> I have a question for you about PHREEQ-C: you mention in the manual that the usage of mole fraction for the activity of surface species instead of molarity (which sounds reasonable to me) does affect multidentate complexes - I'd be interested in how this affects bi- or tridentate complexes. If you consider mass-action equations (1) Surf + Me+ = SurfMe+, K = [SurfMe+]/[Surf][Me+] and (2) 2Surf + Me+2 = Surf2Me+2, K = [Surf2Me+2]/[Surf]^2[Me+2] If mole fraction is used [SurfMe+] = (SurfMe+)/(Surf, total) and [Surf] = (Surf)/(Surf,total), where  indicates activity and () molality. In equation (1) the (Surf, total), the total number of sites, cancel relative to molality. In equation 2, [SurfMe+2] = 2(Surf2Me+2)/(Surf,total) and [Surf] = (Surf)/(Surf, total). In this case a mass-action equation formed from molality and one formed with mole fraction will differ by a factor of 2(Surf, total). I think using molality gives K's that are dependent on the amount of surface sites and use of mole fraction is better. >In addition, I was wondering if it is possible to include surface precipitation as well in PHREEQ-C (accounting for the phenomenon that near the surface concentrations may be high enough for precipitation whereas overall solution concentrations are too low for precipitation to occur). Basically, no, I did not implement Dzombak and Morel's surface precipitation model. There are solid solution capabilities and general kinetics capabilities. It might be possible to use them to try to recreate the effects of surface precipitation, but it would not be straightforward. > I simulated As(V) sorption onto a quartz sand using the diffuse double layer model, and I had some problems at "higher" As(V) concentrations - at 10^-7 and 10^-8 moles/l As(V) the model works fine, but it underpredicts sorption at 10^-6 moles/l and I was wondering if that could be due to bidentate complexes involved (which was observed in other studies) or surface precipitation with dissolved Fe. I'm not sure. Surface complexation is not my specialty. It sounds like you have your ideas and they are probably better than mine. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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