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Organic Compounds

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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

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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

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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

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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

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Ethanol

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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

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Carboxyls

Formic Acid Acetic Acid

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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

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Halogens

DDT TCE

Generally the more halogen atoms, the more resistant to degradation

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Organic Compound Functional Groups

• Amino: NH2

– Better proton acceptors, weak acids– From H bonds with H2O, increase solubility– Amino acids: building blocks of life

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Amino

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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

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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

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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

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EDTA (C10H16N2O8)

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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

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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

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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

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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

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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

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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

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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

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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

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Some Kow values

Compound log Kow

Ethanol -0.284Ethyl acetate 0.6851-Pentanol 1.39

Nitrobenzene 1.84Benzene 2.14

Chlorobenzene 2.80Biphenyl 3.96

Pentachlorobenzene 4.99

Dec

reas

ing

solu

bilit

y

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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

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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

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Different Isotherms

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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

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Freundlich Isotherm

log C*

log Kf

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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

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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

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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