ADSORPTION part A

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ADSORPTION part A

• Reading Sparks, Chap 5 (skip 177 - 182)
• Essington, 7.3 - 7.3.1(skip pp 340 and 341
  7.3.2, through page 351. 7.2.
• and Sposito, 8.1 - 8.2

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Outline

• Mathematical description of sorption
• Charges on surfaces
• Strong adsorption sites on minerals
• Adsorption on pH dependent charge minerals

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Description of "Sorption"

• Sorption
• Surface adsorption
• Absorption in structures
• Precipitation
• E.g. Phosphate sorption
• 10 g of soil suspended in one liter of water
• Add 1.0 mg L-1 (ppm) to the solution
• Mix overnight
• Measure P
• E.g. 0.10 mg L-1

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Mathematical Description of Sorption Batch
Adsorption Experiments

• q quantity adsorbed per unit mass of adsorbent,
• ?C quantity removed from solution,
• V solution volume,
• m mass of solid phase
• This is a macroscopic measurement that does not
  imply any particular mechanism of disappearance.

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How much sorbed in the P example.

• Calculate the quantity in the soil phase in mg
  kg-1.
• Note mg kg-1 ppm
• Should be able to convert among
• mg kg-1
• ppm
• µg kg-1
• ppb
• etc.

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Answer
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Kinetics

• Most true surface adsorption is quite rapid but
  diffusion into aggregates takes at least several
  minutes if not hours.
• Secondary reactions can occur. For example, with
  phosphate adsorption on calcite , calcite will
  dissolve and calcium phosphate phases can
  precipitate.

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Mathematical models for sorption
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Distribution Coefficient KD

• KD q/c where c is the concentration in solution

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LINEAR ADSORPTION

• KD q/c
• c the concentration in solution
• q quantity adsorbed per unit mass of adsorbent.
• KD Distribution Coefficient
• Usually has units of L kg-1
• If the solution is expressed in mass units (e.g.
  kilograms of water rather than liters, KD can be
  a unitless quantity).

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(Fig. 8.1 Sposito)

• Linear adsorption

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• Linear sorption is often used in simple transport
  models for movement of dissolved inorganic and
  organic components in soil and ground water

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Calculate the Kd for the P example.

• 90 mg kg-1 sorbed
• 0.10 mg L-1 in solution

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Answer
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Often sorption is not linear, e.g. phosphorus
(Fig. 8.1 Sposito)
q, m mol kg-1
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Freundlich Equation

• q KcN
• K and N are constants
• Can fit to data with nonlinear least squares
  (NLLS) or by using a log form of the equation.
• ARC - U of M School of Statistics
• Linearize
• log q log K Nlog c
• No maximum adsorption.
• if N 1 then KD constant and sorption is
  linear

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Langmuir Equation Continued

• b maximum adsorption
• KL the Langmuir constant which is proportional
  to the energy of adsorption
• Assumes all binding sites are the same and the
  sites are finite in number.

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Langmuir Equation Continued

• Calculate in terms of fraction of sites occupied
• The solution concentration at equilibrium, c, is
  a function of ?.

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Calculation of Langmuir Parameters

• Can fit the Langmuir data using NLLS approach but
  traditionally the equation is used in a linear
  form.
• Linearization
• There are several ways of expressing the Langmuir
  equation in linear terms.

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Calculation of Langmuir Parameters Linearization

• From Langmiur equation
• q (l KLc) bKLc
• q bKLc - KLcq
• q/c KD bKL -KLq
• plot KD vs. q
• slope -KL
• intercept bKL

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Calculation of Langmuir Parameters Linearization

• Traditional Soil Chemistry Method
• From Langmuir Equation
• plot c/q vs . c
• slope 1/b
• intercept 1/KLb

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Multisite and Multilayer Adsorption

• Many authors have found decreasing slopes in the
  traditional linearized Langmuir plot with
  increasing concentration in solution
• (e.g. for PO43-). They use multisite Langmuir
  plots. A two-site plot has two K values and two
  b values. With four parameters, the fit is quite
  flexible.

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Use of the models for inorganic ions

• Usually used for less abundant cations
• Not for the commonly determined exchangeable
  cations (Ca, Na, K, Mg).
• Use for Cu2, Cd2, Pb2, Zn2, etc
•  Sometimes used for organic compounds
• Used for the less abundant anions in soil
  solution
• Phosphate, selenate, vanadate.
• See example KD values.

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KD values at pH 7 (MPCA Soil Leaching Value. SLV,
worksheet)
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Sorption in soils

• Adsorption of Ions
• On charge sites of permanently charged clays.
• Exchangeable ions.
• On pH dependent charge sites in minerals
• Some ions are exchangeable, most not.
• Sites on SOM
• Some ions are exchangeable, most not.
• Precipitation
• Non ionic organic materials
• Hydrophobic sites in SOM

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Furidone (Sonar) Langmiur and Freundlich on clay.
(Fig 7.16, Essington)
q KcN
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Hard to separate dissolution solubility
equilibrium from adsorption
PO4 added to 0.01 Al3 in variscite, Essington,
Fig. 7.19, with Langmiur fit
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Models of adsorption used in transport solutes in
soils and aquifers

• Retardation
• Where r bulk density (vol/mass)
• f volumetric water content (vol/mass)
• V velocity(distance/time)

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Then
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In class exercise

• Co sorbs strongly in surface soils with a
  moderate Kd value.
• Measured values at pH 7 in a medium textured soil
  tends to be about 100 L kg-1 have been reported.
• A. If the quantity in the soil is 5.0 mg kg-1
  what is the concentration In solution?

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Answer

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• B. Assuming 25 soil moisture and a bulk density
  of 1.25 what is the retardation factor and what
  is the ratio Cd migration relative to the flow of
  water?

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Answer
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Sorption of non polar organic compounds and
organic compounds with low polarity

• Sorption of non polar organic compounds (e.g
  benzene is a is linear and is a function of soil
  organic carbon (SOC)
• KD (q/c)
• and
• KD foc Koc
• Where foc fraction of organic C in soil
• Remember SOC 0.5 SOM
• Koc KD assuming 100 organic C

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Data for Koc (and t1/2 ) for organics

• Tabulated Koc values are available along with
  degradation 1/2 life.
• See HSDB
• Go to TOXNET
• http//toxnet.nlm.nih.gov/

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Behavior of charges near surfaces as described by
diffuse layer models
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Will describe charged surfaces of colloidal
solids suspended in an "indifferent electrolyte"
like NaCl

• Ions like Na and Cl- do not sorb strongly.
• Theory developed to understand colloidal
  stability and to find ways to manipulate
  stability. (late 1800s and early 1900s)
• Fine particle can be dispersed and flocculate
  depending of surface charges.

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Charged surfaces

• Permanent charged clays ( - only)
• pH dependent charged materials ( or -)
• Oxides and hydroxides
• Iron
• Ferrihydrite
• Goethite
• Hematite
• Soil Fe(OH)3
• Aluminum
• Gibbsite
• Amorphous Al(OH)3

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• Edges of silicate clays
• Soil Organic Matter
• Negative charges only.

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Example Permanent Charge Clays

• Monovalent cations near negative charged
  surfaces
• - -
• - -
  
• - -
• - -
  
• - -
• - -
  
• Wet Dry

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Energetics of ions in the diffuse double layer

• ?G ?H - T?S
• ?H electrostatic binding energy
• ?S disorder due to diffuseness
• Boltzmann equation
• C1/C2 exp (-?E/kT)
• Where ?E (?H) is the difference in energy
  between site 1 and site 2 in the diffuse layer
• k is the Boltzmann constant R/Avagadro's

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Let site 2 be at an infinite distance from a
charged plate (bulk solution)

• Assume single ion at site 1
• ?E ze(??)
• Where e the charge on one electron, ?
  electrical potential in volts at site 1 compared
  to infinite distance and z is the number of
  charges on the ion .
• ?E is the energy of moving an ion from Site 2 at
  infinite distance to Site 1 in the charge
  affected zone.

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Energetics of ions in the diffuse layer near a
negative surface (cont.)

• For ions near a negatively charged surface
• Co Concentration in the bulk solution (site
  2, infinite distance)

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The potential at any point in the diffuse layer
is given by a complex function

• Co Concentration in the bulk solution
• d distance from surface
• ? charge density
• Z charge on ion, sign and magnitude
• e charge on one electron (a physical constant)
  (Faradays constant divided by Avogadro's number)
• k Boltzman's constant the universal gas
  constant (R) divided by Avogadro's number.
• See p.147 in Sparks

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Energetics of ions in the diffuse layer (cont.)

• In a NaCl solution

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The counter ion layer
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The net positive charge in the counter ion layer
balances the surface charge

• The net positive charge is the difference between
  Na and Cl-.
• The total counter ion charge is indicted by the
  area under the Na and Cl plots, between the
  surface and the bulk solution.

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Change the charge on the ions to 2 or 3.

• The value of z has a big effect on the C
• Complex because potential is affected by z
• Increasing charge compresses the double layer

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Example, NaCl and MgSO4 near a smectite surface
d, distance from the surface
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Effect of NaCl Concentration
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Effect of electrolyte (e.g. NaCl) concentration
on charge (Sparks Fig 5.13)
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• Increasing concentration decreases the double
  layer thickness.

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Diffuse double layer thickness (DDL)

• DDL is a function of Co and z of cation.
• Example 10-3 mol L-1 NaCl
• DDL for smectite 10 nm 100 Å
• Increasing salt concentration reduces DDL and
  hence reduces swelling pressure (discussed later)
• 10-3 mol L-1 Ca2
• Less than 5 nm (50 Å)

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pH Dependent (variable) Charge on Oxides (and
Clay Edges)

• Explains surface adsorption and desorption of
  protons in an indifferent electrolyte (e.g.
  NaCl).
• An indifferent electrolyte is a salt with
  cation and anion that that do not strongly adsorb
  on the charge sites.
• Protons can adsorb and desorb
• Will use the 2 pK model (Essington uses the much
  less common one pK model)

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pH Dependent Charge on Oxides (and Clay Edges)
(cont.)

• Weak acid oxide or hydroxide surfaces can be
  described as diprotic surfaces
• FeOH FeOH2
• S H S
• FeOH FeOH
• SOH SOH2

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pH Dependent Charge on Oxides (and Clay Edges)
(cont.)

• FeOH FeO-
• S S H
• FeOH FeOH
• SOH SO-

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Titration of Goethite (FeOOH) in NaCl(McBride
Fig. 3.16)
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pH Dependent Charge on Oxides (and Clay Edges)
continued

• More simply SOH H
  SOH 2
• and SOH SO-
  H
• K1 10 5.5 for Fe(III) hydrous oxide, pKA
  5.5)
• K2 10-9.0 for Fe(III) hydrous oxide. pK2 9.0)
• (conditional constants)

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What are intrinsic constants

• The intrinsic constant for reaction of a site on
  a charged surfaces is actually an acidity
  constant. It is a function of the nature of the
  site.
• Equations in the last slide assume ideal behavior
• No charge site interaction
• The surface charge site activity will be affected
  by the surface charge and the salt strength.

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pH Dependent Charge on Oxides (and Clay Edges)
continued

• Re arrange
• At high pH
• Negative charge increases with the increase in pH
  (decrease in (H)).

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At acid pH

• Re arrange
• At low pH
• Positive charge increases with the decrease in pH
  (increase in (H)).

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In general at high pH

• To account for the interaction of charges

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For high pH, in NaCl

• Re arrange negative charge on exponent
• Negative charge increases with the increase in pH
  (decrease in (H)).

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• The exponential charge term is equivalent to the
  Gouy charge term except use F instead of e and R
  instead of k.
• The exponential term is negative because the
  surface charge is negative.

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At low pH in NaCl
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• Positive charge increases with the decrease in pH
  (increase in (H)).

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pH Dependent Charge on Oxides (and Clay Edges)
(continued)

• Net charge, ?
• ? SOH2 - SO- S(? - ?-)
• where ? is the net surface charge in cmolc kg-1
  or mmolc kg-1, S is the total quantity of OH
  sites per kg, and ????the fraction positive or
  negative of sites.

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• The pH at which the net charge is zero is the
  point of zero net charge (pznc).
• The surface potential, ? , decreases with
  increasing salt concentration.
• Increases sheilding
• Both ? and ?- increase
• When an indifferent electrolyte, MX (e.g. NaCl),
  is in the solution ? and ?- are a function of pH
  and ?? only.

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pH Dependent Charge (cont,)

• With increasing NaCl concentration, at fixed pH,
  both ? and ?- are increased (decrease in surface
  potential)
• If no acid or base are added increasing the salt
  concentration causes a shift in pH towards the
  point of zero charge (typically, soil pH
  decreases with increasing salt concentration.)

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For a fixed pH (acid or base added to adjust)
charge the potential decreases with increased
salt at fixed pH, CEC or AEC increase
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For a given charge (no acid or base added) the
potential decreases with increased salt, and pH
increases or decreases
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Cation and anion exchange sites

• In the model presented the Na and Cl- ions are
  charge balancing ions that are exchangeable
• At high pH exchangeable Na is adsorbed
• Variable CEC
• At low pH exchangeable Cl- anions are adsorbed
• AEC
• Particularly important in highly weathered soils
  of the SE of the US and many areas in the tropics.

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Calculating PZNC from surface H Log K values
(varies with source of data)

• PZNC 1/2(logK1 -log K2)

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Point of zero net charge

• Oxides and hydroxides of Fe(III) and Al
• All about 8