Lecture 3-22-11

Report
Redox and Eh
• From electrochemistry: GR = -nF Eh
– E° = -GR° / nF
– For e- on left side of half-reaction;
– If e- on right side: E° = +GR° / nF
• Re-write Nernst Equation:
– Oxidized species on side where e- are
1
Measuring Eh
• The Eh value is usually not very accurate in
natural waters because of a lack of redox
equilibrium
– One half of redox pair often below detection
• Best to use Eh as a semi-quantitative
measurement, giving you a relative idea of
the redox potential of the water
2
Eh – pH Diagrams
• A different type of stability diagram, but using
Eh as variable instead of activity
– Lines indicate equilibrium
– Domains define areas of stability for minerals and
aqueous species
3
O2 and H2 are present
in entire H2O stability
range
Oxidizing and reducing
with respect to SHE
Oxidizing environments
may contain only small
amounts of O2
4
1
We determine what species,
minerals are in diagram
+++
Fe
++
Diagram Fe , T = 25 °C , P = 1.013 bars, a [main] = 10 , a [H2 O] = 1, a [SO 4 ] = 10
–5
(speciates)
Eh (volts)
.5
--
Hematite
Fe
–6
0
Pyrite
–.5
Troilite
0
2
4
6
8
pH
10
12
++
Magnetite
FeO(c)
25°C
14
Walt Fri May 05 2006
5
Evolution of Water Chemistry
6
Source of dissolved species
• Primarily from chemical weathering
• Primary minerals + acid  secondary minerals
+ dissolved ions
– The essential ingredients needed for chemical
weathering are water and acid
7
Precipitation
• Soil water and groundwater start out as
precipitation
– Very dilute (low TDS), in equilibrium with
atmospheric gases (O2, CO2, N2)
• Precipitation passes through the soil zone and
unsaturated zone
8
Soils and Weathering
• In most areas, soils are the first geologic unit to
come into contact with precipitation
– If soil has organic matter, OM decays, consuming O2
and producing CO2
• CH2O + O2(g) → CO2(g) + H2O
• CO2 + H2O  H2CO3  HCO3- + H+
• Soil PCO2 = 10-3 – 10-1 atm
– (atmosphere = 10-3.5)
– Due to production of acid (CO2) soils have the highest
rate of chemical weathering
– TDS increases as minerals dissolve, ions desorbed
9
Unsaturated Zone
• After passing through the soil zone, water
percolates down through the unsaturated
zone
– Thickness of unsaturated zone is primarily a
function of annual precipitation (climate)
• Also affected by lithology, topography, plant species,
nearness to surface water
• Water can move through the unsaturated zone quickly,
or can remain for a long time (years)
– Dissolution/precipitation reactions can occur in
the unsaturated zone, altering water chemistry
10
Groundwater Chemistry Evolution
• By the time water reaches the water table, it has
acquired the chemical signature of the geologic
materials it is flowing through
• As it moves along a groundwater flow path, the
chemistry continues to evolve
• Evolutionary sequence controlled by mineral
availability and solubility
– High availability: carbonates and felsic minerals
– High solubility: gypsum/anhydrite, evaporites
11
Evolution of Groundwater Chemistry
12
Open vs. closed systems
• Soil and shallow groundwater (< 10 ft below water table)
are open systems with respect to gases (CO2 and O2)
– Gaseous exchange with the atmosphere (or soil gas), which is at
or near equilibrium saturation
– As CO2 and O2 are consumed, replaced by CO2 from atmosphere
– As CO2 is generated, it will degas
• Deeper groundwater is a closed system with respect to
gases
– Water is isolated from the atmosphere
– If gases are consumed, their concentrations decrease; if
generated, concentrations increase
13
General trends in groundwater with
increasing age and/or depth
• O2: rapidly consumed by biological activity (oxidation
of organic matter or reduced minerals)
• pH: usually rises along a flow path as H+ is consumed
during weathering reactions
– A closed system has finite acidity
– pH can fall by oxidation of sulfide minerals
• HCO3-: concentration increases because H+ in H2CO3
consumed
14
Trends with age/depth
• As groundwater migrates, concentration of TDS
and most major ions increases
• Anions
– Chebotarev took 10,000 groundwater samples from
large sedimentary basins in Australia and determined
that groundwater evolves towards seawater
composition
– Determined that relative abundances of anions
changed with travel distance/age
• HCO3-  HCO3- + SO42-  SO42- + HCO3-  SO42- + Cl-  Cl- +
SO42-  Cl15
Groundwater Anion Evolution
SO4
0
Tri-linear Diagram:
Used in Piper Diagrams
100
20
80
40
60
60
40
80
20
Young
Very
Old
100
0
HCO3 + CO3
20
40
60
80
0
100
Cl
16
Trends with age/depth
• Cations
– More difficult to generalize trends
– Most common trend: Ca2+, then Ca-Na, Na-Ca,
finally Na+
– Driven by cation exchange and CaCO3 precipitation
• Redox Species
– Sequential reduction of oxidized species
17
Trends with age/depth
• Groundwater Chemistry Zones
– Upper: active groundwater circulation, relatively
weathered (leached) rocks, Ca2+ - HCO3- dominate,
low TDS
• Usually not a lot of soluble minerals (like halite and
gypsum)
• HCO3- dominant anion, Ca2+ commonly dominant
cation, relatively low TDS (< 500 mg/L)
18
Trends with age/depth
• Groundwater Chemistry Zones
– Intermediate: less active flow, unweathered rocks,
SO42- dominant anion, Na+ increases but Ca2+usually still important, higher TDS
– Lower: slow circulation, unweathered rocks, Na+ Cl- dominant ions, high TDS
• Highly soluble minerals common
19
Evolution of Groundwater Chemistry
Low TDS
Intermediate TDS
Aquitard: TDS high relative to aquifers
High TDS
20
Mineralogy and Water Chemistry
• Identity of rocks and minerals along
groundwater flowpath an important variable
affecting water chemistry
21
Mineralogy of Igneous Rocks:
Bowen’s Reaction Series
At/Near Earth’s
Surface:
Less Stable
Everything else being equal,
Ca > Na > K
More Stable
22
Mineralogy of Igneous Rocks:
Bowen’s Reaction Series
Mafics
Felsics
23
Igneous Rock Type and Water Chemistry
• Mafic igneous rocks
– High TDS, high Si
– Mg2+ and Ca2+ dominant cations
– Anions: HCO3-
• Felsic igneous and metamorphic rocks
– Relatively low (< 500 mg/L) TDS
– Anions: HCO3- dominant, F- can be characteristic
– Cations: Ca2+ and Na+ dominant
• Fine-grained or glassy rocks
– High TDS because of high mineral surface area or no
mineral structure
24
Sedimentary Rock Type and Water
Chemistry
• Sandstone
– Variable, dependent on mineral composition and how
“pure” sandstone is
– Most often like felsics, but higher TDS
• Limestone/dolomite
–
–
–
–
–
TDS > igneous
Cations: Ca and Mg, little Na
Anions: HCO3Si varies
Dolomite: Ca and Mg equimolar
25
Sedimentary Rock Type and Water
Chemistry
• Shale
– Main minerals quartz and illite are relatively
unreactive
– Long contact time can lead to high TDS
– Most shales form in marine environments, and
Na+ and Cl- can be elevated from original
porewater
– SO42- if pyrite is present, and from porewater
– Plenty of Si
26
Atmospheric Solids and Water Chemistry
• Atmospheric input (dust, etc.)
– Can provide significant amounts of weatherable
material in all climates
– In arid regions, this can be a dominant source
– Laterites on limestone in Bahamas and Amazon: Al
and Fe from dust
27
Chemical Weathering:
Climate and Topography
• Climate
– As precipitation increases, mineral dissolution increases,
more acid to attack the minerals
– For constant precipitation, weathering rate increases with
temperature
• Topography
– Some debate about this, but the majority of evidence
indicates decreased chemical weathering with increasing
elevation
– Probably related to thinner soils, cooler temperatures
28
Water Chemistry: Information on
Weathering Reactions
• Knowing starting and ending solution chemistry
of a system, we can infer what reactions have
taken place to produce the ending solution
– Reaction-Path Modeling
– In addition to water chemistry, need information on
minerals present
– As groundwater migrates along a flow path, reactions
occur:
• Dissolution adds ions
• Mineral precipitation removes ions
– The change in water chemistry = the sum of all
dissolution/precipitation reactions
29
Water Chemistry: Information on
Weathering Reactions
• Garrels and Mackenzie (1967) first to develop
reaction path modeling concept
– Applied on watershed scale (Sierra Nevadas)
– Initial solution was precipitation (rainfall and
snowmelt)
– Ending solution was spring chemistry
30
Example: granitic springs in Sierra Nevadas
• Information that helped characterize the system:
• Geology: Rocks classified as quartz diorite and quartz
microcline gneiss
• Primary minerals
– Feldspars: albite (Na), microcline (K), anorthite (Ca)
• Average feldspar: andesine (Ca and Na)
– Quartz
– Biotite/hornblende
• Climate: high elevation (2-3 km), cool T, high winter
snowfall, summer thunderstorms
31
Example: granitic springs in Sierra Nevadas
• Start building conceptual model:
• As precipitation recharges the subsurface,
which primary minerals would weather most
readily? Least readily?
32
Mineralogy of Igneous Rocks:
Bowen’s Reaction Series
At/Near Earth’s
Surface:
Less Stable
More Stable
33
Example: granitic springs in Sierra Nevadas
• G&M predict decreasing weatherability: Caplagioclase  Na-plagioclase 
Biotite/hornblende  K feldspar  quartz
• What are expected secondary minerals?
– Clays: kaolinite and smectite
– Amorphous SiO2
– CaCO3?
34
Example: granitic springs in Sierra Nevadas
• Ending solutions: Ephemeral and perennial springs
– Ephemeral: short residence time (up to several years), low
TDS and pH
– Perennial: higher residence time (10-100’s yrs), higher TDS
and pH
• Reaction path model
– Starting point: snow chemistry
– Ending point: spring chemistry
– Difference between the two result of reactions involving
dissolution of primary minerals, precipitation of secondary
minerals
35
Ephemeral springs in Sierra Nevadas
• Began by subtracting snow water chemistry from
spring water chemistry to determine how much of
each ion/species added
SiO2
Ca
Mg
Na
K
HCO3
SO4
Cl
ephemeral
mM
0.273
0.078
0.029
0.134
0.028
0.328
0.01
0.014
- snow water
mM
0.270
0.068
0.022
0.110
0.020
0.310
0
0
36
Ephemeral springs in Sierra Nevadas
• All SO4 and Cl removed; none added in the subsurface
• Remaining species added by reactions
SiO2
Ca
Mg
Na
K
HCO3
SO4
Cl
ephemeral
mM
0.273
0.078
0.029
0.134
0.028
0.328
0.01
0.014
- snow water
mM
0.270
0.068
0.022
0.110
0.020
0.310
0
0
37
Ephemeral springs in Sierra Nevadas
• Hypothesis: plagioclase, biotite and K-feldspar each
weathers to kaolinite, amorphous SiO2, and dissolved
ions
– Allow spring water to back-react with kaolinite to see if
could get original minerals
– First, react Na, Ca, HCO3, and SiO2 with kaolinite to make
plagioclase
• All Na and Ca used up
• Resulting plagioclase composition close to what is found
38
Ephemeral springs in Sierra Nevadas
SiO2
Ca
Mg
Na
K
HCO3
SO4
Cl
ephemeral - snow water -plagioclase
mM
mM
mM
0.273
0.270
0.050
0.078
0.068
0
0.029
0.022
0.022
0.134
0.110
0
0.028
0.020
0.020
0.328
0.310
0.064
0.01
0
0
0.014
0
0
• Next, react all Mg along with K, HCO3, and SiO2 to
make biotite (KMg3AlSi3O10(OH)2)
39
Ephemeral springs in Sierra Nevadas
SiO2
Ca
Mg
Na
K
HCO3
SO4
Cl
ephemeral - snow water -plagioclase
mM
mM
mM
0.273
0.270
0.050
0.078
0.068
0
0.029
0.022
0.022
0.134
0.110
0
0.028
0.020
0.020
0.328
0.310
0.064
0.01
0
0
0.014
0
0
-biotite
mM
0.035
0
0
0
0.013
0.013
0
0
• Remaining K, HCO3, and SiO2 used to form K-feldspar
• 4% of original SiO2 remains, good enough
40
Ephemeral springs in Sierra Nevadas
• Resulting balance worked remarkably well,
explaining the concentration of all ions
• Observations
– All SiO2 could be accounted for by dissolution of
aluminosilicates, no quartz dissolution needed
– Waters gain much of their SiO2 over a very short
distance; action of high CO2
– Despite abundant K-feldspar, 80% of dissolved
ions came from plagioclase weathering
41
Perennial springs
• Can same reactions be assumed to be
occurring in perennial springs?
– Not necessarily
– Look at ratio of ions in solution
42
Ephemeral vs. Perennial Springs
SiO2
Ca
Mg
Na
K
HCO3
SO4
Cl
TDS (ppm)
pH
ephemeral
mM
0.273
0.078
0.029
0.134
0.028
0.328
0.01
0.014
perennial
mM
0.410
0.260
0.071
0.259
0.040
0.895
0.025
0.03
36
6.2
75
6.8
difference
mM
0.137
0.182
0.042
0.125
0.012
0.567
0.015
0.016
43
Ephemeral vs. Perennial Springs
• Differences between spring types
– Cl assumed to come from NaCl, SO4 from CaSO4
• Weak assumptions, but very low concentrations
– SiO2:Na ratio for difference between springs is 1:1
• SiO2:Na ratio in solution for weathering of plagioclase is
2:1
– Some secondary mineral other than kaolinite being
produced to remove SiO2
44
Ephemeral vs. Perennial Springs
• Potential candidates for SiO2: clay mineral
(smectite); amorphous SiO2
– Hypothesized reactions
• Plagioclase and biotite  kaolinite
• Plagioclase  smectite
– Ended up with extra Ca and HCO3-, dissolution of
CaCO3
• Potential sources of CaCO3
– Summer wet/dry deposition
– CaCO3 in fracture fillings
45
Reaction Path Models
• Good for simple systems where flowpaths are
well defined
– The larger and more complex the systems, the
harder it is to constrain potential reactions
• Can consider redox reactions, gas exchange,
isotopic reactions, mixing of waters, etc.
• N.B.: there is no unique solution
– Modeler determines which phases to consider
– Based on available data and “intuition”
46
Redox reactions in Groundwater
• Redox reactions are extremely important in
groundwater and soil water
• Many key elements are redox sensitive:
– C, N, S, Fe, Mn, As, heavy metals
• Very important in terms of water
quality/chemistry
47
Factors Controlling Natural Redox
Conditions
• O2 in recharge
• Organic matter content of solids
– Occasionally dissolved organics (natural)
• Presence of redox buffers, usually minerals
• Groundwater residence time
48
Groundwater Chemistry:
Redox Evolution
• Water tends to become more reducing as it
moves along a flow path
– Isolated from atmosphere, so once O2 consumed it
is not replenished
– Organic matter most commonly oxidized compound
• Sulfide minerals can also be important
– Most rapidly in the shallow zones
49
Microbes and Redox Reactions in
Groundwater
• Almost all redox reactions in groundwater are
biogeochemically mediated
– Microorganisms catalyze almost all redox
reactions and use the energy released
– Microbes also need a carbon source (as well as
other nutrients)
50
Role of Microrganisms
• Microorganisms produce enzymes that bring reactants
into close proximity
• Enzymes specific to substrate: carbon source and
terminal electron acceptor (TEAP) (i.e., O2, NO3-, Fe(OH)3,
…)
– Enzyme induction: ability to create new enzymes to adapt to
new carbon source (i.e., organic contaminants)
• In any soil, there exists a huge variety of microorganisms
but there is usually a dominant species or set of species
– DNA/RNA techniques used to identify dominant species
– Non-dominant species exist in isolated microenvironments
• Biofilms (“slime”): “engineered” microenvironments
51
Groundwater Chemistry:
Redox Evolution
• Dissolved oxygen (DO)
– In clayey/silty soils, DO commonly below detection
in shallow groundwater
– DO is generally detectable in recharge areas and in
sandy soils and karstic limestones
– If there is little or no soil over permeable fractured
rock, detectable DO can persist far into the flow
system
• Occasionally an entire flow system is oxygenated
52
Organic Matter Oxidation
• O2 has low solubility
– 9 mg/L at 25°C (2.8 x 10-4 moles/L)
– 11 mg/L at 5°C
• Half reactions
– OM oxidation: CH2O + H2O  CO2 + 4H+ + 4e– O2 reduction: O2 + 4H+ + 4e-  2H2O
– CH2O + O2  CO2 + H2O
• For every mole of OM oxidized, one mole O2 of reduced
• Therefore, DO typically consumed in the soil zone
and shallow groundwater, resulting in anoxic
groundwater
53
DO Consumption
Flooded soil
54
Groundwater Chemistry: Redox
Evolution
• After DO is consumed, other TEAPs are used by
microbes based on thermodynamics
–
–
–
–
–
–
NO3- reduction (denitrification)
MnO2 [Mn(IV)] reduction
Ferric [Fe(III)] mineral reduction
SO42- reduction
Fermentation and methanogenesis (CO2 reduction)
“Redox ladder”
• The order of the reactions based on obtainable energy
for the microbes
• Kinetics: the less the energy, the slower the reaction
55
Role of Microrganisms
• Microorganisms are subject to the laws of
thermodynamics (as are we)
– They catalyze reactions until equilibrium is reached (ΔG =
0) or until TEAP is consumed (reaction goes to completion)
– For example, when O2 is TEAP
• CH2O(aq) + O2  CO2 + H2O
– When O2 is consumed and NO3- is present, denitrifying
organisms have competitive advantage because they get
more energy from reaction than Fe, Mn, or SO42- reducers
56
Redox Ladder: electron acceptors and
donors
57
Post-DO redox reactions involving OM
• Unbalanced reactions:
• CH2O + NO3-  CO2 + N2 (denitrification)
– 5 CH2O + 4 NO3- + 4 H+  5 CO2 + 2 N2 + 7 H2O
– This reaction causes pH to increase, which is indirect
evidence that denitrification has occurred
• CH2O + NO3-  CO2 + NH3 (ammonification; toxic to
fish)
58
Post-DO redox reactions involving OM
• CH2O + Fe(OH)3  CO2 + Fe2+ (iron reduction;
dissolves Fe(III) minerals)
– CH2O + 4Fe(OH)3 + 8 H+  CO2 + 4 Fe2+ + 11 H2O
• CH2O + SO42-  CO2 + H2S (sulfate reduction)
– 2CH2O + SO42- + H+  2 CO2 + HS- + 2 H2O
– Water from Normal well field has a rotten egg (H2S)
smell—why? High organics and sulfate-reducing bacteria
active
59
Fermentation and Methanogenesis
• Reactions that occur when all external electron
acceptors have been used; methane (CH4) is
produced, CO2 both produced and consumed
• Transformation of complex organics into simpler
compounds
• Fermentation: CH3COOH  CH4 + CO2
– CH3COOH = Acetic acid
– Also produces H2
• 2 H + + 2 e-  H 2
– Fermentation byproducts are used by methanogenic
microbes
60
Fermentation and Methanogenesis
• Methanogenesis: CO2 + 4 H2  CH4 + 2 H2O
– Methane production characterized by increasing H2
– Methanogens need fermenters
• H2 is a reactive intermediate product, produced and
consumed by metabolic processes
– Low at high Eh, higher at lower Eh
– H2 is best indicator of dominant TEAP, but difficult to
measure (field GC)
61
General order of microbially-mediated
redox reactions
Conceptual change in concentrations with time/distance
62
TEAPs in Groundwater
Contaminated
Uncontaminated
FLOW
63
TEAPs
• While thermodynamics predicts an orderly
progression of the dominance of individual
TEAPs, it’s not so simple in nature
– Often have 2 (or more) TEAPs active in same part of
aquifer
• e.g., often have Fe3+ -reduction and SO42--reduction
occurring together, even though Fe3+ reduction more
thermodynamically favorable
– Due to: micro-environments, different microorganisms
responsible, solid vs. aqueous environments
– Where there’s energy to be gained, microbes are
working
64
TEAPs and Eh Ranges
65
Determining
predominant
TEAP
66
Redox Buffering
• The Eh of groundwater does not linearly
decline as oxidizers are consumed along a flow
path
• Instead, the Eh remains relatively constant as
a particular oxidizer is consumed, then the Eh
drops and stabilizes again
67
Redox Buffering
68
Redox Buffering
• System is buffered if oxidizable or reducible
compounds are present that prevent a
significant change in Eh when strong
oxidizing/reducing agents added
– Expect Eh of natural waters to generally be in
buffered ranges
– Values in unbuffered ranges unstable
69
Computed vs. Measured Field Eh
- Vertical bands indicate
buffered ranges; reflect
the standard E°
70
Redox Buffering
• Example: recharging water has dissolved O2, Eh will
remain high until O2 is consumed; after O2 gone, Eh
drops rapidly and stabilizes at the value determined
by next oxidizer
• Buffers can be dissolved species or solid matter
– Dissolved species: usually limited in concentration and
consumed rapidly (if right conditions exist)
– Solid matter: can provide large buffering capacity
– E.g., Fe(OH)3 can provide buffering until equilibrium is
reached with dissolved Fe concentration
71

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