> My question is about the dissolution/precipitation equilibrium constant of iron sulfide (FeS). In the PHREEQC's database- phreeqc.dat and waterq4f.dat, FeS(ppt) FeS + H+ = Fe+2 + HS-, logK = -3.915. Can you tell me where the value come from? > According to Morse et al.(1987, The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters, Earth-science reviews, 24, 1-42.) and Davison (1991, solubility of iron sulfide, Aqut.Sci, 53, 309-329) logK = -2.95. In my own data (in PhD thesis of National Taiwan University), I survey over 100 data from wells and references, show logK is about -2.9 ~ -3.0. Because my data against USGS's phreeqc database, my adivser asked me why the USGS's phreeqc.dat (1995) didn't follow the results from Morse et al.(1987) and Davison(1991). And where the data logK = -3.915 come from. The data are from the compilation of WATEQ4F (Ball, J.W., and Nordstrom, D.K., 1991, User's manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters, U.S. Geological Survey Open-File Report 91-183). The reference for the data are listed as: Berner, R.A., 1967, Comparative dissolution characteristics of carbonate minerals in the presence and absence of aqueous magnesium ion: American Journal of Science, v. 265, p. 45-70. Note that Mackinawite is given a log K of -4.648 for the same reaction. There is almost certainly a range of log K values from fresh precipitation, to more crystalline mackinawite, and ultimately to pyrite. The log K you choose for modeling depends on the time scale at which you are working. The value of -3 may be appropriate for fresh precipitate, but it is less stable (more soluble) than all of the K's that are in the databases for PHREEQC. Data from Lawrence Livermore National Lab(data0.com.V8.R5 and llnl.dat) lists Pyrrhotite with log K -3.7; and Troilite -3.8 for the same reaction. The references are given as 92joh/oel Johnson, J.W., Oelkers, E.H., and Helgeson, H.C., 1992, SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000degC: Computers Geosci., 18, 899-947. 78hel/del Helgeson, H.C., Delany, J.M., Nesbitt, H.W., and Bird, D.K., 1978, Summary and critique of the thermodynamic properties of rock-forming minerals: Amer. J. Sci., 278a, 229p. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 h2osoft Sent by: Kathleen To: alfalf@xxxxxxxxxxxxx, David L M Flynn Parkhurst/WRD/USGS/DOI@xxxx cc: Diane E Brittle/NMD/USGS/DOI@xxxx, h2osoft@xxxxxxxx, ask@xxxxxxxx, William M 03/25/02 09:06 AM Alley/WRD/USGS/DOI@xxxx In-Reply-To: <OF840BACB3.9AEC2338-ON85256B87.00580441@xxxxxxxx> Subject: Re: Iron sulfide's dissolution/precipitation constant(Document link: David L Parkhurst) With this message, I am forwarding your question to Dave Parkhurst, one of the authors of PHREEQC (Dave-with your response, please include a cc to h2osoft@xxxxxxxx for the e-mail archive and to ask@xxxxxxxx for their records.) ask Sent by: Diane E To: "Alf" <alfalf@xxxxxxxxxxxxx>, h2osoft@xxxxxxxx Brittle cc: In-Reply-To: <OF840BACB3.9AEC2338-ON85256B87.00580441@xxxxxxxx> Subject: Re: Iron sulfide's dissolution/precipitation constant(Document link: h2osoft) 03/25/2002 10:28 AM March 25, 2002 >From USGS: I have forwarded this message to the following USGS office, which is identified on the PHREEQC home web page as the technical contact for your question. You should receive a separate response from them. U.S. Geological Survey Hydrologic Analysis Software Support Program 437 National Center Reston, VA 20192 (electronic mail: h2osoft@xxxxxxxx) To the "H2OSOFT" team - please make your response to "Alf" <alfalf@xxxxxxxxxxxxx> We would appreciate a CC sent to "Ask@xxxxxxxx". Thank you, Diane Brittle USGS "ASK@xxxxxxxx" "Alf" <alfalf@xxxxxxxx To: <ask@xxxxxxxx> t.net> cc: In-Reply-To: <OF840BACB3.9AEC2338-ON85256B87.00580441@xxxxxxxx> Subject: Iron sulfide's dissolution/precipitation 03/25/02 01:50 constant PM Dear sir: I have a question about the PHREEQC. Refer to a report: User's guide to PHREEQC - a computer program for speciation, reaction-path advective-transport, and inverse geochemical calculations. by David L. Parkhurst, 1995, USGS Water -resources investigations report 95-4227. My question is about the dissolution/precipitation equilibrium constant of iron sulfide (FeS). In the PHREEQC's database- phreeqc.dat and waterq4f.dat, FeS(ppt) FeS + H+ = Fe+2 + HS-, logK = -3.915. Can you tell me where the value come from? According to Morse et al.(1987, The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters, Earth-science reviews, 24, 1-42.) and Davison (1991, solubility of iron sulfide, Aqut.Sci, 53, 309-329) logK = -2.95. In my own data (in PhD thesis of National Taiwan University), I survey over 100 data from wells and references, show logK is about -2.9 ~ -3.0. Because my data against USGS's phreeqc database, my adivser asked me why the USGS's phreeqc.dat (1995) didn't follow the results from Morse et al.(1987) and Davison(1991). And where the data logK = -3.915 come from. Thank you very much Wen-fu Chen National Taiwan University
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