NAMS 2002 Workshop - ICOM 2008

Nanofiltration and Reverse Osmosis II - Imaging and Characterization – 7
Tuesday July 15, 12:00 PM-12:30 PM, Maui
Characterization of the Polyamide Active Layer in NF/RO Membranes Using
Gold Nanoparticles
F. Pacheco (Speaker), Stanford University, Stanford, CA, USA, fpacheco@stanford.edu
M. Reinhard, Stanford University, Stanford, CA, USA
J. Leckie, Stanford University, Stanford, CA, USA
The goal of this project was to investigate the deposition of nanoparticles during
filtration to better understand how transport and rejection mechanisms occur
within the active layer of a RO membrane. The active layer in state of the art RO
membranes consists of cross-linked networks of fully aromatic polyamide, with
an average thickness of approximately 200 nm and a very heterogeneous
structure that confers the membrane a relatively rough surface, also described in
the field as the peak-and-valley structure. Because of the thinness of this layer,
characterization at the microscale is extremely difficult and as result knowledge
of the transport and separation mechanisms is incomplete. Experiments were
performed with gold nanoparticles in a dead-end filtration system without stirring
at a pressure of 4.8 bar (70 psi). The membrane investigated was a commercial
low pressure RO membrane with a fully aromatic polyamide layer featuring the
characteristic peak- and-valley rough structure.
The ability to separate the polyamide layer from the underlying polysulfone
support was used to develop a novel TEM based technique that allowed us to
image the spatial distribution of the gold nanoparticles with respect to the
projected surface area of the polyamide layer. The resulting images show that
the particles did not accumulate uniformly over the surface of the membrane, but
instead formed distinct clusters around the areas where the polyamide layer was
the thickest, i.e. the areas near the peaks. TEM images of polyamide cross
sections, as well as SEM images of the membrane surface, confirmed that the
particles accumulated preferentially on the peaks rather than in the valleys of the
polyamide structure. Although the deposited nanoparticles only covered about
30% of the projected surface area of the membrane, water flux was significantly
reduced. These results suggest that there are areas within the polyamide layer
that have higher permeability to water and that are the most sensitive to fouling.
The effects of particle size and concentration, pH and ionic strength were
investigated.
The use of nanoparticles in combination with advanced microscopic imaging
techniques can be used to examine which sections of the polyamide layer in NF
and RO membranes are actively involved in the transport and rejection
mechanisms, as well as those that are highly sensitive to the initial stages of