Transcript Title

CHAPTER 7
WATER, THE ULTIMATE GREEN SOLVENT: ITS USES
AND ENVIRONMENTAL CHEMISTRY
From Green Chemistry and the Ten Commandments of
Sustainability, Stanley E. Manahan, ChemChar Research,
Inc., 2006
[email protected]
7.1. H2O: SIMPLE FORMULA, COMPLEX MOLECULE
Angled structure of the water molecule (next slide)
The water molecule is polar
• Positive ends toward anions
• Negative ends toward cations
Water molecule forms hydrogen bonds
The Water Molecule
The properties of water are due to the polar nature of the water
molecule and its ability to form hydrogen bonds.
7.2. IMPORTANT PROPERTIES OF WATER
Excellent solvent for salts, acids, bases, and substances that have H,
O, and N atoms capable of forming hydrogen bonds
Solvent in biological fluids, such as blood or urine
Water weathers minerals and transports dissolved minerals in the
geosphere
Transports nutrients to plant roots in soil
Many industrial uses
Very high surface tension—ducks float
Transparent to visible light enabling photosynthesis to occur in algae
Maximum density as a liquid at 4˚ C causing bodies of water to
become stratified with colder, denser layers on the bottom.
Important Heat Characteristics of Water
High heat capacity of 4.184 joules per gram per ˚ C (J/g-˚C)
Very high heat of fusion of 334 joules per gram (J/g)
Very high heat of vaporization of water is 2,259 J/g, water vapor
carries latent heat
7.3. WATER DISTRIBUTION AND SUPPLY
4.4. Water Utilization (in U.S.)
Trends in U.S, Water Use
Encouraging trends in water use in the U.S., the result of
• Water conservation efforts, especially in industry and agriculture
• Recycling water, including uses through several levels requiring
progressively lower water quality
• Replacement of spray irrigators with direct application of water to
soil including trickle irrigation
• Exact computer control of water usage
Where Earth’s Water is Found
About 97% of Earth’s water is in oceans
Most of the remaining water is in the form of solid snow and ice
Less than 1% of Earth’s water as water vapor and clouds in the
atmosphere, as surface water in lakes, streams, and reservoirs, and as
groundwater in underground aquifers
7.4. Bodies of Water and Life in Water
Stratification of a Body of Water Strongly
Affects Chemical and Biological Processes
Living Organisms in Water
A normal body of water is an ecosystem
• Based upon a food supply consisting of the biomass produced
photosynthetically by the algae and plants living in it:
6CO2 + 6H2O (sunlight)  C6H12O6 + 6O2 (7.4.1)
• Glucose, C6H12O6, is converted to other forms of biomass
• Algae and plants are producers that generate biomass
• Heterotrophic organisms, usually bacteria in water, metabolize
biomass:
C6H12O6 + 6O2  6CO2 + 6H2O
(7.4.2)
Biologically Mediated Processes in Water
Specialized bacteria in water can utilize oxidized chemical species
with high oxygen contents other than molecular O2 for oxygen
sources.
Example: Nitrate ion, NO3-, acts as an oxidizing agent in the
bacterially-mediated biodegradation of biomass:
C6H12O6 + 3NO3- + 6H+  6CO2 + 3H2O + 3NH4+ (7.4.3)
By mediating chemical reactions, such as the one above,
microorganisms, particularly bacteria, largely determine the
chemistry that occurs in water.
Dissolved oxygen in water is very important.
Biodegradable organic pollutants cause biochemical oxygen
demand, BOD.
7.5. CHEMICAL PROCESSES IN WATER
Biochemical processes including photosynthesis
2HCO3- (sunlight energy)  {CH2O} + O2 + CO32- (7.5.1)
• {CH2O} represents biomass
Acid-base reactions
CO32- + H2O  HCO3- + OH- (7.5.2)
Precipitation reactions
Ca2+ + CO32-  CaCO3(s) (7.5.3)
Oxidation-reduction reactions, usually carried out by bacteria are
generally ones in which chemical species gain or lose oxygen
Example: Oxidation of S in H2S
H2S + 2O2 SO42- + 2H+ (7.5.4)
7.6. FIZZY WATER FROM UNDERGROUND
Natural waters contain dissolved gases.
• Dissolved oxygen required by fish
• Dissolved carbon dioxide in some mineral waters
• Carbon dioxide in Lake Nyos in the African country of Cameroon
which asphyxiated 1,700 people in 1986
Henry’s Law for gas solubilities states that the solubility of a gas in
a liquid is proportional to the partial pressure of that gas in contact
with the liquid.
• Gas solubility decreases with increasing temperature
Oxygen in Water
At 25˚ C the concentration of oxygen dissolved in water is only about
8 milligrams per liter of water (mg/L)
• Readily consumed by biodegradation of biomass (abbreviated
{CH2O}) by oxygen-utilizing bacteria:
{CH2O} + O2  CO2 + H2O
(7.6.1)
• Only about 8 mg of {CH2O} consumes 8 mg of O2
7.7. (WEAK) ACID FROM THE SKY
An acid is a substance that contains or produces H+ ion in water,
whereas a base is a substance that accepts H+ ion in water or contains
or produces hydroxide ion, OH-.
Whether water is acidic or basic is expressed by pH:
• pH = -log [H+]
(7.7.1)
• [H+] is the molar concentration of H+ in water, that is, the number
of moles of this ion per liter of water.
[H+], mol/L
log[H+]
pH
0.100
-1.00
1.00
1.00  10-3
-3.00
3.00
1.00  10-5
-5.00
5.00
1.00  10-9
-9.00
9.00
Acid in Water (Continued)
The value of [H+] in pure water at 25˚ C is 1.00  10-7 mol/L and the
pH is 7.00.
• Such water is neutral, neither acidic nor basic.
• Water with a pH less than 7.00 is acidic, whereas water with a pH
greater than 7.00 is basic.
The average global concentration of CO2 gas in air in the year 2001
was about 370 parts per million by volume, and going up by about 1
ppm per year.
• The concentration of dissolved carbon dioxide, [CO2(aq)], in water in
equilibrium with 370 ppm atmospheric air at 25˚ C is 1.21  10-5
mol/L.
• Makes water slightly acidic because
CO2 + H2O  H+ + HCO3-
(7.7.2)
• [H+] = 2.3  10-6 mol/L corresponding to a slightly acidic pH of 5.6
7.8. WHY NATURAL WATERS CONTAIN ALKALINITY AND
CALCIUM
Water alkalinity is the ability of water to react with and neutralize
acid (H+).
• Due to presence of bicarbonate ion, HCO3-, which can react as
follows with H+ ion:
HCO3- + H+  CO2(aq) + H2O
(7.7.3)
Water hardness in the form of dissolved Ca2+ ion
Both water hardness and alkalinity are acquired when water
containing dissolved CO2 reacts with limestone, CaCO3:
• CO2(aq) + CaCO3(s) + H2O  Ca2+(aq) + 2HCO3-
(7.7.4)
Carbon Dioxide and Carbonate Species in Water
Atmospheric CO2 dissolved in water, and from biodegradation
HCO3- dissolved in water
Solid carbonates (CaCO3) in mineral formations in contact with water
7.9. METALS IN WATER
Metal ions in water are present as hydrated ions, such as Ca(H2O)62+.
Bound water molecules can be displaced reversibly by other species.
• Such species include chelating agents, which can bond to metal
ions in 2 or more places to form a metal chelate.
• One such chelating agent is the nitrilotriacetate anion used in some
cleaning formulations and capable of bonding to a metal ion on 4
separate sites
• Chelates tend to be particularly stable, and they are very important
in natural water systems.
• Chelates are involved in life systems; for example, blood
hemoglobin is a chelate that contains Fe2+ ion bonded
simultaneously to 4 N atoms on the hemoglobin protein molecule
Humic Substances in Water
Water in nature may contain naturally-occurring chelating agents
called humic substances that are complex molecules of variable
composition left over from the biodegradation of plant material.
Humic substances bind with Fe2+ ion to produce gelbstoffe (German
for “yellow stuff”) which is very difficult to remove by water
treatment processes.
Humic substances produce trihalomethanes, such as chloroform,
HCCl3 during disinfection of water by chlorine
7.10. Water Interactions With Other Phases
Most important chemical and biochemical processes in water occur
at interfaces between water and another phase (usually solid)
Sediments
Sediments are variable mixtures of minerals, clay, silt, sand, and
organic matter
Formed by
• Erosion
• Sloughing of banks into water
• Washed in from watersheds
Chemical reactions, for example, as the result of photosynthesis:
• Ca2+ + 2HCO3 - + h {CH2O}(s) + CaCO3(s) + O2(g)
• Deposits solid CaCO3 (limestone)
• Deposits biomass, {CH2O}
Colloids in Water
Very small particles suspended in water
Size ranging from very large molecules up to about 1m
Scatter light (Tyndall effect)
Unique characteristics
• High surface/volume • High interfacial energy
• High surface/charge
Behavior and stability of colloids are important in aquatic chemical
phenomena
• Formation of sediments
• Dispersion and agglomeration of bacterial cells
• Dispersion and removal of pollutants
• Waste treatment processes
7.11. HEAVY METAL WATER POLLUTANTS
The heavy metals are those metals of relatively higher atomic
numbers, many of which are toxic
Cadmium, Cd, widely used in metal plating and other industrial
applications, is highly toxic.
Toxic lead, Pb (from its Latin name of plumbum), is the most
common heavy metal pollutant because of its widespread use in
industry, in the manufacture of lead storage batteries, formerly as a
leaded additive to gasoline, as a pigment in white house paint, and as
an anticorrosive primer applied prior to painting steel.
Mercury, Hg, in water caused poisoning in the Minamata Bay area
of Japan.
Arsenic is a toxic metalloid
• Arsenic compounds used for pesticides were lead arsenate,
Pb3(AsO4)2; sodium arsenite, Na3AsO3; and Paris Green,
Cu3(AsO3)2.
• Arsenic-contaminated wells in Bangladesh
Organically Bound Metal Water Pollutants
Organically bound metals may be considerably more mobile and in
some cases more toxic than hydrated metal ions in water.
Metals bound as chelates discussed in Section 7.9.
Metals bonded directly to carbon in hydrocarbon groups such as the
methyl group (-CH3) to produce organometallic compounds.
Methylmercury compounds found in Lake Saint Clair in 1970
released from chloralkali process for making chlorine and sodium
hydroxide
• HgCl2 (action of anoxic bacteria)  CH3HgCl + Cl- (7.11.1)
• The monomethylmercury ion in this compound, CH3Hg+, is soluble
and mobile in water and the dimethylmercury, (CH3)2Hg, also
produced is volatile as well. These mobile species were released
from the sediments and concentrated in fish tissue.
Organotin compounds used as industrial biocides and to prevent
growth of organisms on boat and ship hulls have been common water
pollutants
Heavy Metal Pollutants and Green Chemistry
Regulate releases
Better to find substitutes
7.12. INORGANIC WATER POLLUTANTS
Cyanide as HCN or CN• As little as 60 mg of cyanide can be fatal to a human.
Cyanide is sometimes released to water, especially from metal
extraction operations (especially gold).
• Has caused some large fish kills
Green chemistry calls for use of substitutes for cyanide
Excessive levels of NH4+/NH3 cause water pollution.
Hydrogen sulfide, H2S, is a toxic gas with a foul odor that is
produced by anaerobic bacteria acting upon inorganic sulfate (see
Section 7.4), from geothermal sources (hot springs) and as a pollutant
from chemical plants, paper mills, textile mills, and tanneries.
Inorganic Water Pollutants (continued)
Microbial degradation under ground may generate carbon dioxide,
CO2, that exists as free carbon dioxide in water.
• Can be toxic to aquatic organisms
• Can make water corrosive because of its acidity and tendency to
dissolve protective CaCO3 coatings on pipe.
Nitrite ion, NO2-, generated by bacteria or from industrial sources.
• Can cause methemoglobinemia by converting the hemoglobin in
blood to methemoglobin, a form useless for transporting oxygen.
Nitrate ion, NO3-, is a more common water contaminant.
Acidity
Strong acid pollutants that cause water to have a low pH are very
damaging to organisms living in water and can make water corrosive.
From bacterial oxidation of iron pyrite, FeS2, to produce sulfuric
acid.
• 4FeS2(s) + 2H2O + 15O2  2H2SO4 + 2Fe2(SO4)3 (7.12.2)
Acid rain from dissolved HCl, H2SO4, and HNO3 can cause acidic
water pollution.
• Especially damaging to lakes that are not in contact with basic rock
Alkalinity and Salinity
Alkalinity due to salts such as sodium carbonate, Na2CO3
• Human activities can cause alkaline salts to be leached from soil.
Water salinity is due to dissolved salts, such as sodium chloride and
calcium chloride.
• From municipal water systems and irrigation
Water Pollutants That Are Just Too Nutritious
Growth of algae requires inorganic nitrogen (NH4+, NO3-),
phosphorus (H2PO4-, HPO42-), and potassium (K+).
• Sources include fertilizers put on soil to enhance crop growth, some
industrial pollutants, degradation of sewage in wastewater.
Excessive levels of algal nutrients cause algae to grow too well and
generate too much biomass.
• Biomass eventually dies and decays, which uses up all the oxygen
in the water and clogs a body of water with dead plant matter—
eutrophication.
• Eutrophication is usually curtailed by lowering phosphate (limiting
nutrient) input into bodies of water and streams.
• Lowering levels of phosphate-based detergent builders around 1970
has curtailed eutrophication.
7.13. ORGANIC WATER POLLUTANTS
Two extremes:
• Readily biodegradable organic compounds, such as biomass,
{CH2O}, that consume dissolved O2
• Refractory organic compounds, such as PCBs, that tend to
accumulate in sediments and in the lipid (fat) tissues of fish and
birds that eat fish
Oxygen-Demanding Substances
Biodegradation of biomass, {CH2O}, consumes oxygen.
• {CH2O} + O2CO2 + H2O
(7.13.1)
Biochemical oxygen demand, BOD, refers to the amount of oxygen
consumed in biodegrading the organics in a liter of water.
Sewage
In addition to BOD from fecal matter and food wastes, sewage
contains oil, grease, grit, sand, salt, soap, detergents, degradationresistant organic compounds, disease-causing microorganisms, and
an incredible variety of objects that get flushed down the drain.
Detergents in Sewage
• Foam from poorly degradable surfactants in sewage
• The problem was branched-chain alkyl benzene sulfonate, ABS
H
H C
H
H CH3 H CH3 H CH3 H
CH3 H
C
C C C
C C
C C
C
CH3 H CH3 H CH3 H CH3 H
CH3
• The problem was solved with
linear alkyl sulfonate (LAS)
surfactant that has a readily
biodegraded straight chain.
O
S O- + Na
O AB S
H H H H H H H H H H H H
H C C C C C C C C C C C C H
H H H
H H H H
LAS
O S O
O- + Na
H H H
7.14. PESTICIDES IN WATER
Include insecticides, herbicides, bactericides to control bacteria,
slimicides to control slime-causing organisms in water, and algicides
used against algae.
Originally, highly persistent DDT was the greatest problem, but its
use has been banned.
Now herbicides are the biggest pesticide problem because they are
applied directly to soil and get into water as runoff.
Insecticides That Have Been Water Pollutants
Example synthetic insecticides
Organochlorine compounds
Cl
Cl
H
C
Cl C Cl
DDT
Cl
Cl
Cl
H
Cl
Cl
C
C
C H
Cl C Cl
H
C
C
C
Cl
H
H
C h l ordan e Cl
Organophosphates
H3CO S
P O
H3CO
H O
H C C O C2H5
S
NO2
Hydrolizable bonds
H3C O P S C H
Me th yl parath ion
Mal ath ion
O
C O C2H5
CH3 O
Carbamates
O H
H
O
H
O C N CH3 H
C
N C OH
C arbaryl
H3C
H
O
O H
C arbam ic acid
CH3
C arbofu ran
O C N CH3
Insecticides (continued)
Example insecticides from natural sources
Insecticides from plants
H
H C
H
C H
C H
N H
CH3
Ni cotin e
N
OCH3
H3 CO
O
H
Rote n on e H C
O C
O
C
H
H
H H H H
H
H C C C C C
H
H3 C C
C O
C
O
H3 C
C
H3 C
H
C
C
O C
H H
H
C H
H
Pyre th ri n I
CH3
C C C
CH3
H
O
C H
H C CH3
C
H
H
Insecticides (continued)
Greatest water pollution problems from organochlorine insecticides.
• Dominant from 1940s to 1960s, especially DDT
• Not particularly toxic to humans and other animals
• Very persistent in the environment because they undergo
biodegradation only slowly
• Tendency to undergo bioaccumulation in fish and other organisms,
concentrating in fat tissue
• Undergo biomagnification through food chains
Insecticides (continued)
Organophosphate insecticides replaced organochlorine insecticides.
• Biodegradable with no tendency to undergo bioaccumulation.
• Act by inhibiting acetylcholinesterase enzyme involved with nerve
function (same action as chemically-related military poison “nerve
gases”)
• Can be very toxic; many people have been killed by parathion
• Malathion is only about 1/100 as toxic to mammals as is parathion
because mammals and other animals can break down the molecule
Carbamates are biodegradable and relatively safe
• Carbaryl (Sevin) is used to kill insects on lawns or gardens and,
because of its low toxicity to mammals, can be sprinkled on pets in
(often futile) attempts to rid them of flea infestations.
• Carbofuran is a plant systemic insecticide that is taken up by
roots and leaves and distributed through the plants
Naturally-Occurring Insecticides
Important insecticides derived from plants
• Rotenone extracted from the roots of certain kinds of legumes
• Nicotine from tobacco
• Pyrethrins extracted from dried chrysanthemum or pyrethrum
flowers
• Pyrethroids, synthetic analogs of pyrethrins
• Bt insecticide from Bacillus thuringiensis bacteria transferred to
plants by recombinant DNA techniques so that they can make their
own insecticide
Herbicides
Herbicides to control weeds and grass on cropland
Susceptible to being washed off fields by rainfall with a high
potential to become water contaminants
Commonly found in drinking water supplies, which may require
removal measures
Cl
Cl
C
Typical herbicides:
N H
C
H N
N
H N
CH3
H HN C
H H
C
N
H HN C
N
C N C H H C C
C C H
N
H C C
CH3
H H S im azi n e
H H
Atraz in e
H H
O H N H O
HO C C N C P OH
H
H OH Gl yph os ate
Glyphosate is the active ingredient in Monsanto’s Roundup herbicide.
Some genetically modified soybeans and other crops are “Roundup
ready”
The Infamous Dioxin
Dioxin, known chemically as 2,3,7,8-tetrachlorodibenzo-p-dioxin
H
(TCDD):
H
Cl C
Cl
C
C
O
C
C
C
C
C
C
C
C
H
O
C
H
Cl
Cl
2,3,7,8-Te trach l orodibe n z op-di oxi n (com mon ly cal le d dioxi n)
• Manufacturing byproduct (2,4,5-T herbicide) and generated in some
combustion processes
• Extremely stable and poorly biodegradable
• Accumulates in sediments
• Times Beach
7.15. POLYCHLORINATED BIPHENYLS (PCBs)
Polychlorinated biphenyls (PCBs) consist of a class of 209
compounds made from substituting from 1 to 10 chlorine atoms for H
atoms on biphenyl (see below):
H
H
C C
C
H C
C C
H
H
H
C C
C
C H
C C
H
H
H
Bi ph e n yl
H
Cl
C C
H C
C
C C
Cl
H
Cl
C C
C
C Cl
C C
H
H
H
A typical pol ych l ori n ate d biph e n yl
PCBs are notable for their extreme chemical and thermal stability,
resistance to biodegradation, low vapor pressure, and high dielectric
constants.
Widely dispersed and persistent in the environment
Once widely used for electrical and other applications
Accumulate in sediments
Hudson River sediments due to pollution by General Electric plants
7.16. RADIOACTIVE SUBSTANCES IN WATER
Radioactive isotopes, or radionuclides can get into water from either
natural sources or from the fission of uranium or plutonium in
nuclear power reactors or (formerly) above-ground weapons testing.
Radionuclides have unstable nuclei that evolve ionizing radiation.
• Alpha particles, helium atom nuclei composed of two neutrons and
two protons
• Beta radiation in the form of high-energy electrons
• Gamma rays, which are electromagnetic radiation that behaves like
very short wavelength, high-energy X-rays.
Radionuclides decay with specific half-lives that can range from
fractions of a second to millions of years.
Radionuclides in Water
Greatest concern with respect to radionuclides in water arises from
natural geological sources
• Alpha particle emitter radium-226, 226Ra, half-life 1620 years, is a
particular concern in drinking water.
Radionuclides from above-ground testing of nuclear weapons and
from reactor accidents (only one, 1986 Chernobyl accident)
• Alpha particle emitter radium-226, 226Ra, half-life 1620 years, is a
particular concern in drinking water.
• Strontium-90 (90Sr), half-life 28 years, which is in the same
chemical group as calcium and is incorporated into bone
• Cesium-137, (137Cs), an alkali metal that the body handles much
like sodium and potassium ions
• Iodine-131 (131I), half-life 8 days, that is attracted to the thyroid and
can impair its function and even cause thyroid cancer
Threat to water from nuclear weapons manufacturing and research
operations
• Plume of radioactive water from the Hanford works, Washington
7.17. WATER TREATMENT
Municipal Water
Aeration of water
• 4Fe2+ + 4O2 + 4H2O  4Fe(OH)3(s) + 8H+ (7.17.1)
• Gelatinous Fe(OH)3 product causes coagulation of
colloidal particles.
Municipal Water Treatment (Cont.)
Excessive levels of dissolved calcium along with bicarbonate ion,
HCO3-. (temporary hardness) can be removed from water by adding
lime, Ca(OH)2:
• Ca2+ + 2HCO3- + Ca(OH)2  2CaCO3(s) + 2H2O (7.17.2)
Disinfection of water by adding chlorine
• Cl2 + H2O  H+ + Cl- + HOCl (7.17.3)
Green Ozone for Water Disinfection
Ozone, O3 disinfects water
• 3O2(g) (electrical discharge)  2O3(g) (7.17.5)
Wate ri n
Ozone made from air using clean electrical
energy as needed for water disinfection is a
very good green chemistry practice.
Wate r
con tactin g
oz on e
• No storage of dangerous chemicals
O3 i n ai r
Fil te re d
air
C ompre ss or
Re fri ge rator
an d
de si ccator
Drie d
air
20,000 vol ts
O z on e
ge n erati on ,
3O2 2O3
Di si n fe cte d
wate r
Water for Industrial Use
Wide range of water quality and purification, depending upon use
Water recycling means recycling water back through a system for
essentially the same use.
Sequential use recognizes that several applications may require
water of successively lower quality.
Removal of salts by reverse osmosis
Pre ss u riz e d
wate r
Waste brin e
Me mbran e se l e ctive ly
pe rm eable to wate r
Pu rifie d wate r
Sewage Treatment
The primary objective of sewage treatment is to remove oxygendemanding substances from wastewater.
• Substances of mostly biological origin, abbreviated {CH2O}, that
undergo biodegradation and consumption of dissolved oxygen, thus
exerting a biochemical oxygen demand (BOD)
Three main categories of sewage treatment
(1) Primary treatment to remove grit, grease, and solids
(2) Secondary treatment to reduce BOD
(3) Tertiary treatment to further refine effluent water quality.
Removal of BOD in secondary wastewater treatment by
{CH2O} + O2  CO2 + H2O + biomass
(7.17.6)
• Trickling filter in which sewage is sprayed over rocks coated with
microorganisms
• Activated sludge process (next slide)
Activated Sludge Process for Wastewater Treatment
W as tew ate r con tai n in g
BO D, {C H2O }
Ae ration tan k
De gradati on of BO D
S lu dge
s ettli n g
Mi n e ral iz ati on of C , N, P, an d S
Ai r
Puri fied water
wi th re du ce d
BO D
S e ttle d s l u dge wi th vi able m icroorgan i s ms
Byprodu ct me th an e
fue l gas
Exce s s s lu dge to
an aerobic di ge s te r
2{C H2O }  C H4 + C O2
An ae robic di ges ter