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[OLD] Arsenopyrite - missing data?


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From: Richard Laffers

Subject: Arsenopyrite-missing data

The datablock for Arsenopyrite in the thermo.dat dataset contains log K values as follows,

-15.0049 -14.4888 -14.0906 -13.8475

-13.6831 -13.5466 -232.2766 -237.0440

The latter two seem to be incorrect, moreover, the GWB programs (RXN, REACT) ignore them. E.g. when running the attached script with RXN, it gives Log K at 250 C = -13.5743 (CAUTION -- Value extrapolated from 200 C). Am I missing something? Does anybody have correct data spanning the temperatures > 200 C? (I'm running the GWB 4.0.2)

data = thermo.dat verify

temperature = 250 C

react Arsenopyrite

swap HS- for SO4--

swap AsH3(aq) for O2(aq)


From: Craig Bethke

Subject: Re: Arsenopyrite-missing data

Those two log K's certainly look wrong. The GWB programs, however, ignore them because there is no data at 250C or 300C for the redox coupling reaction between As(OH)4- and AsH3(aq). The only time these log K's might come into play is a decoupled model at high temperature, which is not likely since it is common to assume redox equilibrium under such conditions.

We will, nonetheless, check into this and hopefully find corrected values to include in thermo.data for the next release. (Ominously, the LLNL document that thermo.dat is based on lists the data quality for arsenopyrite as "poor".)

From: Alan Welch

Subject: Re: Arsenopyrite-missing data

This does not directly address the subject, but you may be interested in: Nordstrom, D.K., and Archer, D.G., 2003, Arsenic Thermodynamic Data and Environmental Geochemistry, in Welch, A.H., and Stollenwerk, K.G., eds., Arsenic in Ground Water: Geochemistry and Occurrence: Norwell, Massachusetts, Kluwer Academic Publishers, p. 1-25.

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