Comparing humic acid and protein fouling on polysulfone ultrafiltration membranes: Adsorption and reversibility

Antifouling membranes for water treatment are increasingly described in the literature, often as nanocomposites. Design of such hybrid membranes requires an intrinsic understanding of adsorption behavior coupled with transport-based performance. In the present work, fouling of a polysulfone (PSF) membrane was examined, using adsorption isotherms, cross-flow flux declines and flux recovery ratios (FRRs). Fractional adsorption was characterized using fluorescence excitation emission matrix (EEM) spectroscopy. The foulants used were humic acids (HA) (Aldrich and Suwannee River) and bovine serum albumin (BSA). Adsorption of the HA was insignificant on colloidal PSF, as determined by isotherm data (regardless of addition of Ca2+, Na1+, 7 days of agitation or high adsorbent concentration). EEM spectra showed no evidence of fractional adsorption of Aldrich HA components. Pore blocking and low FRRs occurred on PSF membranes at 2 ppm concentration despite lack of adsorption. At 700 ppm HA, the flux decline and FRRs were high, consistent with the formation of HA micelles and subsequent cake formation. BSA exhibited fluxes and FRRs consistent with pore blocking, which are connected with adsorption in the isotherm studies. A new kinetic trapping mechanism is proposed to explain these observations. Here primary particles of HA diffuse through the membrane, re-aggregate, but then become trapped inside the pores where concentration is high and aggregation is preferred. Another model discussed as well is the classic filter-aid situation.