PhreeqcUsers Discussion Forum

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

Title: Correct pyrite definition minteq
Post 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?
     

Title: Re: Correct pyrite definition minteq
Post by: dlparkhurst on March 25, 2020, 01:46:03 PM
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