Modification of commercial ultrafiltration membranes for improved performance

Abstract

Ultrafiltration (UF) is widely used in food, medicine, water treatment, bio-separation and other fields due to its advantages of low energy consumption, simple operation, mild conditions and environmental friendliness, etc. However, UF also faces huge challenges, such as low permeability, weak selectivity, and severe membrane fouling. These shortcomings are mainly related to the chemical properties of the membrane material and membrane structure. In this thesis, a polysulfone (PSf) membrane substrate with excellent mechanical and thermal properties was modified using three methods: pre-treating the membrane with ethanol or NaOH solution, depositing polyelectrolytes on the membrane surface, and grafting of charged small molecules on the membrane surface. These modifications effectively improved the chemical properties and structural morphology of the UF membrane and brought about a substantial enhancement in membrane permeability, selectivity and fouling resistance. First, we studied the effect of ethanol or NaOH pre-treatment on membrane structure and performance. Using analysis of FTIR spectra, we found that after pre-treatment with hot water, ethanol or NaOH, the preservatives in membrane were completely removed and the flux of membrane was improved. In particular, water flux of membrane treated with absolute ethanol increased by nearly 5-fold. Polymeric materials swell in organic solvents, resulting in large pore size and porosity, which was confirmed by the increase in membrane thickness and the decrease in dextran rejection. However, ethanol treatment causes the hydrophilic additives (PVP) in the membrane to be leached out, resulting in a decrease in membrane hydrophilicity and hence more membrane fouling, especially for protein solutions. For membrane pre-treated with NaOH, membrane permeability was also improved. The PVP additive in the membrane had undergone a hydrolytic reaction to generate more hydrophilic groups, which was verified by the decreased water contact angle and the increase in negative charge. The NaOH treated membrane had a high flux, while retaining a similar fouling resistance to the original membrane. Although pre-treatment enhanced the permeability of membrane, it had no significant effect on the retention of the membrane. In order to further improve the selectivity of UF membranes, we coated different polyelectrolyte layers on the membrane surface and used lysozyme in ultrafiltration experiments to study membrane performance. We found that positively charged lysozyme was adsorbed on membrane surface and pores when the surface was coated with negatively charged polyanions PDA or PAA, which led to sharp flux decline and membrane fouling. In contrast, when the membrane surface was coated with a positively charged polycation PDADMAC, positively charged surface/pores prevented contact with the same charged lysozyme. The modified membrane exhibited a higher flux to protein solution and showed better fouling resistance. As positive charge on the surface of the membrane increased, membrane fouling resistance was also improved. Furthermore, the rejection of lysozyme could be regulated by adjusting surface charge of the membrane. Although polyelectrolyte coating improved the selectivity and fouling resistance of membrane, it severely damaged membrane permeability. In order to retain high permeability, we grafted the small molecules taurine, cysteine and PEI on PSf membrane surfaces. Binary solutions of BSA and bovine hemoglobin (Hb), which have similar molecular weights but different isoelectric points, were used for ultrafiltration experiments. We found that the permeability of the membrane was significantly enhanced after taurine or cysteine grafting. The PEI grafted membrane suffered severe fouling during the filtration process and exhibited undesired transmission rate and selectivity. However, there was pH-related protein transmission and selectivity for taurine and cysteine grafted membranes. Under acidic conditions, a lot of proteins were adsorbed on the membrane surface/pores due to charge attraction, and severe membrane fouling occurred that resulted in low transmission of protein. When pH was higher than the isoelectric point of Hb, membranes grafted with taurine and cysteine exhibited excellent fouling resistance and allowed more protein to pass through. Strong charge repulsion facilitates protein permeation through negatively charged PSf membranes. However, the highest selectivity was obtained at a pH slightly higher than the isoelectric point of Hb for cysteine grafted membrane. Furthermore, under low transmembrane pressure and suitable stirring rate, cysteine modified PSf membrane with a TA coating time of 6 h achieved better selectivity while maintain good permeability and fouling resistance.