Thin film composite (TFC) nanofiltration membranes for water softening

An increasing scarcity in fresh water sources has driven the search for different alternative resources such as ocean water. 97 % of the total water sources on earth is seawater while only 0.5 % is potable, fresh water. Accordingly, desalination of seawater and saline aquifers has become the most promising and significant method for water purification. Membranes came out as an applicable water and wastewater treatment method in the 1960s with the development of high performance synthetic membranes. Since then, more advanced membranes have been produced from new materials and employed in various configurations. In the 1970s, membranes are started to be used for water desalination. After successful production of purified water from salt water, membranes become an applicable alternative to evaporation based technologies. Purified water standards have become stiffer over time and encouraged the use of membrane technology. Membrane known as a selective barrier between two phases should be always associated with its application. Membrane processes are recognized as a great option for water and wastewater treatment to remove solids, viruses, bacteria and other unwanted molecules effectively. Besides water and wastewater treatment, membrane processes are largely used now in the textile, pulp and paper, pharmaceutical, semi-conductor, mining, electroplating, dairy, food and beverage processing, and biotechnology industries. Polymeric membranes are the most widely used membrane type for water treatment due to its evidential pore forming mechanism, higher flexibility, smaller required area for installation, and lower cost contrary to inorganic membranes. However, there is an inherent trade-off between permeability and selectivity, and fouling is a major problem despite its widespread use. Therefore, efforts to produce membranes with high permeability and rejection and high antifouling property continues so that membranes are applicable in terms of energy efficiency and cost effectiveness. The polymeric membranes are classified as isotropic and anisotropic in accordance with membrane morphologies. Isotropic membranes having a uniform composition structure throughout the membrane cross section, which can be nonporous dense, microporous or electrically charged, while anisotropic membranes consist of asymmetric membranes, composite membranes and supported liquid membranes. Separation mechanism in porous membranes is based on sieving, straining, or size exclusion. On the other hand, nonporous (dense) membranes separate molecules depending on the differences in solubility or diffusivity between the solvent and the solute in the membranes. Nitto Denko Corp., Koch, Toray Industries, GE Osmonics, Dow/FILMTEC are the best-known membrane manufacturers today. In water and wastewater treatment, pressure driven membrane processes which are classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are employed. Microfiltration membranes have the largest pore size and typically reject large particles and various microorganisms like colloids, suspensions, microorganisms and emulsions. Ultrafiltration membranes are commonly utilized to eliminate emulsified oils, emulsions, dispersed material, suspended solids, and other large molecular weight materials in the particle size range of about 0.01-0.1 µm, and NF membranes are primarily used for hardness removal, pesticide elimination and color reduction in water treatment while seawater and brackish water desalination is the common application of RO. The required applied pressure increases from microfiltration to reverse osmosis as the pore sizes of the membrane gets smaller due to the increase of the mass transfer resistance of the membrane. Cellulose acetate and polyamide derivatives are most widely used materials in RO and NF membranes. The existence of calcium (Ca) and/or magnesium (Mg) and some other metal cations in water causes water hardness and it leads to several problems in household or industrial water supply systems such as formation of lime-scale, fouling of plumbing and stimulation of galvanic corrosion. Also, it causes membrane scaling which is one of the significant problems in seawater desalination. Hard water reduces the effectiveness of almost all cleaning tasks because of the reaction of calcium and magnesium ions in water with heat, metallic compounds of plumbing and chemical agents like detergents. Water softening is a process to remove water hardness from raw water stream, crucial for domestic and industrial water supply and brackish water and seawater desalination systems. Lime-soda ash treatment and use of ion exchange resins are conventional water softening methods that are still in use. However, membrane softening with nanofiltration technology become a promising alternative to conventional methods in terms of effectiveness and operating costs, primarily for the effective removal of hardness from raw water. NF exhibits performance between RO and UF membranes and it aims to remove divalent and multivalent ions. They are unable to completely separate monovalent ions such as sodium and chloride. Most commercial NF membranes are thin film composite (TFC) flat sheet membranes. Thin film composite NF membranes consist of crosslinked polyamide layer generally with negatively charged chemical groups formed on a substrate. Polysulfone (PSf), polyethersulfone (PES), polyvinyledene fluoride (PVDF), polyacrylonitrile (PAN), and polyether ether ketone (PEEK) are commonly used substrate materials for thin film composite membranes. Thin film composite membranes are fabricated via plasma initiated polymerization, photo-initiated polymerization, photo-grafting, electron beam irradiations, dip-coating, and interfacial polymerization (IP). Interfacial polymerization is the mostly used method for preparation of composite polyamide membranes. In this work, thin film composite nanofiltration membranes were successfully prepared via interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on polysulfone support membranes. Both the polyamide layer and underlying substrate were characterized with respect to chemical and morphological structures. The chemical structure of membrane surface was studied by attenuated total reflectance infrared spectroscopy (ATR-IR) and surface and cross-section morphologies were observed using scanning electron microscopy equipped with field emission gun (FESEM). Surface hydrophilicity of the membranes was characterized by contact angle goniometer. Furthermore, the water flux and salt (MgSO4) rejection rate of the prepared NF composite membranes were measured on a cross-flow filtration apparatus. Parametric studies were conducted by varying interfacial polymerization parameters such as reaction time and monomer concentration in organic phase and effect of acid acceptor addition in PIP solution was investigated in order to optimize the polymerization conditions. It was shown that addition of acid acceptor to aqueous PIP solution was necessary for the successful formation of TFC membrane. Decreasing the TMC concentration from 0.15 % to 0.1 % and decreasing reaction time from 60 seconds to 20 seconds resulted in less cross-linked PA layer on PSf substrate. In addition, increasing reaction time from 60 seconds to 120 seconds lead to increased cross-linking density of the PA layer resulting in improved separation performance. However, the best MgSO4 rejection rate and water flux obtained were only 67 % and 17 LMH, respectively. More work is needed to improve these values