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Kd value calculations

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I am using React in GWB to try and calculate Kd values based on the sorbed component of an element and the dissolved "in fluid" component, and am a bit confused regarding the React output file (attached here).

Essentially, I am curious as to why the sorbed (mg/kg)/in fluid (mg/kg) don't match up with the Kd (L/kg) values at all in the output for each reaction step? I realize that there is a slight conversion of the fluid concentration to mg/L from mg/kg for the Kd value calculation, but it shouldn't be as large as the differences I'm seeing.

Also, while I realize the Kd values assume the mineral content of the system is fully defined, I was wondering how it takes into account inert volume? In my output, it looks like the inert volume doesn't contribute to the mineral mass, so I'm not sure how it affects the Kd value calculations (if at all)?

Any help or insight would be appreciated.



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  • 3 weeks later...

Hello Dietrimj,

Thank you for attaching the file. The sorbed mg/kg unit is mg of sorbed species per kg of fluid, not kg of minerals. The GWB assumes a density of 2.65 g/cm3 for inert minerals. You would need to account for the volume of the inert mineral in the system if you are doing a hand calculation. I did a simple back of the envelope calculation for Kd and got the same value given in the output file.

To solve for Kd,  divide the sorbed concentration (moles sorbed/g solid) by the solute concentration (moles / cm3 fluid). You can find the moles sorbed in your React_output file for each basis species, fluid volume, moles of dissolved species in fluid, mineral mass, and inert mineral volume. You can compute the total mineral mass by adding the mass of your inert mineral to the mineral mass in the existing system. Using Ca++ as an example at the step 0 output, it would look like this:

inert mass = 2.65 g/cm3 * 0.0357 cm3 = 2.5546 g
total mass = 2.5546 g + 0.1522 g = 2.7068 g

S = 0.165 moles Ca / 2.7068 g = 0.006096 moles Ca/g
C = 0.00476 moles Ca / 999 cm3 = 4.765 e -6 moles Ca/ cm3
Kd = (0.006096 moles Ca/g) / (4.765 e -6 moles Ca/ cm3) = 1279.33 cm3/g or L/kg.

The Kd value calculated by the GWB is 1.28 e+03 L/kg.

Hope this helps,
Jia Wang

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