ENVIRONMENTAL CHEMISTRY: FIELD OF STUDY AND APPLICATIONS - ENVIRONMENT - 2024

The environmental chemistry studies the chemical processes that take place at the environmental level. It is a science that applies chemical principles to the study of environmental performance and the impacts generated by human activities.

Additionally, environmental chemistry designs prevention, mitigation and remediation techniques for existing environmental damage.

Figure 1. Diagram of the terrestrial atmosphere, hydrosphere, lithosphere and biosphere. Source: Bojana Petrović, from Wikimedia Commons

Environmental chemistry can be subdivided into three basic disciplines which are:

1. Environmental chemistry of the atmosphere.
2. Environmental chemistry of the hydrosphere.
3. Soil environmental chemistry.

A comprehensive approach to environmental chemistry additionally requires the study of the interrelationships between the chemical processes that occur in these three compartments (atmosphere, hydrosphere, soil) and their relationships with the biosphere.

Environmental chemistry of the atmosphere

The atmosphere is the layer of gases that surrounds the Earth; it constitutes a very complex system, where the temperature, the pressure and the chemical composition vary with the altitude in very wide ranges.

The sun bombards the atmosphere with radiation and high-energy particles; this fact has very significant chemical effects in all layers of the atmosphere, but in particular, in the upper and outer layers.

-Stratosphere

Photodissociation and photoionization reactions occur in the outer regions of the atmosphere. In the region between 30 and 90 km in height measured from the earth's surface, in the stratosphere, there is a layer that mainly contains ozone (O ₃), called the ozone layer.

Ozone layer

Ozone absorbs high-energy ultraviolet radiation that comes from the sun and if it weren't for the existence of this layer, no known life forms on the planet could survive.

In 1995, atmospheric chemists Mario J. Molina (Mexican), Frank S. Rowland (American) and Paul Crutzen (Dutch), won the Nobel Prize in Chemistry for their research on the destruction and depletion of ozone in the stratosphere.

Figure 2. Scheme of depletion in the ozone layer. From nasa.gov

In 1970 Crutzen showed that nitrogen oxides destroy ozone through catalytic chemical reactions. Subsequently Molina and Rowland in 1974, showed that the chlorine in chlorofluorocarbon compounds (CFC's) is also capable of destroying the ozone layer.

-Troposphere

The atmospheric layer near the earth's surface, between 0 and 12 km high, called the troposphere, is mainly composed of nitrogen (N ₂) and oxygen (O ₂).

Toxic gases

As a result of human activities, the troposphere contains many additional chemicals considered air pollutants such as:

• Carbon dioxide and monoxide (CO ₂ and CO).
• Methane (CH ₄).
• Nitrogen oxide (NO).
• Sulfur dioxide (SO ₂).
• Ozone O ₃ (considered a pollutant in the troposphere)
• Volatile organic compounds (VOC's), powders or solid particles.

Among many other substances, which affect human and plant and animal health.

Acid rain

Sulfur oxides (SO ₂ and SO ₃) and nitrogen oxides such as nitrous oxide (NO ₂) cause another environmental problem called acid rain.

These oxides, present in the troposphere mainly as products of the combustion of fossil fuels in industrial activities and transportation, react with rainwater producing sulfuric acid and nitric acid, with the consequent acid precipitation.

Figure 3. Scheme of acid rain. Source: Alfredsito94, from Wikimedia Commons

By precipitating this rain that contains strong acids, it triggers several environmental problems such as acidification of the seas and fresh waters. This causes the death of aquatic organisms; the acidification of soils that causes the death of crops and the destruction by corrosive chemical action of buildings, bridges and monuments.

Other atmospheric environmental problems are photochemical smog, caused mainly by nitrogen oxides and tropospheric ozone.

Global warming

Global warming is produced by high concentrations of atmospheric CO ₂ and other greenhouse gases (GHGs), which absorb much of the infrared radiation emitted by the Earth's surface and trap heat in the troposphere. This generates climate change on the planet.

Environmental chemistry of the hydrosphere

The hydrosphere is made up of all the bodies of water on Earth: surface or wetlands - oceans, lakes, rivers, springs - and underground or aquifers.

-Fresh water

Water is the most common liquid substance on the planet, it covers 75% of the earth's surface and is absolutely essential for life.

All forms of life depend on fresh water (defined as water with a salt content of less than 0.01%). 97% of the water on the planet is salt water.

Of the remaining 3% fresh water, 87% is in:

• The poles of the Earth (which are melting and pouring into the seas due to global warming).
• The glaciers (also in the process of disappearance).
• Groundwater.
• Water in the form of vapor present in the atmosphere.

Only 0.4% of the planet's total fresh water is available for consumption. The evaporation of water from the oceans and the precipitation of rains continuously provide this small percentage.

The environmental chemistry of water studies the chemical processes that occur in the water cycle or hydrological cycle and also develops technologies for the purification of water for human consumption, the treatment of industrial and urban wastewater, the desalination of seawater, recycling and saving this resource, among others.

-The water cycle

The water cycle on Earth consists of three main processes: evaporation, condensation and precipitation, from which three circuits are derived:

1. Surface runoff
2. Plant evapotranspiration
3. The infiltration, in which the water passes to underground levels (phreatic), circulates through aquifer channels and leaves through springs, fountains or wells.

Figure 4. Water cycle. Source: Wasserkreislauf.png: from: Benutzer: Jooooderivative work: moyogo, via Wikimedia Commons

-Anthropological impacts on the water cycle

Human activity has impacts on the water cycle; some of the causes and effects of anthropological action are the following:

Modification of the land surface

It is generated by destruction of forests and fields with deforestation. This affects the water cycle by eliminating evapotranspiration (water intake by plants and return to the environment by perspiration and evaporation) and by increasing runoff.

The increase in surface runoff produces an increase in the flow of rivers and floods.

Urbanization also modifies the land surface and affects the water cycle, as the porous soil is replaced by impermeable cement and asphalt, which makes infiltration impossible.

Water cycle pollution

The water cycle involves the entire biosphere and consequently, human-generated waste is incorporated into this cycle by different processes.

Chemical pollutants in the air are incorporated into the rain. Agrochemicals applied to the soil, suffer leachate and infiltration to aquifers, or run off into rivers, lakes and seas.

Also the waste of fats and oils and the leachates of the sanitary landfills, are carried by infiltration to the groundwater.

Extraction of water supplies with overdraft in water resources

These overdraft practices produce depletion of groundwater and surface water reserves, affect ecosystems and produce local subsidence of the soil.

Soil environmental chemistry

Soils are one of the most important factors in the balance of the biosphere. They provide anchorage, water and nutrients to plants, which are producers in the terrestrial trophic chains.

Soil

The soil can be defined as a complex and dynamic ecosystem of three phases: a solid phase with mineral and organic support, an aqueous liquid phase and a gaseous phase; characterized by having a particular fauna and flora (bacteria, fungi, viruses, plants, insects, nematodes, protozoa).

The properties of the soil are constantly modified by environmental conditions and by the biological activity that develops in it.

Anthropological impacts on the soil

Soil degradation is a process that decreases the productive capacity of the soil, capable of producing a profound and negative change in the ecosystem.

The factors that produce soil degradation are: climate, physiography, lithology, vegetation and human action.

Figure 5. Degraded soil. Source: pexels.com

By human action can occur:

• Physical degradation of the soil (for example, compaction from improper farming and ranching practices).
• Chemical degradation of the soil (acidification, alkalization, salinization, contamination with agrochemicals, with effluents from industrial and urban activity, oil spills, among others).
• Biological degradation of the soil (decrease in the content of organic matter, degradation of the vegetation cover, loss of nitrogen-fixing microorganisms, among others).

Chemical – environment relationship

Environmental chemistry studies the different chemical processes that take place in the three environmental compartments: atmosphere, hydrosphere and soil. It is interesting to review an additional approach on a simple chemical model, which tries to explain the global transfers of matter that occur in the environment.

-Model Garrels and Lerman

Garrels and Lerman (1981) developed a simplified model of the biogeochemistry of the Earth's surface, which studies the interactions between the compartments of the atmosphere, hydrosphere, earth's crust, and the included biosphere.

The Garrels and Lerman model considers seven major constituent minerals of the planet:

1. Gypsum (CaSO ₄)
2. Pyrite (FeS ₂)
3. Calcium carbonate (CaCO ₃)
4. Magnesium carbonate (MgCO ₃)
5. Magnesium Silicate (MgSiO ₃)
6. Ferric oxide (Fe ₂ O ₃)
7. Silicon dioxide (SiO ₂)

The constituent organic matter of the biosphere (both living and dead), is represented as CH ₂ O, which is the approximate stoichiometric composition of living tissues.

In the Garrels and Lerman model, geological changes are studied as net transfers of matter between these eight components of the planet, through chemical reactions and a net balance of mass conservation.

The accumulation of CO

For example, the problem of the accumulation of CO ₂ in the atmosphere is studied in this model, saying that: currently we are burning the organic carbon stored in the biosphere as coal, oil and natural gas deposited in the subsoil in geological times past.

As a result of this intensive burning of fossil fuels, the concentration of atmospheric CO ₂ is increasing.

The increase in CO ₂ concentrations in the Earth's atmosphere is due to the fact that the rate of fossil carbon combustion exceeds the rate of carbon absorption by the other components of the Earth's biogeochemical system (such as photosynthetic organisms and hydrosphere, for example).

In this way, the emission of CO ₂ into the atmosphere due to human activities, surpasses the regulatory system that modulates changes on Earth.

The size of the biosphere

The model developed by Garrels and Lerman also considers that the size of the biosphere increases and decreases as a result of the balance between photosynthesis and respiration.

During the history of life on Earth, the mass of the biosphere increased in stages with high rates of photosynthesis. This resulted in a net storage of organic carbon and emission of oxygen:

CO ₂ + H ₂ O → CH ₂ O + O ₂

Respiration as metabolic activity of microorganisms and higher animals, converts organic carbon back into carbon dioxide (CO ₂) and water (H ₂ O), that is, it reverses the previous chemical reaction.

The presence of water, the storage of organic carbon and the production of molecular oxygen are fundamental for the existence of life.

Environmental Chemistry Applications

Environmental chemistry offers solutions for the prevention, mitigation and remediation of environmental damage caused by human activity. Among some of these solutions we can mention:

• The design of new materials called MOF's (Metal Organic Frameworks). These are very porous and have the capacity to: absorb and retain CO ₂, obtain H ₂ O from the air vapor in desert areas and store H ₂ in small containers.
• The conversion of waste into raw materials. For example, the use of worn tires in the production of artificial grass or shoe soles. Also the use of crop pruning waste, in the generation of biogas or bioethanol.
• Chemical syntheses of CFC substitutes.
• The development of alternative energies, such as hydrogen cells, for the generation of non-polluting electricity.
• The control of atmospheric pollution, with inert filters and reactive filters.
• Desalination of seawater by reverse osmosis.
• The development of new materials for the flocculation of colloidal substances suspended in water (purification process).
• The reversal of lake eutrophication.
• The development of "green chemistry", a trend that proposes the replacement of toxic chemical compounds by less toxic ones, and "environmentally friendly" chemical procedures. For example, it is applied in the use of less toxic solvents and raw materials, in industry, in the dry cleaning of laundries, among others.

References

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