The technology that allows submariners to breathe underwater could someday allow the rest of us to breathe cooler air. Researchers have found a way to suck planet-warming carbon dioxide (CO2) from industrial smokestacks using a chemical technique similar to one scuba divers and submarines use to “rebreathe” CO2-rich exhalations.
The team’s technique “has tremendous potential,” says Kristin Bowman-James, a chemist at the University of Kansas in Lawrence.
The advance relies on a class of organic chemicals called bis(imino guanidines), or BIGs. These chemicals were first discovered more than a century ago, but researchers recently found that they’re really good at binding to negatively charged ions, says Radu Custelcean, a chemist at Oak Ridge National Laboratory in Tennessee. He and his colleagues harness that binding ability to capture CO2.
First, the team dissolves a particular BIG in water, where the substance helps break down H2O molecules into positively charged protons (H+) and negatively charged hydroxide (OH–) ions. The BIG molecules snatch free-floating protons and take on a positive charge. Those BIG ions then react with negatively charged bicarbonate (HCO3–) ions that form when CO2-rich gas bubbles through the solution, Custelcean says. Because the resulting substance doesn’t readily dissolve, it crystallizes and can be separated from the solution.
Those crystals can then be heated to drive off CO2 so it can be collected and stored, rather than emitted to the atmosphere, Custelcean says. The team’s lab tests suggest that process can occur at the relatively low temperature of 120°C. So, the researchers report today in Chem, capturing and recovering CO2 from industrial exhaust using their technique takes about 24% less energy than a process commonly used in smokestack “scrubbers.” Once CO2 has been driven from the crystals, the BIG can be redissolved in the solution, making it available to capture even more CO2.
The particular BIG used by Custelcean’s team sits at what Amar Flood, an organic chemist at Indiana University in Bloomington who was not involved with the work, calls a “magic sweet spot.” Its affinity for bicarbonate ions allows the crystal-forming reaction to readily occur, but the weak hydrogen bonding within the crystal also makes it relatively easy to recover the CO2.
There’s a big difference between demonstrating something in a lab and using the method on a larger scale, of course. For one thing, immense amounts of BIG would be needed to perform the carbon-capture process on an industrial scale. During 2017, for example, coal-fired power plants alone in the United States emitted more than 1.2 billion metric tons of CO2. Although a lot of BIG would be needed to outfit even a single smokestack scrubber, Custelcean says the material is reusable and inexpensive, at about $3 per kilogram.