solute transport ions and molecules being transported in the subsurface often travel at rates slower...

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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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Adsorption

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Fixed SurfaceCharge

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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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• 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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• 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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• Exchange between monovalent and divalent cation:– 2 A-clay + C2+ ↔ C-clay + 2A+

–

–

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• 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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• Vermiculite– Stronger interlayer cation

bonding, slower cation exchange, higher surface charge

– High CEC

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• Illite: most common in nature, makes up most ancient shales– 80% mica, 20% smectite– Low surface charge and CEC

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• 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 3-5Chlorite 10-40Illite 10-40Smectite (montmorillonite) 70-150Vermiculite 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 chargein 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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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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Typical Adsorption Isotherm

Concentration in Solution

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Linear portionof isotherm

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Linear Isotherms

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Linear Isotherms

• Assumptions: – 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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• Advantages: – Based on thermodynamics– Balanced reactions– Law of mass action– Adsorption function of pH and solution chemistry

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• 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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• 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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• 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 + OH-

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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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• 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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• Aromatics: ring structure– e.g., benzene, naphthalene

Benzene (C6H6)

Mercedes Benzene

A (Very) Little Humor

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• Aromatics: ring structure– e.g., benzene, naphthalene– Multi-rings = polyaromatics (PNAs or PAHs)

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Polyaromatics (PAHs)

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• 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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