Aqueous Atmospheric Chemistry: Acid Rain

• Review Henry’s Law: scavenging of water-soluble gasesinto clouds, fogs, and rain

• Review normal pH of rainwater ~ 5.6 due to dissolvedCO2

• Acid precipitation a result of industrial activities:emission of SO2 and NO

One major route to NO x deposition: gas phase oxidation

O3 or RO2 OHNO(g) —> NO2(g) —> HNO3(g) —> HNO3(aq) —> deposition

Several routes to SO2 deposition: gas or aq. phase oxidation H2O

SO2(g) —> H2SO3(aq) —> deposition

OH H2OSO2(g) —> SO3(g) —> H2SO4(aq) —> deposition

H2O [O]SO2(g) —> H2SO3(aq) —> H2SO4(aq) —> deposition

• Acid rain long recognized as a problem; “the” airpollution problem of the ‘80s, but it is still with us

Sources of “acidic gas” emissions

• NOx all combustion processes, but especially:• transportation• power generation• (metal smelting)

N2(g) + O2(g) º 2NO(g)

• SO2S coal combustion (typically 2-3% sulfur by mass)S smelting sulfidic metal ores: many commercially

important metals occur as sulfides: Cu, Ni, Pb, Zn

e.g. 2FeS2(s) + 5½O2(g) —> Fe2O3(s) + 4SO2(g)2NiS(s) + 3O2(g) —> 2NiO + 2SO2(g)

Acidic Deposition – US Data

• Upper panels sulfate; lower panels nitrate• Left hand panels scales are ½ those of right hand panels

Importance of aqueous atmospheric chemistry

• High surface to volume ratio of small droplets assuresrapid approach to equilibrium: S/V = 3/r

• Removal of soluble species from the gas phase reducestheir gas phase concentrations, slowing reaction ratesS scavenging of HO2 slows the rate of gas phase

oxidation of NOS lower concentration of PAN in foggy air because

CH3CO.OO is scavenged into the aqueous phase

• Permanent removal if the droplet falls as rain (e.g.,HNO3)

• Possibility of ionic reaction mechanisms in solution (e.g.,hydrolysis of N2O5; oxidation of SO2 by H2O2: see later)

• Scattering light by droplets reduces light intensity,especially deep in a cloud, lowers J(O3) and J(NO2)

Chemistry of Acid Rain

For CO2:CO2(g) + H2O(l) º H2CO3(aq)

KH = 3.4 x 10-2 mol L-1 atm-1

H2CO3(aq) º H+(aq) + HCO3-(aq)

Ka = 4.2 x 10-7 mol L-1

)))))))))))))))))))))))))))))))))))))))))))

CO2(g) + H2O(l) º H+(aq) + HCO3-(aq)

Kc = 1.4 x 10-8 mol2 L-2 atm-1

For SO2:SO2(g) + H2O(l) º H2SO3(aq)

KH = 1.2 mol L-1 atm-1

H2SO3(aq) º H+(aq) + HSO3- (aq)

Ka = 1.7 x 10-2 mol L-1

)))))))))))))))))))))))))))))))))))))))))))

SO2(g) + H2O(l) º H+(aq) + HSO3- (aq)

Kc = 2.1 x 10-2 mol2 L-2 atm-1

• Low (ppbv) concentrations of SO2(g) change the pH ofrainwater more than 375 ppmv of CO2 because:S SO2 more soluble in water than CO2 (KH)S H2SO3 stronger acid than H2CO3 (Ka)

Oxidation of SO2Major oxidation route for SO2 in dry air:

OH H2OSO2(g) —> SO3(g) —> H2SO4(aq) —> deposition

Details:SO2(g) + OH(g) —> HSO3(g) k = 9×10!13 cm3

molec!1 s!1

HSO3(g) + O2(g) —> SO3(g) + HO2(g)

Oxidation rate: k' ~ 10!6 s!1 —> t½ ~ 7×105 s (8 days)

Major oxidation route for SO2 when the aqueous phase ispresent:

H2O [O]SO2(g) —> H2SO3(aq) —> H2SO4(aq) —> deposition

Details:SO2(g) —> H2SO3(aq)2HO2 —> H2O2 + O2 [in gas or aqueous phase]H2SO3(aq) + H2O2 —> H2SO4(aq) + H2O

[strongly pH dependent; faster at higher pH]Aqueous phase oxidation by O3 is slower

Oxidation rate: up to 10-30% per hour (t½ ~ 2-7 h); typicaloxidation rates 0.01-0.1 h!1 (t½ ~ 2-20 h). Thus acidprecipitation is a regional problem.

Model for rate as oxidation of SO2 as a function of volumefraction of water

SO2 pollution a regional problem• if t½ ~ 2-20 h, and wind speed ~ 20 km/h, then SO2

pollution is occurring over 40-400 km (one half-life)• reasonable to assume that SO2 pollution can extend up to

~ 2000 km

Effects of acidic emissions• effects on plants, on aquatic life, through lowering pH

• susceptible and non-susceptible lakes: CaCO3 as a bufferS natural erosion of caves and gorges

CaCO3(s) + H2CO3(aq) º Ca2+(aq) + 2HCO3!(aq)

K = 5.3×10!5 (mol L!1)2 at 25/C

S lakes and streams underlain by CaCO3(s) have highnatural alkalinity. When acidification occurs:HCO3

!(aq) + H+(aq) —> H2CO3(aq) —> CO2(g)the HCO3

!(aq) consumed is replaced by dissolutionof more CaCO3

S effects on structures, especially limestone and steel Net reaction for limestone can be written as:

CaCO3(s) + H+(aq) º Ca2+(aq) + HCO3!(aq)

K = 1.3×10+2 mol L!1 at 25/C

S in the case of sulfur oxide emissions, “sulfation”leads to flaking off from the surface

CaCO3(s) + ½O2(g) + SO2(g) —> CaSO4(s)

S Read for yourselves text pp. 176-182: natural watersand aluminum solubility

Aluminum solubility• aluminum speciation: solubility minimum near pH 6.5

Al3+(aq) º AlOH2+(aq) º Al(OH)2+(aq) º Al(OH)3(s) º Al(OH)4

!(aq)

• Fluoride raises the overall solubility of aluminum: relevantto aluminum smelters which tend to release HF

Al3+(aq) º AlF2+(aq) º AlF2+(aq)

• Arsenic lowers the concentration of dissolved aluminum:Environ. Sci. Technol. 1990, p. 1774

Al3+(aq) + AsO43! º AlAsO4(s) [oversimplified!]

Abatement of acidic emissionsNOx New technology involving ammonia injection into theexhaust gas stream:

NOx + NH3 —> N2 + H2O (not balanced)

S Proposed use at Southdown gas-fired generatingstation in Mississauga; question of whether highlypolluting Lakeview and Nanticoke stations should bedecommissioned

S Particularly useful for gas-fired plants where there isno SO2 in the flue gases

SO2 from coal combustionCombustion of 1 tonne of coal that is 2% sulfur by mass —>

S 80,000 mol CO2S 320,000 mol N2S 600 mol of SO2 (~0.15% of the total: uneconomic to

recover)

Flue Gas Desulfurization (FGD) technology to remove SO2by passing a slurry of ground lime or limestone down the stackas the hot flue gases pass upwards

SO2 + Ca(OH)2 —> CaSO3 + H2Oalso SO2 + Ca(OH)2 + ½O2 —> CaSO4 + H2O

SO2 + CaCO3 —> CaSO3 + CO2

Improved combustion methods• coal cleaning: separate finely divided coal particles by

froth flotation, since coal has d = 2.3 g cm!3 while pyriteFeS2, the main sulfur species has d = 4.5 g cm!3

• fluidized bed combustion: mix finely ground coal withlimestone and burn the fine [articles on a screen so that theparticles are supported by the combustion air train. Sulfurin the coal —> CaSO3/CaSO4

SO2 from metal refining• Problem is sulfide ores

e.g. 2FeS2(s) + 5½O2(g) —> Fe2O3(s) + 4SO2(g)2NiS(s) + 3O2(g) —> 2NiO + 2SO2(g)

Unlike coal combustion, there is enough SO2 to collect asSO2(l) or to convert into H2SO4. Unfortunately, both these arevery cheap commodity chemicals; H2SO4 by this route mustcompete with purer material from virgin sulfur or natural gassweetening.

SO2(g) + ½O2(g) º SO3(g) [V2O5 catalyst, 450/C]

H2OSO3(g) + H2SO4 (l) —> H2SO4 .SO3(l) —> H2SO4

INCO (Sudbury) has reduced SO2 emissions by 95% since the1970s

The INCO Superstack (photo fromhttp://www.geocities.com/Pentagon/5094/inco.jpg

The stack, erected 1970-1972, is 400 m high and is the tallestfree standing stack in the world ... an example of “dilution isthe solution to pollution”, I’m afraid!!

Land reclamation at the INCO site: Dan Shaw,http://www.hort.agri.umn.edu/h5015/99papers/shaw.htm