|M.Sc Student||Nerovny Anastasya|
|Subject||Membranes for Ultra Filtration: Synthesis and Performance|
|Department||Department of Chemistry||Supervisor||Professor Moris Eisen|
|Full Thesis text|
Ultrafiltration is a membrane process whose nature lies between nanofiltration and microfiltration. The pore size of the membranes used range from 0.05µm (on the microfiltration side) to 1 nm (on the nanofiltration side). Ultrafiltration and microfiltration membranes can both be considered as porous membranes where rejection is determined mainly by size and shape of the solutes relative to the pore size in the membrane and where the transport of solvent is directly proportional to the applied pressure. Ultrafiltration membranes are asymmetric membranes with a dense top-layer (small pore size and low surface porosity) and high hydrodynamic resistance. The toplayer thickness in an ultrafiltration membrane is generally less than 1µm.
Asymmetric membrane created by phase inversion method, when a clear polymer solution is precipitated into two phases: a solid, polymer-rich phase that forms the matrix of the membrane and a liquid, polymer poor phase that forms the membrane pores. If the precipitation process is rapid, the pore-forming liquid droplets tend to be small and the membranes formed are asymmetric. If precipitation proceeds slowly, the pore-forming liquid droplets tend to agglomerate while the casting solution is still fluid, so that the final pores are relatively large. These membranes have a more symmetrical structure.
Most of ultrafiltration membranes used commercially these days are prepared from polymeric materials by a phase inversion process from material like polysulfone/poly (ether sulfone)/sulfonated polysulfone; poly(vinylidene fluoride); polyacrylonitrile (and related block-copolymers); cellulose (e.g. cellulose acetate); polyimide/poly(ether imide) etc. In addition to such polymeric materials, inorganic (ceramic) materials have also been used for ultrafiltration membranes, especially alumina and zirconia.
Industrial applications of UF membranes can be found in such fields like the food and dairy industry, pharmaceutical industry, textile industry, chemical industry, metallurgy, paper industry, and leather industry.
Insertion of functional groups, such as hydrophilic/hydrophobic or charged groups can influence rejection and permeability, separation properties, mechanical strength and other physical properties of the membranes. Hydrophilic groups can improve the flux properties because of a lager adsorption of water by these groups (hydrogen bonding with water molecules) and charged functional groups may increase rejection by the Donnan effect. This effect refers to the observation that charged molecules starting on one side of a semipermeable membrane sometimes will not distribute themselves by diffusion on both sides of the membrane. It probably happens because there are other charged substances already present which cannot move through the membrane themselves and which are creating an electric field that influences the movement of the incoming charged molecules.
We have used SEM (Scanning Electron Microscopy) as the main method for examination of the membrane structures. By this method we can also observe the size of the membrane’s pores and their distribution on the membrane surface. In addition, the SEM analysis of the cross-sections allows us to study how deep the holes are inside the membrane.
The synthesis of various polymers is presented in this work, with different functional groups on its backbone that can improve membrane performance. These groups include carboxylic acid groups, chloromethylated and quaternized (trimethylamine) groups.
The prepared membranes have the properties of UF membranes with improved performance, high fluxes and low operating pressures. Also were measured rejection of organic macromolecules, like PEG (polyethylene glycol), and dependence of the rejection on its concentration.