advanced environmental chemistry 1

8/12/2019 Advanced Environmental Chemistry 1

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ADVANCED

ENVIRONMENTAL

CHEMISTRY

TRUONG QUY TUNG, PhD

HUE UNIVERSITY

Lectures for Master Student of Environmental Sciences

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CHAPTER1.

PRINCIPLESOFCONTAMINANT

BEHAVIORINTHEENVIRONMENT

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1.1. THE BEHAVIOR OF CONTAMINANTS IN

NATURAL WATERS

Every part of our world is continually changing, the unwelcomed

contaminants as well as the essential ecosystems.

Some changes occur imperceptibly on a geological time scale;

others are rapid occurring within days, minutes, or less.

Control of environmental contamination depends onunderstanding how pollutants are affected by environmental

conditions, and learning how to bring about desired changes.

For example, lead dangerous to our health are often more soluble in

water under acidic conditions than under basic conditions.

to remove dissolved lead from drinking water by raising the pH andmaking the water basic, a large part of dissolved lead can be made

to precipitate as a solid and can be removed by settling out or

filtering.

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CONTAMINANTSINTHEENVIRONMENTAREDRIVENTO

CHANGEBY

Physical forces Move contaminants to new locations, often without

significant change in their chemical properties.

Wind, gravity, water flow; increase in temperature;

Electrostatic attractions Chemical changes

Oxidation and reduction: break chemical bonds and allowatoms to rearrange into new compounds.

Biological activity (whereby microbes) Break down contaminant molecules and return their atoms

to the env. cycles that circulate C, O, N, S, P & otherelements repeatedly through our ecosystems.

Biological processes are a special kind of chemical change.4


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IMPORTANT PROPERTIES OF POLLUTANTS

The six most important properties for predicting the

environmental behavior of a pollutant:

1. Solubility in water

2. Volatility3. Density

4. Chemical reactivity

5. Biodegradability

6. Tendency to adsorb to solids

If not known, these properties often can be estimated

from the chemical structure of the pollutant. 5


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WATERPROPERTIES

Temperature

Water quality

Chemical composition,

pH, Oxidation-reduction potential,

Alkalinity, hardness,

Turbidity,

Dissolved oxygen, biological oxygen demand,

Fecal coliforms, etc.

Flow rate and flow pattern6


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PROPERTIESOFSOLIDSANDSOILSINCONTACTWITH

WATER

Mineral composition

Percentage of organic matter

Sorption attractions for contaminants (sorption

coefficients) Mobility of solids (colloid and particulate movement)

Porosity

Particle size distribution

Hydraulic conductivity

The properties of environmental waters and soils are

always site-specific and must be estimated or measured in

the field.

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1. 2. PROCESSES THAT REMOVE POLLUTANTS

FROM WATER

Volati l ization: Dissolved contaminants move from water or

soil into air, in the form of gases or vapors.

Sorpt ion: Dissolved contaminants become bound to solids

by attractive chemical and electrostatic forces.

Precipi tat ion: Dissolved contaminants are caused to

precipitate as solids by changes in pH or oxidation-

reduction potential, or they react with other species in

water to form compounds of low solubility. Precipitationoften produces finely divided solids that will not settle out

under gravity unless sedimentation processes occur.

Sedimentat ion: Small suspended solids in water grow

large enough to settle to the bottom under gravity.

TRANSPORT PROCESSES

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Two stages to sedimentation:

a. Coagulation: Suspended solids generally carry an

electrostatic charge that keeps them apart.Chemicals may be added to lower the repulsive

electrostatic energy barrier between the particles

(destabilization), allowing them to coagulate.

b. Flocculation: Lowering the repulsive energybarrier by coagulation allows suspended solids to

collide and clump together to form a floc. When floc

particles aggregate, they can become heavy

enough to settle out.

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Photo lys is: exposure to sunlight can break chemical bonds and

start chemical breakdown.

Complexat ion and chelat ion: Polar or charged dissolved species

(such as metal ions) bind to electron-donor ligands to form complex

or coordination compounds. Complex compounds are often soluble

and resist removal by precipitation because the ligands must be

displaced by other anions before an insoluble species can be

formed.

Acid-base: Protons are transferred between chemical species.

Oxidat ion-reduct ion (OR, redox): Electrons are transferred

between chemical species, changing the oxidation states and the

chemical properties of the electron donor and the electron acceptor.

Hydro lys is and hy dration: A compound forms chemical bonds to

water molecules or hydroxyl anions.

Precipi tat ion: Two or more dissolved species react to form an

insoluble solid compound.

ENVIRONMENTAL CHEMICAL REACTIONS

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BIOLOGICAL PROCESSES (BIODEGRADATION)

Microbes can degrade organic pollutants by facilitating

oxidation-reduction reactions. Microbial metabolism: (biological reactions) convert

organic compounds into energy and carbon for growth.

Electron acceptors: (present in soil or water env.) receive

e- from a pollutant molecule by Bio.processes. (O2, CO2,NO3

, SO42, Mn2+, and Fe3+).

Aerobic biodegradation: O2electron acceptor is available

and preferred.

Anaerobic biodegradation: is otherwise Organic pollutants are generally toxic because of their

chemical structure. Changing their structure in any way

will change their properties and may make them

innocuous or, in a few cases, more toxic.

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1.3. MAJOR CONTAMINANT GROUPS AND THEIR

NATURAL PATHWAYS FOR REMOVAL FROM

WATER

METALS

Dissolved metals such as Fe, Pb, Cu, Cd, Hg, etc., areremoved from water mainly by sorption and precipitationprocesses.

As, Cd, Hg, Ni, Pb, Se, Te, Sn, and Zn can formvolatile metal-organic compounds in the naturalenvironment by microbial mediation volatilization isimportant mechanism.

Bioaccumulation of metals in animals can lead to toxic

effects but usually is not very significant as a removalprocess.

Phytoremediation: Bioaccumulation in plants

Biotransformation of metals: some metals are caused toprecipitate. 12


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CHLORINATED PESTICIDES

Atrazine, chlordane, DDT, dicamba, endrin, heptachlor,

lindane, etc.,

Removed from water mainly by sorption,

volatilization, and biotransformation.

Chemical processes (oxidation, hydrolysis, and

photolys) is appear to play a usually minor role.

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HALOGENATED ALIPHATIC HYDROCARBONS

1,2-dichloropropane, 1,1,2-trichlorethane,

tetrachlorethylene, ...

Mostly originate as industrial and household solvents.

Removed mainly by volatilization.

Under natural conditions, biotransformation and

biodegradation processes are usually very slow, (half-lives of tens or hundreds of years).

Development of engineered biodegradation

procedures useful remediation techniques with

short enough half-lives.

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FUEL HYDROCARBONS

Gasoline, diesel fuel, and heating oils: mixtures of

hundreds of different organic hydrocarbons.

The lighter weight compounds (benzene, toluene,

ethylbenzene, xylenes, naphthalene,

trimethylbenzenes, smaller alkanes, are removed

mainly by sorption, volatilization, andbiotransformation.

The heavier compounds (PAHs -polycyclic aromatic

hydrocarbons , fluorene, anthracene,

benzo(a)pyrene, phenanthrene, ) are removedmainly by sorption, sedimentation, and

biodegradation.

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INORGANIC NONMETAL SPECIES

Ammonia, chloride, cyanide, fluoride, nitrite, nitrate,

phosphate, sulfate, sulfide, etc.

Removed mainly by sorption, volatilization,

chemical processes, and biotransformation.

Many normally minor pathways can become

important, or even dominant, in special

circumstances (photolysis)

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1.4. CHEMICAL AND PHYSICAL REACTIONS IN

THE WATER ENVIRONMENT

Homogeneous - occurr ing ent i rely among

dissolved species.

Heterogeneous - occu rr ing at the l iquid -so l id-

gas interfaces.

Most environmental water reactions are heterogeneous.

Purely homogeneous reactions are relatively rare in

natural waters and wastewaters.

Among the most important reactions occurring at the

liquid-solid-gas interfaces are those that move pollutantsfrom one phase to another.

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Volatilization: At the liquid-air and solid-air interfaces,

Transfers volatile contaminants from water and solid surfaces into

atmosphere, and into air in soil pore spaces.

Most important for compounds with high vapor pressures.

Dissolution:

At the solid-liquid and air-liquid interfaces,

Transfers contaminants from air and solids to water.

Most important for contaminants of high water solubility.

Environmental mobility of contaminants dissolved in water isgenerally intermediate between volatilized and sorbed

contaminants.

Processes by which a pollutant becomes distributed (or is

partitioned) into all the phases it comes in contact with:

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Sorption: At the liquid-solid and air-solid interfaces,

Transfers contaminants from water & air to soils andsediments.

Most important for compounds of low solubility & lowvolatility.

Sorbed compounds undergo chemical and biologicaltransformations at different rates and by differentpathways than dissolved compounds.

Binding strength depends on the nature of the solidsurface (sand, clays, organic particles,...), and on theproperties of the contaminant.

General term of Sorption: including both adsorption and

absorption.Adsorption: binding to a particle surface.

Absorption: becoming bound in pores and passages within aparticle.

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PARTITIONING BEHAVIOR OF POLLUTANTS

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Some of the liquid diesel oil (commonly called free product)

flows downward under gravity through the soil toward thegroundwater table.

Before the spill, the soil pore spaces above the water table(called the soil unsaturated zone) were filled with air and water,and the soil surfaces were partially covered with adsorbedwater.

As diesel oil, which is a mixture of many different compounds,passes downward through the soil, its different componentsbecome partitioned among the pore space air and water, thesoil particle surfaces, and the oil free product.

After the spill, the pore spaces are filled with air containing

diesel vapors, water carrying dissolved diesel components, anddiesel free product that has changed in composition by losingsome of its components to other phases.

The soil surfaces are partially covered with diesel free productand adsorbed water containing dissolved diesel components. 22

What happens when diesel oil is spilled at

the soils surface


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Each different compound of diesel oil mixture has a uniquepartitioning, or distribution pattern.

Pore space air will contain mainly the most volatile components,the pore space water will contain mainly the most soluble

components, and the soil particles will absorb mainly the leastvolatile and soluble components.

Quantity of the free product diminishes continually as it movesdownward through the soil because a significant portion is lost toother phases. The composition of the free product also changescontinually because the most volatile, soluble, and stronglyabsorbed compounds are lost preferentially.

Chemical distributions attain quasi-equilibrium, with compoundscontinually passing back and forth across each phase interface.

As the remaining free product continues to change by losingcomponents to other phases (part of the weathering process), itincreasingly resists further change.

Since the lightest weight components tend to be the most volatile

and soluble, they are the first to be lost to other phases, and theremaining free product becomes increasingly more viscous andless mobile.

Severely weathered free product is very resistant to furtherchange, and can persist in the soil for decades. It only disappearsby biodegradation or by actively engineered removal

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Depending on the amount of diesel oil spilled, it is possiblethat all of the diesel free product becomes immobilized in thesoil before it can reach the water table. This occurs when themass of free product diminishes and its viscosity increases tothe point where capillary forces in the soil pore spaces canhold the remaining free product in place against the force ofgravity.

There is still pollutant movement, however, mainly in the non-free product phases. The volatile components in the vaporstate usually diffuse rapidly through the soil, moving mostly

upward toward the soil surface and along any highpermeability pathways through the soil.

New water percolating downward, from precipitation or othersources, can dissolve additional diesel compounds from theabsorbed phase and carry it downward. Percolating water canalso displace some soil pore water already carrying dissolved

pollutants, as well as free product held by capillary forces,forcing them to move farther downward.

Although the diesel free product is not truly immobilized, itsdownward movement can become.

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However, if the spill is large enough, diesel free

product may reach the water table before becoming

immobilized. If this occurs, liquid free product beinglighter than water, cannot enter the water-saturated

zone but remains above it, effectively floating on

top of the water table.

There, the free product spreads horizontally on the

groundwater surface, continuing to partition into

groundwater, soil pore space air, and to the

surfaces of soil particles.

In other words, a portion of the free product will

always become distributed among all the solid,liquid and gas phases that it comes in contact with.

This behavior is governed by intermolecular forces

that exist between molecules. 25


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INTERMOLECULAR FORCES

All molecules have attractive forces acting between them.

Volatility, solubility, and sorption processes all result from theinterplay between intermolecular forces.

The attractive forces (electrostatic in nature) created by anonuniform distribution of valence shell electrons around thepositively charged nuclei of a molecule regions carry net

positive and negative charges. Polar attractive forces:A charged region on one molecule is

attracted to oppositely charged regions on adjacent molecules(can be momentary electrostatic repulsive forces).

On average, however, molecular arrangements will favor thelower energy attractive positions, and the attractive forcesalways prevail.

(The most obvious demonstrations of IAF : the phase changes ofmatter that inevitably accompany a sufficient lowering oftemperature, where a cooling gas turns into a liquid and into asolid, when the temperature becomes low enough) 26


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Temperature dependent phase changes:

Attractive forces always work to bring order to molecular

configurations

Gases are always the higher temperature form of any

substance. If the temperature of a gas is lowered enough,

every gas will condense to a liquid, a more ordered state

Condensation manifests intermolecular attractive forces.

Boiling point, Freezing point

Volatility, solubility, and sorption:

Molecules of volatile liquids have relatively weak attractions to

one another. Thermal energy at ordinary env. T is sufficient to

allow the most energetic of the weakly held molecules to

escape from liquid and fly into the gas phase. Molecules in water-soluble solids are attracted to water more

strongly than to themselves. If a water-soluble solid is placed in

water, its surface molecules are drawn from the solid phase

into the liquid phase by attractions to water molecules.27


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PREDICTING RELATIVE ATTRACTIVE FORCES

Predict relative attractive forces between moleculespredictrelative solubility, volatility, and sorption behavior. The water solubility of a compound is related to the strength of the

attractive forces between molecules of water and molecules of thecompound.

The soil-water partition coefficient of a compound indicates the relativestrengths of its attraction to water and soil.

BoilingFreezing

Wax

Compounds that are highly soluble in water have strong

attractions to water molecules.Compounds that are found associated mostly with soils have

stronger attractions to soil than to water.

Compounds that volatilize readily from water and soil haveweak attractions to water and soil. 28


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Uniform and non-uniform electron distributions, resulting in nonpolar and polar

covalent chemical bonds. The use of a delta (d) in front of the + andsigns

signifies that the charges are partial, arising from a non-uniform electron charge

distribution rather than from the transfer of a complete electron.29

advanced environmental chemistry 1

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