Environmental chemistry

Environmental chemistry. study of the effect of human activity on the chemical processes in the environment concerns political and natural borders global issues applied chemistry. Main topics: core. air pollution acid deposition greenhouse effect ozone depletion.

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carbon monoxide
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Environmental chemistry

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Environmental chemistry • study of the effect of human activity on the chemical processes in the environment • concerns political and natural borders • global issues • applied chemistry
Main topics: core • air pollution • acid deposition • greenhouse effect • ozone depletion • dissolved oxygen in water • water treatment • soil • waste
Main topics: AHL • ozone depletion • smog • acid deposition • water and soil
Environmental chemistryE1: air pollution Describe the main sources of carbon monoxide (CO), oxides of nitrogen (NOx), oxides of sulfur (SOx), particulates and volatile organic compounds (VOCs) in the atmosphere. Evaluate current methods for the reduction of air pollution.
the atmosphere
composition of dry clean airhttp://www.dep.state.pa.us/earthdaycentral/97/air_teachers/TAB1.htm
Units used to express amount of a gas % by volume ppm = parts per million/particles per million particles Remember in gases, assuming conditions are the same: same volume = same number of particles So 78.09 % by volume is 78.09 % particles per million particles or 780,900 ppm
primary air pollutants • waste products from human activity • primary = added directly to the air • pollutant = chemical in the wrong concentration in the wrong place • primary air pollutants: CO NOx particulates SOx volatile organic compounds (VOCs)
Air pollutants For each air pollutant you need to know: • sources: natural and man-made • methods of reducing its emissions • any relevant balanced symbol equations
carbon monoxide: sources • Natural: • atmospheric oxidation of methane CH4 + ½ O2  CO + 2H2 • natural forest fires • Man-made: • incomplete combustion of carbon-containing fuels; mainly in cities; localized problem as well as fluctuations during the day – heavy traffic. • forest fires
carbon monoxide: health effect • CO combines with Fe in haemoglobin in blood – bonds 320 times stronger than oxygen – oxygen cannot bond onto heamoglobin. • Less oxygen supplied to body cells. • Effects: • headaches, • shortness of breath, • in case of high concentration (e.g. rush hour): unconsciousness, death.
carbon monoxide: reduction (1) Lean burn engines Decrease fuel /air ratio (in mass) from 1:14.7 (stoichiometric ratio/green band in diagram) to 1:18. Why? (or increase air /fuel ratio to 18:1)
carbon monoxide: reduction (2) catalytic converter in car exhaust system • reduction of nitrogen oxide • oxidation of CO and unburnt hydrocarbons/VOCs 2CO (g) + O2(g) 2CO2 (g) 2NO (g) + 2CO (g) 2CO2 (g) + N2 (g) 2C8H18 (g) + 25O2 (g) 16CO2 (g) + 18H2O (g)
Catalytic converter
catalytic converter To achieve high rate of reaction for this redox process: • exhaust gases are passed over platinum which acts as a heterogeneous catalyst; • converter has a honey comb structure to increase surface area; • very high temperature of the exhaust gases also increase the rate of reaction dramatically.
Thermal exhaust reactor Heat from engine makes CO react with more air to oxidise it further to carbon dioxide.
sulphur oxides: sources • Natural: • volcanoes • sea spray • biological decay of organic matter that contains sulphur • reduction of sulphates • Man-made: • coal-burning power stations (equation starting from S in coal) • roasting of metal sulphides e.g. ZnS and Cu2S (equation)
sulphur oxides: health effects • acidic oxides • lung irritants, affect in particular those suffering from respiratory problems e.g. asthma sufferers • formation of sulphuric acid aerosols (droplets of sulphuric acid) (equation) (often catalysed by metal particulates); effects of aerosols: • irritant to the eyes • irritate vessels in lungs causing impaired breathing
sulphur oxides: methods of reduction • use of “low-sulphur content” fuels or removal of sulphur before burning coal/oil • removal of SO2from fumes before they are released: • limestone based fluidised bed(equations showing decomposition of calcium CaCO3reaction of CaO with SO2) • alkaline scrubbing (wet scrubber) (equations showing reaction of CaO and Mg(OH)2)
Sulphur oxides: reduction (1) Wet scrubber Limestone based fluidised bed
nitrogen oxides: sources • Natural: • Electrical storms release enough energy to cause oxidation of atmospheric nitrogen: (equations showing oxidation of nitrogen and further oxidation of NO) • Decomposition of organic matter containing nitrogen • Man-made: Combustion of fossil fuels in car engines and furnaces of power stations produces hightemperaturesto cause oxidation : (equation showing oxidation of nitrogen)
nitrogen oxides: health effects • choking irritating gas, affects eyes and people with respiratory problems • forms nitric acid aerosols/acid rain (equation showing dissolution of nitric acid in water) • nitric acid also increases the rate of oxidation of SO2 (see later) • plays an important role in the formation of secondary pollutants e.g. ozone and smog
nitrogen oxides: reduction • catalytic converter • lean burn engines: high air/fuel ratio or low fuel/air • recirculation/reburn of exhaust gases: nitrogen oxide emissions are reduced by reintroducing exhaust gases into the fuel mixture, lowering peak combustion temperatures as it is the high temperature in the combustion engine which causes nitrogen oxide production.
Particulates: sources particulates = airborne/suspended liquid and solid particles. • Natural: • volcanic eruptions • large forest fires • Man-made: • burning fossil fuels e.g. diesel • forest fires • industrial emissions; chemical processes • incinerators
Particulates: health effects • particulates penetrate lungs and may block air passages • some are poisonous e.g. Pb and asbestos • adsorb chemicals and can act as catalysts in reactions producing secondary pollutants • by adsorbing also increase concentration and rate of reaction • reduce visibility
Particulates: reduction Electrostatic precipitator Particulates are given a negative charge and then attracted onto positively collection plates .
Particulates: reduction
Volatile organic compounds: sources • Natural sources: • methane: bacterial anaerobic decomposition of organic matter (e.g. in rice paddies) • from plants e.g. terpenes • leakage from natural fossil reserves • Man-made: • evaporation of fuels • partial combustion of fuels • leakage from storage reservoirs
VOCs: health effects • photochemical smog • can lead to carcinogenic compounds • fatigue, weakness • respiratory problems VOCs: reduction • catalytic converter

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