Chapter 9 Sorption to organic matter

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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 wasnt
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!
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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
as more compound is sorbed, sorption becomes
more favorable
linear (Kd cst)
levels off at max value
as more is sorbed, sorption becomes less
favorable
mixed
???
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 isotherms
Freundlich empirical description
Langmuir sorption to a limited number of sites
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Freundlich isotherm
Due to the exponent n, Kd is not constant (unless
n 1)
units of KF depend on units of Ciw
in other words
Linearization (n and KF are fitting factors)
Interpretation multiple types of sorption
sites, exhibiting a diversity of free energies
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Freundlich isotherm shapes
n 1 all sites have equal energy at all
sorbent concs n lt 1 added sorbates are bound
with weaker and weaker energies n gt 1 more
sorbate presence enhances the free energies of
further sorption
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Langmuir isotherm
Not empirical can be derived from first
principles
saturation (Ciw very big)
Gmax
where Gmax 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)
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Langmuir - linearization
y mx b
Note usually Cis,max Gmax
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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)
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Dissolved fraction of a compound in a system
Vw volume of water (out of total volume
Vtot) Ms mass of solids Since
of course, fs 1 - fw
rsw solid/water ratio
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Ways to express the solid/water ratio
rsw solid/water ratio (kg/L) could also use
porosity f
rs is usually about 2.5 kg/L
or use bulk density (rb)
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Example 1,4-DMB (Kd 1 L/kg)
In a lake, rsw 1 mg/L 10-6 kg/L
essentially all dissolved
In an aquifer, rsw 10 kg/L
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
covalently bonded adsorption of ionized form to
mineral surface
exchangeable adsorption of ionized form to
charged surface
adsorption to mineral surface
sorption to organic carbon
s refers to conc of suitable sites (mol/m2)
total amount in dissolved phase consists of
neutral and ionized forms
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Recall
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It gets worse
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
Sorption to organic matter is often the dominant
sorption process for organic chemicals, because
they dont 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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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
Hence
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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 dont 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
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.
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Fig 3.3
Four theories on how humic substances are formed
Pathways 2 3 polymerization of quinones,
probably predominant in forest soils
Pathways 4 Classical theory, probably
predominant in poorly drained soils
Pathway 1 probably not important
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Humic substances

• Fig 3.6

C12H12O9N C10H12O5N Rough chemical formulas
Negative charge comes primarily from ionization
of acid functional groups (esp. carbonyls)
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structures
soil humic acid
black carbon AKA soot carbon AKA elemental carbon
seawater humic
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 lt 5
• Quinones also pKa lt 5
• Phenols pKa lt 8
• SOM constitutes most of the buffering capacity of
  soils

55 of SOM CEC?
30 of SOM CEC?
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Fig 3.13
Strong 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.
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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Achman et al., 1993 Green Bay
slopes ltlt 1 can also mean system is not at
equilibrium
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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 linearity
Recall nonlinear isotherm
High slope, high Kd
Measure here because highconc easy to detect
Low slope, low Kd
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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
for black carbon (bc), an exponent of 0.7 seems
to work We might be able to estimate Kbc for
planar sorbates via
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Effect of T on Kioc
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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
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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
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Effect of cosolvents on Koc
Cosolvents will increase the solubility (decrease
the activity coefficient) of the solute in water
Recall s cosolvency power, depends on solute
and cosolvent If the cosolvent has no effect on
the organic matter, then
However, the cosolvent may dissolve into the
organic carbon phase and change its
properties. We can account for this empirically
by introducing a
a 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 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)
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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 MW UV-light
absorptivities Degree of aromaticity by 13C or
1H NMR Stoichiometric ratios For pyrene
at 280 nmin L/mol-cm
in L/kg OC
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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
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
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PCBs
For PCBs, many models use KDOC mKow 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 polarit
y 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,
Recall
Thus
usually
thus if pH lt 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)
Sorption of the anion important (bigger, more
hydrophobic)
Note that KA-ioc is dependant on pH and sometimes
on the cations present!
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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
sorption of neutral form only
quinoline pKa 4.9 sorption max at this pH
at lower pH, fewer negative sites available
additional contribution from sorption of cation
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Problem 9.1

• what fraction of atrazine is the truly dissolved
  phase
• in lake with 2 mg/L POC
• in marsh with 100mg/L solids, foc 0.2
• in aquifer, where porosity 0.2 by vol, density
  of minerals 2.5 kg/L, foc 0.005