Title: Lecture 10: Welcome to the Wonderful World of Silicates
1Lecture 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.
2Where do most common clays come from?
K2O-Al2O3-SiO2, H2O System Several other
minerals can form in reaction of K-Feldspar and
H2O Al2Si2O5(OH)4 (kaolinite, a type of clay
mineral) Al2Si4(OH)2 (pyrophyllite, also a clay
mineral) Al(OH)3 (gibbsite) Quartz
(SiO2) KAl3Si3O10(OH)2 (muscovite) or KAlSi3O8
(K-Feldspar) might saturate How do we know which
minerals should form?
K-Feldspar to Clay?
3Examine Possible Reactions
K-Feldspar to Clay KAlSi3O8 (K-felds) H
4.5H2O 0.5Al2Si2O5(OH)4 (kaolinite) K
2H4SiO4 At equilibrium Keq aK a2H4SiO4/aH
or log Keq log aK /aH 2log
aH4SiO4 log aK /aH log Keq - 2log aH4SiO4
4Examine Possible Reactions
Muscovite to Clay KAl3Si3O10(OH)2 (muscovite)
H 1.5H2O 1.5Al2Si2O5(OH)4 (kaolinite) K
K-Feldspar to Muscovite KAlSi3O8 (K-felds)
(2/3)H 4H2O (1/3)KAl3Si3O10(OH)2
(muscovite) (2/3)K 2H4SiO4
Gibbsite to Clay Al(OH)3 (gibbsite) 2H4SiO4
0.5Al2Si2O5(OH)4 (kaolinite) 2.5H2O
ALL REACTIONS CAN BE WRITTEN IN THE FORM y
mX b
5K-Feldspar to Muscovite KAlSi3O8 (K-felds)
(2/3)H 4H2O (1/3)KAl3Si3O10(OH)2
(muscovite) (2/3)K 2H4SiO4 log aK /aH
1.5log Keq - 3log aH4SiO4
K-Feldspar to Clay KAlSi3O8 (K-felds) H
4.5H2O 0.5Al2Si2O5(OH)4 (kaolinite) K
2H4SiO4 log aK /aH log Keq - 2log aH4SiO4
6TRACING WATER COMPOSITION DURING WEATHERING
7The Earliest Reaction Results in Forming
Gibbsite (short circuited reaction K-Felds?
Kaol ? Gibbsite)
K-Feldspar to Gibbsite KAlSi3O8 (K-felds) H
7H2O ? Al(OH)3 (gibbsite) K 3H4SiO4 At
equilibrium Keq aK a3H4SiO4/aH or log
Keq log aK /aH 3log aH4SiO4 Initial
Water Simply Dissolves K-felds and ppt the first
sat phase
8K-Feldspar to Gibbsite KAlSi3O8 (K-felds) H
7H2O ? Al(OH)3 (gibbsite) K 3H4SiO4
9With the help of The Geochemists Workbench
(ACT2, p. 41)
swap Kaolinite for Al swap K/H for
K diagram Kaolinite on K/H vs SiO2(aq) x -5
-2 y-5 10 go
10With the help of The Geochemists Workbench
(REACT, p. 91)
SPECIFY initial water composition (e.g.) Al
0.1 mg/kg K0.0 mg/kg Cl-0.2 mg/kg pH 7
. react 1000 mg K-feldspar go
11NaAlSi3O8 H 2.5H2O 0.5Al2Si2O5(OH)4
(kaolinite) Na SiO2,aq
12EFFECT OF MEAN ANNUAL RAINFALL ON RESULTING SOIL
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14Alcoa
15Assignment Albite Dissolution NaAlSi3O8 H
2.5H2O 0.5Al2Si2O5(OH)4 (kaolinite) Na
SiO2,aq
Al(OH)3 (gibbsite) 2SiO2,aq
0.5Al2Si2O5(OH)4 (kaolinite) 0.5H2O
Al2Si4(OH)2 (pyrophyllite) (1) Write balanced
reactions (exclude paragonite) (2) Manually show
reaction lines in log(Na/H) vs log SiO2(aq)
(see Appendix-Kehew for data)
16- Clay Mineralogy
- Electrical Double Layers
- Sorption Isotherms
- Organic Sorption
17- Clay is an operational definition connoting
fine-grained material ? particles lt 2mm (others
include lt 4mm). No compositional connotation. - colloids denote very small materials considered
to be molecular aggregates that stays in
suspension because of the surface charge. - Clay minerals are crystalline, hydrous silicates
with layered structures, usually taken as part of
the larger class of silicate minerals the
phyllosilicates. - Most common products of waterrock reactions
involving silicates. 16 by volume of top 20 km
of the Earth's surface. - Very commonly involved in regulating water
chemistry (sorption, ion-exchangers).
18Building Blocks of Clay Minerals
-1 (corner-shared)
T
-1/3 (edge-shared)
O
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20Tetrahedral Sheets
Octahedral Sheets
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22Types of OCTAHEDRAL Sheets
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26Your assignment problem
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2821 Clays
Smectite
Dioctahedral
Trioctahedral
Tetrahedral Substitution
Octahedral Substitution
29Not All Substitutions are Equal Charge!
- Most commonly (Site of charge imbalance 4 vs
6) - 4 Si4 ---gt 4 Al 3 (tetrahedral site)
- 6 Al3 ---gt 6 Mg 2 (octahendral site)
- 6 Fe3 ---gt 6 Fe 2 (octahendral site)
- Also possible are vacancies 6 Mg2 ---gt 6
0 (octahendral site)
CONSEQUENCE Uncompensated charge
30Charges on Clay Surfaces 1. Uncompensated
Lattice Substitutions 2. Interlayer Cation
Substitutions 3. Dissociation of Surface OH (or
s-complex)
Fixed
Variable (pH-dependence)
31Dealing with Uncompensated Charges INTERLAYER
IONS
Mixed Layer -- illite-smectite,
chlorite-smectite, etc.
32Dealing with Uncompensated Charges Diffused
Surface Charge
Colloidal (21)
Smectite
Flocculent (11)
Kaolinite
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35Dealing with Uncompensated Charges Electrical
Double Layer
Continuum
Thickness determines colloid stability
36Adjacent Double Layers
37Adjacent Double Layers
Anion Exclusion - Membrane Filtration
38Dealing with Uncompensated Charges Zero Point
Charge (ZPC)
Low pH \ Al-OH-H ( charge) / ZPC \
Al-OH0 ?? H (neutral) / High pH \ Al-O-
?? H (- charge)
39CONSEQUENCE pH lt ZPC anions will be attracted
to the surface
40CONSEQUENCE pH ZPC no preference for
cations/anions
41CONSEQUENCE pH gt ZPC cations will be attracted
to the surface
42ZPC 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
43Cation 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
44Ion 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)
45If 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
46Similar 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
47For 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
48This 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
49An 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!!)
50IMPLICATIONS
- 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).
51More SORPTION
52- 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
53 Adsorption
Absorption
Sorbent
Sorbate
54Sorption
Luthy et al. (1997, EST 31, 3341-3347)
55- 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)
56Sorption Effects in Plumes
Retardation
57Solid Water Distribution Ratio (Kd) Kd
Cs(mol/kg) / Cw(mol/L)
Particle in Aqueous Solution
H2O
ORGANIC SORPTION
58Solid Water Distribution Ratio (Kd) Kd
Cs(mol/kg) / Cw(mol/L)
How does Kd vary with concentration? Is a Kd
true CONSTANT?
59- Linear - the simplest adsorption isotherm
- (Kd is independent of concentration)
- Cs Kd Cw (plot of ms vs. mw straight line with
slope Kd)
60Sorption
- 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)
61Sorption
pyrene
naphthalene
phenanthrene
Chiou et al. (1998, EST 32, 264-269)
62Sorption
- Non-linear sorption -- Freundlich
n gt 1
Kd Cs / Cw
Cs Kf Cnw
Cs (mol kg-1)
n lt 1
Cw (mol L-1)
63- 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.
64Freundlich 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))
65Finite total available sites for sorption!
66Langmuir 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
67Hence 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)
68Cfilled 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 !
69Sorption
- 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)
70Sorption
- Langmuir
- 2,4-dinitrotoluene sorption to (A)
montmorillonite and (B) Burkholderia sp. cells - Ortega-Calvo et al. (1999, EST 33, 3737-3742)
71Simple 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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74Sorption
- 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)
75Sorption (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
76Sorption (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
77Sorption
Luthy et al. (1997, EST 31, 3341-3347)
78- 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)
79Sorption Effects in Plumes
Retardation
80Solid Water Distribution Ratio (Kd) Kd
Cs(mol/kg) / Cw(mol/L) L kg-1
Particle in Aqueous Solution
H2O
ORGANIC SORPTION
81Solid Water Distribution Ratio (Kd) Kd
Cs(mol/kg) / Cw(mol/L)
How does Kd vary with concentration? Is a Kd
true CONSTANT?
82Sorption
- 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)
83Sorption
- Non-linear sorption -- Freundlich
n gt 1
Kd Cs / Cw
Cs Kf Cnw
Cs (mol kg-1)
n lt 1
Cw (mol L-1)
84Finite total available sites for sorption!
85Langmuir 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
86Hence 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)
87Cfilled 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 !
88Sorption
- Langmuir
- 2,4-dinitrotoluene sorption to (A)
montmorillonite and (B) Burkholderia sp. cells - Ortega-Calvo et al. (1999, EST 33, 3737-3742)
89Simple Application of Kd
Add 1,4 dimethylbenzene
f 0.2, rs 2.5 kg/L, Kd (1,4 DMB) 1 L/kg
What fraction of DMB will end up in solution at
any time?
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92Sorption
- 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)
93Sorption (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
94Sorption (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
95REALITY CHECK Complex Nature of Kd
- Neutral on OM
- Neutral on Mineral
- Ion Exchange on IES
- Reaction on SRS
96REALITY CHECK Complex Nature of Kd
97REALITY CHECK Complex Nature of Kd
98REALITY CHECK Complex Nature of Kd
99REALITY CHECK Complex Nature of Kd
100Sorption to Organic Matter
- Partial expression for sorption
for neutral (hydrophobic) organic compounds
101REALITY CHECK Complex Nature of Kd
Luthy et al. (1997, EST 31, 3341-3347)
102REALITY CHECK Complex Nature of Kd
103- Kom Com / Caq,neut
- Kd Com fom / Caq,neut
- Kd Kom x fom
KOM constant
fom
104Sorption 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
105Sorption to Organic Matter
Koc
Karickhoff et al. (1979, Water Research 13,
241-248)
106Sorption 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
107Sorption 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
108- 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
109Sorption to Organic Matter
- fom threshold
- if fom too low, other sorption processes
dominate! - fom gt 0.001generallyaccepted asthreshold
Koc
110Sorption to Organic Matter
- Kom assumed property of compound
- relate to Kow
- both are partition processes
- octanol and organic matter
- partially hydrophobic
- partially hydrophilic
Octanol
111Sorption 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
112Sorption 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)
113log 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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115Sorption 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
116Sorption to Organic Matter
- All organic matter not the same!
- soils
- sediments
- lakes
- rives
- aquifer materials
- aquatic organic matter
117Sorption 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)
118Sorption to Organic Matter
- and the sorption of all compounds depends only
on Kow - from Karickhoff et al. (1981 Chemosphere 10,
833-846)
119Sorption 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)
120Sorption to Organic Matter
- Organic matter
- elemental composition
Barron (2001, MS Thesis, University of Colorado)
121Sorption to Organic Matter
- Organic matter
- aromaticity/aliphilicity
- molecular size
- UV absorbance
- fluorescence
Barron (2001, MS Thesis, University of Colorado)
122Sorption to OM
- Effect of Organic Matter on Kom
- phenanthrene
- wide range of sedimentary rocks
- organic facies
Kleineidam et al. (1999, EST 33, 1637-1644)
123Sorption to OM
- Effect of Organic Matter on Kom
- pesticides
- carbaryl
- phosalone
- NMR analysis
Ahmad et al. (2001, EST 35, 878-884)
124REALITY CHECK Complex Nature of Kd Sorption to
Minerals of neutral hydrophobic compounds.
125Sorption to Minerals
Luthy et al. (1997, EST 31, 3341-3347)