CO2 Mineralization in Geologically Abundant Rocks for Storing Carbon

To prevent the worst effects of climate change, humanity has to change when it comes to lowering carbon emissions. If the world is to adhere to the IPCC’s minimum target of keeping global temperature increases lower than 1.5 °C, every possible path for CO₂ remediation must be investigated.

Geological trapping could have a huge role to play here. Earth’s underground rocks and sediments provide a huge potential space for storing carbon for a long period of time. To establish this, a new computational study from a Japanese-led international group at Kyushu University reveals how trapped CO₂ can be transformed into harmless minerals.

The rocks underneath the earth’s surface are extremely porous, and trapping requires injecting CO₂ into the pores after gathering it from its source of emission. Although CO₂ is typically considered very stable to react chemically with rock, it can bind firmly to the surface by physical adsorption. Ultimately it dissolves in water, forming carbonic acid, which can react with aqueous metals to make 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. 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.

Jihui Jia, Study First Author, International Institute for Carbon-Neutral Energy Research (I²CNER), Kyushu University.

As stated in The Journal of Physical Chemistry C, simulations were originally run to predict what occurs when CO₂ bumps into a cleaved quartz surface—quartz (SiO₂) being plentiful in the earth’s crust. When the simulation trajectories were played back, the CO₂ molecules were observed to be bending from their linear O=C=O shape to develop trigonal CO₃ units bonded with the quartz.

In the second round of simulations, H₂O molecules were incorporated to imitate the “formation water” that is frequently present underneath oil and gas drilling areas. Interestingly, the H₂O molecules instinctively attacked the reactive CO₃ structures, breaking the Si-O bonds to create carbonate ions. Similar to carbonic acid, carbonate ions can react with dissolved metal cations (such as Mg₂+, Ca₂+, and Fe₂+) to bind carbon into mineral form forever.

Together, the simulations reveal that the two steps of CO₂ mineralization—carbonation (binding to rock) and hydrolysis (reacting with water)—are promising. Furthermore, free carbonate ions can be created by hydrolysis, not only by dissociation of carbonic acid as was once assumed. These understandings depended on an advanced form of molecular dynamics that models not only the physical collisions between atoms, but also electron transfer, the core of chemistry.

Our results suggest some ways to improve geological trapping. 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.

Takeshi Tsuji, Study Lead Author, I²CNER, Kyushu University.

Confirmation is increasing that captured CO₂ can mineralize a lot faster than formerly believed. While this is stimulating, the Kyushu paper emphasizes how multifaceted and delicate the chemistry can be. For the time being, the team suggests more studies on other abundant rocks, for example, basalt, to map out the role that geochemical trapping can play in the greatest technical challenge civilization is faced with.