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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 complexation reactions. Surface Complexation Reactions.

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Solute Transport

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  1. 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

  2. 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

  3. Sorption Processes

  4. 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

  5. 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

  6. 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

  7. 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

  8. Adsorption Counterions Fixed Surface Charge

  9. 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)

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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?

  15. Ion Exchange Equilibrium • Exchange between monovalent and divalent cation: • 2 A-clay + C2+ ↔ C-clay + 2A+

  16. 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?

  17. Monovalent-divalent effect • In fresh (dilute) waters, the dominant exchangeable cation is Ca2+ • In the ocean, the dominant exchangeable cation is Na+

  18. 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

  19. 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

  20. 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

  21. 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)

  22. 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

  23. Clay Structures

  24. 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)

  25. Kaolinite

  26. 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

  27. 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+

  28. Smectite

  29. 3-Layer Phyllosilicates • Vermiculite • Stronger interlayer cation bonding, slower cation exchange, higher surface charge • High CEC

  30. 3-Layer Phyllosilicates • Illite: most common in nature, makes up most ancient shales • 80% mica, 20% smectite • Low surface charge and CEC

  31. 3-Layer Phyllosilicates • Mica • Muscovite and biotite primary minerals with little substitution or vacancy, little surface charge • Similar structure to illite

  32. CEC values for some clays (pH = 7.0)

  33. 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

  34. Double Layer Theory Net positive charge in Gouy layer

  35. 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)

  36. 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+

  37. 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

  38. 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

  39. 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

  40. Typical Adsorption Isotherm

  41. 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

  42. 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

  43. 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

  44. Typical Adsorption Isotherm Linear portion of isotherm

  45. Linear Isotherms

  46. 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

  47. 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

  48. Langmuir Isotherms Cmax*

  49. 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

  50. Langmuir Isotherms

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