Arsenic co-precipitation

[pia said]

Hi! I'm modelling metal removal with calcite dissolution. In my model I considered the dissolution of calcite (pH incremented), iron oxidation, and Fe(OH)3 precipitation. Now I want to model arsenic co-precipitation with Fe(OH)3 but I didn’t find guide information or a similar example so I don’t know how to start. My question is if someone knows information or a model that can help me to start? Thank you

[dlparkhurst]

I can think of a couple of ways to look at arsenic coprecipitation.

You could define a SURFACE that is related to the EQUILIBRIUM_PHASE Fe(OH)3. Assuming there are arsenic surface species defined in SURFACE_SPECIES in the database or input file, the arsenic will sorb the surface and the amount of surface sites will depend on the amount of Fe(OH)3 that is precipitated.


Alternatively, you could set up a SOLID_SOLUTION with end members Fe(OH)3 and FeAsO4 (with definitions in PHASES). When thermodynamically favorable, a solid solution will form that contains varying amounts of each end member.

[John Mahoney]

I tried this about 15 years ago,  I used scorodite and Fe(OH)3 as the end members and found some data from Robert Robins (c. 1988) I think it worked out as an ideal solid solution but I haven't looked at the details in a long time.  the problem is that the Fe(OH)3 and Scorodite crystal structures are so different that it is not really a replacement of As into Fe(OH)3.  so I would expect a lot of miscibility, at that time I was not as familiar with Solid solutions as I am now.     

[pia said]

Thanks you for your answers, first I'm trying with solid_solution but I have a problem with the code.

I considered the Fe(OH)3 precipitation including the kinetics and when I add:

SOLID_SOLUTIONS 1
    AsFe(OH)3
        -comp Fe(OH)3(a) 90
        -comp Scorodite 0

The Fe(OH)3 precipitates faster (almost instantaneous) than before, according to my knowledge it should be the same time.

So, my question is if solid solution could considers the kinetics or just the thermodynamics?

[dlparkhurst]

SOLID_SOLUTIONs are calculated as equilibrium, in other words, it happens instantaneously. For starters, I would look at the equilibrium distribution of SOLID_SOLUTIONs, and then adjust the formula of your KINETICS so that some arsenic precipitates. The formula will be a fixed ratio of As:Fe, but it would be a beginning. (Once you are using KINETICS again, you should remove SOLID_SOLUTIONs.

[pia said]

Ok I got it. But I have a question I don't know how to fix the formula with ratio of As:Fe in kinetics, do you have an example or something that can help me ? Beforehand thank you so much !!!

[dlparkhurst]

For example:

KINETICS 1
-formula Fe(OH)3 0.95 FeAsO4 0.05
...

[pia said]

Ok, thank you very much !!!!!