We could have a problem with our roads: The researchers discovered that sunlight and rain could turn certain compounds in the asphalt into potentially dangerous hydrocarbons, threatening the environment and the people who use these routes.
In particular, it is the binder (also called asphalt cement) that is the problem. This heavy black glue is used to glue stones, sand, and gravel on paved roads. It is made from surplus crude oil at the end of its distillation process.
While leakage of toxic and carcinogenic polycyclic aromatic hydrocarbons (PAHs) from asphalt around roads and pavements has previously been investigated, so far it has not been considered to be a sufficient problem to affect human health, something the researchers behind the new studio wanted to investigate further.
“The long-term stability of petroleum-based materials in the environment has always been a curiosity of mine,” says chemist Ryan Rodgers, of MagLab at Florida State University (FSU).
“Knowing their compositional and structural complexity, it seemed highly unlikely that they were benign to the environment. How do black and silky roads turn into gray and rough roads? And where the hell did all the asphalt go?”
The team created an experiment where an asphalt binder film was glued to the side of glass, before being submerged in water and exposed to a solar simulator for a week. A sample was kept in the dark for one week to provide a comparison.
Using an ultrahigh resolution technique called Fourier transformed ion cyclotron resonance mass spectrometry (FT-ICR MS), the researchers analyzed the water around the irradiated sample and the control sample.
Solar energy appears to react with oxygen-containing compounds in the water to release potentially dangerous hydrocarbons from the binder. This process, known as photooxidation, also occurs with oil stains.
“We had this road sign and illuminated it with false sunlight in the presence of water,” says chemist Sydney Niles of FSU.
“Then we look at the water and find out that there are all of these compounds that are derived from petroleum, and probably toxic. We also found that more compounds leak out over time.”
Approximately 25 times the amount of hydrocarbons that leaked into the water in the main sample compared to the control, in fact, involves the role of sunlight in the production of the molecule. Importantly, the hydrocarbons also contained more than the usual amount of oxygen atoms, which aids the compound’s solubility.
In total, the irradiated sample ended up with more than 15,000 different carbon-containing molecules.
This is not yet proof that runoff from asphalt exposed to everyday weather is poisonous, but given the general toxicity and carcinogenic nature of PAHs like these, the reactions are definitely a concern.
The next stage is to investigate chemical reactions more closely to see how compounds are being transformed and to establish to what extent the asphalt binder generates water-soluble contaminants.
“Hopefully it will be the motivation for a solution,” says Niles. “I hope that engineers will be able to use this information to find a better alternative, whether it be a sealer that I put on the asphalt to protect it or find something else to pave the roads.”
The research has been published in Environmental science and technology.
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