Transcript Chapter 9 Sorption to organic matter
Chapter 9 Sorption to organic matter
Outline
• Introduction • Sorption isotherms, K d , and
f
• Sorption to POM dissolved • Sorption to DOM • Sorption of acids & bases to NOM
definitions
•
ab
sorption - sorption (penetration into) a 3D matrix •
ad
sorption – 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
C is C iw
equilibrium “constant” describing partitioning between solid and water phases C is = mol/kg solid or mg/kg solid C iw = mol/L water or mg/L liquid K id = 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 (K d cst) levels off at max value as more compound is sorbed, sorption becomes
more
favorable 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
Equations for sorption isotherms
Freundlich – empirical description
C is
K iF
C n i iw
Langmuir – sorption to a limited number of sites
C is
max 1
K K iL iL
C C iw iw
Freundlich isotherm
C is
K iF
C n i iw
Due to the exponent n, K d is not constant (unless n =1):
K id
K iF
C n i iw
1 in other words:
K id
iF
units of K F depend on units of C iw Linearization (n and K F are fitting factors): log
C is
n
log
C iw
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
C is
max 1
K K iL iL
C C iw iw
saturation (C iw very big) max where max = total number of available sites (usually depends on the sorbate) K iL = Langmuir constant K iL = K d C max at low concentrations (linear region) linear region (C iw very small)
Langmuir - linearization
1
C is
C is
, max 1
K iL
1
C iw
1
C is
, max y = mx +b Note: usually C is,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:
C is
K ip
C iw
C is
, max 1
K K iL
iL
C C iw iw
Example: sorption to sediments containing black carbon (important for PAHs)
C is
K ip
C iw
K iF
C n i iw
Dissolved fraction of a compound in a system:
f iw
C iw
C iw V w
V w C is
M s
V w = volume of water (out of total volume V tot ) M s = mass of solids Since:
C is
K id
C iw f iw
V w V w
K id
M s
of course,
f iw
1 (
M s
1 /
V w
)
K id
1 1
r sw
K id f
s = 1 -
f
w r sw = solid/water ratio
Ways to express the solid/water ratio
r sw = solid/water ratio (kg/L) could also use porosity f: f
V w V tot
V w V w
V s
V w
V w M s
/
s
1 1
r sw
/
s V s
M
s s
s is usually about 2.5 kg/L or use bulk density ( b )
b
M V tot s
s
( 1 f )
Example: 1,4-DMB (K
d
= 1 L/kg)
In a lake, r sw = 1 mg/L = 10 -6 kg/L
f iw
1
r sw
1
K id
1 1 10 6 1 1 essentially all dissolved
In an aquifer, r sw = 10 kg/L
f iw
1
r sw
1
K id
1 1 10 1 0 .
09 one molecule in 11 dissolved movement in groundwater retarded by a factor of 11 retardation factor: R f = 1/
f
w
The complex nature of K
d The apparent distribution of a compound between water and solids (K d ) may be a result of many different types of sorption processes.
These processes include: sorption to organic carbon
ad
sorption to mineral surface exchangeable
ad
sorption of ionized form to charged surface covalently bonded
ad
sorption of ionized form to mineral surface
K id
C ioc
f oc
C i
min
A surf
C i
ex surf ex
A surf C iw
,
neut
C iw
,
ion
C i
rxn surf rxn
A surf
refers to conc of suitable sites (mol/m 2 ) total amount in dissolved phase consists of neutral and ionized forms
Recall:
It gets worse:
C ioc
f oc
both
ad
sorption and
ab
sorption to different types of OC
C i
min
A surf
ad
sorption to many different types of minerals (each with different K and different concentrations)
C i
ex surf ex
A surf
ad
sorption to many different types of minerals (each with different surface charge)
C i
rxn surf rxn
A surf
reaction (
ad
sorption) to many different types of reactive sites
Sorption of neutral organics to POM
K id
C ioc
C iw f oc
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.
f
oc = fraction of organic carbon in solid
f
om = 2
f
oc Even at
f
oc = 0.0001, sorption to OC may still dominate
the equilibrium “constant” K d varies over more than an order of magnitude!
K d is strong function of
f
oc Therefore, define the organic-carbon normalized partition coefficient:
K ioc
K id f oc
Hence:
f iw
1
r sw
1
K oc
f oc
Normalizing to
f
oc reduces, but does not eliminate, the variability in K d Thus the
type
of organic carbon does matter Terrestrial organic carbon more polar?
If you don’t actually measure K oc 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
• Fig 3.6
Humic substances
C 12 H 12 O 9 N C 10 H 12 O 5 N Rough chemical formulas Negative charge comes primarily from ionization of acid functional groups (esp. carbonyls)
soil humic acid
structures
seawater humic Structures are guesses based on 13 C NMR 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
Fig 3.13
acid
Relationships between Kow and Koc
Gigliotti et al. 2002 logK oc vs. logK ow for PAHs in Raritan Bay Karickhoff (1981) has agued that the slope of this plot should be one.
Totten et al., 2001 For PCBs in Raritan Bay, slopes one Correction for PCBs sorbed to C T C T DOC and quantified as part of the “apparent dissolved” phase makes the slopes one.
C d C DOC C p C d (1 K DOC DOC K OC TSM
f OC
) for this particular model, assume logK oc = logK ow – 0.21
logK DOC = logK ow –1
What is K d ?
sorption to colloids (DOC) is often the cause of the “solids concentration effect”
Achman et al., 1993
Green Bay
slopes << 1 can also mean system is not at equilibrium
Solids concentration effect
2008
LFERs for K
oc (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 K
oc Adsorption to black carbon can be important for PAHs and other compounds. A mixed isotherm (linear plus Freundlich) is then appropriate:
C is
f oc
K ioc
C iw
f bc
K ibc
0 .
7
C iw
for black carbon (bc), an exponent of 0.7 seems to work We might be able to estimate K bc
for planar sorbates
via: log
K ibc
1 .
6 log
K iow
1 .
4
Effect of T on K
ioc ln '
K ioc
POMw H i RT
cst
ln
K ocT
2
K ocT
1
POMw H R
1
T
1 1
T
2
POMw H i
E H iPOM
E H iw
POMw H i
E H iPOM
E H iw
H E w
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
H E POM
average
excess enthalpy for various sorption sites/matrixes may depend on concentration range
ab
sorption--of apolar compounds, may assume this is small absorption relatively insensitive to temperature
ad
sorption--for H bonding compounds, may be -40-50 kJ/mol double with 10 degree increase in temperature
Effect of salinity on K
oc
Salinity
will increase K oc 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 i s
[
salt
]
to t
Effect of cosolvents on K
oc
Cosolvents
will increase the solubility (decrease the activity coefficient) of the solute in water:
il
(
f v
)
iw
10 Recall
i c
f v
= 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
i s
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
i s
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 K
DOC K DOC 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 13 C or 1 H 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 K DOC
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 K DOC In general, Usually ignore effects of pH, ionic strength and T
LFERs relating K DOC to K ow
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 K ow significant ~ 6-8, sorption to DOC can be (PAHs have log K ow ~ 3-6, sorption to DOC usually insignificant)
For PCBs: K DOC = (0.1-0.2)*K oc
Totten et al. 2001
PCBs
8.0
7.5
7.0
6.5
6.0
5.5
5.0
5.0
5.5
6.0
6.5
log K OW
7.0
7.5
Figure 4. The log apparent K OC vs. log K OW plot for the Zone 2 May 2002 cruise sample. This plot is representative of the other samples and displays the differences between apparent K OC and the theoretical slope of 1 (1:1 line). show the regression line and equation on the plot.
8.0
For PCBs, many models use K DOC = m*K ow 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
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,
D ioc
[
HA
]
oc
[
HA
]
w
[
A
[
A
]
oc
]
w
Recall: a
ia
1 1 10
pH
pK ia
Thus:
D ioc
a
ia
HA K ioc
( 1 a
ia
)
A
K ioc
usually
HA K ioc
A
K ioc
thus if pH < 2 + pK a then sorption of ionized species is usually negligible
2,4,5-trichlorophenol (pK a
D ioc
a
ia
K HA ioc
= 6.94) pentachlorophenol (pK a
D ioc
a
ia
K HA ioc
( 1
= 4.75)
a
ia
)
A
K ioc
Sorption of the anion important (bigger, more hydrophobic)
Note that K A 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 sorption of neutral form only quinoline pK a = 4.9
sorption max at this pH at lower pH, fewer negative sites available additional contribution from sorption of cation
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, f oc = 0.2
c. in aquifer, where porosity = 0.2 by vol, density of minerals = 2.5 kg/L, f oc = 0.005