Solving an intricate jigsaw puzzle hinges on the ability to see
relationships, patience, and a passionate conviction that each piece
matters.
Those qualities also describe Kristopher McNeill, an assistant
professor of chemistry whose research focuses on mechanistic aquatic
chemistry—reactions that occur in water. Like other environmental
scientists, he has an unflagging curiosity about how the world works
and how the pieces of the environmental puzzle fit together.
"I'm interested in what I think are the two most important ways
the environment has to clean itself,” says McNeill. “One is photochemistry,
and one is biochemistry.”
Photochemistry is a vital component of the global carbon cycle
that sustains life on Earth. A finite supply of carbon exists in
the earth and its atmosphere, so carbon (in the form of carbon dioxide)
must be continuously recycled through photosynthesis, respiration,
and decomposition. In simplest terms, this recycling is the exchange
of carbon dioxide between two great reservoirs, the atmosphere and
biosphere. However, since the onset of the industrial age, human
activities—especially the burning of fossil fuels and forests—have
increased the concentration of atmospheric carbon dioxide. That
ongoing buildup has sparked worldwide concern about global warming
and its long-term effect on Earth's climate.
McNeill and his team are conducting several studies involving singlet
oxygen, one of a family of compounds known as reactive oxygen species
(ROS), which are formed when sunlight hits natural waters. A high-energy
version of the oxygen we breathe, singlet oxygen can react more
quickly with substances than normal ground-state oxygen. Singlet
oxygen is formed through an energy-transfer reaction involving a
photon (particle of light), a sensitizer, and ground-state oxygen.
ROS have a fleeting lifespan ranging from microseconds to perhaps
an hour, and they exist at very low concentrations in sunlit waters.
Despite their transience, ROS may play a significant part in the
global carbon cycle. They're thought to be very important to the
photochemical processes that degrade carbon-based compounds in aquatic
systems.
McNeill says his singlet oxygen studies will reveal more details
about the carbon cycle. To effectively model the cycle, he says,
researchers must calculate how much carbon is sequestered by trees
or through aquatic synthesis.
"Those numbers are going to be off if they don't understand the
intricacies of the cycle,” he says. “Without understanding this,
we're not going to be able to model the fate of our planet very
well.”
Although most of the carbon fixed by photosynthesis is returned
to the atmosphere through respiration, the process also yields biomolecules
that resist degradation. Over time, these carbon-rich materials
will settle out of the water column into the sediment at the bottom
of a lake or river. In effect, they're removed from the respiration
subcycle.
"Right now, for our environment, this is a good thing, because
we want to reduce the atmospheric carbon dioxide,” says McNeill.
“This is a natural process that takes carbon from the atmosphere
and puts it into something that is no longer cycling.”
Sometimes, however, these resistant materials are broken down by
sunlight, consumed by microorganisms, and reinjected into the carbon
cycle. McNeill wants to learn how singlet oxygen and other reactive
oxygen species abet this process.
One of McNeill's current projects is designed to take the first
direct measurements of singlet oxygen's concentration in natural
waters. Using materials that react selectively with singlet oxygen,
the team will create molecular probes, bind them to a polymer, and
place them directly in an aquatic system for a specified time. After
removing the probes, the researchers will analyze the degree of
reaction and singlet oxygen concentration.
Another study will examine singlet oxygen's ability to react with
humic substances—decayed organic matter such as leaf litter,
plant matter, and decomposed organisms. Humic substances are poorly
recycled biopolymers that consist largely of lignin, an organic
molecule found in trees, vascular plants, soil, and all natural
waters.
Singlet oxygen breaks down humic substances into bioavailable materials,
but exactly how that occurs isn't clear. McNeill is creating models
of humic substances and exposing them to thermally generated singlet
oxygen to see what happens.
"There already is some literature on this,” he says, “so we have
a pretty good idea of what to expect: cleavage of the carbon-carbon
bonds in the lignin.”
Reactive oxygen species may also help degrade pharmaceutical compounds
that humans add to the environment. Many of these compounds literally
go down the drain after human use and eventually end up in lakes
and streams.
"Natural waters are a sink for pharmaceutical compounds,” says
McNeill. “It's been identified as an emerging problem, but we don't
know the extent of it.” He and Assistant Professor Bill Arnold of
the civil engineering department are conducting a study of personal
care products to see if the compounds react with singlet oxygen.
If so, then photochemistry will also degrade the compounds in an
aquatic environment.
McNeill also has a strong interest in biochemical processes that
clean the environment. His team is building models that mimic a
naturally occurring process in which microorganisms use metal complexes
to dehalogenate chlorocarbons. These common pollutants include dry-cleaning
and industrial solvents as well as chlorinated pesticides, all of
which can contaminate aquifers.
"We are building very simple cobalt, nickel, and zinc complexes
that we can put in and observe this same kind of reaction,” McNeill
says. “We are trying to understand how they work, how we can make
them better, [and] how we can clean up the environment with them.”
McNeill teaches undergraduate and graduate students and says that
environmental issues rank high on the list of students' academic
interests.
He's also pleased that the number of environmental scientists at
the University is growing. “No one person can study an environmental
problem in depth because at some point you're going to run up against
something you don't understand,” he says.
He recalls a time he encountered a very complex fluid mechanics
problem. “[In this lake] there was groundwater coming in, stream
water coming in, another plume of groundwater coming in, and a stream
going out,” he says. “I had to go to people who work on these problems
all the time and appeal to them for help in understanding how this
was working. [As a chemist], everything in my flask is well-stirred.
Lakes aren't well-stirred reactors!”
McNeill relishes the challenges of environmental science. “I think
they're the most complicated problems there are,” he says. “What's
more complicated than the real world? Just about nothing!"
