NAMS 2002 Workshop - ICOM 2008

Membrane Fouling III - RO & Biofouling – 5
Thursday July 17, 4:30 PM-5:00 PM, Maui
Effect of surface charge and pH on fouling and critical flux of MF
membranes during protein filtration
Jochen Meier-Haack (Speaker), Leibniz Institute of Polymer Research Dresden, Dresden,
Germany - mhaack@ipfdd.de
Although subject to research for decades, fouling is still one of the major limiting
factors in membrane applications. Numerous methods have been suggested to
overcome this drawback, like crossflow filtration, backflushing, air sparging, all of
them in combination with chemical cleaning. However many of these techniques
imply off-production cycles, resulting in lower yield, shorter life-time of
membranes and therefore higher costs. In the mid-nineties Field et al. introduced
the concept of the -critical flux [1, 2] and which has been subject of a review
recently [3]. It defines a permeate flux below a critical value - the critical flux -
where no irreversible fouling occurs. The critical flux is determined by several
factors including hydrodynamic forces introduced by the crossflow velocity and
the transmembrane pressure, electrostatic interaction between feed components
and the membrane surface and others [3]. Although mainly important for large
molecules, we have focused our work on the effect of surface charges on fouling
and critical flux.
In static (non-filtration) adsorption experiments we observed a strongly reduced
protein adsorption at the surface in the case of repulsive electrostatic forces
between the membrane surface and feed components and vice-versa [4]. The
same effect was reported in dead-filtration using surface modified MFmembranes
[5]. We now extended our investigations on the effect of surface
modification on the critical flux. Surface modified microporous PP membranes
were obtained by grafting polyacrylic acid onto the surface using a so-called
macroinitiator [6, 7]. These modified membranes showed a strong influence of
pH on the filtration properties (stimuli-response membranes). The surface charge
was reversed by adsorption of a polycation (PDADMAC) onto the graft-layer.
Although the total amount of modificator on the membrane surface was
increased, a slight flux enhancement (at constant pressure) was observed
compared to the "one-layer" membrane. Simultaneously the response on pH
change was reduced dramatically, but still observable. Upon the adsorption of a
third polyelectrolyte layer (PAAc) the response to pH change was recovered to a
small extent while the permeate flux at constant pressure was unchanged (20
l/m 2 h at 1 bar) compared to the two-layer membrane. The surface modification
along with the introduction of surface charges has also a strong effect on the
critical flux. While for the unmodified membrane a critical flux of 20 l/m 2 h was