Chapter 9Sorption to organic matter

Outline

• 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 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!

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)

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

Equations for sorption isothermsFreundlich – empirical description

Langmuir – sorption to a limited number of sites

iniwiFis CKC

iwiL

iwiLis CK

CKC

1max

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

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

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

Langmuir - linearization

max,max,

1111

isiwiLisis CCKCC

Note: usually Cis,max = max

y = mx +b

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

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

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

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

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)

Recall:

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

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

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

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

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

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

Humic substances

• Fig 3.6

Negative charge comes primarily from ionization of acid functional groups (esp. carbonyls)

C12H12O9N C10H12O5N

Rough chemical formulas

structuressoil humic acid

seawater humic

black carbonAKA soot carbonAKA elemental carbon

Structures are guesses based on 13C NMR

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

• SOM constitutes most of the buffering capacity of soils

55% of SOM CEC?

30% of SOM CEC?

Fig 3.13Strong acid

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.

)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

slopes << 1 can also mean system is not at equilibrium

Achman et al., 1993

Green Bay

Solids concentration

effect

2008

LFERs for Koc

(assuming slope 1)

As with similar LFERs, these are compound-class specific

Problem with non linearityRecall nonlinear isotherm

Low slope, low Kd

High slope, high Kd

Measure here because highconc easy to detect

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

Effect of T on Kioc

cstRT

HK iPOMw

ioc

'ln

Eiw

EiPOMiPOMw HHH

211

2 11ln

TTR

H

K

K POMw

ocT

ocT

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

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

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

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)

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

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

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)

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

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,

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

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)

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

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