💧Membrane Technology for Water Treatment Unit 7 – Membrane Fabrication: Techniques & Materials

Membrane Basics & Types

• Membranes act as selective barriers allowing certain substances to pass through while blocking others based on size, charge, or other properties
• Can be classified by their structure as symmetric (uniform throughout) or asymmetric (non-uniform with a thin selective layer supported by a porous substructure)
• Grouped by their separation mechanism into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes
  • Microfiltration membranes have pore sizes ranging from 0.1 to 10 μm and are used for removing suspended solids, bacteria, and protozoa (Cryptosporidium)
  • Ultrafiltration membranes have pore sizes between 0.01 and 0.1 μm and can remove viruses, colloids, and large molecular weight substances (proteins)
• Classified by their material composition as organic (polymeric) or inorganic (ceramic, metallic)
  • Polymeric membranes are commonly made from materials like cellulose acetate, polyamide, and polysulfone due to their flexibility and low cost
  • Inorganic membranes offer high chemical, thermal, and mechanical stability but are more expensive and brittle compared to polymeric membranes
• Can be further categorized by their geometry as flat sheet, hollow fiber, or tubular configurations depending on the application requirements

Key Materials in Membrane Fabrication

• Polymers are the most widely used materials in membrane fabrication due to their versatility, processability, and cost-effectiveness
  • Common polymers include cellulose acetate, polyamide, polysulfone, polyethersulfone, and polyvinylidene fluoride (PVDF)
  • Each polymer has unique properties such as hydrophilicity, chemical resistance, and mechanical strength that influence membrane performance
• Additives and fillers are incorporated into the polymer matrix to enhance specific membrane properties
  • Pore-forming agents (polyvinylpyrrolidone) create a more porous structure and improve permeability
  • Hydrophilic additives (polyethylene glycol) increase surface hydrophilicity and reduce fouling propensity
• Solvents play a crucial role in dissolving the polymer and controlling the phase inversion process during fabrication
  • Common solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)
  • Solvent properties like volatility and miscibility with non-solvents affect the membrane morphology and performance
• Inorganic materials such as ceramics (alumina, zirconia) and metals (stainless steel, titanium) are used for specialized applications requiring high durability and resistance to harsh environments
• Composite materials combine the advantages of different materials to create membranes with enhanced properties
  • Examples include polymer-ceramic composites for improved mechanical strength and thermal stability

Fabrication Techniques Overview

• Phase inversion is the most common technique for fabricating polymeric membranes
  • Involves the controlled transformation of a polymer solution from a liquid to a solid state through solvent-non-solvent exchange
  • Can be achieved through immersion precipitation, vapor-induced phase separation, or thermally-induced phase separation
• Interfacial polymerization is used to create thin-film composite membranes with a selective layer on a porous support
  • Monomers from two immiscible phases react at the interface to form a highly crosslinked and dense selective layer
• Stretching is a technique used to create microporous membranes with high porosity and narrow pore size distribution
  • Involves extruding a polymer film and stretching it under controlled conditions to create a porous structure
• Track-etching utilizes high-energy particle radiation to create well-defined cylindrical pores in a polymer film
  • Provides precise control over pore size and distribution but is limited to small-scale production
• Sintering is a technique for fabricating inorganic membranes by compacting and heating powder particles until they fuse together
  • Commonly used for ceramic membranes and allows for control over pore size and distribution
• Electrospinning produces nanofiber membranes with high surface area and porosity
  • Applies a high voltage to a polymer solution to create a jet that solidifies into nanofibers on a collector

Phase Inversion: The Go-To Method

• Phase inversion is the most versatile and widely used technique for fabricating polymeric membranes
• Involves the controlled transformation of a polymer solution from a liquid to a solid state through the exchange of solvent and non-solvent
• The polymer solution is cast on a support and immersed in a non-solvent bath (immersion precipitation) or exposed to a non-solvent vapor (vapor-induced phase separation)
  • Immersion precipitation is the most common method and involves immersing the cast film in a non-solvent bath (water) to induce phase separation
  • Vapor-induced phase separation exposes the cast film to a non-solvent vapor, allowing for slower and more controlled phase separation
• As the solvent exchanges with the non-solvent, the polymer precipitates and forms a porous structure
  • The rate of solvent-non-solvent exchange determines the membrane morphology and pore size distribution
  • Faster exchange rates result in finger-like macrovoids, while slower rates produce sponge-like structures
• Thermally-induced phase separation (TIPS) uses temperature changes to induce phase separation in a polymer-solvent system
  • The polymer solution is cooled to induce phase separation, and the solvent is removed to create a porous structure
• Advantages of phase inversion include its simplicity, scalability, and ability to control membrane morphology by adjusting fabrication parameters (polymer concentration, solvent-non-solvent system, and coagulation bath composition)
• Limitations include the use of large amounts of solvents and the need for precise control over the fabrication environment to ensure reproducibility

Other Popular Fabrication Methods

• Interfacial polymerization creates thin-film composite membranes with a selective layer on a porous support
  • Involves the reaction of two monomers from immiscible phases at their interface to form a highly crosslinked and dense selective layer
  • Commonly used for reverse osmosis and nanofiltration membranes due to the high selectivity and permeability of the thin selective layer
• Stretching is used to create microporous membranes with high porosity and narrow pore size distribution
  • Involves extruding a polymer film and stretching it under controlled temperature and stress conditions
  • The stretching process creates a porous structure by inducing microvoid formation and orientation of the polymer chains
• Track-etching creates membranes with well-defined cylindrical pores using high-energy particle radiation
  • A polymer film is exposed to high-energy particles (heavy ions) that create damage tracks through the film
  • The tracks are then chemically etched to form cylindrical pores with precise size and distribution
• Sintering is a technique for fabricating inorganic membranes, particularly ceramic membranes
  • Involves compacting and heating powder particles (alumina, zirconia) until they fuse together to form a porous structure
  • Sintering temperature and time can be adjusted to control the pore size and distribution of the resulting membrane
• Electrospinning produces nanofiber membranes with high surface area and porosity
  • A high voltage is applied to a polymer solution, creating a charged jet that stretches and solidifies into nanofibers on a collector
  • Electrospun membranes have applications in water treatment, air filtration, and tissue engineering due to their unique properties

Surface Modification Strategies

• Surface modification alters the membrane surface properties without affecting the bulk structure to improve performance, selectivity, and fouling resistance
• Grafting involves covalently attaching functional groups or polymer chains to the membrane surface
  • Grafting can be initiated by chemical, radiation, or plasma treatments to create active sites on the surface
  • Common grafting monomers include acrylic acid, methacrylic acid, and polyethylene glycol to increase hydrophilicity and fouling resistance
• Coating applies a thin layer of material on the membrane surface to modify its properties
  • Can be done by dip-coating, spin-coating, or spray-coating techniques
  • Coating materials include hydrophilic polymers (polyvinyl alcohol), nanoparticles (silver, titanium dioxide), and biocidal agents (quaternary ammonium compounds) to improve antifouling and antimicrobial properties
• Blending incorporates additives or modifiers into the polymer solution before membrane fabrication
  • Hydrophilic additives (polyvinylpyrrolidone, polyethylene glycol) can increase surface hydrophilicity and reduce fouling propensity
  • Nanoparticles (zeolites, carbon nanotubes) can enhance mechanical strength, thermal stability, and adsorption capacity
• Plasma treatment modifies the membrane surface using ionized gas (oxygen, nitrogen, argon) under vacuum
  • Plasma can induce surface etching, crosslinking, or activation of functional groups depending on the gas composition and treatment conditions
  • Improves surface hydrophilicity, wettability, and adhesion properties for better performance and fouling resistance
• Surface modification techniques can be combined to achieve synergistic effects and optimize membrane performance for specific applications

Quality Control & Characterization

• Quality control ensures that fabricated membranes meet the desired specifications and performance standards
• Pore size and distribution are critical parameters that determine the membrane's separation properties
  • Mercury intrusion porosimetry measures pore size distribution by applying pressure to force mercury into the pores
  • Gas permeation tests (nitrogen, carbon dioxide) can also provide information on pore size and distribution
• Permeability and selectivity are key performance indicators for membranes
  • Pure water permeability is measured by applying pressure and monitoring the water flux through the membrane
  • Solute rejection tests evaluate the membrane's ability to remove specific contaminants (salts, organic compounds) from the feed solution
• Mechanical properties such as tensile strength, elongation at break, and Young's modulus are important for membrane durability and stability
  • Tensile testing applies a uniaxial load to the membrane sample and measures the stress-strain behavior
  • Dynamic mechanical analysis (DMA) can provide information on the membrane's viscoelastic properties and glass transition temperature
• Surface characterization techniques provide insights into the membrane's surface chemistry, morphology, and roughness
  • Scanning electron microscopy (SEM) visualizes the membrane surface and cross-section to assess pore structure and defects
  • Atomic force microscopy (AFM) measures surface roughness and topography at the nanoscale
  • X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analyze the surface chemical composition and functional groups
• Fouling and cleaning studies evaluate the membrane's resistance to fouling and the effectiveness of cleaning methods
  • Fouling tests monitor the decline in permeate flux over time when the membrane is exposed to a fouling solution (proteins, humic acids)
  • Cleaning tests assess the recovery of permeate flux after applying chemical (acids, bases, enzymes) or physical (backwashing, sonication) cleaning methods

Real-World Applications & Case Studies

• Desalination is a major application of membrane technology for producing potable water from seawater or brackish water
  • Reverse osmosis (RO) membranes are widely used in desalination plants worldwide, with examples like the Sorek desalination plant in Israel, which has a capacity of 624,000 m³/day
  • Nanofiltration (NF) membranes can be used for selective removal of divalent ions and organic matter in brackish water desalination
• Wastewater treatment and reuse are increasingly important applications of membrane technology
  • Membrane bioreactors (MBRs) combine biological treatment with ultrafiltration or microfiltration membranes for enhanced wastewater treatment and effluent quality
  • The Changi Water Reclamation Plant in Singapore uses MBRs and RO membranes to produce high-quality reclaimed water (NEWater) for industrial and indirect potable use
• Food and beverage processing relies on membrane technology for various separation and concentration processes
  • Ultrafiltration is used for clarification of fruit juices, wine, and beer, as well as for protein concentration in dairy products (whey protein concentrate)
  • Nanofiltration is applied for sugar concentration in the sweetener industry and for the removal of alcohol from wine and beer
• Pharmaceutical and biomedical industries use membranes for purification, sterilization, and controlled drug delivery
  • Ultrafiltration membranes are used for virus removal and protein purification in the production of biopharmaceuticals (monoclonal antibodies)
  • Dialysis membranes are essential for the treatment of kidney failure, with examples like the Fresenius Medical Care FX-class dialyzers
• Gas separation membranes are used for various industrial applications, such as carbon dioxide capture, hydrogen purification, and nitrogen enrichment
  • Polymeric membranes (cellulose acetate, polyimide) are commonly used for gas separation due to their high permeability and selectivity
  • The Air Products and Chemicals hydrogen production facility in Port Arthur, Texas, uses membrane technology for hydrogen purification and carbon dioxide capture
• These real-world applications demonstrate the versatility and importance of membrane technology in addressing global challenges related to water scarcity, environmental protection, and sustainable industrial processes