After centuries of releasing carbon dioxide into the atmosphere, humans are now looking for ways to remove it (see Physics Today, June 2022, page 26). One obstacle is that despite the climate changes being driven by the rising concentration of CO2 in the atmosphere, that concentration is still relatively dilute, at about 420 ppm. Diffusion naturally moves molecules from high-concentration areas to low-concentration ones, but extracting CO2 from air requires a reversal of that process. Separating an already dilute substance from air thus poses thermodynamic and kinetic challenges that can predispose the task to being energy-intensive and slow.
To reverse the typical diffusion process, cells and other biological systems use an efficient trick to move molecules against a concentration gradient. Cells, for example, pump out hydrogen ions to maintain a specific pH. The trick, known as active transport, works in a manner akin to a waterwheel:...
