PROBLEMS RELATED TO ACIDIC AND BASIC SUBSTANCES IN THE ENVIRONMENT - ENVIRONMENT - 2025

The main problems related to acidic and basic substances that impact the environment are directly associated with the pH changes they induce and their indirect or direct effect on living beings.

Both acidic and basic substances can generate serious environmental problems; particularly environmental acidification causes the problems of acid rain, acidification of oceans, fresh water bodies and soils. Alkalinization manifests itself especially in soil changes to basic pH.

Figure 1. Forest affected by acid rain. Source: Lovecz, from Wikimedia Commons

An environmental problem can be defined as a situation that threatens the integrity of any ecosystem and that occurs as a consequence of a disturbance in the natural environment.

Human activity has caused extreme environmental problems. The current mode of production, with intensive use of natural resources and an overload of pollutants, is violating the carrying capacity and resilience of the environment.

The unique ways of modifying large areas of land, of emitting huge amounts of toxic substances into the atmosphere and of affecting bodies of water, in very short periods and generating dramatic impacts on the environment, are exclusive to the human species.

Acidic substances are discharged into the environment through some industrial effluents, mining activities, the use of soil acidifying fertilizers and gas emissions that react with rainwater or air humidity producing acidic compounds.

Figure 2. Production of polluting industrial emissions. Source: pixabay.com.

Basic or alkaline substances can also come from various industrial effluents and mining activities.

What problems related to acidic and basic substances impact the environment?

-Environmental problems due to acidification: sources

Effluents

Acid effluents from some industries and acid mining drains contain mainly acids: hydrochloric (HCl), sulfuric (H ₂ SO ₄), nitric (HNO ₃) and hydrofluoric (HF).

The metallurgical, plastics, dyes, explosives, pharmaceuticals and resin industries are generators of acid discharges.

Figure 3. Discharge of industrial effluents. Source: Nigel Wylie, via Wikimedia Commons

Emissions

The emissions of carbon dioxide (CO ₂), sulfur dioxide (SO ₂) and nitrogen oxides (NO, NO ₂) into the atmosphere, from the combustion of fossil fuels such as coal, oil and natural gas, are cause not only from global warming of the planet, but from acid rain.

CO ₂ emissions also lead to acidification of the oceans and surface freshwater bodies (lakes and rivers), an environmental problem of catastrophic dimensions.

Fertilizers

Prolonged use of inorganic fertilizers containing ammonia nitrogen and superphosphates have a residual effect of acidifying the soils.

Also, the application of large amounts of organic matter to very humid soils produces acidification due to the effect of humic acids and other organic acids generated.

Among the most worrying environmental problems generated by acidic substances, we will mention acid rain, acidification of soils and acidification of the terrestrial oceans.

Acid rain

Sulfur dioxide gases (SO ₂) and nitrogen oxides (NO and NO ₂), produced in the combustion of fossil fuels in industries, power plants, air, maritime and land transport, and in smelting for the extraction of metals, are the cause of acidic rainfall.

In the troposphere, SO ₂ undergoes oxidation to form sulfuric acid (H ₂ SO ₄), a strong acid, and nitrogen oxides are also transformed into nitric acid, another strong acid.

When it rains, these acids present in the atmosphere in the form of aerosols are incorporated into the rainwater and acidify it.

Buildings

Acidic rainwater corrodes buildings, bridges and monuments, as it reacts with calcium carbonate (CaCO ₃) from limestone in buildings and marble and with metals. Acid rain also acidifies soils and bodies of water on the planet.

Figure 4. Damage to buildings generated by acid rain, gargoyle of the Cathedral of Notre Dame (Paris, France) showing damage to the back. Source: Michael Reeve, via Wikimedia Commons

Metals in soil

Acid rain alters the composition of the soil, displaces toxic heavy metals into the soil solution and into groundwater.

At very acid pH values, an intense alteration of the soil minerals occurs, due to displacement of cations by the H ⁺ ions present in high concentrations. This generates instability in the soil structure, high concentrations of toxic elements and low availability of nutrients for plants.

Acidic soils with a pH lower than 5 contain high concentrations and toxic to plant development of aluminum (Al), manganese (Mn) and iron (Fe).

Additionally, the availability of the nutrients potassium (K), phosphorus (P), sulfur (S), sodium (Na), molybdenum (Mo), calcium (Ca) and magnesium (Mg) is notably reduced.

Microorganisms

Acidic conditions do not allow the development of soil microorganisms (mainly bacteria), which are decomposers of organic matter.

Nitrogen fixing bacteria function optimally at pH values ​​between 7 and 6.5; its fixation rate drops dramatically when the pH is less than 6.

Micro-organisms also favor the aggregation of soil particles, which promotes structuring, aeration and good soil drainage, essential for plant growth.

Acidification of oceans, lakes and rivers

The acidification of surface waters - oceans, lakes and rivers - is produced mainly by the absorption of CO ₂ that comes from the burning of fossil fuels.

The surface waters of the planet act as natural sinks for atmospheric CO ₂. In particular, the oceans constitute the Earth's great carbon dioxide sink. CO ₂ is absorbed by water and reacts with it, producing carbonic acid (H ₂ CO ₃):

CO ₂ + H ₂ O → H ₂ CO ₃

Carbonic acid dissociates in water, contributing H ⁺ ions to ocean water:

H ₂ CO ₃ + H ₂ O → H ⁺ + HCO ₃ ^-

Excessive concentrations of H ⁺ ions produce an increase in the acidity of the planet's marine waters.

Marine ecosystems

This excess acidity dramatically affects marine ecosystems and in particular the organisms that form calcium carbonate exoskeletons (shells, shells and other supporting or protective structures), since H ⁺ ions displace calcium from carbonate and dissolve it, preventing their formation.

The species of corals, oysters, clams, sea urchins, crabs and plankton with exoskeletons are the most directly affected by the acidification of the oceans.

The life of all marine species depends largely on coral reefs, as they are the areas of greatest biodiversity in the sea. A large part of the smaller fauna take refuge and live there, serving as food for the secondary consumers of the marine ecosystem, such as fish, whales and dolphins.

Acidification due to excess CO ₂ in the earth's atmosphere constitutes a serious threat to the entire marine ecosystem. The history of the planet has never recorded an ocean acidification process at current rates - the highest in the last 300 million years - which also decrease its capacity as a sink for CO ₂.

-Environmental problems due to alkalization: sources

Industrial and mining

The detergent and soap, textile, dyeing, papermaking and pharmaceutical industries, among others, generate basic effluents that mainly contain sodium hydroxide (NaOH), strong base, and other bases such as sodium carbonate (Na ₂ CO ₃), which is a weak base.

Treatment of the mineral bauxite with NaOH for the extraction of aluminum, generates highly alkaline red mud. Also oil extraction and the petrochemical industry produce alkaline effluents.

The main environmental problem produced by basic substances is the alkalization of soils.

Soil alkalization

Alkaline soils have pH values ​​greater than 8.5, have a very poor structure, with scattered particles and compact calcareous layers between 0.5 and 1 meter deep, which prevent root growth and infiltration, percolation and drainage of water.

They present toxic concentrations of sodium (Na) and boron (B) and are highly infertile soils.

Figure 5. Alkaline soil. Source: Pixabay.com

References

1. Bowman, AF, Van Vuuren, DP, Derwent, RG and Posch, M. (2002) A global analysis of acidification and eutrophication on terrestrial ecosystems. Water, Air and Soil Pollution. 41,349-382.
2. Doney, SC, Fabry, VJ, Feely, RA and Kleypas, JA (2009). Ocean acidification: The other CO ₂ Annual Review of Marine Sciences. 1, 169-192.
3. Ghassemi, F., Jakeman, AJ and Nix, HA (1995). Salinization of Land and Water Resources: human causes, extent, management and case studies. CAB International, Wallinford, UK. 544pp.
4. Kleypas, JA and Yates, KK (2009). Coral reefs and ocean acidification. Oceanography. 22,108-117.
5. Mason, C. (2002). The Ecology of Freshwater Pollution. Pearson Education Limited. 400pp.