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Organic Chemicals Fate in the Environment

Organic Chemicals Fate in the Environment. Objective to understand the basic chemical, physical and biological factors that affect the fate of organic compounds in the environment. References Sawyer Mannahan. Organic Chemicals. Definition Carbon Chemistry

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Organic Chemicals Fate in the Environment

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  1. Organic Chemicals Fate in the Environment • Objective • to understand the basic chemical, physical and biological factors that affect the fate of organic compounds in the environment. • References • Sawyer • Mannahan

  2. Organic Chemicals • Definition • Carbon Chemistry • Large Variability of Structure and Reactivity • few types of atoms • C, H, N, O, S, P, F, Cl, Br, • Structure gives properties, properties dictate impact in the environment • Nomenclature • IUPAC system • isomers • Functional Groups give chemical reactivity

  3. Origin of Organic Chemicals • Natural • Eg All living things • Eg Breakdown products of the above • complex polymers • aromatics Tryptophan, Tyrosine • Anthropogenic • Manmade chemicals • 95% are organic - crude oil feedstocks • 70,000 daily use • 3 billion tons oil/year • PCBs > 1 million tons total • Red List - EPA priority pollutants

  4. Toxicity • Structures critical • PAH • Benzo(a)pyrene carcinogenic. • pyrene not carcinogenic • Safe Levels • Maximum Contaminant Level (MCL) • generally ppb (< I g/l) in water for pesticide, chlorinated aromatics, PCB, Dioxin (0.01 g/l) • Much higher levels stable in sediments

  5. Physical Factors • Temperature • pH • H+ and OH- are catalytic (e.g. TKN analysis) • Phase Partitioning • adsorption • stabilization - diffusion restricted • destabilization - access, concentration • catalytic properties of clays • absorption • Organic Matter • stabilization, occluded sites (protected)

  6. Physical Factors • Hydrophobicity • partition between hydrophobic solids and water • Octanol Water Distribution units (mole.l-1.mole-1.l) dimensionless low values mean greater affinity for water phase KOW value for PAH c. 104 KOW value for hydrocarbons c. 104

  7. Colloids • Large organic molecules (i.e. small particles) • Size 0.001 to 1 m • Surface Area - 600 m2 per gram • Hydrophobic or Hydrophilic • Charged - cation exchange capacity (CEC) • bind metals, organics • Electrical Double Layer • Stern Layer, Diffuse Layer • stabilizes the colloidal ‘Sol’ • Charge varies with pH • point of zero charge (isoelectric point)

  8. Physical Factors • Volatilization • Henry’s Law 1. Solubility of gases - Kh 2. Volatility of Dissolved Species H is in units (atm.l / mole) i.e. high values (>10 atm.l.mol-1) mean high fugacity (will escape from water to air) • PAH H  10 -2 atm.l.mol-1 • hydrocarbons H  10 2 - 10 4 atm.l.mol-1

  9. Sorbed Xenobiotic Chemicals • Environmental persistence, slow degradation • Organometals • Methyl-mercury, Phenyl-mercury • Organo-arsenic • Polychlorinated Biphenyls (PCB) • Pesticides • DDT, Dieldrin, Lindane • Polycyclic Aromatic Hydrocarbons • Dioxins • Organochlorines • CFC - volatile • Solvents - volatile

  10. Chemical factors • Redox Reactions (abiotic) • Aldehydes - oxidize aldehyde  acid • Dihaloalkanes reductive dehalogenation dichloroethane  ethene + chloride • Nitroaromatics - reduction nitro aromatic  aromatic amine

  11. Light Induced Reactions • Carbon compound excited by quanta of light C C* • Vibrational loss (heat) • Light emission (fluorescence) • Fragmentation, Dimerisation • Intramolecular rearrangement (isomerise) • Redox (H+ abstraction, e- transfer) • Pass on Energy to another compound called Photosensitivity • Sensitizers help mediate this (e.g. acetone) h

  12. h Photo-oxidation may occur in a number of ways Se h h * E Se O2 Organic Xenobiotic O Oxidised Organic * O

  13. Photosensitivity • A Sensitizer can act in two ways 1. Pass on energy directly to Xenobiotic 2. Activate an Oxygen atom to Singlet state -ROO , HO , (H2O2 may be formed) oxygen then reacts with Xenobiotics.

  14. May lead to mineralisation • Many different reactions Oxidation, reduction and hydrolysis • Much faster than spontaneous reactions • Unidirectional • Bacteria derive energy and/or nutrient • Dependent on environment • Catalysed by certain Key Enzymes • Strategy - modify xenobiotic, use existing metabolic pathway Biological (Microbially Mediated Reactions)

  15. Enzymes • Oxygenase • produce Catechol from Xenobiotic • Catechol undergoes ring cleavage by meta-pathway chlorine substituents can give Dead-End Metabolite

  16. Cytochrome P450 Monooxygenase • Wide substrate specificity • Involves activation of Oxygen to O+ (electrophilic oxygen) • Redox state of Iron in Cytochrome drives • O+ reacts with Xenobiotic (addition reaction) • good for hydrophobic Xenobiotics • Creates a more Hydrophilic product which can take part in other enzyme reactions.

  17. Enzymes • Reduction by Cytochromes • A Redox reaction which reduces the Xenobiotic • Hydrolysis • similar to abiotic process but FASTER • e.g. Dehalogenation

  18. Bioaccumulation • Xenobiotics not degraded • Partition in fatty tissues • e.g. DDT DDT Location Concentration (mg/l) water 0.00005 plankton 0.04 fish 0.2 - 0.9 Predatory fish 1.3 - 2.0 Fish eating birds 22.8 • Enrichment Factor or Concentration Factor • Up to 75,000 for some pesticides - Dieldrin, Aldrin

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