unit for total no. of surface sites

[padhi]

Hello,

I am using PHREEQC for simulating fluoride sorption to soil assuming a generic surface site.
This post is about the units used to describe total no. of surface sites.
I read the previous posts but its still not clear to me.

I am assuming a site density of 3.8*10^-6 moles/m2. The surface area of the sample is 7.7 m2/g and mass of solid is 10 g/l.

In this case, what is the site unit?
I input in the line 2 of SURFACE data block as:
SOH  2.926*10^-4   7.7  10; 
assuming no. of sites to be in moles/l and mass in g/l.

Is it correct?

Any explanation is kindly appreciated.

Padhi

[dlparkhurst]

Yes, assuming your solution is about 1 liter (default).

[padhi]

Dear Sir,
Thank you very much.

Now, I want to simulate transport of fluoride in a soil column, where fluoride sorption was described by surface complexation using a generic surface site.
In my experiment, the column was first contaminated with fluoride and then flushed with fluoride free water.

I could simulate fluoride sorption successfully, however, for the desorption part, I found overshooting during the initial stage of desorption.
I took care of the courant and von-neumann conditions and also tried to decrease the grid size.
However, it could not solve the overshooting issue.

Would you help why there is increase in concentration then the initial sorbed fluoride, when the infiltrating solution is having zero fluoride concentration?

Padhi

[dlparkhurst]

Could be the sorption reactions, the log Ks, competing ions, mineral or solid solution formation; maybe you want to start with a simpler isotherm approach; maybe modeling is not the right approach.  Can't really help you; this is your investigation.

[padhi]

Thank you.

[padhi]

Hello,

I am simulating fluoride transport in a soil column.
In my experiment, the column was first contaminated with fluoride and then flushed with fluoride free water.

I want to fix the pH of the initial solution by using a new phase. Because, otherwise, the pH of the initial solution changes to maintain charge balance. After the initial condition is established, the pH is allowed to change during fluoride sorption and desorption.

However, if I use:

PHASES                                           # Fix pH...
   Fix_pH
   H+ = H+
   log_k 0.0

SOLUTION 1-30
   temp   25
   pH     4.5
   units mmol/kgw
   F    2.632e-3
   Al      9.025e-2
   Na    3.064e-1
   Cl    3.949e-1    charge
   water   1 #kg


EQUILIBRIUM_PHASES 1-30
Fix_pH   -4.5     NaOH    1.0

SAVE solution 1-30

END

and then evaluate the transport part, the pH of the solution in the column was equilibrated at every time step during transport and kept constant at 4.5.
How can I make the pH to be constant only during the initial stage (initial solution) and vary according to the chemical reaction during transport?

Any suggestion is kindly appreciated.
Padhi

[dlparkhurst]

I you define SOLUTION 1-30 with a pH of 4.5, then that will be the starting pH of the solution in the column.

Now if you have other reactions and you want the entire system tfor each cell to be in equilibrium at pH 4.5 before the transport begins, you can do that equilibration, and then set the initial column definition.

Something like this:

PHASES                                           
   Fix_pH
   H+ = H+
   log_k 0.0
END
SOLUTION 1
   temp   25
   pH     4.5
   units mmol/kgw
   F    2.632e-3
   Al      9.025e-2
   Na    3.064e-1
   Cl    3.949e-1    charge
   water   1 #kg
END
USE solution 1
EQUILIBRIUM_PHASES 100
Gibbsite 0.0  10
Fix_pH   -4.5  HCl 10
SAVE solution 1-30
END