DURABLE SEQUESTRATION OF CO2 BY CARBONATION

Carbon capture, use, and storage (CCUS) is an emissions reduction technology designed to capture carbon dioxide (CO₂) from fossil fuel combustion before it enters the atmosphere. Direct air capture (DAC) is a carbon dioxide removal (CDR) strategy that seeks to stabilize climate by physically removing CO₂ already in the atmosphere to lower the concentration. Both CCUS and DAC produce quantities of CO₂ gas that must be sequestered from the atmosphere for centuries or longer. This can be done on land, in the oceans, or in geologic formations. Geologic sequestration options include placing CO₂ in depleted oil and gas reservoirs, unmineable coal seams, mafic volcanic rock, and deep saline aquifers. Each has advantages and disadvantages, but all require monitoring, verification, and accounting (MVA) to ensure that the CO₂ remains in place and is not migrating back to the surface.

When dissolved in water, carbon dioxide forms carbonic acid (H₂O + CO₂ → H₂CO₃). The hydrogen cations are readily replaced with metals such as Ca, Mg, or Fe through a process called carbonation to form carbonate minerals such as calcite (CaCO₃), magnesite (MgCO₃) or siderite (FeCO₃). This process has occurred naturally on Earth for billions of years and permanently sequestered large volumes of CO₂ from the Earth’s primitive atmosphere. Once carbon is stored as carbonate, it requires no MVA because it is immobile.

As a geologic process, there are of course concerns that carbonation could take too long to be helpful in the current climate crisis. However, researchers in Iceland found that carbonic acid solutions injected into basalt reacted with the rock and formed substantial amounts of carbonate in as little as two years. Other researchers are investigating the use of microbes to potentially speed up carbonation in both surface and downhole environments. In depleted oil fields, carbonates precipitating in subsurface pore space will immobilize CO₂ while occluding the pores. The minerals will also block fluid movement and halt fugitive emissions from abandoned wells. Carbonate is a valuable building material, and creating slabs, blocks, and bricks from captured CO₂ could provide a saleable product to help improve the economics of DAC. Converting CO₂ into solid, stable carbonate minerals offers a simple and elegant solution for carbon dioxide sequestration.