PhreeqcUsers Discussion Forum

Processes => Reactive transport modelling => Topic started by: peterwadeuk on June 26, 2019, 04:04:19 PM

Title: Where is the phosphate coming from?
Post by: peterwadeuk on June 26, 2019, 04:04:19 PM
Greetings good people,

I am attempting to simulate a soil column which experiences an input of pure water and which emits an effluent.

The full length of the column is ten cells. The first nine are composed of soil, the last cell is effluent exposed to atmospheric concentrations of carbon dioxide.

The first five cells contain basaltic minerals of differing concentrations. These weather according to kinetics and rate laws.

The soil cells contain an inorganic exchanger and an organic exchanger, the latter simulated by the SURFACE function and the T_H.dat database.

Since the input into the column consists of pure water, I would expect phosphate ex apatite to be washed through the column, and the apatite in at least the first cell continuously dissolved, since the only source of phosphate in the system is apatite.

Instead, I am getting apatite initially dissolving, and then remaining at supersaturated state in all five cells throughout the simulation.

If I comment out the description of the organic carbon in the system as SURFACE entities, I seem to have apatite dissolving.

Is there some feature of the SURFACE function that I am misunderstanding?

All the best,
Peter
Title: Re: Where is the phosphate coming from?
Post by: peterwadeuk on June 26, 2019, 05:03:02 PM
I believe that I am misunderstanding the -Donnan feature of the surface.

If I remove the -Donnan feature, I get steady dissolution of apatite.

If I invoke the -Donnan feature, does PHREEQC populate the exchanger with a layer of ions derived from -equilibrate 1, and are these ions depleted as the TRANSPORT function operates?
Title: Re: Where is the phosphate coming from?
Post by: dlparkhurst on June 26, 2019, 08:47:53 PM
If you define a SURFACE; -donnan; -equil 1, then the donnan layer is populated, but the solution composition is unchanged. So, if solution 1 contains phosphorus, phosphorus is essentially added to the surface layer to achieve equilibrium with solution 1 without taking any from the solution.