Environmental chemistry

Environmental chemistry refers to the occurrence, movements, and transformations of chemicals in the environment . Environmental chemistry deals with naturally occurring chemicals such as metals, other elements, organic chemicals, and biochemicals that are the products of biological metabolism . Environmental chemistry also deals with synthetic chemicals that have been manufactured by humans and dispersed into the environment, such as pesticides, polychlorinated biphenyls (PCBs), dioxins, furans , and many others.

The occurrence of chemicals refers to their presence and quantities in various compartments of the environment and ecosystems. For example, in a terrestrial ecosystem such as a forest, the most important compartments to consider are the mineral soil , water and air present in spaces within the soil, the above-ground atmosphere , dead biomass within the soil and lying on the ground as logs and other organic debris, and living organisms, the most abundant of which are trees. Each of these components of the forest ecosystem contains a wide variety of chemicals in some concentration, and in some amount. Chemicals move between all of these compartments, as fluxes that represent elements of nutrient and mineral cycles.

The movements of chemicals within and among compartments often involve a complex of transformations among potential molecular states. There may also be changes in physical states, such as evaporation of liquids, or crystallization of dissolved substances. The transformations of chemicals among molecular states can be illustrated by reference to the environmental cycling of sulfur. Sulfur (S) is commonly emitted to the atmosphere as the gases sulfur dioxide (SO₂) or hydrogen sulfide (H₂S), which are transformed by photochemical reactions into the negatively-charged ion , sulfate (SO ^-2 4 ). The sulfate may eventually be deposited with precipitation to a terrestrial ecosystem, where it may be absorbed along with soil water by tree roots, and later used to synthesize biochemicals such as proteins and amino acids. Eventually, the plant may die and its biomass deposited to the soil surface as litter. Microorganisms can then metabolize the organic matter as a source of energy and nutrients, eventually releasing simple inorganic compounds of sulfur such as sulfate or hydrogen sulfide into the environment. Alternatively, the plant biomass may be harvested by humans and used as a fuel, with the organic sulfur being oxidized during combustion and emitted to the atmosphere as sulfur dioxide. Organic and mineral forms of sulfur also occur in fossil fuels such as petroleum and coal , and the combustion of those materials also results in an emission of sulfur dioxide to the atmosphere.

Contamination and pollution

Contamination and pollution both refer to the presence of chemicals in the environment, but it is useful to distinguish between these two conditions. Contamination refers to the presence of one or more chemicals in concentrations higher than normally occurs in the ambient environment, but not high enough to cause biological or ecological damages. In contrast, pollution occurs when chemicals occur in the environment in concentrations high enough to cause damages to organisms. Pollution results in toxicity and ecological changes, but contamination does not cause those damages.

Chemicals that are commonly involved in pollution include the gases sulfur dioxide and ozone , diverse kinds of pesticides, elements such as arsenic , copper , mercury , nickel , and selenium, and some naturally occurring biochemicals. In addition, large concentrations of nutrients such as phosphate and nitrate can cause eutrophication, a type of pollution associated with excessive ecological productivity . Although any of these chemicals can cause pollution in certain situations, they most commonly occur in concentrations too small to cause toxicity or other ecological damages.

Modern analytical chemistry has become extremely sophisticated, and this allows trace contamination of potentially toxic chemicals to be measured at levels that are much smaller than what is required to cause demonstrable physiological or ecological damages.

Environmental chemistry of the Aamosphere

Nitrogen gas (N₂) comprises about 79% of the mass of Earth's atmosphere, while 20% is oxygen (O₂), 0.9% argon (Ar), 0.035% carbon dioxide (CO₂), and the remainder composed of a variety of trace gases. The atmosphere also contains variable concentrations of water vapor, which can range from 0.01% in frigid arctic air to 5% in humid tropical air.

The atmosphere also can contain high concentrations of gases, vapors, or particulates that are potentially harmful to people, other animals, or vegetation, or that cause damages to buildings, art, or other materials. The most important gaseous air pollutants (listed alphabetically) are ammonia (NH₃), carbon monoxide (CO), fluoride (F, usually occurring HF), nitric oxide and nitrogen dioxide (NO and NO₂, together known as oxides of nitrogen, or NO_x), ozone (O₃), peroxyacetyl nitrate (PAN), and sulfur dioxide (SO₂).

Vapors of elemental mercury and hydrocarbons can also be air pollutants. Particulates with tiny diameters (less than 1μm) can also be important, including dusts containing such toxic elements as arsenic, copper, lead , nickel, and vanadium, organic aerosols that are emitted as smoke during combustions (including toxins known as polycyclic aromatic hydrocarbons ), and non-reactive minerals such as silicates.

Some so-called "trace toxics" also occur in the atmosphere in extremely small concentrations. The trace toxics include persistent organochlorine chemicals such as the pesticides DDT and dieldrin, polychlorinated biphenyls (PCBs), and the dioxin , TCDD. Other, less persistent pesticides may also be air pollutants close to places where they are used.

Environmental chemistry of water

Earth's surface waters vary enormously in their concentrations of dissolved and suspended chemicals. Other than the water, the chemistry of oceanic water is dominated by sodium chloride (NaCl), which has a typical concentration of about 3.5% or 35 g/l. Also important are sulfate (2.7 g/l), magnesium (1.3 g/l), and potassium and calcium (both 0.4 g/l). Some saline lakes can have much larger concentrations of dissolved ions, such as Great Salt Lake in Utah, which contains more than 20% salts.

Fresh waters are much more dilute in ions, although the concentrations are variable among waterbodies. The most important cations in typical fresh waters are calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), ammonium (NH ⁺ 4 ), and hydrogen ion (H⁺; this is only present in acidic waters, otherwise hydroxy ion or OH^- occurs). The most important anions are bicarbonate (HCO_3-) sulfate (SO ²⁺ 4 ), chloride (Cl^-), and nitrate (NO 3 ). Some fresh waters have high concentrations of dissolved organic compounds, known as humic substances, which can stain the water a tea-like color. Typical concentrations of major ions in fresh water are: calcium 15 mg/l, sulfate 11 mg/l, chloride 7 mg/l, silica 7 mg/l, sodium 6 mg/l, magnesium 4 mg/l, and potassium 3 mg/l.

The water of clean precipitation is considerably more dilute than that of surface waters such as lakes. For example, precipitation at a remote place in Nova Scotia contained 1.6 mg/l of sulfate, 1.3 mg/l chloride, 0.8 mg/l sodium, 0.7 mg/l nitrate, 0.13 mg/l calcium, 0.08 mg/l ammonium, 0.08 mg/l magnesium, and 0.08 mg/l potassium. Because that site is about 31 mi (50 km) from the Atlantic Ocean, its precipitation is influenced by sodium and chloride originating with sea spray. In comparison, a more central location in North America had a sodium concentration of 0.09 mg/l and chloride 0.15 mg/l.

Pollution of surface waters is most often associated with the dumping of human or industrial sewage, nutrient inputs from agriculture, acidification caused by acidic precipitation or by acid-mine drainage , and industrial inputs of toxic chemicals. Eutrophication is caused when nutrient inputs cause large increases in aquatic productivity, especially in fresh waters and shallow marine waters into which sewage is dumped or that receive runoff containing agricultural fertilizers. In general, marine ecosystems become eutrophic when they are fertilized with nitrate, and freshwater systems with phosphate. Only 35–100 μg/l or more of phosphate is enough to significantly increase the productivity of most shallow lakes, compared with the background concentration of about 10 μg/l or less.

Freshwater ecosystems can become acidified by receiving drainage from bogs, by the deposition of acidifying substances from the atmosphere (such as acidic rain), and by acid-mine drainage. Atmospheric depositions have caused a widespread acidification of surface waters in eastern North America, Scandinavia, and other places. Surface waters acidified by atmospheric depositions commonly develop pHs of about 4.5–5.5. Tens of thousands of lake and running-water ecosystems have been damaged in this way. Acidification has many biological consequences, including toxicity caused to many species of plants and animals, including fish.

Some industries emit metals to the environment, and these may pollute fresh and marine waters. For instance, lakes near large smelters at Sudbury, Ontario , have been polluted by sulfuric acid , copper, nickel, and other metals, which in some cases occur in concentrations large enough to cause toxicity to aquatic plants and animals.

Mercury contamination of fish is also a significant problem in many aquatic environments. This phenomenon is significant in almost all large fish and sharks , which accumulate mercury progressively during their lives and commonly have residues in their flesh that exceed 0.5 ppm (this is the criterion set by the World Health Organization for the maximum concentration of mercury in fish intended for human consumption). It is likely, however, that the oceanic mercury is natural in origin, and not associated with human activities. Many fresh-water fish also develop high concentrations of mercury in their flesh, also commonly exceeding the 0.5 ppm criterion. This phenomenon has been demonstrated in many remote lakes. The source of mercury may be mostly natural, or it may originate with industrial sources whose emissions are transported over a long distance in the atmosphere before they are deposited to the surface. Severe mercury pollution has also occurred near certain factories, such as chlor-alkali plants and pulp mills. The most famous example occurred at Minamata, Japan, where industrial discharges led to the pollution of marine organisms, and then resulted in the poisoning of fish-eating animals and people.

Environmental chemistry of soil and rocks

The most abundant elements in typical soils and rocks are oxygen (47%), silicon (28%), aluminum (8%), and iron (3–4%). Virtually all of the other stable elements are also present in soil and rocks, and all of these can occur in a great variety of molecular forms and minerals. Under certain circumstances, some of these chemicals can occur in relatively high concentrations, sometimes causing ecological damages.

This can occur naturally, as in the case of soils influenced by so-called serpentine minerals, which can contain hundreds to thousands of ppm of nickel. In addition, industrial emissions of metals from smelters have caused severe pollution. Soils near Sudbury, for example, can contain nickel and copper concentrations up to 5,000 ppm each. Even urban environments can be severely contaminated by certain metals. Soils collected near urban factories for recycling old automobile batteries can contain lead in concentrations in the percent range, while the edges of roads can contain thousands of ppm of lead emitted through the use of leaded gasoline .

Trace toxics

Some chemicals occur in minute concentrations in water and other components of the environment, yet still manage to cause significant damages. These chemicals are sometimes referred to as trace toxics. The best examples are the numerous compounds known as halogenated hydrocarbons, particularly chlorinated hydrocarbons such as the insecticides DDT, DDD, and dieldrin, the dielectric fluids PCBs, and the chlorinated dioxin, TCDD. These chemicals are not easily degraded by either ultraviolet radiation or by metabolic reactions, so they are persistent in the environment. In addition, chlorinated hydrocarbons are virtually insoluble in water, but are highly soluble in lipids such as fats and oils. Because most lipids in ecosystems occur within the bodies of organisms, chlorinated hydrocarbons have a marked tendency to bioaccumulate (i.e., to occur preferentially in organisms rather than in the non-living environment). This, coupled with the persistence of these chemicals, results in their strong tendency to food-chain/web accumulate or biomagnify (i.e., to occur in their largest concentrations in top predators).

Fish-eating birds are examples of top predators that have been poisoned by exposure to chlorinated hydrocarbons in the environment. Some examples of species that have been affected by this type of ecotoxicity include the peregrine falcon (Falco peregrinus ), bald eagle (Haliaeetus leucocephalus ), osprey (Pandion haliaetus ), brown pelican (Pelecanus occidentalis ), double-crested cormorant (Phalacrocorax auritus ), and western grebe (Aechmophorus occidentalis ). Concentrations of chlorinated hydrocarbons in the water of aquatic habitats of these birds is generally less than 1 μg/l (part per billion, or ppb), and less than 1 ng/l (part per trillion, or ppt) in the case of TCDD. However, some of the chlorinated hydrocarbons can biomagnify to tens to hundreds of mg/kg (ppm) in the fatty tissues of fish-eating birds. This can cause severe toxicity, characterized by reproductive failures, and even the deaths of adult birds, both of which can cause populations to collapse.

Other trace toxics also cause ecological damages. For example, although it is only moderately persistent in aquatic environments, the insecticide carbofuran can accumulate in acidic standing water in recently treated fields. If geese, ducks, or other birds or mammals utilize those temporary aquatic habitats, they can be killed by the carbofuran residues. Large numbers of wildlife have been killed this way in North America.

Petroleum

Water pollution can also result from the occurrence of hydrocarbons in large concentrations, especially after spills of crude oil or its refined products. Oil pollution can result from accidental spills of petroleum from wrecked tankers, offshore drilling platforms, broken pipelines, and from spills during warfare, as occurred during the Gulf War of 1991. Other important sources of oil pollution include operational discharges from tankers disposing oily bilge waters, and chronic releases from oil refineries and urban runoff .

The concentration of natural hydrocarbons in seawater is about 1 ppb, mostly due to releases from phytoplankton and bacteria. Beneath a slick of petroleum spilled at sea, however, the concentration of dissolved hydrocarbons can exceed several ppm, enough to cause toxicity to some organisms. There are also finely suspended droplets of petroleum in water beneath slicks, as a result of wave action on the floating oil. The slick and the sub-surface emulsion of oilin-water are highly damaging to organisms that become coated with these substances.

[Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Freedman, B. Environmental Ecology, 2nd ed. San Diego: Academic Press, 1995.

Hemond, H.F., and E.J. Fechner. Chemical Fate and Transport in the Environment. San Diego: Academic Press, 1994.

Manahan, S. Environmental Chemistry, 6th ed. Boca Raton, FL: Lewis Publishers, 1994.