Saturday, June 19, 2010


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.

 common gaseous pollutants released by industries

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.

 common gaseous pollutants released by industries

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 (SO2) and nitrogen oxides (NOx - 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, NOx 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.


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.


- 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.


- 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).


- 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.


- 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 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.