Modelling a mine drainage with inverse model, but problems with mineral sinks

[Juan Sebastian]

Hello,

I am using inverse model  between basic water (solution 1) and ?cid water (mine drainage; solution 2). The mine drainage has a pH of 0.8. I am using also Eh-pH diagrams modeled by PhreePlot, which show the predominance of species (solubles and minerals) of solution 2, using the physocochemial parameters of temperature (23.0?C), potencial redox (Eh = 0.69),  pH = 0.8, major anions, major catios and some metal(oids) (Fe, As, P,  Si, Al). The problem is that almost no metals precipitates at these physicochemical conditions, and metals and metaloids form soluble species such as FeHSO4+, F2+, FeSO4+, AlSO4+, FeH2PO4+2, H2PO4-, H3AsO4 and H2AsO3. For major cations and anions, for example, I have the predominance species Ca+, Mg2+, K+, etc.

The following is my question:

Can I introduce some of this species as a PHASES (e.g., FeHSO4+, AlSO4+, FeH2PO4+2)  and indicate that these ones precipitate? I know that this species are soluble, but I do not have abundant  mineral sinks.

[Juan Sebastian]

In addition, I have been modelled other mine/rock drainage with other physicochemical characteristics, and in these other ones, I know that some minerals precipitates such as Hematite, Goethite, Jarosite, etc. But in the previous question no minerals are predict to precipitate. 

[dlparkhurst]

No, you are accounting for total dissolved concentrations in your SOLUTION definitions. You cannot add phases that represent dissolved species.

In your case, at pH 0.8, you are right that not many minerals will precipitate, maybe jarosite or alunite. In that case, the mole transfers for your minerals should be dissolution.

Assuming P and As are relatively minor, I would not try to account for them in the mole-balance modeling, only the major dissolved elements and the major elements in your minerals.

[Juan Sebastian]

Ok., thank you so much for your quick response, I will tray to adjust the model with the advices that you gave me.