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Re: Adding species to database




> I am trying to model a pyrite system with cyanide so that thiocyanate
forms. MINTEQ is the only database with cyanide species however there is no
thiocyanate.

llnl.dat has a limited amount of data on cyanide and thiocyanide.

How does one enter a new species into the database? or is there an updated
version of the database? (I am using PHREEQC 2.4.2)

You enter new "elements" in the database with the SOLUTION_MASTER_SPECIES
data block. New aqueous species are defined with SOLUTION_SPECIES.

> I would appreciate your feedback on this matter.

I think it may be a bit tricky to model cyanide reactions. I suspect that
they are metastable and are only formed kinetically. The approach in
minteq.dat and llnl.dat are different. In llnl.dat, cyanide is composed of
C and N, and the cyanide species will be in equilibrium with all other C
and N aqueous species. Thus, I would guess, if you enter cyanide in the
initial solution, it will exist in the initial solution calculation, but
will decompose to equilibrium products when you run any kind of reaction
(equilibrate with a mineral, add a reactant (REACTION), etc) because
reaction calculations always impose redox equilibrium. Minteq.dat defines
an "element" named Cyanide. This element does not know it is composed of C
and N, and there is no equilibrium between Cyanide and other C and N
species. Thus, it will exist at the same total concentration unless you add
or remove some of it in the reaction calculations. The  speciation may vary
as pH and ligand concentrations vary through reactions. I am guessing that
you will need to use the minteq.dat approach if you want to keep any
cyanide in solution. I don't know if you can model thiocyanide as an
equilibrium reaction between cyanide and HS-, or you need a separate
element for thiocyanide as well. If separate, you would probably define
reactions between them kinetically with KINETICS and RATES.

David


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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