Our asphalt roads might be harmful: researchers have discovered that sunlight and rain might be able to turn certain compounds in asphalt into a potentially dangerous hydrocarbons, threating the surrounding environment and people using these routes.
In particular, the asphalt cement is the problem. This heavy, black glue is used to stick stones, sand, and gravel together in paved roads. It's made from leftover crude oil at the very end of its distillation process.
Knowing the toxic, carcinogenic polycyclic aromatic hydrocarbons (PAHs) could be leaking from the asphalt around roads, up until now it hasn't been discovered to be enough of a problem to impact human health.
Chemist Ryan Rodgers, from MagLab at Florida State University (FSU) said, "The long-term stability of petroleum-derived materials in the environment has always been a curiosity of mine, Knowing their compositional and structural complexity, it seemed highly unlikely that they would be environmentally benign.”
The team designed an experiment to stick a film of asphalt binder to a glass side, then submerge them in water and expose them to a solar simulator for a week. A sample was kept in the dark for a week to provide a comparison (control sample).
Using an ultra-high resolution technique called Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), the researchers then analyzed the water around both the irradiated sample and the control sample.
It appears that solar energy reacts with oxygen-containing compounds in the water to release potentially dangerous hydrocarbons from the binder. This process, known as photooxidation, also happens with oil slicks.
Chemist Sydney Niles, from FSU said, “We had this road sample and we shined fake sunlight on it in the presence of water, then we analyzed 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.”
In fact, roughly 25 times the amount of hydrocarbons leaked into the water in the main sample compared with the control, implying sunlight's role in the molecule's production. Importantly, the hydrocarbons also contained more than the usual number of oxygen atoms, helping the compound's solubility.
In total, the irradiated sample ended up with more than 15,000 different carbon-containing molecules.
This isn't yet proof that asphalt road exposed to everyday weather is harmful, but given the general toxicity and carcinogenic nature of PAHs like these, the reactions are definitely cause for concern.
The next stage is to investigate the chemical reactions more closely to see how the compounds are transforming, and to establish to what extent asphalt binder is generating water–soluble contaminants.
Niles says, "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."
The research has been published in Environmental Science & Technology.
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