questions on equilibrate vs USE function

[Mark Himenz]

I'm still very, very new to phreeqc and I just started using it to model uranium sorption to mineral surfaces. I am a bit confused on how to set it up. Specifically I'm confused as to what the equilibrate function does. To model sorption I do need to use

USE solution 1
USE surface 1

to get an actual sorption reaction. What solution do I need to use to equilibrate the surface with, though, and why is it needed?

Thanks for helping a beginner get started with phreeq

[dlparkhurst]

First, I like using USE because it avoids knowing some of the default ways that reactions are selected for equilibration. However, USE is not required.

All PHREEQC calculations require a solution. Solutions must be defined with the SOLUTION or SOLUTION_SPREAD keywords. A solution may be pure water, or an analyzed solution composition.

Then we come to the SURFACE definition. The question is what is the starting composition of the surface. There are two main choices. You can assume that nothing is sorbed on the surface and it is initially simply SurfOH, for example. However, for most systems, it is more likely that the initial condition of the surface is one in equilibrium with a solution. If you have a sediment from an aquifer, it is reasonable to assume that the surface is in equilibrium with the water in the aquifer. Similarly, if you do a column experiment and flush the column with multiple volumes of some solution so that the inflow has the same composition as the outflow, then it is likely to be in equilibrium with that solution. In these cases, you can define the initial composition of the surface to be in equilibrium with a solution by using the -eq n option.

So, your file could look like this:

SOLUTION 1 Aquifer water
...
END
SURFACE 1
...
-eq 1
END

In this case, you don't really need to react the surface with the solution because it is defined to be in equilibrium with the
solution.

Alternatively, you could do the following, assuming the surface starts in the SurfOH state:

SOLUTION 1
...
END
SURFACE 1
SurfOH ....
END
USE solution 1
USE surface 1
END

An equivalent calculation can be defined by

SOLUTION 1
...
SURFACE 1
SurfOH ....
END

where PHREEQC assumes you want to react the solution with the surface.