environmental modeling chapter 2: basic chemical processes copyright © 2006 by dbs
TRANSCRIPT
Environmental Modeling
Chapter 2:Basic Chemical Processes
Copyright © 2006 by DBS
Quote“The noblest of the elements is water”
-Pindar, 476 B.C.
Concepts
• Liquid medium• Properties of water• pH• Concentration• Solubility• NOM• Vapor pressure• HLC• Reactions• pC-pH• Redox• Complexation• Equilibrium• Sorption• Transformation
The Blue Marble
71 % water
What is it?
Liquid Medium Water Cycle
Source: NASA
Compartments• Atmosphere• Land• Groundwater• Rivers lakes• Oceans
Liquid Medium Water Cycle
Source: NASA
Liquid Medium Water Cycle
Transport medium
volumes, residence times, fluxes
Largest reservoir – oceans
τ = 40,000 yr
Liquid Medium Uneven distribution of water
Region Total Renewable Water Resources
(km3 yr-1)
Total Water Withdrawals
(m3 yr-1)
Per Capita (m3 person-1)
Average % of Renewable Resources
Average % Used by Agriculture
Average % Used by Industry
World 43,249 3,414,000 650 - 71 20
Asia 11,321 1,516,247 1,028 29 79 10
Europe 6,590 367,449 503 9 25 48
Middle East/N. Africa
518 303,977 754 423 80 5
N. America 4,850 512,440 1,720 14 27 58
Subject to contamination
Using water at a reate faster than it can be supplied (>100 due to use of sea water)
• Water shrinks on melting (ice floats on water)
• Unusually high melting point
• Unusually high boiling point
• Unusually high surface tension
• Unusually high viscosity
• Unusually high heat of vaporization
• Unusually high specific heat capacity
• And more…
Unique Properties
Ice shrinks on melting as 15% H-bonds are lost
A certain mass of ice occupies more space than
the same mass of water
Unique Properties
Unusually high Mpt. and Bpt.
This increase in the ‘thermal window’ of liquid water from 25º to 100 º allows aquatic life to exist over a broader range of temperatures
H-bonding leads to vicosity and surface tension
Unique Properties Why H-Bonding is Important
Concentration Units
• ppm, ppb, etc. (assumes ρ = 1.00 g mL-1)
1 mg/L = 1 ppm
1 μg/L = 1 ppb
1 ng/L = ppt
1000. mg pollutant /L of H2O 1.00 g pollutant
1000mg
1.00 L H2O
1000. g H2O
=
1000. g pollutant
106 g H2O = 0.001 g/g 1000. parts per million
Concentration Units
• Molarity
• Molarity of species A written [A] in mol/L
Molarity (M) = mass (g) of substance
molar mass of substance (g/mol)
Volume of solution (L)
= moles of solute
L of solution
Chemical AspectspH
pH = - log [H+]
• H+ usually surrounded by water of hydration, written H3O+
• ‘Master Variable’ – controls parameters e.g. speciation
• Ranges 5.5 - 9
Chemical AspectsActivity
• Activity: measures the effective concentration of an ion taking into account interactions with ions that may mask it. In an ideal situation – rough calculations, A = [C], not possible in real world
A = [C]
Where = activity coefficient (a function of ionic strength) and [C] = the molar concentration
Ca2+ SO42-
H2O
H2ONO3
-
H2O
Surrounding ions balance charge and make central ion less mobile, activity accounts for this canceling out of concentration
Surrounding ions balance charge and make central ion less mobile, activity accounts for this canceling out of concentration
Fortunately, electronic interactions of ions do not depend on chemical nature, only charge
Chemical AspectsActivity
• Why bother with activity?
• End result of pollutant fate and modeling is to predict risk, risk is based on toxicity
• Water containing higher (nontoxic) dissolved salt content has lower toxicity
• Toxicity is governed by activity NOT concentration
Chemical AspectsActivity
• We can calculate if we know Ionic strength μ due to surrounding ions
μ = 0.500 CiZi2 = 0.500(C1Z1
2 + C2Z22 + …)
(Where Z = charge)
• This leads to the following relationship:
Electrolyte Molarity Ionic Strength (μ)
1:1 M 1 x M
2:1 M 3 x M
3:1 M 6 x M
2:2 M 2 x M
Doesn’t hold for MgSO4
Chemical AspectsActivity
• Mixtures of salts conform to empirically derived Debye-Hückel equation
• Where Z = ionic charge and α = hydrated radius of the ion you are calculating for
• Very important to use with high salt content, can also get from tables
log - 0.512 Z2
1 /305 at 25 oC Eqn 2.3
See example calculation in Dunnivant
Chemical Aspects Solubility
• Solubility in water (moles/L or mg/L)– Inorganic e.g. salts– Organic e.g. short chain alcohols, acetone etc.
• Persistant Organic Pollutnats– DDT, PCB’s listed insoluble but slightly soluble in ppm range– Hydrophobic, actively partition away from water– Bioconcentrate onto surface of MO’s such as algae
25 ppm Bird
2 ppm big fish
1 ppm sm fish
0.04 ppm plankton
0.00005 ppm water
x 106 increase
Woodwell et al., 1967
Bioconcentration and bioaccumulation
Chemical Aspects Solubility - Organics
Chemical Aspects Solubility - Organics
• Solubility of low solubility pollutants difficult to determine, may be off by 10 - 100
See Table 2.6
• Easier to determine relative solubility (one compared to another)
• SPARC – predictive model http://ibmlc2.chem.uga.edu/sparc
Note: wide range of solubilities10-6 to 106
SPARC
SPARC estimation program can be used to
obtain most constants
Chemical Aspects Solubility
• Many organic pollutants are pure liquids known as Non Aqueous Phase Liquids (NAPLs)– More or less dense than water (DNAPL and LNAPL)
• Do not mix with water however compounds will dissolve out of the NAPL into water
Chemical Aspects Solubilty - Inorganics
• Example in text p42 (with activities) but we will make it more complicated in Chapter 3
e.g. find maximum solubility of Cu+ if waste CuBr is in contact with rainwater. Ksp for CuBr = 5.3 x 10-9
CuBr(aq) → Cu+(aq) + Br-
(aq)
Ksp = [Cu+][Br-]
Ksp = 5.3 x 10-9 = x2, x = (Ksp)1/2 = 7.3 x 10-5 M Cu+
(7.3 x 10-5 M Cu+) (63.6 g/mol) = 4.6 x 10-3 g/L = 4.6 mg/L = 4.6 ppm Cu+
Chemical Aspects Solubilty - Inorganics
• Hazardous waste containing PbCO3, PbCl2, PbCrO4, PbF2, PbSO4, PbS
• Which form of lead will determine the maximum concentration of Pb in leachate from the waste?
Highest Ksp = PbCl2 leads to highest Pb2+
x = Pb2+, Cl- = 2x
Ksp = 1.7 x 10-5 = [x][2x]2 = 4x3
x = 0.016 M Pb2+
(0.016 M Pb2+) (207.2 g/mol) (1000 mg/g) = 3400 mg/L
Pollutant Ksp
PbCO3 7.4 x 10-14
PbCl2 1.7 x 10-5
PbCrO4 2.8 x 10-13
PbF2 3.6 x 10-8
PbSO4 6.3 x 10-7
PbS 3 x 10-28
Often reported concentrations exceed known values (due to second ‘dissolved’ phase) – particles (NOM)
Chemical Aspects Solubilty - Inorganics
• Presence of secondary ‘dissolved’ phase of colloidal inorganic or organic particles complicates things
• Dissolved phase is defined operationally as pollutant concentration present in a sample filtered through 0.20 or 0.45 μm filter
• Natural Organic Matter (NOM) found in dissolved phase (DOM)
• Many functional groups and hydrophobic centers
See Fig 2.7
Chemical Aspects Vapor Pressure
• Pressure of a compound’s vapor phase at STP
• Range: 0.77 to 10-12 atm
• Important when modeling atmospheric pollutants
• Determines rate of volatilization
Volatilize quickly
Chemical AspectsHenry’s Law
X(aq) X(g)
KH= P (atm)
M (mol/L)
Many pollutants have low VP but high KH
Preference for gaseous phase over water
Ratio of equilibrium vapor pressure to solubility
End
• Review
Reactions and Equilibrium
• Basic chemical reactions:– Precipitation– Acid-base– Oxidation reduction
Reactions and EquilibriumAcid-Base Chemistry
pC = -log10 [C] where [C] is the concentration of pollutant
Requirements:(i) Concentration of acid or base(ii) Equilibrium equations and dissociation constants
Also known as distribution or alpha plots
Reactions and EquilibriumAcid-Base Chemistry
All pH-PC have:OH- as a function of pHH3O+ as a function of pH
Example pC-pH system
CO32-/HCO3
2- system
System closed from atmosphere
H2O H+ + OH-
Kw = [H+][OH-] = 1.00 x 10-14 (at 25 ºC)
pH + pOH = 14.0
pOH = 14.0 – pH (where OH = C)
Reactions and EquilibriumAcid-Base Chemistry
Total concentration is broken into 2 or more regions depending on no. protons
CT = total concentration of acid or base
e.g, CO32- = 0.0200 M
Vertical lines are guides for adding:
CO32-, HCO3
-, H2CO3
Reactions and EquilibriumAcid-Base Chemistry
Larger the Ka the stronger the acid
Lower the pKa the stronger the acid
HCO3- = most acidic
H2CO3 = second most acidic
Vertical lines are guides for adding species:
HCO3-, H2CO3, CO3
2-
Reactions and EquilibriumAcid-Base Chemistry
• Lines may be derived mathematically (see p63-64)
• We will use computer program on the CD-ROM (pC-pH simulator)
• Usefullness – all ion concentrations can be estimated for any given pH value
Reactions and EquilibriumAcid-Base Chemistry
• Component of system exists as gas
• System is open to input from atmosphere
1. CO2(aq) CO2(g)
2. CO2(g) + H2O H2CO3
3. H2CO3 HCO3- + H+
4. HCO3- CO3
2- + H+
5. H2O H+ + OH-
Example pC-pH system
CO32-/HCO3
2- system
System open to atmosphere
K2 = [H2CO3] = 10-1.47
PCO2
K3 = [H+][HCO3-] = 10-6.35
[H2CO3]
K4 = [H+][CO32-] = 10-10.33
[HCO3-]
K5 = [H+][OH-] = 10-14
Reactions and EquilibriumAcid-Base Chemistry
CT is not constant due to exchange with atmosphere
Mass balance: CT = [H2CO3] + [HCO3-] = [CO3
2-]
Reactions and Equilibrium Acid-Base Chemistry
pC-pH Diagram (open system)
In an open system concentrations of all ions vary with changes in pH and PCO2 (see equations)
Intercept H+ - HCO3- is pH
Reactions and Equilibrium Redox Chemistry
• Electrode potential, EH
EH = 2.3 RT pε (Nernst eqn.) nF
At 25 ºC and 1 mol e-; reduces to EH = 0.059pε
EH range is from -0.25 to 0.74 V
pε = -log{e-}
• pε is typically 14 in aerated water at pH 7, may drop to 4 with reduced iron, or -4 with sulfide
Reactions and Equilibrium Redox Chemistry
• Oxidation in aquatic environment is controlled by MO’s – biologially mediated
• TEA’s – terminal electron acceptors e.g. food digestion
C6H12O6 + 6H2O(l) → 6CO2 + 24H+ + 24 e- -984 kJ/mol
[O2(g) + 4H+ + 4e- → 2H2O(l) -313.2 kJ/mol] x6
C6H12O6 + 6O2 → 6H2O + 6CO2 -2863. kJ/mol
• e- are removed from C, reduced C is oxidized
• Acceptor for these e- is O2, O2 is reduced (TEA)
energy release
Reactions and Equilibrium Redox Chemistry
• Common TEA’s in anoxic environments:
2NO3- + 12H+ + 10e- → N2 + 6H2O -714 kJ/mol
MnO2(s) + HCO3- + 3H+ + 2e- → MnCO3(s) + 2H2O -100.4
kJ/mol
FeOOH(s) + HCO3- + 2H+ + e- → FeCO3(s) + 2H2O 4.6 kJ/mol
SO42- + 9H+ + 8e- → HS- + 4H2O 170.4 kJ/mol
CO2 + 8H+ + 8e- → CH4 + 2H2O 188.0 kJ/mol
• ΔG determines which TEA is used first (combination yielding most energy)
• EH for each is different
Reactions and Equilibrium Redox Chemistry
Highly oxygenated 0.81 V(Oxic)
Anaerobic -0.40 V(Anoxic)
Chemistry (fate and transport) of metals and organics changes depending on EH
Reactions and Equilibrium Redox Chemistry
• EH is obviously important for metals (changes oxidation state)
• What about organics?
e.g. nitrobenzene reductionReducing conditions from high MO activity in water lowers EH
Exponential decrease of 3-chloronitrobenzene
Reactions and Equilibrium Redox Chemistry
• Reduction products increase as 3-chloronitrobenzene is removed
3-chloronitrobenzene → 3-chlorophenylhydroxylamine → 3-chloroaniline
• REDOX changes chemical speciation, creates a more toxic product
Reactions and Equilibrium Redox Chemistry
What are the effects of changing pH and EH?
Transformation of 3-chloronitrobenzne under redox conditions
Reactions and Equilibrium Redox Chemistry
Increasing pH increases reaction rate, k exponentially
Reactions and Equilibrium Redox Chemistry
Decreasing EH increases reaction rate, k exponentially
Becoming more negative
More reducing
Reactions and Equilibrium Redox Chemistry
Experiment described above used natural organic matter
Reactivity differs depending on type of OM
Complexation
• Speciation deals with distribution in environment (forms and phases)
• Need to know physical and chemical properties of form (species)
– Species are subject to interactions with other molecules or solutes
– e.g. metal ions exist as hydrated cations Ca2+.6H2O
Complexation
• Complex: central metal cation and anion ligand
– Example ligands: +ve, -ve or neutral
Cl-, OH-, CO32-, HCO3
2-, HPO42-, H2PO4
-, amino, humic, fulvic acids, EDTA, NTA
– e.g. Pb2+ and Cl- forming PbCl+
• No. ligands = coordination number, unidentate, bidentate, …chelates
• Inner and outer sphere types
End
• Review
Equilibrium Sorption Phenomena
• Constants (Ks) are an equilibrium concentration of a chemical in one phase divided by the concentration of the same chemical in a different phase
• Working at equilibrium is a simplification
surface
Metals Organics (hydrophobic)
adsorption(ion-exchange process)
partitioning (solvation process)
+adsorption if polar
Sorption = attraction = adsorption (metals) = partitiioning (organics)
Equilibrium Sorption Phenomena
Equilibrium SorptionSorption Surfaces
• Colloidal particles - do not settle out may be inorganic/organic combination– Inorganic sorption sites - coated with Fe, Mn oxyhydroxides; High
surface area/volume ratio
– Organics – consist of NOM: dissolved/sorbed/insoluble (humin)
• Classification – no official size 0.45 μm filter fraction called ‘dissolved’ (includes colloidal)
• Particles of interest – clays and NOM coatings for all pollutants• Fe/Mn minerals and precipitants particuarly for metals or polar
organics
Equilibrium SorptionSorption Surfaces - Clays
• Clay particle – any particle smaller than 2 μm
• Clay mineral – specific composition and structure
• Alluminosillicate sheet structure – alternates between SiO2 and Al2O3 structures
Equilibrium SorptionSorption Surfaces - Clays
montmorillonite
• Edge charge, depends on pH– OH group exposed from Si and Al
– Protonated at low pH, deprotonated at high pH (anionic)
• Isomorphic substitution and surface charge– Leads to deficiency of +ve charge, may pull metals from solution
• Measurement of surface charge– Point of zero charge (PZC) = pH of a soil suspension at the point
when net charge vanishes
– Titration measuring mobility under applied voltage
Equilibrium SorptionSorption Surfaces
• Factors affecting sorption– pH and ionic strength e.g. if pH increases, surface loses H+ becomes
anionic and absorbs more metals– Oxidation state of metal– Composition of salts (ionic strength)– Suspended solids
Equilibrium Sorption Organic Matter
• Strongly influences sorption
• Transformed into fulvic and humic acids and humin = NOM
Proteins
Lipds
Carbohydrates
Porphyrins
Plant pigments
NOM
MO’s and abiotic reactions
1000’s of different structures 500 – 5000 amu
NOM: twotypical structures
Note functional groups inTable 2.9. Some are ionicand some are nonpolar
We also have NOMhydrophobic centers
Characterized based on:
Chromatography
pH
Dissolved or sorbed
Fulvic acid – soluble (DOM)
Humic acid – ppt at pH 2
Humin – insoluble
Equilibrium Sorption Organic Sorbates
• Adsorption– Mn+ adsorb to –ve sites of clay mineral or deprotonated functional
group of NOM
• Sorption– Hydrophobic pollutants (e.g. PCBs) attracted and partitions into
hydrophobic centers in NOM molecules and hydrophobic mineral surfaces
Equilibrium Sorption Partition Coefficients
• Adsorption, Kd – metals accumulate at interface between solid and liquid
• Sorption, Kp – organics partition through induced-dipole interactions
• Pollutants on solid phase are less bioavailable = less toxic
• Particles aggregate and settle out (sink) adsorbate
adsorbent
Equilibrium Sorption Partition Coefficients
• PCBs in Lake Hartwell (S. Carolina)
Mixed sediment
PCBs buried by cleaner more recent sediments
Germann, 1988
Equilibrium Sorption Ion Exchange Model
• Mn+ with higher charge displaces resident H+, Na+ or K+ ion• Finite no. of sites expressed as CEC = cation exchange
capacity– The concentration of sorbed cations that can be readily
exchanged for other cations (meq/100 g soil)
• Affinity of soil/sediment for Mn+ increases with tendency to form inner-sphere complexes (small size, high charge)
Irving-Williams order:
Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+
Impacts of NOM on Ks
• Increase [NOM] on sorbate and you increase sorption of pollutant on sorbate
• Increase [DOM] in water you decrease sorption of pollutant to sorbate (you make it more soluble in the water phase)
Transformation/DegradationAbiotic
• Abiotic
• Biotic
• Photochemical
• Nuclear
Transformation/DegradationAbiotic
• Chemical reactions
• Bulk e- donor reduces abiotic e- mediator (Fe/Mn minerals, NOM) which reacts with pollutant
• Easier to study than biotic mechanisms (controlled pH/pE)
Transformation/Degradation Photochemical
• Form of abiotic reaction
• Breaks apart organics
– UV (200 nm, 0.2 μm)
– VIS (750 nm, 0.75 μm)
• UV and VIS break bonds, IR vibrates them
• Molecule may be:
– Excited
– Destroyed
• e.g. formation of OH*
Transformation/DegradationBiotic
• MOs use enzymes to degrade pollutants
• Every organic compound can be degraded by MOs under laboratory settings
– Work by mneralizing pollutants to stable inorganic forms of C, H, P, S etc.
– May not go to completion
• Rate of degradation measured in lab, extrapolated to field
• Study is complicated by the fact that abiotic reactions are also occuring
– Chemists can remove MOs, biologists cannot eliminate chemicals!
Further Reading
Journals
• Bell, J. and Bates, T.H. (1988) Distribution coefficients of radionuclides between soils and groundwater and their dependence on various test paramters. Science of the Total Environment, Vol. 69, pp. 297-317.
• Dunnivant, F.M., Macalady, D.L., and Schwarzenbach, R.P. (1992) Reduction of substituted nitrobenzenes in aqueous solutions containing natural organic matter. Environmental Science and Technology, Vol. 26, pp. 2133-2141.
• Mackay, D., Shiu, W.Y., and Sutherland, R.P. (1979) Determination of air-water Henry’s law constants for hydrophobic pollutants. Environmental Science and Technology, Vol. 13, pp. 333-337.
• Schnitzer, M. (1986) Binding of humic substances by soil mineral colloids. In: Interactions of Soil Minerals with Natural Organic and Microbes, Soil Science Society of America Special Publication No. 17.
• Tichnor, K.V. (1993) Actinide sorption by fracture-infilling materials. Radiochimica Acta, Vol. 60, pp. 33-42
• Woodwell, G.M., Wurster, C.F., and Isaacson, P.A. (1967) DDT residues in an east coast estuary: A case of biological concentration of a persistant insecticide. Science, Vol. 156, pp. 821-824.
Books
• Harris, D.C. (1999) Quantitative Chemical Analysis, 5th edition. W.H. Freeman & Co., New York.
• Hayes, M.H.B., MacCarthy, P., Malcolm, R.L., and Swift, S. Jr. (1989) Humic Substances II: In Search of Structure. Wiley-Interscience, New York.
• Suffet, I.H. and MacCarthy, P. (1989) Aquatic Humic Substances: Influence of Fate and Transport of Pollutants. Advances in Chemistry Series # 219, American Chemical Society, Washington D.C.
• Thurman, E.M. (!995) Organic Geochemistry of Natural Waters. Martinus Nijhoff/Junk, Boston, MA.
• Stumm W. and Morgan, J.J. (1996) Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters, 3rd Edition. John Wiley & Sons, New York.