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Introduction to PHREEQC—Chemistry for PHAST

Introduction to PHREEQC—Chemistry for PHAST. Flow. Transport. Chemistry. Flow. Transport. Chemistry. PHAST. HST3D—Flow and transport PHREEQC—Chemistry Operator splitting—Sequential Non-Iterative Approach. PHREEQC. PHAST chemistry is inherited from PHREEQC

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Introduction to PHREEQC—Chemistry for PHAST

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  1. Introduction to PHREEQC—Chemistry for PHAST

  2. Flow Transport Chemistry Flow Transport Chemistry PHAST • HST3D—Flow and transport • PHREEQC—Chemistry • Operator splitting—Sequential Non-Iterative Approach

  3. PHREEQC • PHAST chemistry is inherited from PHREEQC • PHREEQC is run at the beginning of PHAST • Solutions and reactants for initial conditions • Solutions for boundary conditions • PHREEQC is run cell-by-cell for each time step in the reactive-transport simulation

  4. PHREEQC Reactants • Keyword data blocks define reactants • SOLUTION—Solutions • EQUILIBRIUM_PHASES—Equilibrium minerals and gases • EXCHANGE—Exchangers • SURFACE—Surfaces • KINETICS and RATES—Kinetic reactions • SOLID_SOLUTIONS—Solid solutions • GAS_PHASE—gas bubble (rarely used) • Store reactants on shelves by type and number

  5. SOLUTION—Chemical Composition of a Water • Chemical analysis • pH • Temperature • Major elements Ca, Mg, Na, K, Alkalinity, Cl, SO4 • Trace elements • Nutrients

  6. SOLUTION Data Block • SOLUTION 1: Oklahoma Brine units mol/kgw pH 5.713 temp 25. Ca .4655 Mg .1609 Na 5.402 Cl 6.642 C .00396 S .004725 As .03 (ug/kgw)

  7. EQUILIBRIUM_PHASESMinerals and gases that react to equilibrium Calcite reaction CaCO3 = Ca+2 + CO3-2 Equilibrium K = [Ca+2][CO3-2]

  8. EQUILIBRIUM_PHASES Data Block • Mineral or gas • Saturation state • Amount Example EQUILIBRIUM_PHASES 5: CO2 Log PCO2 = -2, 10 moles Calcite equilibrium 1 moles Dolomite equilibrium 1 moles Fe(OH)3 equilibrium 0 moles

  9. EXCHANGECation exchange composition Reaction: Ca+2 + 2NaX = CaX2 + 2Na+ Equilibrium:

  10. EXCHANGE Data Block • Exchanger name • Number of exchange sites • Chemical composition of exchanger Example EXCHANGE 15: CaX2 0.05 moles (X is defined in databases) NaX 0.05 moles Often X 0.15 moles, Equilibrium with solution 1

  11. SURFACE—Surface CompositionTrace elements Zn, Cd, Pb, As, P Reaction: Hfo_wOH + AsO4-3 = Hfo_wOHAsO4-3 Equilibrium:

  12. SURFACE Data Block • Surface name—Hfo is Hydrous Ferric Oxide • Number of surface sites • Chemical composition of surface Example SURFACE 21: Hfo_wOH 0.001 moles Hfo_sOH 0.00005 moles Often Hfo_w 0.001 moles, Equilibrium with solution 1

  13. KINETICS—Nonequilibrium Reactions • Monod Kinetics • Radioactive decay • Silicate hydrolosis • Biological processes

  14. KINETICS and RATES Data Blocks • Kinetic reaction name • Stoichiometry of reaction • Rate expression (RATES) Example KINETICS 21: DOC_decay formula Doc -1 CH2O +1 RATES 10 Rate = 0.01*TOT(“Doc”) 20 SAVE rate*TIME

  15. Solution 1 Equilibrium phases 5 Surface 21 PHREEQC—Reactions To the beaker From the shelf Kinetic reaction and equilibration

  16. Arsenic in the Central Oklahoma Aquifer • Arsenic mostly in confined part of aquifer • Arsenic associated with high pH • Flow: unconfined to confined back to unconfined

  17. Geochemical Reactions • Brine initially fills the aquifer • Calcite and Dolomite equilibrium • Cation exchange • 2NaX + Ca+2 = CaX2 + 2Na+ • 2NaX + Mg+2 = MgX2 + 2Na+ • Surface complexation Hfo-HAsO4- + OH- = HfoOH + HAsO4-2 Desorption at pH > 8.5

  18. Where we are headed • Make a brine • Define exchanger • Define surface • Define recharge water • Define minerals in aquifer • Simulate inflow of recharge water into brine-filled aquifer

  19. Na SO4 Ca Mg Fe Cl HCO3 Solution Definition and Speciation Calculations Inverse calculations Saturation Indices Speciation calculation Reaction calculations

  20. SOLUTION—Define solution composition SOLUTION_SPREAD—Spreadsheet input for solution composition Other data blocks related to speciation SOLUTION_MASTER_SPECIES—Redox states and gram formula weight SOLUTION_SPECIES—Reaction and log K PHREEQC Data Blocks

  21. Seawater: units are ppm

  22. PHREEQC Names and Default Gram Formula Weights

  23. Solution Data Block

  24. pH, pe, Temperature

  25. Solution Composition • Set default units! • Select analytes • Enter concen-trations • Set “As”, special units • Click OK when done

  26. Run Speciation Calculation Run Select files, phreeqc.dat

  27. Exercise: Speciate seawater Use PhreecI to run a speciation calculation for seawater using phreeqc.dat database.

  28. What is a speciation calculation? • Input: • pH • pe • Concentrations • Equations: Mass-balance—sum of the calcium species = total calcium Mass-action—activity of products divided by reactants = constant Activity coefficients—function of ionic strength • Output • Molalities, activities • Saturation indices

  29. What is pH? Questions 1. How does the pH change when CO2 degasses during an alkalinity titration? 2. How does pH change when plankton respire CO2? 3. How does pH change when calcite dissolves? pH = 6.3 + log[(HCO3-)/(CO2)]

  30. What is pe? Fe+2 = Fe+3 + e- pe = log( [Fe+3]/[Fe+2]) + 13 HS- + 4H2O = SO4-2 + 9H+ + 8e- pe = log( [SO4-2]/[HS-]) – 9/8pH + 4.21 N2 + 6H2O = 2:NO3- + 12H+ + 10e- pe = 0.1log( [NO3-]2/[N2] ) –1.2pH + 20.7 pe = 16.9Eh, Eh platinum electrode measurement

  31. Mass-Action Equations H+ + CO3-2 = HCO3-

  32. Mass Balance Calcium mass balance: Catot = (Ca+2) + (CaSO4) + (CaHCO3+) + (CaCO3) + (CaOH+) + (CaHSO4+) In millimoles per kilogram of water: 10.7 = 9.5 + 1.1 + 0.05 + 0.03 + 0.0009 + 6e-8

  33. Activity Coefficients WATEQ activity coefficient Davies activity coefficient Pitzer activity coefficients High ionic strength Limited model

  34. Results of Speciation Calculation

  35. SATURATION INDEXThe thermodynamic state of a mineral relative to a solution SI < 0, Mineral should dissolve SI > 0, Mineral should precipitate SI ~ 0, Mineral reacts fast enough to maintain equilibrium Maybe • Kinetics • Uncertainties

  36. Useful Mineral ListMinerals that may react to equilibrium relatively quickly

  37. Other SOLUTION Capabilities • Define pe by ratio of redox states—O(0)/H2O, N(5)/N(-3), Fe(3)/Fe(2), S(6)/S(-2) • Charge balance—pH or ionic element • Adjust element concentration to phase boundary—Al to gibbsite • Calculate pH from Alkalinity and C(4) (TDIC) • SOLUTION_SPREAD—Spreadsheet format

  38. Modifying the Database • Problems with arsenic thermo data • Arsenic aqueous model (Nordstrom) not consistent with sorption model (Dzombak and Morel) • Competition for surface sites between minor anion and major cations appears unrealistic

  39. Arsenic Thermodynamic Data from Dzombak and Morel SOLUTION_MASTER_SPECIES As H3AsO4 -1.0 74.9216 74.9216 SOLUTION_SPECIES #H3AsO4 primary master species H3AsO4 = H3AsO4 log_k 0.0 #H2AsO4- H3AsO4 = H2AsO4- + H+ log_k -2.243 delta_h -1.69 kcal #HAsO4-2 H3AsO4 = HAsO4-2 + 2H+ log_k -9.001 delta_h -0.92 kcal #AsO4-3 H3AsO4 = AsO4-3 + 3H+ log_k -20.597 delta_h 3.43 kcal

  40. Arsenic Surface Complexation from Dzombak and Morel SURFACE_MASTER_SPECIES Surf SurfOH SURFACE_SPECIES SurfOH = SurfOH log_k 0.0 SurfOH + H+ = SurfOH2+ log_k 7.29 SurfOH = SurfO- + H+ log_k -8.93 SurfOH + AsO4-3 + 3H+ = SurfH2AsO4 + H2O log_k 29.31 SurfOH + AsO4-3 + 2H+ = SurfHAsO4- + H2O log_k 23.51 SurfOH + AsO4-3 = SurfOHAsO4-3 log_k 10.58

  41. Exercise: Define Arsenic Chemistry • Cut and paste As aqueous species defined above • Cut and paste As surface complexation defined above • Add As to the SOLUTION definition for seawater 0.03 ppb • Run speciation

  42. Arsenic Speciation • Arsenic has been added as a new element • Predominant species is HAsO4-2 at pH 8.22 • Although not used yet, arsenic sorption has been defined

  43. SOLUTION EXCHANGE SURFACE KINETICS MIX REACTION EQUILIBRIUM_PHASES SOLUTION EXCHANGE SURFACE KINETICS EQUILIBRIUM_ PHASES Reaction Calculations + EQUILIBRATION REACTOR

  44. EQUILIBRIUM REACTIONS • Can be used as PHAST initial conditions • SURFACE • EXCHANGE • SOLID_SOLUTIONS • EQUILIBRIUM_PHASES

  45. NON-EQUILIBRIUM REACTIONS • REACTION • REACTION_TEMPERATURE • KINETICS (PHAST initial condition)

  46. SAVE and USE • Save results of calculations • Use previously defined SOLUTIONS, EQUILIBRIUM_PHASES, REACTIONs, etc • Use previously SAVEd SOLUTIONS, EQUILBRIUM_PHASES, etc

  47. Reactions Evaporating Seawater

  48. PHREEQC Processing and Output • Initial-solution calculation • Reaction calculation includes any of the following: • Simulation/END EQUILIBRIUM_PHASES 2 SOLUTION 1 SOLUTION 1 END SOLUTION 1 END MIX REACTION REACTION_TEMP EQUILIBRIUM_PHASES EXCHANGE SURFACE SOLID_SOLUTION GAS_PHASE KINETICS

  49. Exercise: Evaporate Seawater • Append to input file and “save as” • END • USE solution 1 • EQUILIBRIUM_PHASES 1 • Halite • SI = 0 • Alternate formula is H2O • Calcite—SI=0, moles=0 • Dolomite—SI=0, moles=0 • CO2(g)—SI=-1.5, moles=10 • Anhydrite—SI=0, moles=0 • Gypsum—SI=0, moles=0 • SAVE solution 1

  50. Exercise: Evaporate Seawater • How much water remains? • What is the concentration of Na, Cl?

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