Acid Rain and Its Effects

Acid levels are measured using the pH scale (logarithmic scale), where pH = 7 indicates neutral solutions. Typically, a pH of 5.6 (the pH of water saturated with carbon dioxide) is considered the threshold for determining acid rain. This means that any rainfall with an acidity level lower than 5.6 is classified as acid rain. It is also worth mentioning that in professional circles, the term “acid deposition” is sometimes used instead of acid rain. The two terms differ in that acid deposition refers to the deposit of acids from the atmosphere onto the Earth’s surface (including both dry [particulate matter] and wet [acid rain] forms), while acid rain strictly refers to the deposition of acids in wet form.

A record low acidity rainfall (pH = 2.4) occurred in New England. This rain caused the paint on parked cars to wash away, leaving marks from the raindrops on the frames of these vehicles.

Chemical Mechanism of Converting SO₂ and NO_x into Acids

For SO₂

In the Gas Phase: In the gas phase, various reactions convert SO₂ into sulfuric acid. One of these reactions is the photochemical oxidation of SO₂ by UV rays. However, this reaction contributes insignificantly to the formation of sulfuric acid. The second reaction involves the oxidation of SO₂ by atmospheric oxygen, which proceeds as follows:

2 SO₂ + O₂ —> 2 SO₃ (1)
SO₃ + H₂O —> H₂SO₄ (2)

Reaction 2 occurs rapidly, while reaction 1 occurs very slowly; therefore, this type of reaction also plays a minor role in the conversion of SO₂ to sulfuric acid. Other reactions, including oxidation by products of alkene-ozone reactions, oxidation by nitrogen oxides (NO_x), and oxidation by peroxy radicals, also play minor roles.

Only the following reaction plays an important role in converting SO₂ into sulfuric acid:

HO + SO₂(+M) —> HOSO₂(+M)

This reaction occurs very rapidly, and the hydroxy radical needed for the reaction is generated through the photolysis of ozone.

In the Liquid Phase: In the liquid phase, SO₂ exists in three forms:

[S(IV) —> [SO₂ (aq)] + [HSO₃^–] + [SO₃^2-]

The dissociation process proceeds as follows:

SO₂(aq) —> H⁺ + HSO₃^–
HSO₃^–(aq) —> H⁺ + SO₃^2-

The establishment of equilibrium in these two equations depends on pH, the size of water molecules, and the “bonding coefficient” between water and SO₂.

The oxidation of SO₂ in the liquid phase is facilitated by metal catalysts such as Fe³⁺, Mn²⁺, or a combination of both ions. However, the oxidation of SO₂ by ozone is more significant because it does not require a catalyst, and the concentration of ozone in the atmosphere is higher than that of atomic oxygen. The predominant oxidation process of SO₂ in the liquid phase is through hydrogen peroxide, which generates an intermediate (A^–), possibly peroxymonosulfurous acid ion, with the reaction occurring as follows:

HSO₃^– + H₂O₂ —> A^– + H₂O
A^– + H⁺ —> H₂SO₄

For NOx:

In the Gas Phase: The formation of nitric acid primarily results from the reaction of hydroxy radicals, which are highly reactive and prevalent in the atmosphere. The reaction proceeds as follows:

HO + NO₂(+M) —> HONO₂(+M)

In the Liquid Phase: There are three types of reactions that equivalently transform NO_x into nitric acid:

2NO₂(g) + H₂O(L) —> 2 H⁺ + NO^3- + NO^2-
NO(g) + NO₂(g) + H₂O(L) —> 2H⁺ + 2NO^2-
3NO₂(g) + H₂O(L) —> 2H⁺ + 2NO^3- + NO(g)

These three types of reactions depend on the partial pressure of NO_x present in the atmosphere and the very low solubility of NO_x in water. The rates of these reactions can be accelerated in the presence of metal catalysts such as Fe³⁺ and Mn²⁺.

Effects of Acid Rain on Lakes and Aquatic Ecosystems

Acid rain directly or indirectly affects lakes and aquatic ecosystems. When acid rain falls to the ground, it washes away nutrients and carries toxic metals into lakes. Additionally, in spring, when ice melts, acids (in the snow) and heavy metals in the ice flow into lakes, causing sudden changes in pH, a phenomenon known as “spring acid shock.” Aquatic organisms do not have enough time to adapt to these changes. Furthermore, spring is a peak breeding season for many species, and some terrestrial species also lay eggs, with their larvae living in water for extended periods, resulting in significant harm to these species. Sulfuric acid can affect fish in two ways: directly and indirectly. Sulfuric acid impacts the ability of fish to absorb oxygen, salts, and nutrients necessary for survival. For freshwater fish, sulfuric acid affects their salt and mineral balance. Acid molecules in the water create mucus in their gills, hindering their ability to absorb oxygen, leading to suffocation. Calcium imbalance reduces reproductive capabilities, damaging their eggs and weakening their spines. Nitrogen salts also affect fish; when washed into lakes by acid rain, they promote algae growth, and photosynthetic algae produce more oxygen. However, the high mortality rate of fish leads to decomposition, which consumes a large amount of oxygen, further suffocating the fish.

While many fish species can survive in environments with a pH as low as 5.9, at this pH, Al²⁺ in the soil is released into the lakes, becoming toxic to fish. Al²⁺ damages fish gills and accumulates in their livers.

Effects of pH on Aquatic Ecosystems can be summarized as follows:

Moreover, due to the phenomenon of bioaccumulation, when humans consume fish containing toxins, these toxins accumulate in the human body, posing health risks. Amphibians in lakes are also affected, as they cannot reproduce in acidic environments.

Did you know that according to Canada’s food safety standards, the allowable mercury content in rivers and lakes is only 0.005 ppm? However, currently, some Eskimos and Indigenous peoples in certain regions of Canada consume fish and seal meat with mercury levels as high as 17.5 and even 32.7 ppm.

Effects of Acid Rain on Plants and Soil

One of the severe impacts of acid rain is its effect on plants and soil. Acid rain washes away nutrients in the soil. Aluminum-containing compounds in the soil release aluminum ions, which can be absorbed by plant roots and become toxic to plants. As mentioned earlier, not all SO₂ in the atmosphere is converted into sulfuric acid; a portion may deposit back onto the ground as gaseous SO₂. When this gas comes into contact with plant leaves, it can block the stomata, hindering photosynthesis. An experiment on balsam fir (coniferous trees) showed that spraying a mixture of sulfuric acid and nitric acid with a pH ranging from 2.5 to 4.5 on young balsam fir trees causes the appearance and development of brown lesions on their leaves, leading to leaf drop; new leaves eventually grow back but at a very slow rate, severely impacting the photosynthesis process.

Did you know that Canada’s forestry sector generates an annual income of $10 billion? Ten percent of Canada’s workforce depends on forestry. If forests are damaged, it will severely affect income and employment in Canada.

Effects on the Atmosphere

Sulfate and nitrate particles formed in the atmosphere can limit visibility. Acid fog affects the ability to transmit sunlight. In the Arctic, it has impacted the growth of lichens, which in turn affects the populations of reindeer and caribou that feed on lichens.

Effects on Architectural Structures

Acid particles falling on buildings and sculptures corrode them. For instance, the Capitol building in Ottawa has deteriorated due to high SO₂ levels in the air. In 1967, a bridge over the Ohio River collapsed, killing 46 people; the cause was also attributed to acid rain.

Impact on Materials

Acid rain also damages fibers, books, and valuable antiques. The ventilation systems of libraries and museums have brought acid particles indoors, where they come into contact with and destroy the aforementioned materials.

Impact on Humans

The direct harms of pollution from acid gases on humans include respiratory diseases such as asthma, whooping cough, and other symptoms like headaches, eye pain, and sore throat. Indirect harms arise from the bioaccumulation of metals in the human body from food sources contaminated by these metals due to acid rain.

Ways to Reduce SO₂ and NO_x Emissions

For SO₂

Use clean coal—coal that has been gravity classified to remove FeS₂—or use low-sulfur coal (subbituminous).

Employ the fluidized bed combustion method.

Treat exhaust gases using wet filtration methods, employing lime or caustic soda solutions as absorbents. The reaction occurs as follows:

CaCO₃ + SO₂ + H₂O + O₂ —-> CaSO₄ + CO₂ + H₂O

Treatment of Exhaust Gases Using Dry Filtration Methods.

For NO_x

Utilize a combustion method known as “Overfire Air.” In this method, a portion of the air required for combustion is redirected to the upper part of the combustion chamber. This approach allows the combustion process to occur in conditions with less oxygen, thereby reducing the oxidation of nitrogen in the air into NO_x.

Treat exhaust gases using catalysts. In this process, ammonia reacts with NO in a catalytic chamber.

4NO + 4 NH₃ + O₂ —-> 4N₂ + 6 H₂O
2NO₂ + 4 NH₃ + O₂ —-> 3N₂ + 6 H₂O

In vehicles, an additional air filter component shaped like a honeycomb coated with platinum, palladium, or rhodium is installed. This component facilitates oxidation and reduction reactions to convert NO_x, CO₂, and hydrocarbons into non-harmful gases.

Author: Lê Hoàng Việt