Acid precipitation

Pollutants involved in acid rain

- Sulfur dioxide

- Sulfur trioxide

- Ozone

- Hydrogen chloride

- Oxides of nitrogen

Sulfur dioxide dissolves in water to produce...

sulfurous acid

Sulfur dioxide may be oxidised in the atmosphere by gases, such as ozone to produce...

sulfur trioxide

Sulfur trioxide is dissolved to produce...

sulfuric acid, a much more powerful acid than sulfurous acid

Oxides of nitrogen dissolve to produce...

nitrous and nitric acids

What are primary and secondary pollutants?

Primary pollutants are directly emitted, secondary pollutants are primary pollutants that have reacted in air

Name the primary pollutants

• SOx : sulfurous and sulfuric acids

• NOx : nitric acid

Name the secondary pollutant

Ozone

Sources of Sulfur dioxide

• Smelting of sulphide ores.

Sources of oxides of nitrogen

High temperature combustion, mainly in power stations and car engines

Sources of ozone

A secondary pollutant produced by photochemical reactions involving oxides of nitrogen

Sources of hydrogen chloride

The combustion of coal and incineration of wastes containing chlorine (like PVC)

Sources of sulfur trioxide

Oxidation of sulfur dioxide by ozone

Impacts of acid rain on metal

Acid deposition corrodes metals, causing damage to railway lines, metal railings, water pipes, and overhead powerlines

Impacts on limestone structures (like buildings and statues)

They are damaged as the acids dissolve the surface layers and weaken the stone structure of porous limestone

Impacts on cells

• Low pHs denature proteins in cell membranes and can inhibit enzyme action

• Tissues which have living cells exposed to the environment are most likely to be damaged by acid rain, like cells inside leaf stomata, plant root hairs, germinating seeds, fish eggs, fish gills

Impacts on exoskeletons

Invertebrates with exoskeletons may die as the acids dissolve the calcium compounds that form the skeleton

How can lichen be used to monitor acid rain?

They are very sensitive to acidic conditions and their size, state of health, abundance, and diversity, may be used in a biotic index to monitor acid rain pollution

Impacts on breathing

Sulfur dioxide can create breathing difficulties and increase the frequency of respiratory problems, like asthma attacks

What are the indirect impacts of acid rain on living organisms?

• Many metal ions become more soluble at low pH.

• The acidic solutions that percolate through soil can leach metal ions from the soil.

• Now important plant nutrients like Mg²⁺ ions are lost

• This leads to other toxic ions to be mobilised, like Al³⁺, when usually they are adsorbed onto the surface of clay particles to be immobilised.

What do the toxic metal ions do in plants?

They inhibit enzyme action in plant root hair cells and other soil organisms, such as detritivores and decomposers

What else do toxic metal ions do?

These mobilised ions can also be leached into rivers and lakes, where they can harm aquatic organisms

How can toxic metal ions affect human health?

Lead is a neurotoxin, and there is some evidence that aluminium ions may be a factor associated with some neurological disorders.

Controls of SOx

• Fuel (coal, crude oil) desulfurization

• Flue Gas Desulfurization (FGD)

• Dry FGD

• Wet FGD.

Fuel desulfurization

The removal of sulfur from the fuel before it is burnt

How is crude oil desulfurised?

Sulfur compounds are removed from crude oil during distillation, using molybdenum catalysts, in the process of hydrosylfurisation.

What form is sulfur present in coal?

Solid iron pyrites (FeS₂)

How can FeS₂ be removed?

By washing and streaming

Explain the washing and streaming process more.

The coal is crushed and put in a stream of flowing water, where the flow rate is fast enough to carry the coal away, but leaves behind the denser pyrites.

What are flue gases?

The exhaust gases containing SO₂

Flue Gas Desulfurization (FGD)

Processes to remove SO₂ from the power station emissions released after fuel combustion

Wet FGD

These gases are passed through a tall tower (scrubber) where a liquid solution (sodium sulphite) is sprayed on to these gases.

This produces sodium hydrogen sulphite.

What can you do with calcium sulfate?

If the effluent gases have been cleaned to remove smoke particles, then the calcium sulfate produced may be pure enough to make gypsum building plaster

Dry FGD

Flue gases are passed through a bed of crushed calcium carbonate

Sulfur dioxide reacts with the calcium carbonate to produce solid calcium sulfur.

What happens with sodium hydrogen sulphite?

It is heated, which breaks down to produce sodium sulphite and water and concentrated pure sulfur dioxide.

Sodium sulphite and water are reused, and concentrated pure sulfur dioxide is a valuable raw industrial material, since it can be converted to solid sulfur or sulfuric acid.

Controls of NOx

• catalytic converters

• urea sprays

Catalytic converters

A chamber in the exhaust pipes containing a catalyst, such as platinum, which chemically reduces NOx back to nitrogen and oxygen gases.

Urea Sprays

NOx can be removed by reacting it with urea

Control of ozone

Control of NOx reduces ozone formation.