# PhreeqcUsers Discussion Forum

## Beginners => PHREEQC basics => Topic started by: snjaykdubey on May 12, 2015, 12:17:14 PM

Title: Input Definition for Ex-changer
Post by: snjaykdubey on May 12, 2015, 12:17:14 PM
Dear sir,
I have following basic doubts:

If data is given in terms of equivalent percent for  the following elements:
Nna, Nk, Nca and Nmg (sodium,potassium,magnesium and calcium ) and CEC is given as 787 meq/kg and
Dry Density is 1770 kg/m3
then
What will be the input units for Ex-changer keywords in phreeqc?

NaX = mmol/cm3 of Soil or mmol/kg of water.?

Since i was able to calculate the exchange composition in terms of mmol/cm3 of soil.Is it correct?

regards,
Sanjay

Title: Re: Input Definition for Ex-changer
Post by: JG on May 12, 2015, 01:41:41 PM
Hi.

The composition in the EXCHANGE block is given in moles. Think of it as "moles in the system", the system being the solution + the exchanger(s) (+ surfaces + solid + gas phases).

What you need to do here is define a soil/solution ratio (x kg of soil in y kg of water), and input the quantities (moles) of NaX, CaX2, ... according to that ratio.

In short:
equivalent fraction => eq cation/kg soil => mol/kg soil (divide by cation charge) => mol in system (multiply by qty of soil)
Title: Re: Input Definition for Ex-changer
Post by: snjaykdubey on May 12, 2015, 03:09:15 PM

Hey,

I converted till Mol/Kg of Soil but Denominator part is still in Kg of Soil by following way:

meq/kg of soil => (Na+)gm/Kg of Soil => Mol/Kg of Soil => From here i cant understand the step
in order to get Mol/kg of water..

I have porosity 0.43 do we have to multiply it Or this information is useful for calculating mole of exchange?

i hope if u can explain in detail.

regards,
Sanjay

Title: Re: Input Definition for Ex-changer
Post by: dlparkhurst on May 12, 2015, 03:34:01 PM
You need to use porosity and rock density to calculate per liter of water, something like

eq/kgs  * 2.7 kg soil/L soil * (1-0.43) /0.43 (L soil/L water)
Title: Re: Input Definition for Ex-changer
Post by: snjaykdubey on May 12, 2015, 03:57:42 PM
Thanks Sir
Title: Re: Input Definition for Ex-changer
Post by: Angie on December 17, 2020, 03:26:42 PM
Hello!
If instead of porosity I have water retention (%) in the information sheet of the resin. Should I proceed the same way?

Thanks!

Angie
Title: Re: Input Definition for Ex-changer
Post by: dlparkhurst on December 17, 2020, 04:46:36 PM
That sounds like the amount of water remaining when the resin is drained, but I don't know for sure.

I don't think you need to be too accurate. The most dense packing of equal-sized spheres has 26% porosity. So, a guess is 30 to 40 percent porosity.