Chuong Hoa hoc khi quyen (3) pps

1 1 Chapter 11 GASEOUS INORGANIC POLLUTANTS Environmental Chemistry, 9th Edition Stanley E. Manahan Taylor and Francis/CRC Press 2010 11.1 INORGANIC POLLUTANT GASES Aside from CO2, most common from anthrosphere sources are • CO • SO2 • NO • NO2 Others from pollutant sources • NH3 • N2O • N2O5 • H2S • Cl2 • HCl • HF Large quantities from natural sources • NH3 • N2O • H2S 2 3 11.2 PRODUCTION AND CONTROL OF CARBON MONOXIDE About 0.1 parts per million by volume in atmosphere • Residence time of 36-110 days • About 2/3 CO is intermediate in oxidation of CH4 by HO• Most common pollutant problem is localized in urban areas from automotive emissions • Controlled by computerized engine control and exhaust catalysts 4 11.3 FATE OF ATMOSPHERIC CO Oxidized by reaction with hydroxyl radical • CO + HO•→ CO2 + H Production of hydroperoxyl radical • O2 + H + M →HOO• + M Additional reactions • HOO• + NO →HO• + NO2 • HOO• + HOO•→ H2O2 + O2 • H2O2 + hν→HO• Soil micoorganisms metabolize CO • Soil is a sink for CO 5 11.4 SULFUR DIOXIDE SOURCES AND THE SULFUR CYCLE Of the order of 100 million tons of sulfur enter the atmosphere from the anthrosphere each year • Primarily as SO2 from combustion of sulfur-containing fossil fuels Fluxes of sulfur from the geosphere and biosphere are large but uncertain • H2S and SO2 from volcanic activity • H2S from action of anoxic bacteria on organosulfur compounds and SO42- • (CH3)2S from marine microorganisms 6 Figure 11.1 The global sulfur cycle, fluxes in millions of metric tons per year 7 11.5 SULFUR DIOXIDE REACTIONS IN THE ATMOSPHERE SO2 is oxidized to sulfate • H2SO4 in aerosol droplets • Sulfate salts, predominantly NH4HSO4 and (NH4)2SO4 Sulfuric acid and sulfates account for much of the atmospheric haze Oxidation of SO2 under various conditions 1. Very rapid under oxidizing conditions that occur in presence of N oxides, hydrocarbons, and sunlight (photochemical smog conditions) 2. In solution inside water aerosol droplets 3. On catalytic solid surfaces Hydroxyl radicals react with SO2 leading to oxidation Other atmospheric oxidants of SO2 • Ozone, O3 • Hydrogen peroxide, H2O2 Metal ions catalyze SO2 oxidation • Fe(III) • Mn(II) 8 Effects of Atmospheric Sulfur Dioxide Not particularly toxic • People with some respiratory conditions (asthma) are sensitive to SO2 exposure Incidents of acute air pollution have been associated with high SO2 levels • 1930, Meuse River Valley, Belgium • 1948, Donora, Pennsylvania • 1952, London Phytotoxic to some plants • Leaf chlorosis (bleaching) Erosion of surfaces of stone, such as dolomite • CaCO3•MgCO3 + 2SO2 + O2 + 9H2O→CaSO4•2H2O + MgSO4•7H2O + 2CO2 9 Sulfur Dioxide Removal Removed from coal combustion products Fluidized bed combustion of coal in a bed of granular material that sequesters SO2 SO2 removal from stack gas (see Table 11.1) Dry processes using alkaline sorbents, such as Ca(OH)2, are not very effective Lime slurry scrubbing of stack gas is most used • Wet throwaway process • Ca(OH)2 + SO2→CaSO3 + H2O • CaSO3 may be oxidized to produced gypsum, CaSO4•2H2O Wet regenerative systems use alkaline absorbent, such as Na2SO3 solution, from which SO2 is recovered • Some SO2 reduced to H2S • Claus reaction to produce S: 2H2S + SO2→3S + 2H2O (Example of a green process) 11.6 Nitrogen Oxides in the Atmosphere • N2O emitted in relatively large quantities from natural and some pollutant sources • NO from combustion, especially internal combustion engine, lightning • NO2, largely secondary from NO oxidation • NO3 radical, secondary pollutant NO and NO2 are collectively termed NOx • One of the six criteria pollutants regulated by U.S. Environmental Protection Agency Some tens to over 100 million tons of NOx emitted to the atmosphere annually 10