CO2(aq) - brine solns, ion size params, salting effect

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From: Biniam Zerai

Subject: Info on CO2(aq)

I am trying to model the possible reaction of CO2 gas, albite, annite, siderite, calcite and dolomite in 2 molal of brine solution. I made the correction for fugacity of the gas using Duan and Sun equation of state but I am not sure what is the right way as how to correct the salting-out effect using GWB. I have been told to change the the ionic size in the thermodynamic database that contains the aqueous species, CO2(aq) to -0.5 in order to correct the effect of salinity on CO2(g) solubility. Is this the right way to do it? I did it using the above advise and observe a large difference. Any info, input or comment is highly appreciated. I am using GWB 3.2.2.

From: Craig Bethke

Subject: Re: Info on CO2(aq)

The equations used to calculate activity coefficients for electrically neutral species are given in Chapter 7 of the "Geochemical Reaction Modeling" text. The special meaning of the ion size parameter for neutral species is described in the "Thermo Datasets" appendix to the GWB Reference Manual:

The ion size parameter (ao) has special meaning for neutrally charged aqueous species in the thermo dataset. For neutral species with ao = 0, the species' activity coefficient is set to one. When ao = 1/2, the activity coefficient is calculated from the "CO2" coefficients in the data table section, according to equation 7.6 in the "Geochemical Reaction Modeling" text. When ao = 1, the logarithm of the activity coefficient is set to the product B(dot)x I, where B(dot) is given by the data tables above, and I is true ionic strength.