New Science for a New Environment
Carnegie Mellon Scientists Work to Understand, Eradicate Pollution
A gentle catalyst that purifies water. A group of bacteria that eats toxic waste. A plastic that quickly decomposes in three simple ways. These scenarios promise to be realities, according to Mellon College of Science (MCS) investigators working to remediate the effects of persistent pollutants and transform harmful manufacturing practices into cleaner, safer methods. Their studies, done in collaboration with colleagues in the College of Engineering, also should contribute much-needed data to inform policy decisions about air pollutants and climate change.
Carnegie Mellon's research in environmental science is prompted by the unexpected consequences of 20th century productivity - industrial pollution. Public alarm sounded most clearly in 1962's "Silent Spring" and was reinforced less than a decade later with the advent of Earth Day. In the years since then, interdisciplinary teams of chemists, biologists, and engineers have steadily built the case that manufacturing practices which release toxic chemicals threaten our environment and imperil our future. But only recently has environmental science begun to yield practical solutions to some of the most vexing concerns. Some of these solutions involve sophisticated biology and engineering. Others center on radically new, inherently non-toxic approaches, better known as green chemistry.
Designing technologies to clean up environmental mistakes is just one aspect of becoming green, according to Terry Collins, Thomas Lord Professor of Chemistry and Director of the Institute for Green Science.
"Green chemistry is really about reinventing chemistry to be more efficient and non-polluting," he said. "And it is as such one of mankind's most powerful tools for developing a sustainable civilization."
A bucket of sediment from the Hudson River tells a bleak story. In the mid-20th century, companies along the Hudson and other rivers in upstate New York collectively released more than one million pounds of polychlorinated biphenyls (PCBs) into the water. The facilities didn't realize that these pollutants would linger for thousands of years. PCBs accumulate in fish and birds high in the food chain, crippling their ability to reproduce. They also build up in humans, where they are suspected of harming reproduction, causing cancer, injuring the immune system and thyroid gland, and impairing learning and memory.
A mixture of 209 related chemicals, PCBs were generated worldwide largely by industries that manufactured plastics, paints, lubricants, transformers and other materials. While U.S. production ceased in 1978, PCBs continue to endanger waterways across the United States.
Removing a Toxic Legacy
Most solutions to persistent pollutants involve containing the problem, rather than eliminating the offending agent. Key to implementing any kind of permanent cleanup measure is determining how chemicals break down in their environment. A safe chemistry that takes its cue from nature should provide the means to destroy many long-lived toxins, say MCS scientists.
In the case of PCBs, Bill Brown hopes to identify and cultivate naturally occurring bacteria that perform the necessary chemistry. For several years, Brown, a professor of biological sciences, has collaborated with Carnegie Mellon colleagues Edwin Minkley (Biomedical Engineering) and Jeanne VanBriesen (Civil and Environmental Engineering) to study PCB-laden sediments in the Grasse and Hudson Rivers.
"Our work takes advantage of nature's ability to evolve a bug that eats PCBs," said Minkley.
The research is part of a large project on the environmental fate and persistence of PCBs funded by the Packard Foundation and led by David Dzombak, professor of civil and environmental engineering.
According to Brown, the right bacteria could be grown and exploited to catalyze, or accelerate, the breakdown of PCBs into harmless compounds. Although apparently straightforward, the task is challenging.
"There are perhaps one million different kinds of bacteria in sediment from each of these rivers. Somewhere in there is an organism that degrades PCBs," said Brown.
To complicate the picture, bacteria that digest PCBs do so through different chemical pathways. So it may take enzymes from several kinds of PCB-digesting bacteria to remove all the chlorine atoms from a PCB's structure and thereby completely decompose it. Brown's team also has found that different nutrients affect the growth of bacteria taken from river sediment. In fact, different amounts of nitrogen - a major component of fertilizers and animal waste - could account for why the team saw unique bacterial populations inhabiting Grasse and Hudson River sediments. Agricultural runoff into the Hudson could be fostering the outgrowth of one set of microbes versus another, according to Brown.
With this information in hand, the team is currently enriching sediment samples from the Hudson and Grasse rivers with PCBs. Over several months, they hope to coax bacteria with a preference for these chemicals into becoming the dominant life-forms. At the same time, the team is using state-of-the-art molecular analysis to pinpoint specific enzymes employed by PCB-digesting bacteria in removing various chlorine atoms. Ideally, the team could identify a combination of factors or nutrients that could be added to river sediments to accelerate the breakdown of PCBs in situ, without having to dredge a river. But, should dredging be needed, their research also could identify ways to break down PCBs in sediments that are disposed in secure, hazardous waste landfills. Ultimately, the team's work could result in approaches that detoxify PCBs much more thoroughly and much faster than any existing method.
Even bacteria with a modestly improved appetite for PCBs will do, suggests Brown, because this cleanup strategy could be combined with other new approaches, such as iron TAML® (TetraAmidoMacrocyclicLigand) activators, or TAMLs, for short.
Initially conceived by Collins as a safe catalyst to disinfect water, TAMLs appear capable of tackling a vast range of environmental pollutants. In Fall 2003, TAMLs received widespread publicity for their promising potential. TAMLs are comprised of elements that nature commonly uses in biochemistry - iron, carbon, oxygen, nitrogen and hydrogen. Elements in TAMLs lack toxicity, and the TAML design avoids toxic groupings of atoms, according to Collins. Just a pinch of these catalysts activates hydrogen peroxide to oxidize, or break down, a variety of chemicals in water at ambient conditions. While TAMLs currently are being explored for their ability to break down PCBs, initial studies already have proven their ability to rapidly, completely oxidize chlorophenols into harmless byproducts. These dangerous, long-lived chemicals are found in pesticides, disinfectants, wood preservatives and personal care formulations.
Yet a third strategy based on MCS science is being explored to eliminate another pollutant, the organic solvent trichloroethylene (TCE). A suspected carcinogen, TCE is still used extensively to remove grease from metal parts. Approximately 60 percent of the 1,400 contaminated sites on the National Priorities List, which tracks the nation's most hazardous waste sites, are contaminated with TCE. Underground pockets of TCE can steadily release this material into groundwater aquifers that supply 50 percent of the nation's drinking water. Left untreated, billions of gallons of groundwater could be contaminated by TCE, according to Greg Lowry, an assistant professor of civil and environmental engineering.
Working with Lowry are Krzysztof Matyjaszewski (Chemistry), Sara Majetich (Physics) and Robert Tilton and David Sholl (Chemical Engineering). This interdisciplinary team is designing and evaluating smart nanoparticles that act like a targeted drug delivery system to find and destroy TCE. The nanoparticles are made via atom transfer radical polymerization (ATRP), a controlled "living" polymer synthesis developed by Matyjaszewski, professor of chemistry and director of the Center for Macromolecular Engineering.
In ATRP, a special catalyst adds chemical units to the end of a slowly growing polymer chain. This process, which can be shut down or restarted at will, allows investigators to precisely control the formation of polymers at the nanoscale level. Using ATRP, scientists can mass produce high quality materials that combine very different structural and functional properties.
To make the nanoparticles used in the current research, the investigators started with a core of reactive iron that quickly breaks down solvents like TCE into harmless chemicals. Matyjaszewski's team used ATRP to design two polymer coatings - one inner, "water-hating" coat covers the iron core, and an outer, "water-loving" shell enables coated particles to travel through an aquifer. Such a dual coating should make the particles cling to a water-TCE interface, where the iron can most effectively break down TCE. Field tests of this system are expected soon.
From Earth to Sky — and Back
Solving environmental problems on the ground is difficult enough. But imagine trying to conduct research on chemistry that takes place miles above us. We don't yet have a good understanding of how human activity affects the atmosphere, let alone how this activity impacts Earth, says Neil Donahue, assistant professor of chemistry and chemical engineering. What little we do know is disturbing, he points out.
For instance, scientists appreciate that gases in the atmosphere oxidize pollutants released from automobiles and industry. These chemical reactions result in the formation of ﬁne particles that lodge within our lungs.
"Particles are thought to cause 20,000 extra deaths from lung disease in the United States each year," said Donahue.
Air pollutants also speed the oxidation of other gases that are naturally released when plants decompose. This activity, in turn, accelerates the formation of dangerous particles, says Donahue, who with four other faculty members in engineering comprise an interdisciplinary air quality group.
Because particles also seed rain clouds, changes in their abundance and distribution would inevitably be expected to alter our climate, Donahue adds.
While these dire consequences of atmospheric pollution are understood, our knowledge of how gases interact with one another is extremely limited, says Donahue, who is trained in atmospheric chemistry and meteorology. Through studies in physical chemistry and atmospheric chemistry, scientists have tried to sort out the dynamics of atmospheric gases. But each route has its limitations.
Fundamental physical chemistry stops more or less at the elementary chemical reaction. The task of stitching together all the chemical reactions in the atmosphere, at all possible temperatures, is overwhelmingly complex. Furthermore, adds Donahue, the tiniest errors introduced in measuring any one step can propagate through subsequent calculations and undermine results.
Rather than reduce the task at hand to its smallest components, air quality engineers take the opposite approach. Using large, tent-like chambers, they study the chemical reaction kinetics of gases mixed together under speciﬁc temperatures and pressures. This permits the rapid study of these complex systems under realistic conditions. However, various reaction products get jumbled together, and it's very difﬁcult to extend data beyond the speciﬁc conditions of any given experiment, according to Donahue.
A High Pressure Flow System (HPFS) - what Donahue calls a "Goldilocks" system - is neither conﬁned to a single reaction step nor expanded to include a plethora of reaction products. Using the HPFS, which he designed while still a postdoctoral researcher at Harvard University, Donahue's team can study chemical reaction kinetics that occur from a hundredth of a second to 100 seconds over a wide range of temperatures and pressures. In this way, Donahue can record ﬂeeting, unusual chemical behaviors to piece together chemical dynamics needed to learn how given industrial practices alter the atmosphere.
Studying how particles continue to interact with atmospheric gases after they form is especially difﬁcult, according to Donahue, because particles come in all shapes, sizes and compositions. Donahue hopes to use ATRP to quickly and easily generate large amounts of a single type of particle to study how it ages.
"Ultimately, we want to understand the transformation of different particles as they travel through the atmosphere and enter soil or water," he said.
From Remediation to Reinvention
While environmental catastrophes can occur in a few minutes, effective cleanup strategies based on sophisticated bioengineering or green chemistry often require decades of careful research and design, say MCS scientists.
In many cases, we still need to deﬁne pollution for a given setting, says Donahue. "Only by understanding the life cycles of pollutants and how they behave in their surroundings can we effectively address environmental problems."
Despite the challenges facing investigators, dedicated environmental science research can produce big results that extend beyond remediation. TAML activators are a prime example of green chemistry to reinvent not one - but many - industries.
Recycling water from industrial plants is an area where TAMLs could make a big difference. Using miniscule amounts of a TAML activator, textile mills could remove greater amounts of dye from efﬂuent, enabling manufacturers to recycle millions of gallons of water and reduce operating costs, says Collins.
If detergents contained TAMLs to prevent the transfer of dyes among fabrics, manufacturers could market washing machines that use fewer gallons of water per load. With another TAML activator, automotive engines could burn fuel more efﬁciently and release fewer sulfur contaminants that cause serious health problems and contribute to acid rain.
"We believe that TAML activators will reduce the levels of sulfur contaminants to those allowable under new EPA guidelines in a very fast and easy process," assured Collins.
TAML activators also could reinvent the manufacturing of paper products, says Collins. TAMLs decolorize efﬂuent and eliminate signiﬁcant amounts of chlorophenols from wood pulp bleaching. They could replace the current bleaching method altogether, providing a safer, pollutant-free alternative.
Green chemistry approaches could ease landﬁll burdens, too. Take prevalent commodity plastics like Plexiglas® (made from poly(methyl methacrylate)) or Styrofoam (made from polystyrene). Matyjaszewski has used ATRP to introduce regularly spaced breakage points along the length of these polymers. Exposure to light or a mild chemical treatment should essentially break down plastics made from the modiﬁed polymers.
"Newly designed polymers could be used to create a sturdy Styrofoam cup designed to last a speciﬁc length of time before degrading through natural exposure to light," said Matyjaszewski.
What's more, polymers engineered this way would disintegrate into short, uniform fragments that should be more easily digested by bacteria. This design aspect would allow many plastics to decompose much more rapidly than materials now on the market, according to Matyjaszewski, who is working with civil and environmental engineers at Carnegie Mellon on further tests of plastics generated in this way.
Selling green chemistry
Ultimately, selling green chemistry requires scientists to tailor solutions to individual problems. Collins' family of TAMLs exempliﬁes this approach.
Energy costs are another consideration. Chemical manufacturing processes often require high energy input in the form of extreme temperatures and/or pressures. Crafting green chemistry to work under normal temperatures and pressures provides a clear cost-savings advantage.
Increasing efﬁciency through recycling or by reducing the amount of starting materials used are other ways that industry can win with green chemistry. Small amounts of TAMLs used in solution with non-toxic hydrogen peroxide and water are a perfect example. In recent years, Matyjaszewski's group has made advances in ATRP so that it can be performed in water with small amounts of catalyst. Modifying the ATRP catalyst's properties and recycling it are other ways his group has reﬁned ATRP to be greener and commercially appealing.
For some applications, a green chemistry approach must prove more effective than an existing method - however costly - before it is adopted. Anthrax cleanup is a case in which a fail-safe alternative to a conventional method is a must. In late 2003, Collins' group showed that TAMLs can kill 99.99999 percent of an anthrax-like agent in culture. Collins' team is working to expand this remarkable killing ability into a commercial method to clean waterborne anthrax. This advance is a step toward Collins' long- term goal - using TAMLs as a safe, easy method for global water puriﬁcation.
"In industry, there's always competition and compromise between cost and performance. It's a question of what makes sense overall," said Matyjaszewski, who runs an international consortium of academic and corporate partners interested in ATRP.
Environmental research should only intensify in the coming years, according to Collins, who envisions the widespread adoption of green chemistry approaches by industry. "Ultimately, the operation of technologies should not conspire against the welfare of future generations."