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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.

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Presentation on theme: "Solute Transport Ions and molecules being transported in the subsurface often travel at rates slower than water The migration is “retarded” primarily due."— Presentation transcript:

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 complexation reactions 1

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 2

3 Sorption Processes 3

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 4

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 5

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 6

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 7

8 Adsorption 8 Fixed Surface Charge Counterions

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 – mm) 9

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: Al 3+ > Ca 2+ > Mg 2+ > K + = NH 4 + > Na + Reversible reactions 10

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 NH 4 + acetate – Not a precise measurement: pH dependent, organic coatings – Primarily applicable to clays 11

12 Ion Exchange Equilibrium Mass action equation: – B-clay + A + ↔ A-clay + B + Where A + and B + are monovalent cations – a A-clay and a B-clay = activities of A and B on exchange sites a A+ and a B+ = activities in solution K AB = exchange constant – 12

13 Ion Exchange Equilibrium Mass action equation can be rewritten using mole fractions in the solid phase – X A-clay and X B-clay = mole fractions of A and B on clay – X A-clay + X B-clay = 1 K’ AB = selectivity coefficient K’ AB is not a constant because activity coefficients in the solid dependent on composition 13

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 K K+-Na+ = 2. What will the final Na + and K + concentrations be? 14

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

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 K Ca2+-Na+ = 2. What are the mole fractions (X Ca2+ and X Na+ ) in the solid phase? 16

17 Monovalent-divalent effect In fresh (dilute) waters, the dominant exchangeable cation is Ca 2+ In the ocean, the dominant exchangeable cation is Na + 17

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 18

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 19

20 Clay structure Clays have 2 distinct sheet structures – Tetrahedral: 3-sided pyramid, 4 oxygen (O 2- ) atoms (or OH - ) surrounding a silicon atom (Si 4+ ) Al 3+ can substitute for Si 4+, resulting in negative charge 20

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) 21

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 22

23 Clay Structures 23

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 (Al 2 Si 2 O 5 (OH) 4 ) and serpentite (Mg 3 Si 2 O 5 (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) 24

25 Kaolinite 25

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 26

27 3-Layer Phyllosilicates Smectites – Wide interlayer spacing, easily exchange ions/ H 2 O – High substitution/vacancy, high CEC CEC: 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 – Ca 2+  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 CaCO 3 to exchange Ca 2+ for Na + 27

28 Smectite 28

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

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

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

32 CEC values for some clays (pH = 7.0) 32 ClayCEC (meq/100 g) Kaolinite3-5 Chlorite10-40 Illite10-40 Smectite (montmorillonite) Vermiculite

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 33

34 Double Layer Theory 34 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) 35

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 – Fe 3+ > Al 3+ > Co 2+ > Ca 2+ = Sr 2+ > Rb + > Mg 2+ > K + > NH 4 + > Na + > Li + 36

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 37

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 38

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 39

40 Typical Adsorption Isotherm 40

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 41

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 42

43 Linear Isotherms Ratio of adsorbed to dissolved concentration is constant – K d = C* / C K d = distribution coefficient (L 3 /mass) C* = adsorbed species (mass ion /mass solid ) C = dissolved concentration (mass /L 3 ) This approach produces Linear Isotherms Once K d determined, calculate adsorbed “concentration” for any measured dissolved concentration 43

44 Typical Adsorption Isotherm 44 Linear portion of isotherm

45 Linear Isotherms 45

46 Linear Isotherms Assumptions: – Fast reaction (i.e. equilibrium quickly reached) – Reversible reaction – Isothermal – Monolayer adsorption Use K d ’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 46

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 47

48 Langmuir Isotherms 48 C max *

49 Langmuir Isotherms –α = K Lang = adsorption constant (L 3 / mass) – β = maximum amount of adsorption sites (mass/mass) Also C max * 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 49

50 Langmuir Isotherms 50

51 Surface Complexation Adsorption Models Diffuse double layer, triple layer, constant capacitance Best used to describe adsorption of metals and other cationic species 51

52 Surface Complexation Adsorption Models Advantages: – Based on thermodynamics – Balanced reactions – Law of mass action – Adsorption function of pH and solution chemistry 52

53 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 53

54 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 54

55 Surface Complexation Adsorption Models Writing surface complexation reactions; account for free energy based on chemical and electrostatic contributions – ≡S-OH + M 2+ ↔ ≡S-OM + H + ≡S = surface OH = functional group M 2+ = dissolved metal – Anion adsorption ≡S-OH + A - ↔ ≡S-A + OH - 55

56 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 56

57 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 57

58 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 58

59 Organic Compounds 59

60 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 60

61 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 61

62 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) 62

63 Typical Humic Acid 63

64 Humic Acid Chromatography 64

65 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 CO 2, 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 65

66 Organic Compound Nomenclature All organics have carbon skeletons with functional groups attached Aliphatics: straight or branched chains – e.g., propane, methylpropane 66

67 Propane (C 3 H 8 ) and Butane (C 4 H 10 ) 67

68 Organic Compound Nomenclature Aromatics: ring structure – e.g., benzene, naphthalene 68

69 Benzene (C 6 H 6 ) Mercedes Benzene A (Very) Little Humor

70 Organic Compound Nomenclature Aromatics: ring structure – e.g., benzene, naphthalene – Multi-rings = polyaromatics (PNAs or PAHs) 70

71 Polyaromatics (PAHs) 71

72 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 72

73 Pyridine 73


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