Help! Solubility of goethite

[ifeoma]

I need to run PHREEQC stimulations for Ni/ Co sorbed  and substituted goethite solubility as a function of pH. Please how do I set up the input files? Is it possible generate solubility products of the reactions in PHREEQC?
Please any suggestion will be high appreciated.

[dlparkhurst]

If you are using measured concentration of Fe(III) and pH, you can use SOLUTION to calculate saturation indices for goethite. The calculation will give you activities for all aqueous species in solution, including Co and Ni if you include them in the solution definition.

If you know the thermodynamic properties of the endmembers, you can use SOLID_SOLUTION to calculate solution and solid-solution compositions.

SURFACE is used to calculate sorption.

[ifeoma]

Thank you very much.

[ifeoma]

 Dear David,
 Thanks again for you last suggestions. Below are the results from Ni sorption on weak and strong sites.

          pH             Ni        m_(w_OH)2Ni+   m_s_(OH)2Ni+   
           9     8.2677e-06     8.9417e-06     2.2312e-06

   
 Can SURFACE keyword be used to calculate solubility of these Ni complexes formed at lower pH? If not, which keyword can be used?

Thanks in advance. 

[dlparkhurst]

I would not call it "solubility", but rather the distribution of Ni between solution and surface.

As for another pH, you can add acid or base with REACTION, or you can use EQUILIBRIUM_PHASES to fix the pH.

Example 8 is a bit complicated because it writes a file that is then run, but the basic building block is something like this:
PHASES
Fix_H+
H+ = H+
log_k 0.0
END

USE solution 2; USE surface 1
EQUILIBRIUM_PHASES 1
   Fix_H+  -5.2500e+00 NaOH 10.0
END