Effect of secondary phase formation on the carbonation of olivine

Our plain-English summary of Effect of secondary phase formation on the carbonation of olivine, by H.E. King, O. Plümper, A. Putnis (Environmental Science & Technology, 2010).

The study

• Authors: H.E. King, O. Plümper, A. Putnis
• Journal: Environmental Science & Technology
• Published: 2010;44(16):6503-9
• Cited by: 26 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Large-scale olivine carbonation has been proposed as a potential method for sequestering CO(2) emissions. For in situ carbonation techniques, understanding the relationship between the formation of carbonate and other phases is important to predict the impact of possible passivating layers on the reaction. Therefore, we have conducted reactions of olivine with carbonated saline solutions in unstirred batch reactors. Altering the reaction conditions changed the Mg-carbonate morphology. We propose that this corresponded to changes in the ability of the system to precipitate hydromagnesite or magnesite. During high-temperature reactions (200 degrees C), an amorphous silica-enriched phase was precipitated that was transformed to lizardite as the reaction progressed. Hematite was also precipitated in the initial stages of these reactions but dissolved as the reaction proceeded. Comparison of the experimental observations with reaction models indicates that the reactions are governed by the interfacial fluid composition. The presence of a new Mg-silicate phase and the formation of secondary products at the olivine surface are likely to limit the extent of olivine to carbonate conversion.

In plain English

This was a laboratory chemistry study of a mineral, not a study of people or of health effects. The authors reacted forsteritic olivine, a magnesium-rich mineral, with salty bicarbonate solutions in unstirred vessels at 80 to 200 degrees C. The aim was to understand how olivine might be used to store carbon dioxide as magnesium carbonate. Magnesium carbonate formed in all experiments above 120 degrees C. The shape of its crystals changed with temperature and with salt concentration, but not when the air above the solution was replaced with carbon dioxide gas. At 200 degrees C, hematite and an amorphous silica-enriched material also formed on the olivine. Lizardite, a serpentine mineral, formed as the reaction progressed. After long experiments its crystals were curled into scroll-like shapes or were needle-like. The authors remarked that scrolling is more commonly associated with chrysotile. Hematite was no longer found after long experiments. A computer model predicted equilibrium within a month, yet olivine remained after two months. The authors suggested that new phases forming on the olivine surface may slow carbonation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment (hydrothermal reactions in unstirred batch reactors), compared with computer reaction modeling
• Who was studied: No people or animals. Forsteritic olivine (Mg1.8Fe0.2SiO4) was used as single crystals and as powders. Work was done at the University of Münster, Germany, with some analysis at the University of Oslo, Norway.
• Exposure: Olivine was reacted with carbonated saline solutions (based on 0.64 M NaHCO3 + 1 M NaCl) at 80 to 200 degrees C. NaCl concentration was varied between 0.5 and 1.5 M, and some runs were done under CO2 gas instead of air. This is not an asbestos exposure study.
• Main results:
  • Magnesium carbonate formed in every experiment above 120 degrees C.
  • Carbonate crystal shape changed with temperature and ionic strength, but did not change when the headspace gas was switched from air to CO2.
  • At high temperature (200 degrees C), the products included hematite and an amorphous silica-enriched phase containing nanocrystalline lizardite.
  • After long experiments, lizardite crystals were platy with curled, scroll-like edges or were needle-like.
  • The authors remarked that scrolling is more commonly associated with the serpentine mineral chrysotile.
  • No iron oxide (hematite) was identified after long-duration experiments.
  • In the computer model, serpentine (modeled using chrysotile data) was predicted to be the most stable phase.
  • The model predicted equilibrium within a month, but unreacted olivine remained after two months. The authors said this suggests the precipitated phases on the olivine surface may slow the reaction.
• The authors' conclusion: The authors stated that their comparison of model predictions, solution analyses and precipitated phases indicates the reactions are controlled by the fluid at the olivine surface. They suggested, from the slower-than-modeled olivine dissolution, that the precipitation of new phases slows the carbonation process.
• Limitations the authors state:
  • The thermodynamic database used for the modeling does not use Pitzer parameters, which are more accurate at higher ionic strengths.
  • Chrysotile data were used in place of lizardite in the simulations because thermodynamic data for lizardite were absent.
  • After most experiments, the silica-enriched phase was too low in concentration to detect with XRD or Raman spectroscopy.
  • At 160 degrees C, the other precipitated phases were too fine-grained to identify.

Topics

Medical terms in this study, explained in our Lexicon: Olivine, Lizardite, Hematite, Silicate.

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