Chuong Hoa hoc khi quyen (3) pps

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 H

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

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

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

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

Figure 11.1 The global sulfur cycle, fluxes in millions of metric tons per year

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)

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

Sulfur Dioxide Removal

Removed from coal combustion products

Fluidized bed combustion of coal in a bed of granular

material that sequesters SO2

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

Atmospheric Reactions of NOx

Figure 11.3 Principal reactions among NO, NO2, and HNO3

in the atmosphere where ROO • represents an organic

peroxyl radical such as CH3O •

Harmful Effects of Nitrogen Oxides

NO relatively insignificant

NO2 harmful to health

• Fatal bronchiolitisfibrosaobliterans

NO2 leads to formation of corrosive and harmful nitric acid and nitrates (see Figure 11.3)

Most significant reaction of atmospheric NO2 is the

following producing O atoms that participate in important atmospheric chain reactions:

• NO2 + hν→ NO + O

• Leads to harmful photochemical smog formation

• NOx often limiting reactant in production of photochemical smog

Control of Nitrogen Oxides

1 Prevent production by control of combustion conditions

2 Removal from exhaust gas after combustion

Prevention of production

• Low excess air firing to limit O in N2 + O2→ 2NO

• Low combustion temperatures prevent NO formation

Removal from exhaust gas

• Sorption by base relatively ineffective due to low acidity of

N oxides

• Most commonly by catalytic reduction with reducing

agents

• Automotive exhaust catalysts reduce NOx with slight excess CO and hydrocarbons

Biofilters consisting of microorganisms on fixed or fluidized supports are an experimental approach to NOx control

11.7 Acid Rain

Acidic precipitation from strong acids (see Chapter 14)

• HNO3 secondary pollutant from NOx

• H2SO4 secondary pollutant from SO2

• HCl typically from combustion of organohalide polymers

11.8 Ammonia in the Atmosphere

Numerous sources of atmospheric NH3

• Soil microorganisms • Decay of animal wastes

• Ammonia fertilizer • Sewage treatment • NH3 synthesis

• Leakage of ammonia-based refrigeration systems

• Livestock and feedlot operations largest U.S source (more than 100 km/year)

Ammonia is the major base in the atmosphere

Ammonia removed by its water solubility and basicity

• With rain as NH3(aq)

• By reaction with H2SO4, HNO3, HCl

Effects of atmospheric ammonia

• Ammonium sulfate major atmospheric aerosol – visibility

• Ammonium salts cause corrosion

• Damage to foliage

11.9 Fluorine, Chlorine and their Gaseous Compounds

F2 and HF are rare air pollutants

• Extremely toxic

• React even with glass

Plants susceptible to flourides

Sulfur hexafluoride, SF6, is ultrastable

• Very strong greenhouse gas

Chlorine and Hydrogen Chloride

Elemental chlorine, Cl2, was first military poison gas used

• Occasional spills of Cl2 still cause fatalities

Hydrogen chloride, HCl, rapidly forms hydrochloric acid droplets in the atmosphere

• Major acid rain constituent

11.10 Reduced Sulfur Gases

Major reduced sulfur gases in the atmosphere

• Hydrogen sulfide, H2S • Carbonyl sulfide, OCS

• Carbon disulfide, CS2 • Dimethyl sulfide, S(CH3)2

Hydrogen sulfide is most damaging reduced sulfur gas in the atmosphere

• About as toxic as hydrogen cyanide

• Phytotoxic

• Damage to materials

• Formerly lead paint pigments

Most acute hydrogen sulfide releases from natural gas

• 1950 Poza Rica, Mexico, incident killed 22

• 2003 release in China killed 242

Hydrogen sulfide in the atmosphere is quickly oxidized to SO2