Patrick J. Garnett
Edith Cowan University
Tribute to Sir Noel BaylissI would like to begin by paying tribute to Sir Noel Bayliss who was Professor of Chemistry at the University of Western Australia during my time as an undergraduate and postgraduate student in the 1960s. Sir Noel's contribution to Chemistry in this State is immense. During his time as Professor of Chemistry he developed the School of Chemistry into one of the most respected research and teaching schools in the country. Thank you, Sir Noel, for your contribution to Chemistry and for the opportunities you provided to students of several generations.IntroductionIncreasing awareness of environmental issuesThe past two decades have witnessed an astonishing growth in public awareness of environmental issues. Several factors have contributed to this increased interest in the environment.
- The American space missions of the 1960s and 1970s and the accompanying T.V. pictures beamed back to Earth showing the planet as a tiny living sphere in the immensity of space;
- The influence of authors and conservationists such as Rachel Carson, Paul Ehrlich and David Suzuki who have graphically described the effect of human impacts on the environment;
- A series of increasingly serious global and local environmental issues such as
- acid rain
- photochemical smog
- the greenhouse effect
- ozone depletion
- eutrophication of wetlands and waterways
- encroaching salinity
- acute pollution episodes e.g. Exon Valdez, Bhopal, Chernobyl.
This paper focusses on the chemistry of our environment, particularly the hydrosphere and the atmosphere. By better understanding the chemical nature and processes within the environment, we should be able to solve some of the present problems with which we are faced and learn to live and function in a manner which is more harmonious with nature and which is sustainable for future generations.
Elemental distributionI would like to start by going back to the beginning, to the basic building blocks of matter - the elements, and to the beginning of the Universe.The Periodic Table includes the symbols of all the elements which make up the Universe. Of the first 92 elements, 90 occur naturally on the Earth while elements 93 to 106 have been synthesised in the laboratory.
Elemental abundances in the Universe, the Earth and its crust differ quite markedly. Within the Universe hydrogen and to a lesser extent helium are by far the most abundant elements. Indeed, most scientists believe that shortly after the Big Bang which seems to have marked the beginning of the Universe as we know it, virtually all the mass of the Universe existed as hydrogen. Gradually over the 10-20 billion years since that time some of the hydrogen has undergone a complicated series of nuclear reactions in the stars to form the heavier elements from which the Earth and other planets are formed.
In the lower temperature, less hostile conditions which prevail on Earth the elements and their constituent atoms are no longer subject to the transmutation processes which occur in the stars. As a result the elemental composition of the Earth is essentially fixed. While chemical processes can alter the arrangements of the elemental atoms into different combinations and compounds the actual abundances of the elements in the Earth can be considered as fixed.
Environmental cycles
Although the percentages of each element on the Earth are fixed, atoms of the elements are constantly recycled by the various chemical, biological and physical processes through which the biosphere and the environment function on our living planet. This recycling of atoms of the elements can be represented using environmental cycles. These show the distribution of an element in various reservoirs and the movement of the element between reservoirs. Environmental cycles have been developed for several elements including carbon, nitrogen, oxygen, phosphorus and sulfur.
Carbon cycle
The major carbon reservoirs and the processes by which carbon moves between these reservoirs are shown in Figure 2.
Major reservoirs include the atmosphere, plants, dead organic matter, dissolved carbon in oceans, sediments and fossil fuels.
Processes by which carbon moves between these reservoirs include photosynthesis, respiration, decomposition, fossil fuel combustion, dissolving and precipitation. Chemical equations representing these processes are shown below.
Human impacts can profoundly affect the rate of movement of elements between reservoirs and produce significant effects at both the local and global levels. These changes may distort the balanced processes which normally occur in natural systems. For example, human activities such as the combustion of fossil fuels and deforestation are thought to contribute to an increase in CO2 inputs to the atmosphere of about 7 billion tonnes per year. Of this about 4 billion tonnes per year is being taken up by increased amounts dissolved in the oceans and in the Earth's biomass, resulting in a net gradual build up of about 3.5 billion tonnes per year in the atmosphere.
Water and aquatic systemsSimilar to the elemental cycles described previously, water passes through a natural cycle known as the water or hydrologic cycle (Figure 3). The water cycle, like the elemental cycles, is powered by the sun which provides the energy required for evaporation and air transport processes.
Water in natural freshwater systems (lakes and rivers)
Liquid water in natural systems is virtually never pure. This is because water is an excellent solvent. It dissolves gases from the air and, because of its polar nature, it also dissolves ionic salts as it makes its way in rivers or underground systems towards the sea.
Dissolved gases
Gases dissolved in water because of its interaction with the atmosphere include nitrogen, oxygen and carbon dioxide.
Oxygen
The presence of dissolved oxygen is of crucial importance in natural systems as it is critical to the sustainability of aquatic life. The level of dissolved oxygen depends on the temperature, the stability of the water system, and the rates of photosynthesis and respiration. The solubility of oxygen, like other gases, decreases as the temperature increases.
Dissolved oxygen is needed by aquatic plants and animals so that respiration and decomposition processes can take place. The level of dissolved oxygen or D.O. is a useful indicator of water quality in aquatic ecosystems. If the D.O. level falls to less than about half of that needed to saturate the water then the water quality is likely to be poor. This may be the result of high levels of organic matter such as plant and animal wastes which use up the dissolved oxygen as they are oxidised. If the D.O. gets too low this can result in the death of plants and animals and lead to anaerobic decomposition products such as methane, ammonia and hydrogen sulfide.
Carbon dioxideCarbon dioxide from the atmosphere dissolves quite readily in rain, seawater and freshwater. When it dissolves carbon dioxide forms carbonic acid which behaves as a weak acid.CO2 + H2O ⇌ H+ + HCO3-
HCO3- ⇌ H+ + CO32-
This explains why rainwater and distilled water which has been exposed to the atmosphere has a pH of about 6, whilst that of pure water is equal to 7. Carbon dioxide is essential for photosynthesis by aquatic plants and also plays an important role in the formation of limestone deposits, caves and cave formations.
Dissolved Salts- sodium ion, Na+
- potassium ion, K+
- calcium ion, Ca2+
- magnesium ion, Mg2+
- chloride ion, Cl-
- sulfate ion, SO42-
- hydrogencarbonate ion, HCO3-
In Western Australia and in many other parts of Australia salinity is a considerable problem in agricultural areas. This is a result of excessive forest clearing in the past. As a consequence the water table has risen in these areas, mobilising the salt and resulting in saline seeps and increased stream salinity.
Increased salinity decreases the amount of useful land available for agriculture. Increased stream salinity decreases the amount of good drinking water available. Various techniques are now being used in an attempt to reduce this problem including reduced clearing and reafforestation.
Water hardnessAs well, scale formation in kettles and boilers can be a significant problem when used with hard water.
Dissolved nutrients- nitrate ion, NO3-
- ammonium ion, NH4+
- phosphate ion in its various forms, PO43-
While these species are essential to support plant growth in aquatic ecosystems, high levels of nutrients resulting from sewage outlets, animal wastes (eg. piggeries), excessive application or run-off of fertilisers, and the over-use of phosphate-containing detergents can lead to the serious problem of eutrophication. High nutrient concentrations stimulate algal growth, sometimes resulting in the formation of algal blooms. Enhanced algal growth reduces the ability of light to penetrate the system and increases the oxygen demand of the system. Lower level plants may die causing the dissolved oxygen levels to fall further as the dead plants decompose. With reduced levels of dissolved oxygen the system is less able to support higher forms of life which may result in the death of various aquatic organisms including fish. In addition, decomposition in the absence of oxygen tends to occur through the activity of anaerobic bacteria leading to the formation of unpleasant by- products such as methane (CH4 , hydrogen sulfide (H2S) and ammonia (NH3).
Algal blooms may include macroalgae; a range of microalgae including green algae and the notorious blue-green algae; diatoms; and dinoflagellates. The effects of algal blooms include deoxygenation (with resultant deaths of bottom animals and fish), decreasing aesthetic appeal (formation of scum and unpleasant odours), threat to water supplies (unpleasant taste and organic load), and the possible release of toxins (threat to wildlife, stock and human health).
Some recent examples of the problems of eutrophication have occurred in
- The Murray-Darling River system in the Eastern States;
- The Peel-Harvey Estuary at Mandurah - leading to the Dawesville Cut;
- The upper reaches of the Swan River.
Considerable efforts are now underway to minimise these problems by improved catchment management involving reduced fertiliser use and the better management of animal waste and sewage.
Water for drinking- Hills dam storage;
- Unconfined groundwater aquifers e.g. Gnangara and Jandakot Mounds;
- Confined artesian water e.g. Leederville and Yarragadee Formations.
Chlorination involves treatment with chlorine (Cl2), a greenish yellow gas which is a strong oxidising agent, an algicide and bacteriocide. It is used to destroy potential pathogens and also to prevent the growth of algae in water pipes. Sufficient chlorine is usually added to provide a residual minimum of 0.2 to 0.5 mg/L or ppm.
Chlorine is also used in swimming pools for the same reasons. In household pools instead of using chlorine gas which is dangerous and quite difficult to handle it is common to supply chlorine in some other form such as
- calcium hypochlorite (white powder) - Ca(ClO)2
- sodium hypochlorite (solution) - NaClO
- sodium dichloroisocyanurate (white powder) - NaCl2C3N3O3.2H2O
Sodium hexafluorosilicate (Na2SiF6) or fluorosilicic acid (H2SiF6) is added in small amounts to our water supply to provide concentrations of about 1 ppm of fluoride ion (F-). At these levels the fluoride is built into the dental enamel in children's teeth which are consequently much more resistant to dental decay.
Treatment of groundwater- Maintains the Earth at an average temperature of 15°C compared with a temperature of -18°C if the Earth had no atmosphere;
- Shields the Earth's surface from potentially harmful ultraviolet radiation;
- Provides CO2 for photosynthesis;
- Provides O2 for respiration;
- Provides a medium for the hydrologic cycle;
- Pressurises the environment.
- Troposphere - up to about 15 km and containing the Earth's weather systems;
- Stratosphere - 15-50 km and containing the ozone layer
- Mesosphere - 50-85 km;
- Thermosphere - >85 km.
The composition of dry, pure air
| CO2 | 0.03% |
| N2 | 78.1% |
| O2 | 21.0% |
| Ar | 0.9% |
* The atmosphere contains traces of other gases.
Normal rain is slightly acidic and has a pH of about 6 due to the presence of dissolved CO2.
Rain is usually described as acid rain if it has a pH lower than 5. The worst examples of acid rain are probably those in Western Europe and the North East of the United States. These occur in and adjacent to densely populated and highly industrialised regions in these continents. In some areas rain with a pH of 4 is quite common and rainfall events with a pH close to 2 have occurred. In Australia rain with a pH of 3.6 has been recorded in the lower Hunter Valley near Sydney.
Acid rain is usually the result of rain dissolving
- sulfur oxides - SO2 and SO3
- nitrogen dioxide - NO2
Human outputs of sulfur oxides into the atmosphere are usually the result of burning fossil fuels (eg. oil and coal) in electric power stations and the smelting of sulfide ores in smelting plants.
S(in fuel) + O2 → SO2MS(sulfide ore) + 3/2O2 → MO + SO2
2SO2 + O2 → 2SO3
Human outputs of nitrogen oxides are usually the result of high temperature internal combustion processes such as those found in motor cars. These bring about the reaction of atmospheric N2 and O2:
N2 + O2 → 2NO2NO + O2 → 2NO2
When dissolved in water these sulfur and nitrogen oxides produce acids as illustrated below:
SO2 + H2O → H2SO3SO3 + H2O → H2SO4
4NO2 + 2H2O + O2 → 4HNO3
Acid rain can have the following effects:
- Make soils, surface waters and takes acidic with resulting damage to aquatic plants and animals including fish;
- Damage plants including crops and forests;
- Corrosion of metal and stone (especially limestone/marble) buildings.
With regard to nitrogen oxide pollution research has focused on improving technology to reduce the amount of nitrogen oxides produced as a side-effect of the combustion of fossil fuels.
Photochemical smogThe requirements for the formation of photochemical Smog are intense sunlight (containing some ultraviolet light), stagnant air (possibly resulting from a temperature inversion), nitrogen oxides and unburned hydrocarbons from automobile emissions.
A simplified reaction scheme for the formation of photochemical smog is set out below:
1. Production of nitrogen oxides.
N2 + 02 → NO
2NO + 02 → 2NO2
2. Absorption of ultraviolet light by NO2 to form atomic oxygen and ozone.
NO2 + hv → NO + O
O2 + O → O3
3. Reactions of hydrocarbons with atomic oxygen and ozone to form various organic free radicals.
Hydrocarbons + O → R* + ROO*
Hydrocarbons + O3 → R* + ROO*
4. Formation of aldehydes, peroxyacyl nitrates etc.
e.g. CH3CHO (ethanal), CH3COOONO2 (peroxyacetyl nitrate)
The release of hydrocarbons and initial formation of NO and NO2 is followed later in the day by increasing levels of ozone and other oxidants. A diagram of the variations in the concentrations of the various pollutants in photochemical smog during the day is shown in Figure 4.
Increasingly on still summer days in Perth, as in Sydney and Melbourne, we are seeing the beginnings of a photochemical smog problem. This is mainly the result of the extremely high dependence of city dwellers on the use of cars. The development of an improved public transport system and the use of various incentives and disincentives will be essential components of the city's approach if it is to avoid a significant photochemical smog problem in the future.
Ozone depletionA graph of ozone concentration with altitude, (Figure 5) shows a maximum of about 10 ppm at an altitude of about 85 km. As previously mentioned, this "ozone layer" plays a critical role in filtering potentially harmful ultraviolet rays from solar radiation and preventing them from reaching the Earth's surface.
Within the stratosphere ozone is produced and consumed according to the so-called Chapman mechanism which maintains ozone levels at a more or less steady state. The reactions in a simplified Chapman Cycle are as follows:
Production:O2 + hv → 2O
O3 + O → O3
Decomposition:
O3 + hv → O2 + O
O + O → O2
The decomposition reactions can be catalysed by a variety of species such as HO*, NOx and Cl*.
As early as 1974 some scientists were warning of the potential threat which CFCs posed for the ozone layer. However the first definitive evidence was provided by British scientists who reported significant declines in ozone over the Antarctic in 1985. Debate ensued as to the cause of this decline; increased NOx levels, CFCs and changing atmospheric transport processes all being mentioned as possible causes.
A crucial NASA Airborne Antarctic Ozone Experiment conducted in spring 1987 shed considerable light on this debate. This experiment involved about 150 scientists, ground and aeroplane-based laboratories. The experiment involved analysing stratospheric chemical species as a function of latitude. The graph in Figure 6 shows that ozone levels as a function of latitude were almost a mirror image of ClO levels and clearly implicated chlorine as part of the ozone depletion problem. This chlorine is thought to arise from the photochemical decomposition of CFCs as they diffuse to higher altitudes in the atmosphere.
The reason that ozone levels fall so drastically over the Antarctic during Spring seems to result from the peculiar meteorological conditions which exist at this time. Air in the so-called polar vortex is stagnant during winter and early spring. This very cold air produces stratospheric clouds containing ice particles which seem to play a critical part in the catalytic decomposition of the ozone. As a result of the NASA experiment governments around the world are now moving quickly to phase out the use of CFCs in its various applications. A series of international agreements starting with the 1987 Montreal Protocol has now brought forward the phasing out of most CFCs in industrialised countries to 1995. Despite this phasing out it is predicted that it will take the best part of a century for atmospheric chlorine levels to decrease to their 1970 levels.
Greenhouse effectWithin the Earth's atmosphere several gases are increasing in concentration which have the potential to increase the Earth's natural greenhouse effect and produce global warming. These include carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and ozone.
These gases are produced as follows:
- Carbon dioxide from combustion of fossil fuels;
- Nitrous oxide from combustion processes and fertiliser use;
- Ozone in photochemical smog;
- CFCs from air conditioning units, use as refrigerants, foaming agents, and solvents;
- CH4 from enteric fermentation by ruminants (flatulent sheep and cows!) and anaerobic methanogenesis in rice paddy fields and landfills.
Figure 7 show the changes in carbon dioxide over the last 30 years.
The evidence is quite clear that anthropogenic (human) activities are changing the composition of the atmosphere. Changes to the major atmospheric components, nitrogen and oxygen, are insignificant. However the relative changes in the concentrations of the minor atmospheric constituents is quite marked. For example, carbon dioxide levels are projected to increase to about 600ppm by the middle of the next century.
The effect of these changes in the levels of the minor atmospheric components is difficult to predict with certainty. Scientists are using computer models called General Circulation Models (GCMs) which consist of extremely complicated mathematical equations used to predict winds, temperatures, pressures, cloud cover and precipitation as changes in atmospheric composition take place.
These models are both complex and crude. The models contain huge amounts of information but are crude in that one datapoint represents a 500km square grid on the Earth's surface. The different models may consider four, nine or 15 vertical levels in the atmosphere. There are enormous problems in trying to cope with the complexities of atmospheric circulation patterns, the interaction of ocean and atmospheric dynamics, and feedback processes associated with clouds, the biosphere and the Earth's surface albedo.
The postulated consequences of changes in the Earth's atmosphere vary from model to model and there is presently a vigorous and vitriolic debate on the extent of the changes which are likely to take place. The models are tending to suggest the following changes:
- Temperature increases of the order of 2-4°C;
- Increased but uneven rainfall (10%);
- Rising sea levels (10 - 80 cm);
- Poleward shift of climatic zones;
- Increased frequency of extreme weather events;
- Threat to some ecosystems due to the rapidity of climatic change.
While there is no doubt that the atmosphere's composition is changing there is considerable debate over the extent to which climatic change is or is not currently taking place. One extensive study concluded that temperatures had increased by about 0.5°C over the past century but this result has been questioned. The IPCC panel concluded that sea levels have increased by 10-20cm over the same period but once again there is discussion over the validity of this claim. Thus there are competing views on the extent to which we are already seeing climatic changes associated with changing atmospheric composition.
The difficulty of policy setting in this climate of uncertainty is clearly a dilemma for decision makers. The two extremes in position can be summarised as follows:
- Policy makers must wait for greenhouse science to get better. It is premature to make policies based on fragile data especially as the economic consequences could be severe.
- Uncertainty should not be used as an excuse for inaction especially as the longer action is delayed the greater the extent to which the Earth will be committed to irreversible greenhouse effects.
- Encourage and promote global cooperation on environmental issues;
- Achieve population stabilisation;
- Plan for sustainable economic development;
- Develop and promote the use of alternative forms of energy and energy efficient technology;
- Encourage everyone to make an individual commitment to the environment.
Good Luck!
