1 Supporting Information 2 3 Model Predictions of Realgar Precipitation by Reaction of As(III) with Synthetic Mackinawite Under Anoxic Conditions 4 by 5 Tanya J. Gallegos, Young-Soo Han, Kim F. Hayes 6 7 Number of pages: 22 8 Table S-1. Gibbs free energy values for principal components 9 Table S-2. Gibbs free energy of formation values for all species used in the model 10 Table S-3. Tableau listing aqueous species used in the MINEQL+ model 11 Table S-4. Tableau listing dissolved solids used in the model 12 Table S-5. Arsenic reactions used in the model 13 Table S-6. Equations used to generate pe-ph diagram for the Fe-S-As-H 2 O system 14 Table S-7. Comparison of pe values for ph edge simulations 15 Table S-8. Eh measured as a function of ph and mackinawite concentration 16 Table S-9. Sensitivity Analysis for model fit 17 Figure S-1. Fe-As-S-H 2 O pe-ph diagram S1
18 19 Figure S-2. Total iron concentrations measured in 0.1, 0.5, 1, and 10 g/l FeS after equilibration with 1.3 10-5 M As(III) and in the absence of arsenite (1g/L system only) S2
Model Input Data. In order to incorporate the e- in the reactions and allow for conversion of oxidation states based on pe, the reactions were recast in terms of principal components: H 2 O, H +, AsO 3-3, HS -, Fe 2+, Cl -, Na + and e - and their respective formation constants re-computed in light of updated values for Gibbs free energy values of formation resulting from a literature review. The stability constants for all reactions were calculated from G values for all species that best represented the system using the following relationships: G rxn 0 = G f( 0 prod) - G f( 0 react) (S-1) G rxn 0 = RT ln K (S-2) where R = 8.314 J/(mol*K), T=298 K and G 0 0 0 rxn (kj/mol), G f( prod) (kj/mol), and G f( react) (kj/mol), represent the Gibbs free energy of formation values for the reaction, products and reactants, respectfully. G 0 f values for the principal components are listed in Table S-1. The 0 0 G f( prod) and G f( react) are listed in Table S-2. The resulting log K values are listed in Table S-3 and S-4. S3
Table S-1. Gibbs free energy values ( G ) for principal components used to recast all species in terms of principal components. Species G component (kj/mol) Reference e(-) 0.00 Zero reference state H 2 O -237.20 (1) H(+) 0.00 Zero reference state AsO 3 (3-) -447.70 (2) Cl(-) -131.26 (3) Fe(2+) -78.87 (4) Na(+) -261.87 (4) HS(-) 12.59 (1) Table S-2. Gibbs free energy of formation values for all species used in model G species (KJ/mol) Reference Species Charge -157.30 1 OH- (-1) -615.00 2 Fe(OH) 3 - (-1) -441.00 2 Fe(OH) 2 (0) -277.40 2 FeOH + (+1) -524.27 11-2 HAsO 3 (-2) -639.75 11 H 3 AsO 3 (0) -587.03 11 - H 2 AsO 3 (-1) - 17 + H 4 AsO 3 (+1) -648.53 11 AsO 4 (-3) -714.64 11 HAsO 4-2 (-2) -753.14 11 H 2 AsO 4 - (-1) -766.11 11 H 3 AsO 4 (0) -27.60 3 H 2 S (0) -119.24 4 Fe(HS) 2 (0) -118.83 4 Fe(HS)3- (-1) -518.80 5 S 2 O 3 (-2) -744.63 2 SO 4 (-2) -756.01 2 HSO 4 (-1) -823.49 2 FeSO 4 (0) -772.80 2 Fe(III)SO 4 (+1) S4
-1524.60 2 Fe(SO 4 ) 2 (-1) -4.60 2 Fe(III) (+3) 79.50 2 S 2 (-2) 73.60 2 S 3 (-2) 69.00 2 S 4 (-2) 65.70 2 S 5 (-2) 67.20 6 S 6 (-2) -529.10 5 H 2 S 2 O 3 (0) -525.60 5 HS 2 O 3 (-1) -527.810 2 HSO 3 (-1) -486.60 2 SO 3 (-2) -4.60 2 Fe(III) (+3) -229.41 2 FeOH (+2) -438.10 2 Fe(OH) 2 (+1) -481.45 5 Fe 2 (OH) 2 (+4) -653.64 5 Fe(OH) 3 (0) -963.28 5 Fe 3 (OH) 4 (+5) -842.20 2 Fe(OH) 4 (-1) -242.50 8 AsS(OH)(SH) (-1) -440.15 14 As(OH) 2 (SH) (-1) -410.80 14 As(OH) 2 S- (-1) -190.40 14 As(OH)S 2 (-2) 36.64 14 AsS 3 (-3) -8.37 14 HS 3 As (-2) -88.51 14 As(HS) 4 (-1) -120.00 7 FeHS (+1) -125.60 8 (SH) 2 As 3 S 4 (-1) -353.30 5 Fe(II)Cl 2 (0) -143.90 2 Fe(III)Cl (+2) -291.50 5 Fe(III)Cl 2 (+1) -415.02 5 Fe(III)Cl 3 (0) -334.97 5 Fe(III)C l3 (molysite) (0) -93.30 12 MACKINAWITE (0) 0 1 As(s) (0) -76.80 8 As 2 S 3 (am) (0) -575.90 10 ARSENOLITE (0) -576.57 11 CLAUDETITE (0) -84.30 8 ORPIMENT (0) -486.60 2 Fe(OH) 2 (0) -83.70 12 FeS (ppt) (0) -393.04 2 HALITE (0) 0 8 Sulfur (0) -490.00 16 FeOOH(goethite) (0) -1015.36 2 Fe 3 O 4 (magnetite) (0) -1921.40 5 Fe 3 (OH) 8 (0) -712.95 5 Fe(OH)3 soil (0) -726.97 5 Fe 2 O 3 (maghemite) (0) -311.96 13 Fe 3 S 4 (Greigite) (0) S5
-748.50 2 Fe 7 S 8 (pyrrhotite) (0) -820.61 5 FeSO 4 (0) -2510.13 5 Fe 2 (SO4) 3 (0) -2145.00 20 Fe 4 (OH) 8 Cl (0) -3790.00 20 Fe 6 (OH) 12 SO 4 (0) -1799.70 19 Fe 3 (OH) 7 (0) -708.16 5 Fe(OH) 3 (am) (0) -1244.10 19 Fe 2 (OH) 5 (0) -50.00 3 FeAsS(arsenopyrite) (0) -35.57 15 AsS (0) -20.90 15 Fe 2 As (0) -28.03 15 FeAs (0) -52.30 21 FeAs 2 (lollingite) (0) -276.34 2 WUSTITE (-0.11) -162.26 5 PYRITE (0) -483.25 5 Fe(OH) 3 (0) (lepidocrocite) -742.20 2 Fe 2 O 3 (hematite) (0) -100.34 5 FeS 1.053 (pyrhotite) (0) 0 17 Fe(0) metal (0) -158.28 5 FeS 2 (marcasite) (0) -97.91 5 FeS (troilite) (0) -278.40 5 Fe 2 S 3 (0) -251.45 16 FeO (0) -705.54 2 Fe(OH) 3 (c) (0) -1010.39 5 NaSO 4 (0) -1265.70 2 Na 2 SO 4 (0) -359.23 2 FeOCl (0) 1. (5), 2. (4), 3. (6), 4. (7), 5. (3), 6.(8), 7. (9), 8. (10), 9. (11), 10. (12), 11. (2), 12. (13), 13. (14), 14.(15), 15. (16), 16. (17), 17. Original MINEQL+ database, 18. (18), 19. (19), 20. (20), 21. (21) S6
Table S-3. Tableau- Aqueous Species (type III) (*species not included in final model) Aqueous Phases H 2 O H+ Fe 2+ AsO 3 3- e- HS- Cl- Na+ log K OH(-1) 1-1 -14.005 Iron Species Fe(OH) 3 (-1) 3-3 1-30.756 Fe(OH) 2 (aq) 2-2 1-19.679 FeOH(+1) 1-1 1-6.778 Fe(III)(+3) 0 1-1 -13.019 FeOH(+2) 1-1 1-1 -15.190 Fe(OH) 2 (+1) 2-2 1-1 -20.187 Fe2(OH) 2 (+4) 2-2 2-2 -26.413 Fe(OH) 3 (aq) 3-3 1-1 -23.983 Fe3(OH) 4 (+5) 4-4 3-3 -38.935 Fe(OH) 4 (-1) 4-4 1-1 -32.509 FeHS(+1) * 0 1 1 9.413 FeOCl(aq) 1-2 1-1 1-15.442 Fe(II)Cl 2 (aq) 0 1 2 2.088 Fe(II)Cl(+1) 0 1 1 26.460 Fe(III)Cl(+2) 0 1-1 1-11.609 Fe(III)Cl2(+1) 0 1-1 2-8.745 Sulfur Species S(-2) 0-1 1-12.926 H 2 S(aq) 0 1 1 7.041 Fe(HS) 2( aq) 0 1 2 11.483 Fe(HS)3(-1) 0 1 3 13.615 S 2 O 3 (-2) 3-8 -8 2-29.387 SO 4 (-2) 4-9 -8 1-33.583 S7
HSO 4 (-1) 4-8 -8 1-31.588 FeSO 4 (aq) 4-9 1-8 1-33.585 Fe(III)SO 4 (+1) 4-9 1-9 1-42.470 Fe(SO 4 ) 2 (-1) 8-18 1-17 2-74.797 S 2 (-2) 0-2 -2 2-9.529 S 3 (-2) 0-3 -4 3-6.291 S 4 (-2) 0-4 -6 4-3.281 S 5 (-2) 0-5 -8 5-0.500 S 6 (-2) 0-6 -10 6 1.441 H 2 S 2 O 3 (aq) 3-6 -8 2-27.582 HS 2 O 3 (-1) 3-7 -8 2-28.195 HSO 3 (-1) 3-6 -6 1-30.011 SO 3 (-2) 3-7 -6 1-37.235 NaSO 4 (-1) 4-9 -8 1 1 13.002 Arsenic Species HAsO 3 (-2) 0 1 1 13.422 H 3 AsO 3 (aq) 0 3 1 33.665 H 2 AsO 3 (-1) 0 2 1 24.423 H 4 AsO 3 (+1) 0 4 1-78.477 AsS(OH)(SH)(-1) -2 4 1 2 51.594 As(OH) 2 (SH) -1 4 1 1 42.458 As(OH) 2 S(-1) -1 3 1 1 37.314 As(OH)S 2 (-2) -2 3 1 2 42.462 AsS 3 (-3) -3 3 1 3 46.445 HS 3 As(-2) -3 4 1 3 54.335 As(HS) 4 (-1) -3 6 1 4 70.586 (SH) 2 As 3 S 4 (-1) -9 14 3 6 174.01 S8
Table S-4. Tableau - Dissolved Solids (Type V) (*species not included in final model) Solid Phases H 2 O H + Fe 2+ AsO 3 3- e- HS- Cl- Na+ Log K Fe(III)Cl 3 (aq) 0 1-1 3-10.102 Fe(III)Cl 3 (molysite) 0 1-1 3-24.134 FeOOH(goethite) 2-3 1-1 -11.089 Fe 3 O 4 (magnetite) 4-8 3-2 -29.806 Fe 3 (OH) 8 8-8 3-2 -33.285 Fe(OH) 3 (soil) 3-3 1-1 -13.587 Fe 2 O 3 (maghemite) 3-6 2-2 -24.954 Fe 3 S 4 (Greigite) 0-4 3-2 4 22.022 WUSTITE(-0.11) 1-2 0.95 0-6.273 PYRITE * 0-2 1-2 2 19.024 Fe(OH) 3 3-3 1-1 -53.851 (lepidicrocite) Fe 2 O 3 (hematite) 3-6 2-2 -22.285 FeS 1.053 (pyrrhotite)* 0-1.1 0.95-0 1 6.657 Fe(0)metal* 0-2 1 2 2-9.418 FeS 2 (marcasite)* 0-2 1-2 2 18.327 FeS(troilite)* 0-1 1 0 1 5.541 Fe 2 S 3 * 0-3 2-2 3 27.761 FeO 1-2 1 0-11.326 Fe(OH) 3 ( c) 3-3 1-1 -14.886 Na 2 SO 4 4-9 -8 1 2 57.755 MACKINAWITE 0-1 1 1 4.734 Fe(OH) 2 2-2 1-11.685 FeS(ppt) 0-1 1 1 3.050 S9
HALITE 0 1 1 45.888 Sulfur 0-1 -2 1 2.203 Fe 7 S 8 (pyrrhotite) * 0-8 7-2 8 52.056 FeSO 4 4-9 1-8 1-34.090 Fe 4 (OH) 8 Cl 8-8 4-1 1-34.938 Fe 6 (OH) 12 SO 4 16-21 6-10 1-81.649 Fe 3 (OH) 7 * 7-7 3-1 -17.053 Fe(OH) 3 (am) 3-3 1-1 -14.427 Fe 2 (OH) 5 5-5 2-1 -17.463 Arsenic Solids AsS(realgar) -3 5 1 1 1 54.281 FeAsS(arsenopyrite) -3 5 1 1 3 1 43.400 AsS -3 5 1 1 1-54.69 Fe 2 As -3 6 2 1 7 23.521 FeAs -3 6 1 1 5 37.346 FeAs 2 (lollingite) -6 12 1 2 8 87.858 As(s)(native) -3 6 1 3 46.258 As 2 S 3 (am) -6 9 2 3 112.588 ARSENOLITE -6 12 4 36.510 CLAUDETITE -6 12 4 36.628 ORPIMENT -6 9 2 3 113.903 The MINEQL + database was updated by consideration of the thermodynamic data presented by Nordstrom and Archer (2003), recent literature and databases distributed with the Geochemist's Workbench to acquire missing thermodynamic data for solids describing As-Fe, S-Fe and As-S aqueous and solid species. The selection of data for the model was not meant to be a S10
comprehensive critical review, but an attempt to include new information regarding As species and data necessary for the other S and Fe species required to recalculate log K values to assure a self-consistent database. Solid Arsenic Species. Selection of arsenic solid phases for inclusion in the model is limited due to availability of thermodynamic data. Although a number of arsenic sulfide phases and their respective polymorphs are thought to exist in As-S-Fe system such as duranusite, dimorphite, uzonite, and parrarealgar, there is presently no available or known thermodynamic data to describe these formation reactions. Thus, the model database was updated to include Fe 2 As (-20.92 kj/mol= G 0 f ), FeAs (-28.03 kj/mol= G 0 f ) and lollingite (FeAs 2 ) (-52.30 kj/mol= G 0 f ) (16) and As(0) (0.0 kj/mol= G 0 f ), As 2 S 3 (am) (-76.8 kj/mol= G 0 f ) (10), arsenopyrite of G 0 0 0 f = -50.0 kj/mol (6), orpiment ( G f =-84.3 kj/mol) (2, 10) and realgar ( G f =-35.57 kj/mol) (16). Although the thermodynamic free energy of formation constant for arsenopyrite of G 0 f = -50.0 kj/mol (6) used in this study falls at the high end of the spectrum for reported values ranging from G f 0 = -141.6±6 kj/mol (22) to -42 kj/mol (23), it was appropriately selected in comparison to the values for As 2 S 3 (am) ( G f 0 =-76.8 kj/mol) (10), As 2 S 3 ( G f 0 =-84.3 kj/mol) (2, 10) and AsS ( G f 0 =-35.57 kj/mol) (16) to reflect that arsenopyrite does not predominate over the arsenic sulfide solid species that were previously identified by XAS and XPS (24). Table S-5A lists the solid arsenic reactions and respective equilibrium constants included in the model. Table S-5A. Arsenic reactions included in the thermodynamic model for solid species S11
Species and Reaction log K Solid Species 3H 2 O + AsO 3 (3-) + Fe(2+) = 3e- + 2H(+) + FeAsO 4 *2H 2 O (SCORODITE) -22.207 H 2 O + AsO 3 (3-) + Fe(2+) = FeAsO 4 +3e- + 2H(+) 4.899 2H 2 O + 2 AsO 3 (3-) + 3Fe(2+) = Fe 3 (AsO 4 ) 2 +4e- + 4H(+) 51.576 2H(+) + 2 AsO 3 (3-) = H 2 O + 4e- + As 2 O 5 23.262 6H(+) + 4 AsO 3 (3-) = 6H 2 O + As 4 O 6 (ARSENOLITE) 141.736 12H(+) + 4 AsO 3 (3-) = 6H 2 O + As 4 O 6 ( CLAUDETITE) 142.041 6H(+) + AsO 3 (3-) + 3e- = 3H 2 O + As(s) 48.55 5H(+) + AsO3(3-)+ Fe(2+) + HS(-) + 3e-= 3H 2 O + FeAsS (ARSENOPYRITE) 55.796 6H(+) + AsO 3 (3-) + 2Fe(2+) + 7e- = 3H 2 O + Fe2As 21.722 12H(+) + 2AsO 3 (3-) + Fe(2+) + 8e- = 6H 2 O + FeAs 2 (LOLLINGITE) 94.562 9H(+) + 2 AsO 3 (3-) + 3HS(-) = 6H2O + As 2 S 3 (ORPIMENT) 122.855 e(-) + 5H(+) + AsO 3 (3-) + HS(-) = 3H 2 O + AsS (REALGAR) 59.25 Aqueous Arsenic Species. In the absence of sulfur and depending on pe and ph, arsenic is expected to form either arsenate (As(V)) or arsenite (As(III)) and their respective protonated species based on the pk a values. Gibbs free energy of formation for each species was taken from the same source for consistency (2): HAsO -2 3 (-524.27 kj/mol= G 0 f ), H 3 AsO 3 (-639.75 kj/mol= G f 0 ), H 2 AsO 3 - (587.03 kj/mol= G f 0 ), AsO 4 3- (-648.54 kj/mol= G f 0 ), HAsO 4-2 (- 714.64 kj/mol= G 0 f ), H 2 AsO - 4 (-753.14 kj/mol= G 0 f ), and H 3 AsO 4 (-766.11 kj/mol= G 0 f ). Although, the log K values for the formation reactions for each species were recomputed using the above free energy values to achieve self consistent database, the results represented little change from the original MINEQL + database log K formation values. S12
While dissolved iron has not been reported to form important aqueous species with arsenic, thioarsenates and thioarsenites are known to exist in solution. Thioarsenites have recently been studied for their prevalence and stability in As-S systems (15, 25, 26). Nordstrom and Archer (10) suggested refining the values of AsS(OH - )(SH - 0 - ) (( G f = 45.1 kj/mol), and As 3 S 4 (SH) 2 0 ( G f = 31.4 kj/mol)), based on earlier work (26, 27). The values have been adopted here in addition to thermodynamic data from Wilkin et al., which demonstrate a progressive conversion from sulfur-rich to oxygen-rich species depending on S-concentration (15). Although the Gibbs free energy values were not explicitly given, they were derived from formation reaction log K values leading to the following: As(OH)(SH) - 0 ( G f = -242.5 kj/mol), As(OH) 2 (SH) - 0 ( G f = - 440.15 kj/mol), As(OH) 2 S - 0 ( G f = -410.80 kj/mol), As(OH)S 2-0 2 ( G f = -190.40 kj/mol), AsS 3-0 3 ( G f = 36.64 kj/mol), HAsS 2-0 ( G f = -8.37 kj/mol= G 0 f ), As(SH) - 0 4 ( G f = -88.51 kj/mol). The inclusion of these thioarsenite species is important as their presence can prevent the formation of arsenic sulfide solids. Table S-5B lists the aqueous arsenic reactions and respective equilibrium constants included in the model. S13
Table S-5B. Arsenic reactions included in the thermodynamic model for aqueous species Aqueous Species and Reaction log K As(III) species H(+) + AsO 3 (3-) = HAsO 3 (-2) 13.414 3H(+) + AsO 3 (3-) = H 3 AsO 3 34.744 2H(+) + AsO 3 (3-) = H 2 AsO 3 (-) 25.454 4H(+) + AsO 3 (3-) = H 4 AsO 3 (+) 34.439 H 2 O + AsO 3 (3-) = 2H(+) + 2e- + AsO 4 (-2) -5.716 H 2 O + AsO 3 (3-) = H(+) + 2e- + HAsO 4 (-2) 6.084 H 2 O + AsO 3 (3-) = 2e- + H 2 AsO 4 (-) 13.094 H 2 O + H(+) + AsO 3 (3-) = 2e- + H 3 AsO 4 (0) 15.394 Thioarsenite species 14H(+) + 3AsO 3 (3-) + 6HS(-) = As 3 S 4 (SH) 2 (-) + 9 H 2 O 174.01 4H(+) + AsO 3 (3-) + 2HS(-) = AsS(OH)(SH)(-) + 2 H 2 O 51.594 4H(+) + AsO 3 (3-) + HS(-) = As(OH) 2 (SH)(aq)+ H 2 O 42.458 3H(+) + AsO 3 (3-) + HS(-) = As(OH) 2 S(-1) + H 2 O 37.314 3H(+) + AsO 3 (3-) + 2HS(-) = As(OH)S 2 (-2)+ 2 H 2 O 42.462 3H(+) + AsO 3 (3-) + 3HS(-) = AsS 3 (-3) + 3 H 2 O 46.445 4H(+) + AsO 3 (3-) + 3HS(-) = HS 3 As(-2) + 3 H 2 O 54.335 6H(+) + AsO 3 (3-) + 4HS(-) = As(HS) 4 (-1) + 3 H 2 O 70.586 S14
Table S-6. Equations used to generate pe-ph diagram for Fe-As-S-H 2 O system. Note that the numbering/lettering of the equations correspond to those shown on the pe-ph diagram in Figure S-1. 1. Lower Stability Limit for water: H + /H 2 (g) H + + e- = ½ H 2 (g) log K = 0.00 pe = -ph ½ log P H2(g) 2. AsS/As 2 S 3 (<ph 7) 2e- + 2H + + As 2 S 3 = 2AsS + H 2 S log K = 2.53 pe = 1.26 1/2log(H 2 S) - ph 3. AsS/As 2 S 3 (>ph 7) 2e- + 2H + + As 2 S 3 = 2AsS + HS- log K = -4.51 pe = -2.25 1/2 log(hs-) 1/2pH 4. AsS /As 0 ph<6 2e- + 2H + + AsS = As 0 + H 2 S log K = -1.4 pe = -0.70-1/2log(H 2 S) ph 5. AsS + FeS/As 0 <ph7 2e- + 4H + + AsS +FeS = As 0 + 2H 2 S + Fe 2+ log K = 0.91 pe = -0.5log(Fe 2+ ) - log(h 2 S) + 0.46-2 ph 6. AsS + FeS/As 0 >ph7 2e- + 2H + + AsS +FeS = As 0 + 2HS- + Fe 2+ log K = 75.14 pe = -6.59-0.5log(Fe 2+ )-log(hs - )-ph 7. FeS/Fe 2 (OH) 5 (>ph7) e- + 3H + + Fe 2 (OH) 5 + 2HS - = 2FeS + 5H 2 O log K = 26.92 pe=26.92-3ph + 2log(HS - ) 8. Fe 2 (OH) 5 /Fe 3 S 4 (HS-region ph>7) e-+ 2Fe 3 S 4 + 15H 2 O=3Fe 2 (OH) 5 + 8HS- + 7H + log K = -96.44 S15
pe = -96.44-8log(HS-) + 7pH 9. FeS/ Fe 3 S 4 (<ph7) 3FeS+ H 2 S =Fe 3 S 4 + 2H + + 2e- log K = 0.78 pe=-0.39-0.5log(h 2 S) - ph 10. FeS/ Fe 3 S 4 (>ph7) (HS- region) 3FeS+ HS - =Fe 3 S 4 +H + + 2e- log K = 7.82 pe= -3.911-0.5log(HS-) - 0.5pH 11. Fe 2 (OH) 5 /Fe 3 S 4 (SO 4 -region) 63e- + 79H + + 8SO 4 2- + 3Fe 2 (OH) 5 = 2Fe 3 S 4 + 47H 2 O log K = 365.13 pe = 5.7-(79/63)pH+(8/63)log(SO 4 2- ) A. AsS/H 3 AsO 3 6H 2 O + 2AsS = 2H 3 AsO 3 + 2HS - + 4H + + 2e - log K = -42.06 pe =21.03 + log(h 3 AsO 0 3 ) +log(hs-) - 2pH - B. AsS/H 2 AsO 3 6H 2 O + 2AsS = 2H 2 AsO - 3 + 2HS - + 6H + + 2e - log K = -60.55 pe =30.27 + log(h 2 AsO - 3 ) + log(hs-) - 3pH 0 C. As 2 S 3 /H 3 AsO 3 18H 2 O + As 2 S 3 = 2H 3 AsO 3 + 3SO 2-4 + 30H + + 24e - log K = -147.3 pe =6.14 + 1/12(log(H 3 AsO 0 3 )) + 1/8(log(SO 2-4 )) - 1.25pH D. FeS/Fe 2+ and H 2 S (<ph7) (pe is independent of ph) 2H + + FeS = H 2 S + Fe 2+ log K = 2.31 S16
ph = -0.5log(H 2 S )- 0.5log(Fe 2+ ) + 1.15 E. Fe 3 S 4 /Fe 2+ 3Fe 2+ + 4H 2 S= Fe 3 S 4 + 8H + + 2e- log K = 6.14 pe =3.07-3/2(log(Fe 2+ )) - 2log(H 2 S) - 4pH i. SO 2-4 / HS - 8e- + 9H + + SO 2-4 = HS - + 4 H 2 O log K = 33.04 pe= 1/8(log(SO 2-4 )) -1/8(log(HS - )) + 4.13-9/8pH ii. SO 2-4 / H 2 S 8e- + 10H + + SO 2-4 = H 2 S + 4 H 2 O log K = 40.62 pe = -1/8(log(H 2 S)) + 5.08-5/4pH + 1/8(log(SO 2-4 )) iii. H 2 S /HS - HS - + H + = H 2 S log K = 7.042 ph =-log(h 2 S) + log(hs - ) + 7.04 iv. H 2 AsO - 3 /H 3 AsO 3 H 2 AsO - 3 + H + = H 3 AsO 3 log K = 9.24 ph=9.24 + log(h 2 AsO - 3 ) - log( H 3 AsO 3 ) Table S-7. Comparison of pe values for ph edge simulations [FeS] Acidic Region Alkaline Region (g/l) (mol/l) ph 5 ph 8 ph 9 ph 10 40 0.4550-5.0-8.62-9.59-10.8 10 0.1137-4.5-7.5-8.45-10.1 1 0.0114-3.7-6.7-7.85-10.0 0.5 0.0057-3.0-6.0-7.74-9.98 0.1 0.0011-2.5-5.4-7.63-9.96 S17
Table S-8. E h measured as a function of ph and mackinawite concentration ph 5 ph 7 ph 9 g FeS/L Eh(mV) pe (Eh*0.0169) 0.1-127.4-2.15 0.2-180.5-3.05 0.5-209.8-3.55 1-225 -3.80 5-261.1-4.41 10-251.5-4.25 g FeS/L Eh(mV) pe (Eh*0.0169) 0.1-234.5-3.96 0.2-240.6-4.07 0.5-273.5-4.62 1-276.4-4.67 5-331.9-5.61 10-360.2-6.09 g FeS/L Eh(mV) pe (Eh*0.0169) 0.1-371 -6.27 0.2-371.4-6.28 0.5-391.9-6.62 1-411.8-6.96 5-436.8-7.38 10-454.7-7.69 Table S-9. Sensitivity Analysis for model fits. Deviation from best fit Sample + 0.01 pe - 0.01 pe + 0.02 pe - 0.02 pe 0.1g/L (acidic) 3.0% 3.0% 3.0% 3.0% 1g/L (acidic) 0.08% 0.08% 0.04% 0.05% 10g/L (acidic) 0.01% 0.01% 0.01% 0.01% 0.1 g/l (alkaline) 4.0% 3.0% 8.0% 7.0% 1 g/l (alkaline) 3.0% 3.0% 6.0% 6.0% 10 g/l (alkaline) 2.0% 2.0% 4.0% 4.0% S18
pe-ph diagram for Fe-As-S-H 2 O system Figure S-1. pe-ph diagram for Fe-As-S-H 2 O system corresponding to equations in Table S-6. Boxed numbers indicate boundary between solids, boxed uppercase letters indicate stability between aqueous and solid phases and roman lowercase numerals indicate boundary between dissolved species. See Table S-6 for list of reactions corresponding to the letters/numbers shown in the boxes. pe -1-3 -5-7 -9-11 As 2 S 3 Fe 2+ + Fe 2+ + AsS 4 1 Fe 2+ + H 2 S As 0 E 2 C D 5 9 ii 3 iii Fe 3 S 4 AsS H 3 AsO 3 SO 4 2- Fe 2 (OH) 5 6 FeS AsS Fe 2+ + HS - As 0 i H 2 AsO - 3 HS - Fe 2 (OH) 5 5 6 7 8 9 10 10 11 A 8 7 iv B ph S19
Figure S-2. Total iron concentrations measured in 0.1, 0.5, 1, and 10 g/l FeS after equilibration with 1.3 10-5 M As(III) and in the absence of arsenite (1g/L system only). S20
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