Page 382 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 367

associated with the deformation of diffuse layers within these cakes
and the flow of permeate across these cakes may be manifest as a sig-
nificant electroviscous effect in which the viscosity of the fluid appears
to be greater than the bulk viscosity due to the back-migration of ions.

Active Membrane Systems
Future convergence between nanochemistry and membrane science will
likely yield a generation of active membrane systems. Nanomaterials
might be used to develop membranes in the future with the capability
to simultaneously sense and separate contaminants in a fashion that
allows membranes to vary their selectivity as a function of the condi-
tions in the feed stream. For example, self-regulating membranes might
allow membranes to operate in a high permeability/low energy mode
during periods where high rejections of small molecular weight
materials are not required. Seasonal peaks in concentration of a given
contaminant (for example, a target pesticide) would be detected by
the membranes and trigger an increase in the membrane molecular
weight cutoff.
Nanomaterials might also be incorporated into membranes to impart
properties that are activated by an electrical or chemical signal. For
example, a membrane composite that is capable of producing reactive
oxygen in the presence of an electron donor might be activated by the
introduction of such a compound with the purpose of periodically clean-
ing the membrane. Membranes might also be engineered to allow for
local heating of the membrane skin with the purpose of promoting mem-
brane distillation.
Living organisms are the ultimate nanotechnology. The ability of cell
membranes to selectively transport materials, often against concentra-
tion gradient, and to avoid fouling is impressive. As the field of
nanochemistry advances, engineered biomimetic systems based on selec-
tive transport or rejuvenating layers of self-organizing materials may
be developed for performing critical separations in energy and envi-
ronmental applications.

References
1. Onsager, L., Reciprocal relations in irreversible processes I. Physics Review, 1931a. 37:
p. 405–426.
2. Onsager, L., Reciprocal relations in irreversible processes II. Physics Review, 1931b.
38: p. 2265–2279.
3. Kedem, O., and A. Katchalsky, Thermodynamic analysis of the permeability of biolog-
ical membranes in non-electrolytes. Biochem. Biophys. Acta, 1958. 27: p. 229.
4. Spiegler, K.S., and O. Kedem, Thermodynamics of hyperfiltration (reverse osmosis):
criteria for efficient membranes. Desalination, 1966. 1: p. 311.
5. Lonsdale, H.K., U. Merten, and Riley, R.L., Transport Properties of Cellulose Acetate
Osmotic Membranes. Journal of Applied Polymer Science, 1965. 9: p. 1341–1362.