NAMS 2002 Workshop - ICOM 2008

Nanofiltration and Reverse Osmosis II - Imaging and Characterization – 1 –
Keynote
Tuesday July 15, 8:15 AM-9:00 AM, Maui
On the Correlation Between MWCO Values for Nanofiltration Membranes
and Quantitative Porosity Analysis Using Variable Energy Positron Beams.
J. De Baerdemaeker (Speaker), Ghent University, Gent, Leuven, Belgium,
jeremie.debaerdemaeker@ugent.be
K. Boussu, KU Leuven, Leuven, Belgium
B. Van der Bruggen, KU Leuven, Leuven, Belgium
M. Weber, Washington State University, Pullman WA, USA
K. Lynn, Washington State University, Pullman WA, USA
Correlations between the MWCO (Molecular Weight Cut Off) and the pore size in
the skin layer of nanofiltration membranes is still under debate due to the lack of
independent techniques to determine in a quantitative way the porosity of the
skin layer. It might even be stated that progress in nanofiltration (NF) is tempered
by the lack of knowledge of fundamental properties such as porosity.
Using positron and positronium spectroscopy, valuable information can be
gained regarding the true influence of porosity on the transport of molecules
through nanofiltration membranes (NFM). The use of variable energy positron
beams enables depth profiling of the porosity in NFM. By measuring the lifetime
of the positronium in the skin layer of the membrane the size and distribution of
the pores can be determined. This techniques has only very recently been
introduced into the nanofiltration field[1,2,3].
As will be demonstrated complementary information is gained by comparing the
depth profile porosity evolution with high resolution cross section images using a
dualbeam FIB/SEM (Focused Ion Beam - Scanning Electron Microscope).
The results presented within the scope of this research focuse on the correlation
of MWCO values measured for different commercial nanofiltration membranes
with positron results. This comparison not only indicates that the absolute size of
the pores does not seem to be the crucial parameter for the understanding of NF
but presents strong evidence on how the pore distribution determines the
selectivity in these membranes.
This implies that the current models which describe NF should be reexamined.
These findings are crucial for the modeling of NF and might open the path to the
final goal of NF, the production of tailor made NFM. This new study should also
stimulate the membrane community to consider and use positronium