ibchenvironment

IB Chemistry — 10 Environmental Chemistry

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10.1 — Primary Air Pollution Air pollutants are chemicals that are normally not present in the atmosphere, or chemicals that are at significantly higher levels than are normal. Primary air pollutants are chemicals that are released into the air before they undergo further reactions to become secondary air pollutants (see smog and acid rain).

Carbon Monoxide: CO Carbon monoxide is toxic. It binds with haemoglobin in red blood cells and prevents them from reacting in equilibrium with oxygen. Low levels of CO cause headaches and dizziness. Concentrations of ~1% causes death in minutes. CO is produced by the incomplete combustion of hydrocarbons. Naturally this can occur in the incomplete oxidation of methane: CH4(g) +3/2 O2(g) CO(g) + 2 H2O(g) Incomplete combustion can also occur in large fires where there is insufficient oxygen: Forest fires or large wildfires. Man made sources are the incomplete combustion of fossil fuels: C8H18(g) +17/2 O2(g) 8 CO(g) + 9 H2O(g) This type of reaction is commonly seen in internal combustion engines where insufficient oxygen can be present in rich (higher fuel to air ratio) burning engines. The concentration of the man-made sources is the primary danger. Local levels can be very high. To limit Primary air pollution, there are four primary approaches:

1. Limit the use of the technology that produces the pollution. 2. Remove the cause of the pollution in the process. 3. Improve the process to limit the pollution. 4. Remove the pollutant after it has been produced.

Carbon monoxide production can be limited by burning less fossil fuels. This requires less use of internal combustion engines, or using fuels other than hydrocarbons. In the use of an internal combustion engine, the engine can be tuned lean, to allow more oxygen in the combustion chamber to completely combust the CO (see problem with NOx production).


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After the CO has been produced, it can be removed with a thermal exhaust reactor. This adds extra air to the exhaust gasses. Due to the heat of the exhaust gasses, the CO in the exhaust will react with oxygen to produce CO2. CO(g) + 1/2 O2(g) CO2(g) A second and very effective way to remove CO is a catalytic converter. A catalytic converter used platinum or palladium as a catalyst to oxidize the CO to CO2. 2 CO(g) + 2 NO(g) 2 CO2(g) + N2(g) Unfortunately, platinum and palladium are very expensive.

Oxides of Nitrogen NOx: N2O, NO, NO2 Oxides of nitrogen are respiratory tract irritants that lead to tissue damage and susceptibility to infections. Oxides of nitrogen react to form nitric and nitrous acid upon contact with water. This causes the respiratory irritation. Natural sources of nitrogen oxides are some biological processes, and lightning. Biologically, NOx can be produced by some types of decomposition of organic molecules. The high energy of lightning is required to overcome the very high activation energy of: N2(g) + O2(g) 2 NO(g) Man made sources of NOx are primarily due to combustion processes. High temperature inside internal combustion engines can produce NOx (mostly NO). The nitrogen gas in air is combusted with oxygen: N2(g) + O2(g) 2 NO(g)

Jet engines can also produce NO. Once again, the concentration of man-made NOx production is the biggest problem. Because of the high levels of nitrogen in air, removal of nitrogen from the process is not feasible. Like most pollution related to cars and industry, using less, driving less or not using internal combustion engines is a long term solution.


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An engine tuned to run rich (higher fuel to air ratio) as this will provide less oxygen to react with the nitrogen. Unfortunately, the conditions to limit NOx levels (less Oxygen) are the conditions that increase CO levels (insufficient oxygen). To limit CO levels often produces more NOx. These two factors must be balanced. A catalytic converter is the best solution to deal with the CO and the NOx. An engine can be tuned to produce regular levels of both and they can be reacted in the catalytic converter: 2 CO(g) + 2 NO(g) 2 CO2(g) + N2(g)

Hydrocarbons The last pollutants that is primarily associated with cars and internal combustion engines are hydrocarbons. Hydrocarbons are produced naturally by many plants: rice, evergreen trees (terpenes) and other plants. These are produced at low levels over all of the earth. Many of these compounds are toxic or can produce toxic compounds (benzene). Man-made sources are due to leakages of fuels and solvents, and partially combusted hydrocarbons from engines. Limiting these pollutants requires stopping the escape or stopping there use. In an engine, a catalytic converter will oxidize partially combusted hydrocarbons to CO2.

Sulfur dioxides SO2 Sulfur dioxides are a very serious primary air pollutant that is produced by many industrial processes. Sulfur oxides are respiratory tract irritants. Natural sources of sulfur oxides are decay products (rotten eggs) and volcanoes. Man made sources are the burning of coal that has high sulfur levels, and smelting of ores that contain sulfides. These activities are concentrated in industrial areas.


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Sulfur coals are now cheaper than low sulfur coals. Diesel fuel can contain sulfur, and it costs more to produce diesel fuel that is low sulfur. The best method to lower sulfur oxide emissions is to remove the sulfur before combustion or to combust low sulfur fuels. Both of these options are expensive. It is easy and cheap to remove sulfur from gasoline (bubble hydrogen gas through it removes the sulfur). It is more difficult and costly to remove the sulfur from solid coal. Sulfur oxides can be removed from flue gasses in a process called scrubbing. Exhaust gasses are exposed to an alkaline limestone (CaCO3) or lime (CaO) slurry. CaCO3(s) + SO2(g) CaSO3(s) + CO2(g) CaO(s) + SO2(g) CaSO3(s) The CaSO3 can be oxidized to CaSO4 (gypsum) that is a sailable product. A newer method is a “fluidized bed combustion”. The coal is combusted on a bed of limestone (CaCO3) and the sulfur is converted to CaSO3 or CaSO4 during the combustion. These processes are very expensive. There must be laws that force industries to implement these measures. Particulates are the last primary air pollutant that we will look at. Particulates affect the respiratory system and can cause lung diseases. Natural sources: dust, ash, smoke, bacterial, fugal spores and pollen. The greatest man-made sources that are problematic are burning of fossil fuels (primarily diesel and coal). These are produced in concentrated areas and also in areas of other pollution. Particulates also can act as catalysts for the production of secondary air pollutants. Large particulates can be separated by settling. Smaller particulates can be separated by electrostatic precipitation Ionized particles are attracted to a charged plate collector.


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10.2 — Ozone Ozone (O3) is a toxic pollutant near the earth and a vital gas protecting us from ultraviolet (uv) radiation from the sun in the upper atmosphere (stratosphere, 12-50 km above the earth). Oxygen and ozone together block most of the damaging uv light. Oxygen molecules absorb higher energy uv, and ozone absorbs the lower energy uv light. Oxygen (O = O) has a double bond that requires 496 kJ/mol to break. This requires light with a wavelength of 242 nm or less to break (high uv, UVA). Ozone has a delocalized 1.5 order bond that requires 362 kJ/mol to break. This requires light with a wavelength of less than 330 nm to break (low uv, UVB). The bond in oxygen gas is broken by UVA

The bond in ozone gas is broken by UVB

This is the natural process that consumes and produces ozone in the upper atmosphere. This process has been interfered with in the last decades to weaken the protective affect of the “ozone layer”. The increase of uv radiation on the earth causes problems because the high energy radiation can break the bonds of molecules in living organisms on the earth. Animals:

• Sunburn • Skin cancer • Cataracts • Single celled organisms

have altered growth rates and patterns

Plants: • Growth inhibited and

weakened organisms • Disease susceptibility • Interferes with

photosynthesis • Marine plants effected

(affects CO2 storage)

O=O(g)UVA, high energy! "!!!!! 2O(g)

•

O(g)• +O2 (g) " O3 (g)

O ! O ! O(g)UVB, low energy! "!!!! O2 (g)+ O(g)

•

O(g)• +O2 (g) " 2O2 (g) or O(g)

• +O2 (g) " O3 (g)


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CCl2F2(g) ! CClF(g)• + Cl(g)

•

Cl(g)• + O3(g) ! ClO(g)

• + O2(g)

ClO(g)• + O(g)

• ! Cl(g)• + O2(g)

The ozone layer has been degraded by chemicals that have been released into the atmosphere on industrial scales. The primary culprits are CFC’s (chloro-fluoro carbons) and NO2. The C—Cl bond can be broken by the uv light in the upper atmosphere to form a chlorine radical. NO2 can also produce an oxygen radical. These radical can catalytically break ozone into oxygen. CFC’s: NO2: CFC’s were used primarily as a refrigerant, in aerosol sprays, and in the plastics industry. Freon was a common chemical. They are stable (freon lasts for over 80 years), inflammable, and inert (in the lower atmosphere). Replacements for CFC’s must have the above properties, and have fewer or no C—Cl bonds. Some replacements are: Chlorodifluoromethane, 1,1,1,2-tetrafluoroethane, and 3-methylpropane. The first has only one C—Cl bond, but the second and third are flammable. They are less stable than Freon, so they will not stay in the atmosphere as long. All are also strong infrared absorbers, that makes them greenhouse gasses (next topic) In the arctic regions, ozone depletion is worst in spring. In winter, ice crystals in the atmosphere catalyze the production of Cl2 and HClO molecules. In spring, as the ice crystals melt, these molecules are released into the atmosphere to produce chlorine radicals and decompose ozone in the polar regions. The immediate short term response to ozone depletion in the increased use of sunscreen products. Naturally, the body produces a brown pigment (melanin) that absorbs uv light. Monomers of melanin:

NO2(g) ! NO(g) + O(g)•

O(g)• + O3(g) ! 2 O2(g)


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The alternating single and double C—C bond allow for bond delocalization. These delocalized electrons allow a molecule to absorb uv light. A common ingredient in sunscreens is PABA (para-aminobenzoic acid). It is also carcinogenic. Other sunscreen products also have this bond delocalization Another option is to stay indoors, as glass blocks most uv light. Alternately, sun blocking compounds (titanium oxide) will reflect visible and uv light. 10.3 — Global Warming The greenhouse effect is a normal and necessary condition of the atmosphere. The basic principle is:

• Short wavelength light passes through the atmosphere and warms the earth.

• Infrared light is radiated from the earth. • Some of the ir light is absorbed by molecules (greenhouse gasses)

in the atmosphere and reradiated in random directions.


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Global warming is a result of the increase of the greenhouse effect that is probably due to human activity since the industrial revolution. A greenhouse gas must be an infrared absorber. The degree that a gas contributes to the greenhouse effect depends on its effectiveness as an ir absorber, and its prevalence in the atmosphere. Chemicals that human activity have added to the atmosphere have increased the greenhouse effect.

Gas

Source

Ir absorber

Effect on warming

H2O

Evaporation

0.1

-

CO2

Combustion of fossil fuels

1

50 %

CH4

Anaerobic decay (cows)

30

18 %

N2O

Fertilizers and combustion

150

6 %

O3

Smog and pollution

2000

12 %

CFC’s

Refrigerants

10,000+

14 %

There is little debate that the mean temperature of the earth is increasing. There is also little doubt that carbon dioxide levels have changed similarly to temperature changes


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There are some serious scientific problems that are arising from this issue: • Politics driving science • Abandoning of the scientific method • The drive to find the evidence to prove the required conclusion.

The effects from global warming are not clear. The models used to predict consequences are not complex enough and produce conflicting results. It is reasonable certain that there will be large scale changes in local climate, rainfall levels and temperatures (increases and decreases). This will have major impact on human populations in already marginal agricultural areas. Sea levels will also rise due to the melting of ice caps and the increase in the volume of the oceans because of thermal expansion. For island nations and low level areas (Victoria, Miami, Bangladesh), there will be considerable dislocations. Particulates in the atmosphere have the opposite effect as the greenhouse effect. They block light before it has gotten to the earth. This decrease in radiation coming to the earth decreases the temperature. The temperature dip in the 1940’s and the 1960’s is probably due to volcanic activity.


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10.4 Acid Rain and Smog Acid rain and smog are secondary pollutants. They result from chemical reactions to primary pollutants. Acid rain results when pollution has decreased the pH of rain to below 5.6 Natural rain has a pH down to 5.6 because of carbon dioxide normally in the air. Covalent oxides react with water to form acidic compounds. Carbon dioxide forms carbonic acid with water. CO2(g) + H2O(l) H2CO3(aq) This naturally results in rain that is acidic. Carbonic acid is a weak acid and normally reaches concentrations to provide a pH of 5.6-6.0. Primary pollutants that form oxides can contribute to acid rain. Acid rain is rain with a pH < 5.6 and is not caused by CO2. The combustion and oxidation of sulfur can produce sulfur oxides S(s) + O2(g) SO2(g) SO2(g) + 1/2O2(g) SO3(g) Sulfur dioxide and sulfur trioxide will react with water to form sulfurous acid and sulfuric acid. SO2(g) + H2O(l) H2SO3(aq) SO3(g) + H2O(l) H2SO4(aq)

These contribute to acid rain. The combustion of nitrogen produces nitrogen monoxide which is oxidized to nitrogen dioxide. N2(s) + O2(g) NO(g) 2 NO(g) + O2(g) 2 NO2(g) The reaction with water can produce nitrous acid or nitric acid: 2 NO2(g) + H2O(l) HNO3(aq) + HNO2(aq) 4 NO2(g) + O2(g) + 2 H2O(l) 4 HNO3(aq)


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The mechanism for acid formation involves the formation of a hydroxyl free radical OH•.

They form from the action of ozone or an oxygen free radical O• produced by ozone.

These hydroxyl radicals react with the oxides and water to form

the acids: Ammonia in the environment can precipitate with acids to form

ammonium salts. (NH4)2SO4, NH4NO3 These salts can be deposited on the ground where they can react to

form nitrates and acidify the soil: NH4

+(aq) + 2O2(g) → 2H+

(aq) + NO3-(aq) + H2O(l)

A map of North America shows where the affect of acid rain is the greatest Most of the problem is industrial. Emission controls on cars have limited acid rain due to car pollution to only periodic times. The worst acid rain recorded in North American had a pH ~2.4 in Los Angeles before any pollution restriction were in place for cars. Acid rain is detrimental to living things. Plants: • Critical nutrients are leached

from the soil. • Leaves are damaged by acid

exposure. • Toxic aluminum ions are

leached from rocks into ground water.

Buildings: • Buildings made with marble

will react with the acid: • CaCO3(s) + H2SO4(aq) • CaSO4(aq) + CO2(g) + • H2O(l)

Lake and Rivers: • Toxic aluminum ions in

water. • Aquatic animals have

damage to exposed tissues. • Small animals and plants in

the food chain die. • pH of 4 is a dead lake. Humans: • Oxides that react with water

in the respiratory tract result in acids witch cause irritation.

• Higher levels of toxic ions can be dissolved from rocks, pipes etc. (Pb2+, Cu2+)

To limit oxides in the atmosphere sulfur compounds must be removed from fuels before or after combustion.

H2O(g)+O3(g) ! 2HOi(g)+O2(g)

H2O(g)+Oi(g)! 2HOi(g)

HOi(g)+ NO(g) ! HNO2(g)

HOi(g)+ SO2(g) ! HOSO2 i(g)

SO3(g) + H2O(g) ! H2SO4(g)

HOSO2 i(g)+O2(g) ! HO2 i(g) + SO3(g)

HOi(g)+ NO(g) ! HNO2(g)


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A better solution is to stop burning fossil fuels as an easy source of energy. As a remedy for damaged areas, Calcium oxide or Calcium hydroxide could be used to neutralize the acid. This has been successful in some lakes. Smog is a toxic combination of smoke, fog, organic molecules and oxides. This combination can create very dangerous conditions in a city. Smog formation is common is large cities. In cities surrounded by mountains, where there are periods of little wind, the smog can build up to dangerous levels. Rain will dissolve many of the components of smog, and wild will blow them away. A thermal inversion can produce concentrated smog. Photochemical smog is an oxidizing smog that forms with sunlight. It is usually worst in summer, with little winds, no rain, and plentiful sunlight from high-pressure systems. Sunlight oxidizes compounds and produces radicals that can further result in a multitude of dangerous chemicals. The primary fault in photochemical smog is automobile pollution. There are an infinite number of possibilities of reactions and products that can form. Some of the most dangerous are produced by oxygen radicals that are formed by sunlight acting on NO2. NO2(g) NO(g) + O•(g) The oxygen radicals can produce ozone, hydroxide radicals, acids, aldehydes, and peroxides. Peroxides are oxygen atoms single bonded with an oxidation state of -1. They are very reactive.


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If this combination of chemicals is allowed to concentrate, the air can cause severe irritations and kill people with previously damaged lungs. One of the worst products are PANs. Peroxy acetyl nitrates. PANs form from a peroxy acid and NO2. They are irritants at parts per billion (µg per kg), are toxic to plants, and are known mutagens at very low levels. 10.5 Dissolved Oxygen in Water Oxygen gas dissolves in water. Microscopic oxygen bubbles are surrounded by hydrogen bonded water spheres in the water. Any splashing or bubbling will allow oxygen to dissolve in water. At 20°C oxygen can dissolve in water to a concentration of ~9ppm (~9mg/L). Fish need a minimum of 3 ppm to survive, and an aquatic environment requires at least 6 ppm to thrive. There are many factors that affect the amount of oxygen that is dissolved or can dissolve in a sample of water. Biological Oxygen Demand (BOD) is one of the most important factors in the quality of water from an oxygenation perspective. Biological oxygen demand is a measure of the oxygen required to biologically decompose any organic material in a sample of water. It is usually measured over a period of five days in a sealed container. The oxygen content of the water is measured after this time to find what is left. “Pure Water” has a BOD < 1ppm. A BOD over 5 ppm is considered polluted. High BOD can remove all the oxygen from a water sample. A river can re-oxygenate itself through mechanical action, but a lake or ocean may remain de-oxygenated for a long period of time. High levels of nitrates (fertilizer runoff) can cause large algae blooms. As these die, their decomposition can consume all the available oxygen. Further decomposition occurs anaerobically which produces toxic and fouls products. Living things that enter this area die, decomposing and consuming any oxygen left. A dead zone forms. This is called eutrophication.


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Another important factor in oxygen solubility is the temperature. Dissolved oxygen requires a stable, hydrogen bonded sphere. Higher temperatures do not favour stable hydrogen bonds. Higher temperature water will have a lower oxygen solubility. In warm water organisms will consume oxygen faster when there is less oxygen. Thermal pollution can damage water bodies. Additionally, a warm layer of water can form a low-solubility blanket over a colder body of water that limits the transfer of oxygen to the colder water below. 10.6 Water Use Drinking water is one of the most precious resources on the earth. About 97% of the water on the earth is salt water not suitable for drinking. Of the fresh water, ~2.1% is in ice caps and glaciers; ~0.6% is in rivers and lakes; and ~0.6% is groundwater. Only ~1.0% of the fresh water on the earth is readily available for human use. With the growth in the world’s population, demand for fresh water is growing. However, water is distributed very unevenly. Some countries have an abundance (Canada, Russia) others have very little (Northern Africa, Middle east). Water use per person varies per country, and the what the water is used for also varies for different countries. The main difference is the degree of industrialization. Conserving water is a matter of social and political will. If wasting water is costly, conserving water becomes a matter of profit. Describe ways that water could be conserved in: Industry Agriculture Domestic Use


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Water that is considered safe for drinking must be: Free of pathogens Free of unpleasant odours, tastes, colours, and turbidity Free of dangerous dissolved substances. Filtering can reduce turbidity. Ozone, UV light , and chlorine are the other common method of treatment for drinking water. Chlorine treatment uses chlorine gas Cl2(aq) + H2O(l) HCl(aq) + HClO(aq)

Or HClO (hypochlorous acid, active ingredient in bleach) is added to the water directly. HClO is a strong oxidizing agent that oxidizes many pathogens (not all viruses): HClO(aq) + 2H+

(aq) + 2e- HCl(aq) + H2O(l) Ozone is more effective than chlorine. The ozone itself oxidizes the pathogen: O3(aq) + 2H+

(aq) + 2e- O2(g) + H2O(l)

High intensity ultraviolet light is ionizing radiation. This ionizes the pathogens due to the intense energy delivered by the short wavelength light. Chlorine is very cheap, has a long retention time (if pathogens enter the water system after the treatment), but has a bad taste. There is a possibility that toxic chlorine compounds can form. Ozone and UV treatment is more effective and leaves no bad taste in the water. It is more expensive and has a very short retention time. There has been much research into desalination of sea water. The oldest and simplest method is distillation.


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The problem is the high heat capacity of liquid water, and the high heat of vaporization to boil water. This is very costly. The largest current distillation plant is in Saudi Arabia. It produces ~ 150 x106 m3/year. It cost over one billion dollars to build Saudi demands for water are about 3 x109 m3/year Reverse osmosis can also be used to desalinate sea water. Osmosis is the diffusion of water through a semi-permeable membrane. The water molecules will flow from a pure water sample to a salty water sample. The membrane will allow water molecules to pass through, but not ions. Reverse osmosis plants are expensive to build. The largest is in Israel that produces ~ 100 x106 m3/year. The cost is about C$0.65 per m3. (This is a similar cost to distillation depending on the cost of the material that is burned.) Another method of desalinization is through an ion exchange column. A resin removes the sodium and chloride ions and replaces then with hydrogen and hydroxide ions. This is a very expensive method, but it can be used to produce very pure water. After water is used by people it must be treated so it is safe to return to the environment. Water used in industry should have toxic substances removed. Water from domestic use is sewage. It can be returned untreated to the environment, where it is eventually broken down by natural processes. There can be dangerous problems due to untreated wastes. Sewage treatment aims to remove solids from the water, lower the biological oxygen demand (BOD) of the water, kill any pathogens in the water, and remove any other chemicals or ions that have been added to the water. There three main stages to sewage treatment: Primary, secondary and tertiary. Primary treatment involves coarse filtering and settling. Large solids are filtered out. Sand and grit are allowed to settle out in a “grit chamber”.


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Suspended solids are allowed to settle through a process of flocculation. Al2(SO4)2(aq) + 3Ca(OH)2(aq) 2Al(OH)3(s) + 3CaSO4(aq) The precipitate settles with the suspended solids forming a sludge that is treated further. Oils and grease is skimmed of the settling ponds. (Has been used in cosmetics.) Primary treatment removes ~60% of solid material and about 30% of BOD. Secondary treatment is intended to remove the BOD from the waste water. The most effective method is to inject oxygen into the mixture to aerobically break down organic material with aerobic bacteria. The activated sludge that forms can be recycled to break down other material. Up to 90% of the BOD can be removed this way. Tertiary treatment is designed to remove ions and other organic molecules from the waste water. There are many methods for tertiary treatment. Here are some examples. Many ions can be removed by precipitation. Phosphates: Al3+

(aq) + PO43-

(aq) AlPO4(s) 3Ca2+

(aq) + 2PO43-

(aq) Ca3(PO4)2(s) Metal ions can often be precipitated with hydroxide ions or sulfide ions: Cr3+

(aq) + 3 OH-(aq) Cr(OH)3(s)

Cd2+

(aq) + H2S(g) CdS(s) + 2 H+(aq)

Activated carbon can absorb organic compounds. Upon heating, the activated carbon is reactivated and the organics are oxidized to carbon dioxide and water. Nitrates are very difficult to remove as all nitrate compounds are soluble. If nitrates are a concern ion exchange can be used (very expensive), or a slower process can use bacteria to break down the nitrates to nitrogen. Similarly, high nitrate ponds can have algae growth that consume the nitrates for growth. This is a very slow process.


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10.7 Toxins in Water In our examination of toxins in water, we will look at three categories: Heavy metals Organic molecules Nitrates Three of the most serious heavy metal pollutants are: cadmium, mercury, and lead. Cadmium has entered the environment as a waste product of zinc mining, discarded batteries (NiCad), electroplating, and as a pigment. It is toxic because it replaces zinc in many enzymes. It can cause stomach pain, vomiting, diarrhea, kidney damage, and damage to red blood cells. LD50 = 225 mg/kg (rats) LC50 = 500mg/m3 (rats) (lethal concentration in the air) 20µg/m3 for kidney/respiratory damage. Mercury has entered the environment as a fungicide, battery ingredient, and many industrial processes. It also concentrates in the food chain in fatty tissues. Organo-mercury compounds are very dangerous as they are quickly taken up by metabolic processes. Mercury is a neurotoxin that can damage the liver, kidneys, brain and blood. LD50 = 40 mg/kg (rats) Lead is in the environment mostly from processes that are now banned in most countries: leaded gasoline (tetraethyl lead) pigments in paint, lead pipes, and car batteries. Lead causes kidney damage and failure. It can cause developmental problems and other brain damage in children. LD50 = 70 mg/kg (rats) Some recent studies have found a strong synergistic affect between lead and mercury. If the LD1 level for mercury and lead are combined in the same population, the result was 100% fatality. Since lead and mercury are both in the environment, this synergistic effect is likely to be actualized. Many organic compounds constitute toxic pollutants in water. Herbicides and pesticides can persist in water and cause many and varied problems in the environment and humans.


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These toxic chemicals can continue to kill plants and animals after they have spread beyond their intended purpose. As most organisms have similar cellular mechanisms, these chemicals can create unintended consequences. Dioxins are present in waste water that have organochlorine compounds that have been incinerated at too low temperatures. They are also produced in pulp-mills. It is very toxic (LD50 = 0.02 mg/kg rat). It is persistent in the environment and accumulates in fatty tissues and liver cells. It is also known to cause birth defects. Polychlorinated biphenyls (PCB) were used in transformers because of their low electrical conductivity. They are very stable in the environment. They collect in fatty tissues and cause developmental problems in children. They are also a probable carcinogen. The number of chlorines can vary from 4 to 10. Nitrates are difficult to remove from water, and are constantly added to the environment as fertilizers. Nitrate levels are to be below 50 mg/L for safe drinking water (WHO). High nitrate levels can be dangerous for babies. Lack of acid in their stomach can allow bacteria to reduce nitrates to nitrites. Nitrites can oxidize the iron in haemoglobin and oxygen starved babies. Nitrite levels in adults can also be a problem. Nitrites are found in preserved meats and beer. Nitrites can be converted into nitrosamines that are known carcinogens. Heavy metals and phosphates (not nitrates) can be removed from water by

precipitation. Many of these are only slightly soluble in water with certain ions. The addition of appropriate ions can precipitate the unwanted ion which can be removed by filtration.

The slightly soluble ion compounds form a solubility equilibrium:

MX(s )!M(aq)+ + X(aq)

!

K sp = M+"# $% X!"# $%


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A table of equilibrium values will be provided to you in questions and is available in your text. (A-10) Example 1: A compound M2X has a solubility of 1.5 x 10-7 M. What is the solubility constant for this compound? Example 2: What is the concentration of lead in an saturated solution of lead chloride? Example 3: Lead ions are removed from a water system by bubbling Hydrogen sulfide

gas through the water. The sulfide concentration is 0.028 M. What is the highest concentration of lead ions that can be left in the water?


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Example 4: To remove phosphate ions, Magnesium ions are added to a water solution.

Will a precipitate form if [Ca2+] = 5.2 x10-4M, and [PO43-]=2.4x10-6M?

Follow-up Problems: 19.4-19.9 10.8 Soil and Waste Soil Degradation is the lowering of crop yields due to natural or

anthropogenic causes. Salinization: due to over-irrigation or lowering of natural river flow. Salts

concentrate in top soil. Plants are unable to take up water through osmosis.

Nutrient depletion: constant intensive farming removes nutrients and micro-nutrients that are not replaced.

Soil Pollution: Intentional or accidental dumping of toxic substances or

the slow accumulation of toxic substances can lower the yield of agricultural land. Accumulation of toxins can also kill many organisms in non agricultural lands.

Can also lead to ground water toxicity. Soil Organic Matter (SOM) The presence of organic matter at various levels of decay (humus) is an

important factor in healthy soil. Source of important nutrients for future growth. Absorbs moisture and heat to keep soil healthy and fertile. Humus and clay can have a high ion-exchange capacity. This can trap

ions in water and exchange them with other ions. This can trap nutrient ions for future growth or toxic ions to remove them

from the water supply. This can have positive and negative consequences.

The ability of clay and humus to do this is called the ion-exchange capacity.


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Hydrocarbons (VOC), (Semi VOC, SVOC) from transport and industry. Argichemicals - Pesticides, herbicides and fungicides. Polyaromatic hydrocarbons from incomplete combustion of hydrocarbons PCBs from electrical equipment Organotin compounds used as bactericides and fungicides in wood, paper

and paint processing. Method Advantages Disadvantages Landfill

• Cheap, efficient • Reclaim land after use

• Gross • Methane problem

Open Dumping

• Cheap, efficient

• Gross • Water and air pollution • Infestations

Ocean Dumping

• Cheap, efficient • Nutrients returned to

ocean

• Toxins directly to the sea • Danger to marine organisms

Incineration

• Reduce volume, uses minimal space

• Low level of toxicity • Can recycle some of

the energy

• expensive • Partial combustion of

hydrocarbons (dioxins) • Produces greenhouse gasses???

Recycling

• Step towards sustainable used of resources

• Very expensive • Not efficient • Can cause more pollution than it

removes

Nuclear Waste Low Level Waste: Low level of radioactivity - short half lives. (medical

waste, or equipment in contact with radioactive materials) Storage until much of the radioactivity is gone. Often dumped into the sea. Ions exchange to remove cesium and strontium (most common radioactive

isotope) High Level Waste: Highly radioactive with potentially long half lives

(waste from spent fuel rods, and processing fuel…) Most waste in melted and vitrified. Long term storage is the problem. Currently it is stored underground in

geologically stable areas.

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