Researchers term the membrane a "memzyme" because it acts like a filter but is near-saturated with an enzyme, carbonic anhydrase, developed by living cells over millions of years to help rid themselves of carbon dioxide efficiently and rapidly.

"To date, stripping carbon dioxide from smoke has been prohibitively expensive using the thick, solid, polymer membranes currently available," says Jeff Brinker, a Sandia fellow, University of New Mexico regents' professor and the paper's lead author.

"Our inexpensive method follows nature's lead in our use of a water-based membrane only 18 nanometers thick that incorporates natural enzymes to capture 90 percent of carbon dioxide released. (A nanometer is about 1/700 of the diameter of a human hair.) This is almost 70 percent better than current commercial methods, and it's done at a fraction of the cost."

Coal power plants are one of the United States' largest energy producers, but they have been criticized by some for sending more carbon dioxide into the atmosphere than any other form of electrical power generation. Still, coal burning in China, India and other countries means that U.S. abstinence alone is not likely to solve the world's climate problems.

But, says Brinker, "maybe technology will."

The device's formation begins with a drying process called evaporation-induced self-assembly, first developed at Sandia by Brinker 20 years ago and a field of study in its own right.

The procedure creates a close-packed array of silica nanopores designed to accommodate the carbonic anhydrase enzyme and keep it stable. This is done in several steps. First, the array, which may be 100 nanometers long, is treated with a technique called atomic layer deposition to make the nanopore surface water-averse or hydrophobic. This is followed by an oxygen plasma treatment that overlays the water-averse surface to make the nanopores water-loving or hydrophilic, but only to a depth of 18 nanometers. A solution of the enzyme and water spontaneously fill up and are stabilized within the water-loving portion of the nanopores. This creates membranes of water 18 nanometers thick, with a carbonic anhydrase concentration 10 times greater than aqueous solutions made to date.

The solution, at home in its water-loving sleeve, is stable. But because of the enzyme's ability to rapidly and selectively dissolve carbon dioxide, the catalytic membrane has the capability to capture the overwhelming majority of carbon dioxide molecules that brush up against it from a rising cloud of coal smoke. The hooked molecules then pass rapidly through the membranes, driven solely by a naturally occurring pressure gradient caused by the large number of carbon dioxide molecules on one side of the membrane and their comparative absence on the other. The chemical process turns the gas briefly into carbonic acid and then bicarbonate before exiting immediately downstream as carbon dioxide gas. The gas can be harvested with 99 percent purity—so pure that it could be used by oil companies for resource extraction. Other molecules pass by the membrane's surface undisturbed. The enzyme is reusable, and because the water serves as a medium rather than an actor, does not need replacement.

The nanopores dry out over long periods of time due to evaporation. This will be checked by water vapor rising from lower water baths already installed in power plants to reduce sulfur emissions. And, enzymes damaged from use over time can easily be replaced.

Says Brinker, "The very high concentration of carbonic anhydrase, along with the thinness of the water channel, result in very high carbon dioxide flux through the membrane. The greater the carbonic anhydrase concentration, the greater the flux. The thinner the membrane, the greater the flux."

The membrane's arrangement in a generating station's flue would be like that of a catalytic converter in a car, suggests Brinker. The membranes would sit on the inner surface of a tube arranged like a honeycomb. The flue gas would flow through the membrane-embedded tube, with a carbon dioxide-free gas stream on the outside of the tubes. Varying the tube length and diameter would optimize the carbon dioxide extraction process.

"Energy companies and oil and gas utilities have expressed interest in optimizing the gas filters for specific conditions," says Susan Rempe, Sandia researcher and co-author, who suggested and developed the idea of inserting carbonic anhydrase into the water solution to improve the speed by which carbon dioxide could be taken up and released from the membrane. "The enzyme can catalyze the dissolution of a million carbon dioxide molecules per second, vastly improving the speed of the process. With optimization by industry, the memzyme could make electricity production cheap and green," she says.

The separation process could increase the amount of fuel obtained by enhanced oil recovery using carbon dioxide injected into existing reservoirs.

A slightly different enzyme, used in the same process, can convert methane—an even more potent greenhouse gas—to the more soluble methanol for removal, she says.

Prior cleansing by industrial scrubbers means that the rising smoke will be clean enough not to significantly impair membrane efficiency, says University of New Mexico professor and paper co-author Ying-Bing Jiang, who originated and developed the idea of using watery membranes based on the human body's processes to separate out carbon dioxide. The membranes have operated efficiently in laboratory settings for months.

The procedure also could sequester carbon dioxide on a spacecraft, the authors mention, because the membranes operate at ambient temperatures and are driven solely by chemical gradients.

April 18, 2018

Eight of the USA's 10 most-polluted cities, in terms of ozone pollution, are in California, according to the American Lung Association's annual "State of the Air" report, released Wednesday.

The Los Angeles/Long Beach area took the dubious distinction of being the nation's most ozone-polluted city as it has for nearly the entire 19-year history of the report.

Overall, the report said about 133 million Americans — more than four of 10 — live with unhealthful levels of air pollution, placing them at risk for premature death and other serious health effects such as lung cancer, asthma attacks, cardiovascular damage and developmental and reproductive harm.

"We still have a lot to do in this country to clean up air pollution," said Lyndsay Moseley Alexander, director of the Association's Healthy Air Campaign.

The report looked at pollution levels from 2014 to 2016. Ozone pollution was worse overall in this report than it was in last year's report.

Bakersfield, Calif., was in second place for ozone pollution. Other California cities on the list include Fresno, Sacramento and San Diego. The only non-California metro areas in the top 10 list were Phoenix and New York City.

Of the 10 most-polluted cities, seven cities did worse in this year's report, including Los Angeles and the New York City metro area.

Bakersfield took the top spot in a list of cities with another variety of air pollution — small particulate matter, aka soot. Increased heat, changes in climate patterns, drought and wildfires — many related to climate change — contributed to the high number of days with unhealthy particulate matter.

Since California is known for its strict environmental regulations, why are so many cities from the state typically on this list? It's because the state would be far worse off without its strict laws on tailpipe pollution and eliminating coal-fired power plants. California has done more than any other state to counteract air pollution, the Lung Association said.

With this report, the Lung Association also calls out Congress and the Environmental Protection Agency (EPA) for ongoing threats to the nation’s air quality, including steps to roll back or weaken enforcement of the Clean Air Act.

EPA administrator Scott Pruitt has taken many steps to roll back or put in loopholes to the Clean Air Act, Alexander said, adding that "the association is concerned and we will continue to fight for healthy air," she said.

Some good news in the report was that particulate pollution "generally continued to improve in 2014-16," the report said. This was true for both short-term particulate pollution and for year-round particulate pollution.

For year-round particle pollution, Fairbanks, Alaska, was the most-polluted city.

The Lung Association also lists the nation's cleanest cities, meaning ones that experience no high ozone or high particulate pollution days.

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