Composition of acid rain - Essay Example

Acid Rain Acid rain or acid precipitation is the "atmospheric deposition of acidified rain, snow, sleet, hail, acidifying gases and particles, as well as acidified fog and cloud water" (Likens, 2007). It also refers to precipitation that is much more acidic than natural, unpolluted rain. Measurement of Acid Rain "The pH scale is used to measure the acidity or alkalinity of an aqueous solution and is determined by the hydrogen ion (H+) content" ("Measuring Acid Rain"). This scale ranges from 0 to 14, moving from strongly acid to strongly alkaline, respectively, with the point 7 representing neutral. (source: Environment Canada) "The pH scale is logarithmic rather than linear" ("Measuring Acid Rain"). Hence, there is a tenfold increase in acidity with each pH point. Natural rain is mildly acidic because of the presence of carbon dioxide present in atmospheric moisture, which react together to form weak carbonic acid (H2CO3): CO2(g) + H2O(l) H2CO3(aq) H2CO3 is a weak acid and so it partially dissociates to release H+ (aq), resulting in the reduction of pH of the system. This partial ionization occurs in equilibrium, since carbonic acid only ionizes 1.7% ("Sources of Acid Rain"): H2CO3 H+ (aq) + HCO3 - (aq) This results in the PH of about 5.6 for the unpolluted, natural rain. Therefore, only rain having a pH of less than 5.0 is considered to be acid rain ("Sources of Acid Rain"). Composition of Acid Rain The two dominant acids found in acid rain are sulfuric acid (H2SO4) and nitric acid (HNO3). These acids are created from the primary pollutants sulfur dioxide SO2 and nitrogen oxides such as NO2. These pollutants are usually carried over long distances from their primary source, where they finally result in acid rain. The chemistry of both these pollutants from this source to the creation of acid rain is shown below: 1. Sulfur dioxide reacts with water to form sulfurous acid (H2SO3): SO2 (g) + H2O (l) H2SO3 (aq) Sulfurous acid molecules then react with water producing equilibrium with H+ (aq) and hydrogen sulfite: H2SO3 (aq) + H2O (l) H+ (aq) + HSO3- (aq) Sulphur dioxide (SO2) can also be oxidized gradually to sulfur trioxide (SO3): 2SO2 (g) + O2 (g) 2SO3 (g) Or react with ozone to form sulfur trioxide (SO3): SO2 (g) +O3 SO3 (g) +O2 (g) Sulfur trioxide (SO3) then acts with water to form sulfuric acid (H2SO4): SO3 (g) + H2O (l) H2SO4 (aq) The sulfuric acid produced, by the above reactions, is a very strong acid that ionizes 100% in atmospheric precipitation and produce H+ (aq) ions: H2SO4 (aq) H+ (aq) + HSO4 - (aq) These hydrogen ions are responsible for the acidic effects of the resultant acidic rain. ("Sources of Acid Rain") Sulfur dioxide is mainly released into the atmosphere through combustion of fossil fuels. The world over, sulfur dioxide is also released by volcanoes and also by the oxidation of sulfur gases released by the decomposition of plants. This natural sulfur dioxide is released very high into the atmosphere, and hence the concentration of the gas is very minimal in unpolluted air. But the sulfur dioxide produced from the combustion of fossil fuels, for example in electric power plants and mined coal containing 1 to 5% sulfur, is released into the ground level air. The concentration of sulfur dioxide in the ground level air is, thus, quite high in certain areas, particularly in the northern hemisphere. This contributes to acid rain over large areas. 2. Nitrogen monoxide is produced as a byproduct of the burning of gasoline in car engines using air as the source of oxygen. The nitrogen present in air, when exposed to high temperatures inside car engines, is converted into the pollutant nitrogen monoxide (NO). A series of secondary reactions in the presence of sunlight produces nitrogen dioxide oxide and ground level ozone, which further precipitate acid rain reactions. NO2 (g) + O3 (g) NO3 (g) + O2 (g) The gaseous NO3 then combines with any reactive hydrogen donor (X) in the atmosphere, producing aqueous nitric acid (HNO3): NO3 (g) + XH (g) HNO3 (aq) The resultant nitric acid is also a very strong acid which completely ionizes into aqueous hydrogen and nitrate ions: HNO3 (aq) H+ (aq) + NO3- (aq) The hydrogen ions again are responsible for the acidic affects of the acid rain ("Sources of Acid Rain"). Sources of Acids The bevy of acids produced in clean air and polluted air are given below: (i) Carbonic acid (H2CO3): In clean air, the source of the acid is carbon dioxide (CO2) produced during plant and animal respiration. In polluted air, the pollutant carbon dioxide is released from the complete combustion of fossil fuels. For example, Coal: C (s) + O2 (g) CO2 (g) Petrol (Octane) : 2C8H18(l)+25O2(g) 16CO2(g)+18H2O(g) Ethanol : C2H5OH(l)+3O2(g) 2CO2(g)+3H2O(g) ("The Chemistry of Acid Rain"). (ii) Formic Acid (Methanoic Acid) (HCOOH): The source of Formic Acid in acid rain is oxidation of natural methane (CH4) in clean air. In polluted air, the source is increased oxidation. (iii) Sulfuric Acid (H2SO4): As explained above, in clean air the source of sulfuric acid is the natural decay of organic matter resulting in release of hydrogen sulfide gas (H2S) which can be further oxidized to sulfur dioxide (SO2): 2H2S(g)+3O2(g) 2SO2(g)+2H2O(l) Sulfur dioxide is then further oxidized to sulfur trioxide (SO3), reacts with water to form sulfuric acid. This has been already explained above. Volcanoes also emit sulfur dioxide which then undergoes transformation into sulphuric acid. Another source is ocean algae, which release sulfur gases, like dimethyl sulfide. This is then oxidized to form sulfuric acid ("The Chemistry of Acid Rain"). In polluted air, the burning of coal and other fossil fuels is the main source of sulfur dioxide in the atmosphere. They account for about 80% of the manmade sulfur dioxide. These are being released from coal fired power stations. Motor vehicle emissions a responsible for only 1% of manmade sulfur dioxide present in the atmosphere. Sulfur dioxide is also produced during roasting of sulfur ores: 2ZnS(s)+3O2(g) 2ZnO(s)+2SO2(g) Sulfur dioxide is also released during the manufacture of sulfuric acid through contact process, in petroleum refineries giving refining process, and during the manufacture of coke from coal ("The Chemistry of Acid Rain"). (iv) Nitric Acid (HNO3): In clean air, lightning flashes precipitate a reaction between atmospheric nitrogen and oxygen in the presence of water vapor resulting in nitric acid. In polluted air, nitrogen monoxide (NO) is the result of the reaction between oxygen and nitrogen at high temperatures in internal combustion engines. Nitrogen monoxide is then easily oxidized into nitrogen dioxide (NO2). Nitrogen dioxide connects with water to form nitrous acid and nitric acid. 2NO2(g)+H2O(l) HNO2(aq)+HNO3(aq) (v) Methanesulphonic Acid: In the clear air, ocean algae release dimethyl sulfide, which then oxidizes into methanesulphonic acid. This acid is produced naturally only. ("The Chemistry of Acid Rain") (vi) Ammonium: Ammonia (NH3) is a byproduct of certain natural processes and also agricultural sources, like use of nitrogen fertilizers; confined animal feed spaces. This, then, combines with water to form ammonium (NH4), which contributes to the acidity of waters through nitrification (Likens, 2007). "In addition to wet deposition rain, snow, and fog, acid deposition also includes the deposition of dry, particulate, and gaseous acid precursors that becomes acidic in contact with moisture" (Likens, 2007). This dry deposition comprises about 20% to 80% of the total deposition of acids to the landscape. Impact of acid rain Impact on Surface Water Chemistry The surface waters become acidic when the addition of acids into the surface waters, from atmospheric deposition and watershed processes, is much more than the capacity of watershed minerals and the drainage waters to neutralize the acidity. "The chemical conditions that define acidity are that acid anion concentrations (sulfate, nitrate, organic acids) are present in excess of concentrations of base cations (typically calcium or magnesium" (Likens, 2007). The aqueous ionization of the acids present in acid rain produces hydrogen ions H+ (aq). The processes are shown as under: H2SO4(aq) H+(aq) + HSO4-(aq) HNO3(aq) H+(aq) + NO3-(aq) H2SO3(aq) H+(aq) + HSO3-(aq) H2CO3(aq) H+(aq) + HCO3-(aq) As the concentration of hydrogen ions increases in a body of water, the pH of the body drops. The toxic heavy metal ions, such as Cd2+, Pb2+, and Hg2+, which are present in the underlying bedrock of the body of water, are usually present in combination with sulfur. The acid rain solubilizes the heavy metals through reaction of hydrogen ions in the rain with the basic anion present with the heavy metal ion. This occurs according to the LeChateliers's Principle. This is shown below: CdS(s) + Excess 2H+(aq) Cd2+(aq) + H2S(aq) PbS(s) + Excess 2H+(aq) Pb2+(aq) + H2S(aq) HgS(s) + Excess 2H+(aq) Hg2+(aq) + H2S(aq) The heavy metals released, as a result of the "solubilization", are very toxic to the living organisms in their cation form ("Heavy Metal Leaching"). When these metal cations enter into a living organism, with a pH of about 7-8, through ingestion of polluted acidified water, they outcompete hydrogen ions for sulfur. As a result, the "sulfhydral" groups (-SH), present in the enzymes controlling the rate of critical metabolic reactions in the human body, attach readily to the heavy metal cations. Cd2+(aq) + H - S - H CdS(s) + 2H+(aq) R - S - H + Pb2+(aq) + H - S - R R - S - Pb - S - R + 2H+(aq) This adversely affects the shape and function of the critical enzyme preventing it from acting normally and adversely affecting the health of the living organism. This could also turn fatal ("Heavy Metal Leaching"). The other metals, which may also be leached from the catchment area of a lake and result in contamination, are aluminum, manganese, iron, zinc, copper, nickel, and vanadium. Even very low concentrations of these metals can be fatal for both fish and humans, also hamper their growth and reproductive ability. Mercury, even at very small input rates, "bio magnifies" from the bottom of the food chain to the top (MacIvar, 1998). Neutralization of Acidified Body of Waters The acidic precipitation can be neutralized with the natural alkaline buffer such as limestone (CaCo3): 2H+(aq) + CaCO3(s) Ca2+(aq) + CO2(g) + H2O(l) CO2(g) + H2O(l) H2CO3(aq) H2CO3(aq) H+(aq) + HCO3-(aq) ("Effects on Natural Waters") This process results in the release of carbonic acid. This is a weak acid compared to sulfuric acid as it only ionizes 1.7%. Thus, the presence of hydrogen ions is reduced to a great extent. Men have tried to reduce the effects of acid rain through addition of alkaline materials like lime (CaO), limestone (CaCO3) and slaked lime (Ca(OH)2) to lakes. The neutralization process is shown below: Ca(OH)2(s) + 2H+(aq) 2H2O(l) + Ca2+(aq) ("Effects on Natural Waters") Certain types of bacteria can also oxidize sulfur compounds and reduce sulfur. They are also being used to reduce lake acidification. Impact on Buildings Corrosion of Metals Acid rain impacts the metal and steel structures with extensive corrosion of constituent metals, more specifically copper and iron. 1. Iron: "The iron and steel structures are highly susceptible to corrosion, and their protection requires billions of dollars annually" ("Corrosion of Iron"). An increase in acid precipitation causes a substantial increase in corrosion. Rusting is a redox reaction occurring at the impurity sites in the iron. 2Fe(s) 2Fe2+(aq) + 4e- O2(g) + 4H+(aq) + 4e- 2H2O(l) 4Fe2+(aq) + 3O2(g) + 6H2O(l) 2Fe2O3 .6H2O(s) (Rust) ("Corrosion of Iron") Since rust does not adhere tightly to the metal, so it allows further corrosion of the metal. Galvanized steel, which is protected with zinc, is also the susceptible to corrosion in acid rain conditions. Copper: The copper and copper alloys form a layer of copper carbonate (CuCO3) in the presence of carbonic acid. This is a green substance and is also called patina. This tightly adheres to the surface of the metal, and prevents further corrosion from acid rain. However, it gives an ugly green color to the copper structures and monuments. The process of creation of patina is: CO2(g) + H2O(l) H2CO3(aq) H2CO3(aq) + Cu(s) CuCO3(s) + H2(g) ("Corrosion of Copper Metal") As the concentration of carbon dioxide in the atmosphere increases from emissions, the rate of corrosion of copper also increases, resulting reducing the age of copper structures. The restoration process is very costly. Buildings made of limestone, marble, and sandstone are particularly vulnerable to acid rain. Building stone is damaged when the calcium carbonate in stone dissolves in the presence of sulfur dioxide in acid rain to form a crust of calcium sulfate or gypsum. These sulphated layers are very easily washed away by rainfall or by the action of roasted and other weather conditions, there by exposing more stone ("Buildings"). This process is known as sulphation. When nitrogen dioxide is present along with sulfur dioxide, there is an increase in collusion rate because nitrogen dioxide oxidizes sulfur dioxide to sulfite, hence resulting in further sulfur dioxide absorption. Recommendations 1. Chemical Recovery: This involves the reduction of Concentrations of sulfate, nitrate and aluminum in soils and surface waters. These will, finally, result in an increase in their pH and their acid-neutralizing capacity (ANC), and a higher concentration of base cations. The chemical recovery of water bodies and soils hastens the biological recovery (Likens, 2007). 2. Use of Technology: This involves the use of instruments and processes which reduce the release of primary pollutants like carbon dioxide, sulfur dioxide, nitrogen dioxide etc. This can be done through the use of catalytic converters in internal combustion engines; the use of processes like Flue gas desulphurization (FGD) in coal burning power plants ("Acid Rain"). Works Cited Likens, Gene. 2007. "Acid rain." Environmental Protection Agency (Content source); Wayne Davis, Lori Zaikowski and Stephen C. Nodvin (Topic Editors). 2007 " Read More