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Organic Compounds

Organic Compounds. Organic Compound Properties. In general, not very soluble in water Uncharged or weakly charged Can exist as dissolved, solid, or gaseous phases Organic matter in water is composed of an almost infinite variety of compounds

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Organic Compounds

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  1. Organic Compounds

  2. Organic Compound Properties • In general, not very soluble in water • Uncharged or weakly charged • Can exist as dissolved, solid, or gaseous phases • Organic matter in water is composed of an almost infinite variety of compounds • Most dissolved organic matter in groundwater are humic acids • Very resistant to further biodegradation

  3. Measuring Organic Compounds in Groundwater • Dissolved organic carbon (DOC) (water passed through 0.45 μm filter) • DOC in groundwater typically low, ≤ 2 mg/L • Swamps and other wetlands can have much higher DOC values, ~60 mg/L

  4. Organic Compound Nomenclature • All organics have carbon skeletons with functional groups attached • Aliphatics: straight or branched chains • Aromatics: ring structure • Multi-rings = polyaromatics (PNAs or PAHs) • Heterocyclic: ring structure with atoms other than C in skeleton

  5. Organic Compound Functional Groups • Besides H, skeleton can have other functional groups attached to it which effect compound properties • Sites of reactivity or function • Impart important properties to organic compounds • Charge, polarity (sharing of electrons, affects solubility), acidity, adsorption, chelation • Alcohol (or hydroxyl): OH group • Most common • H dissociates, weak acid

  6. Ethanol

  7. Organic Compound Functional Groups • Carboxyl: R – COOH • Weak acids; e.g., acetic acid (CH3COOH) • Strong H+ donors/acceptors, increase solubility because of charge • Easily degraded

  8. Carboxyls Acetic Acid Formic Acid

  9. Organic Compound Functional Groups • Halogens (Cl-, Br-, I-, F-) • Can be naturally occurring, but contaminants associated with anthropogenic production such as pesticides (DDT), solvents (TCE), refrigerants (CFCs, PCBs) • Halogens strongly bonded to C atoms, stable compounds in the environment • Low solubility because weak H-bonding with H2O • Trihalomethanes form in chlorinated drinking water

  10. Halogens DDT TCE Generally the more halogen atoms, the more resistant to degradation

  11. Organic Compound Functional Groups • Amino: NH2 • Better proton acceptors, weak acids • From H bonds with H2O, increase solubility • Amino acids: building blocks of life

  12. Amino

  13. Organic complexes • Organic compounds can bond with ions • Especially important with respect to metals • Can increase metal solubility and mobility • e.g., natural waters commonly have Fe concentrations several orders of magnitude greater than the equilibrium solubility of iron hydroxide • Fe may form dissolved complexes with naturally occurring organic substances • Ligands = ion or molecule (usually organic) that binds to a metal atom • Have a negative charge

  14. Chelation • A special type of aqueous complex (strong bonds) • Most ligands: single bond site (unidentate) • Chelation: multidenate (2 or more bonds with cation/metal) • Multiple bonds decrease entropy, increase bond stability • Can increase mobility of metals significantly

  15. Chelation • Natural chelating agents • Humic and fulvic acids • Citric acid • Anthropogenic chelating agents • Polyphosphates: water softeners that complex with Ca2+ to inhibit precipitation of CaCO3 • NTA and EDTA: cleaning compounds, detergents, metal plating baths • Very stable in environment (EDTA also food preservative) and have been implicated in 60Co transport at Oak Ridge National Lab

  16. EDTA (C10H16N2O8)

  17. DOC in Natural Environments • Soils: O and A horizons are major source of DOC to soil water and groundwater • DOC decreases with depth in soil profile • Decomposition • Adsorption • Precipitation as a solid • Organic acids can control pH of soil and therefore mineral weathering • Al/Fe can complex with organics, increasing transport to lower horizons

  18. DOC in Natural Environments • Groundwater: • Usually < 2 mg/L since most is removed in soil zone • Can be higher under certain conditions, e.g., buried paleosols (ancient soils) • Can be important in increasing transport of metals and radioactive elements • Implicated in solubility of As in sand and gravel aquifers in Illinois • Rivers • Varies by climate, season, vegetation, and discharge • Low discharge, groundwater main source of water • High discharge, increasing % of soil water

  19. Organic Pollutants • Large number of synthetic organics, many of which find their way into the environment • Relevant properties: • Solubility • Adsorption • Density • Liquid/gas partitioning • Biodegradability

  20. Organic Pollutants • 3 main groups which cause most problems (due to abundance and toxicity) • Aromatic hydrocarbons: fuels, BTEX (benzene, toluene, ethylbenzene, xylene) • Chlorinated hydrocarbons: solvents, pesticides • PAHs: low solubility, but carcinogenic

  21. Solubility of organics • (Considering only synthetic organics of known chemical composition) • In general, hydrophobic • Repelled by water • Low solubility • Even though only slightly soluble in water, their equilibrium solubility can be 1000 – 1 million times greater than the regulatory MCL

  22. Solubility of organics • Usual measure of hydrophobicity (literally, “fear of water”) is octanol-water partitioning coefficient • Octanol (CH3(CH2)7OH) is a liquid (alcohol) • Octanol and water are immiscible fluids • i.e., they don’t mix (like oil-water) • Use of octanol is arbitrary, but it is a non-polar organic liquid (water is polar) • Polar solutes dissolve in polar solvents • e.g., alcoholic beverages are aqueous solutions of ethanol • Non-polar solutes dissolve better in non-polar solvents • e.g., hydrocarbons such as oil and grease that easily mix with each other, while being incompatible with water

  23. Octanol-water partitioning coefficient • Determine using batch tests • Mix octanol, water, and organic of interest, and measure concentration in both phases • Kow = Coctanol / CH2O • Kow = octanol-water partitioning coefficient • Coctanol = equilibrium concentration of compound in octanol • CH2O = equilibrium concentration of compound in water • Kow came from biosciences field, to determine behavior of organic compounds in living organisms

  24. Octanol-water partitioning coefficient • Kow correlated to water solubility • As Kow increases, hydrophobicity increases and solubility decreases • In general, Kow good 1st approximation for solubility, and also indicator of adsorption and bioaccumulation

  25. Some Kow values Decreasing solubility

  26. Adsorption of Organics • Also known as partitioning (between aqueous and solid phases) • Since most organics are hydrophobic, they tend to adsorb onto aquifer solids • Organics are attracted to the solid organic matter • Unlike charged surfaces, there is not a theoretical limit to the amount of adsorption that can occur • What often occurs is multilayer adsorption

  27. Adsorption of Organics • Perform batch experiments to determine partitioning coefficient between the dissolved and adsorbed amounts of a given organic • Plots of data have linear and nonlinear parts • Linear partitioning coefficient may be appropriate at low concentrations • At higher concentrations, Freundlich isotherms used • C* = KfCN • Kf = Freundlich partitioning coefficient • N = fitting parameter

  28. Different Isotherms

  29. Freundlich Isotherm • C* = KfCN • Make log-log plot of batch test data • Take the log of both the aqueous (C) and adsorbed (C*) concentrations • log C* = N log C + log Kf • N (slope) commonly 0.9 – 1.4 • log Kf = y-intercept

  30. Freundlich Isotherm log C* log Kf

  31. Adsorption of Organics • Can calculate Kd from Kow • As Kow increases, solubility decreases, Kd increases • Organic adsorption is a function of the amount of organic matter in the soil or aquifer material (foc) • As foc increases, adsorbed mass increases, Kd increases • foc usually ranges 1 – 5 % in soil • When foc < 1%, organic adsorption ≈ inorganic adsorption • Kd = Kocfoc • Koc = organic carbon partitioning coefficient

  32. Adsorption of Organics • Kd = KOCfOC • KOC = the ratio of the mass of a chemical that is adsorbed in the soil per unit mass of organic carbon in the soil per the equilibrium chemical concentration in solution • Normalized • KOC values useful in predicting the mobility of organic soil contaminants; • High KOC values = less mobile organic compounds • Low KOC values = more mobile organic compounds

  33. Adsorption of Organics • Koc is calculated from Kow values • Empirical equations developed based on type of organic • e.g., for aromatic/PAHs, log Koc = 0.937 log Kow – 0.006 • Steps for converting Kow to Kd • Look up Kow; Calculate Koc; Measure foc; Calculate Kd • For many organic chemicals, Kd strongly correlated to their aqueous solubilities • Main advantages: simple (look up), and foc easily measured

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