Gas transport in the surface layer of a water pond

[Alberto_S]

Hello.

I am currently modelling the oxidation of a reduced  solution (reduced species: HS-, CN-, SCN- and their decay products) using KINETICS and RATES.

So far I assumed in the GAS_PHASE an O2 partial pressure of 0.1 to 0.2 atm. However, as the O2 concentration in solution will drop, there will be a gradient in O2 (and H2S + HCN) concentration in the layer separating the two phases. There will be diffusion of O2 from the gas phase to the aqueous solution and also H2, HCN and H2S from the liquid phase to the gas phase.

I guess there will be (assuming no wind) a layer where gas/liquid diffusion will happen. I think I will have the following system:


(main) LIQUID PHASE (dissolved HCN , H2, H2S, O2, etc.) with fixed composition (at a given timestep)

(surface layer) LIQUID PHASE (dissolved HCN , H2, H2S, O2, etc.) with composition (at a given timestep) given by diffusion with the main liquid phase and the surface gas layer

(surface layer) GAS PHASE (gaseous HCN , H2, H2S, O2, etc.) with composition (at a given timestep) given by diffusion with the surface liquid layer  and the main gas phase.

(main) GAS PHASE (dissolved HCN , H2, H2S, O2, etc.) with fixed composition (at a given timestep)

Could PHREEQC help me with this kind of problem, or at least part of it (with some assumptions). If yes, how?

[dlparkhurst]

PHREEQC can simulate diffusion in the liquid phase, reactions, and gas-water equilibrium, but it does not have the capability to simulate gas diffusion.

You could set up a column such that cell 1 was in equilibrium with a GAS_PHASE (surface layer gas phase), and cell n was subject to reactions (main liquid phase). Diffusion would occur between cell n and cell 1, which could also be subject to reactions.

I think that would cover everything but exchange of gas between the main gas phase and the surface layer gas phase.  If you have a gas transport code, you might be able to hook it to IPhreeqc or PhreeqcRM to do the liquid part of the calculations. It also is possible you could set up a KINETICS reaction that removed or added gases to solution 1 based on a difference in gas composition between the main gas phase and the surface layer gas phase.