NAMS 2002 Workshop - ICOM 2008

Nanostructured Membranes II – 1 – Keynote
Tuesday July 15, 8:15 AM-9:00 AM, Moloka’i
Nanofiltration of Electrolyte Solutions by Sub-2nm Carbon Nanotube
Membranes
F. Fornasiero (Speaker), Lawrence Livermore National Laboratory, Livermore CA, USA,
fornasiero1@llnl.gov
H. Park, Lawrence Livermore National Laboratory, Livermore CA, USA
J. Holt, Lawrence Livermore National Laboratory, Livermore CA, USA
M. Stadermann, Lawrence Livermore National Laboratory, Livermore CA, USA
S. Kim, University of California at Davis, Davis, CA, USA
J. In, University of California at Berkeley, Berkeley, CA, USA
C. Grigoropoulos, University of California at Berkeley, Berkeley, CA, USA
A. Noy, Lawrence Livermore National Laboratory, Livermore CA, USA
O. Bakajin, Lawrence Livermore National Laboratory, Livermore CA, USA
MD simulations have shown that liquid and gas flow through carbon nanotubes
with nanometer size diameter is exceptionally fast compared to the predictions of
continuum hydrodynamic theories and, also, compared to conventional
membranes with pores of similar size, such as zeolites. This unique property has
been attributed to their exceptionally smooth pore walls allowing nearly
frictionless transport, and to fluid molecular ordering at nanoscale. Recently, the
availability of membranes made of well-aligned carbon-nanotube (CNT) arrays
embedded in an impermeable filling matrix has allowed experimental
confirmation of MD predictions on a laboratory scale. For applications in
separation technology, selectivity is required together with fast flow. In particular,
for water desalination, coupling the enhancement of the water flux with selective
ion transport could drastically reduce the cost of brackish and seawater
desalting.
In this study, we use pressure-driven filtration experiments, coupled with capillary
electrophoresis analysis of permeate and feed to quantify ion exclusion in silicon
nitride/CNT composite membranes as a function of solution ionic strength, pH,
and ion valence. The pores of the membranes used in this study are sub-2-nm
diameter CNTs whose entrance is decorated by negatively charged carboxylic
groups.
We show that carbon nanotube membranes exhibit significant ion exclusion that
can be as high as 98% under certain conditions. Our results support a Donnantype
rejection mechanism, dominated by electrostatic interactions between fixed
membrane charges and mobile ions, while steric and hydrodynamic effects
appear to be less important. Comparison with commercial nanofiltration
membranes for water softening reveals that our carbon nanotube membranes
provides far superior water fluxes for similar ion rejection capabilities.