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From: Megan Elwood Madden

Subject: Charge Balancing

I'm just getting started using GWB and I'm a bit confused about the "balance" command in React. I'm trying to model the reaction of basalt with a dilute water solution with a fixed CO2 fugacity. The input i've used is below. If I simply set the initial pH, charge balance with Cl-, and fix the CO2 fugacity I get a charge balance warning from the model and indeed when I check the charge equivalents from the resulting fluid, I have a negatively charged fluid. However if I fix the pH along with all the other factors above, the resulting fluid is charge balanced. In natural systems though, it doesn't make sense to me to fix the pH, it should change over the course of the reaction. What does the "balance" command actually do if the resulting fluid is not charge balanced? Is there a way to get a charge balanced fluid without fixing the pH? Here's my input variables: Basalt-Dilute Cl Brine fix CO2

itmax0 5000

itmax 5000

chdir “e:Program FilesGwbGtdata�

T=25

balance Cl-

Na+= 10e-12 molal

Cl-= 10e-12 molal

K+= 10e-12 molal

Ca++= 10e-12 molal

Al+++= 10e-12 molal

SiO2(aq)= 10e-12 molal

Fe++= 10e-12 molal

Mg++= 10e-12 molal

O2(aq)= 10e-12 molal

pH=5.0

react .1295 mole Diopside

react .00136 mole K-feldspar

react .04816 mole Albite

react .11586 mole Anorthite

react .02851 mole Enstatite

react .08565 mole Ferrosilite

react .01738 mole Magnetite

react 0.02215 mole Fayalite

react 0.02215 mole Forsterite

HCO3-= 10e-12 molal

swap CO2(g) for HCO3-

f CO2(g)=.006

suppress Annite, Tremolite, Diopside, Andradite, Hedenbergite, Wollastonite, Antigorite,

suppress Greenalite, Albite, K-feldspar, “Albite low“, “Maximum Microcline�

suppress Minnesotaite, Phengite, Phlogopite, Epidote, Epidote-ord, Muscovite

fix f CO2(g)

 

From: Craig Bethke

Subject: Re: Charge Balancing

Unless you set "balance = off", React should enforce charge balance on your chemical system initially and over the course of any reaction path. It does this by adjusting the concentration of the basis entry you specify (Cl-, by default). The adjustment is necessary only for the initial system, unless you have fixed the activity ofa charged species, or the pH. When I ran your script using the latest release, I received no warnings and found no problems with charge balance. Could you please be more specific about what release you are running, and what warnings you get?

 

From: Megan Elwood Madden

Subject: RE: Charge Balancing

Here's a bit more information:

I'm running Release 3.0 . I just copied and pasted directly from my email into react and got this statement after the completed reaction path: "Warning, system has a charge imbalance of 1.047e-008 faradays". When I examined the charge equivalent concentrations of the equilibrium solutes, there was a significant imbalance in charged species, with about half of the anions (Cl- and HCO3-) unbalanced. It seems a bit odd that I am having the opposite symptoms Dr. Bethke described, where it is charge balancing only when the pH is fixed and not when the pH is allowed to vary.

 

From: Craig Bethke

Subject: RE: Charge Balancing

First of all, I wouldn't worry about that warning message. A Faraday is a mole of electrical charge. The message is set to appear whenever the charge imbalance is 10^-8 Faradays. In your case, it's 1.018^-8, which is still a very small number. You are getting this message because the solute concentrations in your initial solution are very small (have you thought of using the average composition of rainwater for your initial system?). Second, I believe that your system is charge balanced. Here're the predominant species from your output:

Aqueous species

molality

mg/kg sol'n

act. coef.

log act.

 

CO2(aq)

0.0002117

9.317

1.0000

-3.6742

 

H+

1.004e-005

0.01012

0.9963

-5.0000

 

HCO3-

9.149e-006

0.5582

0.9963

-5.0402

 

Cl-

8.872e-007

0.03145

0.9963

-6.0536

 

OH-

1.035e-009

1.760e-005

0.9963

-8.9868

Note that the negative charge of the anions (HCO3-, Cl-, OH-) is compensated by the positive charge of the hydrogen ions (H+). And remember that you can't figure charge balance by adding concentrations in mg/kg – you have to use molal units.

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