Organic contaminants interact with soil in complex ways. Their solubility, log Kow, pKa, and Henry's Law constant determine how they partition between air, water, and organic matter in soil over time. This partitioning and other degradation processes influence the bioavailability and environmental fate of contaminants in the ecosystem.

1. Soil:Organic interactions
Dr. Stephen Johnson
s.j.johnson@gre.ac.uk

2. n-Alkanes
H2 H2 H2
CH4 Methane C C C CH3
H3C C C C Octane
H2 H2 H2
H3C CH3 Ethane
H2 H2 H2 H2
C C C C
H2 H3C C C C CH3 Nonane
Propane H2 H2 H2
C
H3C CH3
H2 H2 H2 H2 H2
C CH3 C C C C CH3 Decane
H3C C
Butane H3C C C C C
H2 H2 H2 H2 H2
H2 H2 H2 H2 H2 H2 H2
C C C C C C C Undecane
H3C C CH3 Pentane H3C C C C C CH3
H2 H2 H2 H2 H2
H2 H2 H2 H2 H2 H2 H2
C C CH3 C C C C C CH3
H3C C C Hexane H3C C C C C C Dodecane
H2 H2 H2 H2 H2 H2 H2
H2 H2 H2 H2 H2 H2
H2 H2 H2 C C
C C C C Tridecane
C C C Heptane H3C C C C C C CH3
H3C C C CH3 H2 H2 H2 H2 H2
H2 H2

3. Aromatics
EPA Priority PAHs
Naphthalene 9H-Fluorene
Acenaphthylene Acenaphthene
Phenanthrene Anthracene Fluoranthene Pyrene
Benzene Toluene Ethyl-benzene
Benzo[a]anthracene Benzo[e]acephenanthrylene
Chrysene
= Benzo[b]fluoranthene
o-Xylene m-Xylene
Benzo[k]fluoranthene Benzo[def]chrysene
= Benzo[a]pyrene
p-Xylene Dibenzo[a,h]anthracene
Benzo[ghi]perylene Indeno[123cd]pyrene

4. Polar molecules
O
O S O
O
O
O S O
O
Sulphuric acid mono-[4-(1-ethyl-decyl)-phenyl] ester
Linear alkylbenzene sulphonate
Sulfuric acid monododecyl ester
Sodium dodecyl sulphate
Sodium lauryl sulphate

5. Halogenated compounds
Cl CH3 Cl CH2
Cl Cl
Cl
1,1,1-Trichloroethane 1,1-Dichloroethene
Cl
H
Cl C Cl
Cl Cl
Cl Cl
1,1,2-Trichloroethene 1,1,2,2-Tetrachloroethene

6. Pesticides
H
H H O
H
O H H P
S
H
CH3 H SO
O N H
H O O H
H H H O
H
H O
H
H H
Malathion
Methyl-carbamic acid naphthalen-1-yl ester
Sevin(R)

7. Contaminant loss in soil
12
Contaminant concentration
10
8
A
6 B
C
4
2 ?
0
0 2 4 6 8 10
Time

8. Partitioning
Air
Aqueous Sorbed to organic
solution matter and clays
NAPL Bioaccumulation etc.

9. Partitioning
• Lipophilicity/hydrophobicity
• Air:water partitioning
• Solubility
• Ionising potential

10. Lipophilicity/hydrophobicity
• Octanol-water partition coefficient (Kow) is a measure of the
equilibrium concentration of a compound between octanol and water
that indicates the potential for partitioning into soil organic matter
(i.e., a high Kow indicates a compound which will preferentially
partition into soil organic matter rather than water)
• Kow is inversely related to the solubility of a compound in water.
• Log Kow is used in models to estimate plant and soil invertebrate
bioaccumulation factors
• Sorption to soil
– Log Koc = (0.52 x Log Kow + 0.62) x 1.72
• Uptake by plants
– Log (Root concentration factor – 0.82) = 0.77 x log Kow -1.52
• Bioconcentration factor (earthworms)
– Log BCF = 0.48 log Kow + 1.04

11. Soluble pollutant
Vadose Zone
Water table
Saturated Zone
Bedrock

12. LNAPL plume
Vadose Zone
Water table
Saturated Zone
Bedrock

13. DNAPL plume
Vadose Zone
Water table
Saturated Zone
Bedrock

14. pKa
• pH at which 50% is ionised
• Soil usually has a net –ve charge
• Low pKa → ↑ sorption
• High pKa → ↓ sorption
• http://www.chem.wisc.edu/areas/reich/pka
table/

15. Sorption
From http://www.ualberta.ca/~gray/Projects/Bioreactors.htm

16. Microorganisms
From http://cse.naro.affrc.go.jp/tomoki/bdcontents/microorganisms-e.html

17. Air/water partitioning
• Henry’s Law
• Describes partitioning between air and aqueous
solution
• p = K' c
c
– p is the partial pressure of the gas
– c is its molar concentration
– K'c is the Henry's law constant on the molar
concentration scale.
• Accurate when concentrations and partial
pressures are reasonably low.

18. “Aging”
100
90
Contaminant concentration
80
70
Degradable
60
50
40 Readily available
30
20 Recalcitrant
10
Non-extractable
0
0 2 4 6 8 10
Time

19. Summary
• Hydrophobic effects depend on solubility
and log Kow
• Electrostatic effects (e.g. sorption) depend
on pKa
• Volatilisation depends on Henry’s Law
constant
• All these factor influence “Bioavailability”