Coal-fired power plants as nanoparticle spinning

Exotic titanium oxide nanoparticles could pose a health hazard

For coal combustion, it appears that undetected exotic titanium suboxide nanoparticles have been formed for decades. © Danicek / thinkstock
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Disquieting discovery: The burning of coal apparently releases large amounts of previously unrecognized mineral nanoparticles. Researchers have detected these titanium suboxide particles in coal ash and sediments around coal-fired power plants and in laboratory coal-burning experiments. A first test suggests that these exotic nanoparticles could have deleterious effects. Further studies are therefore urgently needed, according to the researchers in the journal "Nature Communications".

Around 30 percent of the world's energy needs are derived today from the burning of coal. Especially in power generation and steelmaking, coal is still the leader. However, this has an equally double negative effect on the environment: in the long term, the carbon dioxide released increases the greenhouse effect. In the short term, coal combustion releases soot and other particulate matter that damage the environment and human health.

Unknown titanium oxide variant

And it could get fatter: Yi Yang of the Virginia Virginia Polytechnic Institute (Virginia Tech) and his colleagues have discovered a previously unknown type of nanoparticles that are formed and released during coal burning. The first clues came when they analyzed river sediments and coal ash near a coal-fired power plant.

Surprisingly, in the samples, the researchers discovered lots of nanoparticles of a certain group of titanium oxides, the so-called Magnéli phases. These compounds are characterized by the chemical formula Ti x O 2x-1 - and extremely rare. "So in the beginning I could not believe what we had found there, " says Yang.

Included in all coal ash

It is common knowledge that this variant of titanium suboxide nanoparticles is virtually absent in nature. So far, they have only been found in some meteorites, in lunar rocks and in a single rock formation in western Greenland. All the more puzzling was the finding of up to six grams per kilogram of Magnéli phases in the samples from the vicinity of the coal-fired power plant. display

Electron micrographs of titanium suboxide nanoparticles from sediment (top) and coal ash from power plant near, as well as electron diffraction image of the cutaway section. Yang et al. Nature Communications, CC-by-sa 4.0

Was this possibly just a peculiarity of this particular power plant? To test this, the researchers are analyzing ash deposits from twelve other coal-fired power plants in various regions of the USA and China. The result: All samples contained one to three different variants of these titanium suboxide nanoparticles, the most common being Ti 6 O 11, as they report.

Product of coal combustion

The obvious suspicion of the researchers: It is possible that these nanoparticles are produced by the combustion of coal as previously unrecognized by-products. Coal often contains titanium-containing minerals in the form of rutile, anatase or brookite, they explain. "Their salary varies from a few tenths of a percent to several percent, " said Yang and his colleagues.

In laboratory experiments, the scientists tested whether and under what conditions these titanium minerals are converted into Magn li phases during coal combustion. It turned out that when pulverized coal is burned at at least 900 degrees, these titanium suboxide nanoparticles form. In the case of coal combustion in power stations and also in the production of coke, these nanoparticles can therefore be produced and then be released into the air and the environment with the exhaust gas.

Already released a billion tons?

If confirmed, large quantities of these nanoparticles could have been released into the environment unrecognized since the beginning of the Industrial Revolution. Scientists are protecting the amount of Magn li phases that have been released for around 200 years to at least one billion tonnes.

In the US and other countries with stricter environmental regulations, at least some of this and other nanoparticles are now being removed by filters from the power station exhaust gas, Hochella explains. "But in countries where particulate matter is less effectively or not filtered out, these titanium suboxide particles continue to be released into the atmosphere - for example, in India or China."

Detrimental to organisms?

But what does this mean for the environment and health? Titanium dioxide nanoparticles (TiO 2 ) are already known to damage cells by oxidative stress and cause inflammation in lung tissue and intestines.

In a first toxicity test with titanium suboxide Ti 6 O 11 in zebrafish embryos, Yang and colleagues found that these nanoparticles reduce the survival rate of fish embryos and are biologically active in the dark. The underlying mechanism is still unknown, but the researchers suspect that these titanium oxide variants could work differently than titanium dioxide.

"Whether and how the titanium suboxide nanoparticles are toxic to the lungs must now be carefully investigated in upcoming studies but that could take years, " says Hochella. "Given the potential danger, this is a sobering thought." (Nature Communications, 2017; doi: 10.1038 / s41467-017-00276-2)

(Nature / Virginia Tech, 09.08.2017 - NPO)