A process for removing carbon dioxide from ambient air emerged during another experiment, and provides what may be a new option for direct air capture and sequestering of carbon dioxide.

The original research focus for scientists at Oak Ridge National Laboratory was to find methods to eliminate chemical contaminants, such as sulfate and chromate, from water. These negatively charged ions bind with guanidine, a nitrogenous analog of carbonic acid. The resulting crystals are easily separated from water.

Prism-like crystals containing carbonate. Credit: ORNLPrism-like crystals containing carbonate. Credit: ORNLThe researchers left an aqueous solution of guanidine open to ambient air in their laboratory. Over time they noticed that prism-like crystals formed in the solution. X-ray diffraction analysis confirmed that the crystals contained carbonate, formed when carbon dioxide and water interact.

Releasing CO2 from sorbents currently in use requires heating them to 900 degrees Celsius. This process takes considerable energy and often releases more CO2 than the sorbent captured. The guanidine sorbent released its CO2 at 80-120 degrees Celsius, requiring less energy. It also survived three complete cycles of absorption and release.

The team is conducting advanced analysis of the crystal structure using ORNL’s Spallation Neutron Source (SNS). Their goal is to understand the process of CO2 capture and release so that knowledge can inform the design of next-generation sorbents.

Much of the body of carbon capture and sequestration research focuses on strategies to reduce carbon dioxide emission. Frequently the process involves capturing the CO2 at its source. The process the ORNL researchers developed could also work as a source capture method.

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