Surface Bonding Sites on Soil Minerals

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Surface Bonding Sites on Soil Minerals
Sorption Part B.

• Assigned Sparks Chapter 5
• pp. 162-167

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Cations on permanent charge sites in silicate
clays

• Outer sphere
• In M generally complete dispersion of clays
  (diffuse counter ion layer)
• M2 and M3 Can be somewhat diffuse on outer
  surfaces but tend to form interlayer bridges
  where possible.

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M2 Interlayer ions in permanent charge clay
(Fig. 3.10, McBride)
Example Ca2 or Mg2  "Outer sphere Complex"
Exchangeable cations
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Complex system in soils Ca - Na system
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Ions near hydrous oxide surfaces (and silicate
clay edges)

• Comparison with permanent charge clays
• The charge sites are at the surface and covalent
  bonding is possible.
• However, many ions are exchangeable.
• Oxides and clay edges contribute to CEC.
• Alkali metal cations (e.g. Na) are diffuse.
• Some singly charged anions like nitrate and
  chloride are diffuse.
• Alkaline earth cations like Mg2 are likely much
  less diffuse but are outer sphere.

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Many are inner sphere

• Much more strongly bonded.
• Transition metals cations.
• E.g. Cu2, Co2 and Zn2
• Many oxyanions.
• PO43-, SeO32-, and AsO42-

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Surface spectroscopy

• Reflectance infrared (IR)
• Diffuse reflectance (DRS)
• Attenuated total reflectance (ATR)
• X-ray spectroscopy
• Use the intense x-rays available at synchrotron
  accelerators
• X-ray absorption fine structure (XAFS)
• Can determine inter atomic distances and the
  coordination environment of an atom.
• E.g Coordination of Cu on Al(OH)3 surfaces
• Used x-ray wavelengths absorbed by Cu.

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Periodic table
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Inner sphere, cation, Pb2 on Al oxide or
hydroxide (octahedra)
Bidentate
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Inner sphere anion, arsenate I(AsO3-4)

• Mononuclear and binuclear.

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Mathematical modeling for inorganic anion and
cation adsorption
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Surface complexation equations

• First we will first look at simple case where the
  electrostatic effects are ignored.
• These constants are sometimes called
  conditional constants

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Metal Ions (e.g. M2)

• 1) Monodentate
• gtS-OH gtS-O- H log K2
• gtS-O- M2 gtS-OM log Km
• gtS-OH M2 gtS-OM H log K2Km

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Metal Ions (e.g. M2) Continued

• 2) Bidentate
• 2gtS-OH gtSO- H 2
  log K2
• 2gtSO- M2 (gtS-O)2M log K2m
• 2 gtSOH M2 (gtSO)2M 2H log K22K2m
• Note Protons are displaced by metal ion
  adsorption

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Anion Adsorption (e.g. A2-)

• 1) Mononuclear
• gtSOH H gtS-OH2 log K1
• gtSOH2 A2- gtS-OH2 A- log KA
• gtSOH H A2- gtSOH2A- log KAK1

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Anion Adsorption (e.g. A2-) Continued

• 2) Binuclear
• 2gtS-OH 2H A2- (gtS-OH2)2A
• Note Protons are consumed (OH- displaced) by
  anion adsorption

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Mathematical Model for Adsorption of Cations
(ignoring charge effects)

• Consider the monodentate adsorption of Co2 in a
  solution with NO3- or other non-specifically
  adsorbed anion. Total sites for the adsorption of
  Co2
• gtSt gtS-OH2 gtS-OH gtS-O-
  gtS-O-Co) Eqn. 1
• gtSt can be determined (e.g. by titration)
• Substitute equations for Km, K1, and K2 into the
  mass balance equation.
• We want to describe the adsorption in terms of
  the quantity sorbed, q, as a function of the
  concentration in solution

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Mathematical Model for Adsorption of Cobalt

• Express all terms in the sum of sites in terms of
  (S-OCo) and (Co2).
• gtS-OH H gtS-OH2 log K1
• gtS-O- H gtS-OH -log K2
• gtS-OCo gtS-O- Co2
  -log Km
• gtS-OCo 2H gtSOH2 Co2 logK1/K2Km

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Mathematical Model for Adsorption of Cobalt

• From the equation defining Km

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Mathematical Model for Adsorption Cobalt

• The monodentate complex can be defined in terms
  of gtS-OH .

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Mathematical Model for Adsorption Cobalt

• Substitute equations into equation 1
• Divide by S-OCo and multiply by (Co2)

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Mathematical Model for Adsorption Cobalt

• Invert and multiply through by (Co2). Then the
  fraction of sites with Co
• With decreasing pH increasing (H) adsorption
  is decreased
• At constant pH

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Mathematical Model for Adsorption Cobalt

• where
• Then

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Mathematical Model for Adsorption Cobalt (cont.)

• This is a Langmuir equation where B (B1/K) is
  the inverse of a pH dependent Langmuir binding
  constant. Thus the Langmuir model should work
  quite well at constant pH.

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Mathematical Model for Adsorption Cobalt (cont.)

• If Km 1.0 x 104, Log K1 5.5, and logK2
  -9.5 then
• pH B
• 5.5 2
• 6.5 0.11
• 7.5 0.01
• 8.5 0.0011
• At pH 7.5, if ? 0.5 (Co2) 0.01 M

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Adsorption on hematite, Fig. 4.3
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Predicted monodentate Cu adsorption on aluminum
oxide (Fig. 4.4)
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Adsorption of anions
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Models that incorporate electrostatic effects

• Diffuse layer
• Surface complexation models
• Constant capacitance
• Simplifies the diffuse layer into a plane.
• Triple layer
• Diffuse double layer model
• Etc.

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Constant Capacitance Models (CCM)

• Two types of counter ions
• Indifferent electrolyte
• Surface bound ions that neutralize charge
• Usually thought to be inner sphere

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Components of charge

• Particle charge has 2 components
• ? ?H ?is
• Where ?H is the proton charge and sis is the
  charge of the inner sphere sorbed ions.
• ? can be positive or negative
• Co2 example
• ? gtS-OH2 - (gtS-O- gtS-O-Co)
• Orange - proton charge
• Red- is charge due to metal ions

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CCM proton charge assuming no strongly bound
ions. Start with acidity equations
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E.g. A net negative charge particle

• Potential difference described by using a simple
  parallel plate condenser model
• Charge on condenser in moles per surface area
• ? (area)C?0/F
• The simple definition of capacitance is
• C ? ?0-1
• but we want to use molar units pre unit surface
  area

---------------
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Surface Potential

• Calculate the relationship between the surface
  charge and the surface potential assuming the
  surface acts like a parallel plate condenser and
  the particle charge density, ? (mol m-2),
  specific surface area A (m2 g-1), a is the
  suspension density in g L-1 and capacitance, in
  farads m-2 . Then
• ??????????????????????????????? (CAa?0/F)
• Solve for surface potential, ?0
•  ?0 ?F/CAa
• Substitute into equilibrium equation
• Note C increases with increasing salt
  concentration

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• For a complete discussion, see the discussion of
  the Constant Capacitance Model (CCM), see Sparks
  Chapter 5 and Sposito (pp. 164 -167 )

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Then
See equation 5.4i in Sparks The negative on F is
because the exponential term was in the devisor
on the right hand side. As ? goes to zero, as a
the PZC the exponential term goes to 1.
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For the Co2 example need the mass balance
equation for ?

• ? gtS-OH2 - (gtS-O- gtS-O-Co)
• Have 4 equilibrium equations and total charge
  equation. 5 equations and 5 unknowns. Can be
  solved easily by iteration using Vminteq.

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Except for very simple cases CCM requires a
computer program

• Increase in the concentration of an indifferent
  electrolyte increases the capacitance (decreases
  the surface potential)
• With increase in salt concentration the surface
  charge must be greater at a given pH. (const.
  potential at constant pH)

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Hypothetical oxide surface charge in NaCl
(McBride, Fig.3.20) m mol kg-1
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Inputs for the CCM

• Proton binding constants (K1 and K2)
• Metal and anion binding constants
• Total number of sites
• Obtained from acid base titration
• May have to correct for dissolution of the
  hydrous oxide at very low and very high pH.
• E.g Al(OH)3
• Capacitance
• Often is used as a model fitting parameter.

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Acidity of Oxide Surfaces

• A function z/r or valance/coordination no.
• The greater z/r, the lower the pKA values for
  both the first and second ionization
• Al3 gt Fe3gt Ti4 gt Si4
• more acid ---gt
• (see Table 4.1 in McBride)
• High charge with small size destabilizes the O-H
  bond.

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pKA values for surfaces
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Point of zero net charge

• Arithmetic mean of the pK values
• PZNC 1/2(logK1 -log K2)

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Point of zero net charge
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Metal constants
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Cation affinity for permanent charge clays

• M3 gt M2 gt M
• Alkali Metals
• Cs gt Rb gt K NH4 gt Na gt Li gt
• Alkaline Earth Metals
• Ba2 gt Sr2 gt Ca2 gt Mg2
• Ions with lower hydration energy, lower z/r, are
  preferred

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Affinity due to charge and hydration

• Related to the distance of closest approach to
  the surface (electrostatic bonding).
• Related to hydrated radius

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Cation affinity on oxides

• At pH of pznc and higher, alkaline earth metals
  and alkali metals are held as outer sphere, low
  specificity are exchangeable.
• Transition metal 2 cations and 3 cations mostly
  held in inner sphere complexes, i.e. are
  specifically adsorbed.
• The bonding is partial covalent.
• Preference similar to the order of hydrolysis to
  form hydroxy ions (first hydrolysis constant).
  For divalent metals
• CugtNigtCogtZn
• First row transition metals follow Irving
  -Williams

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Anion Affinity

• Evaluate electronegativity or "shared charge"
• Halides (pH lt pznc)
• F- gt Cl- gt Br- gt I-
• Follows electronegativity
• For Cl- to I- mostly outer sphere and are
  exchangeable, F- has a radius similar to OH- and
  is bound much more strongly.

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Anion Affinity

• For oxyanions the less the shared charge, the
  greater the affinity (Charges shared with a given
  number of O atoms)
• PO43- gt SeO32- gt SO42- gt SeO42-
• 5/4 4/3 6/4 shared
  charge
• For sulfate and selenate there is greater shared
  charge
• Mostly outer sphere binding to oxides
• With greater shared charge electrons are pulled
  away from the O atom and O atoms are poorer
  electron donors (poorer Lewis bases).

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Influence of Protonation on Adsorption of Anions

• Adsorption of weak acid anions is influenced by
  the protonation of the anion in solution.
• Example F-, a strongly adsorbed monovalent anion
• Increased adsorption with decreasing pH because
  of the increasing positive charge.
• Maximum adsorption near pKA (pKA 3.8) because
  at lower pH F- is protonated to form molecular HF

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Adsorption of Anions (continued)

• Example phosphate
• Strength of adsorption
• PO43- gt HPO42- gt H2PO4- (pK1 2.1, pK2 7.2,
  pK3 12.2)
• Adsorption is increased with decreasing pH down
  to pH 2 but with decreasing slope.

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Adsorption of some anions
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Adsorption of anions (Sposito Chap. 8)
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Role of SOM in cation sorption
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Sorption of Cations by Organic Matter

• The most abundant sites are the "hard acid"
  carboxylic acid and phenolic sites. Carboxylic
  acids are more acidic, more important.
• N and S can be important for small quantities of
  ions adsorbed.
• Sulfhydryl (thiol RSH) groups important at low
  levels of soft ions (e.g. Hg2,where Km can be gt
  1032)
• N might be important for Cu2 at low levels
  (stronger complex than for carboxyl groups)

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Soil organic matter

• Ionization of carboxyl sites produces negative
  charges
• Some ionization of phenolic groups is also
  possible

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Titration of humic acid in NaCl (McBride Fig.
3.21)
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See modified Henderson Hasselbach equation
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McBride Fig. 3.22
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McBride Fig. 3.22
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SOM and pH dependent cation exchange sites

• In glaciated regions SOM clearly accounts for
  most of the pH dependent charge.
• Even in soils high in oxides surface soil SOM is
  generally more important than the oxide
  components.
• Oxides might be dominant in subsoils

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Adsorption of Cations on Organic Matter

• Alkaline earth metals and alkali metals are held
  as outer sphere, low specificity are
  exchangeable.
• Unlike oxides SOM is an important contributor
  cation exchange even at pH 4
• Order is usually similar to clays.

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• Transition metal 2 cations and 3 cations mostly
  held in inner sphere complexes, i.e. are
  specifically adsorbed.
• The bonding is partial covalent.
• Preference similar to the order of hydrolysis to
  form hydroxy ions (first hydrolysis constant).
  For divalent metals
• CugtNigtCogtZn
• First row transition metals follow Irving
  -Williams

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The surface complexation model can be used for SOM

• For solid organic matter surface complexation
  reactions like used for the calculation of
  binding to oxide surfaces can be used.
• No positive charges sites
• E.g.. Co2 with a monodentate and bidentate
  carboxylate sites.
• St RCOO-RCOOH RCOOCo (RCOO)2Co

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Adsorption of Cations by Dissolved Organic Matter
(DOC)

• Complexation with soluble organic acids (fulvic
  acidic and monomeric acids) is important for the
  mobility of some metal ions in solution (e.g.
  Fe(III), Hg2, and Cu2)
• Increases solubility of some very insoluble ions
  like Fe3 which precipitates as Fe(OH)3. Helps
  plants get Fe.

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Ions bound to Soil Organic Matter (SOM)

• Important ions bound to SOM.
• Al3
• Fe3 is less important on O.M. than Al3 because
  of low solubility of Fe(OH)3 (ferrihydrite)
• Cu2
• Hg2

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Example Adsorption control of Zn (Fig. 4.14)
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Short Summary

• Model sorption
• KD
• Freundlich
• Langmiur
• Koc for organic compounds

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Short Summary (cont.)

• Gouy diffuse double layer model helps explain
  colloidal stability and adds useful electrostatic
  term for adsorption models.
• Cations and soluble ligands (anions) can
  participate in complex formation on surfaces.
• Surface complex theory can be used to model
  surface sorption on oxides hydroxides and clay
  edges.
• Alkali metal ions are indifferent cations and do
  not adsorb on oxides except as weakly held ions
  in the diffuse layer.
• Alkaline earth cations generally are sorbed in
  outer sphere (and weak inner sphere) complexes
  and are readily exchangeable.

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• Anion sorption of weak acids is influenced by the
  formation of lower charged protonated forms.
  (e.g. F- protonation to HF)
• Surface complexation theory can be used to
  calculate the adsorption of cations by organic
  matter.
• Adsorption and precipitation can take place
  simultaneously,
• Eg. Phosphate

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• Precipitation and dissolution are generally
  slower than adsorption and desorption.
• Precipitation is complicated by formation of
  mixed solids.
• P precipitates and adsorbs in soils.
• Precipitation occurs in several forms.

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