Sun and rain transform asphalt binder into potentially toxic compounds


Date:
July 15, 2020
Source:
Florida State University
Summary:
Chemists show that asphalt binder, when exposed to sun and
water, leaches thousands of potentially toxic compounds into
the environment.



FULL STORY ==========================================================================
A dramatic oil spill, such as the Deepwater Horizon accident in the
Gulf of Mexico a decade ago, can dominate headlines for months while scientists, policymakers and the public fret over what happens to all
that oil in the environment. However, far less attention is paid to the
fate of a petroleum product that has been spread deliberately across
the planet for decades: asphalt binder.


==========================================================================
Now a study by chemists at the Florida State University-headquartered
National High Magnetic Field Laboratory shows that asphalt binder,
when exposed to sun and water, leaches thousands of potentially toxic
compounds into the environment. The study was published in the journal Environmental Science & Technology.

Asphalt binder, also called asphalt cement, is the glue that holds
together the stones, sand and gravel in paved roads. The heavy, black,
sticky goo is derived from bottom-of-the-barrel crude oil at the tail
end of the distillation process.

The MagLab, funded by the National Science Foundation and the State of
Florida, is a world leader in the field of petroleomics, which studies
the mind- numbingly complex hydrocarbons that make up crude oil and its byproducts. Using high-resolution ion cyclotron resonance (ICR) mass spectrometers, chemists there have developed expertise in identifying
the tens of thousands of different types of molecules that a single drop
can contain, and how that composition can be changed by time, bacteria
or environmental conditions.

Ryan Rodgers, director of petroleum applications and of the Future Fuels Institute at the MagLab, had wanted for years to study asphalt binder
using the ICR instruments. It was a logical next step in his group's
years-long effort to better understand the structure and behavior
of petroleum molecules and their potentially toxic effects. Previous
studies had shown that soils and runoff near paved roads exhibit higher concentrations of polycyclic aromatic hydrocarbons (PAHs), which are
known to be carcinogenic. Rodgers suspected there were dots connecting
those PAHs and asphalt binder, and he wanted to find them.

"The long-term stability of petroleum-derived materials in the environment
has always been a curiosity of mine," said Rodgers, who grew up on
the Florida Gulf Coast. "Knowing their compositional and structural
complexity, it seemed highly unlikely that they would be environmentally benign. How do silky smooth black roads turn into grey, rough roads? And
where the heck did all the asphalt go?" He finally acquired a jug of
asphalt binder from a local paving company and handed the project off
to Sydney Niles, a Ph.D. candidate in chemistry at Florida State, and
MagLab chemist Martha Chaco'n-Patin~o. They designed an experiment in
which they created a film of binder on a glass slide, submerged it in
water, and irradiated it in a solar simulator for a week, sampling the
water at different timepoints to see what was in it. They suspected that
the sun's energy would cause the reactive oxygen-containing compounds
in the water to interact with the hydrocarbons in the binder, a process
called photooxidation, thus creating new kinds of molecules that would
leach into the water.



==========================================================================
"We had this road sample and we shined fake sunlight on it in the presence
of water," explained Niles, lead author on the paper. "Then we looked
at the water and we found that there are all these compounds that are
derived from petroleum, and probably toxic. We also found that more
compounds are leached over time." The hydrocarbons they found in the
water contained more oxygen atoms. The scientists were confident that
the sun was indeed the mechanism behind the process because far fewer
compounds leached into a control sample that had been kept in the dark,
and those had fewer oxygen atoms. In fact, the amount of water-soluble
organic compounds per liter that the team found in the water of the
irradiated sample after a week was more than 25 times higher than in the
sample that had been left in the dark. And, using the lab's ICR magnets,
they detected more than 15,000 different carbon-containing molecules in
the water from the irradiated sample.

Given the general toxicity of PAHs, these results are cause for concern,
Niles and Rodgers said. But the team will need to do more experiments
to investigate that toxicity.

"We have definitively shown that asphalt binder has the potential to
generate water-soluble contaminants, but the impact and fate of these
will be the subject of future research," Rodgers said.

They also plan more studies to look at exactly how the compounds are transforming and if different categories of petroleum molecules behave differently.

Niles worries about hydrocarbons in and out of the lab. If she forgets
to bring her reusable produce bags to the grocery store, she'd rather
juggle her veggies on the way to the register than use a store-furnished plastic bag. Although these findings aren't good news for the planet,
she said, they could lead to positive change.

"Hopefully it's motivation for a solution," she said. "I hope that
engineers can use this information to find a better alternative, whether
it's a sealant you put on the asphalt to protect it or finding something
else to use to pave roads."

========================================================================== Story Source: Materials provided by Florida_State_University. Original
written by Kristen Coyne. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Sydney F. Niles, Martha L. Chaco'n-Patin~o, Samuel P. Putnam,
Ryan P.

Rodgers, Alan G. Marshall. Characterization of an Asphalt
Binder and Photoproducts by Fourier Transform Ion Cyclotron
Resonance Mass Spectrometry Reveals Abundant Water-Soluble
Hydrocarbons. Environmental Science & Technology, 2020; DOI:
10.1021/acs.est.0c02263 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200715123140.htm

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