1. Partitioning of pollutants
Sorption involving organic matter

2. Aims
To address general aspects of solid-aqueous solution exchange involving natural sorbents
To address aspects needed to quantify sorption equilibrium in natural environments and to predict partition coefficients
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

3. Outcomes
Students will be able to evaluate compound partitioning between water, dissolved organic matter, and sediment organic matter based on physico-chemical properties of compounds
Students will be able to estimate partition coefficients on the basis of compound's chemical structure and physico-chemical properties
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

4. sorbed molecules are not bioavailable
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.)
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
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

5. Sorption is complex phenomenon because sorbents in the natural environment are complex, and sorption may occur via several different mechanisms.
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

6. Solid-water distribution coefficient
equilibrium “constant” describing partitioning between solid and water phases
Cis = mol/kg solid or mg/kg solid
Ciw = mol/L water or mg/L solid
Kid = L/kg
This partitioning model assumes:
All sorption sites have equal energy
An infinite number of sorption sites exist
The problem with sorption is that these two assumptions are generally not true!
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

7. surrounded by water molecules (dissolved)
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)
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

8. Sorption isotherms
Sorption takes place via many different mechanisms, even in the same system.
The shape of the isotherm does not prove which sorption mechanism is operating.
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

9. Sorption models Freundlich model (b)
ni – factor of nonlinearity (dimensionless)
KiF – Freundlich constant or sorption capacity (units depend on units of Ciw and Cis)
Due to the isotherm nonlinearity, Kd is not constant over the whole concentration range (unless n =1):
multiple types of sorption sites, exhibiting a diversity of free energies, empirical
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

10. all sites have equal energy at all sorbent concentrations
more sorbate enhances the free energies of further sorption
n < 1
added sorbates are bound with weaker and weaker energies
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

11. KiL – Langmuir constant
Langmuir isotherm (c)
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)
saturation (Ciw very big)
Gmax
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

12. Adsorption plus absorption: Example 1: Langmuir plus linear
In natural environments 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.
Adsorption plus absorption:
Example 1: Langmuir plus linear
Example 2: Freundlich plus linear (sorption to sediments containing black carbon (important for PAHs))
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

13. Dissolved fraction of a compound in a system:
Vw – volume of water (out of total volume Vtot)
Ms – mass of solids
Retardation factor:
rsw = solid/water ratio (kg/L)
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

14. Environmental processing / Partitioning of pollutants / Sorption involving organic matter

15. s is usually about 2.5 kg/L
Porosity:
s is usually about 2.5 kg/L
b – bulk density
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

16. Example: 1,4-DMB (Kd = 1 L/kg) In a lake, rsw = 1 mg/L = 10-6 kg/L
Dissolved
In an aquifer, rsw = 10 kg/L
one molecule in 11 dissolved
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

17. The complex nature of Kd
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
s refers to c of suitable sites (mol/m2)
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

18. 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
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

19. Sorption of hydrophobic (neutral) organics to natural organic matter (NOM)
foc = fraction of organic carbon in solid
Even at foc = , sorption to OC may still dominate
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

20. Not only quantity but also quality of OC matters!
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

21. LFERs for Koc The examples of compound class-specific LFERs:
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

22. Sorption of hydrophobic (neutral) organics to dissolved organic matter (DOC)
Effects of DOC:
increases apparent solubility
decreases air/water distribution ratio
may decrease bioavailability
may affect interactions of compounds with light
KDOC is hard to measure because it is difficult to separate the dissolved and sorbed phases of organic compounds
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

23. LFERs relating KDOC to Kow
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

24. 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:
Van der Waals interactions
polarity
H-bonding
When considering sorption of charged species, must ALSO consider electrostatic interactions and formation of covalent bonds with the NOM
D instead of K for distribution ratio
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

25. For weak acids with only one acidic group:
Usually:
thus if pH < 2 + pKa then sorption of ionized species is usually negligible
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

26. therefore the sorption isotherm is non-linear and
Sorption of bases:
sorption of the cationic form to negatively charged sites in the NOM may dominate the overall sorption of the compound:
therefore the sorption isotherm is non-linear and
competition with other cations can occur
quinoline pKa = 4.9
sorption max at this pH
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

27. References
“Environmental Chemistry (a global perspective)” Gary W. vanLoon, Stephen J. Duffy; Oxford University Press, New York (2nd edition 2005), ISBN
“Environmental Soil Chemistry” Donald L. Sparks; Academic Press, Published ISBN
“Environmental Organic Chemistry ” ; Rene P. Schwarzenbach, Philip M. Gschwend and Dieter M. Imboden; 2nd Edition, John Wiley &L Sons, Inc. Copyright ISBN:
Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M. (2003). 2nd Edition John Wiley and Sons, New Jersey, ISBN
Environmental processing / Partitioning of pollutants / Sorption involving organic matter

28. References
Aboul-Kassim, T.A.T., Simoneit, B.R.T., Chemistry and Modeling. Springer-Verlag, Berlin. P , ISBN:
Allen-King, R.M., Grathwohl, P., Ball, W.P., Advances in Water Resources 25,
Environmental processing / Partitioning of pollutants / Sorption involving organic matter