Investigating Emerging Contaminants from Self-Cleaning Concretes
2016 GSA Annual Meeting Paper 13-3
Boulder, CO, USA: The use of nanoparticles in self-cleaning concretes is growing rapidly, but could be contributing novel nanoscale contaminants to waterways and drinking water supplies, according to new research being presented on Sunday, 25 Sept., at the annual meeting of the Geological Society of America in Denver.
Self-cleaning concretes contain nanometer-scale particles of titanium dioxide (TiO2). The same compound is used to make bright "Titanium White" paint. But with a diameter less than 100 nanometers (1,000 times smaller than the diameter of a human hair), nano-TiO2 behaves differently than it does in paint, explains Army Lt. Col. Christopher Gellasch, assistant professor at Uniformed Services University of the Health Sciences.
It is not uncommon for the chemical and physical properties to differ between nanoscale and larger scale (bulk) versions of a material. For example, gold nanoparticles are red or purple and have a much lower melting point than bulk gold.
"Titanium dioxide nanoparticles are activated by ultraviolet light, commonly sunlight," Gellasch says, and the activated nano-TiO2 cause water to form a thin film on the surface that will carry away dirt and other contaminants, thereby creating a self-cleaning concrete. They also react with nitrous oxides in the air, which are pollutants from internal combustion engines.
So not only does nano-TiO2 clean concrete, it can clean the air by removing one of the key components of ground-level ozone formation. What is not known, however, is to what degree nano-TiO2 is being released from concrete surfaces (sidewalks, buildings, and roads) due to weathering and entering surface waters. And once in the water, what are the ecological effects and the effects on human health?
"These are emerging contaminants," says Gellasch. "We already hear about pharmaceuticals and personal care products entering the water supply, but titanium dioxide is something that is not as well known."
To begin to answer those questions, Gellasch and his colleagues have been working on determining how much nano-TiO2 might escape concrete over time. They conducted experiments with nano-TiO2 concrete pucks using a machine at the U.S. Army Engineer Research and Development Center that exposed them to high-intensity UV light. The machine simulates a month of sunlight exposure in just 24 hours. These exposed pucks, along with a group of unexposed control pucks, were then soaked in water for 35 minutes to simulate a rain event.
"We soaked the pucks and looked for calcium, magnesium, and titanium in the water," Gellasch says. The calcium and magnesium concentrations help establish how much of the concrete is weathering away, which they compared to the titanium concentrations in the water.
The first thing they found is that the pH of the simulated rainwater did not affect the amount of nano-TiO2 dissolving into the water. Acidic rainwater, for instance, makes contact with the concrete surface and is quickly buffered by calcium carbonate, or lime, from the cement in the concrete.
On the other hand, they did see titanium in the water after exposure, but a second laboratory instrument was unable to determine if it was in the bulk or nano form of TiO2 because it could not determine if the larger particles detected were actually nanoparticles that had clumped together. They also found that the UV light exposed pucks released more titanium into the water.
"We have definitely seen the titanium being released from the concrete," says Gellasch. "If that gets into a drinking water supply, there is a potential for an increased human health risk. There have been other studies showing titanium can accumulate in various animal organs but the toxicity is not well understood. That is one reason nano-TiO2 is considered an emerging contaminant — because we do not know enough about this exposure pathway."
Emerging contaminants in urban environments: From leaking sewers to nanoparticles in concrete
Sunday, 25 September 2016: 8:35 a.m.
Colorado Convention Center, Room 501
Author: Christopher A. Gellasch, firstname.lastname@example.org
The Geological Society of America, founded in 1888, is a scientific society with more than 25,000 members from academia, government, and industry in more than 100 countries. Through its meetings, publications, and programs, GSA enhances the professional growth of its members and promotes the geosciences in the service of humankind. Headquartered in Boulder, Colorado, GSA encourages cooperative research among earth, life, planetary, and social scientists, fosters public dialogue on geoscience issues, and supports all levels of earth science education.
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