CHEM/TOX 336 Lecture 12/13 Acid Rain Aqueous Atmospheric Chemistry: Acid Rain Review Henry s Law: scavenging of water-soluble gases into clouds, fogs, and rain Review normal ph of rainwater ~ 5.6 due to dissolved CO 2 Acid precipitation a result of industrial activities: emission of SO2 and NO One major route to NO x deposition: gas phase oxidation O 3 or RO 2 OH NO(g) NO 2 (g) HNO 3 (g) HNO 3 (aq) deposition Several routes to SO 2 deposition: gas or aq. phase oxidation H 2 O SO 2 (g) H 2 SO 3 (aq) deposition OH H 2 O SO 2 (g) SO 3 (g) H 2 SO 4 (aq) deposition H 2 O [O] SO 2 (g) H 2 SO 3 (aq) H 2 SO 4 (aq) deposition
Acid rain long recognized as a problem; the air pollution problem of the 80s, but it is still with us Sources of acidic gas emissions NO x all combustion processes, but especially: transportation power generation (metal smelting) N 2 (g) + O 2 (g) 2NO(g) SO 2 smelting sulfidic metal ores: many commercially important metals occur as sulfides: Cu, Ni, Pb, Zn e.g. 2FeS 2 (s) + 5½O 2 (g) > Fe 2 O 3 (s) + 4SO 2 (g) 2NiS(s) + 3O 2 (g) > 2NiO + 2SO 2 (g) coal combustion (typically 2-3% sulfur by mass)
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 assures rapid approach to equilibrium: S/V = 3/r Removal of soluble species from the gas phase reduces their gas phase concentrations, slowing reaction rates scavenging of HO 2 slows the rate of gas phase oxidation of NO lower concentration of PAN in foggy air because CH 3 CO OO is scavenged into the aqueous phase Permanent removal if the droplet falls as rain (e.g. HNO 3 ) Possibility of ionic reaction mechanisms in solution (e.g. hydrolysis of N 2 O 5 ; oxidation of SO 2 by H 2 O 2 : see later)
Scattering light by droplets reduces light intensity, especially deep in a cloud, lowers J(O 3 ) and J(NO 2 ) Chemistry of Acid Rain For CO 2 : Net: CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) K H = 3.4 x 10-2 mol L -1 atm -1 H 2 CO 3 (aq) H + (aq) + HCO 3 - (aq) K a = 4.2 x 10-7 mol L -1 CO 2 (g) + H 2 O(l) H + (aq) + HCO 3 - (aq) K c = 1.4 x 10-8 mol 2 L -2 atm -1 For SO 2 : Net: SO 2 (g) + H 2 O(l) H 2 SO 3 (aq) K H = 1.2 mol L -1 atm -1 H 2 SO 3 (aq) H + (aq) + HSO 3 - (aq) K a = 1.7 x 10-2 mol L -1 SO 2 (g) + H 2 O(l) H + (aq) + HSO 3 - (aq) K c = 2.1 x 10-2 mol 2 L -2 atm -1 Low (ppbv) concentrations of SO 2 (g) change the ph of rainwater more than 375 ppmv of CO 2 because: SO 2 more soluble in water than CO 2 (K H ) H 2 SO 3 stronger acid than H 2 CO 3 (K a ) Oxidation of SO 2 Major oxidation route for SO 2 in dry air: OH H 2 O SO 2 (g) SO 3 (g) H 2 SO 4 (aq) deposition
Details: SO 2 (g) + OH(g) HSO 3 (g) k = 9 10-13 cm 3 molec -1 s -1 HSO 3 (g) + O 2 (g) SO 3 (g) + HO 2 (g) Oxidation rate: k ~ 10-6 s -1 t ½ ~ 7 10 5 s (8 days) Major oxidation route for SO 2 when the aqueous phase is present: H 2 O [O] SO 2 (g) H 2 SO 3 (aq) H 2 SO 4 (aq) deposition Details: SO 2 (g) H 2 SO 3 (aq) 2HO 2 H 2 O 2 + O 2 [in gas or aqueous phase] H 2 SO 3 (aq) + H 2 O 2 H 2 SO 4 (aq) + H 2 O [strongly ph dependent; faster at higher ph] Aqueous phase oxidation by O 3 is slower Oxidation rate: up to 10-30% per hour (t ½ ~ 2-7 h) typical oxidation rates 0.01-0.1 h -1 (t ½ ~ 2-20 h) Thus acid precipitation is a regional problem.
Model for rate as oxidation of SO 2 as a function of volume fraction of water SO 2 pollution a regional problem if t ½ ~ 2-20 h, and wind speed ~ 20 km/h, then SO 2 pollution is occurring over 40-400 km (one half-life) reasonable to assume that SO 2 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: CaCO 3 as a buffer natural erosion of caves and gorges CaCO 3 (s) + H 2 CO 3 (aq) Ca 2+ (aq) + 2HCO 3 - (aq) K = 5.3 10-5 (mol L -1 ) 2 at 25 C lakes and streams underlain by CaCO3(s) have high natural alkalinity. When acidification occurs: HCO 3 - (aq) + H + (aq) H 2 CO 3 (aq) CO 2 (g) the HCO 3 - (aq) consumed is replaced by dissolution of more CaCO 3 effects on structures, especially limestone and steel Net reaction for limestone can be written as: CaCO 3 (s) + H + (aq) Ca 2+ (aq) + HCO 3 - (aq) K = 1.3 10 +2 mol L -1 at 25 C in the case of sulfur oxide emissions, sulfation leads to flaking off from the surface CaCO 3 (s) + ½O 2 (g) + SO 2 (g) CaSO 4 (s) Read for yourselves text pp. 176-182: natural waters and aluminum solubility
Aluminum solubility aluminum speciation: solubility minimum near ph 6.5 Al 3+ (aq) AlOH 2+ (aq) Al(OH) 2 + (aq) Al(OH) 3 (s) Al(OH) 4 - (aq) Fluoride raises the overall solubility of aluminum: relevant to aluminum smelters which tend to release HF Al 3+ (aq) AlF 2+ (aq) AlF 2 + (aq) Arsenic lowers the concentration of dissolved aluminum: Environ. Sci. Technol. 1990, p. 1774 Al 3+ (aq) + AsO 4 3- AlAsO4 (s) [oversimplified!]
Abatement of acidic emissions NO x New technology involving ammonia injection into the exhaust gas stream: NO x + NH 3 N 2 + H 2 O (not balanced) Proposed use at Southdown gas-fired generating station in Mississauga; question of whether highly polluting Lakeview and Nanticoke stations should be decommissioned Particularly useful for gas-fired plants where there is no SO 2 in the flue gases SO 2 from coal combustion Combustion of 1 tonne of coal that is 2% sulfur by mass 80,000 mol CO 2 320,000 mol N 2 600 mol of SO 2 (~0.15% of the total: uneconomic to recover) Flue Gas Desulfurization (FGD) technology to remove SO 2 by passing a slurry of ground lime or limestone down the stack as the hot flue gases pass upwards SO 2 + Ca(OH) 2 CaSO 3 + H 2 O also SO 2 + Ca(OH) 2 + ½O 2 CaSO 4 + H 2 O SO 2 + CaCO 3 CaSO 3 + CO 2 Improved combustion methods coal cleaning: separate finely divided coal particles by froth flotation, since coal has d = 2.3 g cm -3 while pyrite FeS 2, the main sulfur species has d = 4.5 g cm -3 fluidized bed combustion: mix finely ground coal with limestone and burn the fine [articles on a screen so that the particles are supported by the combustion air train.
Sulfur in the coal CaSO 3 /CaSO 4 SO 2 from metal refining Problem is sulfide ores e.g. 2FeS 2 (s) + 5½O 2 (g) Fe 2 O 3 (s) + 4SO 2 (g) 2NiS(s) + 3O 2 (g) 2NiO + 2SO 2 (g) Unlike coal combustion, there is enough SO 2 to collect as SO 2 (l) or to convert into H 2 SO 4. Unfortunately, both these are very cheap commodity chemicals; H 2 SO 4 by this route must compete with purer material from virgin sulfur or natural gas sweetening. SO 2 (g) + ½O 2 (g) SO 3 (g) [V 2 O 5 catalyst, 450 C] H 2 O SO 3 (g) + H 2 SO 4 (l) H 2 SO 4 SO 3 (l) H 2 SO 4 INCO (Sudbury) has reduced SO 2 emissions by 95% since the 1970s The INCO Superstack (photo from http://www.geo.utexas.edu/students/cmcfar/thestack.htm The stack, erected 1970-1972, is 400 m high and is the tallest free standing stack in the world... an example of dilution is the solution to pollution, I m afraid!! Land reclamation at the INCO site: Dan Shaw, http://www.hort.agri.umn.edu/h5015/ 99papers/shaw.htm