NAMS 2002 Workshop - ICOM 2008

macromolecules adsorbed on neighboring silica colloid surface was
hypothesized to be the cause of the greatly enhanced deposition of silica colloids
in the presence of Ca 2+ . Monte-Carlo simulations of the BSA silica system are
currently being performed to better understand how BSA or other
macromolecules affect silica aggregation and deposition behaviors.
Results from these molecular-level characterizations were combined with those
from a series of cross-flow filtration experiments to reveal the mechanisms
involved in the combined effect of model organic and colloidal foulants on NF flux
decline. The enhanced deposition of silica colloids observed in the QCM-D
experiments agreed well with the increased initial flux decline rate during crossflow
filtration of the colloid-organic mixture compared to the additive sum of the
effects of the two individual foulants.
In addition to changing the deposition rate of silica colloids and hence increasing
the initial fouling rate, adsorption of organic macromolecules also alters the
structure of the colloidal cake layer. Transmission electron microscopy (TEM)
imaging of fouled membranes was employed to visualize the structure of the
fouling layer formed with and without the model organic macromolecules. This
effect was manifested at a later stage of the filtration process. Although BSA was
found to significantly increase initial membrane fouling rate, higher quasi-steady
state flux was observed in the presence of BSA due to a more porous fouling
layer.
Results from this study clearly demonstrate that different organic
macromolecules affect colloidal fouling differently. The overall fouling potential of
a complex suspension may not be predicted based on the fouling potential of
each individual foulant.