environmental modeling chapter 2: basic chemical processes copyright © 2006 by dbs

Environmental Modeling

Chapter 2:Basic Chemical Processes

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

Source: NASA

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-

H2ONO3

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

• 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

• 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

• 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

• 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 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-

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+

• 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)

• 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

• 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

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)

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

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

• 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

• 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

K3 = [H+][HCO3-] = 10-6.35

[H2CO3]

K4 = [H+][CO32-] = 10-10.33

[HCO3-]

K5 = [H+][OH-] = 10-14

CT is not constant due to exchange with atmosphere

Mass balance: CT = [H2CO3] + [HCO3-] = [CO3

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

• 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

• 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

Highly oxygenated 0.81 V(Oxic)

Anaerobic -0.40 V(Anoxic)

Chemistry (fate and transport) of metals and organics changes depending on EH

• 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

• Reduction products increase as 3-chloronitrobenzene is removed

3-chloronitrobenzene → 3-chlorophenylhydroxylamine → 3-chloroaniline

• REDOX changes chemical speciation, creates a more toxic product

What are the effects of changing pH and EH?

Transformation of 3-chloronitrobenzne under redox conditions

Increasing pH increases reaction rate, k exponentially

Decreasing EH increases reaction rate, k exponentially

Becoming more negative

More reducing

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

• 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

Carbohydrates

Porphyrins

Plant pigments

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

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!

environmental modeling chapter 2: basic chemical processes copyright © 2006 by dbs

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