Research ArticlePHYSICAL SCIENCES

The fate of carbon dioxide in water-rich fluids under extreme conditions

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Science Advances  12 Oct 2016:
Vol. 2, no. 10, e1601278
DOI: 10.1126/sciadv.1601278
  • Fig. 1 High-pressure and high-temperature conditions reached in experiments on dissolved carbon in supercritical water, together with the conditions simulated in the present work: Frantz (10), Caciagli and Manning (30), Martinez et al. (19), Facq et al. (31, 36), and Schmidt (11).

    The melting curve of ice is from the study by Datchi et al. (51). The shaded area shows the P-T conditions of the upper mantle (7).

  • Fig. 2 Mole percents of CO2 (orange), Embedded Image (red), Embedded Image (black), and H2CO3 (blue) as functions of simulation time in first-principles MD simulations.

    (A) Na2CO3 solution [0.9 m (molality)] at 0.2 GPa and 823 K. (B) Na2CO3 solution (0.9 m) at 11 GPa and 1000 K. (C) CO2 solution (0.9 m) at 11 GPa and 1000 K. Calculations carried out with two xc functionals: PBE (22) and PBE0 (23) are compared.

  • Fig. 3 The solvation structures of carbon species and ion pairing distances in the 0.9 m Na2CO3 solutions.

    (A) RDFs of carbon atoms (C) versus oxygen atoms of water (Ow). (B) Probability distributions of the distances between carbon atoms and sodium ions. The MD simulations on the 0.9 m Na2CO3 solutions were conducted at 0.2 GPa, 823 K and 11 GPa, 1000 K. Two xc functionals were compared: PBE and PBE0.

  • Fig. 4 Probability distributions of positions of protons hopping between Embedded Image and H2O in the Na2CO3 solution at 11 GPa and 1000 K.

    The unit is Å−2. The reaction coordinate RO−O is the distance between the two neighboring oxygen atoms, Oc and Ow, in carbonate ions and water molecules, respectively, and δ is the proton displacement ROc−HRH−Ow. Two xc functionals were compared: PBE and PBE0.

  • Table 1 Carbon species in the 0.9 m Na2CO3 and 0.9 m CO2 solutions in first-principles MD simulations.

    The concentrations are shown as mole percents of the total dissolved carbon. The mole percents of Embedded Image and Embedded Image include those of ion pairs formed by them, respectively. The experimental results (Expt.) are from the Raman data in the study by Frantz (10) (see text). N/A, not applicable.

    SolutionP (GPa)T (K)MethodCarbon species (%)
    CO2Embedded ImageEmbedded ImageH2CO3
    Na2CO30.2823PBE0.616.782.40.1
    PBE00.226.872.90.1
    Expt.N/A3268N/A
    111000PBE0.140.857.91.2
    PBE00.151.847.50.6
    CO2111000PBE0.68.279.811.4
    PBE00.01.775.023.3
  • Table 2 The ion pairing in the first solvation shells of the carbon species in the 0.9 m Na2CO3 solutions.

    The concentrations are shown as mole percents of the total dissolved carbon.

    P (GPa)T (K)MethodIon pairing species (%)
    Embedded ImageEmbedded ImageEmbedded ImageEmbedded Image
    0.2823PBE3.713.059.77.5
    PBE08.418.240.818.2
    111000PBE18.15.928.97.5
    PBE022.410.020.412.5

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/10/e1601278/DC1

    fig. S1. The vibrational density of states of the 0.9 m Na2CO3 solution at ~11 GPa and 1000 K.

    fig. S2. Probability distributions of positions of protons hopping between Formula and H2O in the Na2CO3 solution at 0.2 GPa and 823 K.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. The vibrational density of states of the 0.9 m Na2CO3 solution at ~11 GPa and 1000 K.
    • fig. S2. Probability distributions of positions of protons hopping between CO2−3 and H2O in the Na2CO3 solution at 0.2 GPa and 823 K.

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