Acid Rain: Chemistry, Impacts and Control

Acid rain refers to the precipitation with elevated levels of hydrogen ions or a low pH. When fossil fuel is burnt, some of the elements within their molecules combine with oxygen and form oxides. The oxide produced in a large quantity is carbon dioxide, followed by smaller quantities of Sulphur Oxides and Nitrogen Oxides. All of these oxides form the acidic solution when they dissolve in water. Out of these oxides, the Nitrogen oxides can also be produced naturally by lightning strikes. Similarly, the Sulfur Dioxide can also be produced by the volcanic eruptions. But these natural phenomena did not contribute the acidic rains as compared to the anthropogenic activities. The Acid Rains have been seen in many parts of the world more prominently since 1970s. In some parts of the world the Acid Rains with as low as 1.5pH has been witnessed.

Chemistry of Acid Rains

The three kinds of Oxides matter in the Acid Rains. They are Oxides of Sulphur, Oxides of Nitrogen and Oxides of Hydrogen viz. water and Hydroxyl Radicals.

The sulphur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction shown below:

SO2 + OH → HOSO2

HOSO2 is unstable and it reacts with atmospheric Oxygen as follows:

HOSO2 + O 2 → HO2 + SO3

The Sulfur Trioxide SO3 produced so quickly reacts with the water and forms the Sulphuric acid as follows:

SO3 (g) + H 2O (l) → H2 SO4 (l)

The Nitrogen Dioxide also reacts with the OH to produce the Nitric Acid

NO2 + OH → HNO 3

The above reactions may take in the cloud drops as follows:

SO2 (g) + H 2O ↔SO2·H2O

SO2·H 2O ↔H++ HSO3

HSO3  ↔ H++ SO3-−

Impacts of Acid Rain

Impact on Biota and Human Life

The Acid Rains show adverse impact on the forests, freshwaters, soil and aquatic life forms. The acid rain eliminates the insect life in the lakes and ponds. It kills the soil organisms and thus changes the soil chemistry.

Impact on Soil Chemistry:

In the soil, there is an adverse impact on the nutrients such as Magnesium. This is because, Calcium and Magnesium are leached away by the Hydronium ion of the acids.

Impact on Buildings

Acid rain is capable of damaging the buildings and historic monuments which are made up of rocks such as limestone and marble. This is because these rocks contain a large amount of Calcium Carbonate, which reacts with the Sulfuric Acid to create Gypsum. Gypsum flakes off easily. This is shown in the following reaction:

CaCO3 (s) + H 2SO4 (aq) ↔CaSO4 (aq) + CO 2 (g) + H2 O (l)

How to control Acid Rain?

The best approach to combat acid rain is to reduce the amount of NOX and SO2 being released into the atmosphere. Fitting a catalytic converter a catalytic converter to a car can reduce the emission of NOx by up to 90%, but they very expensive, and cause more carbon dioxide to released, which contributes to the greenhouse effect. SO2 emissions from power stations can be reduced before, during, or after combustion. In addition there are several methods to controls SO2 and NOx in the environment. Acid rain may be controlled by

  • When fuel with low sulphur content (such as North Sea gas or oil) is burnt, not much sulphur dioxide will be formed. However, low sulphur fuels are more expensive because they are in greater demand and although high-sulphur fuels can be treated to reduce their sulphur content, it is very expensive
  • The SO2 created during combustion can be absorbed if an appropriate chemical (such as limestone) is present while the fuel burns.
  • Once the fuel has been burned, the SO2 can be removed from the exhaust gases. Most system spray a mixture of limestone and water onto the gases. This mixture reacts with the SO2 to form gypsum, useful building materials
  • Another option is not to burn fossil fuels, but to use alternative energy sources.
  • All these methods for reducing acid gases are expensive, and have draw-backs, so have been passed to use them. The best way to reduce them is not to use as much energy in the first place. One can save energy by turning off-lights when you leave a room, avoiding short journeys by car, insulating the house properly and using electric and related appliances which use less energy.