Supporting Information. Model Predictions of Realgar Precipitation by Reaction of As(III) with Synthetic

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1 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

2 18 19 Figure S-2. Total iron concentrations measured in 0.1, 0.5, 1, and 10 g/l FeS after equilibration with M As(III) and in the absence of arsenite (1g/L system only) S2

3 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 = J/(mol*K), T=298 K and G 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

4 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 (1) H(+) 0.00 Zero reference state AsO 3 (3-) (2) Cl(-) (3) Fe(2+) (4) Na(+) (4) HS(-) (1) Table S-2. Gibbs free energy of formation values for all species used in model G species (KJ/mol) Reference Species Charge OH- (-1) Fe(OH) 3 - (-1) Fe(OH) 2 (0) FeOH + (+1) HAsO 3 (-2) H 3 AsO 3 (0) H 2 AsO 3 (-1) H 4 AsO 3 (+1) AsO 4 (-3) HAsO 4-2 (-2) H 2 AsO 4 - (-1) H 3 AsO 4 (0) H 2 S (0) Fe(HS) 2 (0) Fe(HS)3- (-1) S 2 O 3 (-2) SO 4 (-2) HSO 4 (-1) FeSO 4 (0) Fe(III)SO 4 (+1) S4

5 Fe(SO 4 ) 2 (-1) Fe(III) (+3) S 2 (-2) S 3 (-2) S 4 (-2) S 5 (-2) S 6 (-2) H 2 S 2 O 3 (0) HS 2 O 3 (-1) HSO 3 (-1) SO 3 (-2) Fe(III) (+3) FeOH (+2) Fe(OH) 2 (+1) Fe 2 (OH) 2 (+4) Fe(OH) 3 (0) Fe 3 (OH) 4 (+5) Fe(OH) 4 (-1) AsS(OH)(SH) (-1) As(OH) 2 (SH) (-1) As(OH) 2 S- (-1) As(OH)S 2 (-2) AsS 3 (-3) HS 3 As (-2) As(HS) 4 (-1) FeHS (+1) (SH) 2 As 3 S 4 (-1) Fe(II)Cl 2 (0) Fe(III)Cl (+2) Fe(III)Cl 2 (+1) Fe(III)Cl 3 (0) Fe(III)C l3 (molysite) (0) MACKINAWITE (0) 0 1 As(s) (0) As 2 S 3 (am) (0) ARSENOLITE (0) CLAUDETITE (0) ORPIMENT (0) Fe(OH) 2 (0) FeS (ppt) (0) HALITE (0) 0 8 Sulfur (0) FeOOH(goethite) (0) Fe 3 O 4 (magnetite) (0) Fe 3 (OH) 8 (0) Fe(OH)3 soil (0) Fe 2 O 3 (maghemite) (0) Fe 3 S 4 (Greigite) (0) S5

6 Fe 7 S 8 (pyrrhotite) (0) FeSO 4 (0) Fe 2 (SO4) 3 (0) Fe 4 (OH) 8 Cl (0) Fe 6 (OH) 12 SO 4 (0) Fe 3 (OH) 7 (0) Fe(OH) 3 (am) (0) Fe 2 (OH) 5 (0) FeAsS(arsenopyrite) (0) AsS (0) Fe 2 As (0) FeAs (0) FeAs 2 (lollingite) (0) WUSTITE (-0.11) PYRITE (0) Fe(OH) 3 (0) (lepidocrocite) Fe 2 O 3 (hematite) (0) FeS (pyrhotite) (0) 0 17 Fe(0) metal (0) FeS 2 (marcasite) (0) FeS (troilite) (0) Fe 2 S 3 (0) FeO (0) Fe(OH) 3 (c) (0) NaSO 4 (0) Na 2 SO 4 (0) 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

7 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) Iron Species Fe(OH) 3 (-1) Fe(OH) 2 (aq) FeOH(+1) Fe(III)(+3) FeOH(+2) Fe(OH) 2 (+1) Fe2(OH) 2 (+4) Fe(OH) 3 (aq) Fe3(OH) 4 (+5) Fe(OH) 4 (-1) FeHS(+1) * FeOCl(aq) Fe(II)Cl 2 (aq) Fe(II)Cl(+1) Fe(III)Cl(+2) Fe(III)Cl2(+1) Sulfur Species S(-2) H 2 S(aq) Fe(HS) 2( aq) Fe(HS)3(-1) S 2 O 3 (-2) SO 4 (-2) S7

8 HSO 4 (-1) FeSO 4 (aq) Fe(III)SO 4 (+1) Fe(SO 4 ) 2 (-1) S 2 (-2) S 3 (-2) S 4 (-2) S 5 (-2) S 6 (-2) H 2 S 2 O 3 (aq) HS 2 O 3 (-1) HSO 3 (-1) SO 3 (-2) NaSO 4 (-1) Arsenic Species HAsO 3 (-2) H 3 AsO 3 (aq) H 2 AsO 3 (-1) H 4 AsO 3 (+1) AsS(OH)(SH)(-1) As(OH) 2 (SH) As(OH) 2 S(-1) As(OH)S 2 (-2) AsS 3 (-3) HS 3 As(-2) As(HS) 4 (-1) (SH) 2 As 3 S 4 (-1) S8

9 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) Fe(III)Cl 3 (molysite) FeOOH(goethite) Fe 3 O 4 (magnetite) Fe 3 (OH) Fe(OH) 3 (soil) Fe 2 O 3 (maghemite) Fe 3 S 4 (Greigite) WUSTITE(-0.11) PYRITE * Fe(OH) (lepidicrocite) Fe 2 O 3 (hematite) FeS (pyrrhotite)* Fe(0)metal* FeS 2 (marcasite)* FeS(troilite)* Fe 2 S 3 * FeO Fe(OH) 3 ( c) Na 2 SO MACKINAWITE Fe(OH) FeS(ppt) S9

10 HALITE Sulfur Fe 7 S 8 (pyrrhotite) * FeSO Fe 4 (OH) 8 Cl Fe 6 (OH) 12 SO Fe 3 (OH) 7 * Fe(OH) 3 (am) Fe 2 (OH) Arsenic Solids AsS(realgar) FeAsS(arsenopyrite) AsS Fe 2 As FeAs FeAs 2 (lollingite) As(s)(native) As 2 S 3 (am) ARSENOLITE CLAUDETITE ORPIMENT 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

11 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 ( kj/mol= G 0 f ), FeAs ( kj/mol= G 0 f ) and lollingite (FeAs 2 ) ( 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 f = kj/mol (6), orpiment ( G f =-84.3 kj/mol) (2, 10) and realgar ( G f = kj/mol) (16). Although the thermodynamic free energy of formation constant for arsenopyrite of G 0 f = kj/mol (6) used in this study falls at the high end of the spectrum for reported values ranging from G f 0 = ±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 = 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

12 Species and Reaction log K Solid Species 3H 2 O + AsO 3 (3-) + Fe(2+) = 3e- + 2H(+) + FeAsO 4 *2H 2 O (SCORODITE) H 2 O + AsO 3 (3-) + Fe(2+) = FeAsO 4 +3e- + 2H(+) H 2 O + 2 AsO 3 (3-) + 3Fe(2+) = Fe 3 (AsO 4 ) 2 +4e- + 4H(+) H(+) + 2 AsO 3 (3-) = H 2 O + 4e- + As 2 O H(+) + 4 AsO 3 (3-) = 6H 2 O + As 4 O 6 (ARSENOLITE) H(+) + 4 AsO 3 (3-) = 6H 2 O + As 4 O 6 ( CLAUDETITE) H(+) + AsO 3 (3-) + 3e- = 3H 2 O + As(s) H(+) + AsO3(3-)+ Fe(2+) + HS(-) + 3e-= 3H 2 O + FeAsS (ARSENOPYRITE) H(+) + AsO 3 (3-) + 2Fe(2+) + 7e- = 3H 2 O + Fe2As H(+) + 2AsO 3 (3-) + Fe(2+) + 8e- = 6H 2 O + FeAs 2 (LOLLINGITE) H(+) + 2 AsO 3 (3-) + 3HS(-) = 6H2O + As 2 S 3 (ORPIMENT) e(-) + 5H(+) + AsO 3 (3-) + HS(-) = 3H 2 O + AsS (REALGAR) 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 ( kj/mol= G 0 f ), H 3 AsO 3 ( kj/mol= G f 0 ), H 2 AsO 3 - ( kj/mol= G f 0 ), AsO 4 3- ( kj/mol= G f 0 ), HAsO 4-2 ( kj/mol= G 0 f ), H 2 AsO - 4 ( kj/mol= G 0 f ), and H 3 AsO 4 ( 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

13 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 ) (( 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 = kj/mol), As(OH) 2 (SH) - 0 ( G f = kj/mol), As(OH) 2 S - 0 ( G f = kj/mol), As(OH)S ( G f = kj/mol), AsS ( G f = kj/mol), HAsS 2-0 ( G f = kj/mol= G 0 f ), As(SH) ( G f = 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

14 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) H(+) + AsO 3 (3-) = H 3 AsO H(+) + AsO 3 (3-) = H 2 AsO 3 (-) H(+) + AsO 3 (3-) = H 4 AsO 3 (+) H 2 O + AsO 3 (3-) = 2H(+) + 2e- + AsO 4 (-2) H 2 O + AsO 3 (3-) = H(+) + 2e- + HAsO 4 (-2) H 2 O + AsO 3 (3-) = 2e- + H 2 AsO 4 (-) H 2 O + H(+) + AsO 3 (3-) = 2e- + H 3 AsO 4 (0) Thioarsenite species 14H(+) + 3AsO 3 (3-) + 6HS(-) = As 3 S 4 (SH) 2 (-) + 9 H 2 O H(+) + AsO 3 (3-) + 2HS(-) = AsS(OH)(SH)(-) + 2 H 2 O H(+) + AsO 3 (3-) + HS(-) = As(OH) 2 (SH)(aq)+ H 2 O H(+) + AsO 3 (3-) + HS(-) = As(OH) 2 S(-1) + H 2 O H(+) + AsO 3 (3-) + 2HS(-) = As(OH)S 2 (-2)+ 2 H 2 O H(+) + AsO 3 (3-) + 3HS(-) = AsS 3 (-3) + 3 H 2 O H(+) + AsO 3 (3-) + 3HS(-) = HS 3 As(-2) + 3 H 2 O H(+) + AsO 3 (3-) + 4HS(-) = As(HS) 4 (-1) + 3 H 2 O S14

15 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 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 = /2log(H 2 S) - ph 3. AsS/As 2 S 3 (>ph 7) 2e- + 2H + + As 2 S 3 = 2AsS + HS- log K = pe = /2 log(hs-) 1/2pH 4. AsS /As 0 ph<6 2e- + 2H + + AsS = As 0 + H 2 S log K = -1.4 pe = /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) ph 6. AsS + FeS/As 0 >ph7 2e- + 2H + + AsS +FeS = As 0 + 2HS- + Fe 2+ log K = pe = log(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 = pe= ph + 2log(HS - ) 8. Fe 2 (OH) 5 /Fe 3 S 4 (HS-region ph>7) e-+ 2Fe 3 S H 2 O=3Fe 2 (OH) 5 + 8HS- + 7H + log K = S15

16 pe = log(HS-) + 7pH 9. FeS/ Fe 3 S 4 (<ph7) 3FeS+ H 2 S =Fe 3 S 4 + 2H + + 2e- log K = 0.78 pe= log(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= log(HS-) - 0.5pH 11. Fe 2 (OH) 5 /Fe 3 S 4 (SO 4 -region) 63e- + 79H + + 8SO Fe 2 (OH) 5 = 2Fe 3 S H 2 O log K = 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 = pe = log(h 3 AsO 0 3 ) +log(hs-) - 2pH - B. AsS/H 2 AsO 3 6H 2 O + 2AsS = 2H 2 AsO HS - + 6H + + 2e - log K = pe = 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 H e - log K = pe = /12(log(H 3 AsO 0 3 )) + 1/8(log(SO 2-4 )) pH 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

17 ph = -0.5log(H 2 S )- 0.5log(Fe 2+ ) E. Fe 3 S 4 /Fe 2+ 3Fe H 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 H 2 O log K = pe= 1/8(log(SO 2-4 )) -1/8(log(HS - )) /8pH ii. SO 2-4 / H 2 S 8e- + 10H + + SO 2-4 = H 2 S + 4 H 2 O log K = pe = -1/8(log(H 2 S)) /4pH + 1/8(log(SO 2-4 )) iii. H 2 S /HS - HS - + H + = H 2 S log K = ph =-log(h 2 S) + log(hs - ) iv. H 2 AsO - 3 /H 3 AsO 3 H 2 AsO H + = H 3 AsO 3 log K = 9.24 ph= 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 S17

18 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) g FeS/L Eh(mV) pe (Eh*0.0169) g FeS/L Eh(mV) pe (Eh*0.0169) Table S-9. Sensitivity Analysis for model fits. Deviation from best fit Sample pe pe pe 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

19 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 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) A 8 7 iv B ph S19

20 Figure S-2. Total iron concentrations measured in 0.1, 0.5, 1, and 10 g/l FeS after equilibration with M As(III) and in the absence of arsenite (1g/L system only). S20

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