Correlation between membrane surface properties, polymer nature and fouling in skim milk ultrafiltration

Abstract

The fouling of the membranes in skim milk ultrafiltration with the nominal molecular weight cut-off (MWCO) of 100 kDa fabricated from different polymers (polysulfone (PSF), polysulfonamide (PSA), aromatic polyamide (PA), polyacrylonitrile (PAN), cellulose acetate (CA) and regenerated cellulose (RC)) was studied. The membrane structure and physical-chemical properties of the selective layer were analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle (θ, °) measurements, free surface energy measurements and bovine serum albumin (BSA) adsorption. Additionally, the flux of the skim milk at the constant product concentration, protein adsorption, resistance of the gel layer of the membranes were determined. It was found that according to the decrease in water contact angle of the membrane selective layers membranes can be arranged in the series as follows: PSF > PSA > PA > PAN > CA > RC. It was revealed that there was no direct correlation between the membrane hydrophilicity and the protein adsorption. It was noted, that the studied membranes featured significantly different hydraulic resistances of the protein gel-layer, which can be considered as a secondary dynamic membrane. Comparison of the parameters – water contact angle and polar component of the free surface energy of the membrane selective layer, and normalized dipole moment of the membrane polymers - with the adsorption values of the proteins during ultrafiltration proves that the protein adsorption to the membrane surface increases with an increase in hydrophobicity and polarity of the membrane. The high protein adsorption by the moderately hydrophilic PAN membrane is due to the contribution of the high normalized dipole moment of the polymer. In the case of the polar RC-100 membrane, the influence of the membrane polarity was shown to be counter-balanced by its high hydrophilicity. The study highlights the impact of the physical-chemical properties and structure of the membrane on the protein gel-layer and thus their importance in membrane fouling control in dairy applications.