environmental chemistry chapter 6: environmental organic chemistry copyright © 2009 by dbs

Environmental Chemistry

Chapter 6:Environmental Organic Chemistry

Copyright © 2009 by DBS

Contents

• Introduction• The Diversity Of Organic Compounds• The Fate of Organic Compounds• Chemical Partitioning• Chemical Transformation and Degradation• Chemical Transformation Through Photochemistry• Conclusions

Environmental Organic ChemistryIntroduction

• EOC – Environmental Organic Chemistry

• Study of both natural and man-made organic chemicals

• Developed as an area of interest through developments in gas chromatography

• ‘Organic chemist’ – synthesizes organic chemicals under lab conditions, produces highest possible yield, at a designated purity

• ‘EOC’ – works under Earth’s near surface conditions, quantifies the same chemicals, by-products from environmental media (e.g. soil, water, biota)

Environmental Organic ChemistryThe Diversity of Organic Compounds

• Categorized according to molecular weight, volatility and/or reactivity



• More volatile C1-C6 compounds have impact on atmospheric photochemistry

• Oxidation of alkanes with OH radical to form peroxy (HO2) and alkoxy (RO2) radicals important for converting NO to NO2 and formation of ozone (Chp. 2)

• Compounds of higher molecular weight react o to form hydroxyl carbonyl products which comprise organic aerosols

• Semi-volatile organic compounds include the persistent organic pollutants (POP’s), aliphatic compounds (C chains >C15), normal, branched and cyclic alkanes and polycyclic compounds

• Humic substances in soils

POP’s

Phase out of POP’s by the 2004 Stockholm Convention


• Over 217,000 contaminated sites have been identified by EPA

• Chlorinated phenols and organochlorines enter the environment as emissions from manufacture, incineration, use as a wood and leather preservative (Creosote) and biocide. - find their way into soil, sludges and sediments)

• The scale and complexity of remediation sites pose problems in applying common treatment processes such as bioremediation

• Incineration is effective, but can produce harmful chemicals during incomplete combustion. Projected cost is also high ($1200/ton).

Environmental Organic Chemistry The Diversity of Organic Compounds

Identifying Sources of Hydrocarbons• Source appointment requires being able to distinguish between material of different

origins (biogenic vs anthropogenic)

• Carbon Preference Index (CPI) – concentration ration of odd:even numbered alkane chain lengths

e.g. recent biogenic n-alkanes favor odd-carbon numbered chain lengths, whereas aged organic material does not

CPI > 1 suggests that the aliphatic HC’s are biogenically derived

CPI < 1 suggests petrogenic (e.g. fossil fuel) derived sources

• PAHs and their derivatives are used to distinguish between pyrolytic and petrogenic sources

• Ratios of parent PAHs and parent PAH to alkylated versions may be used to identify petrogenic vs pyrolytic sources

Environmental Organic Chemistry The Fate Of Organic Contaminants

• 1000’s of organic chemicals of interest

• EPA’s US Office for Pollution Prevention and Toxics (OPPT) maintains lists of high production volume chemicals as well as persistent bioaccumulative toxins (PBTs)

• All must be risk assessed

• Knowledge of both the usage (quantity released) and its behavior (transport, partitioning and transformation)

• Once present in the environment EOCs are subject to two main processes:– Transport within and between phases– Chemical transformation driven by chemical and/or biochemical processes

Environmental Organic ChemistryChemical Partitioning

• Equilibrium partitioning

e.g. – between solid and liquid phases (dissolution)– Liquid and gas phases (volatilization)– Solution and solid (adsorption)– Solution and immiscible liquid (solvent partitioning)

• Net transport of organic chemical between phases is quantified according to a partition coefficient (K)

K1,2 = C1/C2

• Where C1 = concentration of chemical in phase 1, C2 = concentration of chemical in phase 2

• Units are usually mol/L (mg/L) for liquids and mol/kg (mg/kg) for solids


• Partition coefficient refers to one chemical species in each phase

• Ionizable chemicals in water may be present in both neutral and dissociated forms, each would have a separate partition coefficient

• For well-defined phases (pure water or pure liquid chemical) partitioning with another well-defined phase results in the use of the term partitioning constant (e.g. Henry’s Law constant)

• Distribution ratios (e.g. soil-water distribution ratio Kd) describe partitioning between a heterogeneous solid phase (soil) and soil-water (containing a variety of compounds)

• Distribution ratios account for all the speciated forms of a chemical and sorption mechanisms – it describes the distribution of a chemical rather than a partitioning


Important Partitioning Coefficients

Vapor pressure (solubility in air) and aqueous solubility explain the partitioning of chemicals

Kaw

Koa

Kow



Octanol-water Partition Coefficient

• (KOW) is a measure of the lipophilicity/hydrophobicity of a substance

KOW = concentration in octanol

concentration in water

• Non-polar chemicals partition in octanol and give a high KOW

• 1000’s to 10,000’s, usually use log scale, pKOW


• Dependence of KOW on increasing molecule size

Contaminant KOW Mol. Weight

Benzene 2.13 78

Napthalene 3.35 128

Phenanthrene 4.57 178

Pyrene 5.18 202

Benzo(a)pyrene 252

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• Chlorine substitution in organic compounds results in larger KOW values

• Due to a steric effect

Contaminant KOW Mol. Weight

Benzene 2.13 78

Monochlorobenzene 2.8 113

Hexachlorobenzene 5.5 178

Dec

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• Bioconcentration – uptake of a chemical by an aquatic organism via respiratory surface (gils/skin)

• BCF = bioconcentration factor (related to KOW)

• Usually normalized to lipid content of the organisms to account for differences between organisms


Bioconcentration factor, BCF

BCF = concentration of solute in organism

concentration of solute in water

From empirical studies:

BCF KOW x % by weight of fat

(assumes fatty tissues have reached equilibrium, assume 5 % fat content)

For DDT

log KOW = 6 orKw= 1000000

BCF for DDT lies20000 - 400000

Hence KOw can be used to predict BCF

Higher the KOw morelikely chemical is boundto organic matter in soiland fatty materials

Question

Fish (5.0 % body fat) taken from a particular lake were tested and found to contain 200 ppm DDT in their tissues.

Determine the concentration of DDT (pKOW = 6.2) in this lake.

Log KOW = 6.2, KOW = 106.2

BCF = KOW x (% body fat/100)

BCF = 1.6 x 106 x (5/100) = 7.9 x 104

BCF = concentration in fish / concentration in lake

Concentration in lake = 200 / 7.9 x 104 = 2.5 x 10-3 ppm = 2.5 ppb


• Organic carbon to water partition coefficient (Koc)

Koc = Coc (mol/kg) / Cw (mol/L)

• Units of Koc are L kg-1

• Koc can be estimated from Kow according to an empirically derived relationship

Koc = 0.35Kow (units are L kg-1)

• To describe partitioning as it relates to conc. Organic matter

Koc= Kd or Kp

Fraction of organic matter in sample

KOC = Kp/fOC

Environmental Organic Chemistry Chemical Partitioning

• Lab work: solution of known pollutant mass and water, soil/sediment, or DOM is mixed together for 3 days

• Solid and aqueous phases are separated by 0.45 μm filtration

Measurement in lab is an approximation!

Kd depends on pH, type of cations present, ionic strength, surface charge, solids-to-water ratio

Assumes reversible sorption


Total mass (g) of pollutant added to flask

0.00725

Mass of pollutant recovered in blank (mg)

0.00720

Mass of pollutant measured in water phase (mg)

0.00542

Volume of wate r (L)

0.0300

Conc. Of pollutant in water phase (mg/L)

0.181

Mass of pollutant on solid phase (mg)

0.00178

Mass of solid phase (kg)

3.58 x 10-5

Conc. Of pollutant on solid phase (mg/kg)

49.7

Kd 275

Table 3.2: Cd2+ on sediment

Designates exp. Measurements


• Kp usually more constant

• Methoxychlor (pesticide) on clay

• Hydrophobic organic pollutant

Karickhoff et al., 1979


Karickhoff et al., 1979

• Hydrophobic pollutants sorb more with greater OM content

Slope here is: Kp/fOC = KOC



Henry’s Law Constant and Air-Water partitioning (Kaw)

• Henry’s Law constant (H, units Pa/mol m3 )

H = p / Cw

• Where p = partial pressure of chemical in air (Pa), Cw = concentration in water (mol/m3)

• If PV = nRT

n/V = Ca = P/RT

• If Kaw = Ca/Cw = P/RTCw



Henry’s Law Constant and Air-Water partitioning (Kaw)

• Henry’s Law constant for organic chemicals varies tremendously

• e.g. short chain alkanes with high VP and low aqueous solubilities have high H compared to alcohols which have lower VP and higher aqueous solubility

• Pesticides generally have low VP, but lower solubility can result in high H values, these chemicals appreciably partition to air from bodies of water


Pesticides generally have low VP, but have very much lower solubility, can result in higher than expected H values, these chemicals appreciably partition to air from bodies of water


Octanol-Air Partition Coefficient• A


Temperature Dependence• A


Partition Maps• A

Environmental Organic ChemistryChemical Transformation and Degradation

• A

Environmental Organic ChemistryChemical Transformation Through Photochemistry

• A

Environmental Organic ChemistrySummary

• A

References

• Karickhoff et al., 1979• Mackay, D. (1982) Environmental Science and Technology, Vol. 16, pp. 274-274.• Maldonado, C. et al (1999) EST, Vol. 33, pp. 2693-2702.• Schwarzenbach, R.P., Gschwend, P.H. and Imboden, G.M. (2003) Environmental Organic Chemistry, 2nd

ed., Wiley Interscience.• Yunker, M.B. and Macdonald, R.W. (2003) Organic Geochemistry, Vol. 33, pp. 2693-2702.

environmental chemistry chapter 6: environmental organic chemistry copyright © 2009 by dbs

Documents

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cyclic alkanes

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lab conditions