pressure effect on molar volumes

[manofhope]

Dear Forum,

when I am running a simple decompression (see below) of a geothermal water from 200 bar to 10 bar, SI calcite rises from 0.00 to 0.16 as expected due to reduction of CO2 partial pressure. However, I don't get why the molar volumes of dissolved CaCO3, CO3^2-, Ca2+ and HCO3^- decrease during pressure decrease, as the output shows. Why is that? Probably, someone knows already without running the file.

Thank you very much!



DATABASE /usr/share/phreeqc/database/phreeqc.dat

# ======================================   
TITLE 1: Geothermal water

SOLUTION 1

    units mg/L

    temp   118
    pH     6.6
    pe     -1.15633024277113

    Na     115.8
    K      19.02
    N(-3)  0.95 as NH4+
    Ca     28.6
    Mg     4.68
    Fe     0.0225
    Mn     0.00119

    F      2.295
    Cl     77.937
    S(6)   3.042 as SO4-2
    S(-2)  5.93
    Alkalinity  301.7 as HCO3-

REACTION_PRESSURE
 200                  # Reservoir pressure

REACTION_TEMPERATURE
118

SAVE solution 99

END

# ============================================================
TITLE 2: Decompression at constant temperature

USE solution 99

REACTION_TEMPERATURE
118

REACTION_PRESSURE
10

END

[dlparkhurst]

All I can say is that the molar volumes are parameterized in phreeqc.dat and pitzer.dat with the -vm option in SOLUTION_SPECIES.

References for -vm of individual species are noted in the database files, and here is an excerpt from the comments at the bottom of phreeqc.dat:

Code: [Select]

# Temperature- and pressure-dependent volumina of aqueous species are calculated with a Redlich-
#   type equation (cf. Redlich and Meyer, Chem. Rev. 64, 221), from parameters entered with
#                        -Vm a1 a2 a3 a4 W a0 i1 i2 i3 i4
# The volume (cm3/mol) is
#    Vm(T, pb, I) = 41.84 * (a1 * 0.1 + a2 * 100 / (2600 + pb)  + a3 / (T - 228) +
#                            a4 * 1e4 / (2600 + pb) / (T - 228) - W * QBrn)
#                   + z^2 / 2 * Av * f(I^0.5)
#                   + (i1 + i2 / (T - 228) + i3 * (T - 228)) * I^i4
#   Volumina at I = 0 are obtained using supcrt92 formulas (Johnson et al., 1992, CG 18, 899).
#   41.84 transforms cal/bar/mol into cm3/mol.
#   pb is pressure in bar.
#   W * QBrn is the energy of solvation, calculated from W and the pressure dependence of the Born equation,
#     W is fitted on measured solution densities.
#   z is charge of the solute species.
#   Av is the Debye-Hückel limiting slope (DH_AV in PHREEQC basic).
#   a0 is the ion-size parameter in the extended Debye-Hückel equation:
#     f(I^0.5) = I^0.5 / (1 + a0 * DH_B * I^0.5),
#     a0 = -gamma x for cations, = 0 for anions.
# For details, consult ref. 1.
#
# ref. 1: Appelo, Parkhurst and Post, 2014. Geochim. Cosmochim. Acta 125, 49–67.
# ref. 2: Procedures from ref. 1 using data compiled by Laliberté, 2009, J. Chem. Eng. Data 54, 1725.
# ref. 3: Appelo, 2017, Cem. Concr. Res. 101, 102-113.
#
# =============================================================================================
# It remains the responsibility of the user to check the calculated results, for example with
#   measured solubilities as a function of (P, T).