Pollutants

Pollutants are ingredients causing pollution. They may be gases, liquids, solids of different chemical composition or may be high pitched sounds. The pollutants are not deliberately manufactured. But they are the by products of manufacturing processes. At different stage of manufacturing, these pollutants may be generated. As they are not useful to manufacture any useful product of commercial value, they are sent out of the industries.

The pollutants from industries are of different types and carry varying importance. The type of pollutant generated out of manufacturing process depends upon the nature of materials used during manufacturing process as wall as technology adopted to process them.

Gaseous Pollutants

During the process of manufacture, most industries produce pollutants. Most common gaseous pollutants released by industries are:

Carbon monoxide

Highly dangerous gas with a high potentiality of causing cancer and respiratory ailments. Generally, it is evolved, when a fuel is burnt under limited supply of oxygen.

Carbon dioxide

It is also a pollutant, when generated in large quantity - because it can reduce the oxygen concentration in the air. It is a gas produced when fuel is burnt in adequate oxygen.

Normally, the atmospheric air will have 0.03% CO₂. When its concentration increases due to pollution, it reduces the percentage of oxygen in atmosphere.

Sulphur dioxide

Sulphur dioxide(SO₂) is one of the dangerous pollutant with a potentiality to form sulphuric acid, when mixed with water vapour. Water vapour commonly available in air can mix with SO₂ gas and produce sulphuric acid.

Such sulphuric acid is the cause of acid rain.

Dry and Wet Acid Deposition or Acid Rain

Hydrogen sulphide

Hydrogen Sulphide(H₂S) is one of the dangerous, pungent pollutant gas with many deleterious effects respiratory tract. It is released from many industries as a part of sulphonation process.

Hydrogen Cyanide (HCN)

When cyanide chemicals are used in manufacturing process, hydrogen cyanide is formed as a by product. It is a highly corrosive and poisonous gas leading to eye irritations, respiratory problems as well as other complications. In high doses, it can cause death.

Ammonia Gas

In many industries, ammonia gas (NH₃) is used as reducing agent. It is a corrosive gas with a high potentiality of causing throat irritations, burning of pharynx, oesophagus, etc. It can also cause digestive problems - when excess gas is inhaled.

Liquid Pollutants

Many liquid pollutants are evolved during the process of manufacturing pesticides, medicines and other chemicals. Liquid pollutants are also released during the process of refinement and processing. Some of them are dangerous and cause serious implications on drinking water quality.

The types of liquid pollutants can be

• acids - hydrochloric, acetic, fumaric, sulphuric, nitric, benzoic

• bases - hydroxides of sodium, calcium, magnesium

• carboxylic compounds

• nitrogenous wastes

Oil refineries

Crude oil contains sulphur. When sulphur is burned, it produces sulphur dioxide. If this gets into the atmosphere, it can cause what is called 'acid rain'.

Acid rain is a broad term to describe the effects of acidic compounds in the atmosphere - even though they don't necessarily fall as rain. Sulphur dioxide (SO^₂) and nitrogen oxides (NO_x - sometimes called 'nox') are released into the atmosphere when we burn fossil fuels (particularly coal). They return to earth on dry dust particles or dissolved in rain drops.

Acid rain is thought to be responsible for damaging large areas of forests and degrading the soil. It also damages materials including limestone and exposed metals.

Therefore, wherever possible, we want to remo

ve sulphur from fuels before we burn them. There are now strict controls in the UK on sulphur and 'nox' emissions from cars. These can be met, in part, thanks to the removal of sulphur from naphtha in the refinery.

Treat gas is used to remove the sulphur from fuel fractio ns (naphtha, light gas oil and kerosine). The treat gas is mainly hydrogen and is a by-product of other processes.
	The hydrogen reacts with the sulphur compounds in the hot hydrocarbons. The reaction produces hydrogen sulphide (which may be familiar to you because of its 'rotten eggs' smell). The hydrogen sulphide is removed and taken to the sulphur processing plant. Here, it is oxidised to make liquid sulphur. this is solidified and sold to the chemical industry.

Effects of Acid Rain

We generally consider acid rain to affect areas which are downwind of pollution generating sites. The northeastern United States, for instance, suffers from acid precipitation generated both locally and by coal fired plants in the mid-western states. As a result, ecosystem damage is localized. However, acid precipitation can be caused by some natural events (volcanic eruptions, erosion and oxidation of organic-rich sedimentary rocks) and some catastrophic events (bolide impact) which increase the amounts of CO₂, NO_x and SO₂ in the atmosphere. As a result, it is important to understand the effects of acid rain on animals inorder to evaluate both possible causes for past extinction events, as well as the potential for modern ecosystem damage.

Acid Formation in the Atmosphere

First, let us review some basic chemistry as it applies to acid precipitation.

Carbonic acid forms naturally in the atmosphere due to the reaction of water (H2O) and carbon dioxide (CO2),

H₂O + CO₂ -> H₂CO₃

_{while the burning of coal and other organics adds sulfur dioxide (SO2) and Nitrous oxides (NOx) to the atmosphere where they react to form sulfuric acid and nitric acid,}

2SO₂ + H₂O + O₂ -> 2H₂SO₄

4NO₂ + 2H₂O + O₂ -> 4HNO₃

_{All of these acids will be buffered by reacting with rocks, minerals, etc. on the earth's surface. The most important (and fastest) buffering comes from the reaction with (weathering of) calcite in the form of limestone, dolomite or marble.}

H₂CO₃ + CaCO₃ -> 2HCO₃^- + Ca⁺²

When this reaction occurs, the acid is neutralized and the calcite dissolved. While the reaction with calcite is very fast (the standard test for calcite in introductory geology labs is to put very dilute acid on a sample to see if it bubbles (reacts)), the reaction with other rocks is very slow, so most of the acid is not affected. This is why ponds in the Adirondacks became acidified (non-calcite rock in those areas), while Lake Champlain (abundant calcitic bedrock) did not.

The degree of acidification is the pH of the water, which is defined as the negative logarithm of the concentration of hydrogen ion (H⁺), or

pH = -log [H⁺].

(This to a certain degree comes from the old definition of an acid as a proton donor. A hydrogen ion is little more than a proton, so think of it as the amount of free protons floating around).

A pH of 7 is considered neutral, while a pH less than 7 is considered acidic. For example, wine has a pH of about 3.5 and your stomach digestive fluids have a pH of about 1.9.

We should also be aware that increased acidity does not have to be constant, but instead can be episodic. High surface water discharge events (storms, snowmelts) can increase the pH of streams and ponds to dangerous levels for short times.

Effects of Acidity on Plants and Animals

As a first example of the effects of acid rain, we can examine a case which is not obvious - effects on non-aquatic, tree nesting birds. This study was carried out in the Netherlands. It was observed that the proportion of birds laying defective eggs rose from roughly 10% in 1983-84 to 40% by 1987-88. The defective eggs had thin and highly porous egg shells, which resulted in eggs failing to hatch because of shell breakage and desiccation. As a result, there was also a high proportion of empty nests and clutch desertion. It was also observed that these effects were limited to areas of acid rain.

Since the birds did not appear to be directly affected by the acidity, the food chain was examined (these birds are positioned at the upper part of the local food chain). The difference between areas of normal soil pH (buffered by high calcium content due to limestone and marble outcrops and bedrock) and those with acidic soil appeared to be the presence of snails. The snails depend on the soil as their calcium source as they secrete their shells. With much of the CaCO₃ leached out of the soil by the acid precipitation, the snails could not survive in the area. The birds did not, at first, appear to be affected, because they continued to eat spiders and insects which, while supplying a sufficiently nutritious diet for the birds, where a poor source of calcium.

To test the hypothesis that the lack of calcium was the cause of the bird's laying defective eggs, ecologists "salted" the area with chicken egg shell fragments. The birds began to eat the chicken egg shells, and those that did laid normal eggs.

In this case, acid precipitation had affects that passed on up the food chain.

AFFECTS ON AQUATIC SYSTEMS

Mollusks - snails and clams.

- these invertebrates are highly sensitive to acidification because of their shells which are either calcite or aragonite (both forms a CaCO₃) which they must take from the water.

- in Norway, no snails are found in lakes with a pH of less than 5.

- of 20 species of fingernail clams, only 6 were found in lakes with pH of less than 5.

Arthropods

- crustaceans are not found in water with a pH less than 5.

- crayfish are also uncommon in water where the pH is less than 5. This is an important consideration because crayfish are an important food source for many species of fish.

- many insects also become rare in waters with a pH less than 5.

Amphibians

- as you may know, many species of amphibians are declining. To what extent acid rain is contributing to this decline is not exactly known. However, one problem is that in places like northeastern North America amphibians breed in temporary pools which are fed by acidified spring meltwater. In general, eggs and juveniles are more sensitive to the affects of acidity.

Zooplankton in lakes

- changes in diversity among zooplankton have been noted in studies carried out in lakes in Ontario, Canada. These studies found that in lakes where the pH was greater than 5 the zooplankton communities exhibited diversities of 9 - 16 species with 3 - 4 being dominant. In lakes where the pH was less than 5, diversity had dropped to 1 - 7 species, with only 1 or 2 dominants.

Periphytic algae

- many acidified lakes exhibit a large increase in the abundance of periphytic algae (those that coat rocks, plants and other submerged objects). This increase has been attributed to the loss of heterotrophic activity in the lake (i.e., the loss of both microbial and invertebrate herbivores in the lake).

Fish

- as a result of acidification, fish communities have suffered significant changes in community composition attributed to high mortality, reproductive failure, reduced growth rate, skeletal deformities, and increased uptake of heavy metals.

Mortality

- effects on embryos and juveniles:

- Atlantic salmon fry have been observed to die when water with pH <>

- in fish embryos, death appears to be due to corrosion of epidermal cells by the acid. Acidity also interferes with respiration and osmoregulation. In all fish at a pH of 4 to 5 the normal ion and acid/base balance is disturbed. Na+ uptake is inhibited in low pH waters with low salinity. Small fish are especially affected in this way because due to their greater ratio of body and gill surface area to overall body weight, the detrimental ion flux proceeds faster.

- in all fish low pH water causes extensive gill damage. Gill laminae erode, gill filaments swell, and edemas develop between the outer gill lamellar cells and the remaining tissue.

- at pH <3>

Reproductive Failure

Reproductive failure has been suggested as the main reason for fish extinction due to acidity. In Ontario, Canada it was observed that in acidified lakes female fish did not release ova during mating season. When examined, the fish were found to have abnormally low serum calcium levels which appears to have disrupted their normal reproductive physiology.

Growth

Growth may increase or decrease depending on resistance of a species to acidity. For resistant species, growth can increase due to the loss of competing non-resistant species. On the other hand, growth can decrease due to increase in metabolic rate caused by sublethal acid stress. In this case the organism's rate of oxygen consumption goes up because the excess CO₂ in the water increases the blood CO₂ level which decreases the oxygen carrying capacity of the hemoglobin.

Skeletal Deformity

This occurs in some fish as a response to the lowered blood pH caused by increase in CO₂ described above. Bones decalcify in response to a buildup of H₂CO₃ in the blood as the body attempts to maintain its normal serum osmotic concentration (i.e., the body attempts to return to a normal blood pH level).

How is acid rain measured?

Acid rain is measured through pH tests that determine the concentration of hydrogen ions in a liter of fluid. The pH (potential for hydrogen) scale is used to measure acidity or alkalinity. It runs from 0 to 14. Water has a neutral pH of 7. (The greater the concentration of hydrogen ions and the lower the pH number, the more acidic a substance is; the lower the concentration of hydrogen ions and the higher the pH number, the more alkaline—or basic—a substance is.) So a pH greater than 7 indicates an alkaline substance while a pH less than 7 indicates an acidic substance.

It is important to note that a change of only one unit in pH equals a tenfold change in the concentration of hydrogen ions. For example, a solution of pH 3 is 10 times more acidic than a solution of pH 4.

Normal rain and snow measure about pH 5.60. In environmental science, the definition of acid precipitation refers to a pH less than 5.65.

Acid rain can erode structures such as buildings, even if they are made out of stone. (Reproduced by permission of JLM Visuals.)

Measured values of acid rain vary according to geographical area. Eastern Europe and parts of Scandinavia have rain with pH 4.3 to 4.5; rain in the rest of Europe ranges from pH 4.5 to 5.1; rain in the eastern United States and Canada ranges from pH 4.2 to 4.6, and the Mississippi Valley has a range of pH 4.6 to 4.8. The worst North American area, analyzed at pH 4.2, is centered around Lake Erie and Lake Ontario.

When pH levels are drastically upset in soil and water, entire lakes and forests are endangered. Evergreen trees in high elevations are especially vulnerable. Although the acid rain itself does not kill the trees, it makes them more susceptible to disease. Also, high acid levels in soil causes leaching (loss) of other valuable minerals such as calcium, magnesium, and potassium.

Small marine organisms cannot survive in acidic lakes and rivers, and their depletion (reduced numbers) affects the larger fish who usually feed on them, and, ultimately, the entire marine-life food chain. Snow from acid rain is also damaging; snowmelt has been known to cause massive, instant death for many kinds of fish. Some lakes in Scandinavia and New York's Adirondack Mountains are completely devoid of fish life. Acid rain also eats away at buildings and metal structures. From the Acropolis in Greece to Renaissance buildings in Italy, ancient structures are showing signs of corrosion from acid rain. In some industrialized parts of Poland, trains cannot exceed 40 miles (65 kilometers) per hour because the iron railway tracks have been weakened from acidic air pollution.

Impacts of Acid Rain on Soils

Soil is the basis of wealth upon which all land-based life depends.
The damage that occurs to ecosystems from acidic deposition is dependent on the buffering ability of that ecosystem. This buffering ability is dependent on a number of factors, the two major ones being soil chemistry and the inherent ecosystem sensitivity to acidification. Indirect damage to ecosystems is largely caused by changes in the soil chemistry. Increasing soil acidity can affect micro-organisms which break down organic matter into nutrient form for plants to take up. Increasing soil acidity also allows aluminium (a common constituent of soil minerals) to come into solution. In its free organic form, aluminium is toxic to plant roots and can lock up phosphate, thereby reducing the concentrations of this important plant nutrient.

What Effect Does the Soil and Underlying Bedrock Have on Acid Rain?
Soils containing calcium and limestone are more able to neutralise sulphuric and nitric acid depositions than a thin layer of sand or gravel with a granite base.
If the soil is rich in limestone or if the underlying bedrock is either composed of limestone or marble, then the acid rain may be neutralised. This is because limestone and marble are more alkaline (basic) and produce a higher pH when dissolved in water. The higher pH of these materials dissolved in water offsets or buffers the acidity of the rainwater producing a more neutral pH.

Acid Sensitive Areas
In regions where the soil is not rich in limestone or if the bedrock is not composed of limestone or marble, then no neutralising effect takes place, and the acid rainwater accumulates in the bodies of water in the area. This applies to much of the north-eastern United States where the bedrock is typically composed of granite. Granite has no neutralising effect on acid rainwater. Therefore over time more and more acid precipitation accumulates in lakes and ponds.

The water bodies most susceptible to change due to acid precipitation are those whose catchments have shallow soil cover and poorly weathering bedrock, for example granite and quartzite. These soil types are characterised by the absence of carbonates that could neutralise acidity. The run-off water from such areas is less buffered than from areas such as limestone catchments, with an adequate level of carbonate. Such catchments and waters are termed acid-sensitive (poorly buffered), and can suffer serious ecological damage due to artificially acidified precipitation from air masses downwind of major emissions.

Notable high-risk areas in Canada and the United States are the Canadian Shield, the Adirondack Mountains, the Laurentians, the Appalachians, and the Green Mountains of Vermont. These areas are vulnerable because of their high elevations, small watersheds, and naturally acidic soils. Different types of bedrock contain variable amounts of alkaline chemicals. Regions with bedrock containing less alkali have a lower capacity for reducing acidity, and thus are more sensitive to acid deposition.

Effects of soil on vegetation
When acid rain falls, it can affect forests as well as lakes and rivers. To grow, trees need healthy soil to develop in. Acid rain is absorbed into the soil making it virtually impossible for these trees to survive. As a result of this, trees are more susceptible to viruses, fungi and insect pests.
Long-term changes in the chemistry of some sensitive soils may have already occurred as a result of acid rain. As acid rain moves through the soils, it can strip away vital plant nutrients through chemical reactions, thus posing a potential threat to future forest productivity.

Poisonous metals such as aluminium, cadmium and mercury, are leached from soils through reacting with acids. This happens because these metals are bound to the soil under normal conditions, but the added dissolving action of hydrogen ions causes rocks and small-bound soil particles to break down.
Plant life in areas where acid rain is common may grow more slowly or die as a result of soil acidification. In the Green Mountains of Vermont and the White Mountains of New Hampshire in the United States 50% of the red spruce have died in the past 25 years. There has also been noted a reduced amount of growth in existing trees as measured by the size of growth rings of the trees in these areas.

These effects occur because acid rain leaches many of the existing soil nutrients from the soil. The number of micro-organisms present in the soil also decreases as the soil becomes more acidic. This further depletes the amount of nutrients available to plant life because the micro-organisms play an important role in releasing nutrients from decaying organic material. In addition, the roots of plants trying to survive in acidic soil may be damaged directly by the acids present. Finally, if the plant life does not die from these effects, then it may be weakened enough so that it will be more susceptible to disease or other harsh environmental influences like cold winters or high winds.

Critical Loads
Environmental response to pollutants depends on many factors. Some regions cope with acidification better than others, having larger 'critical loads'. Critical load refers to the greatest assault that an ecological system can withstand before showing measurable degradation.

Scientists determine critical load by examining rock and soil type, land use and rainfall. If soil is fertile with a pH greater than 4.5, and rainfall is relatively low, the critical load will be high. The terrain can withstand moderately large additions of acidity without undue suffering. Conversely, in low pH soils, acidification mobilises toxic aluminium ions. If coniferous forests predominate, or if land is devoted to rough grazing, the result is a low critical load. Even minor acid deposition is undesirable.

There are very few long-term UK monitoring studies of soil acidification and none of soil biota. Chemical data are available from a few specific sites, from a small number of regional studies and from three national studies. From the limited information available, the National Expert Group on Transboundary Pollution has concluded that there is evidence that acid deposition has resulted in widespread acidification of acid sensitive soils in the UK. Further critical loads modelling research suggests that soil recovery from acidification may take many years or even decades.

Acid Rain – Is It A By-Product of Global Warming?

Acid rain: two words that are not very pretty. Instead of the romantic rain that most of us would like to imagine, acid rain brings to mind frightening images of a future wrought with pollution and other problems. But what is acid rain and what is it caused by? And is acid rain really a by-product of global warming? The short answer is both yes and no. Acid rain has causes that are rooted both in nature and in the human activity that is causing the effects of global warming to become more pronounced.

In scientific terms, acid rain refers to any kind of precipitation, including mist, snow, fog, and of course, rain, that is more acidic than normal. Most rain is naturally a bit acidic, but acid rain contains an above average level of acid in it. Generally speaking, acid rain is caused by emissions of sulfur dioxide and nitrogen oxides that react with hydroxyl radicals and water vapor that exist in many industrial environments. When this combination exists, the acid rain may come down as either dry acid deposition or, when it is mixed with water, it is known as acid rain.

What is most acid rain composed of? Acid rain as it falls in the eastern part of North America and parts of Europe is composed mostly of sulfuric acid and nitric acid. How do these things make up acid rain? Acid rain generally occurs when the burning of fuels produces sulfur dioxide and nitrogen oxides. These different oxides get into our atmosphere because of both natural environmental activity as well as human activity. When these oxides reach the troposphere, they become oxidized by the hydroxyl radicals in the atmosphere that then break down the oxides into sulfuric and nitric acids. These acids will usually break down readily into water that is then brought down in the form of precipitation, or acid rain.

So is acid rain a by-product of global warming? It is not so simple. Many natural sources are also a part of acid rain. Many tons of sulfur is released into the earth's atmosphere each year from natural sources, including volcano eruptions, microbial processes, and sea sprays. Nitrogen oxides are also released into the earth's atmosphere in a natural manner, including from burning, lightning, the burning of biomass, and many microbial processes.

However, in a sense, acid rain is indeed a type of by-product of global warming because human activity often is responsible for some kinds of acid rain. It is estimated that human beings release up to 100 to 130 million tons of sulfur dioxide into the atmosphere. Human beings are also estimated to be responsible for roughly 60 to 70 million tons of the nitrogen oxides that are released into the earth's atmosphere each year. Most acid rain occurs in highly industrialized areas where these oxides are released into the earth's atmosphere on a regular basis. However, human activity has caused more oxides to be released into the earth's atmosphere in certain concentrated areas. Thus, human activity is definitely a strong factor in the occurrence of acid rain, especially in highly concentrated areas.

The effects of acid rain are becoming recognized as a growing problem, especially around highly industrial areas. Areas that have been highly industrialized for more than 100 years are considerably more susceptible to experiencing acid rain. However, all parts of the world are susceptible to some kind of acid rain. Acid rain is especially having an effect on many fragile ecosystems, including many of the earth's aquatic ecosystems. Acid rain can also have a devastating effect on forests.

Normally while rain travels through the air, it dissolves floating chemicals and washes down particles that are suspended in air. At the start of its journey raindrops are neutral (pH = 7). In clean air, rain picks up materials that occur naturally such as dust, pollen, some CO₂ and other chemicals produced by lightening or volcanic activities. These substances make rain slightly acidic (pH = 6), which is not dangerous. However, when rain falls through polluted air, it comes across chemicals such as gaseous oxides of sulphur (SO_x), oxides of nitrogen (NO_x), mists of acids such as hydrochloric and phosphoric acid, released from automobile exhausts industrial plants, electric power plants etc.

These substances dissolve in falling rain making it more acidic than normal with pH range between 5.6 -3.5. In some case, it's pH gets lowered to the extent of 2. This leads to acid rain. The term acid rain is used here to describe all types of precipitation, namely, rain, snow, fog and dew more acidic than normal.

Chemistry of acid rain

In the natural processes of volcanic eruptions, forest fires and bacterial decomposition of organic oxides of sulphur and nitrogen, production and reductions of gases naturally tend to an equilibrium. Power plants, smelting plants, industrial plants, burning of coal and automobile exhausts, release additional sulphur dioxide, nitrogen oxides and acidic soot, causing pollution. Sulphur dioxide and nitrogen dioxide interact with water vapours in presence of sunlight to form sulphuric acid and nitric acid mist.

The formed sulphuric acid and nitric acid remain as vapour at high temperatures. These begin to condense as the temperature falls and mix with rain or snow, on the way down to the Earth and make rain sufficiently acidic.

Harmful effects of acid rain

SO_x, NO_x mixed with water as acid rain causes plant, animal and material damage. Some of the significant ill effects of acid rain are:

Damage to animals

Acid rain chemically strips waterways of necessary nutrients and lowers the pH to levels where plants and animals cannot live. Most of the aquatic animals cannot survive when the pH is less than 4. Some species of fish, such as salmon, die even when the pH is less than 5.5. Certain species of algae and zooplankton are eliminated at pH less than 6. A reduction in the zooplankton and bottom fauna ultimately affects the food availability for the fish population. The problem is most severe downwind of industrial areas where fishing and tourism are major sources of income such as in Norway and Sweden.

Damage to plants

Acidic water is dangerous to plants. Sulphuric and nitric acid rain washes nutrients out of the soil, damages the bark and leaves of trees and harms the fine root hairs of many plants which are needed to absorb water. Leaf pigments are decolorized because acid affects green pigment (chlorophyll) of plants. Agricultural productivity is also decreased. Several non-woody plants, such as barley, cotton and fruit trees like apple, pear, etc., are severely affected by acid rain. Since the acid concentration increases near the base of clouds by density, high altitude trees and vegetation may be exposed to pH levels as low as 3. Unique areas such as the Black Forest in Germany and sugar maples in Vermont (USA) are particularly threatened.

Material damage

Metallic surfaces exposed to acid rain are easily corroded. Textile fabrics, paper and leather products lose their material strength or disintegrate by acid rain.

Building materials such as limestone, marble, dolomite, mortar and slate are weakened on reaction with acid rains because of the formation of soluble compounds.

Thus, acid rain is dangerous for historical monuments.