Fouling and rejection behavior of ceramic and polymer-modified ceramic membranes for ultrafiltration of oil-in-water emulsions and microemulsions

The effectiveness of poly(vinylpyrrolidone) (PVP)-modification of a zirconia-based ultrafiltration membrane was investigated for the treatment of oil-in-water (o/w) emulsions. Fouling, hydraulic permeability, flux decline, and solute rejection for modified and native membranes were evaluated using a diagnostic o/w emulsion as well as an emulsion prepared using a commercial cutting oil. The native membrane was irreversibly fouled by both the o/w microemulsion and the commercial cutting oil emulsion. In contrast, irreversible fouling was not observed for the PVP-modified membrane. The main fouling agent for the diagnostic o/w microemulsion was identified as the anionic surfactant octanoate, which lead to an irreversible decline of initial membrane hydraulic permeability of up to 20%. Adsorption of surfactant species onto the native membrane surface were attributed to charge screening due to high ionic strength environment and presence of hydroxyl groups on the zirconia surface. Analysis of permeate flux decline and membrane resistance behavior suggests that membrane fouling was mainly due to a solute cake (gel) layer buildup at the membrane surface. However, the possibility of some internal pore plugging, due to deposition of small surfactant molecules, micelles or oil droplets, during the initial few minutes of filtration runs, could not be ruled out. Improved oil rejection (two-fold for the microemulsion and over 20% for the cutting oil emulsion) with the modified membrane compared to the native membrane was attributed to repair (or narrowing) of defects (or ‘pin-holes’) of the native membrane upon polymer grafting.