monday tuesday
Download
Skip this Video
Download Presentation
Monday-Tuesday

Loading in 2 Seconds...

play fullscreen
1 / 178

Monday-Tuesday - PowerPoint PPT Presentation


  • 95 Views
  • Uploaded on

Monday-Tuesday. Solutions Thermodynamics of aqueous solutions Saturation indices Mineral equilibria Cation exchange Surface complexation Advective transport Diffusive transport Acid mine drainage. 1. Carbonate reactions 2. Ion exchange 3. Organic carbon oxidation

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Monday-Tuesday' - noam


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
monday tuesday
Monday-Tuesday
  • Solutions
    • Thermodynamics of aqueous solutions
    • Saturation indices
  • Mineral equilibria
  • Cation exchange
  • Surface complexation
  • Advective transport
  • Diffusive transport
  • Acid mine drainage
processes that control major 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

Processes that Control Major Element Chemistry

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

Processes that Control Minor Element Chemistry

phreeqc programs
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
solution definition and speciation calculations

Na

SO4

Ca

Mg

Fe

Cl

HCO3

Reactions

Saturation Indices

Inverse Modeling

Transport

Solution Definition and Speciation Calculations

Speciation calculation

initial solution 1 questions
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
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 Gram Formula Mass

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

databases
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)
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

PHREEQC Databases
solutions
Solutions
  • 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
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
mass balance equations
Analyzed concentration of sulfate = (SO4-2) + (MgSO40) + (NaSO4-) + (CaSO40) + (KSO4-) + (HSO4-) + (CaHSO4+) + (FeSO4) + (FeSO4+) + (Fe(SO4)2-) + (FeHSO4+) + (FeHSO4+2)

() indicates molality

Mass-Balance Equations
mass action equations
Mass-Action Equations

Ca+2 + SO4-2 = CaSO40

[] indicates activity

activity
Activity

WATEQ activity coefficient

Davies activity coefficient

uncharged species
Uncharged Species

bi, called the Setschenow coefficient

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

pitzer activity coefficients
Pitzer Activity Coefficients

ma concentration of anion

mc concentration of cation

Ion specific parameters

F function of ionic strength, molalities of cations and anions

sit activity coefficients
SIT Activity Coefficients

mk concentrations of ion

Interaction parameter

A = 0.51, B = 1.5 at 25 C

aqueous models
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 models1
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 models2
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
Solution Composition

Set units!

Default is mmol/kgw

Select elements

Set concentrations

“As”, special units

Click when done

seawater exercise
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)
results of 2 speciation calculations
Results of 2 Speciation Calculations

Tile

Ion Association

Pitzer

questions
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
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 index the thermodynamic state of a mineral relative to a solution
SATURATION INDEXThe thermodynamic state of a mineral relative to a solution

IAP is ion activity product

K is equilibrium constant

saturation index
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
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
saturation indices
Saturation Indices
  • SI(Calcite)
  • SI(CO2(g))

= log(PCO2)

data tree
Data Tree
  • Files (double click to edit)
    • Simulation (END)
      • Keywords (double click to edit)
        • Data
edit screen
Edit Screen
  • Text editor
tree selection
Tree Selection
  • Input
  • Output
  • Database
  • Errors
  • PfW
keyword data blocks
Keyword Data Blocks

Also right click in data tree—Insert keyword

alkalinity

Total Inorganic Carbon

Alkalinity
  • Number of moles of carbon of valence 4
  • Approximately

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

  • Alkalinity is independent of PCO2
carbon and alkalinity solution spread pqi
Carbon and Alkalinitysolution_spread.pqi

SOLUTION_SPREAD

SELECTED_OUTPUT

USER_GRAPH

ph and pe
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

slide49

USE

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

Alkalinity is independent of PCO2

what is ph
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)]

slide54

USE

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
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
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
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
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
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)
berner s redox environments
Berner’s Redox Environments
  • Oxic
  • Suboxic
  • Sulfidic
  • Methanic

Thorstenson (1984)

summary
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
summary1
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)
phreeqc reactions in a beaker

SOLUTION

EXCHANGE

SURFACE

KINETICS

MIX

REACTION

EQUILIBRIUM_PHASES

GAS_PHASE

SOLUTION

EXCHANGE

SURFACE

GAS_PHASE

EQUILIBRIUM_

PHASES

+

PHREEQC: Reactions in a Beaker

REACTION BEAKER

REACTION_TEMPERATURE

REACTION_PRESSURE

reaction simulations
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
keywords
Keywords

SOLUTION 1

END

USE solution 1

REACTION_PRESSURE

USER_GRAPH

END

use item on shelf
USE—Item on shelf

To the beaker

Item number on shelf

use all of these reactants are numbered
USEAll of these Reactants are Numbered
  • SOLUTION
  • EQUILIBRIUM_PHASES
  • EXCHANGE
  • GAS_PHASE
  • KINETICS
  • SOLID_SOLUTIONS
  • SURFACE
  • REACTION
  • REACTION_PRESSURE
  • REACTION_TEMPERATURE
reaction pressure
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
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 in the Central Oklahoma Aquifer
  • Arsenic mostly in confined part of aquifer
  • Arsenic associated with high pH
  • Flow:
    • Unconfined
    • Confined
    • Unconfined
geochemical reactions
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

more reactions and keywords
More Reactions and Keywords

EQUILIBRIUM_PHASES

SAVE

EXCHANGE

SURFACE

equilibrium phases minerals and gases that react to equilibrium
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
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
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 20 ignoring no3 and nh4
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
Brine
  • Oil field brine
solution data block
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)

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

Ion Exchange Calculations (#1)

  • 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 2

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)

ion exchange 3

“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)

Ion Exchange (#3)

exchange cation exchange composition
EXCHANGECation exchange composition

Reaction:

Ca+2 + 2NaX = CaX2 + 2Na+

Equilibrium:

exchange data block
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
EXCHANGE
  • Calculate the composition of an exchanger in equilibrium with the brine
  • Assume 1 mol of exchange sites
exchange composition
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
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.
some simple models

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
slide99
Sorption on variable charge surfaces:
    • “Surface complexation”
    • Occurs on Fe, Mn, Al, Ti, Si oxides & hydroxides, carbonates, sulfides, clay edges.
slide100

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

slide101

Examples of Surface Complexation Reactions

outer-sphere complex

inner-sphere complex

bidentate inner-sphere complex

surface complexation

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

surface surface composition trace elements zn cd pb as p
SURFACE—Surface CompositionTrace elements Zn, Cd, Pb, As, P

Reaction:

Hfo_wOH + AsO4-3 = Hfo_wOHAsO4-3

Equilibrium:

surface data block
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
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
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
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
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
Explicit Approach
  • Repeat
    • USE carbonate groundwater
    • USE equilibrium_phases
    • USE exchange
    • USE surface
    • SAVE equilibrium_phases
    • SAVE exchange
    • SAVE surface
1d solute transport
1D Solute Transport
  • Terms
    • Concentration change with time
    • Dispersion/diffusion
    • Advection
    • Reaction
phreeqc transport calculations

1

2

3

4

5

6

n

1

1

2

2

3

3

4

4

5

5

6

6

n

n

Advection

PHREEQC Transport Calculations

Dispersion

Reaction

advection data block

1

2

3

4

5

6

n

1

2

3

4

5

6

n

Brine

ADVECTION Data Block

Carbonate groundwater

Reaction

Minerals, Exchange, Surface

advection
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
advection1
ADVECTION
  • Number of cells
  • Number of shifts
  • If kinetics—time step
advection2
ADVECTION
  • Output file
    • Cells to print
    • Shifts to print
  • Selected-output file
    • Cells to print
    • Shifts to print
complete simulation
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
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
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 reactions1
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
Diffusive TRANSPORT and Kinetics
  • Potomac River Estuary data
  • KINETICS
    • Non-equilibrium reactions
    • Biogeochemical
    • Annual cycle of sulfate reduction
  • TRANSPORT capabilities
thermodynamics vs kinetics
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
kinetics is concentration versus time
Kinetics is Concentration versus Time

Dissolution “half-life”

Appelo and Postma, 2005

half life ph 5 dissolution of the solid phase
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
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
KINETICS—Nonequilibrium Reactions
  • Monod Kinetics
  • Radioactive decay
  • Silicate hydrolosis
  • Biological processes
kinetics and rates data blocks
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
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
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
Organic Decomposition
  • Sequential removal of electron acceptors, usually in the sequence:
    • O2
    • NO3-
    • MnO2
    • Fe(OH)3
    • SO4-2
    • Organic carbon
rate equation ch 2 o
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

kinetics
KINETICS

KINETICS 1-4

CH2O

-formula (CH2O)8NH3

END

transport
TRANSPORT
  • 20 cells
  • 100 shifts
  • 0.1 y time step
transport1
TRANSPORT
  • Diffusion only
  • Diffusion coefficient
  • Constant boundary (1/2 seawater)
  • Closed boundary
transport2
TRANSPORT
  • Cell lengths

0.025 m

  • Dispersivities

0.0 m

transport3
TRANSPORT
  • Output file
  • Selected output file and USER_GRAPH
transport options
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—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 options1
TRANSPORT Options
  • Stagnant cells/dual porosity

-One stagnant cell

-Multiple stagnant cells

  • Dump options
v3 pqi
V3.pqi
  • Check periodic steady state
  • Adjust parameters
    • More SO4 consumption
    • Greater depth range
adjust parameters so envelope is like the green curves
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
One Choice
  • Diffusion coefficient
  • RATES k
  • RATES cycle
  • Cells
so 4 2
SO4-2

Multicomponent diffusion

Fixed diffusion coefficient

slide148
NH4+

Fixed diffusion coefficient

Multicomponent diffusion

h 2 s
H2S

Fixed diffusion coefficient

Multicomponent diffusion

sulfide oxidation
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
Iron Mountain, California
  • Sulfide deposits at the top of a mountain
  • Lots of rain and snow
  • Unsaturated conditions
  • Tunnels drain
picher oklahoma
Picher, Oklahoma
  • Flat topography
  • Mines 200 to 500 ft below land surface
  • Saturated after dewatering ceased
  • Cut off the supply of oxygen
simplified reactions
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
Additional reactions
  • Hydrous ferric oxides
    • Ferrihydrite
    • Goethite
    • Jarosite
  • Aluminum hydroxides
    • Alunite
  • Carbonates
  • Gypsum
modeling pyrite oxidation
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
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 mine mg l
Picher Oklahoma Abandoned Pb/Zn Minemg/L
  • Mines are suboxic
  • Carbonates are present
  • Iron oxidizes in stream
pyrite oxidation

Pyrite Oxidation

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 chemistry1
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

processes that control minor element chemistry2

1. Redox

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

Processes that Control Minor Element Chemistry

other major keywords
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
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.
simple mole balance equations
Simple Mole Balance Equations

Cfinal = Cinitial + DCO2 + DCaCO3

Cafinal = Cainitial + DCaCO3

phreeqc equations
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
Example Equations
  • Generalized mole-balance:
  • Solution charge balance:
slide171
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
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
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 modeling3
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 modeling4
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

ad