E5 Dissolved oxygen in water

For animals to survive in aquatic systems, water must contained a minimum concentration of dissolved oxgyen as the organisms require oxygen for aerobic respiration. Oxygen is a diatomic non-polar molecule that is only slightly soluble in water, as water is polar. At 20°C, the solubity of oxygen is ≈0.00028cm3, which is 9mg of water per million mg of water (9ppm). Solubility decreases as temperature rises.

Quality of water is dependent on the dissolved-oxygen (DO) content of a body of water. Water of good quality has a DO content of 8-9ppm, while moderately polluted water is about 4.5ppm, and anything below that concentration is highly polluted. Quality of water depends on factors like oxygen demanding wastes, like organic substances (plants, animals, human waste) and waste from industrial processes (meat packing and food processing plants and paper mills), and diseases causing microorganisms (pathogens).

E.5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen demanding wastes.

BOD is the measure of the amount of oxygen consumed by biodegradable organic wastes and ammonia in a given amount of water over a time period, normally 5 days, at 20°C. A sample is diluted in oxygen saturated water and enclosed without any air in a BOD sample bottle for a 5 day incubation period, after which the decrease in dissolved oxygen is measured using an oxygen electrode. Almost pure water has a ppm BOD of less than 1, while water of doubtful purity has a ppm BOD of 5, and water of unacceptable purity a ppm BOD of 20 (e.g. untreated sewage has a BOD of 100-400ppm, while food processing plants can be up to 10000ppm).

E.5.2 Distinguish between aerobic and anaerobic decomposition of organic material in water.


Aerobic decomposition
-Oxidation process where complex organic matter is broken down into simple organic material, carbon dioxides and water. Simple organic material can be further converted to nitrates, sulphates, and phosphates.

-DO decreased as oxygen is used up, and if too much organic matter is present, DO may decrease to zero and kill aquatic life that depends on oxygen.

Anaerobic decomposition
-Typical products include ammonia and amines from nitrogen and hydrogen (strong fishy smell), methane/biogas/marsh gas from carbon and hydrogen, hydrogen sulphide (rotten egg smell) from organic sulphur, and phosphine (PH3) phosphorus.
-Oxygen is not used up.

E.5.3 Describe the process of eutrophication and its effect.

Plant nutrients are often soluble and leach into water bodies. This leads to excessive growth of aquatic plant life, often in the form of algal blooms.

Effects:

-water smells and tastes bad, becomes lifeless.
-red tides: marine plankton produce chemical toxins.

-Dead plants are decomposed anaerobically, depleting oxygen

-Fish die from asphyxiation (lack of oxygen)

-Anaerobic processes produce toxic substances like phosphine and hydrogen sulfide.

E.5.4 Discuss the source and effects of thermal pollution of water.

Thermal pollution occurs when water that is heated in power plants/industrial processes is dumped into streams, rivers or lakes. Two major effects would be on dissolved oxygen and the metabolic rates of aquatic life.

OXYGEN:

Concentration of oxygen decreases as temperature increases. Furthermore, warm water is less dense than cool water and stays near the top. This water is unable to absorb as much oxygen from the atmosphere; at 0°C, DO is 15ppm, at 20°C, DO is 9ppm, and at 40°C, DO is 6.5. This is for pure water (polluted water will have even lower DO contents).

AQUATIC LIFE:

Increased temperatures increase the rate of biochemical processes, and hence the metabolic rates of aquatic animals increase and, in so doing, require more oxygen. The rate of consumption of oxygen increases.

E4 Ozone Depletion

E.4.1 Describe the formation and depletion of ozone in the stratosphere by natural processes.

Ozone is a naturally occuring component of the stratosphere 15 to 45 km above the Earth's surface. It is a powerful oxidising agent and pale blueish gas with an acrid odour. Ozone can have harmful effects on living matter but is also essential to life and health (ozone layer).

Ozone is formed from the photo-dissociation of molecular oxygen by uv light. Free radicals are formed from the splitting of oxgyen molecules into oxygen atoms by high energy, short wavelength UV light from the sun. These oxygen atoms then react with other oxygen molecules to form ozone.

The resonance structure of ozone is as follows:

It suggests that the two bonding electrons from the pi bonds are spread over the entire structure of the molecule with a bond order of 1.5 (weaker bond than oxygen). As a result, less energy (i.e. longer wavelength light) is required to break the bonds in ozone than in oxygen.

The depletion of ozone hence occurs when the reverse reaction occurs and ozone absorbs longer wavelength UV light to form an oxygen molecule and an oxygen free radical. An oxygen free radical reacts with ozone to form two molecules of oxygen gas.

Ozone is hence constantly being formed and broken down, and acts as a shield by absorbing 93-99% of the sun's harmfulUV light of longer wavelength than that absorbed by oxygen and nitrogen.

E.4.2 List the ozone-depleting pollutants, and their sources.

Depletion of the ozone layer has been observed from satellite data. There are holes in the ozone layer over the north and south poles, although this is partially seasonal (e.g. with lowest levels in the antarctic spring).

Ozone is depleted by chlorofluorocarbons. CFCs are chemically inert when released but become chemically reactive chlorine atoms when they reach the unfiltered UV rays of the sun in the stratosphere.

The bond enthalphy for C-F is higher than that of the C-Cl bond, and as a result a chlorine free radical is formed, that reacts readily with ozone to produce oxygen.

CCl2F2 + uv light --> ∙Cl +CClF2
∙Cl + O3 --> ClO∙ + O2

Nitric oxide (from aircraft exhausts) react with ozone to form nitrogen dioxide and oxygen.

The depletion of ozone means that more harmful UV rays reach the Earth, increasing skin cancer, eye cataracts, genetic mutations, sunburn, damage to animals and plant (suppression of plant growth) and causing changes in the world's climate.

E.4.3 Discuss the alternatives to CFCs in terms of their properties.

Properties of CFCs that make it popularly used in aerosols and as solvents would be their lack of reactivity, low toxicity and low flammability. Alternatives to CFCs should have no C-Cl bonds and show little absorption of infrared radiation so as to to behave like a greenhouse gas.

1. Hydrocarbons are used as refrigerator coolants and do not cause ozone depletion, but are flammable and are greenhouse gases.

2. Hydrochloroflurocarbons decompose less easily because of the presence of hydrogen as the bond enthalpy for the C-H bond is higher than that of the C-Cl bond. However, ozone is still depleted because of the presence of a C-Cl bond.

3. Fluorocarbons have low reactivity, are neither toxic nor flammable and are stable to UV radiation because of the strong C-F bond. However, these are greenhouse gases.

4. Hydrofluorocarbons are also ideal as they do not contain chlorine atoms and have low reactivity, low toxicity and low flammability but contribute to global warming.

For more on ozone depletion, click here.

E3 Greenhouse Effect

E.3.1 Describe the greenhouse effect.

The Greenhouse Effect is the trapping of heat in the atmosphere. Short wavelength sunlight (visible light and UV) penetrates the atmosphere to warm the Earth's surface which partially absorbs the energy before radiating it back out as longer wavelength radiation (infrared). This long-wavelength radiation is partially retained in the atmosphere and only some is re-emitted back out.

http://arcticclimatemodeling.org/Movies/greenhouse_effect_dvd_sample.html

Greenhouse gases, like carbon dioxide, methane, CFCs, water vapour and nitrous oxides, trap as a one-way filter, trapping heat like in a glass/plastic in a greenhouse. This heating effect is necessary for the maintanence of life on Earth and is what maintains global temperatures. However, with increased concentrations of greenhouse gases, the greenhouse effect is 'enhanced', with even more radiation being trapped in the atmosphere, causing a significant rise in global temperatures known as global warming. The rate of temperature change is significantly faster than any observed in the last 10000 years.

E.3.2 List the main greenhouse gases and their sources, and discuss their relative effects.

Water vapour: main source is the evaporation of water bodies and the combustion of hydrocarbons. Occupy 0-4% of the atmostphere, and

E.3.3 Discuss the influence of increasing amounts of greenhouse gases on the atmosphere.

Over the last century worldwide:

1. Increase in temperatures by 0.5ºC

2. 1% increase in precipitation

3. 15-20cm rise in sea levels from glacial meltdown and expansion of ocean water by warmer temperatures.

--> Particularly in the Arctic, global warming from rising levels of carbon dioxide is particularly irreversible because of how the melting of permafrost results in further decomposition of the previously frozen matter, causing a continual increase in carbon dioxide and methane levels.

http://arcticclimatemodeling.org/Movies/permafrost_dvd_sample.html

Scientific models are also being exceeded- the prediction of scientific models are taking place 30 years sooner than expected.

IMPACT OF CLIMATE CHANGE ON HEALTH, AGRICULTURE, FORESTS, WATER RESOURCES, COASTAL AREAS, SPECIES DIVERSITY, SPECIES NUMBERS AND NATURAL AREAS:

-Health
Life cycles of pathogens and insects affected (e.g. Mosquitoes). Water-borne diseases may become more prevalent.
-Agriculture
Crop yield and crop distribution will be affected. Flooding of land from sea water may result in salination of the water table and affect crops requiring fresh water.

-Forests
Insects and diseases may increase, summer droughts may produce forest fires, higher temperatures and higher increased precipitation may cause increase in vegetation growth, but plants requiring little rainfall may become extinct.

-Water Resources
Decreased water quality due to flooding, more resources needed to turn water into potable sources. Both floods and droughts are more likely, from increased precipitation and increased rates of evaporation.

-Coastal Areas
Eroding of beaches, flooding of low lands and coastal flooding, and resulting loss of such ecosystems.

-Species and Natural Areas
Loss of cold water fish habitat, shift in ecological areas (organisms in temperate regions may migrate upwards to previous uninhabitable regions). Desertification. Loss of habitats and species.

E2 Acid Deposition

E.2.1 State what is meant by the term acid deposition and outline its origin.



Acid deposition refers to acidic particles and gases that deposit or fall to the Earth.

e.g. WET deposition: acidic gases like oxides of sulphur and nitrogen and acidic particles brought down by precipitation by rain, fog and snow. DRY deposition: absense of precipitation.

Pure rain water has a pH of ≈5.6, because carbon dioxide dissolves in rain to form carbonic acid. As a result, environmental pollution called acid rain is any rain with pH less than 5.6, and occurs commonly in industrailised areas (pH≈4-5 [4 to 40 times greater than pure rain water]). Extreme case of acid rain resulted in pH of 1.7 (LA, Dec 1982), and also a pH of 4.0 can kill fish life to form dead lakes (North America, China, Russia).

Acid deposition is associated with parts of a country with heavy industries and down-wind from industrial sites. Oxides of sulphuur and nitrogen are mostly responsible for this acidity and precipated become contaminated with acids when these oxides are present as pollutants.

Formation from sulphur oxides:

- sulphur dioxide + water --> sulphurous acid (stronger than carbonic acid)

-sulphur dioxide+ oxgyen--> sulphur trioxide (formed in the presence of oxygen, ozone, hydroxyl free radical and sunlight)

sulphur trioxide + water --> sulphuric acid

Formation from nitrogen oxides:

-nitrogen dioxide + water --> nitric acid + nitrous acid (absence of oxygen)

-nitrogen dioxide + water + oxygen --> nitric acid (consists of more complicated steps, summarised in this equation)

- other pollutants like the hydroxyl free radical (formed during photochemical smog) can react with nitrogen dioxide (also a free radical with a lone electron) to form nitric acid [accelerates conversion of nitrogen dioxide to nitric acid].

E.2.2 Discuss the environmental effects of acid deposition and possible methods to counteract them.

Acid deposition affects humans, aquatic life, materials, soil, vegetation and visibility.

Humans:
· Irritates whole respiratory tract and the eyes.
· Sulphate particles penetrate the lungs where they become embedded, adverse effects on asthmatics, the elderly and the young. (Sulphate aerosols are powerful irritants)
· Increased concentrations of aluminum ions in water due to acidic conditions may lead to Alzheimer’s disease.

Aquatic life:
· Eutrophication from nitrates in acid deposition.
· Aluminum ions from leaching of soil by acid rain affect the function of the gills.
· Acidification of water bodies may result in the death of some species of fish.

Materials:
· Corrosion of basic materials (e.g. marble, limestone, dolomite).
· Insoluble carbonates are converted to more soluble sulphates that then dissolve- destroys structural and artists’ stone (stone leprosy).
· Corrosion of iron and steel promoted by acid rain, worsened by high humidity, high temperatures, presence of particulates.
· Leaching of toxic heavy metals (e.g. Lead, Cadmium, Mercury) into the water system.
· Bleaching and weakening of fabrics and leather.
· Dicolouration and embrittlement of paper.
· Deterioration of electrical appliances.

Soil and vegetation:
· Acute injury from short term exposure to high acid concentrations consists of dead areas of leaves which dry out and usually become bleached.
· Chronic injury from long term exposure to low acid concentrations consists of bleached spots, yellowing of leaves, suppression of plant growth and reduction in yield (acid rain disrupts chlorophyll synthesis).
· Leaching and removal of important nutrients such as magnesium ions from the soil.

Visibility:
· Mist can causes great loss of visibility and affect air flights.

METHODS OF COUNTERACTING ACID DEPOSITION:
-pre-combustion methods or post-combustion methods can be used to prevent the formation of acidic particles/gases.
-Promotion of alternative energy sources and conserving energy would decrease combustion of fossil fuels.
-Adding calcium carbonate or calcium hydroxide to soil and lakes neutralizes acidity.

For more on acid deposition, click here.

E1 Air pollution

E.1.1 Describe the main sources of carbon monoxide (CO), oxides of nitrogen (NOx) oxides of sulphur (SOx), particulates and volatile organic compounds (VOCs) in the atmosphere.

Unpolluted air contains 78% nitrogen, 21% oxygen, 1% argon, 0.04% carbon dioxide and trace amounts of other gases with up to 4% water vapour. An air pollutant is a substance normally absent in air, or a substance that is normally present but in excess amounts. Primary pollutants are carbon monoxide, oxides of nitrogen, sulphur dioxide, particulates and hydrocarbons. Secondary pollutants are compounds formed when primary pollutants react in the air.

CARBON MONOXIDE:
-natural source: incomplete oxidation of methane
-man-made source: incomplete combustion of fossil fuels
-effect on health: prevents haemoglobin from carrying oxygen by binding irreversibly to form carboxyhaemoglobin.

-methods of reduction: use lean burn engine, thermal exhaust reactor, or catalytic converter.

OXIDES OF NITROGEN:
-natural source: electrical storms and biological processes
-man-made source: at high temp.s inside internal combustion engines.
-effect on health: respiratory irritant that results in infections
-methods of reduction: use of lean burn engine, recirculation of exhaust gases or catalytic converter.

SULPHUR DIOXIDE [can be oxidised to form sulphur trioxide]:
-natural source: oxidation of H2S produced by volcanoes and decay of organic matter
-man-made source: combustion of sulphur-containing coal and smelting of sulphide ores
-effect on health: respiratory irritant that results in infections
-methods of reduction: removal of sulphur from fossil fuels before combustion, alkaline scrubbing, fluidised bed converter.

PARTICULATES:
-natural source: soot, ash, dust, asbestos, sand, smoke, pollen, bacterial and fungal spores
-man-made source: burning of fossil fuels (esp. coal and diesel)
-affects respiratory system, causing lung diseases (e.g. emphysema, bronchitis, cancer)
-methods of reduction: sedimentation chambers, electrostatic precipitation.

VOCs (CxHy or R-H):
-natural source: plants (like rice; emit unsaturated hydrocarbons called terpenes)
-man-made source: unburned or partially burned gasoline and other fuels, solvents.
-effect on health: carcinogens (e.g. benzene), can form toxic secondary pollutants (e.g. PANs)
-methods of reduction: catalytic converter

E.1.2 Evaluate current methods for the reduction of air pollution.

1) Thermal exhaust reactor: Exhaust from car engine is combined with more air and reacts (at high temperature from the heat of the exhaust gas). CO becomes CO2 and unburned hydrocarbons are combusted.

2) Lean burn engines: Carburettor is adjusted altering ratio of air:fuel. Higher ratios results in less CO emitted (more complete combustion). However, this results in higher temperatures that increase the formation of oxides of nitrogen. Conversely, lower ratios cause less oxides of nitrogen to form but more carbon monoxide.

3) Catalytic converter:

Hot exhaust gases are passed over catalysts (i.e. platinum/palladium/phodium) that complete combust CO and unburned VOCs, as well as catalysing the reaction between nitrous oxide and carbon monoxide to give carbon dioxide and nitrogen gas.

2CO + 2NO --> 2CO2 + N2

4) Removal of sulphur from fossil fuels (e.g. metal sulphides in coal) [pre-combustion]

i) Coal gasification removes most of the sulphur in high sulphur coal when converting it to synthetic natural gas (SNG). Any sulphur is converted to H2S (acidic and easily removed). However, this process requires a lot of energy.

ii) use low sulphur, cleaner burning coal (e.g. anthracite) with high heat content. However, such coal is scarcer and more expensive.

iii) remove sulphur in petroleum refining and natural gas processing with a catalyst to hydrogen sulphide.

iv) coal washing: sulphur removed from coal by crushing the coal and mixing it with water, so that the denser sulphides sink and the cleaned coal can be skimmed from the surface. However, a lot of sulphur is trapped below the surface of the particles and organic sulphur (chemically bonded) cannot be physically removed.

5) Alkaline scrubbing [post-combustion]: alkaline slurry of limestone and lime (large amounts required, and heating to convert limestone to lime requires large amounts of energy which is expensive) [or magnesium hydroxide] are used to remove sulphur dioxide from the exhausts of coal-burning plants, with the resulting sludge used to for landfills, or gypsum [CaSO4.2H2O] for plasterboards. A wet alkaline scrubber uses liquid (usually water-based) to remove contaminants by sprayin ghte alkaline liquid in downwards as the gas streams upwards).

CaCO3(s) + SO2(g) --> CaSO3(s) + CO2(g) (note CO2 is a greenhouse gas)

CaO(s) + SO2(g) --> CaSO3(s)

2CaSO3(s) + O2(g) + 4H2O(g) --> 2CaSO4.2H2O(s)

6)Fluidized bed combustion [post-combustion]: burning of coal on a bed of limestone removes sulphur as calcium sulphite or sulphate as the coal burns.

7) Electrostatic precipitation: Soild or liquid particles are suspended in the air, with larger particles allowed to settle in sedimentation chambers (gravity) while an electrostatic precipitation chamber can be used for smaller particles. Charged particles are attracted to oppositely charged electrodes which are shaken periodically to remove aggregated particulates from the bottom of the precipitator.

Environmental Chemistry (Option E)

This blog aims at covering the Environmental Chemistry Option in the IB Diploma using computer simulations and data bases. Be awed.

* Information taken from Chemistry 3rd Edition, by John Green and Sadru Damji.

Databases:
http://www.agnet.org/library/
http://geodata.grid.unep.ch/
http://www.brighthub.com/environment/science-environmental/articles/16695.aspx