CHEMISTRY AND THE ENVIRONMENT
Elective Topic: HSC course
The purpose of this elective is to illustrate the interaction between chemistry and our
environment, and the relevance of chemistry in dealing with issues perceived to have
environmental significance.
1. Water
comparative analysis of samples of water from three or more different sources (eg rain, tap, sea, creek, pond, river, swimming pool, industrial effluent) - this analysis must include tests for pH, oxygen content, solid content, halides, metallic ions, microorganisms
hardness
(a) causes
(b) effect
(c) control.
2. The atmosphere
a survey of atmospheric composition resulting from an equilibrium involving natural cycles, ie carbon, nitrogen, oxygen cycles
a survey of the effect of humans on the cycles, including:
(a) production of carbon dioxide and
(b) the use of organic fertilisers
the effect of industrial effluents, motor car, freons, supersonic aircraft on the atmosphere
laboratory work involving gas tests, in the field (if possible), but otherwise on laboratory samples: CO, CO2, SO2, H2S, O2.
3. Radioactivity
a survey of:
(a) natural sources
(b) decay products
(c) nature and disposal of radioactive wastes
(d) industrial extraction and use of uranium.
4. Effects on living things
minimising human environmental impact in the areas mentioned; water treatment; sewage treatment; industrial waste; exhaust control; freon and supersonic aircraft control and role of ozone layer; radioactive waste disposal
concept of pollution:
(a) pollution in an industrial society and
(b) questions of standard of our living.
5. Mandatory experiences
collecting and analysing water samples
analysis of water samples for pH, oxygen, metallic ions, solids, halides; examination for micro-organisms.
6. Suggested experiences
the chemical determination of ions producing hardness and their removal
constructing and interpreting flow diagrams of cycles in nature
carrying out gas tests; CO, CO2, SO2, H2S, O2
consideration of human environmental impact in areas 1, 2 and 3.
Chemistry and the Environment Considers both the Natural and man-made processes which go on in our environment
Environmental degradation occurs when either natural processes or the agricultural, industrial and commercial activities of humans render the environment less amenable to living systems.
A pollution problem occurs when:
a) the flow of waste materials and emissions into the environment exceeds the capacity of natural systems to absorb or degrade them.
b) Synthetic substances, for which there are no efficient pathways for dispersal or degradation, are introduced into the environment.
Pollutants
A pollutant is a substance that has a net detrimental affect upon the environment or upon living organisms. Pollutants may be natural (but introduced) or synthetic.
Sources of Pollution
The major causes of man made pollution are traceable to the following interrelated factors:
* industrialisation - raw materials and energy
* urbanisation - clearing, concentrated wastes
* population - increasing demands
* lifestyle - wasteful consumption of energy and materials
* economic growth - increases consumption and production of wastes and pollutants.
Water is an essential resource for living systems (plant and animal). The principal user of water are:
* aquatic and other life
* public water supplies
* agriculture
* industry
* recreation and aesthetics.
Water pollutants can be classified into one (or more) of seven categories:
Wastes may be classified as:
Water treatment is divided into three categories, primary, secondary, tertiary:
Primary treatment aim to separate water from suspended solids by screening and sedimentation. Then the effluent is aerated and then it is chlorinated to kill pathogens.
Secondary treatment is based on continuos aerobic biological oxidation. Then it goes through either activated sludge procession or trickle filter processing
tertiary filtering aids to reduce suspended solids, soluble organics or soluble in-organics. Treatments include:
pH
rain water � 6
sea water � 8
Test for pH
Universal indicator:
Red Acidic
Green: Neutral
Purple: Basic
Water can also be measured for both total acidity and alkalinity (both ionised and unionised), the ability of the water to neutralises the opposite.
Natural Sources of Alkalinity:
OH- + CO2 HCO3- H+ + CO32-
Man made sources of alkalinity
Natural sources of acidity
Man made sources of acidity
Dissolved Oxygen Content (DO)
Oxygen gas is essential for aquatic life and breakdown of organic matter. Therefore DO is an indicator of a waterways health and capacity to assimilate waste discharge.
DO is measured in mg/L, the same as ppm. At 25 degrees the saturated content is 8 mg/L.
DO can be measured by a procedure called the Winkler method in which:
O2 + 4Mn2+ + 8OH- --> 2Mn2O3(s) + 4H20 (dissolved)
Mn2O3 + 6H+ --> 2Mn3+ + 3H2O
2Mn3+ +2I- --> 2Mn2+ I2
I2 + 2S2O3 2- --> 2I- + S4O6 2-
Step 4 involves the iodine being titrated with thiosulphate ions using starch as an indicator (turns blue)
From these equations 1 mole of S2O3 2- corresponds to 0.25 mole of O2.
water quality can be judged using % saturation values.
Main causes of oxygen depletion
* presence of biodegradable wastes
* industrial discharge
* eutrophication - Eutrophication can occur after an algal bloom in waterways. The resultant massive decay of the plants leads to DO depletion, so aerobic bacteria as well as animal and plant life die. Further decay by anaerobic bacteria causes the release of CH4 and H2s gasses.
Biochemical Oxygen Demand (BOD) (not in syllabus)
BOD measures the amount of oxygen used by microorganisms, including bacteria. A high BOD points towards a high level of contamination due to microorganisms.
Test for Biochemical oxygen demand:
Typical BOD for a clean river are 1-5 mg/L (ppm).
>5 mg/L is undesirable.
Microorganisms such as bacteria use the DO in the water to break down organic matter.
What affects Dissolved Oxygen?
The solubility of oxygen in water decreases as:
Solid Content (turbidity)
In turbid water:
* light penetration is reduced, with less photosynthesis occurring, so the DO is lowered.
* frog and fish eggs, and larvae are destroyed.
* the water temperature increases.
Suspended solids often present in water systems include: mud and clay, algae and bacteria, and minerals such as silica, calcium carbonate and ochre (iron oxide)
Activities that can increase the level of suspended solids include:
Test for total solid content:
Halides
Tests:
Cl- Silver Nitrate solution
AgNO3 + Cl- - > AgCl + NO3-
white turbidity - darkens on exposure to light
Metallic Ions
Naturally occurring: Ca2+, Na+, Mg2+
Heavy metal ions: Pb2+, Cd2+, Hg2+
Testing for dissolved lead with sodium sulphide:
Pb2+ + Na2S -> PbS(s) + 2Na+
Black
Also: Pb2+ + 2KI -> PbI2 + 2K+
Bright yellow
Flame tests can also be used to test for Cations
Na+ yellow
Ca2+ brick-red
K+ lilac
Common heavy metal ions:
Cadmium, Cd2+:
from electroplating, paints, ceramic glazes
Lead, Pb2+:
leaded petrol, old paints, car batteries.
Pb2+ inhibits the synthesis of haemoglobin and thus inhibit the oxygen carrying capacity of the blood
Mercury, Hg2+:
from mercury switches, batteries, industrial catalysts.
Organic mercury (C2H5HgCl)is very toxic because it can bioaccumulate
Atomic absorption spectrometers (AAS) are also used to test for metallic ions
Microorganisms
Fecal coliforms - E.coli (bacteria that normally lives in guts of humans and other mammals such cows. Purple dots
Test:
The excessive growth of algae can degrade water quality because firstly DO is lowered, subsequently the aerobic bacteria and algae die and are decomposed by anaerobic bacteria producing a foul smelling waterway.
HARDNESS
Causes
Hard water is groundwater which contains Ca2+, Mg2+ together HCO3-, SO42- and Cl-. Water exceeding 200 mgL-1 of these salts is hard.
The main causes of Ca2+ and Mg2+ are minerals such as gypsum (CaSO4.2H2O), calcite (CaCO3) and dolomite [CaMg(CO3)2]
Effect
Ca2+ + 2HCO3- --> CaCO3 +H2O + CO2
Scale
Control
Several reactions take place between the chemicals and the Mg2+, Ca2+, HCO3-. End products are CO32-, Mg(OH)2 and CaCO3 (and Na+).
2. The Atmosphere
a survey of atmospheric composition resulting from an equilibrium involving natural cycles, ie carbon, nitrogen, oxygen cycles
a survey of the effect of humans on the cycles, including:
(a) production of carbon dioxide and
(b) the use of organic fertilisers
Organic Fertilisers
Nitrogen fertilisers produce N2O which may eventually drift into the upper atmosphere.
the effect of industrial effluents, motor car, freons, supersonic aircraft on the atmosphere
The ozone layer is in the lower stratosphere, 20-50 km up. Max concentration occurs from 25-35 km up. In the lower atmosphere it is a pollutant because it is a strong oxidant.
O2 ->[U.V. light] O + O
O + O2 -> O3
Ozone absorbs much of the damaging UV light (290-320 nm). This protects living organisms from damage e.g. increased skin cancers, and eye damage, and increased mutation rate in plants and animals.
The ozone layer is under threat from organic fertilisers, freons and supersonic aircraft.
Freons
Freon were (and are) widely used in aerosol cans and fridges.
The major freons affecting the ozone layer are CCl3F, and CCl2F2.
Freons affect the ozone layer because they decompose in to free radicals which deplete ozone:
CCl3F -> Cl. + CCl2F.
Cl- + O3 -> ClO + O2
Chlorine radicals react with ozone
ClO + O. -> Cl. + O2.
ClO reacts with free oxygen in the atmosphere that would otherwise form ozone.
A Halon is similar to CFC but with some Bromine atoms present.
Supersonic aircraft
Supersonic Aircraft release NO and NO2 into the upper atmosphere which breaks down the ozone.
Acid Rain
Acid rain is caused by NOX and SO2 which form sulphuric and nitric acids, H2SO4 and HNO3.
laboratory work involving gas tests, in the field (if possible), but otherwise on laboratory samples: CO, CO2, SO2, H2S, O2.
Test for CO
Another test is the blood test: blood goes bright red because Hb+ bonds to CO much better that O2.
Test for CO2
Limewater Ca(OH)2
Ca(OH)2 + CO2 -> CaCO3 + H2O
Test for H2S
Test for SO2 - sulphur dioxide:
Also can use K2Cr2O7
Cr2O72- + SO2 -> Cr3+ + SO42-?
Orange Brown
Test for O2
Glowing splint test.
Note: Anything with nitrate or acetate is soluble (NO3-, CH3COO-)
3. Radioactivity
a survey of:
(a) natural sources
(b) decay products
(c) nature and disposal of radioactive wastes
(d) industrial extraction and use of uranium.
Radioactivity is harmful to life. radioactivity can cause the following:
The Nuclear Fuel Cycle
The spent fuel can then be encased in Pyrex or synrock inside stainless steel containers and placed in underground storage chambers.
Use of Uranium
Two examples of nuclear reactions are:
23592U + 1on -> 14256Ba + 9136Kr + 310n
23592U + 1on -> 13752Te + 9740Zr + 210n
Some hazardous isotopes are:
plutonium-239 (24k years)
cesium-137 (27 years)
xenon-135 (9 hours)
4. Effects on Living Things
minimising human environmental impact in the areas mentioned; water treatment; sewage treatment; industrial waste; exhaust control; freon and supersonic aircraft control and role of ozone layer; radioactive waste disposal
Role of the ozone layer
The ozone layer protects the earths surface from harmful UV radiation. UV radiator:
A 1% reduction in the ozone layer would lead to a 4% increase in skin cancers
concept of pollution:
(a) pollution in an industrial society and
(b) questions of standard of our living.
Primary Treatment
The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. Such materials are removed by screens or vertical bars, and the debris is burned or buried after manual or mechanical removal. The wastewater then passes through a comminutor (grinder), where leaves and other organic materials are reduced in size for efficient treatment and removal later.
Grit Chamber
In the past, long and narrow channel-shaped settling tanks, known as grit chambers, were used to remove inorganic or mineral matter such as sand, silt, gravel, and cinders. These chambers were designed to permit inorganic particles 0.2 mm (0.008 in) or larger to settle at the bottom while the smaller particles and most of the organic solids that remain in suspension pass through. Today, spiral-flow aerated grit chambers with hopper bottoms, or clarifiers with mechanical scraper arms, are most commonly used. The grit is removed and disposed of as sanitary landfill. Grit accumulation can range from 0.08 to 0.23 cu m (3 to 8 cu ft) per 3.8 million litres (about 1 million gal) of wastewater.
Sedimentation
With grit removed, the wastewater passes into a sedimentation tank, in which organic materials settle out and are drawn off for disposal. The process of sedimentation can remove about 20 to 40 per cent of the BOD5 and 40 to 60 per cent of the suspended solids.
The rate of sedimentation is increased in some industrial waste-treatment stations by incorporating processes called chemical coagulation and flocculation in the sedimentation tank. Coagulation is the process of adding chemicals such as aluminium sulphate, ferric chloride, or polyelectrolytes to the wastewater; this causes the surface characteristics of the suspended solids to be altered so that they attach to one another and precipitate. Flocculation causes the suspended solids to coalesce. Coagulation and flocculation can remove more than 80 per cent of suspended solids.
Flotation
An alternative to sedimentation that is used in the treatment of some wastewaters is flotation, in which air is forced into the wastewater under pressures of 1.75 to 3.5 kg per sq cm (25 to 50 lb per sq in). The wastewater, supersaturated with air, is then discharged into an open tank; there the rising air bubbles cause the suspended solids to rise to the surface, where they are removed. Flotation can remove more than 75 per cent of the suspended solids.
Digestion
Digestion is a microbiological process that converts the chemically complex organic sludge to methane, carbon dioxide, and an inoffensive humuslike material. The reactions occur in a closed tank or digester that is anaerobic-that is, devoid of oxygen. The conversion takes place through a series of reactions. First the solid matter is made soluble by enzymes, then the substance is fermented by a group of acid-producing bacteria, reducing it to simple organic acids such as acetic acid. The organic acids are then converted to methane and carbon dioxide by bacteria. Thickened sludge is heated and added as continuously as possible to the digester, where it remains for 10 to 30 days and is decomposed. Digestion reduces organic matter by 45 to 60 per cent.
Drying
Digested sludge is placed on sand beds for air drying. Percolation into the sand and evaporation are the chief processes involved in the dewatering process. Air drying requires dry, relatively warm weather for greatest efficiency, and some plants have a greenhouselike structure to shelter the sand beds. Dried sludge in most cases is used as a soil conditioner; sometimes it is used as a fertilizer because of its 2 per cent nitrogen and 1 per cent phosphorus content.
Secondary Treatment
Having removed 40 to 60 per cent of the suspended solids and 20 to 40 per cent of the BOD5 in primary treatment by physical means, the secondary treatment biologically reduces the organic material that remains in the liquid stream. Usually the microbial processes employed are aerobic-that is, the organisms function in the presence of dissolved oxygen. Secondary treatment actually involves harnessing and accelerating nature's process of waste disposal. Aerobic bacteria in the presence of oxygen convert organic matter to stable forms such as carbon dioxide, water, nitrates, and phosphates, as well as other organic materials. The production of new organic matter is an indirect result of biological treatment processes, and this matter must be removed before the wastewater is discharged into the receiving stream.
Several alternative processes are also available in secondary treatment, including a trickling filter, activated sludge, and lagoons.
Trickling Filter
In this process, a waste stream is distributed intermittently over a bed or column of some type of porous medium. A gelatinous film of micro-organisms coats the medium and functions as the removal agent. The organic matter in the waste stream is absorbed by the microbial film and converted to carbon dioxide and water. The trickling-filter process, when preceded by sedimentation, can remove about 85 per cent of the BOD5 entering the plant.
Activated Sludge
This is an aerobic process in which gelatinous sludge particles are suspended in an aeration tank and supplied with oxygen. The activated-sludge particles, known as floc, are composed of millions of actively growing bacteria bound together by a gelatinous slime. Organic matter is absorbed by the floc and converted to aerobic products. The reduction of BOD5 fluctuates between 60 and 85 per cent.
An important companion unit in any plant using activated sludge or a trickling filter is the secondary clarifier, which separates bacteria from the liquid stream before discharge.
Stabilization Pond or Lagoon
Another form of biological treatment is the stabilization pond or lagoon, which requires a large land area and thus is usually located in rural areas. Facultative lagoons, or those that function in mixed conditions, are the most common, being 0.6 to 1.5 m (2 to 5 ft) in depth, with a surface area of several acres. Anaerobic conditions prevail in the bottom region, where the solids are decomposed; the region near the surface is aerobic, allowing the oxidation of dissolved and colloidal organic matter. A reduction in BOD5 of 75 to 85 per cent can be attained.
Advanced Wastewater Treatment
If the receiving body of water requires a higher degree of treatment than the secondary process can provide, or if the final effluent is intended for reuse, advanced wastewater treatment is necessary. The term tertiary treatment is often used as a synonym for advanced treatment, but the two methods are not exactly the same. Tertiary, or third-stage, treatment is generally used to remove phosphorus, while advanced treatment might include additional steps to improve effluent quality by removing refractory pollutants. Processes are available to remove more than 99 per cent of the suspended solids and BOD5. Dissolved solids are reduced by processes such as reverse osmosis and electrodialysis. Ammonia stripping, denitrification, and phosphate precipitation can remove nutrients. If the wastewater is to be reused, disinfection by ozone treatment is considered the most reliable method other than breakpoint chlorination. Application of these and other advanced waste-treatment methods is likely to become widespread in the future in view of new efforts to conserve water through reuse. See Absorption; Precipitation.
"Sewage Disposal," Microsoft(R) Encarta(R) 99 Encyclopaedia. (c) 1993-1998 Microsoft Corporation. All rights reserved.
One impact that the burning of fossil fuels has had on the Earth's environment has been the increase of carbon dioxide (CO2) in the Earth's atmosphere. The amount of atmospheric CO2 apparently remained stable for millennia, at about 260 ppm (parts per million), but over the past 100 years it has increased to 350 ppm. The significance of this change is its potential for raising the temperature of the Earth through the process known as the greenhouse effect. Carbon dioxide in the atmosphere prevents the escape of outgoing long-wave radiation from the Earth to outer space; as more heat is produced and less escapes, the temperature of the Earth increases.
A significant global warming of the atmosphere would have profound environmental effects. It would speed the melting of polar ice caps, raise sea levels, change the climate regionally and globally, alter natural vegetation, and affect crop production. These changes would, in turn, have an enormous impact on human civilization. Since 1850 there has been a mean rise in global temperature of about 1� C (1.8� F). Most scientists have predicted that rising levels of CO2 and other "greenhouse gases" will cause temperatures to continue to increase, with estimates ranging from 2� to 6� C (4� to 11� F) by the mid-21st century. However, some scientists who research climate effects and trends dispute the theories of global warming, and attribute the most recent rise to normal temperature fluctuations.
"Environment," Microsoft(R) Encarta(R) 99 Encyclopaedia. (c) 1993-1998 Microsoft Corporation. All rights reserved.
During the 1980s, scientists began to find that human activity was having a detrimental effect on the global ozone layer, a region of the atmosphere that shields the Earth from the Sun's harmful ultraviolet rays. Without this gaseous layer, which is found at about 40 km (25 mi) above sea level, no life could survive on the planet. Studies showed the ozone layer was being damaged by the increasing use of industrial chemicals called chlorofluorocarbons (CFCs, compounds of fluorine) that are used in refrigeration, air-conditioning, cleaning solvents, packing materials, and aerosol sprays. Chlorine, a chemical by-product of CFCs, attacks ozone, which consists of three molecules of oxygen, by taking one molecule away to form chlorine monoxide. Chlorine monoxide then reacts with oxygen atoms to form oxygen molecules, releasing chlorine molecules that break up other molecules of ozone.
It was initially thought that the ozone layer was being reduced gradually all over the globe. In 1985, however, further research revealed a growing ozone hole concentrated above Antarctica; 50 per cent or more of the ozone above this area of the Earth was being depleted seasonally (beginning each October). A thinning of the ozone layer is the key factor in the greenhouse effect, and exposes life on Earth to excessive ultraviolet radiation, which can increase skin cancer and cataracts, reduce immune-system responses, interfere with the photosynthetic process of plants, and affect the growth of oceanic phytoplankton. Because of the growing threat of these dangerous environmental effects, many nations are working towards eliminating the manufacture and use of CFCs at least by the year 2000. However, CFCs can remain in the atmosphere for more than 100 years, so ozone destruction will continue to pose a threat for decades to come.
"Environment," Microsoft(R) Encarta(R) 99 Encyclopaedia. (c) 1993-1998 Microsoft Corporation. All rights reserved.
Describe the processes used to separate the U 235 from U-238.
The 235- UF6 diffuses more rapidly through a porous barrier. By using a multi stage separator the two types of molecules can be separated from one another to obtain relatively pure U-235 F6.
Gaseous Diffusion
UF6 (gas) is injected into the centre of a chamber. This gas is collected from around the edges and from centres. Because lighter isotopes diffuse faster than heavier ones, the gas around the edges will contain U-235 more than normal and vice versa for centre. This is called gaseous diffusion (ie. more U-235 than usual).