Lecture 10: Welcome to the Wonderful World of Silicates

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Lecture 10 Welcome to the Wonderful World of
Silicates

• SUMMARY
• Clays and Weathering
• Silicate reactions and clay formation.
• Writing chemical reactions between minerals.
• Constructing activity diagrams.

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Charges on Clay Surfaces 1. Uncompensated
Lattice Substitutions 2. Interlayer Cation
Substitutions 3. Dissociation of Surface OH (or
s-complex)
Fixed
Variable (pH-dependence)
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Dealing with Uncompensated Charges INTERLAYER
IONS
Mixed Layer -- illite-smectite,
chlorite-smectite, etc.
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Dealing with Uncompensated Charges Diffused
Surface Charge
Colloidal (21)
Smectite
Flocculent (11)
Kaolinite
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Dealing with Uncompensated Charges Electrical
Double Layer
Continuum
Thickness determines colloid stability
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Adjacent Double Layers
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Adjacent Double Layers

Anion Exclusion - Membrane Filtration
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Dealing with Uncompensated Charges Zero Point
Charge (ZPC)
Low pH \ Al-OH-H ( charge) / ZPC \
Al-OH0 ?? H (neutral) / High pH \ Al-O-
?? H (- charge)
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CONSEQUENCE pH lt ZPC anions will be attracted
to the surface
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CONSEQUENCE pH ZPC no preference for
cations/anions
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CONSEQUENCE pH gt ZPC cations will be attracted
to the surface
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ZPC describes pH-dependent sorption (contrast
lattice substitutions)
What does this all mean in terms of common
minerals?
What is the pH of a regular podzol? 3-6
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Cation Exchange Capacity (CEC) Al gt H gt Ca gt Mg
gt K gt NH4 gt Na Def mEq/100g of cations
displaced by 1M NH4 _at_ pH7
Acid Rain?
(compare Table 4-6, Kehew)
Function of ZPC and Surface Charge Density
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Ion Exchange
Consider the exchange equilibrium of two
monovalent cations (A and B) between a clay
surface and water A-clay B(aq) B-clay
A(aq) aA-clay/ aB-clay KAB aA/ aB The
clay surface terms can be re-written in terms of
the fraction of the CEC filled (XA-clay
A-CEC/Total CEC converting surface activity to
tangible measure), again assuming a
C XA-clay/ XB-clay K'AB mA/ mB K'AB is
called the selectivity coefficient (see Table
4-7, Kehew)
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If A and B are the only cations
present XA-clay/ XB-clay K'AB mA/
mB mA-clay/ (CEC - mA-clay) K'AB mA/
(M-mA) where mA-clay is the surface
concentration of A on the clay (in meq/kg solid)
and M is the total concentration of cations in
solution (in meq/kg solution). If A
concentration (both surface and solution) is very
low, i.e., trace component mA-clay K'AB
CEC mA/ M or mA-clay Kd mA
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Similar suggestions for divalent-divalent ion
exchange, with one suggested formulation XCa-cla
y/ XMg-clay K (mCa/ mMg)p with p
varying from 0.7 to 0.9 (because of non-ideal
exchange).
You can start with this and work your way with
the Kd derivation
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For monovalent-divalent exchange 2A-clay
C(aq) C-clay 2A(aq) aC-clay/ a2A-clay
KAC aC/ a2A As before, the clay surface
terms can be re-written in terms of the fraction
of the CEC filled (X) filled XC-clay/ X2A-clay
K'AC mC/ m2A
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This formulation has important consequences to
displacement reactions in natural sediments. For
example, if we assign (arbitrary) K'AC 1 mC
1 mA 1 We can solve the above equation
subject to the condition XA XC
1 (1-XA-clay)/ X2A-clay 1 (1/1) XC-clay/
X2A-clay KAC mC/ m2A X2A-clay XA-clay
- 1 0 A quadratic formula of the form
aX2 bX c 0, which has the
solution XA-clay -b /- (b2 - 4ac)1/2 /
2a XA-clay -1 (14)1/2 / 2 0.618 Implying
XC-clay 0.382
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An interesting consequence of the square term
thus follows. If we dilute the water by 103
without altering the A/C ratio We can solve
the above equation again subject to the
condition XA XC 1 (1-XA-clay)/ X2A-clay
10-3 /10-6 XC-clay/ X2A-clay KAC mC/
m2A 103X2A-clay XA-clay - 1 0
Another quadratic formula that can be solved
yielding XA-clay -1 (14)1/2 / 2
0.03 Implying XC-clay 0.97 (the divalent
ion almost completely displaced the monovalent
cation on the clay surface!!)
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IMPLICATIONS

• It is possible to lower the ratio of adsorbed
  cation by lowering the concentration of dissolved
  A and C (even while maintaining a constant ratio
  of A and C in solution).
• FACT Upon dilution, a greater proportion of
  higher valence ions will be taken up by the solid
  phase!
• Analytical consequence For determining the
  exchange cation of clay in sea water, rinsing or
  dilution of solution will alter the sorbed
  components (e.g., gt Mg ions in the clays).

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Aqueous Organics SORPTION
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• Definitions
• Adsorption/Sorption - attachment of a solute to
  the surface of a solid (or accumulation of
  solutes at the solid/solution interface)
• Sorbate - solute being sorbed
• Sorbent - solid accepting the sorbate
• Adsorption can be physical, chemical or
  electrostatic

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Adsorption
Absorption
Sorbent
Sorbate
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Sorption
Luthy et al. (1997, EST 31, 3341-3347)
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• Why do we care?
• Air-water partitioning (sorbed are "excluded")
• Deposition (sorbed can be sequestered by
  sediment or filtration)
• Photolysis (sorbed are less accessible to light)
• Biodegradation (cells rely on diffusion to bring
  chemicals in for processing)

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Sorption Effects in Plumes
Retardation
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Solid Water Distribution Ratio (Kd) Kd
Cs(mol/kg) / Cw(mol/L)
Particle in Aqueous Solution
H2O
ORGANIC SORPTION
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Solid Water Distribution Ratio (Kd) Kd
Cs(mol/kg) / Cw(mol/L)
How does Kd vary with concentration? Is a Kd
true CONSTANT?
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• Linear - the simplest adsorption isotherm
• (Kd is independent of concentration)
• Cs Kd Cw (plot of ms vs. mw straight line with
  slope Kd)

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Sorption

• Linear sorption
• infinite capacity for sorption
• partitioning process like KH and Kow
• good assumption for most nonpolar organic
  compounds
• Solids soil, sediments, particles, activated
  carbon, chitin, peat, saw dust, bark mulch,
  bacteria, etc. ...

Kd Cs / Cw
Cs (mol kg-1)
Cwsat
Cw (mol L-1)
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Sorption
pyrene

• Linear sorption (PAH)

naphthalene
phenanthrene
Chiou et al. (1998, EST 32, 264-269)
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Sorption

• Non-linear sorption -- Freundlich

n gt 1
Kd Cs / Cw
Cs Kf Cnw
Cs (mol kg-1)
n lt 1
Cw (mol L-1)
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• Freundlich Isotherm
• Cs Kf Cnw
• (where n is a constant that is usually less than
  1).
• Kf is Freundlich Kd
• Although the form is empirical, it could be
  justified mechanistically by suggesting
• (a) the adsorption to the surface is non-ideal
  (harder to sorb later as sites fill up, e.g.,
  repulsive interaction with other sorbed ions).
• (b) there is a hierarchy of site binding energy,
  hence the sites with the strongest binding energy
  is filled first and later sorbates have to occupy
  sites with lower binding energy. Easy early,
  harder late.

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Freundlich Isotherm

• Freundlich
• phenanthrene on smectite clays
• low organic matter content
• phenanthrene sorption in interlayers
• Hundal et al. (2001, EST 35, 3456-3461)

(Cw/Cwsat(L))
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Finite total available sites for sorption!
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Langmuir Isotherm The adsorption reaction can be
written as vacant site iwater filled
site KLangmuir Cfilled site / Cvacant site
Cw If Ci,ads,max is the total adsorption site
for "i" available at the surface of the
solid Ci,ads,max Cfilled site Cvacant
site
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Hence KLangmuir Cfilled site / (Ci,ads,max -
Cfilled site) Cw Cfilled site Ci,ads,max
KLangmuir Cw- Cfilled site KLangmuir Cw Cfilled
site (1 Cw KLangmuir) Ci,ads,max KLangmuir Cw
Hence Cfilled site Ci,ads,max KLangmuir
Cw /(1 KLangmuir Cw)
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Cfilled site Ci,ads,max KLangmuir Cw /(1
KLangmuir Cw)
Ci,filled sites Ci,ads,max

• Note how at high Cw, Cfilled site flattens to a
  maximum of Ci,ads,max i.e., at very high Cw,
  "KLangmuir Cw /(1 KLangmuir Cw)" ? 1
• At very low Cw, "(1 KLangmuir Cw)" ? 1
• i.e., at low Cw, Cfilled Ci,ads,max
  KLangmuir Cw K Cw !

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Sorption

• Non-linear sorption -- Langmuir

Cfilled site Ci,ads,max KLangmuir Cw /(1
KLangmuir Cw)
Cfilled site, max
Cs (mol kg-1)
K Cs / Cw
Cw (mol L-1)
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Sorption

• Langmuir
• 2,4-dinitrotoluene sorption to (A)
  montmorillonite and (B) Burkholderia sp. cells
• Ortega-Calvo et al. (1999, EST 33, 3737-3742)

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Simple Application of Kd
Add 1,4 dimethylbenzene
f 0.2, rs 2.5 kg/L, Kd (1,4 DMB) 1L/kg
What fraction of DMB will end up in solution at
any time?
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Sorption

• Retardation
• property of contaminant
• velocity of water relative to velocity of
  contaminant
• assumes linear, reversible (equilibrium) sorption
• bulk density ?b Ms/(VwVs) for saturated porous
  medium
• porosity ? Vw/(VwVs) for saturated porous
  medium
• R 1/0.09 11 (i.e., 11x slower than the water)
  

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Sorption (Try This)

• Fraction sorbed, fraction in water (fw)
• PCB and PCE in slightly turbid lake water
• 2,2,5,5-tetrachlorobiphenyl, Kd 107.0 L kg-1
• tetrachloroethene, Kd 102.0 L kg-1
• 1 mg L-1 particle concentration (rsw)

fw 1 / (1 rswKd) R 1/ fw
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Sorption (And This)

• Fraction sorbed, fraction in water (fw)
• PCB and PCE in ground water
• 2,2,5,5-tetrachlorobiphenyl, Kd 107.0 L kg-1
• tetrachloroethene, Kd 102.0 L kg-1
• 2 kg porous medium per liter of volume of
  saturated porous medium (rsw)

fw 1 / (1 rswKd) R 1/ fw
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REALITY CHECK Complex Nature of Kd

• Neutral on OM
• Neutral on Mineral
• Ion Exchange on IES
• Reaction on SRS

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REALITY CHECK Complex Nature of Kd
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REALITY CHECK Complex Nature of Kd
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REALITY CHECK Complex Nature of Kd
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REALITY CHECK Complex Nature of Kd
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Sorption to Organic Matter

• Partial expression for sorption

for neutral (hydrophobic) organic compounds
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REALITY CHECK Complex Nature of Kd
Luthy et al. (1997, EST 31, 3341-3347)
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REALITY CHECK Complex Nature of Kd
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• Kom Com / Caq,neut
• Kd Com fom / Caq,neut
• Kd Kom x fom

KOM constant
fom
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Sorption to Organic Matter

• Kom (or Koc)
• property of the compound
• tendency to flee water for organic matter
• fom (or foc)
• property of the solid
• organic matter is about 50 carbon
• fom 2 foc
• Kom 0.5 Koc

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Sorption to Organic Matter
Koc
Karickhoff et al. (1979, Water Research 13,
241-248)
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Sorption to Organic Matter

• Measuring fom
• combustion of organic matter
• furnace (450?C 24 h)
• gravimetric difference
• chemical oxidation of organic matter
• persulfate, permanganate
• IR detection of CO2
• removal of inorganic carbon
• acidification
• purging with N2

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Sorption to Organic Matter

• Typical values of fom
• peat
• nearly all organic matter
• fom 0.5 to 1.0
• soils
• depends on layer
• fom 0.01 to 0.5
• coarse aquifer sediments (next slide)
• most organic matter mineralized
• fom 0.00001 to 0.05

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• The normal range for TOC in soils is from 0.5 to
  5 (foc 0.005 to 0.05 of fom 0.01 to 0.1),
  examples of measured TOC concentrations include
• Coarse soil - 4.2 (plant litter)
• Clayey silty loam - 0.4
• Silty Loam - 1.6
• Silty Clayey Loam - 2.95
• Silty Loam - 5.2
• Clayey Loam - 0.38
• Glaciofluvial - 0.02 to 1.0

HENCE, if we know TOC, we know fOC, and we can
estimate Kd from known KOM for any compound. IF
we have an estimate of Kd, R 1 (rs/f)Kd
where R vwater/vcontaminant
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Sorption to Organic Matter

• fom threshold
• if fom too low, other sorption processes
  dominate!
• fom gt 0.001generallyaccepted asthreshold

Koc
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Sorption to Organic Matter

• Kom assumed property of compound
• relate to Kow
• both are partition processes
• octanol and organic matter
• partially hydrophobic
• partially hydrophilic

Octanol
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Sorption to Organic Matter

• Kom (or Koc) correlated to Kow
• log Koc 1.00 log Kow 0.21
• Kom or Koc inversely correlated to Cwsat(l,L)
• log Koc 0.54 log xwsat(l,L) 0.44

Dependence on hydrophobicity and solubility
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Sorption to Organic Matter

• Kom (or Koc) correlated to Cwsat(l,L)

log Koc 4.04 - 0.557 log Cwsat(l,L) (?M)
Chiou et al. (1979, Science 206, 831-832)
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log Kom a log Kow b (e.g., log Kom 0.82
log Kow 0.14) The value of a ranges from 1 to
0.54, b ranges from 1.32 to -0.21 or log Kom
-a log CsatW b
Octanol
Good Within Compound Classes
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Sorption to Organic Matter

• Limitations
• all organic matter the sameThe close fit
  suggests that the makeup of the organic matter in
  soil is not critical in determining log Kom
  values for neutral chemicals.
  Chiou et al. (1981)
• limited to equilibrium applications, low
  solubility compounds

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Sorption to Organic Matter

• All organic matter not the same!
• soils
• sediments
• lakes
• rives
• aquifer materials
• aquatic organic matter

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Sorption to Organic Matter

• According to Chiou and Karickhoff, all organic
  matter is the same
• Kdf(compound)
• for neutral (hydrophobic) organic compounds
• Karickhoff et al. (1979) and Chiou et al. (1979)

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Sorption to Organic Matter

• and the sorption of all compounds depends only
  on Kow
• from Karickhoff et al. (1981 Chemosphere 10,
  833-846)

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Sorption to OM

• or is it?
• Wide variety of organic matter!
• phytoclasts
• coal, charcoal
• amorphous OM
• particulate OM
• glassy/rubbery

Karapanagioti et al. (2000, EST 34, 406-414)
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Sorption to Organic Matter

• Organic matter
• elemental composition

Barron (2001, MS Thesis, University of Colorado)
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Sorption to Organic Matter

• Organic matter
• aromaticity/aliphilicity
• molecular size
• UV absorbance
• fluorescence

Barron (2001, MS Thesis, University of Colorado)
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Sorption to OM

• Effect of Organic Matter on Kom
• phenanthrene
• wide range of sedimentary rocks
• organic facies

Kleineidam et al. (1999, EST 33, 1637-1644)
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Sorption to OM

• Effect of Organic Matter on Kom
• pesticides
• carbaryl
• phosalone
• NMR analysis

Ahmad et al. (2001, EST 35, 878-884)
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REALITY CHECK Complex Nature of Kd Sorption to
Minerals of neutral hydrophobic compounds.
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Sorption to Minerals
Luthy et al. (1997, EST 31, 3341-3347)