dissolution with solution agitation

[Cauchy]

Hello Dear all,

I've found in litterature that agitation contribute to the increasing of brucite dissolution. I would like to model this reaction with Phreeqc.
Can one help me how to introduce agitation of the solution into kinetics reaction ?

Thank you in advance !!

[dlparkhurst]

How would you adjust the rate expression?

[Cauchy]

I have some experimental results so i can use these results to adjust my rates parameters for future analysis.
but for instense, I use rates parameters from Palandri & Kharaka, 2004

[dlparkhurst]

I assume you would use another factor that multiplies your rate. If you multiply by 2, your rate is twice as fast.

[Cauchy]

yes I Know.

But changing the rate doesn't fix the problem. In fact, the rate drive to the accelaration of the process. but if the concentration of reactant is not sufficient in the vicinity of the solid, Nothing will happen.

I think the mechanical agitation help to homogenize the concentration of the reactant in the solution, so that in an finite solution, the pH curve of any reactant in the infilling solution is the same evrywhere.
But how to simulate this process

My second question is about the infilling solution. I don't understand why the mole fraction of the infilling solution is set to zero in a dispersive-advective transport process. I define mys solution (Solution 0) but the result look like this solution has not been taking into acount, so no diffusion and advection.

thank in advance

[dlparkhurst]

I guess I don't know how you are trying to model this process. I assumed you were modeling a batch reaction of a SOLUTION with KINETICS, which is probably the way to start.

But you seem to be talking about diffusive transport. For diffusive transport, agitation effectively makes the diffuse layer thinner.

If you are modeling advective transport with immobile zones, the effect of agitation may be to increase the exchange constants for transport in the immobile zones.

And, if you are modeling diffusion_only, solution 0 is used when the boundary condition is constant, in which case the composition of solution 0 should make a difference. If you are modeling forward advection, again solution 0 is used. Only if you are modeling diffusion with a closed boundary would solution 0 have no effect.

[Cauchy]

Thanks a lot David

Yes I'm modelling diffusion process by taking into account Kinetics.

First I tried a batch reaction with kinetics but my result doesn't match very well the experimental result. it look like the Brucite dissolution attend rich a maximum value a stop.
concerning the pH valu of the solution increse from 8.22 to almost 9.3 and become constant, which is in acordance with the experimental results.
so I though that my constant rate are good but I didn't have good mineral mass balance because of the solution agitation which contribute to homogenize the solution and increase the amount of reactant at the vicinity of the material block.

I tried also to add some reactant into my solution (using REACTION Keyword) but this change my pH curve and the amount of the mineral dissolve and precipitate is not better.

What can I do ?

[dlparkhurst]

Sorry, I still don't know what your physical situation is and how you are modeling it. Without seeing the input file and the data you are trying to match, I can't do much, but really, it is your responsibility to decide how to model your situation.

Is there a block of material that is dissolving with a stirred solution, or a set of particles agitated in a solution, both?

Are you using the TRANSPORT keyword with -diffusion_only? Just SOLUTION and KINETICS? other keywords?