With several proposals related to the conversion of sequestered CO2 to carbonates being considered by the science community, a team of University of California at Davis researchers say their new discoveries about the transformational and crystallization of amorphous calcium carbonate will help fill in a few more pieces of the puzzle of what happens when CO2 is stored underground.
The group, lead by Alexandra Navrotsky, a professor of ceramic, earth, and environmental materials at the school, wanted to understand more about how ACC transforms with aging or heating into a less hydrated form and, with time, changes into calcite. They studied both synthetic and biogenic ACC using isothermal acid solution calorimetry and differential scanning calorimetry.
They identified that ACC changes typically follow an energetically downhill sequence:
- More metastable hydrated ACC converts into less metastable hydrated ACC.
- The less metastable hydrated ACC converts to anhydrous ACC.
- The anhydrous ACC then can go through a serious of conversions, to vaterite, then to aragonite and finally to calcite.
They note that In a given reaction sequence, not all these phases need to occur, but that each lowers the enthalpy of the system. They say the crystallization of the dehydrated amorphous material lowers the enthalpy the most. They go on to report that the structural reorganization during dehydration is exothermic and irreversible, and the initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO2 sequestration.
Navrotsky, an ACerS emeritus member, and the others have been motivated to deepen this understanding by proposals to sequestor and store CO2 underground. Implicit in some of these proposals is that the dense CO2 gas would eventually turn into solid, stable calcium carbonate.
“Calcium carbonate is the major long-term sink for atmospheric carbon dioxide. By measuring the heat liberated during these transformations, we can study the process by which carbon dioxide is trapped and transformed to stable carbonate minerals,” Navrotsky says.