Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Human impact on the nitrogen cycle wikipedia , lookup
No-till farming wikipedia , lookup
Terra preta wikipedia , lookup
Soil salinity control wikipedia , lookup
Total organic carbon wikipedia , lookup
Soil microbiology wikipedia , lookup
Canadian system of soil classification wikipedia , lookup
Organosulfur compounds wikipedia , lookup
Soil contamination wikipedia , lookup
Organic Compounds 1 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 2 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 3 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 4 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 5 Ethanol 6 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 7 Carboxyls Formic Acid Acetic Acid 8 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 9 Halogens DDT TCE Generally the more halogen atoms, the more resistant to degradation 10 Organic Compound Functional Groups • Amino: NH2 – Better proton acceptors, weak acids – From H bonds with H2O, increase solubility – Amino acids: building blocks of life 11 Amino 12 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 13 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 14 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 15 EDTA (C10H16N2O8) 16 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 17 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 18 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 19 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 20 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 21 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 22 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 23 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 24 Compound Ethanol Ethyl acetate 1-Pentanol Nitrobenzene Benzene Chlorobenzene Biphenyl Pentachlorobenzene log Kow -0.284 0.685 1.39 1.84 2.14 2.80 3.96 4.99 Decreasing solubility Some Kow values 25 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 26 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 27 Different Isotherms 28 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 29 Freundlich Isotherm log C* log Kf 30 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 = Koc foc • Koc = organic carbon partitioning coefficient 31 Adsorption of Organics • Kd = KOC fOC – 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 32 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 33