Solute transport
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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. Surface Complexation Reactions.

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Solute Transport

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Solute transport

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 complexationreactions


Surface complexation reactions

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


Sorption processes

Sorption Processes


Surface charge

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


Surface charge1

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


Surface charge2

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


Adsorption

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


Adsorption1

Adsorption

Counterions

Fixed Surface

Charge


Ion exchange

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)


Ion exchange1

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


Cation exchange capacity cec

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


Ion exchange equilibrium

Ion Exchange Equilibrium

  • Mass action equation:

    • B-clay + A+ ↔ A-clay + B+

      • Where A+ and B+ are monovalentcations

      • aA-clay and aB-clay = activities of A and B on exchange sites

      • aA+ and aB+ = activities in solution

      • KAB = exchange constant


Ion exchange equilibrium1

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


Ion exchange equilibrium2

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?


Ion exchange equilibrium3

Ion Exchange Equilibrium

  • Exchange between monovalent and divalent cation:

    • 2 A-clay + C2+ ↔ C-clay + 2A+


Ion exchange equilibrium4

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?


Monovalent divalent effect

Monovalent-divalent effect

  • In fresh (dilute) waters, the dominant exchangeable cation is Ca2+

  • In the ocean, the dominant exchangeable cation is Na+


Cation exchange capacity and groundwater composition

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


Clay mineralogy

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


Clay structure

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


Clay structure1

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)


Clay structures

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


Clay structures1

Clay Structures


Types of clays

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)


Kaolinite

Kaolinite


Types of clays1

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


3 layer phyllosilicates

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+


Smectite

Smectite


3 layer phyllosilicates1

3-Layer Phyllosilicates

  • Vermiculite

    • Stronger interlayer cation bonding, slower cation exchange, higher surface charge

    • High CEC


3 layer phyllosilicates2

3-Layer Phyllosilicates

  • Illite: most common in nature, makes up most ancient shales

    • 80% mica, 20% smectite

    • Low surface charge and CEC


3 layer phyllosilicates3

3-Layer Phyllosilicates

  • Mica

    • Muscovite and biotite primary minerals with little substitution or vacancy, little surface charge

    • Similar structure to illite


Cec values for some clays ph 7 0

CEC values for some clays (pH = 7.0)


Double layer theory

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


Double layer theory1

Double Layer Theory

Net positive charge

in Gouy layer


Double layer theory2

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)


Double layer theory3

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+


Strength of adsorption

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


Desorption

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


Measuring adsorption

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


Typical adsorption isotherm

Typical Adsorption Isotherm


Depicting adsorption mathematically

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


Adsorption2

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


Linear isotherms

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


Typical adsorption isotherm1

Typical Adsorption Isotherm

Linear portion

of isotherm


Linear isotherms1

Linear Isotherms


Linear isotherms2

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


Langmuir isotherms

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


Langmuir isotherms1

Langmuir Isotherms

Cmax*


Langmuir isotherms2

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


  • Langmuir isotherms3

    Langmuir Isotherms


    Surface complexation adsorption models

    Surface Complexation Adsorption Models

    • Diffuse double layer, triple layer, constant capacitance

    • Best used to describe adsorption of metals and other cationic species


    Surface complexation adsorption models1

    Surface Complexation Adsorption Models

    • Advantages:

      • Based on thermodynamics

      • Balanced reactions

      • Law of mass action

      • Adsorption function of pH and solution chemistry


    Surface complexation adsorption models2

    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


    Surface complexation adsorption models3

    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


    Surface complexation adsorption models4

    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 + OH-


    Surface complexation adsorption models thermodynamically based

    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


    Surface complexation adsorption models5

    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


    Adsorption3

    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


    Organic compounds

    Organic Compounds


    Organic compounds1

    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


    Organic compound properties

    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


    Organic compound properties1

    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)


    Typical humic acid

    Typical Humic Acid


    Humic acid chromatography

    Humic Acid Chromatography


    Measuring organic compound in groundwater

    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


    Organic compound nomenclature

    Organic Compound Nomenclature

    • All organics have carbon skeletons with functional groups attached

    • Aliphatics: straight or branched chains

      • e.g., propane, methylpropane


    Propane c 3 h 8 and butane c 4 h 10

    Propane (C3H8) and Butane (C4H10)


    Organic compound nomenclature1

    Organic Compound Nomenclature

    • Aromatics: ring structure

      • e.g., benzene, naphthalene


    A very little humor

    A (Very) Little Humor

    Benzene (C6H6)

    Mercedes Benzene


    Organic compound nomenclature2

    Organic Compound Nomenclature

    • Aromatics: ring structure

      • e.g., benzene, naphthalene

      • Multi-rings = polyaromatics (PNAs or PAHs)


    Polyaromatics pahs

    Polyaromatics (PAHs)


    Organic compound nomenclature3

    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


    Pyridine

    Pyridine


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