Synthesis, characterization, and application of a novel thin film composite (TFC) forward osmosis (FO) membrane for seawater desalination

With extreme population growth, industry expansion, prolonged droughts, climate change, declining of both quality and quantity of conventional water resources due to overuse, pollution, or salinization, scientists found their selves in front of the huge challenges to develop new emerging technologies that can comply with these needs. Seawater desalination played the crucial role in providing water supply during the last 4 decades. Dominate initially with thermal based technology, sea water desalination is mainly membrane based technology nowadays, thanks to developing unique membrane fabrication techniques and processes with continuously decline and affordable prices. This thesis is a part of the Project entitled fabrication of innovative Forward Osmosis (FO) and low pressure Reverse Osmosis (RO) membranes and module development for Production of drinking water which supported by THE SCIENTIFIC AND TECHNOLOGICAL RESEARCH COUNCIL OF TURKEY (TÜBİTAK). The Project aims to the production of economic, new generation and low-pressure desalination membranes and membrane systems which enable drinking water production from seawater as a novel membrane technology. The thesis scope goes in line with the project goal. It is on one hand targeted the fabrication and characterization of high performance FO membrane that would enjoy high water permeation, low salt passage, reasonable mechanical strength, and low fouling propensity. On the other hand, its encompass the application of the developed FO membrane for some real-life applications, and in particular regard for seawater desalination. This thesis starts with an introduction that included general background, the importance of the thesis, and the thesis purpose, scope, unique aspects, impacts, and organization. Two main topics are highlighted in the literature review, which are forward osmosis process and electrospinning technique. The forward osmosis theory, forward osmosis membrane, membrane characteristic parameters, membrane orientation, forward osmosis applications, and draw solutions are presented in the forward osmosis process section where using electrospun nanofiber membrane in the different applications defined in the electrospinning technique section. Chapter three was a preliminary work on electrospinning based membrane fabrication, characterization and trail to enhance its mechanical stability. In this chapter, polyacrylonitrile (PAN) was utilized to synthesize electrospun nanofiber membranes on the top of a non-woven fabric. To increase the structural integrity and mechanical stability of the as-spun yarns web, 300 nm Polyvinylidene fluoride (PVDF) small particles were incorporated in the electrospun nano fiber web. PVDF solutions with various concentrations were prepared. Nanofiber webs flooded by and then immersed in the solutions to integrate the PVDF in its matrix. Finally, the membrane exposed to heat treatment for a certain period at about one hundred seventy grades centigrade, to fuse the PVDF particles. The fused particles function as a cement material that would join the fiber junctions and result in better membrane mechanical stability. Characterization was done to evaluate the membrane surface morphology, filtration efficiency, and mechanical stability. In particular regards, the impact of PVDF concentrations and submersion time on the membrane mechanical properties and filtration efficiency was evaluated. Results revealed enhancement of the membrane mechanical characteristics in terms of Young's modulus and tensile strength. In the PVDF-treated membranes, Young's modulus and tensile strength enhanced by approximately 6.6 and 20 folds, respectively, in comparison to as-spun membrane. While, the strain ratio reduced by 5.47 folds. Over the tested membranes, a permeability reduction of 12.7 to 31.7% registered PVDF treated membranes. The best membrane improvements can be obtained using 0.01% PVDF solution concentration for one second submersion time. The tensile strength and Young's modulus achieved by it were 126 MPa and 7.08 MPa respectively. Comparing to two different studies used other techniques to improve the membrane stability; this technique is less complex, applicable, and cost-time saving. Work in chapter four targeted fabrication of high performance Forward Osmosis Thin Film Composite (FO-TFC) Nanofibers based substrate membrane for potential application of saline water desalination. TFC polyamide top separation film was formed on the support layer of the by interfacial polymerization reaction between two different monomers (m-Phenylenediamine (MPD) and 1,3,5-trimesoylchloride (TMC)). Sulfonated polysulfone (sPSf) with definite sulfonation level was used to fabricate support layer. Tubular bedless fiber network owning scaffold-like structure with a fiber diameter of 247nm was formed. Polysulfone was sulfonated by heterogenous method using chlorosulfonic acid as a sulfonation agent. Characterization of both substrate and TFC separation layers have been done by means of SEM, water permeation flux, porometry, contact angle, FTIR as well as other tests. While the characterization of TFC separation layer restricted to SEM and FTIR. The characterization illustrates that the sPSf support layer is highly porous with a narrow pore size distribution. FO Performance evaluation of two commercial and newly developed membranes was probed using FO and PRO modes with co-current and counter-current flow scheme. The active layer presents excellent intrinsic properties with A/B of 17.31 and a high salt separation ratio of 99.54%. The newly developed membrane can achieve a high FO and PRO water flux of 65.7 and 313 LMH respectively, using a 1 M NaCl draw solution and DIO water feed solution. The corresponding salt flux is only 2.5 and 5.3 gMH. The reverse flux selectivity represented by the ratio of water flux to reverse salt flux (Jw/ Js) was able to be kept as high as 26.3 and 58.8 L/g for FO and PRO modes. To the best of our knowledge, the performance of the current work developed membrane is superior to all FO membranes previously reported in the literature. Performance evaluation of TFC –FO nanofibers based substrates membrane against synthetic/model and real seawater was investigated in chapter 5 as a real life application. Desalination of seawater performed using 2M NaCl as a draw solution. Performance data showed that the newly fabricated FO-TFC membrane have a water flux of 15.1 and 49.4 in both Co-current FO and Co-current PRO respectively when real seawater used as a feed solution. Two different synthetic/model seawater solution (NaCL and MgSO4), have a salt concentration equal to that of the real seawater sample, were prepared to characterize the performance of the fabricated membrane against them under the same operating conditions. The membrane also achieved a water flux of ̴ 8/54 LMH against 1.1% synthetic/model NaCl and 10.3/45.6 LMH against a synthetic/model 1.1% MgSO4 solution using 2 M NaCl solution as the draw solution under both forward osmosis (FO) and pressure retarded osmosis (PRO) modes, respectively. The structural parameter (S) of the sPSf thin-film-composite membrane estimated to be 125 µm Which consider one of the smallest values ever reported in the literature.