Lecture 4-5-11

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Solute Transport
• Ions and molecules being transported in the
subsurface often travel at rates slower than
water
• The migration is “retarded” primarily due to
their interactions with mineral surfaces
• Surface complexation reactions
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Surface Complexation Reactions
• Reactions occurring at the mineral-water
interface (mineral surface)
• Important for:
– Transport and transformation of metals and
organic contaminants
– Nutrient availability in soils
– Formation of ore deposits
– Acidification of watersheds
– Global cycling of elements
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Sorption Processes
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Surface Charge
• Solids typically have an electrically charged surface
• There are 2 main sources of surface charge
• (1) Chemical reactions
– pH dependent: surfaces tend to have positive charges at low
pH, negative charges at high pH
– For most common solid phases at natural pHs, the surface
charge is negative
• (2) Lattice imperfections and substitutions in the solid
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Surface Charge
• Clays: substitution or vacancy result in negative
charge, which is the dominant charge
• Al/Fe hydroxides adsorb both cations and anions
depending on pH: Amphoteric
– Low pH: positive surface charge
– High pH: negative surface charge
• Organic compounds can also have pH dependent
charge
– DOM can be important in transport of low solubility
metals
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Surface Charge
• The interfacial system (surface – water) must
be electrically neutral
• Electrical Double Layer
– Fixed surface charge on the solid
– Charge distributed diffusely in solution
• Excess of counterions (opposite charge to surface) and
deficiency of ions of same charge as surface
• Counterions attracted to the surface
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Adsorption
• Adsorption refers to a dissolved ion or
molecule binding to a charged surface
• All ions (including H+ and OH-) are continually
competing for sites
• Reversible reactions; i.e., if conditions change,
the ion can desorb
• Kinetically fast reactions; equilibrium often
assumed
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_
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Solid _
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Phase _
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Fixed Surface _
Charge
_
Counterions
Adsorption
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+
+
+
_
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+
Solution
+
_
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Ion Exchange
• Ion exchange refers to exchange of ions between
solution and solid surfaces
• It differs from adsorption in that an ion is released
from the surface as another is adsorbed
– AX + B+  BX + A+
– X refers to a mineral surface to which an ion has adsorbed
– Most important for cations, anions less so, because most
mineral surfaces are negatively charged
• Primarily occurs on clay minerals of colloidal size (10-3
– 10-6 mm)
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Ion Exchange
• Ion size (radius) and charge affect how they
exchange
– Smaller ions from stronger bonds on surfaces
– Ions with more positive charge form stronger
bonds on surfaces
– Stronger to weaker, increasing ionic radii:
• Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+
• Reversible reactions
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Cation Exchange Capacity (CEC)
• CEC is the capacity of a mineral to exchange one
cation for another
– Depends on charge imbalances in the crystal lattice
• Amount of exchange sites per mass of solid
(meq/100 g)
– Measured in lab by uptake and release of NH4+ acetate
– Not a precise measurement: pH dependent, organic
coatings
– Primarily applicable to clays
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Ion Exchange Equilibrium
• Mass action equation:
– B-clay + A+ ↔ A-clay + B+
• Where A+ and B+ are monovalent cations
–
• aA-clay and aB-clay = activities of A and B on exchange sites
• aA+ and aB+ = activities in solution
• KAB = exchange constant
–
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Ion Exchange Equilibrium
• Mass action equation can be rewritten using
mole fractions in the solid phase
–
• XA-clay and XB-clay = mole fractions of A and B on clay
– XA-clay + XB-clay = 1
• K’AB = selectivity coefficient
• K’AB is not a constant because activity coefficients in the
solid dependent on composition
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Ion Exchange Equilibrium
• Example: Mix 10 g of a Na-saturated smectite
with CEC = 100 meq/100 g with 1 liter of
water containing 20 mg/L Na+ and 20 mg/L K+
as the only cations. Assume KK+-Na+ = 2.
• What will the final Na+ and K+ concentrations
be?
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Ion Exchange Equilibrium
• Exchange between monovalent and divalent
cation:
– 2 A-clay + C2+ ↔ C-clay + 2A+
–
–
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Ion Exchange Equilibrium
• Example: Suppose a solution in contact with a
clay is at equilibrium and has a Ca2+
concentration = 35 mg/L and Na+ = 10 mg/L.
Assume KCa2+-Na+ = 2.
• What are the mole fractions (XCa2+ and XNa+) in
the solid phase?
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Monovalent-divalent effect
• In fresh (dilute) waters, the dominant
exchangeable cation is Ca2+
• In the ocean, the dominant exchangeable
cation is Na+
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Cation Exchange Capacity and
Groundwater Composition
• Ion exchange reacts important control on
groundwater chemistry
• Typically CEC value in aquifer of 5 meq/100 g
gives an exchange capacity of ~500 meq/L
– Much larger than concentration of dissolved
cations in dilute groundwater
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Clay Mineralogy
• Clays are fine-grained, crystalline, hydrous
silicates with sheet structures
– Phyllosilicates
• Most common type of secondary mineral
• Have surface charge, usually negative
– Charge attracts cations to surface where they are
bound by electrostatic forces
– Not part of crystal structure so they can easily
exchange with other ions in solution
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Clay structure
• Clays have 2 distinct sheet structures
– Tetrahedral: 3-sided pyramid, 4 oxygen (O2-) atoms
(or OH-) surrounding a silicon atom (Si4+)
• Al3+ can substitute for Si4+, resulting in negative charge
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Clay structure
– Octahedral: two 3-sided pyramids joined at the
base
• Surface charge results from substitution or vacancy in
central cation (usually Al, Mg, Fe)
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Clay Structures
• The tetrahedrons and octahedrons are joined
to each other in sheets
• The sheets join in 2 main patterns to create
different clays: 2-layer and 3-layer
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Clay Structures
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Types of clays
• 2-layer phyllosilicates
– Alternating tetrahedral and octahedral layers (T:O or
1:1)
– Each T and O sheet are strongly bound, while T:O’s are
held together by weak van der Waal’s forces
– Kaolinte (Al2Si2O5(OH)4) and serpentite
(Mg3Si2O5(OH)4) groups
– Relatively pure clays, close to stoichiometric
– Low substitution results in low surface charge, no
interlayer adsorption sites
– Low CEC (kaolinite: 3-5 meq/100 g)
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Kaolinite
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Types of clays
• 3-layer phyllosilicates
– Each layer consists of 2 tetrahedrons and one
octahedron (T:O:T or 2:1)
– Interlayers can be adsorption sites
– Smectite, vermiculite, and mica groups
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3-Layer Phyllosilicates
• Smectites
– Wide interlayer spacing, easily exchange ions/ H2O
– High substitution/vacancy, high CEC
• CEC: 70-150 meq/100 g
– Shrink/swell: as moisture content increases, more
water in interlayer expands; vice versa as water
content decreases
• Due to type of cation
– Ca2+  Na+ exchange
– 2 ions for 1, increases interlayer thickness
• Road salt can cause expansion of smectities next to roads
due to increased Na+, resulting in engineering problems
• Solution: add lime or CaCO3 to exchange Ca2+ for Na+
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Smectite
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3-Layer Phyllosilicates
• Vermiculite
– Stronger interlayer cation
bonding, slower cation
exchange, higher surface charge
– High CEC
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3-Layer Phyllosilicates
• Illite: most common in nature, makes up most
ancient shales
– 80% mica, 20% smectite
– Low surface charge and CEC
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3-Layer Phyllosilicates
• Mica
– Muscovite and biotite primary minerals with little
substitution or vacancy, little surface charge
– Similar structure to illite
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CEC values for some clays (pH = 7.0)
Clay
CEC (meq/100 g)
Kaolinite
3-5
Chlorite
10-40
Illite
10-40
Smectite (montmorillonite)
70-150
Vermiculite
100-150
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Double Layer Theory
• Describes the distribution of charge near a charge
surface and how charge is neutralized
– Stern layer: closest to surface where cations bonded
by weak electrostatic forces (van der Waals)
• Cations can exchange relatively rapidly and easily
– Gouy layer: further from surface, thickness related to
ionic strength of solution
• High I, thin Gouy layer; more ions can neutralize charge over
shorter distance
• Low I, thick Gouy layer
– Adsorption can occur in both layers
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Double Layer Theory
Net positive charge
in Gouy layer
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Double Layer Theory
• The likelihood of attachment of a charged
species approaches a surface is controlled by
the sum of attractive and repulsive forces
– Attractive: van der Waals between species of
opposite charge
– Repulsive: net positive charge in Gouy layer repels
incoming cations
– Sum of these 2 is the energy barrier (or lack
thereof) needed to be overcome before a species
can adsorb at the surface (Stern layer)
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Double Layer Theory
• Attachment also dependent of charge density
of an ion and ionic strength
– As charge density increases, attraction increases
– As I decreases, Gouy layer thickness increases,
repulsion moves further away from surface where
attraction is weaker
• Adsorption preference
– Fe3+ > Al3+ > Co2+ > Ca2+ = Sr2+ > Rb+ > Mg2+ > K+ >
NH4+ > Na+ > Li+
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Strength of adsorption
• Outer (Gouy) layer complexes: cation still
surrounded by sphere of hydration
– Weakly bound to surface, easily exchanged
• Inner (Stern) layer: no sphere of hydration,
strongly bound directly to solid surface
– Not easily exchange, may be effectively
irreversible
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Desorption
• Reversible reactions: desorption can be
caused by:
– Decreasing ionic strength
– Change in composition of ions in solution
• Ions with higher charge density are more likely to
adsorb
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Measuring adsorption
• Adsorption is measured in the laboratory by
mixing a solution containing an ion with a solid
phase (batch experiments)
–
–
–
–
–
Mix solution of known concentration with solid
Agitate until equilibrium is reached
Measure final dissolved concentration
Initial – final = amount adsorbed
Repeat at different initial concentrations
• Plot data, and a graph called an adsorption
isotherm is prepared
– Isotherm = experiments done at constant temperature
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Concentration adsorbed
Typical Adsorption Isotherm
Concentration in Solution
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Depicting Adsorption Mathematically
• Can be represented in terms of relatively
simple empirical formulas, or more
sophisticated models like double layer, triple
layer, or constant capacitance theories
• Most often, the simple empirical formulas are
used because we don’t have the data for more
sophisticated approaches
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Adsorption
• Since adsorption is a chemical process, we can
write chemical reactions to describe it:
– C + S ↔ CS
• C = ion (mg/L)
• S = surface (g)
• CS = adsorbed ion (mg/g)
– Adsorbed ion measure with respect to amount of solid
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Linear Isotherms
• Ratio of adsorbed to dissolved concentration is
constant
– Kd = C* / C
• Kd = distribution coefficient (L3/mass)
• C* = adsorbed species (massion/masssolid)
• C = dissolved concentration (mass /L3)
• This approach produces Linear Isotherms
• Once Kd determined, calculate adsorbed
“concentration” for any measured dissolved
concentration
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Concentration adsorbed
Typical Adsorption Isotherm
Linear portion
of isotherm
Concentration in Solution
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Linear Isotherms
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Linear Isotherms
• Assumptions:
–
–
–
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Fast reaction (i.e. equilibrium quickly reached)
Reversible reaction
Isothermal
Monolayer adsorption
• Use Kd’s with great care because:
–
–
–
–
–
Reactions are pH, temperature, and Eh dependent
Species specific, don’t account for competition
Ionic strength dependent
Surface dependent
Can’t be universally applied
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Langmuir Isotherms
• These recognize that there are a limited
number of adsorption sites for charged
species
– Take into account that batch experiments at
higher concentration do not result in linear
increases in adsorption
– Plots go non-linear as they approach a maximum
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Langmuir Isotherms
Cmax*
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Langmuir Isotherms
•
– α = KLang = adsorption constant (L3 / mass)
– β = maximum amount of adsorption sites (mass/mass)
• Also Cmax*
• α and β can be obtained by plotting C/C* vs. C
– Slope = 1/β
– Intercept pt = 1/αβ
– Still specific to species, site, water chemistry
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Langmuir Isotherms
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Surface Complexation Adsorption
Models
• Diffuse double layer, triple layer, constant
capacitance
• Best used to describe adsorption of metals
and other cationic species
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Surface Complexation Adsorption
Models
• Advantages:
– Based on thermodynamics
– Balanced reactions
– Law of mass action
– Adsorption function of pH and solution chemistry
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Surface Complexation Adsorption
Models
• Recognizes that all exchange sites are not
equal (inner vs. outer)
• Types of exchange sites:
– Aluminosilicates: crystal damage results in
permanent change, “exchange” sites
– Surface functional groups: usually a hydroxyl (OH-)
on mineral edge
• Surface charge is pH dependent
• Positive and negative sites can co-exist
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Surface Complexation Adsorption
Models
• So there are a variety of sites based on surface
type, charge varies between types
– The same surface type can have sites with
different bond strength
• Inner sphere complex: strong covalent bond, bonds
directly to surface
• Outer sphere complex: cation still surrounded by
sphere of hydration; held by weaker electrostatic forces
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Surface Complexation Adsorption
Models
• Writing surface complexation reactions;
account for free energy based on chemical
and electrostatic contributions
– ≡S-OH + M2+ ↔ ≡S-OM + H+
• ≡S = surface
• OH = functional group
• M2+ = dissolved metal
– Anion adsorption
• ≡S-OH + A- ↔ ≡S-A + OH55
Surface Complexation Adsorption Models:
Thermodynamically Based
• Law of mass action for reaction:
– This reaction accounts for chemical ΔG, but not
electrostatic
– Based on activity of species in bulk solution
– Work is necessary to move ions through charged
Gouy layer
– Close to the surface, the diffuse layer has excess
of cations, therefore activity (concentration) of
cations increases
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Surface Complexation Adsorption
Models
• The equations from these models take into account
multiple site types, multiple species, changes in
solution chemistry
• These surface complexation models have been shown
to realistically model adsorption in lab experiments
– However, most lab experiments use pure mineral phases
and artificial solutions.
• Surface complexation models require measurement of
numerous parameters on heterogeneous materials, so
their field application may not be practical
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Adsorption
• Adsorption is an enormously complicated subject
• It is usually very difficult to apply laboratory derived
values to the field
– It is very difficult to get meaningful field data on surface
properties
– Beware!
• Changing conditions can lead to changing behavior
– e.g., contaminated sites and plumes
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Organic Compounds
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Organic Compounds
• Definition: molecules with a carbon skeleton
– Usually have H and O as well
• Importance:
– Weathering and diagenesis
– Redox conditions of water
– Transport of trace metals
– Contaminants: organic contaminants and
biodegradation
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Organic Compound Properties
• In general, organic matter is not very soluble in water.
– Organics are non-polar or slightly polar while water is
highly polar
• Uncharged or weakly charged
• Can exist as dissolved, solid, or gaseous phases
• Organic matter in water is composed of an almost
infinite variety of compounds
– With current technology, can determine the general
chemical composition of organics, but don’t know specific
formulas
– The exception is anthropogenic compounds where we
know exact formulas
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Organic Compound Properties
• Most dissolved organic matter in groundwater
are humic acids
– Substances are formed by the microbial degradation
of dead plant matter, such as lignin
– Very resistant to further biodegradation
• Easy stuff already degraded
• Explains why old groundwater still has organic matter
– Defined operationally: extracted into a strongly basic
aqueous solution, then precipitated from solution
when pH adjusted to 1 with HCl
• Remaining organics in solution = fulvic acids (dominate
surface water)
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Typical Humic Acid
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Humic Acid
Chromatography
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Measuring Organic Compound in
Groundwater
• Dissolved organic carbon (DOC) (water passed through
0.45 μm filter)
– Arbitrary division between dissolved and suspended
material
– DOC can be converted to CO2, which is how it’s typically
measured
– Can also measure DON and DOP
• Total organic carbon (TOC)
– Same procedures, but not filtered
• DOC in groundwater typically low, ≤ 2 mg/L
• DOC visible in water at about 10 mg/L (dark color)
• Swamps and other wetlands have some of the highest
DOC values, ~60 mg/L
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Organic Compound Nomenclature
• All organics have carbon skeletons with
functional groups attached
• Aliphatics: straight or branched chains
– e.g., propane, methylpropane
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Propane (C3H8) and Butane (C4H10)
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Organic Compound Nomenclature
• Aromatics: ring structure
– e.g., benzene, naphthalene
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A (Very) Little Humor
Benzene (C6H6)
Mercedes Benzene
Organic Compound Nomenclature
• Aromatics: ring structure
– e.g., benzene, naphthalene
– Multi-rings = polyaromatics (PNAs or PAHs)
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Polyaromatics (PAHs)
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Organic Compound Nomenclature
• Aromatics: ring structure
– e.g., benzene, naphthalene
– Multi-rings = polyaromatics (PNAs or PAHs)
– Heterocyclic: ring structure with atoms other than
C in skeleton
• e.g. pyridine
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Pyridine
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