Humanity needs to improve when it comes to reducing carbon emissions to prevent the worst effects of climate change. If the world is to meet the IPCC's minimum target of keeping global temperature increases below 1.5 °C, every possible avenue for CO 2 remediation must be explored.

Geological trapping can play a major role here. Our planet's underground rocks and sediments offer a vast potential space for long-term carbon storage. To support this, a recent computational study from a Japanese-led international group at Kyushu University shows how trapped carbon dioxide can be converted into harmless minerals.

The rocks beneath the earth's surface are highly porous, and trapping involves injecting CO 2 into the pores after collecting it from its emission source. Although CO 2 is usually considered too stable to react chemically with rock, it can bind tightly to the surface by physical adsorption. Eventually it dissolves in water, forming carbonic acid, which can react with aqueous metals to form carbonate minerals.

"Mineralization is the most stable method of long-term CO 2 storage, locking CO 2 into a completely secure form that can't be re-emitted," explains Jihui Jia of the International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, first author of the study. "This was once thought to take thousands of years, but that view is rapidly changing. The chemical reactions are not fully understood because they're so hard to reproduce in the lab. This is where modeling comes in."

As reported in The Journal of Physical Chemistry C, simulations were initially run to predict what happens when carbon dioxide collides with a cleaved quartz surface -- quartz (SiO 2 ) being abundant in the earth's crust. When the simulation trajectories were played back, the CO 2 molecules were seen bending from their linear O=C=O shape to form trigonal CO 3 units bonded with the quartz.

In a second round of simulations, H 2 O molecules were added to mimic the "formation water" that is often present beneath oil and gas drilling sites. Intriguingly, the H 2 O molecules spontaneously attacked the reactive CO 3 structures, breaking the Si-O bonds to produce carbonate ions. Just like carbonic acid, carbonate ions can react with dissolved metal cations (such as Mg2+, Ca2+, and Fe2+) to bind carbon permanently into mineral form.

Together, the simulations show that both steps of CO 2 mineralization -- carbonation (binding to rock) and hydrolysis (reacting with water) -- are favorable. Moreover, free carbonate ions can be made by hydrolysis, not just by dissociation of carbonic acid as was once assumed. These insights relied on a sophisticated form of molecular dynamics that models not just the physical collisions between atoms, but electron transfer, the essence of chemistry.

"Our results suggest some ways to improve geological trapping," says study lead author Takeshi Tsuji. "For quartz to capture CO 2 , it must be a cleaved surface, so the silicon and oxygen atoms have reactive 'dangling' bonds. In real life, however, the surface might be protected by hydrogen bonding and cations, which would prevent mineralization. We need a way to strip off those cations or dehydrogenate the surface."