5.1 Nanofiltration membrane characteristics and materials

Nanofiltration membranes are key players in water treatment. They use size exclusion and charge interactions to remove dissolved solutes from liquids. With pore sizes between 0.5 to 2 nanometers, they can filter out smaller particles than other membrane types.

These membranes are made from various materials, with thin-film composites being the most common. Surface properties like hydrophilicity are crucial for performance. Techniques like grafting and coating can improve fouling resistance, making nanofiltration more effective for water purification.

Membrane Characteristics

Nanofiltration Membrane Properties

Top images from around the web for Nanofiltration Membrane Properties

• 

  A durable thin-film nanofibrous composite nanofiltration membrane prepared by interfacial ... View original

  Is this image relevant?

• 

  Performance and properties of coking nanofiltration concentrate treatment and membrane fouling ... View original

  Is this image relevant?

• 

  Molecular engineering of high-performance nanofiltration membranes from intrinsically ... View original

  Is this image relevant?

• 

  A durable thin-film nanofibrous composite nanofiltration membrane prepared by interfacial ... View original

  Is this image relevant?

• 

  Performance and properties of coking nanofiltration concentrate treatment and membrane fouling ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Nanofiltration Membrane Properties

• 

  A durable thin-film nanofibrous composite nanofiltration membrane prepared by interfacial ... View original

  Is this image relevant?

• 

  Performance and properties of coking nanofiltration concentrate treatment and membrane fouling ... View original

  Is this image relevant?

• 

  Molecular engineering of high-performance nanofiltration membranes from intrinsically ... View original

  Is this image relevant?

• 

  A durable thin-film nanofibrous composite nanofiltration membrane prepared by interfacial ... View original

  Is this image relevant?

• 

  Performance and properties of coking nanofiltration concentrate treatment and membrane fouling ... View original

  Is this image relevant?

1 of 3

• Nanofiltration is a pressure-driven membrane separation process that removes dissolved solutes from water and other liquids
• Pore size of nanofiltration membranes typically ranges from 0.5 to 2 nanometers (nm), allowing them to remove smaller particles and molecules compared to microfiltration and ultrafiltration membranes
• Molecular weight cut-off (MWCO) is a measure of the smallest molecular weight solute that a membrane can reject with 90% efficiency, commonly ranging from 200 to 1000 Daltons (Da) for nanofiltration membranes
• Charge selectivity enables nanofiltration membranes to preferentially remove ions based on their charge, with negatively charged membranes rejecting multivalent anions (sulfates, phosphates) and positively charged membranes rejecting multivalent cations (calcium, magnesium)

Rejection Mechanisms

• Nanofiltration membranes combine size exclusion and charge interactions to achieve selective rejection of solutes
• Size exclusion occurs when solutes larger than the membrane pores are physically prevented from passing through, while smaller solutes can permeate the membrane
• Donnan exclusion is a charge-based rejection mechanism where the membrane surface charge repels similarly charged ions, enhancing the rejection of multivalent ions (calcium, magnesium, sulfates)
• Dielectric exclusion is another charge-based mechanism that arises from the interaction between ions and the polarization charges induced at the membrane-solution interface, further contributing to the rejection of charged solutes

Membrane Materials

Thin-Film Composite Membranes

• Thin-film composite (TFC) membranes are the most widely used type of nanofiltration membranes, consisting of a thin selective layer on top of a porous support layer
• The selective layer is typically made of polyamide, formed by interfacial polymerization of two monomers (m-phenylenediamine, trimesoyl chloride) on the surface of the porous support
• The porous support layer provides mechanical stability and is usually made of polysulfone or polyethersulfone
• TFC membranes offer high permeability, selectivity, and stability, making them suitable for various nanofiltration applications (water softening, color removal, micropollutant removal)

Polymeric Materials

• Polymeric materials are the primary choice for nanofiltration membranes due to their versatility, ease of fabrication, and cost-effectiveness
• Polyamide is the most common selective layer material in TFC nanofiltration membranes, offering excellent salt rejection, chemical stability, and fouling resistance
• Cellulose acetate is another polymeric material used in nanofiltration membranes, known for its hydrophilicity and chlorine tolerance
• Other polymeric materials explored for nanofiltration membranes include polyethersulfone, sulfonated polyethersulfone, and polypiperazine amide, aiming to improve specific properties (permeability, selectivity, fouling resistance)

Surface Properties

Surface Modification Techniques

• Surface modification techniques are employed to improve the performance and fouling resistance of nanofiltration membranes
• Grafting involves covalently attaching functional groups or polymer chains onto the membrane surface to alter its properties (hydrophilicity, charge, fouling resistance)
• Coating is another surface modification approach where a thin layer of material (polyvinyl alcohol, polyethylene glycol) is deposited onto the membrane surface to enhance its characteristics
• Plasma treatment uses ionized gas to modify the membrane surface chemistry and morphology, introducing functional groups and increasing surface roughness for improved fouling resistance

Hydrophilicity and Fouling Resistance

• Hydrophilicity refers to the affinity of the membrane surface towards water, with more hydrophilic surfaces exhibiting better wettability and lower fouling propensity
• Fouling resistance is a critical surface property that determines the membrane's ability to resist the accumulation of foulants (organic matter, inorganic scalants, microorganisms) on its surface
• Increasing the surface hydrophilicity through surface modification techniques (grafting, coating, plasma treatment) can significantly enhance the fouling resistance of nanofiltration membranes
• Hydrophilic surfaces form a hydration layer that prevents foulants from directly interacting with the membrane, reducing adsorption and facilitating foulant removal during cleaning processes