Environmental chemistry

Environmental chemistry is the study of the chemical interactions that take place in a natural environment, how to control them when necessary, and what happens when unnatural chemicals are introduced into a natural environment. Air, water, and soil chemistry are all affected by materials injected into the corresponding environmental systems by way of human activity. All natural environments function according to the same chemical principles that apply in a chemistry laboratory. Regular testing and monitoring of environmental systems determines how those materials affect and are affected by environmental system processes.

The Chemical Nature of the Environment

Modern chemical science is based on the quantum mechanical model of the atom, the underlying tenet being that all material in the universe is matter composed of atoms. Accordingly, anything and everything in the physical universe is chemical in nature. Matter interacts with matter and energy, as the atoms of matter interact with each other and with energy. The quantum mechanical model of the atom requires that the atoms interact in specific ways, however, and according to strict mathematical rules that are neither entirely known nor entirely understood.

The quantum mechanical model describes atoms of matter as consisting of a small, dense nucleus. This nucleus contains almost the entire mass of the atom in a specific number of discrete particles called protons and neutrons. The protons each bear a single positive charge, while the electrically neutral neutrons fulfill the role of a sort of “nuclear glue,” countering the mutual repulsion of the protonic charges. For the atom itself to be electrically neutral, it contains a number of negatively charged electrons equal to the number of protons. The electrons occupy a space around the nucleus that is about 100,000 times larger in diameter than the nucleus.

According to the mathematical rules of quantum mechanics, electrons are allowed only to have specific energies depending on their position in the space around the nucleus; each specific energy-position combination is referred to as an orbital. The lowest-energy electrons, and hence the most tightly bound electrons, are those situated closest to the nucleus, while any other electrons in the atom range outward in order of energy; therefore, the outermost electrons are the most energetic and the least tightly bound by the nucleus. Interaction between atoms takes place only at the level of the outermost electrons involved. Within that restriction, any interaction that can take place will take place. This means that atoms of different elements can combine in an infinite number of ways, producing stable combinations of atoms with specific spatial arrangements called molecules. Electrons in molecules also must adhere to stringent energy levels.

The interaction of matter, as atoms, with energy is somewhat more complicated, but must nevertheless obey the same rule system of quantum mechanics as applies to atomic structure. The most common interaction of atoms occurs through the direct action of light energy on the electrons within the atoms and molecules. Absorption of light energy by an electron in one atomic or molecular orbital raises the level of the orbital that it must occupy. This is often sufficient to break the molecule apart and, in so doing, drive uncontrolled reactions that are usually undesirable. The fading of color and the embrittlement of plastics left in bright sunlight are examples of matter interacting with light energy. In other cases, the interaction of light energy with specific molecules also may be highly desirable, as is the case when chlorophyll is activated by sunlight to produce glucose and oxygen from carbon dioxide and water.

The Physical Interface

Substances exist normally in one of three states: solid, liquid, or gas. Earth is an agglomeration of substances that are found in any and all of these states. As determined by ancient philosophy, the four basic elemental forms of water, air, earth, and fire represent the three basic states of matter plus energy, all of which are the material medium of environmental chemistry. The physical environment of Earth can thus also be considered in the context of physical interaction between matter in all of these states, and in the context of chemical interactions.

It is an unavoidable reality that the physical and chemical contexts affect each other directly in various ways. The rates of chemical reactions, for example, are affected by the extent to which different chemical materials come into contact with each other at a physical interface, such that the larger the interface area, the faster an overall chemical reaction occurs. Similarly, the more intimately mixed two gases become, such as ozone and fluorocarbon compounds, the more rapidly and completely they react.

The concept of the physical interface between materials is complex. In the case of gases, no definitive interfacial surface can be defined as existing between two masses. Diffusion occurs where the two masses come into contact; the only real definable interface for interaction between them is the “surface” of the individual atoms or molecules as they interact. At the other end of the interface surface spectrum are the massive interfaces between air and water, air and land surface, and water and the solid earth. In the overall structure of Earth, one could also include the more diffuse divisions between the crust and the underlying mantle of the planet. As the source of volcanic and seismic activity, these also have significant roles in the environmental chemistry of the planet.

Environmental Chemistry and Human Activities

While there is nothing humans can do to control or even affect the natural geochemical processes associated with volcanic or seismic activity, the practical field of environmental chemistry focuses much more closely on those aspects of human society that enact change gradually. It has become increasingly apparent that human activities since the Industrial Revolution have had a significant impact on the natural workings of the planetary environment, especially with regard to air and water quality. Agriculture, industry, and transportation are the components of human activity that affect the environment most significantly. In all cases, the principal means of these changes is the ejection of waste and unnatural materials into the environment.

Since the early nineteenth century, the main gaseous component has been gaseous carbon dioxide from the combustion of fuels. Additionally, significant environmental effects have been observed as a result of other materials produced through combustion, unnatural quantities of naturally occurring materials (such as methane), and the indiscriminate use and release of chlorofluorocarbon (CFC) compounds as propellants and refrigerants. Unnatural materials and unnatural quantities of normal materials can now be found in the environment everywhere in the world because of the release of liquids and dissolved materials into the environment—as effluents from production and processing facilities, mining operations, industrial and transportation accidents, warfare, and many other human activities.

Solid materials produced as a result of human activities are also problematic with regard to environmental chemistry. Not the least of these materials is the mass quantity of solids placed in landfills every year. Even more insidious environmental effects have been wrought through the nanoparticulate matter deposited in the environment by combustion fuels—namely, the millions of tons of lead and manganese oxides that have been produced by internal combustion engines.

Human energy use and production also play a role in environmental chemistry. Excess city light and heat have affected air and water quality, while the production of electrical energy through nuclear processes has led to an entirely separate environmental chemistry problem of its own: the management of waste and spent materials and the release of radioactive materials in quantity into the environment.

Environmental Chemistry of the Atmosphere

Perhaps the most recognizable of atmospheric chemical effects is the generation of smog: an accumulation of noxious gases, smoke, and nanoparticulate matter that forms throughout cities. Gas-phase chemical reactions occurring between components of smog and normal atmospheric gases are known to contribute to an assortment of undesirable effects, including acid rain and asthma. Nitrogen oxides and sulfur dioxide react with water and oxygen to produce nitrous, nitric, sulfurous, and sulfuric acids, all of which become dissolved in raindrops as water vapor condenses in the atmosphere. The resulting acidic solutions, often appearing long distances from their sources, react chemically with carbonate stone such as limestone, marble, and concrete. This results in the slow chemical erosion of infrastructure, directly bringing about destruction and occasioning structural upkeep that costs billions of dollars.

Acidic solutions also dissolve solid materials on the surface, percolating through soils. These materials are then carried into waterways and into other sources of drinking water. In more extreme or prolonged cases, acidic precipitation has been identified as the agent responsible for the ecological deaths of otherwise pristine bodies of water, rendering them unable to support an aquatic ecology.

Other causes of smog include chlorine, bromine, and ground-level ozone, all of which react with oxygen and with each other in a multitude of ways by radical mechanisms. These interactions produce various oxyacids and other chemical compounds that have inimical environmental effects. Breathing air that contains these reactive materials causes damage to lung tissue and can lead to emphysema, asthma, and other respiratory difficulties. In some cities around the world, smog has become a serious health issue, requiring constant monitoring of air quality. Government environmental regulatory agencies now issue daily statements of air quality as a regular feature of official meteorological reports.

While ground-level ozone is considered a health hazard, upper atmosphere ozone is essential to the survival of life on Earth. The diffuse layer of ozone at very high altitudes acts as a shield for the planet against the continuous influx of ultraviolet radiation. Absorption of the energy of ultraviolet light by the ozone molecule results in the formation of an electronically excited species that re-emits that energy at a different wavelength that is harmless with regard to living tissue.

In the twentieth century, the chemical compound known as CFC (chlorofluorocarbon) was commonly used as a propellant and refrigerant. The low boiling point and ready compressibility in the gaseous state of CFCs made them ideal for that application, but at the same time also made them easy to dispose of as they dissipated readily into the atmosphere. Ignorance of their environmental effects allowed them to be used for many years, until satellite-based monitoring of the upper atmosphere revealed that the accumulation of CFCs had seriously depleted the ozone shield by way of chemical reactions that produced oxygen dichloride and other exotic chemical species that did not interact with ultraviolet radiation.

Air quality and gaseous effluent monitoring is carried out through a number of methods. Direct sampling for instrumental analysis, using tandem gas chromatography-mass spectrometry (GC-MS), can detect airborne materials in the range of parts per billion. This is especially important when the materials of interest are highly carcinogenic materials (such as dioxins) that do not exist naturally in the environment. Other monitoring methods include ground-based and satellite-based infrared detection of composition and temperature and the use of monitoring tags for exposure to specific chemical compounds. Such tags use the occurrence of a specific chemical reaction to bring about a color change in the surface of the tag.

Environmental Chemistry of Water

Airborne materials can become dissolved in condensate water to produce acidic precipitation. Both acidic and nonacidic precipitation, including agricultural irrigation, dissolve materials in the soil and carry that material into the nearest waterways or into the water table as the liquid percolates through the soil. The leachates so formed can include every compound and element in the soil. For naturally occurring materials in their normal natural proportions, this is not problematic. However, the injection into the environment, through human activities, of many materials makes it essential that the chemical effects of those materials in the environment be monitored and understood.

For example, the use of tetraethyl lead in gasoline over a period of fifty years (before its use in North America was banned) had deposited some nine billion kilograms of lead in the environment as nanoparticulate lead oxides from vehicle exhaust gases. This resulted in a sudden, drastic increase in the amount of lead detectable in drinking water. In Europe, where estate wineries keep meticulous records of production, the amount of lead detectable in wine could be traced through analysis of vintage wine samples to the exact year that the use of leaded gasoline began in that region. In North America, the uptick in lead accumulation coincided with an increase in the levels of birth defects and disorders, and in mental health issues in children.

Lead is a known neurotoxin, as is the manganese that was used to replace the lead in gasoline fuels. Another neurotoxin is mercury, a common waterborne effluent of the mining and pulp and paper industries. Mercury has the insidious property of accumulating in the fatty tissue of organisms, as do many other heavy metals such as cadmium. The process of bioaccumulation works to concentrate those materials in organisms as it progresses up the food chain. By this process, creatures that consume smaller creatures as food, such as fish, may contain quantities of bioactive heavy metals that greatly exceed the baseline amount of those materials in the environment. Chemical compounds such as pesticides (for example, dichloro-diphenyl-trichloroethane, or DDT), industrial chemicals (such as polychlorinated biphenyls, or PCBs) and carcinogenic chemicals (dioxin) also can be concentrated by bioaccumulation.

In agriculture, both organic and mineral fertilizers are used to augment the nutritive value of soils to increase or maintain crop yields. The primary components of mineral fertilizers, and to a lesser extent the organic fertilizers, are nitrogen (as nitrate) and phosphorus (as phosphate). Without this addition, mineral nutrients essential for plant growth in soils can become depleted as successive crops remove them faster than natural processes can replenish them.

Irrigation also works toward the depletion of minerals and other nutrients from the soil by dissolving and carrying them away as the water percolates downward to the water table. Coupled with the overuse of inorganic fertilizers, this can and has led to elevated amounts of nitrates and phosphates in waterways and water tables. The presence of these extra nutrients contributes to the excess growth of algae and aquatic plants, and to the eutrophication of bodies of water. To an extent, this problem is alleviated by the practice of crop rotation, in which different crops are grown on the same plots of land in successive years.

Typically, a crop that draws nutrients from the soil in one year will be succeeded in the following year by a crop that incorporates nutrients naturally by nitrogen fixation or deep-root mineral extraction. For example, corn is often rotated with beans on heavy soils with a high clay component, and tobacco is generally rotated with rye grain on light, sandy soils. Additionally, the likelihood of crop disease increases when the same crop is grown on the same fields year after year without rotation through other crops.

Water Testing

Water testing is carried out both regularly and upon request using a much broader variety of techniques. Because materials are dissolved in water, numerous instrumental analytical techniques are available to measure contents and properties of samples brought in for testing.

Spectrophotometric methods detect specific ions and organic compounds both qualitatively and quantitatively. Samples are prepared for analysis in different ways. In some cases, specific compounds are added that combine with dissolved ions or organic compounds to form specific complexes. Such complexes are often colored, whereas the dissolved ions and organic compounds are not. Colorimetric analysis can then be used to determine the concentration of the complexed material in the prepared sample, and hence the original concentration of the ions and organic compounds of interest.

Spectrophotometric analysis can be used to determine the presence and concentration of specific materials directly. In each case, analysis depends on the measurement of the absorption of light at characteristic wavelengths that are uniquely associated with the specific materials for which testing is being carried out. The Beer-Lambert law describes the relationship between the absorption of light energy and the concentration of the material that is absorbing the light.

Methods such as high-performance liquid chromatography (HPLC) and tandem GC-MS can determine concentrations as low as only a few parts per billion. Chromatographic methods are extremely versatile. They are based on the equilibrium between the differential adsorption of dissolved materials on a solid surface and the solubility of those materials in a fluid phase (either gas or liquid).

In chromatographic separations, the fluid phase containing the dissolved materials passes through a second, solid phase. Components of the gas or liquid solution equilibrate between being dissolved in the fluid medium and being adsorbed on the solid medium. As they progress through the column of the solid medium, they become increasingly separated from each other. Effective separation brings each component of the solution out of the column at different times.

When chromatographic methods such as gas chromatography or HPLC are connected in tandem with a second procedure, such as mass spectrometry, it becomes possible to identify and measure the components of complex mixtures such as environmental water samples in a single step. In tandem processes the efflux from the first stage becomes the input material for the second stage without any intermediate isolation or purification step. In some cases, such as the analysis of organic compounds in aqueous solutions, it is expedient to extract the organic compounds by partitioning to eliminate physical interference by water in the analysis.

As a general rule, the presence of water makes organic analysis very difficult simply because of the physical properties of that material. Organic solvents, being much more volatile than water, are also much more easily removed than water and so make analysis much easier.

Principal Terms

absorption: the capture of light energy of a specific wavelength by electrons in a specific atom or molecule

bioaccumulation: the process whereby a pollutant material becomes concentrated in the body tissues of organisms as they consume other organisms

bioactive: those materials that are capable of biological or biochemical activity when present in living systems

chlorofluorocarbons: carbon-based compounds to which various numbers of fluorine and chlorine atoms are bonded instead of the hydrogen atoms of the parent hydrocarbon compounds

colorimetric: relating to the measurement of the intensity of absorption of a specific color, or the light of a specific visible wavelength

eutrophication: the depletion of dissolved oxygen in water by the growth of algae and aquatic plants, and the subsequent decomposition of vegetable matter

interface: the surface between two materials at their point of contact with each other—as, for example, air and water, liquid and solid

partitioning: a physical process by which components of a solution are extracted from one solvent directly into a second solvent immiscible with the first

spectrophometric: relating to the measurement of the intensity of absorption of light in a specific wavelength that may or may not be in the visible range, including infrared and ultraviolet wavelengths

turbidity: the degree to which a fluid scatters or diffuses light, caused by solid microparticles or liquid microglobules suspended in the fluid medium

Bibliography

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Hites, Ronald A. Elements of Environmental Chemistry. Hoboken, N.J.: John Wiley & Sons, 2007.

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Weiner, Eugene. Applications of Environmental Aquatic Chemistry: A Practical Guide. 2d ed. Boca Raton, Fla.: CRC Press, 2008.

Zhu, Shang. "Improved Environmental Chemistry Property Prediction of Molecules with Graph Machine Learning." Green Chemistry, vol. 17, 2023, pubs.rsc.org/en/content/articlelanding/2023/gc/d3gc01920a. Accessed 9 Feb. 2025.