PhreeqcUsers Discussion Forum
Beginners => PHREEQC basics => Topic started 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 Exchanger 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

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)

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

You need to use porosity and rock density to calculate per liter of water, something like
eq/kgs * 2.7 kg soil/L soil * (10.43) /0.43 (L soil/L water)

Thanks Sir

Hello!
If instead of porosity I have water retention (%) in the information sheet of the resin. Should I proceed the same way?
Thanks!
Angie

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 equalsized spheres has 26% porosity. So, a guess is 30 to 40 percent porosity.