Solute Transport

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