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 Chemistry Introduction 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 Chemistry The Diversity of Organic Compounds Categorized according to molecular weight, volatility and/or reactivity

Environmental Organic Chemistry The 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

Environmental Organic Chemistry The Diversity of Organic Compounds 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 Categorized according to molecular weight, volatility and/or reactivity

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 Chemistry Chemical 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

Environmental Organic Chemistry Chemical Partitioning 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

Environmental Organic Chemistry Chemical Partitioning Important Partitioning Coefficients Vapor pressure (solubility in air) and aqueous solubility explain the partitioning of chemicals Kaw Koa Kow

Environmental Organic Chemistry Chemical Partitioning Important Partitioning Coefficients 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

Environmental Organic Chemistry Chemical Partitioning 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 Dec. aqueous solubility

Environmental Organic Chemistry Chemical Partitioning 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. aqueous solubility

Environmental Organic Chemistry Chemical Partitioning 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 Baird and Harrison

Environmental Organic Chemistry Chemical Partitioning 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 or Kw= 1000000 BCF for DDT lies 20000 - 400000 Hence KOw can be used to predict BCF Higher the KOw more likely chemical is bound to organic matter in soil and fatty materials See Bunce (1994) for a detailed look at pharmacokinetics

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

Environmental Organic Chemistry Chemical Partitioning 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

Environmental Organic Chemistry Chemical Partitioning Table 3.2: Cd2+ on sediment Designates exp. Measurements

Environmental Organic Chemistry Chemical Partitioning Kp usually more constant Methoxychlor (pesticide) on clay Hydrophobic organic pollutant Karickhoff et al., 1979

Environmental Organic Chemistry Chemical Partitioning Hydrophobic pollutants sorb more with greater OM content Slope here is: Kp/fOC = KOC Karickhoff et al., 1979

Environmental Organic Chemistry Chemical Partitioning Important Partitioning Coefficients 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

Environmental Organic Chemistry Chemical Partitioning Important Partitioning Coefficients 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

Environmental Organic Chemistry Chemical Partitioning 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

Environmental Organic Chemistry Chemical Partitioning

Environmental Organic Chemistry Introduction Octanol-Air Partition Coefficient A

Environmental Organic Chemistry Introduction Temperature Dependence A

Environmental Organic Chemistry Introduction Partition Maps A

Environmental Organic Chemistry Chemical Transformation and Degradation

Environmental Organic Chemistry Chemical Transformation Through Photochemistry

Environmental Organic Chemistry Chemical Transformation Through Photochemistry

Environmental Organic Chemistry Chemical Transformation Through Photochemistry

Environmental Organic Chemistry Chemical Transformation Through Photochemistry

Environmental Organic Chemistry Chemical Transformation Through Photochemistry

Environmental Organic Chemistry Summary

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.