Acid rain and its effects

Acidity is measured by a pH scale (logarithmic scale), where pH = 7 refers to neutral solutions. Normally pH = 5.6 (pH 5.6 is the pH level of CO ₂ saturated water) which is considered to be the basis for determining acid rain. This means that any rain with an acidity lower than 5.6 is called acid rain. It should be added that in professional circles, sometimes people use the term "acid deposition" (Acid deposition), instead of acid rain (acid rain). These two terms differ in that acid deposition is the deposition of acid in the atmosphere on the surface of the Earth (either in the dry form [dust particles] or the wet form [acid rain]), while the acid rain is purely speaking about atmospheric acid deposition to the Earth's surface in wet form.

Rain has a low acidity at a record (pH = 2.4) taking place in New England. This rain caused the paint of the cars parked in the rain to be washed away and left a trail of raindrops on the frames of these cars.

Chemical mechanism of the conversion of SO ₂ and NO _x into acid

For SO ₂

At the gas phase: At the gas phase there are many different reactions to convert SO2 to sulfuric acid. One of these reactions is the SO2 photochemical reaction by UV rays. However, this reaction plays an unimportant part in making sulfuric acid. The second type of reaction is the oxidation of SO ₂ by oxygen in the atmosphere, the reaction takes place as follows:

2 SO ₂ + O ₂ ---> 2 SO ₃ (1)
SO ³ + H ₂ O ---> H ₂ SO ₄ (2)

Reaction 2 happens at a rapid rate, while reaction number 1 is very slow, so this type 2 reaction also plays a minor role in converting SO ₂ to sulfuric acid. Some other reactions also play an unimportant role in the conversion of SO2 to sulfuric acid including oxidation reactions by ozone-ozone reaction products, oxidation by reaction of N _x O _y substances, oxidized by peroxy radicals.

Only the following type of reaction plays an important role in converting SO2 to sulfuric acid, the reaction takes place as follows:

HO + SO ₂ (+ M) ---> HOSO ₂ (+ M)

This reaction takes place very quickly, the hydroxy radical required for the reaction is produced by ozone decomposition.

In liquid phase: In SO ₂ liquid phase exists in 3 forms:

[S (IV) ---> [SO ₂ (aq)] + [HSO ₃ ^- ] + [SO ₃ ^2- ]

The separation process takes place as follows:

SO ₂ (aq) ---> H ⁺ + HSO ^3-
HSO ₃ - (aq) ---> H ⁺ + SO ₃ ^2-

The equilibrium setting of the two equations depends on the pH, the size of the water particles, the "coefficients" between the water and SO2 _.

SO2 oxidation reaction in liquid phase thanks to metal catalysts such as ion Fe ³⁺ , Mn ²⁺ or combination of the above 2 ions. However, the oxidation of SO ₂ by ozone is more important because it does not require catalysis and the ozone content in the atmosphere is higher than that of the atomic oxygen in the atmosphere. The most dominant oxidation of SO ₂ in the liquid phase is the oxidation by hydrogen peroxide, which forms an intermediate (A-), possibly a peroxymonosulfurous acid ion, the following reaction:

HSO ₃ - + H ₂ O ₂ ---> A- + H ₂ O
A- + H ⁺ ---> H ₂ SO ₄

For NOx:

At the gas phase: The formation of nitric acid is mainly due to the reaction of the hydroxy radical, which is highly active and highly present in the atmosphere. The reaction takes place as follows:

HO + NO ₂ (+ M) ---> HONO ₂ (+ M)

In the liquid phase: There are 3 types of reactions that play a similar role in converting NO _x to nitric acid

2NO ₂ (g) + H ₂ O (L) ---> 2 H ⁺ + NO ^3- + NO ^2-
NO (g) + NO ₂ (g) + H ₂ O (L) ---> 2H ⁺ + 2NO ^2-
3NO ₂ (g) + H ₂ O (L) ---> 2H ⁺ + 2NO ^3- + NO (g)

These three types of reactions depend on the partial pressure of NO _x present in the atmosphere and very low solubility of NO _x in water. The above reactions can increase speed with the presence of metal catalysts such as Fe ³⁺ , Mn ²⁺ .

Effect of acid rain on ponds and aquatic organisms

Acid rain directly or indirectly affects lakes and aquatic organisms. Acid rain falling on the ground will wash away the nutrients on the ground and bring toxic metals into ponds and lakes. Also in the spring when the ice melts, acid (in the snow) and heavy metals in the ice follow water into lakes and causes sudden changes in pH in the pond, this phenomenon is called "shock" phenomenon in the season. Spring. Aquatic creatures do not have enough time to adapt to this change. In addition, Spring is the season when many species lay eggs and some other terrestrial species also lay their eggs and larvae living in the water for a long time, so these species are severely damaged. Sulfuric acid can affect fish in two ways: direct and indirect. Sulfuric acid directly affects the ability to absorb oxygen, salt and nutrients for survival. For freshwater fish, sulfuric acid affects the salt and mineral balance in their bodies. The acid molecules in the water that make up the gums in their gills prevent the ability to absorb oxygen by making the fish suffocate. Calcium salt imbalance reduces the fertility of the eggs, its eggs will be damaged . and their spine is weakened. Nitrogen salt also affects fish, when it is washed away by acid rain into ponds and lakes, it will promote the development of algae, photosynthetic algae will produce more oxygen. However, because fish die a lot, decomposing them will consume a large amount of oxygen, which reduces the oxygen of the water body and makes the fish suffocate.

Although many types of fish can live in pH environments as low as 5.9, up to pH this Al ²⁺ in the soil is released into ponds and is toxic to fish. Al2 + damages fish gills and accumulates in fish liver.

The effects of pH on aquatic organisms can be summarized as follows

Moreover, due to the phenomenon of bioaccumulation, when people eat fish containing toxins, these toxins will accumulate in the human body and endanger human health. In ponds, amphibians are also affected, they cannot reproduce in acidic environments.

Did you know that according to Canada's food safety standards, the amount of mercury salt in lakes and rivers is only 0.005 ppm. But now Eskimos and Indians in some parts of Canada eat fish and seals that have mercury levels of up to 17.5 or even 32.7 ppm.

Effect of acid rain on plants and soil

One of the serious effects of acid rain is the harmful effects on plants and soil. When acid rain is present, nutrients in the soil will be washed away. Aluminum compounds in the soil will release aluminum ions and these ions can be absorbed by plant roots and toxic to plants. As we said above, not all SO ₂ in the atmosphere is converted into sulfuric acid, but part of it can deposit back to the ground in the form of SO ₂ gas. When this gas comes into contact with the leaves, it will turn off somatic forms of the leaves that hinder photosynthesis. An experiment in Spruce (coniferous) showed that when spraying a mixture of sulfuric acid and nitric acid with a pH of 2.5 - 4.5 on the spruce plants, the appearance and development of the stain will appear. The lesion is brown on its leaves and then the leaves fall off, new leaves will grow later but at a very slow rate and photosynthesis is severely affected.

Do you know Canada's forestry industry has an annual income of $ 10 billion. 10% of Canada's workforce is dependent on forestry. If the forest is damaged, it will seriously affect income and employment in Canada.

Influence on the atmosphere

Sulphate particles, nitrates that form in the atmosphere will limit visibility. Acid mist affects the ability to spread sunlight. In the Arctic, it affected the development of lichen, thus affecting populations of Reindeer and Snow Deer - lichen-eating animals.

Impact on architectural works

The acid particles that fall into the house and the sculptures will corrode them. For example, the Capitol building in Ottawa has been disintegrated by too high levels of SO ₂ in the air. In 1967, the bridge across the Ohio River collapsed, killing 46 people; The cause is also due to acid rain.

Effect on materials

Acid rain also spoiled fabrics, books and precious antiques. The ventilation systems of libraries and museums have brought acid particles into the house and they come in contact with and destroy the aforementioned materials.

Influence on people

The direct effects of acid gas pollution on humans include respiratory diseases such as asthma, whooping cough and other symptoms such as headache, eye pain, sore throat . Harmful effects indirectly caused by bioaccumulation of metals in the human body from food sources contaminated with these metals due to acid rain.

How to reduce emissions of SO ₂ and NO _x

For SO ₂

Use clean coal - coal that has been classified by gravity to remove FeS ₂ - or use low sulfur coal (subbituminuos).

Use fluidized bed burning method.

Waste gas treatment by wet filtration method, using lime or caustic solution to absorb. The reaction occurs as follows:

CaCO ₃ + SO ₂ + H ₂ O + O ₂ ----> CaSO ₄ + CO ₂ + H ₂ O

Waste gas treatment by dry filtration method.

For NO _x

Using the burning method called "Air Overfire". Under this method, a portion of the air required for combustion will be diverted to the upper part of the combustion chamber. In doing so, the combustion process will take place in the presence of less oxygen and reduce the oxidation of nitrogen in the air to NO _x .

Treatment of emissions with catalysts. In this process ammonia is applied to NO in a catalytic chamber.

4NO + 4 NH ₃ + O ₂ ----> 4N ₂ + 6 H ₂ O
2NO ₂ + 4 NH3 + O ₂ ---> 3N ₂ + 6 H ₂ O

In car engines, a honeycomb-shaped air filter element is attached with platinum, pallorium or Rhodium. At this part, oxidation and reduction reactions are used to turn NO _x , CO2 and HCs into non-harmful gases.

Author: Le Hoang Viet