1. Reaction Path Modeling Chpt. 8 Zhu & Anderson

2. 2 Reaction Path Models Used to model open systems Variable composition; relative time scale (reaction progress variable ξ) Types: –Titration or mixing (most common) (B Fig. 2.3) –Polythermal (B Fig. 2.2) –Buffering or sliding fugacity (B Fig. 2.4) –Flow-through: tracks evolution of fluid composition as it flows through rock. Reaction products left behind, isolated from further reaction (like fractional crystallization). –Flush: tracks chemical evolution of system through which fluid migrates; unreacted fluid displaces reacted fluid (B Fig. 2.7, ZA Fig. 2.5). –Kinetic reaction path model

3. Reaction Path Models Describe irreversible reactions or processes using a series of partial equilibrium states. Thought of as a very slow titration. Results do not always correspond exactly to reality because we are using thermodynamics to describe a series of disequilibrium states. But still provide insights into the processes involved.

4. Alkalinity titration (Z&A 8.2) # React script, saved Sun Feb 23 2003 by Dabieshan title = "Titrate Bear Creek sample MW-36 with HCl" data = "E:\Program Files\Gwb\Gtdata\thermo.com.v8.r6+.dat" verify temperature = 15 1 kg free H2O total mg/l Ca++ = 158 total mg/l Fe++ =.01 total mg/l Mg++ = 21 total mg/l Mn++ =.11 total mg/l K+ = 17 balance on Na+ total mg/l Na+ = 61 total mg/l SO4-- = 425 total mg/l Cl- = 25 total mg/l Al+++ =.01 total mg/l SiO2(aq) = 5.6 pH = 7.4 total molality HCO3- =.00305 react.004 mol of HCl(aq)

5. Compare w/ Fig. 8.2 calculated using Phreeqc (script in Table 8.1)

6. Acidity of Acid Mine Water (Z&A 8.3) # React script title = "Titrate Bear Creek sample TS-3 with Calcite" data = “C:\Program Files\Gwb\Gtdata\thermo.com.v8.r6+.dat" verify temperature = 16 1 kg free H2O total mg/l Ca++ = 310 total mg/l Fe++ = 1950 total mg/l Mg++ = 1000 total mg/l Mn++ = 66.3 total mg/l K+ = 60 balance on Na+ total mg/l Na+ = 89 total mg/l SO4-- = 16500 total mg/l Cl- = 550 total mg/l Al+++ = 1020 total mg/l SiO2(aq) = 40.5 pH = 3.8 total mg/l HCO3- = 5 react.25 mol of Calcite suppress ALL unsuppress Fe(OH)3 Gibbsite Gypsum For acidic water the carbonate acidity is a small part of total acidity. Titrate solution with calcite to measure the“operational acidity”, the amount of CaCO3 required to saturate the solution.

7. Compare with Fig. 4 Zhu & Anderson.

8. All minerals suppressed except gypsum, gibbsite & Fe(OH)3. Compare with Fig. 8.4 Zhu & Anderson.

10. Weathering of K-feldspar If water/rock ratio high, all K-feldspar converted to kaolinite, composition of weathered rock determined by its environment, i.e., the composition of water. If water/rock ratio low, water dissolves soluble primary minerals (K-spar) & precipitates insoluble secondary minerals (kaolinite), concentrations of dissolved K + and H 4 SiO 4 ↑: 2 KAlSi 3 O 8 + 9 H 2 O + 2 H + ⇌ Al 2 Si 2 O 5 (OH) 4 + 2 K + + 4 H 4 SiO 4

11. log K eq = 4 log [H 4 SiO 4 ] + 2 log [K + ] – 2 log [H + ] log K eq = 4 log [H 4 SiO 4 ] + 2 log [K + ]/[H + ] log [K + ]/[H + ] = ½ log K eq – 2 log [H 4 SiO 4 ] plot y = [K + ]/[H + ] and x = log [H 4 SiO 4 ], get straight line with m = -2 and b = ½ log K eq.

13. React Script: K-feldspar weathering data = "e:\program files\gwb\gtdata\thermo.dat" verify work_dir = E:\Users\Ayers\GEO320\GWB temperature = 25 1 kg free H2O total mg/kg Na+ = 5 total mg/kg K+ = 1 total mg/kg Ca++ = 15 total mg/kg Mg++ = 3 total ug/kg Al+++ = 1 total mg/kg SiO2(aq) = 3 total mg/kg Cl- = 30 total mg/kg SO4-- = 8 total mg/kg HCO3- = 50 pH = 5 react 2 mmol of K-feldspar suppress Clinoptil-K Mordenite-K

16. Extracting the Overall Reaction When mmoles of secondary minerals produced are plotted as a function of mmoles of primary mineral consumed, the slopes of the lines give the reaction coefficient for each species and mineral in the overall reaction (only if both axes use linear scales and consistent units). Three reaction segments: 1) precipitation of kaolinite, 2) transformation of kaolinite to muscovite, 3) further formation of muscovite once kaolinite exhausted.

17. Segment 1Segment 2Segment 3 CO2(aq)0-.67 HCO3-+10+.67 K++10+.67 H2O-1.5+1-.67 Quartz+2 Kaolinite+.5 Muscovite+1.33 1)KAlSi 3 O 8 + 3/2 H 2 O + CO 2 (aq) ⇌ 2 SiO 2 + ½ Al 2 Si 2 O 5 (OH) 4 + HCO 3 - + K + 2)KAlSi 3 O 8 + Al 2 Si 2 O 5 (OH) 4 ⇌ 2 SiO 2 + KAl 3 Si 3 O 10 (OH) 2 + H 2 O 3)KAlSi 3 O 8 + 2/3 CO 2 (aq) + 2/3 H 2 O ⇌ 2 SiO 2 + 1/3 KAl 3 Si 3 O 10 (OH) 2 + 2/3 HCO 3 - + 2/3 K +

18. K-feldspar weathering: Ex6 in Phreeqci Example illustrates how to locate: –points representing intersection of reaction trace with phase boundaries. –trace reaction path between phase boundaries. –use kinetic equations to calculate amount of time required for each segment of reaction path. Other features: –can use “Phases” keyword block to add new phases or modify existing phases in thermodynamic database (allows calculation at temperatures significantly different from 25C). –can’t create activity diagram in Phreeqci; use same database (LLNL) to create diagram in GWB Act2 and import trace from reformatted Phreeqci output file.

19. K-feldspar weathering script in Phreeqci SOLUTION_SPREAD -redox O(-2)/O(0) -units mg/kgw Na K Ca Mg Al Si Cl S(6) pH pe C(4) ug/kgw charge CO2(g) -2 O2(g) -0.7 5 0.0001 1 3 1 0.0001 30 8 5 10 50 INCREMENTAL_REACTIONS True EQUILIBRIUM_PHASES 1 Gibbsite 0 0 Kaolinite 0 0 Muscovite 0 0 Pyrophyllite 0 0 SiO2(am) 0 0 K-Feldspar 0 0 REACTION 1 K-feldspar 1 0.2 millimoles in 100 steps SELECTED_OUTPUT -file Kfeldspar.out -reset false -step true -ph true -pe true -reaction true -totals Si -activities H+ K+ SiO2 -equilibrium_phases Kaolinite K-feldspar Muscovite Gibbsite Pyrophyllite SiO2(am) -saturation_indices Gibbsite Kaolinite K-Feldspar Muscovite Pyrophyllite SiO2(am)

20. Figure 6.--Phase diagram for the dissolution of K-feldspar (microcline) in pure water at 25 o C showing stable phase-boundary intersections (example 6A) and reaction paths across stability fields (example 6B). Diagram was constructed using thermodynamic data for gibbsite, kaolinite, K-mica (muscovite), and microcline from Robie and others (1978).

24. Hydrothermal Fluids: Fluorite Deposits # React script, saved Wed Mar 31 2004 by ayersj data = "c:\program files\gwb\gtdata\thermo.dat" verify temperature initial = 175, final = 125 1 kg free H2O total mol SiO2(aq) =.00157131343 total mol Al+++ =.271987822 total mol F- =.767745894 total mol SO4-- =.00144353827 total mol H+ =.236805901 total mol Ca++ = 1.09283049 total mol Na+ = 3.71859882 total mol K+ =.524567232 total mol Mg++ =.0233140992 total mol Cu+ =.0116570496 total mol HCO3- =.233140992 balance on Cl- total mol Cl- = 6.53610768 TDS = 247500 density = 1.14

26. AlF 3 + ½ CaCl + = AlF 2 + + ½ CaF 2 + ½ Cl -

27. Are results reliable? Fluorite is the only phase that precipitates in significant amounts, consistent with observation that deposit is ~90% fluorite. Decrease in concentration (stability) of neutral complex AlF3 is consistent with known behavior of the dielectric constant of water (charged species favored at low T). However, –get very different results using Phreeqci (see script C:\Documents and Settings\Administrator\My Documents\GMEAP\Phrqci\Fluorite_deposition.pqi). –Ionic strength much higher than recommended range of B-dot equation used to calculate activities in GWB. –Thermodynamic data required to accurately calculate values of equilibrium constants of critical reactions at these elevated temperatures are not present in the GWB databases (see output from program Rxn using LLNL database on next slide).

28. Rxn input # Rxn script, saved Thu Apr 01 2004 by ayersj data = "C:\Program Files\Gwb\Gtdata\thermo.com.v8.r6+.dat" verify react Fluorite swap CaCl+ for Ca++ swap AlF3(aq) for F- swap AlF++ for Al+++ activity CaCl+ = ? activity AlF3(aq) = ? activity Cl- = ? activity AlF++ = ? long

29. Rxn output Fluorite + Cl- + AlF++ = CaCl+ + AlF3(aq) Log K's: 0 °C: ??? 150 °C: ??? 25 °C: -1.0326 200 °C: ??? 60 °C: ??? 250 °C: ??? 100 °C: ??? 300 °C: ??? Polynomial fit: log K = -1.033 + 0 × T + 0 × T^2 + 0 × T^3 + 0 × T^4 Equilibrium equation: log K = log a[CaCl+] + log a[AlF3(aq)] - log a[Cl-] - log a[AlF++]

30. Rxn script # Rxn script, saved Thu Apr 01 2004 by ayersj data = "C:\Program Files\Gwb\Gtdata\thermo.com.v8.r6+.dat" verify react Fluorite swap Anhydrite for Ca++ swap Muscovite for Al+++ swap Quartz for SiO2(aq) swap Alunite for SO4-- swap Kaolinite for H+ swap AlF3(aq) for F- activity AlF3(aq) = ? activity K+ = ? activity H2O = ? long

31. Rxn output Fluorite +.8333 Quartz +.5 Alunite +.3333 Kaolinite = Anhydrite +.6667 AlF3(aq) +.5 Muscovite + 1.667 H2O Log K's: 0 °C: ??? 150 °C: ??? 25 °C: -2.6268 200 °C: ??? 60 °C: ??? 250 °C: ??? 100 °C: ??? 300 °C: ??? Polynomial fit: log K = -2.627 + 0 × T + 0 × T^2 + 0 × T^3 + 0 × T^4 Equilibrium equation: log K =.6667 × log a[AlF3(aq)] + 1.667 × log a[H2O]

32. Seawater Evaporation # React script, saved Tue Feb 25 2003 by Dabieshan data = "e:\program files\gwb\gtdata\thermo_hmw.dat" verify temperature = 25 decouple ALL swap CO2(g) for H+ 1 kg free H2O fugacity CO2(g) =.000316227766 total mg/kg Na+ = 10760 total mg/kg Mg++ = 1290 total mg/kg Ca++ = 411 total mg/kg K+ = 399 balance on Cl- total mg/kg Cl- = 19350 total mg/kg SO4-- = 2710 total mg/kg HCO3- = 142 TDS = 35000 react -996 gram of H2O flow-through delxi =.001 linear dxplot = 0 Flow-through model described on pg. 118 of GWB Users Guide. Equivalent to fractional crystallization – solids separated from liquid, traces chemical evolution of liquid.

34. Calcite precipitation in hot water heater # React script, saved Wed Mar 03 2004 by ayersj data = "c:\program files\gwb\gtdata\thermo.dat" verify temperature initial = 25, final = 50 swap Calcite for Ca++ swap CO2(g) for HCO3- 1 kg free H2O free gram Calcite = 1 balance on H+ fugacity CO2(g) =.000316227766 fix fugacity of CO2(g)

35. Ca++ + 2HCO3- = CaCO3 + H2CO3

36. Heat calcite-saturated water w/ 0.1 mmolal HCl First run previous script at T = 25C, then “pickup” fluid # React script, saved Wed Mar 03 2004 by ayersj data = "c:\program files\gwb\gtdata\thermo.dat" verify temperature initial = 25, final = 50 swap CO2(g) for HCO3- 1 kg free H2O total mol Ca++ =.00044154404 fugacity CO2(g) =.000316227766 balance on H+ total mmolal H+ =.1 total mmolal Cl- =.1

37. Effect of 0.1 mmolal HCl inhibitorSaturation Indices