The
decomposition of plant, animal and microbial material
in soil and water produces a variety of complex
organic molecules, collectively called natural organic
matter. These compounds play many important roles in
the environment.
By studying the molecular mechanisms responsible for
the complex behavior of natural organic matter,
researchers at the University of Illinois at
Urbana-Champaign are finding new ways to prevent the
compounds from fouling water purification and
desalination facilities.
Natural organic matter is ubiquitous in soils, waters
and sediments. In agriculture, natural organic matter
is important because of its positive effects on the
structure, water retention and nutrient properties of
soils.
Natural organic matter also interacts with metal ions
and minerals to form complexes of widely differing
chemical and biological nature. Solubility, mobility
and toxicity of many trace metals are strongly
correlated with the concentration of natural organic
matter in soil and water.
Natural organic matter creates problems for the water
supply industry, however, requiring removal to
minimize water color and giving rise to potentially
harmful chemical byproducts as a result of
chlorination. Through a process called bio-fouling,
natural organic matter is also a major culprit in
degrading the performance of membrane filtration
systems used for water purification and desalination.
However persistent and universal natural organic
matter molecules are in the environment, they are
little understood. Natural organic matter has no
unique structure or composition, cannot be
crystallized and is extremely difficult to
characterize.
Illinois researcher Andrey Kalinichev and geology
professor James Kirkpatrick have used computer
simulations and nuclear magnetic resonance
spectroscopy to investigate some of the factors that
contribute to the complex behavior of dissolved
natural organic matter. They will present their
findings at the American Geophysical Union meeting in
San Francisco, Dec. 11-15. A paper reporting their
findings has been accepted for publication in the
European Journal of Soil Science.
Bio-fouling is one of the most important problems in
developing advanced membrane technologies for water
purification and desalination, Kalinichev said. It
creates great complications for the industry.
Because of its acidic nature, natural organic matter
can form complexes with dissolved metal ions. The
binding of ions such as calcium, sodium, magnesium and
cesium to natural organic matter, and their potential
effects on bio-fouling were studied using molecular
dynamic computer simulations performed by Kalinichev,
and nuclear magnetic resonance measurements performed
by Kirkpatrick and former student Xiang Xu.
Membrane researchers know that when calcium is
present, bio-fouling occurs very fast and filters clog
quickly, Kalinichev said. But when only magnesium or
sodium is present, the filters clog more slowly, if at
all.
Using relatively simple but realistic molecular models
of natural organic matter dissolved in ionic
solutions, Kalinichev and Kirkpatrick found that
sodium and magnesium ions have very weak interactions
with natural organic matter. Cesium interacts more
strongly, but calcium has the strongest interaction
with natural organic matter.
The strength of the interactions of ions with natural
organic matter is dependent upon multiple factors such
as ion size, electric charge, and the energy it takes
to break the hydration shell of water molecules around
the ions, Kalinichev said.
Metal ions in water are usually hydrated, which means
they are surrounded by water molecules. For these ions
to form strong complexes with natural organic matter,
the attached water molecules must be removed.
When this happens, several negatively charged
molecules of natural organic matter can simultaneously
attach to the same ion, creating much larger
aggregates. These aggregates are responsible for the
formation of bio-fouling layers on membrane surfaces.
In addition to clogging filters, natural organic
matter can change the mobility of certain toxic metals
in soil and water.
We performed our experiments and computer simulations
with ions such as sodium and calcium because their
behavior in water is already well studied, Kalinichev
said. Our next step is to use these models to study
the effects of natural organic matter interaction with
less common but more toxic metals such as strontium,
lead, mercury, zinc and nickel.
This work was funded by the U.S. Department of Energy
and the National Science Foundation.
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Source: |
University Of Illinois At
Urbana-Champaign |
Published on 2 January,
2007
