acid rain(t15)

ACID RAIN

Topic 15

ACID RAIN• Acid rain is also known as acid precipitation or

acid deposition.• Acid precipitation is commonly used to mean

the deposition of acidic components in rain, snow, dew or dry particles.

• Acid rain occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds.

• The droplets then fall to earth as rain, snow, mist,dry dust, hail or sleet.

ACID RAIN• The term “acid rain” is sometimes used more

generally to include all forms of acid deposition – both wet and dry deposition.

• Wet deposition – occurs when any form of precipitation (rain, snow, etc) removes acid from the atmosphere and delivers it to the earth’s surface. This can result from the deposition of acids in raindrops or by the precipitation, removing the acids either in clouds or below clouds. Wet removal of both gases and aerosol

are both of importance for wet deposition.

ACID RAIN• Dry deposition – occurs via deposition in

the absence of precipitation and can be responsible for as much as 20 to 60% of total acid deposition. This occurs when particles and gases stick to the ground, plants or other surfaces.

ACID RAIN/ACID PRECIPITATION

• Acid precipitation is not a recent discovery.• Robert Angus Smith, Britain’s first Air

Pollution Inspector, first used the phrase in the late 1800s.

• He was the first person to make the link between sulfur pollution and acidic rainfall.

NORMAL RAIN/PRECIPITATION• Precipitation is usually somewhat acidic, as the

carbon dioxide (CO2) occurring naturally in the air dissolves in it, creates a solution of carbonic acid (H2CO3)

H2O + CO2 H2CO3

• Carbonic acid is not stable and dissociates in water forming hydronium ions and hydrogen carbonate with a pH around 5.6H2CO3 + H2O HCO3

+ H3O+

So normal rainfall is slightly acidic (pH 5.6)

ACID RAIN/ACID PRECIPITATION

• Generally, the pH of 5.6 has been used as the baseline in identifying acid rain because this is the pH value of carbon dioxide in equilibrium with distilled water.

• Hence, acid rain is defined as any rainfall that has an acidity level beyond which is expected in non-polluted rainfall. (below pH 5.6)

CAUSES OF ACID RAIN• Sulfur dioxide and oxides of nitrogen are the

primary molecules that contribute to acid rain.• SO2 emissions are responsible for 60-70% of the

acid deposition that occurs globally• More than 90% of the sulfur in the atmosphere is

of human origin.• Some 95% of the elevated levels of nitrogen

oxides in the atmosphere are the result of human activities.

• The remaining 5% comes form several natural processes.

SOURCES OF SO2

Main sources of sulfur dioxide include:1. coal burning – coal typically contains 2-3%

sulfur so when it is burned, sulfur dioxide is liberated.

2. The smelting of metal sulfide ores to obtain the pure metals. Metals such as zinc, nickel and copper are all commonly obtained in this manner.

3. volcanic eruption4. organic decay

5. ocean spray6. petroleum

SOURCES OF NITROGEN OXIDES

• The major sources of nitrogen oxides include:

1. Combustion of oil, coal and gas. 2. Bacterial action in soil 3. Forest fires 4. Volcanic action 5. Lightning

ACID DEPOSITION FORMATION• Commonly due to secondary pollutants that form

from the oxidation of nitrogen oxides (NOx) or sulfur dioxide (SO2) gases that are released into the atmosphere.

• Reactions at the earth’s surface or within the atmosphere can convert these pollutants into nitric acid, sulfuric acid and particles of acid-forming sulfate and nitrate salts

• The process of altering these gases into their acid counterparts can take several days, and during this time these pollutants can be transported hundred of kilometers from their original source.

ACID DEPOSITION FORMATION

• SO2 and nitrogen oxides are converted into acids by a complex atmospheric processes involving several chemical reactions.

• It is important to consider both aqueous and gaseous phase chemistries in the formation of acid rain.

AQUEOUS PHASE CHEMISTRY OF SO2

• Partially soluble in water• Consequently, only a fraction of

atmospheric SO2 exists in the dissolved aqueous form when there is a cloud or mist content in the air.

• In the aqueous phase, SO2(aq) exists in equilibrium with sulfite, SO3

2-(aq) and bisulfite, HSO3

-(aq) ions.


The dissociation of gaseous sulfur dioxide in water occurs by 3-fold process, as follows:

1. SO2(g) SO2(aq)2. SO2(aq) + 2H2O(l) H3O+(aq) +HSO3

-(aq)3. HSO3

-(aq) + H2O(l) H3O+(aq) + SO32-(aq)

These equilibria are critically dependent upon the pH of the precipitation (since hydrogen ions are involved in reactions 2 and 3 as well as droplet size. The sulfite and bisulfite ions may be oxidized by a number of atmosphericmechanisms to sulfuric acid


• The oxidation of aqueous bisulfite or sulfite by molecular oxygen relies on a metal catalyst such as Fe3+ or Mn2+ or a combination of both.

• Oxidation by ozone is more appreciable as it does not require a catalyst .

• However, the dominant oxidation process occurs by the action of hydrogen peroxide (formed in the gas phase from free radicals) This reaction involves the formation of an intermediate, possibly HSO4

- and may proceed as follows:

HSO3-(aq) + H2O2(g) HSO4

-(aq) + H2O (l) HSO4

-(aq) + H+(aq) H2SO4(aq)


• Gaseous sulfur dioxide will also dissolve in water to form sulfurous acid (H2SO3)

SO2(g) + H2O(l) H2SO3(aq)

• The relative concentration of SO2 and H2SO3(aq) are related by the equilibrium constant for this reaction.

GASEOUS PHASE CHEMISTRY OF SO2

SO2 .OH, O3 several steps

H2SO4 SO42-

SO2 can be oxidized into sulfuric acid in the gaseous state, Ozone, .OH radicals and H2O2 play a role here.

AQUEOUS PHASE CHEMISTRY OF NOX

• There are three equilibria to consider in the aqueous oxidation of NOx:

1. 2NO2(g) + H2O(l)2H+(aq) + NO3-(aq) + NO2

-(aq)2. NO(g) + NO2(g) + H2O(l) 2H+(aq) + 2NO2

-(aq)

3. 3NO2(g) + H2O(l) 2H+(aq) + 2NO3-(aq) + NO(g)

• These reactions are limited by their dependence upon the partial pressures of NOx present in the atmosphere, and the low solubility of these oxides. Potential for increase in reaction rate exists with the use of metal catalysts, similar to those used in the aqueous oxidation of SO2

GASEOUS PHASE CHEMISTRY OF NO

NO O3, OH2 NO2 OH, M

HNO3 NO3-

The oxidation of NO via ozone and HO2 radicals produces NO2 firstly and then nitric acid after the interaction of impact partner M via OH radicals

EFFECTS OF ACID PRECIPITATION

Acid precipitation has been a major environmentalconcern for many decades now. Studies show that iteffects:• lakes and aquatic ecosystems• trees and soils• the atmosphere• architecture• materials• humans

EFFECTS ON LAKES AND AQUATIC ECOSYSTEMSSeveral routes through which acidic chemicals can enter lakes.

• They may enter directly as dry particles falling through the air or as wet particles falling as precipitation, including rain, snow, sleet, hail, dew or fog.

• Lakes can be thought of as ‘sinks’ of the earth, whereby precipitation falling onto the land is drained into them via surface run-off and ground water. Acid precipitation falling on to the earth washes nutrients out of soil and carries toxic metals that have been released from soil into lakes.

An example of this latter method is spring acid shock when snow melts rapidly due to a sudden temperature change. This releases acid and chemicals into the soils . The melted snow then runs off into streams and rivers and gradually makes it way into the lakes. The aquatic ecosystem does not have time to adjust to the sudden change. Spring is when many aquatic species like amphibians, fish and insects reproduce. This sudden pH change is dangerous because the acids can cause serious deformities in the immature creatures or annihilate whole species since the young of many such species spend a significant part of their lifecycle in water.

EFFECTS ON LAKES AND AQUATIC ECOSYSTEMSAcids in water may affect the fish in lakes in two ways:

a) Direct effects• Acid directly interferes with the fish’s ability to take in

oxygen, salt and nutrients. For freshwater fish, maintaining osmoregulation is key to their survival. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules in water irritate the gills of fish, causing mucus to form that prevents the fish absorbing oxygen efficiently and eventually suffocate.

• Low pH affects balance of salts in fish tissues. Salts of Ca2+ migrate away from acid waters, depleting calcium in fish tissue which leads to weak spines and deformities e.g weak exoskeleton and brittle or weak eggs.

EFFECTS ON LAKES AND AQUATIC ECOSYSTEMSb) Indirect effects

• Acids cause metals to be dissociated and released e.g Al3+ .

Like acid molecules, Al3+ ions irritate the gills of fish, causing mucus to form and accumulate in their organs to toxic levels.

• In general as acidification starts, and the pH of the lake decreases, crustaceans start to die out, mainly because of reproductive problems.

At pH 5.6, algal growth is hindered and some species die. As the pH falls, larger fish start to die through suffocation and

reproductive problems. At pH 5, any surviving fish tend to be thin, deformed and

unable to reproduce. At pH 4.5, lakes are nearly sterile.

EFFECTS ON LAKES AND AQUATIC ECOSYSTEMS

EFFECT ON AQUATIC ECOSYSTEM

EFFECTS OF ACID PRECIPITATION ON HUMANS

Acid deposition can influence human health through the following methods:

• Toxic metals, such as mercury and aluminum, can be released into the environment through the acidification of soils. The toxic metals can then end up in the drinking water, crops, and fish, and are then ingested by humans through consumption. If ingested in great quantities, these metals can have toxic effects on human health. One metal, aluminum, is believed to be related to the occurrence of Alzheimer's disease.

• Metals can be leached into public water supplies from pipes and soils under acidic conditions. Long term exposure to elevated concentrations of metals may lead to bioaccumulation in tissues and subsequent detrimental health effects e.g kidney damage.

EFFECTS OF ACID PRECIPITATION ON HUMANS

• Increased concentrations of sulfur dioxide and oxides of nitrogen have been correlated to increased hospital admissions for respiratory illness.

• Research on children from communities that receive a high amount of acidic pollution show increased frequencies of chest colds, allergies, and coughs.

EFFECTS ON TREES AND SOILS• SO2 and acidic aerosols affect the stomata in tree leaves

and hinder photosynthesis.• Minerals in soils (K, Mg, Ca, Na) can be washed away

and replaced with hydrogen ions which inhibit photosynthesis. These minerals are transferred to soil solutions and removed as run-off or in groundwater. If this continues, the silicate structure of the soil is gradually destroyed.

• If soils become very acidic, aluminium from clay minerals is freed and is absorbed by tree roots. So trees in acidic soils will gradually become starved of their vital nutrients (calcium and magnesium) and poisoned by the aluminium.

EFFECTS ON TREES AND SOILS• Reductions in soil pH can cause germination of seeds and

the growth of young seedlings to be inhibited. • Many important soil organisms cannot survive is soils below a

pH of about 6.0. The death of these organisms can inhibit decomposition and nutrient cycling.

• High concentrations of nitric acid can increase the availability of nitrogen and reduce the availability of other nutrients necessary for plant growth. As a result, the plants become over-fertilized by nitrogen (a condition known as nitrogen saturation).

• Dry deposition of SO2 and NOx has been found to affect the ability of leaves to retain water when they are under water stress.

• Acid precipitation can cause direct damage to the foliage on plants especially when the precipitation is in the form of fog or cloud water which is up to ten times more acidic than rainfall.

ATMOSPHERIC EFFECTS• Constituents of acid pollution –

atmospheric nitrates, sulfates can contribute to haze.

• Haze can reduce visibility and interfere with atmospheric energy processes by absorbing sunlight. E.g. in the Artic, haze limits growth of lichen, thus reducing its availability as a foodstuff for carbon and reindeer.

EFFECTS ON ARCHITECTURE• Acid particles and precipitation deposited onto

stone buildings, monuments and statues cause corrosion and stone damage.

• Many historic buildings have been disfigured or damaged irreparably by acid deposition.

• Many ancient buildings and monuments are composed of limestone, sandstone and marble. Limestone (CaCO3) and turn to a crumbling substance called gypsum (calcium sulfate) upon contact with the acid, which explains the corrosion of buildings and statues. Gypsum is readily soluble and will be washed off the stone surface by the action of rain, thus leaving a fresh surface of limestone exposed to further attack

EFFECTS ON ARCHITECTURE• Building materials such as paint, plastic

and steel may be damaged by acid attack.• Any iron that is exposed to acid in the

presence of oxygen will rapidly dissolve and wash away, so weakening the building structure and exposing fresh iron to further corrosion.

• Economically taxing to replace costly protective coatings with greater frequency.

EFFECTS ON MATERIALS• Acid rain may damage materials such as fabrics such as

flags being ‘eaten away’ by acidic chemicals in the precipitation.

• Many old books and works of art have deteriorated because the ventilation systems of the libraries and museums that hold them do not prevent acidic particles from entering the building.

• Acidic water corrodes water pipes so leading to more frequent pipe replacement.

• Copper solubility increases sharply below pH 5.0 and also with increasing temperature.

• Metals leached from pipe walls can be consumed and bio-accumulated by humans, leading to detrimental health effects.

EXAMPLES OF INDUSTRIES EMITTING SO2

• Transport• Chemical• Cement• Fertilizer• Paper & Pulp• Glass• Ceramics• Iron & Steel• Nonferrous metals

• Petroleum• Refineries• Paper Manufacturers• Metal Smelting• Food Preparation• Power Plants• Petroleum Products• Coal Mining &

Production

EXAMPLES OF INDUSTRIES EMITTING NOX

• Petroleum• Paper & Pulp• Cement• Ceramics• Iron & Steel

• Nonferrous metals• Petroleum Products• Refineries• Transport

REDUCING ACID DEPOSITION• Using low-sulfur lignite coal.

However, low-sulfur lignite coal has low heating value, so more coal must be burnt to generate the same amount of electricity. This increases air pollution by emitting more CO2, toxic mercury, and radioactive particles into the troposphere. Also increase costs.

REDUCING ACID DEPOSITION

• Looking into alternate affordable and cleaner energy sources.Using wind turbines and burning natural gas in turbines to produce electricity instead of coal.

Other alternative energy sources e.g. solar, hydroelectric, nuclear, geothermal etc.

REDUCING ACID DEPOSITION• By controlling acid deposition

However, this is a political issue as people and ecosystem affected by acid rain are usually quite distant from those who cause the problem.

• Reduce coal useCountries with large supplies of coal e.g

China, India and U.S have a strong incentive to use it as a major energy resource.

REDUCING ACID DEPOSITION• Adding pollution control equipment

1. Catalytic converters in cars to remove NOx

2. Remove SO2 particulates and NOx from smokestack gases. Expensive. Flue Gas Desulfurization (FGD)

3. Adding lime to neutralize acidified lakes or surrounding soil. Several problems with liming. It is expensive and temporary remedy that usually must be repeated annually. It can kill some types of plankton and aquatic plants and can harm wetland plants that need acidic water. Finally, it is difficult to know how much lime to put and where to put.

FLUE GAS DESULFURIZATION (FGD)• Tall flue gas stacks disperse the emissions by

diluting the pollutants in ambient air and transporting them to other regions.

• As a result of stringent environmental protection regulations, SO2 is now being removed from flue gases by a variety of methods.

• Is the current state –of –the art technology used for removing sulfur dioxide (SO2) from the exhaust flue gases in power plants that burn coal or oil.

• FGD can remove 95% or more of the SO2 in the flue gases.

FLUE GAS STACKS

FLUE GAS DESULFURIZATION

METHODS USED TO REMOVE SO2 FROM FLUE GASES

• Wet scrubbing using a slurry of sorbent, usually limestone or lime to scrub the gases.

• Spray-dry scrubbing using similar sorbent slurries.

• Dry sorbent injection systemsThe highest SO2 removal efficiencies (greater than 90%) are achieved by wet scrubbers.

WET SCRUBBING• Wet scrubbers remove dust particles by capturing them

in liquid droplets. • In a wet scrubber, the polluted gas stream is brought into

contact with the scrubbing liquid, by spraying it with the liquid, by forcing it through a pool of liquid, or by some other contact method, so as to remove the pollutants.

• The design of the wet scrubbers depends on the industrial process conditions and the nature of the air pollutants involved.

• Inlet gas characteristics and dust properties (if particles are present) are of primary importance.

• Scrubbers can be designed to collect particulate matter and/or gaseous pollutants.

• If the gas stream contains both particle matter and gases, wet scrubbers are generally the only single air pollution control device that can remove both pollutants.

ADVANTAGES OF WET SCRUBBERSFor particulate control, wet scrubbers whenevaluated against fabric filters and electrostatic precipitators (ESPs)• Have the ability to handle high temperatures and

moisture.• The inlet gases are cooled, resulting in smaller

overall size of equipment.• Can remove both gases and particulate matter.• Can neutralize corrosive gases.

DISADVANTAGES OF WET SCRUBBERS

• Corrosion• The need for entrainment separation or

mist removal to obtain high efficiencies.• The need for treatment or reuse of spent

liquid.

FGD CHEMISTRY WITH WET SCRUBBERS

Scrubbing with basic solid or solution.SO2 is an acid gas and thus the typical sorbent

slurries or other materials used to remove the SO2

from the flue gases are alkaline. • The reaction taking place in a wet scrubbing

using a CaCO3 (limestone) slurry produces CaSO3 (calcium sulfite) and can be expressed as:CaCO3(s) + SO2(g) CaSO3(s) + CO2(g)

FGD CHEMISTRY WITH WET SCRUBBERS

• When wet scrubbing with Ca(OH)2 (lime) slurry, the reaction also produces CaSO3 (calcium sulfite) and can be expressed asCa(OH)2(s) + SO2(g) CaSO3(s) + H2O(l)

• When wet scrubbing with a Mg(OH)2 (magnesium hydroxide) slurry, the reaction also produces MgSO3 (magnesium sulfite) and can be expressed as:Mg(OH)2(s) + SO2(g) MgSO3(s) + H2O(l)

• Some FGD systems go a step further and oxidize the CaSO3 to produce marketable CaSO4.2H2O (gypsum)

CaSO3(s) + ½O2(g) + 2H2O(l) CaSO4.2H2O(s)

2006

KEASIDAN AIR HUJAN TAHUN 2006

2008Keasidan Air Hujan 2008

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acid rain(t15)

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