kinetic exchange

[zsszabo86]

Hi!

I am trying to fit my experimental solution composition data on phreeqc modeled values. The data originate from batch experiments of a Na-montmorillonite standard. The measured and calculated data get any close to each other only if, besides kinetically control the dissolution reactions, I incorporate EXCHANGE block. However, the observed exponential curves for example for Na cannot be described, the values just jump to their maximum in the first step. I assume it is because the exchange reaction is not controlled kinetically in the model, even though the mineral dissolution precipitation is.

I found connected example:
http://www.hydrochemistry.eu/exmpls/chro_kin.html
http://www.hydrochemistry.eu/exmpls/chro_kin.phr

BUT:
I am not sure if I understand everything clearly, for example in the code, why formula is Na 1.0? all of them 1.0. and why m0 0.55e-3?
I actually would like to control the amount of exchanger by the amount of Na-montmorillonite. is there a way to do that with kinetic solution?
I know that it would look like this in EXCHANGE block:
NaX  Na-Montmorillonite kinetic_reactant 0.285
but i cannot use it here with kinetics.

Thanks in advance for any help

[dlparkhurst]

The kinetic exchange example uses a tiny amount of exchanger in EXCHANGE to determine the equilibrium exchange distribution. It then takes each cation, Na for example, and calculates what the equilibrium would be for the desired amount of exchanger 0.0011 exchange sites, which is 1e7 times the tiny amount of exchanger actually defined. The amount kinetically exchanged is equal to the amount of kinetic reactant, initially for Na it is 0.55e-3. The rate of the kinetic exchange reaction is proportional to the difference between the calculated equilibrium exchange (derived from the EXCHANGEr) and the current amount exchanged from KINETICS.

I am not sure how to make it proportional to Na Montmorillonite, but the same approach may work. You can do the equilibrium calculation with a tiny amount of exchanger as you have stated (NaX  Na-Montmorillonite kinetic_reactant 0.285e-10). The kinetic reactions should adjust to equilibrium as indicated by the EXCHANGEr.