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### Processes that Control Major Element Chemistry

### Processes that Control Minor Element Chemistry

### Solution Definition and Speciation Calculations

### PHREEQC: Reactions in a Beaker

### Ion Exchange Calculations (#1)

### Ion Exchange (#2)

### Ion Exchange (#3)

### Surface Complexation

### Pyrite Oxidation

### Processes that Control Minor Element Chemistry

Monday-Tuesday

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

2. Ion exchange

3. Organic carbon oxidation

O2/Nitrate reduction

Iron oxyhydroxide reduction

Sulfate reduction

Methanogenesis

4. Pyrite oxidation

5. Gypsum dissolution

6. Seawater evaporation

7. Silicate weathering

- Oxyanions
- Trace metals
- Nitrate
- 2. Surface complexation
- Phosphate
- Oxyanions
- Trace metals
- 3. Cation exchange
- 4. Solid solutions
- 5. Minerals

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

SO4

Ca

Mg

Fe

Cl

HCO3

Reactions

Saturation Indices

Inverse Modeling

Transport

Speciation calculation

Initial Solution 1. Questions

- What is the approximate molality of Ca?
- What is the approximate alkalinity in meq/kgw?
- What is the alkalinity concentration in mg/kgs as CaCO3?
- What effect does density have on the calculated molality?

PHREEQC results are always moles or molality

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.

- 412/40 ~ 10 mmol/kgw ~ 0.01 molal
- 142/61 ~ 2.3 meq/kgw ~ 0.0023 molal
- 2.3*50 ~ 116 mg/kgw as CaCO3
- None, density will only be used when concentration is specified as per liter.

Default Gram Formula Mass

Default GFW is defined in 4th field of SOLUTION_MASTER_SPECIES in database file.

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)

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

PHREEQC DatabasesSolutions

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

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

Analyzed concentration of sulfate = (SO4-2) + (MgSO40) + (NaSO4-) + (CaSO40) + (KSO4-) + (HSO4-) + (CaHSO4+) + (FeSO4) + (FeSO4+) + (Fe(SO4)2-) + (FeHSO4+) + (FeHSO4+2)

() indicates molality

Mass-Balance EquationsUncharged Species

bi, called the Setschenow coefficient

Value of 0.1 used in phreeqc.dat, wateq4f.dat.

Pitzer Activity Coefficients

ma concentration of anion

mc concentration of cation

Ion specific parameters

F function of ionic strength, molalities of cations and anions

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

Aqueous Models

- 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

Aqueous Models

- 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?

Solution Composition

Set units!

Default is mmol/kgw

Select elements

Set concentrations

“As”, special units

Click when done

Seawater Exercise

Units are ppm

- Use phreeqc.dat to run a speciation calculation for file seawater.pqi
- 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)

Questions

- Write the mass-balance equation for calcium in seawater for each database.
- What fraction of the total is Ca+2 ion for each database?
- What fraction of the total is Fe+3 ion for each database?
- What are the log activity and log activity coefficient of CO3-2 for each database?
- What is the saturation index of calcite for each database?

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

SATURATION INDEXThe thermodynamic state of a mineral relative to a solution

IAP is ion activity product

K is equilibrium constant

SATURATION INDEX

SI < 0, Mineral should dissolve

SI > 0, Mineral should precipitate

SI ~ 0, Mineral reacts fast enough to maintain equilibrium

Maybe

- Kinetics
- Uncertainties

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

Useful Mineral ListMinerals that may react to equilibrium relatively quickly

Data Tree

- Files (double click to edit)
- Simulation (END)
- Keywords (double click to edit)
- Data

Edit Screen

- Text editor

Tree Selection

- Input
- Output
- Database
- Errors
- PfW

Keyword Data Blocks

Also right click in data tree—Insert keyword

Alkalinity

- Number of moles of carbon of valence 4

- Approximately

HCO3- + 2xCO3-2 + OH- - H+

- Alkalinity is independent of PCO2

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

REACTION

Solution 1

CO2 1.0

1, 10, 100, 1000 mmol

-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

USER_GRAPH

Input filepH.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

pH is the ratio of HCO3- to CO2(aq)

Alkalinity is independent of PCO2

What is pH?

pH = 6.3 + log[(HCO3-)/(CO2)]

pH = 10.3 + log[(CO3-2)/(HCO3-)]

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 = logK + log[(PO4-3)/(HPO4-2)]

REACTION

Solution 1

FeCl2 1.0

1, 10, 100, 1000 mmol

-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

USER_GRAPH

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

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 = 2NO3- + 12H+ + 10e-

pe = 0.1log( [NO3-]2/[N2] ) –1.2pH + 20.7

pe = 16.9Eh, Eh in volts (platinum electrode measurement)

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

Seawater Initial Solution

Fe total was entered. How were Fe(3) and Fe(2) concentrations calculated?

For initial solutions

For “reactions”

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)

Summary

SOLUTION and SOLUTION _SPREAD

- Units
- pH—ratio of HCO3/CO2
- pe—ratio of oxidized/reduced valence states
- Charge balance
- Phase boundaries
- Saturation indices
- Uncertainties
- Useful minerals
- Identify potential reactants

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)

EXCHANGE

SURFACE

KINETICS

MIX

REACTION

EQUILIBRIUM_PHASES

GAS_PHASE

SOLUTION

EXCHANGE

SURFACE

GAS_PHASE

EQUILIBRIUM_

PHASES

+

REACTION BEAKER

REACTION_TEMPERATURE

REACTION_PRESSURE

Reaction Simulations

- SOLUTION, SOLUTION_SPREAD, MIX, USE solution, or USE mix

Equilibrium

Nonequilibrium

- EQUILIBRIUM_PHASES
- EXCHANGE
- SURFACE
- SOLID_SOLUTION
- GAS_PHASE
- REACTION_TEMPERATURE
- REACTION_PRESSURE

- KINETICS
- REACTION

- END

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

USEAll of these Reactants are Numbered

- SOLUTION
- EQUILIBRIUM_PHASES
- EXCHANGE
- GAS_PHASE
- KINETICS
- SOLID_SOLUTIONS
- SURFACE

- REACTION
- REACTION_PRESSURE
- REACTION_TEMPERATURE

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

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+”),…

Arsenic in the Central Oklahoma Aquifer

- Arsenic mostly in confined part of aquifer
- Arsenic associated with high pH
- Flow:
- Unconfined
- Confined
- Unconfined

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

EQUILIBRIUM_PHASESMinerals and gases that react to equilibrium

Calcite reaction

CaCO3 = Ca+2 + CO3-2

Equilibrium

K = [Ca+2][CO3-2]

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

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

Oklahoma Rainwater x 20Ignoring NO3- and NH4+

SOLUTION 0 20 x precipitation

pH 4.6

pe 4.0 O2(g) -0.7

temp 25.

units mmol/kgw

Ca 0.191625

Mg 0.035797

Na 0.122668

Cl 0.133704

C 0.01096

S 0.235153 charge

Brine

- Oil field brine

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)

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+, NH4+, Sr+2, Al+3) fit in the interlayers

- 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.

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.

“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), q is the porosity and rB is the bulk density of the soil in kg/L. (If sw = 2.65 & q = 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)

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

EXCHANGE

- Calculate the composition of an exchanger in equilibrium with the brine
- Assume 1 mol of exchange sites

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

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.

Linear Adsorption (constant Kd):

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), q is porosity, rb is bulk density. Kd is usually expressed in ml/g and measured in batch tests or column experiments.

Some Simple Models- Assumptions:
- Infinite supply of surface sites
- Adsorption is linear with total element aqueous conc.
- Ignores speciation, pH, competing ions, redox states…
- Often based on sorbent mass, rather than surface area

Sorption on variable charge surfaces:

- “Surface complexation”
- Occurs on Fe, Mn, Al, Ti, Si oxides & hydroxides, carbonates, sulfides, clay edges.

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

Examples of Surface Complexation Reactions

outer-sphere complex

inner-sphere complex

bidentate inner-sphere complex

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—Surface CompositionTrace elements Zn, Cd, Pb, As, P

Reaction:

Hfo_wOH + AsO4-3 = Hfo_wOHAsO4-3

Equilibrium:

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_wOH 0.001 moles, 600 m2/g, 30 g

Hfo_sOH 0.00005 moles

Often

Hfo_w 0.001 moles, Equilibrium with solution 1

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

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

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

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

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

Explicit Approach

- Repeat
- USE carbonate groundwater
- USE equilibrium_phases
- USE exchange
- USE surface
- SAVE equilibrium_phases
- SAVE exchange
- SAVE surface

1D Solute Transport

- Terms
- Concentration change with time
- Dispersion/diffusion
- Advection
- Reaction

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

ADVECTION

- Number of cells
- Number of shifts
- If kinetics—time step

ADVECTION

- Output file
- Cells to print
- Shifts to print
- Selected-output file
- Cells to print
- Shifts to print

Complete simulation

- Define As aqueous and surface model
- Define brine (SOLUTION 1)
- Define EXCHANGE 1 in equilibrium with brine
- Define SURFACE 1 in equilibrium with brine
- Define EQUILIBRIUM_PHASES 1 with 1.6 mol dolomite and 0.1 mol calcite
- Define carbonate groundwater (SOLUTION 0)
- Pure water
- EQUILIBRIUM_PHASES calcite, dolomite, CO2(g) -1.5
- SAVE solution 0

Complete simulation (continued)

- Define ADVECTION
- 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_scalex_axis 0 200

-axis_scaley_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+")

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

Geochemical Reactions

- Cation exchange
- 2NaX + Ca+2 = CaX2 + 2Na+
- 2NaX + Mg+2 = MgX2 + 2Na+
- Calcite and dolomite equilibrium
- CaCO3 + CO2(aq) + H2O = Ca+2 + 2 HCO3-
- CaMg(CO3)2 + 2CO2(aq) + 2H2O = Ca+2 + Mg+2 + 4 HCO3-
- Surface complexation

Hfo-HAsO4- + OH- = HfoOH + HAsO4-2

Diffusive TRANSPORT and Kinetics

- Potomac River Estuary data
- KINETICS
- Non-equilibrium reactions
- Biogeochemical
- Annual cycle of sulfate reduction
- TRANSPORT capabilities

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

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

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

KINETICS—Nonequilibrium Reactions

- Monod Kinetics
- Radioactive decay
- Silicate hydrolosis
- Biological processes

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

Organic decomposition KINETICS

- 2CH2O + SO4-2 = 2HCO3- + H2S

WRONG!

-formula

CH2O -2

SO4-2 -1

HCO3- +2

H2S +1

- RIGHT!
- -formula
- CH2O 1
- Or perhaps,
- -formula
- CH2O 1
- Doc -1

Organic Decomposition in PHREEQC

- Mole balance of C increases
- H and O mole balances increase too, but equivalent to adding H2O
- If there are electron acceptors, C ends up as CO3-2 species
- Electron acceptor effectively gives up O and assumes the more reduced state
- The choice of electron acceptor is thermodynamic

Organic Decomposition

- Sequential removal of electron acceptors, usually in the sequence:
- O2
- NO3-
- MnO2
- Fe(OH)3
- SO4-2
- Organic carbon

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

TRANSPORT

- 20 cells
- 100 shifts
- 0.1 y time step

TRANSPORT

- Diffusion only
- Diffusion coefficient
- Constant boundary (1/2 seawater)
- Closed boundary

TRANSPORT

- Output file
- Selected output file and USER_GRAPH

TRANSPORT Options

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

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

TRANSPORT Options

- Stagnant cells/dual porosity

-One stagnant cell

-Multiple stagnant cells

- Dump options

V3.pqi

- Check periodic steady state
- Adjust parameters
- More SO4 consumption
- Greater depth range

Adjust parameters so envelope is like the green curves

- Rate expression
- K controls rate of reaction
- Cycle controls periodicity
- Rate is overall rate of reaction (mol/s)
- TRANSPORT
- Diffusion coefficient
- KINETICS
- Cells with kinetics

One Choice

- Diffusion coefficient
- RATES k
- RATES cycle
- Cells

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

Iron Mountain, California

- Sulfide deposits at the top of a mountain
- Lots of rain and snow
- Unsaturated conditions
- Tunnels drain

Picher, Oklahoma

- Flat topography
- Mines 200 to 500 ft below land surface
- Saturated after dewatering ceased
- Cut off the supply of oxygen

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-

Additional reactions

- Hydrous ferric oxides
- Ferrihydrite
- Goethite
- Jarosite
- Aluminum hydroxides
- Alunite
- Carbonates
- Gypsum

Modeling Pyrite Oxidation

FeS2 + 15/4O2 + 7/2H2O = Fe(OH)3 + 2SO4-2 + 4H+

- Pick the irreversible reactant: FeS2
- Oxygen rich environment of a tailings pile
- We are going to react up to 50 mmol FeS2
- Equilibrium reactions

REACTION Exercise

- React pure water with 10 mmol of pyrite (REACTION) in 20 steps, maintaining equilibrium with atmospheric oxygen (log PO2 = -0.7) and goethite.
- Limestone can neutralize acid mine drainage. Rerun assuming calcite equilibrium (maintain equilibrium with O2, goethite, and atmospheric CO2).
- Add the possibility of gypsum precipitation in the treatment process.

Picher Oklahoma Abandoned Pb/Zn Minemg/L

- Mines are suboxic
- Carbonates are present
- Iron oxidizes in stream

Requires

Pyrite/Marcasite

O2

H2O

Bacteria

Produces

Ferrihydrite/Goethite, jarosite, alunite

Gypsum if calcite is available

Evaporites

Possibly siderite

Acid generation

Pyrite > FeS > ZnS

Processes that Control Minor Element Chemistry

1. Carbonate reactions

2. Ion exchange

3. Organic carbon oxidation

O2/Nitrate reduction

Iron oxyhydroxide reduction

Sulfate reduction

Methanogenesis

4. Pyrite oxidation

5. Gypsum dissolution

6. Seawater evaporation

7. Silicate weathering

- Oxyanions
- Trace metals
- Nitrate
- 2. Surface complexation
- Phosphate
- Oxyanions
- Trace metals
- 3. Cation exchange
- 4. Solid solutions
- 5. Minerals

Other Major Keywords

- Selected output
- SELECTED_OUTPUT
- USER_PUNCH
- GAS_PHASE
- SOLID_SOLUTIONS
- MIX
- REACTION_TEMPERATURE
- INVERSE_MODELING
- RUN_CELLS (not yet in PhreeqcI)
- COPY, DELETE (not yet in PhreeqcI)

Principles

- Objective: Describe the chemical and isotopic evolution observed on a hydrologic flow path in terms of sets (models) of balanced reactions.
- Input:
- Analytical data for one (or more if mixing occurs) “initial” solutions and one “final” solution
- Stoichiometry of plausible reactants and products
- Output:
- Mass-balance models – heterogeneous reactive mass transfers (mineral/gas dissolution/exsolution, sorption/exchange) that account for changes in chemistry and isotopic composition.

PHREEQC Equations

- Mole balance and valence state balance
- Isotope mole balance
- Electron balance
- Solution charge balance
- Alkalinity balance
- Water balance
- Uncertainty constraints (elements, pH, isotopes)

Example Equations

- Generalized mole-balance:
- Solution charge balance:

Algorithm developed by Barrodale and Roberts (1980) solves the sets of linear equality and inequality constraints using a minimization technique.

- The unknowns solved for are: mixing fractions, phase and redox mass transfers, and element/valence/isotope adjustments.

Mineralogical Determinations

- Thin Sections or ion microprobe or Scanning Electron Microscope with Energy Dispersive X-rays
- Mineralogy and composition of specific minerals.
- Poor job of fine grained secondary phases such as clays and oxy-hydroxides
- X-ray diffraction
- Gives mineralogy, including fine grained phases and clays.
- Does not give the specific mineral compositions.
- Quantitation techniques are improving
- Knowledge of geologic framework, geochemistry and hydrology

Tar Creek problem

- Evolution of a recharge water during pyrite and sphalerite oxidation, carbonate, albite and halite dissolution.
- Use file “tarcreek.pqi”

Tar Creek Inverse Modeling

Summary of inverse modeling:

Number of models found: 4

Number of minimal models found: 1

Number of infeasible sets of phases saved: 19

Number of calls to cl1: 31

Tar Creek Inverse Modeling

Solution fractions: Minimum Maximum

Solution 1 1.001e+000 0.000e+000 0.000e+000

Solution 2 1.000e+000 0.000e+000 0.000e+000

Phase mole transfers: Minimum Maximum

Calcite 2.740e-003 0.000e+000 0.000e+000 CaCO3

Dolomite 1.033e-002 0.000e+000 0.000e+000 CaMg(CO3)2

CO2(g) -1.293e-003 0.000e+000 0.000e+000 CO2

Sphalerite 2.306e-003 0.000e+000 0.000e+000 ZnS

Pyrite 1.391e-002 0.000e+000 0.000e+000 FeS2

O2(g) 5.542e-002 0.000e+000 0.000e+000 O2

Albite 3.096e-003 0.000e+000 0.000e+000 NaAlSi3O8

Quartz -6.245e-003 0.000e+000 0.000e+000 SiO2

Kaolinite -1.522e-003 0.000e+000 0.000e+000 Al2Si2O5(OH)4

Halite 7.936e-004 0.000e+000 0.000e+000 NaCl

Fe(OH)3(a) -8.512e-003 0.000e+000 0.000e+000 Fe(OH)3

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