Meet the Scientists
While most agree that desalination is only one solution for helping to bring fresh, potable water to those who need it, innovation continues. We need membranes that are more
permeable and let water pass with even lower pressures and reduced energy consumption.
We need membranes that are stronger and more durable, less prone to getting clogged with
minerals and requiring less frequent replacement.
Benny D. Freeman, Ph.D.
Big Challenges/Small Solutions
Some desalination researchers are responding to those challenges in a small way with huge
implications for providing a drought-proof source of water. That response involves nanotechnology — technology built from atoms and single molecules where dimensions are measured
in nanometers, or billionths of a meter. One nanometer is about 50,000 times smaller than the
width of a human hair.
The next-generation reverse osmosis membranes may be nanocomposite membranes.
These polymers contain small particles that produce major improvements in the polymers’ ability to transport and separate saltwater. Nanocomposites may be the key to solving
another problem with existing membranes. That is the tendency to foul, or clog up, with
salts so that more pressure is needed to force seawater through the membrane. Here is Dr.
Benny D. Freeman, of the University of Texas at Austin, who reported on nanocomposites
this year in ACS’s peer-reviewed journal Macromolecules:
“Fouling is recognized to be one of the most ubiquitous and highest-priority problems
facing membranes in virtually any water purification application that is currently
practiced or envisioned. Membrane fouling is the process by which small particu-
lates and other components in water that is being treated deposit on the surface
of membranes. They act to impede the flow of water through the membranes. As a
result, the amount of membranes that are needed to process a given amount of water
is increased. One of the examples of fouling is the flux, or amount of water coming
through a unit surface area per unit time of a membrane such as an ultra filtration
membrane, which can decrease more than a thousand fold as a membrane becomes
fouled with particulate matter (or proteins or emulsified oil droplets, depending on
the application).”
David Klanecky
Water “Wires”
Researchers are discovering that another product of nanoscience — carbon nanotubes —
shows great promise as a new filter membrane. These hollow tubes of pure carbon have an
inner channel or pore 100,000 times smaller than a human hair. Water can pass through, but
not salt. And water molecules move with amazing speed through these tubes, which have
been called “water wires.” In a study in ACS’s Journal of Physical Chemistry B, Dr. Ben Corry
and colleagues at the University of Western Australia reported on the potential of these
“water wires” in nanotube membranes that require less pressure and energy.
“Indeed, a membrane made from nanotubes could be expected to obtain 95 percent
desalination with a flow rate over 1,500 times that of existing membranes. Lower
pressures could be used in carbon nanotube membranes, provided they overcome the
osmotic pressure difference between seawater and freshwater.”
Wolfram Kloppmann, Ph.D.
Scientists are taking one step forward toward that challenge with a new desalination process called . . . well . . . forward osmosis. Forward osmosis saves energy and money because
it does not need the high pressures for reverse osmosis. Rather than pushing water through
membranes with pumps, forward osmosis draws it though with the chemical energy of
osmotic pressure.
“It’s all about chemistry — in forward osmosis desalination, the main principle is the
use of ammonia and CO that dissolves in water. They form ammonium salts, ionic
2
species, which create such huge osmotic pressure that it can drive the water through the
membrane.”
