Acid rains

Rain more acid than about pH 5.7 results from reactions of the acid gases SO2, NO2, NO, and, to a lesser extent, HCl. The S and species are derived chiefly from the combustion of fossil fuels. Important reactions forming sulfuric acid are 

The 1980 and 1995 maps show pH values above 5.7 at sites in 5 western states and western Canada. These high values reflect the absence of nearby industrial activity, but also reflect neutralization of the acidity by wind-blown carbonates. Calcium carbonate and sulfate salts accumulate in the topsoil of arid western prairies and can be transported in windblown dust for distances up to 300 to 650 km (Berner and Berner 1996). The highest resulting Ca concentrations in precipitation 

Genera) Controls on Natural Water Chemistry Chap. 8 

Initial rainfall from a given event is the most acid, because of rain washout of atmospheric acid gases and aerosols. This effect produces initial rain pH s below 3 in central Pennsylvania, for example. In cities such as Los Angeles and London, when fog has been present and the air stagnant, pH values between 2 and 3 have been observed, due in part to evaporative concentration, with serious consequences for people with respiratory problems. 

Acid rain is a serious environmental problem that affects large parts of many countries in Europe and North America. As shown in the diagram in Figme 10.4, 

SCHEME 10.4 Formation of acid rain from pollutants (a) from SO2 and (b) from NO. The acids formed in these reactions give rise to acid rain. The symbol [O] represents an oxygen-based species (such as O2) without specifying its exact composition. 

FIGURE 10.4 A drawing summarizing the formation of acid rain and its consequences for the environment. Adapted with kind permission from the Hubbard Brook Research Foimda-tion, 32 Pleasant Street, Woodstock, VT 05091. 

Acid rain is defined as any atmospheric precipitation that is more acidic than usual. The increase in acidity might be from natural or industrial sources. Rain acidity varies throughout the world and across the United States. The pH of rain is generally lower in the eastern United States and higher in the West. Unpolluted rain has a pH of 5.6, and so is slightly acidic. This acidity results from the dissolution of carbon dioxide in the water producing carbonic acid  

Chemical reactions between the oxides and water, forming sulfuric acid (H2SO4) and nitric acid (HNO3) 

Rain or snow, which carries the acids to the ground 

The oxides may also be deposited directly on a dry surface and become acidic when normal rain falls on them. 

This ancient stone carving on the Burgos Cathedral in Northern Spain shows the destructive power of acid rain over time. 

We may think of rain as a way that nature has to clean the atmosphere by washing-off substances from it. The cleaning process is carried out in two forms one, by dissolution of the soluble compounds, and the other by sweeping away suspended particles. 

Under normal conditions the pH of rain is acidic (approximately 5.7) due to the dissociation of dissolved carbon dioxide in water, as discussed in Chapter 6  

However, in polluted environments there are also oxides of sulfur and nitrogen present, which are stronger acids than carbon dioxide. When these oxides dissolve in rain droplets, the pH reaches values lower than 5.7, thus forming what is known as acid rain or — in a broader sense — acid deposition. 

Sulfur dioxide from the combustion of sulfur-containing fossil fuels (e.g., diesel oil and coal) is oxidized in the atmosphere to form sulfur trioxide that reacts with water to form sulfuric acid. 

Although the oxidant in combustion reactions is dioxygen, nitrogen oxides form because dioxygen comes from air which contains approximately 78% 

Oxides of nitrogen and sulfur can combine with the moisture in the atmosphere to form rain that can be highly acidic — acid rain. This rain can affect the pH of lakes and streams and has been known to seriously affect aquatic life. In fact, it s made some lakes devoid of life altogether. 

Acid rain is a good example of a non-point source of pollution. It s difficult to pinpoint a single entity as the cause. Air pollution controls have decreased 

This category of contamination includes fecal coliform bacteria from human wastes and the wastes of birds and other anim2ds. Fecal coliform bacteria was once a major problem in the United States and most parts of the world. Epidemics of tjqjhoid, cholera, and dysentery were common. Treatment of wastewater has minimized this problem in industrialized nations, but it s still a definite problem in underdeveloped nations. 

Many experts think that more than three-quarters of the sicknesses in the world are related to biological water contaminates. And even now in the United States, beaches and lakes are still closed at times because of biological contamination. 

Stricter controls on municipal water treatment, septic tanks, and runoff from feedlots will help decrease the biological contamination of our water. 

If the pH of natural rain were measured, you might expect a pH of around 7.0. Because a pH of 7.0 indicates neutral conditions, many people assume this to be the pH of rain. The theoretical pH of pure rainwater is actually about 5.6. Pure rain is acidic due to the equilibrium established between water and carbon dioxide in the atmosphere. Carbon dioxide and water combine to give carbonic acid  

Acid rain or precipitation refers to rain, snow, fog, or gaseous particles that have a pH significantly below 5.6. There is no absolute pH that defines acid rain, but a general guideline that can be used is that precipitation below 5.0 can be considered acidic. Although the term acid rain is used 

The principal cause of acid rain is the combustion of fossil fuels that produce sulfur and nitrogen emissions. The primary sources are electrical power plants, automobiles, and smelters. Power plants produce most of the sulfur emissions and automobiles most of the nitrogen emissions. Other sources of acid rain include nitrogen fertilizers, jet aircraft, and industrial emissions. Just as in our discussion of ozone, numerous reactions are involved in the formation of acid rain. The process can be understood by considering the transformation of sulfur and nitrogen oxides into their respective acidic forms sulfuric acid and nitric acid. Sulfur, present up to a few percent in fuels such as coal, is converted to sulfur dioxide when the fuel is burned. The sulfur dioxide reacts with water to produce sulfurous acid, H,SO ,, that is then oxidized to sulfuric 

Nitric acid, HNO3, and nitrous acid, HNO, form when nitrogen dioxide reacts with water  

Many reactions contribute to the overall formation of acid rain. These reactions often involve a number of complicated steps that depend on the atmospheric conditions. The reactions just shown represent general reactions that form sulfuric and nitric acids, and do not show the numerous reactions that actually occur. 

Volume-weighted H concentration in precipitation in Wilmington, NC, decreased by a factor of 2 between 1985 and 2005, typical of the nationwide decrease of rainwater acidity. Error bars are 11 standard deviation. [From. D. Willey, R. ]. Kieber, and G. B. Avery, r., Environ. Sci. Technol. 2006,40, 5675.] In volume weighting, [H ] in each rainfall is multiplied by the volume of that rainfall to get moles. Total moles for the year are divided by total volume for the year. 

V ombustion products from automobiles and power plants include nitrogen oxides and sulfur dioxide, which react with oxidizing agents in the atmosphere to produce acids and fall to the Earth as acid rain. 

I he chemistry of acids and bases is probably the most important topic you will study in chemical equilibrium. It is difficult to have a meaningful discussion of subjects ranging from protein folding to the weathering of rocks without understanding acids and bases. It will take us several chapters to provide meaningful detail to the study of acid-base chemistry. 

In aqueous chemistry, an acid is a substance that increases the concentration of H30 (hydronium ion). Conversely, a base decreases the concentration of H30 in aqueous solution. As we shall see shortly, a decrease in H30 concentration necessarily requires an increase in OH (hydroxide) concentration. Therefore a base is also a substance that increases the concentration of OH in aqueous solution. 

A more general definition of acids and bases given by Br0nsted and Lowry is that an acid is a proton donor and a base is a proton acceptor. This definition includes the one already stated. For example, HCl is an acid because it donates a proton to H2O to form H30  

As we have seen, all fossil fuels contain sulfur, and sulfur dioxide is also produced when they are burnt, alongside the usual products of hydrocarbon combustion. This is one of the causes of acid rain pollution  

Rain is always been slightly acidic due to atmospheric CO2 dissolving in it, together with very small quantities of nitric acid from natural sources of oxidation of nitrogen in times of thunderstorms  

In recent years the sulfur dioxide content of the air has risen, and so also have the nitrogen dioxide levels. This is due to increased use of fossil fuels and the fact that oxides of nitrogen are produced when some fuels and other materials burn in petrol engines. Catalytic converters on car exhausts help to reduce the amount of the oxides of nitrogen and carbon monoxide in the air, and so help to lower car emission pollution . 

Volcanoes also contribute to the concentration of particulates and of SO2 and other gases in the air. 

What happens to the carbon dioxide Is it let out into the atmosphere to increase the concentration of greenhouse gases  

Strategy To hehave as a greenhouse gas, either the molecule must possess a dipole moment or some of its vibrational motions must generate a temporary dipole moment. These conditions immediately rule out homonuclear diatomic molecules and atomic species. 

Solution Only CO, NO, and NO2, which are aU polar molecules, qualify as greenhouse gases. Both CI2 and H2 are homonuclear diatomic molecules, and Ne is atomic. These three species are all IR-inactive. 

Practice Exercise Which of the following is a more effective greenhouse gas CO or H2O  

Scientists have known about add rain since the late nineteenth century, but it has been a public issue for only about 30 years. 

The metal oxide can be reduced more easily than the sulfide (by a more reactive metal or in some cases by carbon) to the free metal. 

Under similar conditions, nitrogen compounds in the fuel react as follows  

Acid rain harms the environment in a number of ways it dissolves many rocks and metals, alters the composition of soils, groundwaters, and lakes, and alters the environmental conditions of living organisms. Acid rain is also particularly harmful to ancient objects and structures, as it plays an important role in their deterioration and sometimes total destruction. Unprotected limestone, marble, and sandstone, all of them widely used in ancient times for building and making statuary, are disintegrated by acid rain, which 

Changes in temperature alter most of the physical properhes of materials and affect the rates of chemical reactions. Thus heat and cold may advance or inhibit the deterioration of antiquities (see Textbox 75). 

All in all, some 50 million to 60 million tons of SO2 are released into the atmosphere each year In the troposphere, SO2 is almost all oxidized to H2SO4 in the form of aerosol, which ends np in wet precipitation or acid rain. The mechanism for the conversion of SO2 to H2SO4 is qnite complex and not fully understood. The reaction is believed to be initiated by the hydroxyl radical (OH)  

The sulfur trioxide formed would then rapidly react with water to form sulfuric acid  

SO2 can also be oxidized to SO3 and then converted to H2SO4 on particles by heterogeneous catalysis. Eventually, the acid rain can corrode limestone and marble (CaC03). A typical reaction is 

Snlfur dioxide can also attack calcium carbonate directly  

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Figure 10.1 Products of combustion contribute in vanous ways to the greenhouse effect, acid rain, and smog. (From Smith and Petela, Chem. Eng., 523 32, 1992 reproduced by permission of the Institution of Chemical Engineers.)...

Ophardt, G. E. Acid Rain Analysis by Standard Addition Titration, /. Chem. Educ. 1985, 62, 257-258. 

Acid phosphatase Acid phosphatases Acid pickling Acid rain... 

Selection of pollution control methods is generally based on the need to control ambient air quaUty in order to achieve compliance with standards for critetia pollutants, or, in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. For instance, tall stacks which disperse effluent modify the transport of pollutants and can thus reduce nearby SO2 deposition from sulfur-containing fossil fuel combustion. Although better dispersion aloft can solve a local problem, if done from numerous sources it can unfortunately cause a regional one, such as the acid rain now evident in the northeastern United States and Canada (see Atmospheric models). References 3—15 discuss atmospheric dilution as a control measure. The better approach, however, is to control emissions at the source. 

Hydroxypivalyl hydroxypivalate or 3-hydroxy-2,2-dimethylpropyl 3-hydroxy-2,2-dimethylpropionate (9) is a white crystalline solid at room temperature. It is used to manufacture polyester resias for use ia surface coatiags where good resistance to weatheting and acid rain are of particular importance (6). 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

California land-use regulations and growth control Latin American environmental initiatives environmental issues, acid rain, recycling, greenhouse effect... 

As worldwide attention has been focused on the dangers of acid rain, the demand to reduce sulfur dioxide [7446-09-5] emissions has risen. Several processes have been developed to remove and recover sulfur dioxide. Sulfur can be recovered from sulfur dioxide as Hquid sulfur dioxide, sulfuric acid, or elemental sulfur. As for the case of hydrogen sulfide, sulfur dioxide removal processes are categorized as adsorption, absorption, or conversion processes. 

Municipal incinerators are often targeted as a primary cause of acid rain. In fact, power plants burning fossil fuels, which produce sulfur dioxide and nitrogen oxide, are actually the leading cause of acid rain, along with automotive exhaust (176,177). In Europe and Japan, studies show that only about 0.02% of all acid rain can be traced to incineration of PVC (178). 

P. Lightowlers and J. N. Cape, "Does PVC Waste Incineration Contdbute to Acid Rain " Chem. Ind. (June 1987). 

Fig. 9. Genesis of acid tain (13). From the oxidation of C, S, and N during the combustion of fossil fuels, there is a buildup in the atmosphere (gas phase, aerosol particles, raindrops, snowflakes, and fog) of CO2 and the oxides of S and N, which leads to acid—base interaction. The importance of absorption of gases into the various phases of gas, aerosol, and atmospheric water depends on a number of factors. The genesis of acid rain is shown on the upper right as an acid—base titration. The data given are representative of the environment in the vicinity of Zurich, Switzedand.

Even rain is not pure water. Reports from the U.S. Geological Survey show that it contains 2.3—4.6 ppm of soflds, or a yearly precipitation of 2.5—5 t/km. Recently (ca 1997), work conducted ia the United States and Europe has underscored the rather dangerous results of iacreased use of fossil fuels, where the SO and NO emissions that end up ia the rain lower its pH from 5.6 (slightly acidic) for uncontaminated rain, to surface waters (1). About 40 x 10 t of SO and 25 x 10 t of NO were emitted ia the United States ia 1980. There are, however, encouragiag trends the 1970 Clean Air Act has led to a gradual reduction ia these emissions, bringing the SO emissions down from the previous levels cited by 10% by 1990, and the NO emissions down by 6%, with a consequent slight decrease ia rain acidity. A part of the Clean Air Act is also iatended to cap SO emissions from major poiat sources at 13.5 x 10 t (2). Between 1994 and 1995, total SO emissions ia the U.S. decreased remarkably by 13% and total NO emissions by 8%. 

Flue Ga.s Desulfuriza.tion. Citric acid can be used to buffer systems that can scmb sulfur dioxide from flue gas produced by large coal and gas-fired boilers generating steam for electrical power (134—143). The optimum pH for sulfur dioxide absorption is pH 4.5, which is where citrate has buffer capacity. Sulfur dioxide is the primary contributor to acid rain, which can cause environmental damage. 

Biological processes are also being studied to investigate abiHty to remove sulfur species in order to remove potential contributors to acid rain (see Air pollution). These species include benzothiophene-type materials, which are the most difficult to remove chemically, as weU as pyritic material. The pyrite may be treated to enhance the abiHty of flotation processes to separate the mineral from the combustible parts of the coal. Genetic engineering (qv) techniques are being appHed to develop more effective species. 

The usage pattern in Europe and Japan is more dependent upon the automotive industry. However, with the recent concern about acid rain, the European and U.S. markets should show increased interest in fluorocarbon elastomers for pollution control appHcations. On the other hand, the Japanese market has a sizable outlet in electrical and general machinery manufacturing (eg, copiers). Petroleum appHcations are of Htfle interest outside the United States. 

G. J. Heij and J. W. Erisman, Acid Rain Research Do We Have Enough Answers , Elsevier, Amsterdam, 1995. 

Review Group on Acid Rain, Department of Environment, London, 1990. 

K. N. Pilegaai d, N. O. Jensen and P. Huinmelshoj, in Acid Rain Research Do We Have Enough... 

A whole science, called metallography, is devoted to this. The oldest method is to cut the alloy in half, polish the cut faces, etch them in acid to colour the phases differently, and look at them in the light microscope. But you don t even need a microscope to see some grains. Look at any galvanised steel fire-escape or cast brass door knob and you will see the grains, etched by acid rain or the salts from people s hands. 

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