In Situ Natural Abundance 17 O and 25 Mg NMR Investigation of Aqueous Mg(OH) 2 Dissolution in the Presence of Supercritical CO 2 .

We report an in situ high-pressure NMR capability that permits natural abundance ¹⁷ O and ²⁵ Mg NMR characterization of dissolved species in aqueous solution and in the presence of supercritical CO ₂ fluid (scCO ₂ ). The dissolution of Mg(OH) ₂ (brucite) in a multiphase water/scCO ₂ fluid at 90 atm pressure and 50 °C was studied in situ, with relevance to geological carbon sequestration. ¹⁷ O NMR spectra allowed identification and distinction of various fluid species including dissolved CO ₂ in the H ₂ O-rich phase, scCO ₂ , aqueous H ₂ O, and HCO ₃ ^- . The widely separated spectral peaks for various species can all be observed both dynamically and quantitatively at concentrations as low as 20 mM. Measurement of the concentrations of these individual species also allows an in situ estimate of the hydrogen ion concentration, or pC _H ⁺ values, of the reacting solutions. The concentration of Mg ²⁺ can be observed by natural abundance ²⁵ Mg NMR at a concentration as low as 10 mM. Quantum chemistry calculations of the NMR chemical shifts on cluster models aided in the interpretation of the experimental results. Evidence for the formation of polymeric Mg ²⁺ clusters at high concentrations in the H ₂ O-rich phase, a possible critical step needed for magnesium carbonate formation, was found.