Chemical Engineering April 2013 - 11

Edited by Gerald Ondrey
April 2013
A new way to desalt water
without using RO or distillation
C
apacitive desalination, in which salt
ions are electrically removed from
saltwater, has been researched for decades
as a potentially cheaper alternative
to energy-intensive reverse osmosis
(RO) and distillation, but so far its
application has been limited to waters
with very low salt concentrations. A new
method, which may change that, is being
developed by researchers at Lawrence
Livermore National Laboratory (LLNL,
Livermore, Calif.; www.llnl.gov).
In capacitive desalination, saltwater
is typically pumped through a channel
between two capacitor-electrodes (diagram).
The electrodes remove the Na+
and Cl- ions from the saltwater by capturing
them on the surface of the electrodes.
LLNL has built a flow-through
electrode (FTE) module that has no
channel, but consists of two highly porous
blocks of carbon aerogel (with
negative and positive charges), divided
by a thin polymer membrane. Saltwater
is pumped through the entire capacitor
and the ions, which can pass through
the membrane, are accumulated by the
appropriate electrode.
Earlier experiments (in the 1990s)
used aerogels with small pores and had
Flow-between architecture
LLNL flow-through
electrode architecture
Feed
water
Feed
water
Electrode
Separator
Desalted
water
Desalted
water
Separator
<100 nm
~10 nm
~1 µm
a channel between the electrodes. This
limited ion-transport times from the
channel into the electrodes, says Michael
Stadermann, an LLNL staff scientist.
The FTE uses a newly developed
aerogel that combines large pores, of
1-3-µm dia. with small pores of 1-2-nm
dia. The large pores allow easy flow at
low pressure, while the small ones give
the aerogel a vast surface area of up to
3,000 m2/g for capturing ions. The structure
permits elimination of the channel,
thereby reducing the transport distance
from millimeters to micrometers, with a
corresponding decrease in desalination
time, he says.
The aerogel is made by sol-gel polymerization
of resorcinol with formalSimplified
CO2-to-methanol process
uses copper-oxide nanowires
A
process developed by researchers at the
University of Texas at Arlington (www.
uta.edu) uses copper-based semiconductor
nanoscale-rod arrays and solar radiation
to carry out a photoelectroreduction
of carbon dioxide to methanol. The process
offers a pathway to utilize the greenhouse
gas CO2 as a feedstock to synthesize a
widely manufactured chemical (CH3OH)
under much milder conditions that those
needed for steam reforming of methanol
from hydrocarbon-based feedstock. Also,
the researchers note that no co-catalysts
are needed for the selective conversion
process. Since the method uses copperbased
semiconductor materials, less abundant
(Te and Ga) and more toxic (As and
Note: For more information, circle the 3-digit number
on p. 68, or use the website designation.
Cd) materials are not necessary. The two
types of copper oxide were used because
both are photo-active and the two have
complementary solar-light absorption, the
researchers say.
The semiconductor arrays are prepared
using a two-step approach that consists of a
thermal process of growing CuO nanorods,
and a controlled electrodeposition of Cu2O
crystallites on the walls of the nanorods.
The hybrid CuO-Cu2O nanoarrays are constructed
on a copper substrate. When the
nanorod arrays were submerged in a water-based
solution rich in CO2 and exposed
to simulated sunlight, methanol was produced
without the need for excess energy
input (overpotential).
dehyde in water, using acetic acid as a
catalyst. The water is removed by washing
with acetone, which is then evaporated
by heating. Finally, the material
is pyrolized at 950°C and activated by
a carbon dioxide etch. Pyrolysis stabilizes
the pores and makes for a robust
structure, says Stadermann. Laboratory
tests with the FTE module achieved salt
removal of 80 mmol/L in a single pass
and indicate the process would require
less than a hundredth of the pressure of
conventional RO, he says, but the higher
electricity use would even out the operating
costs. However, he expects the
capital cost would be lower and that the
process would have an advantage over
RO for processing brackish water.
Biosteel
The world's first artificial-silk
fiber that is entirely made of recombinant
spider-silk proteins
has been produced by AMSilk
GmbH (Planegg/Martinsreid,
Germany; www.amsilk.com), a
spin-off company of the Technical
University of Munich.
The fiber's tensile strength is
comparable to that of natural
spider silk, and has thus been
tradenamed Biosteel. Fiber
prototypes are smooth to the
touch, pleasant to the skin and
shine like silk, says the company.
Potential applications
include high-performance
technical textiles, sporting
goods, medical textiles and
surgical products.
(Continues on p. 12)
CHeMiCAl enGineerinG www.CHe.CoM APril 2013 11
Electrode
http://www.llnl.gov http://www.amsilk.com http://www.uta.edu http://www.CHe.CoM
Chemical Engineering April 2013

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