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Albite Dissolution in Soils as a Function of pCO2


ats5482
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Hi,

I'm trying to reproduce Fig. 3 from Ibarra et al. (2019; attached). However, I'm getting a component bicarbonate concentration that is too high. In the text, they describe how they model the weathering of soil containing the primary mineral Plagioclase Feldspar (An20Al80), which forms secondary minerals such as Halloysite and Kaolinite. The system is in equilibrium with atmospheric CO2 at a fixed fugacity and the initial fluid composition is dilute rainwater.

For simplicity, I am using 100% Albite as the primary mineral, and Kaolinite as the secondary mineral with GWB's default thermo database (i.e., I am trying to approximate the orange line from panel B). My React script is as follows:

4 kg free H2O
T = 25

swap Quartz for SiO2(aq)
swap CO2(g) for HCO3-
P CO2(g) = 0.0002 bar
fix fugacity CO2(g)

# rainwater 
SiO2(aq) = 9e-7 molal
Al+++ = 2e-7 molal
Ca++ = 9e-5 molal
K+ = 1.6e-5 molal
Mg++ = 6.8e-5 molal
Na+ = 4.2e-4 molal
Cl- = 1e-7 molal
NO3- = 5.1e-7 molal
SO4-- = 4.6e-4 molal

balance H+

react 10 g Albite
react 30 g Quartz

suppress all
unsuppress Kaolinite
unsuppress Quartz

This script gives an HCO3- concentration of ~8,000 umol/L (attached), but it should be ~1,200 umol/L (for pCO2 of 200 ppm and T of 25 C). How can I decrease the concentration of bicarbonate? I am also confused about why Quartz and Kaolinite seemingly become saturated instantaneously (attached).

Note: the amount of water, Qtz, and Al are to give a water/rock ratio of 100

Thanks!

-- Adam

logQ-K.png

HCO3-_umolar.png

Ibarra+2019 Fig 3.png

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Hello,

I am not sure if I understand some of the commands in your script. I am not sure why Quartz is swapped in for SiO2(aq) at the top of your input file. Was this intentional? When you later set SiO2(aq) = 9e-7 molal, you effectively unswapped your mineral. Again, you titrate in some mass of quartz but your description of the problem and the caption of the figure does not mention quartz being titrated. Could you double check the conditions for your initial system?

A general tip when recreating these types of results is to try to use as much of the same conditions as possible. In general, you should check the equilibrium constants for the reactions in your dataset against what they use. You would do best if you are able to use their exact mineral reaction instead of approximating with an alternative dataset. Perhaps the authors provided additional information regarding how they performed the calculation in supplementary materials or appendices. If you believe that your set up is correct, you might inquire one of the paper's original authors for the input file to compare.

Best regards,
Jia Wang
Aqueous Solutions LLC

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