Processes > Acid mine/rock drainage

Use the keyword REACTION to simulate solution loss in leaching experiment.

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Joe:
Hi, phreeqcusers.  I'm working on a model for a leaching experiment. I noticed that "Loss of solution due to pore water retention and weekly evaporation was modeled using the keyword REACTION. At each shift, 0.28 mol of water (5 gs) is removed to maintain an average recovered effluent volume of 950 mL" in a paper (doi.org/10.1016/j.advwatres.2019.04.008).

I'd like to know how the keyword REACTION is achieved for this purpose. Anyone knows bout this problem, please? Thanks!

dlparkhurst:
I can't say exactly what they did from the excerpt that you posted.

Evaporation can be modeled easily with REACTION:


--- Code: ---REACTION n
H2O 1
-0.28 moles

--- End code ---

Water is simply removed from the system, which will increase the concentrations of solutes.

However, water retention is not so simple. In this case, you need to remove both water and solutes. The most straightforward way would be to use MIX


--- Code: ---MIX n
n 0.95

--- End code ---

This would snippit would remove 0.05 of solution--water and solutes--for solution n.

It is also possible to remove water and solutes with REACTION, but it takes some work. You would need to get the total moles of H, O, and all solutes in the solution (assuming charge balance), and then include all of these elements in the REACTION definition. If you do it by hand, you could print TOTMOL of each element, and then develop the REACTION definition. Alternatively, to automate the process, you could use USER_PUNCH to write the REACTION definition to a file, and then INCLUDE$ that file. I don't know if they are using this approach, or whether they are simply removing H2O.

Joe:
That's great, thank you very much!

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