mschock Posted October 30, 2008 Share Posted October 30, 2008 Hi; I'm trying to develop an upper-bounds estimate for the solubility constant and/or Gibbs free energy of formation of vanadinite (as found in lead pipe corrosion scales), from some drinking water monitoring data and some [hopefully wise] estimations of dissolved lead and vanadium levels in places that can't be directly measured. My strategy is to use SpecE8 with known background water chemistry (which varies somewhat for a few constituents) to derive the activities of Pb+2, VO4-3 and Cl- for some expected ends of chemistry ranges, to get an estimated log Ksp. From that, I can use Act2 to construct an Eh-pH diagram for some different water treatment scenarios. I thought I was OK up until I tried to add vanadinite into the thermodynamic database. I am using a modified version of thermo.com.v8.r6 which I have used for a couple of years, having extensively added to and modified the Pb and Cu sections. It is too big to attach, so I exerpted just the section where I added the vanadinite, and I incremented the number of solids by 1. I wrote the reaction in terms of Cl-, Pb++ and VO4---, but when trying to read the new database into SpecE8, it apparently tries to do something with the VO++/VO4--- couple and crashes with a "reaction mass imbalance for vanadinite" error (attached Word file). I am also attaching one of the representative SpecE8 script files which ran with the thermo database without the added vanadinite. Is there an error in the way I entered the solid, or other changes I need to the aqueous species for V(V) as well? I think I noted a redundancy in the database, too, though I don't think it's part of this problem. Both HVO4-- and VO3OH-- are in the database. As an aside, I originally tried to set Eh and include oxygen, but at the high Eh I tried (0.95 V), the calculations got way off, I think because it might have been oxidizing water. I couldn't find a way to make the calculations work by specifying Eh, which would be useful for looking at Pb solubility vs Eh at different pHs or carbonate concentrations, as the phase changes from cerussite or hydrocerussite to plattnerite. One problem that has some bearing on how I can approach this is that realistically, is many drinking water systems are in redox disequilibrium with the stability domain of water, thanks to the use of highly oxidizing disinfectants like hypochlorous acid. However, modelling based on short-term metastable equilibrium represents the systems well. Observed and inferred (from the presence of PbO2 and other highly oxidized mineral phases) Eh's are often in the 800-1000 mV range. Other systems use chloramines, a weaker oxidant, which are tough to characterize and enter into any thermodynamic database. Therefore, it would be best if I could use basis species that I know co-exist in the systems I want to model, such as VO4-3 and Pb+2, and try to ignore actual redox reactions in the key speciation calculations. Any help with this addition of vanadinite and setup of the V speciation problem would be appreciated. --Mike Schock Quote Link to comment Share on other sites More sharing options...
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