Cation Binding Saturating Strong Sites

[awillou4]

Hello PHREEQC community,

I'm working on modeling heavy metal laboratory batch isotherms for my master's thesis.  Most of my fits work well; however my only cation of significance [Cr(III)] appears to saturate all my strong sites when modeling batches with less solid media (resulting in a lowercase "r" shaped isotherm).  This makes sense from the modeling perspective; however experimental data suggests no such saturation.

I can avoid this saturation by upping my HFO_s sites; however doing so takes me outside Dzombak's type 1 site density range of 0.001 to 0.01 mol/mol Fe.  I find that 0.03 mol/mol Fe generally fits my data without HFO_s saturation.  Since I'd prefer not to have to calibrate my model with strong site densities outside Dzombak's range, I want to check if anyone knows other work arounds when dealing with HFO_s saturation.  Any help is much appreciated.

Thanks,
Austin

[awillou4]

I'm posting again in hopes of revisiting my aforementioned question.  As of now I'm planning on increasing HFO_s sites for Cr isotherms in order to attain more meaningful results (so Cr doesn't easily saturate all my HFO_s sites).

Any input is much appreciated!

[dlparkhurst]

You only have so many parameters:

Number of strong sites
Number of weak sites
Surface area
log K for  protonation/deprotonation
log K for metal binding