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Chapter 9Sorption to organic matter
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Outline
• Introduction
• Sorption isotherms, Kd, and f dissolved
• Sorption to POM
• Sorption to DOM
• Sorption of acids & bases to NOM
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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
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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)
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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
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sorption is a difficult subject because sorbents in the natural environment are complex, and sorption may occur via several different mechanisms
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the solid-water distribution coefficientor: the equilibrium constant that wasn’t
iw
isid
C
CK
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 energyAn infinite number of sorption sites
The problem with sorption is that these two assumptions are generally not true!
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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
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sorption isotherms can have many shapes
linear (Kd cst)
levels off at max value
mixed as more is sorbed, sorption becomes less favorable
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
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Equations for sorption isothermsFreundlich – empirical description
Langmuir – sorption to a limited number of sites
iniwiFis CKC
iwiL
iwiLis CK
CKC
1max
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Freundlich isothermin
iwiFis CKC Due to the exponent n, Kd is not constant (unless n =1):
1 iniwiFid CKK
iFid KK in other words:
units of KF depend on units of Ciw
iFiwis KCnC logloglog Linearization (n and KF are fitting factors):
Interpretation: multiple types of sorption sites, exhibiting a diversity of free energies
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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
Freundlich isotherm shapes
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Langmuir isotherm
Not empirical: can be derived from first principles
iwiL
iwiLis CK
CKC
1max
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)
saturation (Ciw very big)
max
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Langmuir - linearization
max,max,
1111
isiwiLisis CCKCC
Note: usually Cis,max = max
y = mx +b
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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:
Example: sorption to sediments containing black carbon (important for PAHs)
iwiL
iwiLisiwipis CK
CKCCKC
1
max,
iniwiFiwipis CKCKC
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Dissolved fraction of a compound in a system:
siswiw
wiwiw MCVC
VCf
Vw = volume of water (out of total volume Vtot)
Ms = mass of solids
Since:
sidw
wiw MKV
Vf
iwidis CKC
idswidwsiw KrKVM
f
1
1
)/(1
1
rsw = solid/water ratio
of course,
fs = 1 - fw
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Ways to express the solid/water ratio
rsw = solid/water ratio (kg/L)
could also use porosity
sswssw
w
sw
w
tot
w
rMV
V
VV
V
V
V
/1
1
/
s
ss
MV
s is usually about 2.5 kg/L
or use bulk density (b)
)1( stot
sb V
M
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Example: 1,4-DMB (Kd = 1 L/kg)
In a lake, rsw = 1 mg/L = 10-6 kg/L
11101
1
1
16
idsw
iw Krf
essentially all dissolved
In an aquifer, rsw = 10 kg/L
09.01101
1
1
1
idswiw Kr
f
one molecule in 11 dissolved
movement in groundwater retarded by a factor of 11
retardation factor: Rf = 1/fw
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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:
ioniwneutiw
surfisurfisurfiociocid
CC
ACACACfCK
,,
rxn surfrxnex surfexmin
total amount in dissolved phase consists of neutral and ionized forms
sorption to organic carbon
adsorption to mineral surface
exchangeable adsorption of ionized form to charged surface
covalently bonded adsorption of ionized form to mineral surface
refers to conc of suitable sites (mol/m2)
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Recall:
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It gets worse:ocioc fC
surfi AC rxn surfrxn
surfi AC min
surfi AC ex surfex
both adsorption and absorption to different types of OC
adsorption to many different types of minerals (each with different K and different concentrations)
adsorption to many different types of minerals (each with different surface charge)
reaction (adsorption) to many different types of reactive sites
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Sorption of neutral organics to POM
iw
ociocid C
fCK
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
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Kd is strong function of foc
Therefore, define the organic-carbon normalized partition coefficient:
oc
idioc f
KK
the equilibrium “constant” Kd varies over more than an order of magnitude!
Hence:ococsw
iw fKrf
1
1
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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?
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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)
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Not all organic carbon is created equal
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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
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table 3.1
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Table 3.2
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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
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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
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Figure 3.1
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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
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Table 3.3
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Definitions
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.
Cellulose
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Fig 3.3
Four theories on how humic substances are formed
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
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Humic substances
• Fig 3.6
Negative charge comes primarily from ionization of acid functional groups (esp. carbonyls)
C12H12O9N C10H12O5N
Rough chemical formulas
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structuressoil humic acid
seawater humic
black carbonAKA soot carbonAKA elemental carbon
Structures are guesses based on 13C NMR
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Properties of SOM
• Voids can trap– Water– Minerals– Other organic molecules
• Hydrophobicity/hydrophilicity
• Reactivity
• H-bonding, chelation of metals
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Fig 3.8
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Conformation and macromolecular structure of HS depend on
– pH– Electrolyte concentration– Ionic strength– HA and FA concentrations
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Fig 3.10
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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
• SOM constitutes most of the buffering capacity of soils
55% of SOM CEC?
30% of SOM CEC?
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Fig 3.13Strong acid
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Relationships between Kow and Koc
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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
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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.
)TSMKDOCK(1CC
CCCC
OCDOCdT
pDOCdT
OCf
for this particular model, assume logKoc = logKow – 0.21logKDOC = logKow –1
What is Kd?
sorption to colloids (DOC) is often the cause of the “solids concentration effect”Totten et al., 2001
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slopes << 1 can also mean system is not at equilibrium
Achman et al., 1993
Green Bay
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Solids concentration
effect
2008
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LFERs for Koc
(assuming slope 1)
As with similar LFERs, these are compound-class specific
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Problem with non linearityRecall nonlinear isotherm
Low slope, low Kd
High slope, high Kd
Measure here because highconc easy to detect
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Nonlinear Koc
Adsorption to black carbon can be important for PAHs and other compounds.
A mixed isotherm (linear plus Freundlich) is then appropriate:
7.0iwibcbciwiococis CKfCKfC
for black carbon (bc), an exponent of 0.7 seems to work
We might be able to estimate Kbc for planar sorbates via:
4.1log6.1log iowibc KK
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Effect of T on Kioc
cstRT
HK iPOMw
ioc
'ln
Eiw
EiPOMiPOMw HHH
211
2 11ln
TTR
H
K
K POMw
ocT
ocT
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Eiw
EiPOMiPOMw HHH
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 smallabsorption relatively insensitive to temperature
adsorption--for H bonding compounds, may be -40-50 kJ/moldouble with 10 degree increase in temperature
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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:
totsi saltK
iocsaltioc KK ][, 10
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Effect of cosolvents on Koc
vci f
iwvil f 10)(
Cosolvents will increase the solubility (decrease the activity coefficient) of the solute in water:
Recall = cosolvency power, depends on solute and cosolvent
If the cosolvent has no effect on the organic matter, then:v
si f
iocwsolvioc KK 10/,
However, the cosolvent may dissolve into the organic carbon phase and change its properties.
We can account for this empirically by introducing v
si f
iocwsolvioc KK 10/,
quantifies how the cosolvent changes the nature of the sorbent
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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 solubilitydecreases air/water distribution ratiomay decrease bioavailabilitymay affect interactions of compounds with light
Effects are seen at low concentrations (below cosolvent range)
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Relationship between DOC properties and KDOC
KDOC is tough to measure because it is difficult to separate the dissolved and sorbed phases.
Characterizing DOC:MWUV-light absorptivitiesDegree of aromaticity by 13C or 1H NMRStoichiometric ratios
For pyrene:14.1)/(70.1log45.1log COK iDOC
in L/kg OCat 280 nm
in L/mol-cm
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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
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LFERs relating KDOC to Kow
For a given DOC and a set of closely related compounds, LFERs can work
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PCBs
For PCBs:
KDOC = (0.1-0.2)*Koc
Totten et al. 2001
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)
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PCBs
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.
5.0
5.5
6.0
6.5
7.0
7.5
8.0
5.0 5.5 6.0 6.5 7.0 7.5 8.0
log KOW
log
ap
par
ent
KO
C
For PCBs, many models use KDOC = m*Kow
Where m = 0.1 for Hudson, many other systems
Rowe calculated m necessary to give a slope of 1 and got m = 0.14 0.076
Except for March 2002, when DOC was high and m = 0.014 0.015
Rowe, PhD dissertation, 2006
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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
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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)
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For weak acids with only one acidic group,
ww
ococioc AHA
AHAD
][][
][][
Recall:
iapKpHia
101
1
Thus:
Aiocia
HAiociaioc KKD )1(
usually
Aioc
HAioc KK
thus if pH < 2 + pKa then sorption of ionized species is usually negligible
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2,4,5-trichlorophenol (pKa = 6.94) pentachlorophenol (pKa = 4.75)HAiociaioc KD
Aiocia
HAiociaioc KKD )1(
Note that KA-ioc is dependant on pH and sometimes on the cations present!
Sorption of the anion important (bigger, more hydrophobic)
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Sorption of basessorption 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
sorption of neutral form only
additional contribution from sorption of cation
at lower pH, fewer negative sites available
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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