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Surface complexation modeling

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[admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 6/13/2006]

Posted by: Rachana Agar

Hi all:

I am a new user to Geochemist Workbench and trying to

work with adsorption of arsenic as a function of pH

using "FeOH.dat" data file (Input file is enclosed).

I wanted to maintain background electrolyte as 0.01M

NaCl. Now in React (basis) when i entered these

species (Na+= 10mmol, Cl-= 10mmol, As(OH)3= 10mmol) it

is showing charge imbalance. In order to balance that

i added K+ species and it started working, but the

problem is it is changing the ionic strength of the

solution. So can you please guide me how can i fix the

ionic strength and charge balance together for this

type of situation.

The second problem is I want to work with Aluminum

hydroxide instead of FeOH sorbing surface. Is there

any data file for AlOH also in geochemist or if not

where can I get log K values for AlOH and arsenic

related surface species? And if I am modifying the

data file (FeOH.dat) for aluminum hydroxide, do I need

to change the thermo file (thermo.dat) also and how

can I do that.

Here are specifications for this problem:

- Software version: Geochemist's Workbench Standard 6.0

- The program in question- React

- Platform- Window XP

- Dataset- FeOH.dat

- File: Attached

Thanks a lot in advance for your help and guidance.

Rachana

Posted by: Craig Bethke

Hi Rachana,

There are several issues with this input. First, I'm pretty sure you want to set the free mass of Fe(OH)3 to 4 mg, not the total mass. Second, you set the As concentration to 20 mmolal, not 10. If you make these two changes, the program converges.

The underlying issue, however, is that you have set a background electrolyte concentration as NaCl and specified an acidic pH as the starting point. But you can't get to pH 3 simply by adding H+; you must also enter a counterbalancing anion like Cl-. So your initial Cl- concentration should reflect not only the background electrolyte but the acid needed to attain the initial pH.

Hope this helps,

Craig

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