Findings from a new study from Kyushu University’s International Institute for Carbon-Neutral Energy Research (I2CNER) showed that trapped carbon dioxide can be converted into harmless minerals.
The rocks below the earth’s surface are highly porous, and trapping involves injecting CO2 into the pores after collecting it from its emission source. Although CO2 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 CO2 storage, locking CO2 into a completely secure form that can’t be re-emitted,” explained first study author Jihui Jia. “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.”
The team conducted a first round of simulations to predict what happens when carbon dioxide collides with a cleaved quartz surface—quartz (SiO2) abundant in the earth’s crust. When the simulation trajectories were played back, the CO2 molecules were observed to bend from their linear O=C=O shape to form trigonal CO3 units bonded with the quartz.
In the next series of simulations, H2O molecules were added to mimic the “formation water” that is often present beneath oil and gas drilling sites. The H2O molecules spontaneously attacked the reactive CO3 structures, breaking the Si–O bonds to produce carbonate ions. 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 CO2 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 believed. 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.
According to lead study author Takeshi Tsuji. “For quartz to capture CO2, 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.”