaction with MA and amidation reaction with EDA. Using Fourier-transform infrared spectroscopic analysis(FT-IR) and thermal gravimetric analysis(TGA), the accuracy of nanoparticle synthesis was checked. Next, TFC and TFN membranes were prepared by dip-coating method. At first, Polycarbonate(PC) support membranes were prepared through phase inversion[31]. The influence of different PAMAM-G2 nanostructure addition amounts on the structure and function properties of PC/PVA TFC membranes was then investigated. Therefore, Attenuated total reflection spectroscopic analysis(ATR), contact angle measurement analysis, field emission scanning electron microscope(FE-SEM), and atomic force microscope(AFM) were used for structural evaluation. Next, the rejection rates of all TFC and TFN membranes were evaluated using ultraviolet-visible (UV-Vis) spectrophotometers in an immersed membrane system for the removal of HA under various transmembrane pressure (TMP) conditions.