Chapter 9
Sorption to organic matter
Outline
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Introduction
Sorption isotherms, Kd, and f dissolved
Sorption to POM
Sorption to DOM
Sorption of acids & bases to NOM
definitions
• absorption - sorption (penetration into) a
3D matrix
• adsorption – sorption to a 2D surface
• Sorbate: the molecule ad- or absorbed
• Sorbent: the matrix into/onto which the
sorbate ad- or absorbs
identical molecules behave very
differently, depending on whether
they are:
• in the gas phase (gas)
• surrounded by water molecules (dissolved)
• clinging onto the exterior of solids
(adsorbed)
• buried within a solid matrix (absorbed)
sorption affects transport:
• generally, molecules which are sorbed are less
mobile in the environment
• sorbed molecules are not available for phase
transfer processes (air-water exchange, etc)
and degradation:
• sorbed molecules are not bioavailable
• sorbed molecules usually shielded from UV light
(less direct photolysis)
• sorbed molecules cannot come into contact with
indirect photoxidants such as OH
• rates of other transformation reactions may be
very different for sorbed molecules
sorption is a difficult
subject because
sorbents in the natural
environment are
complex, and sorption
may occur via several
different mechanisms
the solid-water distribution coefficient
or: the equilibrium constant that wasn’t
K id 
Cis
Ciw
equilibrium “constant”
describing partitioning between
solid and water phases
Cis = mol/kg solid or mg/kg solid
Ciw = mol/L water or mg/L liquid
Kid = L/kg
This type of equilibrium constant assumes:
All sorption sites have equal energy
An infinite number of sorption sites
The problem with sorption is that these two assumptions are
generally not true!
sorption isotherms
• describe equilibrium partitioning between
sorbed and desorbed phase
• the sorption isotherm is a plot of the
concentration sorbed vs. the concentration
desorbed
• sorption isotherms can have many shapes
sorption isotherms can have many shapes
linear
(Kd cst)
as more is sorbed,
sorption becomes less
favorable
levels off at
max value
mixed
as more compound is
sorbed, sorption
becomes more favorable
???
the shape of the isotherm must be consistent with
the mechanism of sorption
BUT the shape of the isotherm alone does not
prove which sorption mechanism is operating
Equations for sorption isotherms
Freundlich – empirical description
Cis  K iF  C
ni
iw
Langmuir – sorption to a limited number of sites
max  K iL  Ciw
Cis 
1  K iL  Ciw
Freundlich isotherm
Cis  K iF  C
ni
iw
Due to the exponent n, Kd is not constant (unless n =1):
K id  K iF  C
ni 1
iw
in other words:
Kid  KiF
units of KF
depend on
units of Ciw
Linearization (n and KF are fitting factors):
log Cis  n log Ciw  log K iF
Interpretation: multiple types of sorption sites,
exhibiting a diversity of free energies
Freundlich isotherm shapes
n = 1 all sites have equal
energy at all sorbent concs
n < 1 added sorbates are
bound with weaker and weaker
energies
n > 1 more sorbate presence
enhances the free energies of
further sorption
Langmuir isotherm
Not empirical: can be derived from first principles
max  K iL  Ciw
Cis 
1  K iL  Ciw
saturation
(Ciw very big)
max
where max = total number of available
sites (usually depends on the sorbate)
KiL = Langmuir constant
KiL = KdCmax at low concentrations
(linear region)
linear region
(Ciw very small)
Langmuir - linearization
 1
1 
1
1




Cis  Cis,max  K iL  Ciw Cis,max
y = mx +b
Note: usually Cis,max = max
In the real world…
Sorption takes place via many different mechanisms, even in
the same system.
Thus, a combination of isotherms may be necessary to
adequately describe sorption behavior.
Example: Adsorption plus absorption: Langmuir plus linear:
Cis  K ip  Ciw 
Cis,max  K iL  Ciw
1  K iL  Ciw
Example: sorption to sediments containing black carbon
(important for PAHs)
Cis  Kip  Ciw  KiF  Ciwni
Dissolved fraction of a compound in a system:
Ciw  Vw
f iw 
Ciw  Vw  Cis  M s
Vw = volume of water (out of total volume Vtot)
Ms = mass of solids
Since:
Cis  Kid  Ciw
Vw
f iw 
Vw  K id  M s
1
1
f iw 

1  ( M s / Vw )  K id 1  rsw  K id
rsw = solid/water ratio
of course,
fs = 1 - fw
Ways to express the solid/water ratio
rsw = solid/water ratio (kg/L)
could also use porosity f:
Vw
Vw
Vw
1
f



Vtot Vw  Vs Vw  M s /  s 1  rsw /  s
Vs 
Ms
s
s is usually about 2.5 kg/L
or use bulk density (b)
Ms
b 
  s (1  f )
Vtot
Example: 1,4-DMB (Kd = 1 L/kg)
In a lake, rsw = 1 mg/L = 10-6 kg/L
1
1
f iw 

1
6
1  rsw  K id 1  10 1
essentially all dissolved
In an aquifer, rsw = 10 kg/L
1
1
f iw 

 0.09
1  rsw  K id 1  10 1
one molecule in 11 dissolved
movement in groundwater retarded by a factor of 11
retardation factor: Rf = 1/fw
The complex nature of Kd
The apparent distribution of a compound between
water and solids (Kd) may be a result of many
different types of sorption processes.
These processes include:
sorption to
organic carbon
Kid 
adsorption
to mineral
surface
exchangeable
adsorption of
ionized form
to charged
surface
covalently
bonded
adsorption of
ionized form to
mineral surface
Cioc  f oc  Ci min  Asurf  Ciex   surf ex  Asurf  Cirxn   surf rxn  Asurf
 refers to conc
of suitable sites
(mol/m2)
Ciw,neut  Ciw,ion
total amount in dissolved phase consists of
neutral and ionized forms
Recall:
It gets worse:
Cioc  f oc
both adsorption and absorption to
different types of OC
Ci min  Asurf
adsorption to many different types of minerals
(each with different K and different
concentrations)
Ciex   surf ex  Asurf
adsorption to many different types of minerals
(each with different surface charge)
Cirxn   surf rxn  Asurf reaction (adsorption) to many different
types of reactive sites
Sorption of neutral organics to POM
Cioc  f oc
K id 
Ciw
Sorption to organic matter is often the dominant sorption
process for organic chemicals, because they don’t have to
compete with water molecules for a charged surface.
foc = fraction of organic carbon in solid
fom = 2 foc

Even at foc = 0.0001, sorption to OC may still dominate
the equilibrium
“constant” Kd varies
over more than an
order of magnitude!
Kd is strong function of foc
Therefore, define the organic-carbon normalized partition
coefficient:
1
K id
Hence: f iw 
K ioc 
f oc
1  rsw  K oc  f oc
Normalizing to foc reduces, but does not eliminate,
the variability in Kd
Thus the type of organic carbon does matter
Terrestrial organic carbon more polar?
If you don’t actually measure Koc for your system, you can
choose a literature value and be accurate to about a factor of 2
(0.3 log units)
Not all organic carbon is created equal
Soil Organic Matter
• SOM = Humus
• Content:
–
–
–
–
–
~0 to 5% of most soils
Up to 100% of organic soils (histosoils)
Higher in moist soils and northern slopes
Lower in drier soils and southern slopes
Cultivation reduced SOM
• High surface area and CEC
• Lots of C and N
table 3.1
Table 3.2
Carbon sequestration
• Soils sequester carbon in SOM and
carbonate minerals
• About 75% of the terrestrial carbon pool is
SOM
• Declines in the SOC pool are due to:
– Mineralization of SOC
– Transport by soil erosion
– Leaching into subsurface soil or groundwater
Sequestration of Carbon by Soils can be
increased via:
• Changing agricultural practices:
– No-till agriculture or organic agriculture
– Limited used of N fertilizer (C released during
N fertilizer manufacture)
– Limited irrigation (fossil fuels burned to power
irrigation)
• Soil restoration
Figure 3.1
Composition of SOM
• Major: lignins and proteins
– Also: hemicellulose, cellulose, ether and
alcohol soluble compounds
– “nonhumic” substances = “juicy” carbon that is
quickly digested
• (carbohydrates, proteins, peptides, amino acids, fats,
waxes, low MW acids)
• Most SOM is not water-soluble
Table 3.3
Definitions
Cellulose
Lignin
= a practically indigestible compound
which, along with cellulose, is a major
component of the cell wall of certain
plant materials, such as wood, hulls,
straws, etc.
Hemicellulose: A carbohydrate
resembling cellulose but more
soluble; found in the cell walls
of plants.
Four theories
on how humic
substances are
formed
Fig 3.3
Pathway 1:
probably not
important
Pathways 2 & 3:
polymerization of
quinones, probably
predominant in
forest soils
Pathways 4:
Classical theory,
probably
predominant in
poorly drained soils
Humic substances
• Fig 3.6
C12H12O9N
C10H12O5N
Rough chemical formulas
Negative charge comes primarily from ionization of
acid functional groups (esp. carbonyls)
soil humic acid
seawater humic
Structures are guesses based on 13C NMR
structures
black carbon
AKA soot carbon
AKA elemental carbon
Properties of SOM
• Voids can trap
– Water
– Minerals
– Other organic molecules
• Hydrophobicity/hydrophilicity
• Reactivity
• H-bonding, chelation of metals
Fig 3.8
Conformation and macromolecular structure
of HS depend on
–
–
–
–
pH
Electrolyte concentration
Ionic strength
HA and FA concentrations
Fig 3.10
Functional groups and charge
characteristics
• PZC ~ 3 (pH of zero charge)
• Up to 80% of CEC in soils is due to SOM
• Acid functional groups
– Carbonyls pKa < 5
– Quinones also pKa < 5
– Phenols pKa < 8
55% of SOM CEC?
30% of SOM CEC?
• SOM constitutes most of the buffering
capacity of soils
Strong
acid
Fig 3.13
Relationships between Kow and Koc
logKoc vs. logKow for
PAHs in Raritan Bay
Karickhoff (1981) has
agued that the slope of
this plot should be one.
Gigliotti et al. 2002
For PCBs in Raritan Bay, slopes 
one
Correction for PCBs sorbed to
DOC and quantified as part of the
“apparent dissolved” phase makes
the slopes one.
CT  Cd  C DOC  C p
CT  Cd (1  K DOC  DOC  K OC  TSM  f OC )
for this particular model, assume
logKoc = logKow – 0.21
logKDOC = logKow –1
What is Kd?
sorption to colloids (DOC) is
often the cause of the “solids
concentration effect”
Totten et al., 2001
Achman et al.,
1993
Green Bay
slopes << 1 can also mean system is not at equilibrium
2008
Solids
concentration
effect
LFERs for Koc
(assuming slope  1)
As with similar LFERs, these are compound-class specific
Problem with non linearity
Recall nonlinear isotherm
High slope, high Kd
Measure here because high
conc easy to detect
Low slope, low Kd
Nonlinear Koc
Adsorption to black carbon can be important
for PAHs and other compounds.
A mixed isotherm (linear plus Freundlich) is
then appropriate:
Cis  f oc  K ioc  Ciw  f bc  K ibc  Ciw0.7
for black carbon (bc), an exponent of 0.7 seems to work
We might be able to estimate Kbc for planar sorbates via:
log Kibc  1.6 log Kiow  1.4
Effect of T on Kioc
ln K
'
ioc
 POMwH i

 cst
RT
K ocT 2  POMwH  1 1 
  
ln

K ocT1
R  T1 T2 
E
 POMwH i  H iPOM
 H iwE
E
 POMwH i  H iPOM
 H iwE
HEw excess enthalpy of dissolution in water
For small organic compounds, small
For polar compounds, may be negative by –20-30 kJ/mol
For large apolar compounds may be positive by 20-30 kJ/mol
HEPOM
average excess enthalpy for various sorption sites/matrixes
may depend on concentration range
absorption--of apolar compounds, may assume this is small
absorption relatively insensitive to temperature
adsorption--for H bonding compounds, may be -40-50 kJ/mol
double with 10 degree increase in temperature
Effect of salinity on Koc
Salinity will increase Koc by decreasing the solubility
(increasing the activity coefficient) of the solute in water.
Account for salinity effects via Setschenow constant:
K ioc, salt  K ioc 10
 K is [ salt]tot
Effect of cosolvents on Koc
Cosolvents will increase the solubility (decrease the activity
coefficient) of the solute in water:
 il ( f v )   iw 10
 ic  f v
Recall  = cosolvency power, depends on solute and cosolvent
If the cosolvent has no effect on the organic matter, then:
K ioc, solv/ w  K ioc 10
 is  f v
However, the cosolvent may dissolve into the organic carbon phase
and change its properties.
We can account for this empirically by introducing a:
K ioc, solv/ w  K ioc 10
a is  f v
a quantifies how the cosolvent changes the nature of the sorbent
Sorption of Neutral Compounds
to “Dissolved” Organic Matter
Dissolved organic matter = anything that passes through the filter
usually measured as dissolved organic carbon (DOC)
may be truly dissolved
may be very small particles (colloids) (1 nm to 1 um in size)
Effects of DOC:
increases apparent solubility
decreases air/water distribution ratio
may decrease bioavailability
may affect interactions of compounds with light
Effects are seen at low concentrations (below cosolvent range)
Relationship between
DOC properties and KDOC
KDOC is tough to measure because it is difficult to
separate the dissolved and sorbed phases.
Characterizing DOC:
MW
UV-light absorptivities
Degree of aromaticity by 13C or 1H NMR
Stoichiometric ratios
For pyrene:
log K DOC  1.45 log  i  1.70(O / C )  1.14
in L/kg OC
at 280 nm
in L/mol-cm
Effect of pH, ionic strength, and T on KDOC
Interactions of DOC with ions can be complex
DOC has polar functional groups which can become ionized
introducing electrostatic attraction or repulsion,
functional groups can complex cations
It is difficult to predict effects of pH and ionic strength on KDOC
In general,
Usually ignore effects of pH, ionic strength and T
LFERs relating KDOC to Kow
For a given DOC and
a set of closely related
compounds, LFERs
can work
PCBs
DOC levels often ~5 mg/L in surface waters
Because PCBs have log Kow ~ 6-8, sorption to DOC can be
significant
(PAHs have log Kow ~ 3-6, sorption to DOC usually insignificant)
For PCBs:
KDOC = (0.1-0.2)*Koc
Totten et al. 2001
PCBs
For PCBs, many models
use
KDOC = m*Kow
8.0
log apparent KOC
7.5
Where m = 0.1 for
Hudson, many other
systems
7.0
6.5
Rowe calculated m
necessary to give a slope
of 1 and got
m = 0.14  0.076
6.0
5.5
5.0
5.0
5.5
6.0
6.5
7.0
7.5
log KOW
Figure 4. The log apparent KOC vs. log KOW plot for the Zone 2 May 2002
cruise sample. This plot is representative of the other samples and displays
the differences between apparent KOC and the theoretical slope of 1 (1:1
line). show the regression line and equation on the plot.
8.0
Except for March 2002,
when DOC was high and
m = 0.014  0.015
Rowe, PhD dissertation,
2006
Sorption of acids and bases to NOM
acids and bases may partially or fully ionized at ambient pH
when considering sorption of neutral species, must consider:
vdW interactions
polarity
H-bonding
when considering sorption of charged species, must ALSO
consider electrostatic interactions and formation of covalent
bonds with the NOM
use D = the distribution ratio, to avoid confusion with K
Character of NOM
at ambient pH, NOM is negatively charged due to
carboxylic acid functional groups
NOM acts as a cation exchanger
Negatively charged species will sorb more weakly to
NOM than their neutral counterparts, and in some cases,
sorption of negatively charged species can be ignored.
Positively charged species will sorb more strongly to
NOM than the neutral form
Sorption due to these electrostatic attractions is usually
fast and reversible (unless covalent bonding occurs)
For weak acids with only one acidic group,
[ HA]oc  [ A ]oc
Dioc 
[ HA]w  [ A ]w
Recall:
1
a ia 
1  10 pH  pKia
Thus:
Dioc  a ia  K
usually
K
HA
ioc
HA
ioc
 (1  a ia )  K
 K
A
ioc
A
ioc
thus if pH < 2 + pKa then sorption of ionized
species is usually negligible
2,4,5-trichlorophenol (pKa = 6.94)
HA
Dioc  a ia  K ioc
pentachlorophenol (pKa = 4.75)
Dioc  a ia  K
HA
ioc
 (1  a ia )  K
A
ioc
Sorption of the anion important
(bigger, more hydrophobic)
Note that KA-ioc is dependant on pH and sometimes on the cations present!
Sorption of bases
sorption of the cationic form to negatively charged sites in the
NOM may dominate the overall sorption of the compound
in other words, there are a limited number of sorption sites…
therefore the sorption isotherm is non-linear
competition with other cations can occur
quinoline pKa = 4.9
sorption max at this pH
at lower pH, fewer
negative sites available
additional contribution
from sorption of cation
sorption of
neutral form
only
Problem 9.1
what fraction of atrazine is the truly dissolved phase
a. in lake with 2 mg/L POC
b. in marsh with 100mg/L solids, foc = 0.2
c. in aquifer, where porosity = 0.2 by vol, density of
minerals = 2.5 kg/L, foc = 0.005