Environmental Chemistry notes

Environment and Environmental Pollution

The environment consists of all living and non-living components surrounding organisms. The main components are the atmosphere, hydrosphere, lithosphere and biosphere. Human activities can disturb the natural balance of these components.

Environmental pollution is the undesirable change in the physical, chemical or biological properties of air, water or land. Pollutants may be gases, liquids, solids, microorganisms, heat or radiation.

• Atmosphere is the layer of gases surrounding Earth.
• Hydrosphere includes oceans, rivers, lakes, groundwater and water vapour.
• Lithosphere includes the solid outer part of Earth, including soil and rocks.
• Biosphere includes all living organisms and the regions where life exists.
• Major sources of pollution include industries, vehicles, fossil fuel combustion, sewage, agricultural fertilizers and pesticides.
• Dust storms are natural events and are not usually classified as an anthropogenic source of pollution.
• A pollutant is a substance or form of energy present in an amount that produces harmful effects.

Atmospheric Pollution

Atmospheric pollution is the presence of harmful gases, vapours or suspended particles in air. Important air pollutants include carbon monoxide, carbon dioxide, sulphur oxides, nitrogen oxides, hydrocarbons, ozone, smoke, dust and particulate matter.

Fossil fuel combustion in power plants, industries and vehicles is a major source of nitrogen oxides. It produces NO and NO2 mainly at high temperatures. Nitrous oxide, N2O, is also associated with combustion and biological processes.

Coal contains approximately 1 to 3 percent sulphur. During burning, sulphur is oxidized to sulphur dioxide, which may further form sulphur trioxide and sulphuric acid in the atmosphere.

• Major air pollutants include CO, CO2, SO2, SO3, NO, NO2, hydrocarbons, ozone, smoke and suspended particles.
• Fossil fuel combustion is a major source of NO, N2O and NO2 in the atmosphere.
• N2O5 is not generally listed as a common atmospheric pollutant among the oxides of nitrogen.
• Coal contains about 1 to 3 percent sulphur.
• Sulphur dioxide can cause respiratory irritation and contributes to acid rain.
• Carbon monoxide is a poisonous gas formed by incomplete combustion of carbon-containing fuels.
• Petroleum is called black gold because of its economic value and its dark colour.

Acid Rain and Greenhouse Effect

Acid rain is rainwater having a pH lower than the natural value of about 5.6. Normal rain is slightly acidic because carbon dioxide dissolves in water and forms weak carbonic acid. Sulphur dioxide and nitrogen oxides make rain more acidic by forming sulphuric acid and nitric acid.

The greenhouse effect is the warming of Earth caused by gases that absorb outgoing infrared radiation. A natural greenhouse effect is necessary for life, but an increased concentration of greenhouse gases causes global warming and climate change.

• The pH of truly acidic rain is less than 5.6.
• SO2 and NOx are major contributors to acid rain.
• Acid rain can damage crops, forests, aquatic life, buildings and marble structures.
• Important greenhouse gases include CO2, CH4, N2O, water vapour and ozone.
• Besides carbon dioxide and nitrous oxide, methane is a significant greenhouse gas.
• The residence time of methane in the atmosphere is approximately 3 to 7 years.
• Burning fossil fuels increases the atmospheric concentration of carbon dioxide.

Ozone Layer and Ozone Depletion

Ozone, O3, is present in greatest concentration in the stratosphere. The ozone layer absorbs much of the Sun's harmful ultraviolet radiation and protects living organisms from excessive exposure.

Chlorofluorocarbons are stable in the lower atmosphere but can reach the stratosphere. Ultraviolet radiation breaks their bonds and releases chlorine radicals. These radicals catalytically destroy ozone molecules, so one chlorine radical can destroy many ozone molecules.

• The protective ozone layer is present in the stratosphere.
• Ozone absorbs harmful ultraviolet radiation from the Sun.
• Ozone depletion increases exposure to ultraviolet radiation.
• Skin cancer is a disease associated with depletion of the ozone layer.
• CF2Cl2, also written as CCl2F2, is a chlorofluorocarbon responsible for ozone depletion.
• Ultraviolet energy is sufficient to break the Cl-F bond in CCl2F2.
• Chlorine radicals participate in chain reactions that convert ozone into oxygen.
• Ozone in the stratosphere is protective, whereas ozone near ground level is an air pollutant.

Water Pollution and Water Pollutants

Water pollution is the contamination of water by substances that make it harmful or unsuitable for drinking, domestic use, agriculture or aquatic life. Water pollutants may be physical, chemical or biological.

Major causes include domestic sewage, industrial effluents, agricultural fertilizers, pesticides, petroleum products, detergents, plastics, heavy metals and disease-causing microorganisms. Phosphate pollution commonly comes from sewage and agricultural fertilizers.

Biodegradable organic matter provides food for microorganisms. These microorganisms consume dissolved oxygen during decomposition. The amount of oxygen needed by microorganisms to decompose biodegradable organic matter is called biochemical oxygen demand, or BOD.

• Causes of water pollution include sewage, industrial waste, fertilizers, pesticides, oil, detergents and pathogenic organisms.
• Phosphate pollution is caused mainly by sewage and agricultural fertilizers.
• Excess phosphates promote algal growth and cause eutrophication.
• Eutrophication can reduce dissolved oxygen and kill fish and other aquatic organisms.
• High BOD indicates a high amount of biodegradable organic matter in water.
• Sewage and organic industrial waste increase the BOD of water.
• Polycyclic hydrocarbons are carcinogenic water pollutants.
• Chloroform can cause cancer in the human liver and may be formed as a harmful chlorination by-product.

Water Purification and Potable Water

Potable water is water that is safe for drinking. Water treatment normally involves screening, sedimentation, coagulation, filtration and disinfection. The exact arrangement may vary, but the purpose is to remove suspended matter, dissolved impurities and microorganisms.

During coagulation, a chemical causes fine suspended particles and colloids to join together into larger particles called flocs. These flocs settle more easily. Alum is commonly used as a coagulant in the purification of potable water.

Chlorination is used to kill many disease-causing microorganisms. However, chlorine can react with organic substances in water to form chlorinated organic compounds, including chloroform and other trihalomethanes. It can also react with ammonia to form chloramines.

• Alum is the common coagulant used in the purification of potable water.
• Coagulation removes fine suspended and colloidal particles.
• Sedimentation allows heavier flocs to settle at the bottom.
• Filtration removes remaining suspended particles through sand, gravel or other filter media.
• Chlorination disinfects water by killing many bacteria and other microorganisms.
• Chlorination may be harmful when water contains organic matter or ammonia.
• Chloroform is a chlorination by-product associated with liver toxicity and cancer risk.
• Potable water should be free from harmful pathogens and dangerous concentrations of toxic chemicals.

Solid Waste and Landfill Pollution

Solid waste includes household refuse, industrial waste, agricultural waste, construction waste and biomedical waste. Improper disposal contaminates land and may pollute surface water and groundwater.

A landfill is a site where solid waste is deposited and covered. When groundwater or rainwater passes through waste, it dissolves and carries soluble substances, suspended particles and microorganisms. The contaminated liquid formed is called leachate.

• Leachate is a mixture of dissolved, suspended and microbial contaminants formed in a landfill.
• Leachate can enter groundwater and make it unsafe.
• Landfill gases may include methane and carbon dioxide produced during anaerobic decomposition.
• Waste management includes reduction, reuse, recycling, composting and safe disposal.
• Recycling reduces the amount of waste sent to landfills.
• Newspaper can be recycled approximately five times because paper fibres become shorter and weaker after repeated processing.
• Biodegradable waste can be decomposed by microorganisms, while many plastics persist for long periods.

Green Chemistry and Industrial Ecology

Green chemistry is the design of chemical products and processes that reduce or eliminate the use and formation of hazardous substances. It aims to prevent pollution at its source rather than treating pollution after it has formed.

Industrial ecology applies ecological ideas to industrial systems. Waste from one process may become a raw material for another process. This reduces the consumption of raw materials and energy and lowers the production of waste.

• Green chemistry favours prevention of waste rather than waste treatment.
• Safer solvents and safer chemical products should be used whenever possible.
• Green processes should use less energy and preferably operate at ordinary temperature and pressure.
• Renewable raw materials are preferred over non-renewable resources.
• Catalysts are preferred because they increase reaction rate without being consumed in stoichiometric amounts.
• Atom economy aims to incorporate the maximum number of reactant atoms into the desired product.
• Industrial ecology includes waste minimization, recycling, energy efficiency and reuse of by-products.
• A sustainable industrial system tries to imitate natural cycles in which waste from one process becomes a resource for another.