Tuesday, July 18, 2017

Key to Speeding Up Carbon Sequestration Discovered

Scientists at Caltech and USC identify how to encourage the chemical reaction by which carbon dioxide is locked away in the ocean.

Scanning electron microscopy image of calcite. (Credit: Adam Subhas/Caltech) Click to Enlarge.
Scientists at Caltech and USC have discovered a way to speed up the slow part of the chemical reaction that ultimately helps the earth to safely lock away, or sequester, carbon dioxide into the ocean.  Simply adding a common enzyme to the mix, the researchers have found, can make that rate-limiting part of the process go 500 times faster.

A paper about the work appears online the week of July 17 ahead of publication in the Proceedings of the National Academy of Sciences.

"While the new paper is about a basic chemical mechanism, the implication is that we might better mimic the natural process that stores carbon dioxide in the ocean," says lead author Adam Subhas, a Caltech graduate student and Resnick Sustainability Fellow.
Calcite is a mineral made of calcium, carbon, and oxygen that is more commonly known as the sedimentary precursor to limestone and marble.  In the ocean, calcite is a sediment formed from the shells of organisms, like plankton, that have died and sunk to the seafloor.  Calcium carbonate is also the material that makes up coral reefs -- the exoskeleton of the coral polyp.

As atmospheric carbon dioxide levels have risen past 400 parts per million--a symbolic benchmark for climate scientists confirming that the effects of the greenhouse gas in the atmosphere will be felt for generations to come--the surface oceans have absorbed more and more of that carbon dioxide.  This is part of a natural buffering process -- the oceans act as a major reservoir of carbon dioxide.  At the present time, they hold roughly 50 times as much of the greenhouse gas as the atmosphere.

However, there is a second, slower, buffering process that removes carbon dioxide from the atmosphere.  Carbon dioxide is an acid in seawater, just as it is in carbonated sodas (which is part of why they eat away at your tooth enamel).  The acidified surface ocean waters will eventually circulate to the deep where they can react with the dead calcium carbonate shells on the sea floor and neutralize the added carbon dioxide.  However, this process will take tens of thousands of years to complete and meanwhile, the ever-more acidic surface waters eat away at coral reefs.  But how quickly will the coral dissolve?
Studying the process with a secondary ion mass spectrometer (which analyzes the surface of a solid by bombarding it with a beam of ions) and a cavity ringdown spectrometer (which analyzes the 13C/12C ratio in solution), Subhas discovered that the slow part of the reaction is the conversion of carbon dioxide and water to carbonic acid.

"This reaction has been overlooked," Subhas says.  "The slow step is making and breaking carbon-oxygen bonds.  They don't like to break; they're stable forms."

Armed with this knowledge, the team added the enzyme carbonic anhydrase -- which helps maintain the pH balance of blood in humans and other animals -- and were able to speed up the reaction by orders of magnitude.

Read more at Key to Speeding Up Carbon Sequestration Discovered

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