1. Monday-Tuesday Solutions –Thermodynamics of aqueous solutions –Saturation indices Mineral equilibria Cation exchange Surface complexation Advective transport Diffusive transport Acid mine drainage 1

2. Processes that Control Major Element Chemistry 1. Carbonate reactions 2. Ion exchange 3. Organic carbon oxidation O 2 /Nitrate reduction Iron oxyhydroxide reduction Sulfate reduction Methanogenesis 4. Gypsum dissolution 5. Pyrite oxidation 6. Seawater evaporation 7. Silicate weathering

3. Processes that Control Minor Element Chemistry 1. Redox Oxyanions Trace metals Nitrate 2. Surface complexation Phosphate Oxyanions Trace metals 3. Cation exchange 4. Solid solutions 5. Minerals

4. PHREEQC Programs PHREEQC Version 3 –PHREEQC: Batch with Charting –PhreeqcI: GUI with Charting –IPhreeqc: Module for programming and scripting PHAST –Serial—soon to be Multithreaded –Parallel—MPI for transport and chemistry –TVD (not done) –4Windows—GUI just accepted WEBMOD-Watershed reactive transport 4

5. Solution Definition and Speciation Calculations Ca Na SO 4 Mg Fe Cl HCO 3 Reactions Saturation Indices Speciation calculation Inverse ModelingTransport 5

6. ConstituentValue pH pe Temperature Ca Mg Na K Fe Alkalinity as HCO3 Cl SO4 8.22 8.45 10 412.3 1291.8 10768 399.1.002 141.682 19353 2712 SOLUTION: Seawater, ppm 6

7. Periodic_table.bmp 7

8. Initial Solution 1.Questions 1.What is the approximate molality of Ca? 2.What is the approximate alkalinity in meq/kgw? 3.What is the alkalinity concentration in mg/kgs as CaCO 3 ? 4.What effect does density have on the calculated molality? PHREEQC results are always moles or molality 8

9. Initial Solution 1. For most waters, we can assume most of the mass in solution is water. Mass of water in 1 kg seawater ~ 1 kg. 1.412/40 ~ 10 mmol/kgw ~ 0.01 molal 2.142/61 ~ 2.3 meq/kgw ~ 0.0023 molal 3.2.3*50 ~ 116 mg/kgw as CaCO3 4.None, density will only be used when concentration is specified as per liter. 9

10. Default Gram Formula Mass Element/Redox StateDefault “as” phreeqc.dat/wateq4f.dat AlkalinityCaCO3 C, C(4)HCO3 CH4 NO3-N NH4+N PO4P SiSiO2 SO4 Default GFW is defined in 4 th field of SOLUTION_MASTER_SPECIES in database file. 10

11. Databases Ion association approach –Phreeqc.dat—simplest (subset of Wateq4f.dat) –Amm.dat—same as phreeqc.dat, NH3 is separated from N –Wateq4f.dat—more trace elements –Minteq.dat—translated from minteq v 2 –Minteq.v4.dat—translated from minteq v 4 –Llnl.dat—most complete set of elements, temperature dependence –Iso.dat—(in development) thermodynamics of isotopes Pitzer specific interaction approach –Pitzer.dat—Specific interaction model (many parameters) SIT specific interaction theory –Sit.dat—Simplified specific interaction model (1 parameter) 11

12. PHREEQC Databases Other data blocks related to speciation SOLUTION_MASTER_SPECIES—Redox states and gram formula mass SOLUTION_SPECIES—Reaction and log K PHASES—Reaction and log K 12

13. Solutions Required for all PHREEQC calculations SOLUTION and SOLUTION _SPREAD –Units –pH –pe –Charge balance –Phase boundaries Saturation indices –Useful minerals –Identify potential reactants 13

14. What is a speciation calculation? Input: –pH –pe –Concentrations Equations: –Mass-balance—sum of the calcium species = total calcium –Mass-action—activities of products divided by reactants = constant –Activity coefficients—function of ionic strength Output –Molalities, activities –Saturation indices 14

15. Mass-Balance Equations Analyzed concentration of sulfate = (SO 4 -2 ) + (MgSO 4 0 ) + (NaSO 4 - ) + (CaSO 4 0 ) + (KSO 4 - ) + (HSO 4 - ) + (CaHSO 4 + ) + (FeSO 4 ) + (FeSO 4 + ) + (Fe(SO 4 ) 2 - ) + (FeHSO 4 + ) + (FeHSO 4 +2 ) () indicates molality 15

16. Mass-Action Equations Ca +2 + SO 4 -2 = CaSO 4 0 [] indicates activity 16

17. Activity WATEQ activity coefficient Davies activity coefficient 17

18. Uncharged Species 18 b i, called the Setschenow coefficient Value of 0.1 used in phreeqc.dat, wateq4f.dat.

19. Pitzer Activity Coefficients m a concentration of anion m c concentration of cation Ion specific parameters F function of ionic strength, molalities of cations and anions 19

20. SIT Activity Coefficients m k concentrations of ion 20 Interaction parameter A = 0.51, B = 1.5 at 25 C

21. Aqueous Models Ion association –Pros Data for most elements (Al, Si) Redox –Cons Ionic strength < 1 Best only in Na, Cl medium Inconsistent thermodynamic data Temperature dependence 21

22. Aqueous Models 22 Pitzer specific interaction –Pros High ionic strength Thermodynamic consistency for mixtures of electrolytes –Cons Limited elements Little if any redox Difficult to add elements Temperature dependence

23. Aqueous Models 23 SIT –Pros Possibly better for higher ionic strength than ion association Many fewer parameters Redox Actinides –Cons Poor results for gypsum/NaCl in my limited testing Temperature dependence Consistency?

24. PhreeqcI: SOLUTION Data Block 24

25. Number, pH, pe, Temperature 25

26. Solution Composition Set units! Default is mmol/kgw Click when done Set concentrations “As”, special units Select elements 26

27. Run Speciation Calculation Run Select files 27

28. Seawater Exercise A.Use phreeqc.dat to run a speciation calculation for file seawater.pqi B.Use file seawater- pitzer.pqi or copy input to a new buffer Ctrl-a (select all) Ctrl-c (copy) File->new or ctrl-n (new input file) Ctrl-v (paste) ConstituentValue pH pE Temperature Ca Mg Na K Fe Alkalinity as HCO3 Cl SO4 8.22 8.45 10 412.3 1291.8 10768 399.1.002 141.682 19353 2712 Units are ppm 28

29. Ion Association Model Results 29

30. Results of 2 Speciation Calculations Tile 30 Ion Association Pitzer

31. Questions 1.Write the mass-balance equation for calcium in seawater for each database. 2.What fraction of the total is Ca +2 ion for each database? 3.What fraction of the total is Fe +3 ion for each database? 4.What are the log activity and log activity coefficient of CO 3 -2 for each database? 5.What is the saturation index of calcite for each database? 31

32. Initial Solution 2. Answers () indicates molality 1a. Ca(total)= 1.066e-2 = (Ca+2) + (CaSO4) + (CaHCO3+) + (CaCO3) + (CaOH+) + (CaHSO4+) 1b. Ca(total) = 1.066e-2 = (Ca+2) + (CaCO3) 2a. 9.5/10.7 ~ 0.95 2b. 1.063/1.066 ~ 1.0 3a. 3.509e-019 / 3.711e-008 ~ 1e-11 3b. No Fe+3 ion. 4a. log activity CO3-2 = -5.099; log gamma CO3-2 = -0.68 4b. log activity CO3-2 = -5.091; log gamma CO3-2 = -1.09 5a. SI(calcite) = 0.76 5b. SI(calcite) = 0.70 32

33. SATURATION INDEX The thermodynamic state of a mineral relative to a solution 33 IAP is ion activity product K is equilibrium constant

34. SATURATION INDEX SI < 0, Mineral should dissolve SI > 0, Mineral should precipitate SI ~ 0, Mineral reacts fast enough to maintain equilibrium Maybe –Kinetics –Uncertainties 34

35. Rules for Saturation Indices Mineral cannot dissolve if it is not present If SI < 0 and mineral is present—the mineral could dissolve, but not precipitate If SI > 0—the mineral could precipitate, but not dissolve If SI ~ 0—the mineral could dissolve or precipitate to maintain equilibrium 35

36. Saturation Indices SI(Calcite) SI(CO2(g)) = log(P CO2 ) 36

37. Useful Mineral List Minerals that may react to equilibrium relatively quickly 37

38. Data Tree Files (double click to edit) –Simulation (END) Keywords (double click to edit) –Data 38

39. Edit Screen Text editor 39

40. Tree Selection Input Output Database Errors PfW 40

41. Keyword Data Blocks 41 Also right click in data tree—Insert keyword

42. PfW Style 42

43. Alkalinity Approximately HCO 3 - + 2xCO 3 -2 + OH - - H + Alkalinity is independent of PCO 2 Total Inorganic Carbon Number of moles of carbon of valence 4 43

44. SOLUTION_SPREAD 44

45. Carbon and Alkalinity solution_spread.pqi SOLUTION_SPREAD SELECTED_OUTPUT USER_GRAPH 45

46. Carbon Speciation and Alkalinity 46

47. pH and pe Keywords SOLUTION—Solution composition END—End of a simulation USE—Reactant to add to beaker REACTION—Specified moles of a reaction USER_GRAPH—Charting 47

48. ConstituentValue pH pe Temperature Alkalinity Na 7 4 25 1 1 charge SOLUTION, mmol/kgw 48 END

49. USE 49 Solution 1 REACTION CO2 1.0 1, 10, 100, 1000 mmol USER_GRAPH -axis_titles "CO2 Added, mmol" "pH" "Alkalinity" -axis_scale x_axis auto auto auto auto log -axis_scale sy_axis 0 0.002 -start 10 GRAPH_X rxn 20 GRAPH_Y -LA("H+") 30 GRAPH_SY ALK -end

50. Input file pH.pqi SOLUTION 1 temp 25 pH 7 pe 4 redox pe units mmol/kgw density 1 Alkalinity 1 Na 1 charge -water 1 # kg END USE solution 1 REACTION 1 CO2 1 1 10 100 1000 millimoles USER_GRAPH 1 -axis_titles "CO2 Added, mmol" "pH" "Alkalinity" -axis_scale x_axis auto auto auto auto log -axis_scale sy_axis 0 0.002 -start 10 GRAPH_X rxn 20 GRAPH_Y -LA("H+") 30 GRAPH_SY ALK -end END 50

51. pH is the ratio of HCO3- to CO2(aq) 51 Alkalinity is independent of P CO2

52. What is pH? Questions 1. How does the pH change when CO 2 degasses during an alkalinity titration? 2. How does pH change when plankton respire CO 2 ? 3. How does pH change when calcite dissolves? pH = 6.3 + log[(HCO 3 - )/(CO 2 )] pH = 10.3 + log[(CO 3 -2 )/(HCO 3 - )] 52 pH = logK + log[(PO 4 -3 )/(HPO 4 -2 )]

53. ConstituentValue pH pe Temperature Fe(3) Cl 2 4 25 1 1 charge SOLUTION, mmol/kgw 53 END

54. USE 54 Solution 1 REACTION FeCl2 1.0 1, 10, 100, 1000 mmol USER_GRAPH -axis_titles "FeCl2 Added, mmol" "pe" "" -axis_scale x_axis auto auto auto auto log -start 10 GRAPH_X rxn 20 GRAPH_Y -LA("e-") -end

55. Input file SOLUTION 1 temp 25 pH 3 pe 4 redox pe units mmol/kgw density 1 Cl 1 charge Fe(3) 1 -water 1 # kg END USE solution 1 REACTION 1 FeCl2 1 1 10 100 1000 millimoles USER_GRAPH 1 -axis_titles "FeCl2 Added, mmol" "pe" "" -axis_scale x_axis auto auto auto auto log -start 10 GRAPH_X rxn 20 GRAPH_Y -LA("e-") -end END 55

56. pe 56

57. What is pe? Fe+2 = Fe+3 + e- pe = log( [Fe +3 ]/[Fe +2 ] ) + 13 HS- + 4H2O = SO4-2 + 9H+ + 8e- pe = log( [SO 4 -2 ]/[HS - ] ) – 9/8pH + 4.21 N2 + 6H2O = 2NO3- + 12H+ + 10e- pe = 0.1log( [NO 3 - ] 2 /[N 2 ] ) –1.2pH + 20.7 pe = 16.9Eh, Eh in volts (platinum electrode measurement) 57

58. Redox and pe in SOLUTION Data Blocks When do you need pe for SOLUTION? –To distribute total concentration of a redox element among redox states [e.g. Fe to Fe(2) and Fe(3)] –A few saturation indices with e - in dissociation reactions Pyrite Native sulfur Manganese oxides Can use a redox couple Fe(2)/Fe(3) in place of pe Rarely, pe = 16.9Eh. (25 C and Eh in Volts). pe options can only be applied to speciation calculations; thermodynamic pe is used for all other calculations 58

59. Iron Speciation with PhreePlot 59

60. Redox Elements ElementRedox state Species CarbonC(4)CO 2 C(-4)CH 4 SulfurS(6)SO 4 -2 S(-2)HS - NitrogenN(5)NO 3 - N(3)NO 2 - N(0)N2N2 N(-3)NH 4 + OxygenO(0)O2O2 O(-2)H2OH2O HydrogenH(1)H2OH2O H(0)H2H2 ElementRedox state Species IronFe(3)Fe +3 Fe(2)Fe +2 ManganeseMn(2)Mn +2 ArsenicAs(5)AsO 4 -3 As(3)AsO 3 -3 UraniumU(6)UO 2 +2 U(4)U +4 ChromiumCr(6)CrO 4 -2 Cr(3)Cr +3 SeleniumSe(6)SeO 4 -2 Se(4)SeO 3 -2 Se(-2)HSe - 60

61. Seawater Initial Solution Fe total was entered. How were Fe(3) and Fe(2) concentrations calculated? For initial solutions For “reactions” 61

62. Final thoughts on pe pe sets ratio of redox states Some redox states are measured directly: –NO3-, NO2-, NH3, N2(aq) –SO4-2, HS- –O2(aq) –Sometimes Fe, As Others can be assumed: –Fe, always Fe(2) except at low pH –Mn, always Mn(2) –As, consider other redox elements –Se, consider other redox elements –U, probably U(6) –V, probably V(5) 62

63. Berner’s Redox Environments Oxic Suboxic Sulfidic Methanic Thorstenson (1984) 63

64. 64

65. Parkhurst and others (1996) 65

66. Summary SOLUTION and SOLUTION _SPREAD –Units –pH—ratio of HCO 3 /CO 2 –pe—ratio of oxidized/reduced valence states –Charge balance –Phase boundaries Saturation indices –Uncertainties –Useful minerals Identify potential reactants 66

67. Summary Aqueous speciation model –Mole-balance equations—Sum of species containing Ca equals total analyzed Ca –Aqueous mass-action equations—Activity of products over reactants equal a constant –Activity coefficient model Ion association with individual activity coefficients Pitzer specific interaction approach –SI=log(IAP/K) 67

68. PHREEQC: Reactions in a Beaker SOLUTIONEQUILIBRIUM _PHASES EXCHANGESURFACEKINETICSMIXREACTION REACTION BEAKER + SOLUTION EQUILIBRIUM_ PHASES EXCHANGESURFACE GAS_PHASE 68 REACTION_TEMPERATUREREACTION_PRESSURE

69. Reaction Simulations SOLUTION, SOLUTION_SPREAD, MIX, USE solution, or USE mix Equilibrium Nonequilibrium 69 EQUILIBRIUM_PHASES EXCHANGE SURFACE SOLID_SOLUTION GAS_PHASE REACTION_TEMPERATURE REACTION_PRESSURE END KINETICS REACTION

70. Calculate the SI of Calcite in Seawater at Pressures from 100 to 1000 atm 70

71. Keywords SOLUTION 1 END USE solution 1 REACTION_PRESSURE USER_GRAPH END 71

72. USE—Item on shelf Item number on shelf To the beaker 72

73. USE All of these Reactants are Numbered SOLUTION EQUILIBRIUM_PHASES EXCHANGE GAS_PHASE KINETICS SOLID_SOLUTIONS SURFACE REACTION REACTION_PRESSURE REACTION_TEMPERATURE 73

74. REACTION_PRESSURE List of pressures 100 200 300 400 500 600 700 800 900 1000 Or Range of pressure divided equally 100 1000 in 10 steps 74

75. USER_GRAPH 10 GRAPH_X PRESSURE 20 GRAPH_Y SI(“Calcite”) 30 GRAPH_SY expr Expressions are defined with Basic functions Basic—+-*/, SIN, COS, EXP,… PHREEQC—PRESSURE, SI(“Calcite”), MOL(“Cl-”), TOT(“Cl-”), -LA(“H+”),… 75

76. Plot the SI of Calcite with Temperature Seawater-p.pqi 76

77. SI Calcite for Seawater with P 77

78. Arsenic in the Central Oklahoma Aquifer Arsenic mostly in confined part of aquifer Arsenic associated with high pH Flow: –Unconfined –Confined –Unconfined 78

79. Geochemical Reactions Brine initially fills the aquifer Calcite and dolomite equilibrium Cation exchange –2NaX + Ca +2 = CaX 2 + 2Na + –2NaX + Mg +2 = MgX 2 + 2Na + Surface complexation Hfo-HAsO4 - + OH - = HfoOH + HAsO4 -2 79

80. More Reactions and Keywords EQUILIBRIUM_PHASES SAVE EXCHANGE SURFACE 80

81. EQUILIBRIUM_PHASES Minerals and gases that react to equilibrium Calcite reaction CaCO 3 = Ca +2 + CO 3 -2 Equilibrium K = [Ca +2 ][CO 3 -2 ]

82. EQUILIBRIUM_PHASES Data Block Mineral or gas Saturation state Amount Example EQUILIBRIUM_PHASES 5: CO 2 Log PCO 2 = -2, 10 moles Calciteequilibrium 1 moles Dolomiteequilibrium 1 moles Fe(OH) 3 equilibrium0 moles

83. Let’s Make a Carbonate Groundwater SOLUTION—Pure water or rain EQUILIBRIUM_PHASES –CO2(g), SI -1.5, moles 10 –Calcite, SI 0, moles 0.1 –Dolomite, SI 0, moles 1.6 SAVE solution 0 83

84. Oklahoma Rainwater x 20 Ignoring NO3- and NH4+ SOLUTION 0 20 x precipitation pH4.6 pe4.0 O2(g) -0.7 temp25. unitsmmol/kgw Ca0.191625 Mg0.035797 Na0.122668 Cl0.133704 C0.01096 S0.235153 charge 84

85. Limestone Groundwater 85

86. Brine Oil field brine 86

87. SOLUTION Data Block SOLUTION 1: Oklahoma Brine units mol/kgw pH 5.713 temp 25. Ca 0.4655 Mg 0.1609 Na 5.402 Cl 6.642 C 0.00396 S 0.004725 As 0.03 (ug/kgw)

88. PHREEQC “speciates” the “exchanged species” on the exchange sites either: – Initial Exchange Calculation: adjusting sorbed concentrations in response to a fixed aqueous composition – Reaction Calculation: adjusting both sorbed and aqueous compositions. Ion Exchange Calculations (#1) Layers of clays have a net negative charge Exchanger has a fixed CEC, cation exchange capacity, based on charge deficit Small cations (Ca +2, Na +, NH 4 +, Sr +2, Al +3 ) fit in the interlayers

89. PHREEQC uses 3 keywords to define exchange processes – EXCHANGE_MASTER_SPECIES (component data) – EXCHANGE_SPECIES (species thermo. data) – EXCHANGE First 2 are found in phreeqc.dat and wateq4f.dat (for component X - and exchange species from Appelo) but can be modified in user-created input files. Last is user-specified to define amount and composition of an “exchanger” phase. Ion Exchange (#2)

90. “SAVE” and “USE” keywords can be applied to “EXCHANGE” phase compositions. Amount of exchanger (eg. moles of X - ) can be calculated from CEC (cation exchange capacity, usually expressed in meq/100g of soil) where: where sw is the specific dry weight of soil (kg/L of soil),  is the porosity and  B is the bulk density of the soil in kg/L. (If sw = 2.65 &  = 0.3, then X - = CEC/16.2) CEC estimation technique (Breeuwsma, 1986): CEC (meq/100g) = 0.7 (%clay) + 3.5 (%organic carbon) (cf. Glynn & Brown, 1996; Appelo & Postma, 2005, p. 247) Ion Exchange (#3)

91. EXCHANGE Cation exchange composition Reaction: Ca +2 + 2NaX = CaX 2 + 2Na + Equilibrium:

92. 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) NaX0.05 moles Often X 0.15 moles, Equilibrium with solution 1

93. EXCHANGE Calculate the composition of an exchanger in equilibrium with the brine Assume 1 mol of exchange sites 93

94. Input File 94

95. Exchange Composition ------------------------------------------------------- Beginning of initial exchange-composition calculations. ------------------------------------------------------- Exchange 1. X 1.000e+000 mol Equiv- Equivalent Log Species Moles alents Fraction Gamma NaX 9.011e-001 9.011e-001 9.011e-001 0.242 CaX2 4.067e-002 8.134e-002 8.134e-002 0.186 MgX2 8.795e-003 1.759e-002 1.759e-002 0.517 95

96. Sorption processes Depend on: –Surface area & amount of sorption “sites” –Relative attraction of aqueous species to sorption sites on mineral/water interfaces Mineral surfaces can have: –Permanent structural charge –Variable charge Sorption can occur even when a surface is neutrally charged.

97. Some Simple Models Linear Adsorption (constant K d ): where q is amount sorbed per weight of solid, c is amount in solution per unit volume of solution; R is the retardation factor (dimensionless),  is porosity,  b is bulk density. K d is usually expressed in ml/g and measured in batch tests or column experiments. Assumptions: 1)Infinite supply of surface sites 2)Adsorption is linear with total element aqueous conc. 3)Ignores speciation, pH, competing ions, redox states… 4)Often based on sorbent mass, rather than surface area

98. Thermodynamic Speciation-based Sorption Models

99. Sorption on variable charge surfaces: –“Surface complexation” –Occurs on Fe, Mn, Al, Ti, Si oxides & hydroxides, carbonates, sulfides, clay edges.

100. Surface charge depends on the sorption/surface binding of potential determining ions, such as H +. Formation of surface complexes also affects surface charge.

101. Examples of Surface Complexation Reactions outer-sphere complex inner-sphere complex bidentate inner-sphere complex

102. pH “edges” for cation sorption

103. PHREEQC uses 3 keywords to define exchange processes – SURFACE_MASTER_SPECIES (component data) – SURFACE_SPECIES (species thermo. data) – SURFACE First 2 are found in phreeqc.dat and wateq4f.dat (for component Hfo and exchange species from Dzombak and Morel) but can be modified in user-created input files. Last is user-specified to define amount and composition of a surface. Surface Complexation

104. SURFACE—Surface Composition Trace elements Zn, Cd, Pb, As, P Reaction: Hfo_wOH + AsO 4 -3 = Hfo_wOHAsO 4 -3 Equilibrium:

105. SURFACE Data Block Surface name—Hfo is Hydrous Ferric Oxide Number of surface sites Chemical composition of surface Multiple sites per surface Example SURFACE 21: Hfo_wOH0.001 moles, 600 m 2 /g, 30 g Hfo_sOH 0.00005 moles Often Hfo_w0.001 moles, Equilibrium with solution 1

106. SURFACE Calculate the composition of a surface in equilibrium with the brine Assume 1 mol of exchange sites Use the equilibrium constants from the following slide 106

107. Dzombak and Morel’s Model 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 10.58 107 SOLUTION_MASTER_SPECIES As H3AsO4 -1.0 74.9216 74.9216 SOLUTION_SPECIES H3AsO4 = H3AsO4 log_k 0.0 H3AsO4 = AsO4-3 + 3H+ log_k -20.7 H+ + AsO4-3 = HAsO4-2 log_k 11.50 2H+ + AsO4-3 = H2AsO4- log_k 18.46

108. Input File 108

109. Surface Composition ------------------------------------------------------ Beginning of initial surface-composition calculations. ------------------------------------------------------ Surface 1. Surf 5.648e-002 Surface charge, eq 3.028e-001 sigma, C/m**2 4.372e-002 psi, V -1.702e+000 -F*psi/RT 1.824e-001 exp(-F*psi/RT) 6.000e+002 specific area, m**2/g 1.800e+004 m**2 for 3.000e+001 g Surf 7.000e-002 moles Mole Log Species Moles Fraction Molality Molality SurfOH2+ 5.950e-002 0.850 5.950e-002 -1.225 SurfOH 8.642e-003 0.123 8.642e-003 -2.063 SurfHAsO4- 9.304e-004 0.013 9.304e-004 -3.031 SurfOHAsO4-3 6.878e-004 0.010 6.878e-004 -3.163 SurfH2AsO4 2.073e-004 0.003 2.073e-004 -3.683 SurfO- 2.875e-005 0.000 2.875e-005 -4.541 109

110. Modeling the Geochemistry Central Oklahoma Reactants –Brine –Exchanger in equilibrium with brine –Surface in equilibrium with brine –Calcite and dolomite –Carbonate groundwater Process –Displace brine with carbonate groundwater –React with minerals, exchanger, and surface 110

111. Explicit Approach Repeat –USE carbonate groundwater –USE equilibrium_phases –USE exchange –USE surface –SAVE equilibrium_phases –SAVE exchange –SAVE surface 111

112. 1D Solute Transport  Terms  Concentration change with time  Dispersion/diffusion  Advection  Reaction

113. PHREEQC Transport Calculations 123456n Advection Dispersion 123456n Reaction 123456n

114. ADVECTION Data Block 123456n Carbonate groundwater Reaction 123456n Brine Minerals, Exchange, Surface

115. ADVECTION Cells are numbered from 1 to N. Index numbers (of SOLUTION, EQUILIBRIUM_PHASES, etc) are used to define the solution and reactants in each cell SOLUTION 0 enters the column Water is “shifted” from one cell to the next

116. ADVECTION Number of cells Number of shifts If kinetics—time step

117. ADVECTION Output file –Cells to print –Shifts to print Selected-output file –Cells to print –Shifts to print

118. Complete simulation 1.Define As aqueous and surface model 2.Define brine (SOLUTION 1) 3.Define EXCHANGE 1 in equilibrium with brine 4.Define SURFACE 1 in equilibrium with brine 5.Define EQUILIBRIUM_PHASES 1 with 1.6 mol dolomite and 0.1 mol calcite 6.Define carbonate groundwater (SOLUTION 0) 1.Pure water 2.EQUILIBRIUM_PHASES calcite, dolomite, CO2(g) -1.5 3.SAVE solution 0 118

119. Complete simulation (continued) 7.Define ADVECTION 8.Define USER_GRAPH X—step or pore volume Y—ppm As, and molality of Ca, Mg, and Na SY—pH USER_GRAPH Example 14 -headings PV As(ppb) Ca(M) Mg(M) Na(M) pH -chart_title "Chemical Evolution of the Central Oklahoma Aquifer" -axis_titles "PORE VOLUMES OR SHIFT NUMBER" "Log(CONCENTRATION, IN PPB OR MOLAL)" "pH" -axis_scale x_axis 0 200 -axis_scale y_axis 1e-6 100 auto auto Log 10 GRAPH_X STEP_NO 20 GRAPH_Y TOT("As")*GFW("As")*1e6, TOT("Ca"), TOT("Mg"), TOT("Na") 30 GRAPH_SY -LA("H+") 119

120. Keywords in Input File SURFACE_MASTER_SPECIES SURFACE_SPECIES SOLUTION_MASTER_SPECIES SOLUTION_SPECIES SOLUTION 1 Brine END EXCHANGE 1 END SURFACE 1 END EQUILIBRIUM_PHASES 1 END SOLUTION 0 EQUILIBRIUM_PHASES 0 SAVE solution 0 END ADVECTION USER_GRAPH Example 14 END 120

121. Advection Results 121

122. Geochemical Reactions Cation exchange –2NaX + Ca +2 = CaX 2 + 2Na + –2NaX + Mg +2 = MgX 2 + 2Na + Calcite and dolomite equilibrium –CaCO 3 + CO 2 (aq) + H 2 O = Ca +2 + 2 HCO 3 - –CaMg(CO 3 ) 2 + 2CO 2 (aq) + 2H 2 O = Ca +2 + Mg +2 + 4 HCO 3 - Surface complexation Hfo-HAsO4 - + OH - = HfoOH + HAsO4 -2 122

123. Diffusive TRANSPORT and Kinetics Potomac River Estuary data KINETICS –Non-equilibrium reactions –Biogeochemical –Annual cycle of sulfate reduction TRANSPORT capabilities 123

124. Thermodynamics vs. Kinetics Thermodynamics predicts equilibrium dissolution/precipitation concentrations Probably OK for “reactive” minerals (Monday’s useful minerals list) and groundwater Need kinetics for slow reactions and/or fast moving water 124

125. Kinetics is Concentration versus Time Appelo and Postma, 2005 Dissolution “half-life” 125

126. Half-life (pH 5 dissolution of the solid phase) Gypsum – hours Calcite – days Dolomite – years Biotite, kaolinite, quartz – millions of years If half-life is << residence time then equilibrium conditions can be used If half-life is >> residence time then kinetics will need to be considered 126

127. Appelo and Postma, 2005 127

128. Rate Laws Mathematically describes the change in concentration with time (derivative) Simple if constant rate (zero order - linear) Complex if rate constant changes with time due to multiple factors (i.e., concentration, temperature, pH, etc.), thus higher order, non-linear Remember that experimental data may not represent real world conditions 128

129. Organic decomposition KINETICS WRONG! -formula CH2O -2 SO4-2 -1 HCO3- +2 H2S +1 2CH2O + SO4-2 = 2HCO3- + H2S RIGHT! -formula CH2O 1 Or perhaps, -formula CH2O 1 Doc -1 129

130. Organic Decomposition in PHREEQC Mole balance of C increases H and O mole balances increase too, but equivalent to adding H 2 O If there are electron acceptors, C ends up as CO 3 -2 species Electron acceptor effectively gives up O and assumes the more reduced state The choice of electron acceptor is thermodynamic 130

131. RATE EQUATION CH2O RATES CH2O -start 10 sec_per_yr = 365*24*3600 20 k = 1 / sec_per_yr 30 pi = 2*ARCTAN(1e20) 40 theta = (TOTAL_TIME/sec_per_yr)*2*pi 50 cycle = (1+COS(theta))/2 60 rate = k*TOT("S(6)") * cycle 70 moles = rate*TIME 80 SAVE moles -end END 131

132. (1+COS(theta))/2 132

133. KINETICS KINETICS 1-4 CH2O -formula (CH2O)8NH3 END 133

134. TRANSPORT 20 cells 100 shifts 0.1 y time step 134

135. TRANSPORT Diffusion only Diffusion coefficient Constant boundary (1/2 seawater) Closed boundary 135

136. TRANSPORT Cell lengths 0.025 m Dispersivities

137. TRANSPORT Output file Selected output and USER_GRAPH

138. TRANSPORT Options At end of exercise we will try multicomponent diffusion, where ions diffuse at different rates Capability for diffusion in surface interlayers

139. TRANSPORT Options Stagnant cells/dual porosity -One stagnant cell -Multiple stagnant cells Dump options

140. TRANSPORT—Charge- Balanced Diffusion TRANSPORT -multi_d true 1e-9 0.3 0.05 1.0 SOLUTION_SPECIES H+ = H+ log_k 0.0 -gamma 9.0 0.0 -dw 9.31e-9 Multicomponent diffusion—true Default tracer diffusion coefficient—1e-9 m2/s Porosity—0.3 Minimum porosity—0.05 (Diffusion stops when the porosity reaches the porosity limit) Exponent of porosity (n) –1.0. (Effective diffusion coefficient–De = Dw * porosity^n) -dw is tracer diffusion coefficient in SOLUTION_SPECIES

141. V3.pqi Check periodic steady state Adjust parameters –More SO 4 consumption –Greater depth range 141

142. Options Rate expression –K controls rate of reaction –Cycle controls periodic function –Rate is overall rate of reaction (mol/s) TRANSPORT –Diffusion coefficient KINETICS –Cells with kinetics 142

143. One Choice Diffusion coefficient RATES k RATES cycle Cells 143

144. SO 4 -2 Multicomponent diffusion 144 Fixed diffusion coefficient

145. NH 4 + 145 Multicomponent diffusionFixed diffusion coefficient

146. H2SH2S 146 Multicomponent diffusionFixed diffusion coefficient

147. Acid Mine Drainage 147

149. Sulfide Oxidation Pyrite/Marcasite are most important reactants Need Pyrite, Oxygen, Water, and bugs Oxidation of pyrite and formation of ferric hydroxide complexes and minerals generates acidic conditions

150. Iron Mountain, California Sulfide deposits at the top of a mountain Lots of precipitation Unsaturated conditions Tunnels drain

151. Picher, Oklahoma Flat topography Mines 200 to 500 ft below land surface Saturated after dewatering ceased Cut off the supply of oxygen

152. Simplified Reactions High pH FeS2 + 15/4O2 + 4HCO3- = Fe(OH)3 + 2SO4-2 + 4CO2 + 1/2H2O Or FeS2 + 15/4O2 + 7/2H2O = Fe(OH)3 + 2SO4-2 + 4H+ Low pH FeS2 + 15/4O2 + 1/2H2O = Fe+3 + SO4-2 + HSO4-

153. Additional reactions Hydrous ferric oxides –Ferrihydrite –Goethite –Jarosite Aluminum hydroxides –Alunite Carbonates Gypsum

154. Modeling Pyrite Oxidation FeS2 + 15/4O2 + 7/2H2O = Fe(OH)3 + 2SO4-2 + 4H+ Pick the irreversible reactant: O2 or FeS2 –Oxygen rich environment of a tailings pile –We are going to react up to 50 mmol FeS2 Equilibrium reactions

155. REACTION 18. Exercise 1.React the pure water with 10 mmol of pyrite, maintaining equilibrium with atmosphreric oxygen. 2.React the pure water with 10 mmol of mackinawite, maintaining equilibrium with atmosphreric oxygen. 3.React the pure water with 10 mmol of sphalerite, maintaining equilibrium with atmosphreric oxygen.

156. REACTION 18. Questions 1.Write qualitative reactions that explain the pH of the 3 solutions. 2.What pH buffer starts to operate at pHs below 3? 3.Run the input file with wateq4f.dat database. What minerals may precipitate during pyrite oxidation?

157. Reaction 18. Answers 1.Question 1 Pyrite oxidation: FeS2 + xO2 + yH2O -> Fe+3 + 2SO4-2 + H+ In addition, ferric iron hydrolizes to make additional H+: Fe+3 + H2O = FeOH+2 + H+ With net acid production to give pH 2. Mackinawite oxidation: FeS + 2.25O2 + H+ -> Fe+3 + SO4-2 +.5H2O But ferric iron hydrolizes Fe+3 + H2O = FeOH+2 + H+ With a net acid production that give pH 4. Sphalerite oxidation: ZnS + 2O2 -> Zn+2 + SO4-2 Zinc hydrolosis is minimal Zn+2 + H2O = FeOH+ + H+ Net result is pH 7. 2. HSO4-/SO4-2 3. Iron oxyhydroxides, goethite (and often Fe(OH)3(a)) and jarosite. There is also a potassium jarosite and other solid solutions of jarosites. Aluminum has analogous minerals named alunite.

158. REACTION 20.Extra Credit Exercise 1.React the pure water with 20 mmol of pyrite, maintaining equilibrium with atmospheric oxygen and goethite. 2.Acid mine drainage is usually treated with limestone. Use the results of part 1 and equilibrate with O2, goethite, and calcite.

159. REACTION 20.Questions 1.Write a net reaction for the PHREEQC results for the low-pH simulation. 2.What are the pH values with and without calcite equilibrium. 3.Looking at the results of the calcite- equilibrated simulation, what additional reactions should be considered?

160. Reaction 20. Answers 1.20FeS 2 + 75O 2 = 19FeOOH +.8Fe(+3) + 27HSO 4 - + 12SO 4 -2 + 50H + 2.pHs are 1.4 and 5.8 without and with calcite equilibrium 3.Gypsum is supersaturated, and probably would precipitate. pCO 2 is 1 atmosphere. If O 2 reacts to equilibrium with the atmosphere, logically, CO 2 would also.

161. Picher Oklahoma Abandoned Pb/Zn Mine mg/L Mines are suboxic Carbonates are present Iron oxidizes in stream

162. Pyrite Oxidation Requires Pyrite/Marcasite O 2 H 2 O Bacteria Produces Ferrihydrite/Goethite, jarosite, alunite Gypsum if calcite is available Evaporites Possibly siderite Acid generation Pyrite > FeS > ZnS

163. SOLID_SOLUTIONS—Composition of one or more solid solutions Trace elements and isotopes List of solid solutions Components of each solid solution Example SOLID_SOLUTION 21: Calcite solid solution Ca[13C]O3 CaCO3 163

164. GAS_PHASE—Finite gas phase in equilibrium with solution Gas bubbles that grow Gas bubbles that fill a finite volume 164

165. GAS_PHASE—Composition of the gas phase Fixed volume or Fixed pressure Initial volume Initial pressure Temperature Partial pressure of each gas Example GAS_PHASE 1: Fixed pressure CO2(g) 0.0 CH4(g) 0.0 165