PhreeqcUsers Discussion Forum
Conceptual Models => Database selection and modification => Topic started by: frenzy_geochem on March 25, 2020, 10:19:39 AM

Hi all,
I have noticed how the definition of pyrite in minteq:
Pyrite
FeS2 + 2H+ + 2e = Fe+2 + 2HS
log_k 18.479
delta_h 11.3 kcal
differs from the definition used in relevant papers to calculate log_k (Rickard, 2007; Ning et al., 2013), namely:
Pyrite
FeS2 + H+ = Fe+2 + HS + S(0)
log_k 16.4
I tried to overwrite the pyrite phase from minteq, however, phreeqc is telling me that no phase (I used 'Sulfur' as S(0) is not stated in minteq) can be stated on the right hand side of the equation. How to solve this?

Here is the definition of the mineral native sulfur from minteq.v4.dat
Sulfur
S + H+ + 2e = HS
log_k 2.1449
and here is you definition of pyrite
Pyrite
FeS2 + H+ = Fe+2 + HS + S(0)
log_k 16.4
You can add these two reactions to get an alternate definition of pyrite that is equivalent:
FeS2 + 2H+ + 2e = Fe+2 + 2HS
log_k 18.5449
So, the minteq definition that you give is only slightly different.
To use one of the more obscure features of PHREEQC, it is possible to use solids in definitions of minerals, but it is necessary to use "(s)" to designate them, so the following will work:
PHASES
Sulfur(s)
S + H+ + 2e = HS
log_k 2.1449
Pyrite
FeS2 + H+ = Fe+2 + HS + Sulfur(s)
log_k 16.4
END