Transcript Evolution of Groundwater Chemistry

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