Pollutants, Resources and Nanotechnology

The embryonic field of nanotechnology, which has been receiving much attention
of late, aspires to the control of matter at molecular and near-molecular scales,
in emulation of the capabilities of biosystems. Element separation technologies
are both an obvious early application of nanotechnology and an obvious motivator
for its funding, as high-value economic drivers exist. Moreover, some molecular-level
technologies are largely passive systems, and hence will be considerably simpler
to design and build than the elaborate “molecular machines” some envision as
the goal of mature nanotechnology, but which for now are the stuff of science
fiction.

At present, pollution control and purification are the economic drivers of
greatest interest to researchers and eventually to the energy industry. As separation
technologies mature, however, they will blur the distinction between a “pollutant”
and a “resource.” Recovered pollutants will begin to have an impact on resource
extraction. After all, zinc extracted from a wastewater stream, for example,
is zinc that does not have to be mined.

Indeed, nanotechnological separation technologies are likely to become “disruptive
technologies.” Such a technology is initially more expensive and less capable,
but becomes quietly established in niche markets where it has distinct advantages.
There it improves until it can invade established markets – to the surprise
of those in those markets (see Figure 1). A familiar example is high-end PCs
supplanting mainframes.

Also, many aqueous solutions, of both natural and artificial origin, become
nontraditional resources given mature separation technologies. Wastewater streams,
acid-mine drainage, seawater, concentrated natural brines such as those in oilfields
or saline lakes – all become potential sources of materials. The largest unconventional
tungsten deposit in the United States, for example, is the brines of Searles
Lake, Calif.

Molecular-level element separation technologies would be much more practical
if there were better control of their fabrication at the nanoscale. As it is,
much of their promise is currently unrealized. Indeed, one reason for the efficiency
of biological systems is that biosystems do organize themselves at the molecular
level.

The Thermal Paradigm

Let’s look at how it’s done now. Conventional technology used for resource
extraction and processing are thought of as intrinsically energy intensive,
due to the inchoate impressions of gigantic open pits, fiery smelters and armies
of heavy equipment moving tons of dirt. Even in the U.S., which imports a great
deal of its primary materials, primary metal and nonmetal production still accounted
for some 3.5 quads (quadrillion BTU) of energy consumption in 2003, about 4
percent of total U.S. energy usage, according to the U.S. Department of Energy’s
2004 Annual Energy Review.

Conventional resource extraction is so energy-expensive because it relies on
vast flows of heat: energy in its most wasteful and disorganized form. Elements
are separated by melting, crystallization, vaporization and so on; the desired
element might separate into a melt; be left behind while other elements are
driven off as vapor; be extracted out as a new compound crystallizes; or something
similar.

For example, iron is extracted by “cooking” iron oxide with carbon (coke).
At elevated temperatures the oxygen combines with the carbon, which wafts off
as gaseous carbon dioxide, leaving behind molten iron. Any silicate (rocky)
impurities form a melt, or “slag,” that is immiscible in the molten iron, analogous
to oil and water, and so can be poured off separately. Even so, the process
is not economic unless the starting material is nearly pure iron oxide. Despite
the fact that iron is the fourth-most abundant element in Earth’s crust, only
a tiny fraction of that iron could be a “resource” with current technology.

Copper is another example. Extracting copper typically begins with “roasting”
a copper-iron sulfide in air. The sulfur is driven off as gaseous sulfur dioxide,
which was vented in the old days but is now recovered because of pollution regulations.
(It is now used to make by-product sulfuric acid.) Left behind is a mixture
of molten copper metal and iron oxide. Silica (silicon dioxide) is then added;
it combines with the iron oxide to form an immiscible slag, which can be poured
off to leave the copper. The iron content is simply discarded. Furthermore,
roasting a sulfide ore wastes energy: it would be thermodynamically possible
to extract useful energy from the sulfide oxidation while also obtaining metal
as a byproduct. (Certain bacteria nearly do this.)

Not only are such processes grossly energy intensive, they are intrinsically
polluting, not just from the combustion necessary to generate the heat, but
also because the element separation is never complete. Some of the desired element
is always left behind. Moreover, byproducts containing geochemically abundant
elements are usually uneconomic and also discarded, as with the iron in the
copper smelting described above. This pyrometallurgy – smelting – hasn’t changed
in essence since antiquity; its only virtue is its relative simplicity. A further
source of expense is the preprocessing necessary to purify the ore. Crushing
and grinding dissipate a great deal of mechanical energy as heat, and physical
separation (“beneficiation”) of the ore minerals from waste minerals (the “gangue”
minerals) also leaves a lot of waste (“tailings”) that must be discarded.

The Biological Example

It is often claimed that the large energy costs of conventional resource extraction
are dictated by the laws of thermodynamics. Yet, biological systems can extract
materials with low energy expenditures. Plant roots extract both nutrients and
water at low concentrations from the surrounding soil. Vertebrate kidneys extract
only certain solutes out of the blood from a background of many other solutes.
For photosynthesis, plants extract carbon dioxide from the air, where its concentration
is only about 350 parts per million (ppm), and do so using only the diffuse
and intermittent energy of sunlight. “Shell builders” such as mollusks (snails,
clams, oysters and so on) extract calcium carbonate from the surrounding water
to build their shells. Diatoms, a type of single-celled alga, are particularly
impressive: their shells are a low-temperature silica glass, made from silica
extracted from the ambient water where it occurs at ppm levels.

Obviously organisms do not carry out thermally driven separation. Instead,
they literally move individual atoms or molecules, using specialized molecular
mechanisms. This is not only vastly less costly energetically but allows separation
from considerably lower concentrations.

Technological Approaches

Molecular-level separation is attractive for applications, such as pollution
control, for which thermal separation would be either prohibitively expensive
or would not work at all because of the low concentrations involved. In its
simplest form, molecular separation does require that the material being separated
be free to flow, as a gas or a liquid. Therefore, solutes in water solution
are technologically easier to deal with. For one thing, the issue of grinding
and crushing of solid materials do not arise.

Some approaches to molecular separation already have practical applications.
Semipermeable membranes are in essence “molecular filters” that strain out one
or more solutes. In reverse osmosis the separation is driven by a pressure difference
across the membrane, while in electrodialysis charged solutes (“ions”) are driven
across a membrane in response to an electric field. Both processes are now used
in the purification of brackish water and in desalination.

Molecular sieves, such as zeolites, can extract oxygen from the air. Instead
of oxygen tanks, for example, people requiring supplemental oxygen can now use
a machine that plugs into the wall that uses such a sieve to extract oxygen
from the air.

Ion exchange is a well-established technology for “swapping out” ions in solution.
Water softeners are a familiar example: calcium ions in tap water, which make
it “hard,” are taken up by the exchanger in exchange for sodium ions. Ion-exchange
materials include zeolites and various polymers (resins) that have electrically
charged molecular groups to which the ions are attracted.

Finally, in recent decades much research effort has been directed toward molecules
that can bind tightly with other chemical species to form highly stable structures.
Such molecules can be chemically linked to form a highly selective binding surface
for extracting particular solutes from solution. For example, the valuable metal
palladium is commercially recovered by dissolving scrap catalytic converters
in acid. Tethered molecules on a silica surface bind the dissolved palladium,
while much more abundant but nearly valueless metals such as iron remain in
solution. Current (early 2005) prices for palladium are around $200 per troy
ounce.

Solute Selectivity

As the above shows, extracting one particular solute out of a background of
many others is fundamental to a great many applications, both of environmental
and of resource interest. The solute might be valuable or toxic (e.g., lead,
which in dissolved form has chemical similarities to innocuous but much more
abundant calcium). Typically, too, the solute of interest is much less common
than all the others in solution. Not only would it usually be prohibitive economically
to extract everything, but in many cases it doesn’t solve the problem. After
all, the idea is to separate that solute out.

This is the problem of “selectivity.” Chemists have become very skilled in
designing molecules that bind only to certain solutes. In this, of course, they’re
only imitating what biology does already.

However, there is a major problem with “binding” approaches to separation.
Breaking up the binding so that the binding molecules can be used again typically
takes extreme chemical measures – ones that generate a much larger volume of
waste that in turn becomes a serious disposal problem. Indeed, further separation
step(s) are now typically required.

Switchable Binding

“Switchable” binding is a way to solve the “elution problem”: under one set
of circumstances binding occurs, but changing some environmental variable causes
the solute to unbind again. Again, biology has anticipated technology. Hemoglobin,
for example, binds strongly to oxygen in the lungs, but under the different
chemical conditions elsewhere in the body gives up the oxygen to the tissues.
The hemoglobin molecule actually changes its configuration in doing so.

A simple example of switchable binding is electrosorption, which is based on
straightforward electrostatic attraction and repulsion. Charging an electrode
attracts out dissolved ions having the opposite charge; reversing the charge
of the electrode desorbs the ions again. Electrosorption was first proposed
in the 1960s for desalination, but had remained impractical until the recent
advent of very high surface area electrodes. Since the “filled up” electrodes
look like a charged capacitor, too, a great deal of the electrical energy can
be recovered when the electrode polarity is switched to desorb the ions.

More subtly, certain electrodes can selectively take up particular ions, typically
because the crystal structure of the electrode has voids into which only those
ions will fit. The lithium ion, for example, is much smaller than the sodium
ion, and a process for drawing lithium ion into a form of manganese oxide having
lithium-sized voids has been patented. Sodium ion, which is always much more
abundant, cannot fit in the voids, so the lithium is selectively extracted.
Reversing the electrical polarity expels the lithium again. Since lithium is
both rare and has growing applications – as in lithium batteries – such a process
has obvious economic promise.

An alternative potential “switch” for binding is light. One approach envisions
using certain molecules that change their structure drastically on absorbing
light. Some research has been carried out on using that structural change to
go from a binding to nonbinding form. So far, however, the molecules used break
down too rapidly for practical separations.

A different approach envisions using the absorption of light energy by a semiconductor
surface. For example, a surface might adsorb ions from a solution in the dark,
but on illumination desorb those ions. Indeed, sunlight might be the trigger.
Merely illuminating a surface to desorb its solutes would obviously be considerably
cleaner and “greener” than flushing it with strong acid or brine solutions.

Conclusion

Molecular-based approaches to element separation constitute a promising economic
driver for nanotechnology research and development in the near future. Some
approaches are even now revolutionizing pollution control and resource recovery.
Moreover, not only will the difference between a “pollutant” and a “resource”
become one of context, but the present paradigm of “dig it up and cook it” for
resource extraction will gradually become obsolete. The obsolescence of that
paradigm, which has been unchanged for millennia, also means that the energy
costs of extracting primary materials will decrease drastically over the coming
decades.

This paper has been adapted from Nanotechnology for Clean Energy and Resources,
available online at the Foresight Institute: www.foresight.org.