Thin-layer nanofiltration membranes using engineered biopolymers for seawater desalination pre-treatment processes

摘要

Nowadays water demand already exceeds supply and water scarcity is a global problem. So it isnecessary to develop novel technologies to be able to use poorer quality source waters for drinkingwater production. Once considered as an expensive, ultimate solution for water supply, desalination isbecoming affordable. The two most commonly used seawater desalination methods are Multi-stageFlash Distillation (MSF) and Seawater Reverse Osmosis (SWRO). SWRO is less energy demandingcompared to MSF, which makes it economically attractive. However there is no backpulsing of theexpensive and delicate reverse osmosis (RO) membranes with air or water, so they are susceptible tofouling, causing the loss of their performance. Therefore cleaning the feed water to the highest levelpossible by nanofiltration, before it reaches the RO membranes would highly increase the efficiency ofthe process.Nanofiltration (NF) as a feed pre-treatment step is a pressure driven membrane separation process thattakes place on a selective layer formed by a semipermeable membrane with properties between ROand ultrafiltration. The objective of this project is the developement of highly efficient thin-film composite(TFC) membranes for SWRO pre-treatment processes based on low-fouling cyanobacterial extracellularpolymeric substances (EPS). TFC membranes combine high flux and mechanical strenght, and they areexpected to be the key components of any water purification technology in the future.Cyanobacterial EPS are complex heteropolysaccharides with putative antimicrobial and antiviralproperties and a particular affinity to bind metal ions [1,2].Within this work, the unicellular N2-fixingmarine cyanobacterium Cyanothece sp. CCY 0110 was chosen for RPS production, since it is amongthe most efficient released polysaccharide (RPS) producers and the polymer has been previouslyextensively characterised [3]. RPS was produced by growing Cyanothece CCY 0110 in 10L bioreactors,in conditions previously defined and the polymer was isolated following the standard methodology [3].A polyvinyl alcohol (PVA) / cyanobacterial EPS blend nanofibrous membranes were fabricated byelectrospinning using polyvinylidene fluoride (PVDF) as a basal membrane, in order to obtain thin-layercomposite nanofiltration membranes. The production of the nanofibers using EPS and PVA asplasticizer in different ratios was produced in a NF-103 MECC Nanon electrospinning equipment with anapplied electric field between 15 and 25 kV and a flow of 0,2 mL/h.Morphological, mechanical, chemical and thermal characterization of the electrospun fibers depositedon the basal membranes, were evaluated by atomic force microscopy (AFM), scanning electronmicroscopy (SEM) and energy dispersive spectroscopy (EDS), dynamical and mechanical analysis(DMA), thermogravimetry (TGA) and differential scanning calorimetry (DSC).The AFM and SEM results show the presence of fibers with dimensions between 54 and 121 nm withlow bead formation. In the EDS analysis presence of sulfur elements was observed confirming theinclusion of EPS in the nanofibers. The morphology and diameter of the nanofibers were mainly affectedby the concentration of the blend solution and the weight ratio of the blend, respectively. The bestPVA/EPS nanofibers were achieved in a ratio of 12 % PVA and 0.4 % EPS. The solution conductivitywas ranging 1500 to 3500 μS/cm with a viscosity of about 100 to 500 cP. The DMA results confirmedthe miscibility of PVA/EPS blends. The elastic modulus of the nanocomposite mats increasedsignificantly as a consequence of the reinforcing effect of EPS. Thermal and mechanical analysisdemonstrated that there were strong intermolecular hydrogen bonds between the molecules EPS-PVAin the blends. The heat-treated electrospun blended membranes showed better tensile mechanicalproperties when compared with PVA alone, and resisted more against disintegration. A lab-scalenanofiltration was performed in a bench stainless steel Sterlitech tangential flow stirred cell (200 mL)connected to an air pressure system that allow pressure driven filtration up to 10 BAR.Bactericidal activity and biofilm formation were tested using Escherichia coli and Sthaphylococcusaureus as pathogenic microorganisms.