Sorption on goethite or ferrihydrite?

[Max1]

Hi all,
I’m trying to model the adsorption of particles (in this case, molybdate) on the surface of a goethite mineral. As such, I define goethite at the EQUILIBRIUM_PHASES. At SURFACE_SPECIES, I define the reaction between a surface functional group on ferrihydrite and molybdate. At the keyword SURFACE, I link this mineral goethite to the weak and strong surface sites of ferrihydrite. So I say there is goethite in my system, yet model it as if the adsorption is on ferrihydrite? I took this from literature but I don’t completely understand this method…

SURFACE_SPECIES
     Hfo_sOH + MoO4-2 + H+ = Hfo_sMoO4- + H2O
          log_k       9.5   
EQUILIBRIUM_PHASES 0      
     Goethite             0 0.0674
SURFACE 0
         Hfo_wOH Goethite      0.174689 53400             
         Hfo_sOH Goethite        0.004367225       
     -equil with solution 0

Thanks in advance.

[dlparkhurst]

 Ferrihydrite is more soluble than goethite, which means ferrihydrite has a larger log K. The difference in stoichiometry between goethite (FeOOH) and ferrihydrite [Fe(OH)3] is just one water, so the formulation used for the mineral is not too important (because the activity of water is about 1), but the log K will affect your calculations because the amount of mineral will differ.

Ferrrihydrite has more sorption sites per mole than goethite, and its specific area is different. So, you can use either mineral for the calculations, but if you are intending to model ferrihydrite, you need to adjust the log K of goethite to match ferrihydrite, and you need to define the surface characteristics, moles of sites per mole of mineral, and surface area per mole of mineral to be consistent with ferrihydrite. Or you can just define a Ferrihydrite phase and avoid confusion.

[Max1]

But if you wanna model sorption on goethite, why wouldn't you not just model it as goethite instead of ferrihydrite. This just seems more complicated. Or is this because of the database (i.e. more data on ferrihydrite)?

[dlparkhurst]

I am sorry. I misread your post. I thought you were using goethite to represent ferrihydrite. I agree, just use goethite for the mineral. However, the Hfo reactions in the database are for ferrihydrite. I am not sure if the reactions for HfoO- and HfoOH2+ are different than they would be for goethite or not. I think people sometimes assume that these reactions are the same for goethite but the surface area and site density are different.