Heavy metals and heavy metal poisoning

Heavy metals are generally defined as environmentally stable elements of high specific gravity and atomic weight. They have such characteristics as luster, ductility, malleability, and high electric and thermal conductivity. Whether based on their physical or chemical properties, the distinction between heavy metals and non-metals is not sharp. For example, arsenic , germanium, selenium, tellurium, and antimony possess chemical properties of both metals and non-metals. Defined as metalloids, they are often loosely classified as heavy metals. The category "heavy metal" is, therefore, somewhat arbitrary and highly non-specific because it can refer to approximately 80 of the 103 elements in the periodic table. The term "trace element" is commonly used to describe substances which cannot be precisely defined but most frequently occur in the environment in concentrations of a few parts per million (ppm) or less. Only a relatively small number of heavy metals such as cadmium , copper , iron, cobalt, zinc, mercury , vanadium, lead , nickel , chromium, manganese, molybdenum, silver, and tin as well as the metalloids arsenic and selenium are associated with environmental, plant, animal, or human health problems.

While the chemical forms of heavy metals can be changed, they are not subject to chemical/biological destruction. Therefore, after release into the environment they are persistent contaminants. Natural processes such as bedrock and soil weathering , wind and water erosion , volcanic activity, sea salt spray, and forest fires release heavy metals into the environment. While the origins of anthropogenic releases of heavy metals are lost in antiquity, they probably began as our prehistoric ancestors learned to recover metals such as gold, silver, copper, and tin from their ores and to produce bronze. The modern age of heavy metal pollution has its beginning with the Industrial Revolution. The rapid development of industry, intensive agriculture, transportation , and urbanization over the past 150 years, however, has been the precursor of today's environmental contamination problems. Anthropogenic utilization has also increased heavy metal distribution by removing the substances from localized ore deposits and transporting them to other parts of the environment. Heavy metal by-products result from many activities including: ore extraction and smelting, fossil fuel combustion , dumping and landfilling of industrial wastes, exhausts from leaded gasolines, steel, iron, cement and fertilizer production, refuse and wood combustion. Heavy metal cycling has also increased through activities such as farming, deforestation , construction, dredging of harbors, and the disposal of municipal sludges and industrial wastes on land.

Thus, anthropogenic processes, especially combustion, have substantially supplemented the natural atmospheric emissions of selected heavy metals/metalloids such as selenium, mercury, arsenic, and antimony. They can be transported as gases or adsorbed on particles. Other metals such as cadmium, lead, and zinc are transported atmospherically only as particles. In either state heavy metals may travel long distances before being deposited on land or water.

The heavy metal contamination of soils is a far more serious problem than either air or water pollution because heavy metals are usually tightly bound by the organic components in the surface layers of the soil and may, depending on conditions, persist for centuries or millennia. Consequently, the soil is an important geochemical sink which accumulates heavy metals rapidly and usually depletes them very slowly by leaching into groundwater aquifers or bioaccumulating into plants. However, heavy metals can also be very rapidly translocated through the environment by erosion of the soil particles to which they are adsorbed or bound and redeposited elsewhere on the land or washed into rivers, lakes or oceans to the sediment .

The cycling, bioavailability, toxicity, transport, and fate of heavy metals are markedly influenced by their physico-chemical forms in water, sediments, and soils. Whenever a heavy metal containing ion or compound is introduced into an aquatic environment, it is subjected to a wide variety of physical, chemical, and biological processes. These include: hydrolysis, chelation, complexation, redox, biomethylation, precipitation and adsorption reactions. Often heavy metals experience a change in the chemical form or speciation as a result of these processes and so their distribution, bioavailability, and other interactions in the environment are also affected.

The interactions of heavy metals in aquatic systems are complicated because of the possible changes due to many dissolved and particulate components and non-equilibrium conditions. For example, the speciation of heavy metals is controlled not only by their chemical properties but also by environmental variables such as: 1) pH ; 2) redox potential; 3) dissolved oxygen ; 4) ionic strength; 5) temperature; 6) salinity ; 7) alkalinity; 8) hardness; 9) concentration and nature of inorganic ligands such as carbonate, bicarbonate, sulfate, sulfides, chlorides; 10) concentration and nature of dissolved organic chelating agents such as organic acids, humic materials, peptides , and polyamino-carboxylates; 11) the concentration and nature of particulate matter with surface sites available for heavy metal binding; and 12) biological activity.

In addition, various species of bacteria can oxidize arsenate or reduce arsenate to arsenite, or oxidize ferrous iron to ferric iron, or convert mercuric ion to elemental mercury or the reverse. Various enzyme systems in living organisms can biomethylate a number of heavy metals. While it had been known for at least 60 years that arsenic and selenium could be biomethylated, microorganisms capable of converting inorganic mercury into monomethyl and dimethylmercury in lake sediments were not discovered until 1967. Since then, numerous heavy metals such as lead, tin, cobalt, antimony, platinum, gold, tellurium, thallium, and palladium have been shown to be biomethylated by bacteria and fungi in the environment.

As environmental factors change the chemical reactivities and speciation of heavy metals, they influence not only the mobilization, transport, and bioavailability, but also the toxicity of heavy metal ions toward biota in both freshwater and marine ecosystems. The factors affecting the toxicity and bioaccumulation of heavy metals by aquatic organisms include: 1) the chemical characteristics of the ion; 2) solution conditions which affect the chemical form (speciation) of the ion; 3) the nature of the response such as acute toxicity, bioaccumulation, various types of chronic effects , etc.; 4) the nature and condition of the aquatic animal such as age or life stage, species, or trophic level in the food chain. The extent to which most of the methylated metals are bioaccumulated and/or biomagnified is limited by the chemical and biological conditions and how readily the methylated metal is metabolized by an organism. At present, only methylmercury seems to be sufficiently stable to bioaccumulate to levels that can cause adverse effects in aquatic organisms. All other methylated metal ions are produced in very small concentrations and are degraded naturally faster than they are bioaccumulated. Therefore, they do not biomagnify in the food chain.

The largest proportion of heavy metals in water is associated with suspended particles, which are ultimately deposited in the bottom sediments where concentrations are orders of magnitude higher than those in the overlying or interstitial waters. The heavy metals associated with suspended particulates or bottom sediments are complex mixtures of: 1) weathering and erosion residues such as iron and aluminum oxyhydroxides, clays and other aluminosilicates;
2) methylated and non-methylated forms in organic matter such as living organisms, bacteria and algae, detritus and humus ; 3) inorganic hydrous oxides and hydroxides, phosphates and silicates; and 4) diagenetically produced iron and manganese oxyhydroxides in the upper layer of sediments and sulfides in the deeper, anoxic layers.

In anoxic waters the precipitation of sulfides may control the heavy metal concentrations in sediments while in oxic waters adsorption, absorption , surface precipitation and coprecipitation are usually the mechanisms by which heavy metals are removed from the water column. Moreover, physical, chemical and microbiological processes in the sediments often increase the concentrations of heavy metals in the pore waters which are released to overlying waters by diffusion or as the result of consolidation and bioturbation. Transport by living organisms does not represent a significant mechanism for local movement of heavy metals. However, accumulation by aquatic plants and animals can lead to important biological responses. Even low environmental levels of some heavy metals may produce subtle and chronic effects in animal populations.

Despite these adverse effects, at very low levels, some metals have essential physiological roles as micronutrients. Heavy metals such as chromium, manganese, iron, cobalt, molybdenum, nickel, vanadium, copper, and selenium are required in small amounts to perform important biochemical functions in plant and animal systems. In higher concentrations they can be toxic, but usually some biological regulatory mechanism is available by means of which animals can speed up their excretion or retard their uptake of excessive quantities.

In contrast, non-essential heavy metals are primarily of concern in terrestrial and aquatic systems because they are toxic and persist in living systems. Metal ions commonly bond with sulfhydryl and carboxylic acid groups in amino acids, which are components of proteins (enzymes) or polypeptides. This increases their bioaccumulation and inhibits excretion. For example, heavy metals such as lead, cadmium, and mercury bind strongly with -SH and -SCH₃ groups in cysteine and methionine and so inhibit the metabolism of the bound enzymes. In addition, other heavy metals may replace an essential element, decreasing its availability and causing symptoms of deficiency.

Uptake, translocation, and accumulation of potentially toxic heavy metals in plants differ widely depending on soil type, pH, redox potential, moisture, and organic content. Public health officials closely regulate the quantities and effects of heavy metals that move through the agricultural food chain to be consumed by human beings. While heavy metals such as zinc, copper, nickel, lead, arsenic, and cadmium are translocated from the soil to plants and then into the animal food chain, the concentrations in plants are usually very low and generally not considered to be an environmental problem. However, plants grown on soils either naturally enriched or highly contaminated with some heavy metals can bioaccumulate levels high enough to cause toxic effects in the animals or human beings that consume them.

Contamination of soils due to land disposal of sewage and industrial effluents and sludges may pose the most significant long term problem. While cadmium and lead are the greatest hazard, other elements such as copper, molybdenum, nickel, and zinc can also accumulate in plants grown on sludge-treated land. High concentrations can, under certain conditions, cause adverse effects in animals and human beings that consume the plants. For example, when soil contains high concentrations of molybdenum and selenium, they can be translocated into edible plant tissue in sufficient quantities to produce toxic effects in ruminant animals. Consequently, the U. S. Environmental Protection Agency has issued regulations which prohibit and/or tightly regulate the disposal of contaminated municipal and industrial sludges on land to prevent heavy metals, especially cadmium, from entering the food supply in toxic amounts. However, presently, the most serious known human toxicity is not through bioaccumulation from crops but from mercury in fish, lead in gasoline , paints and water pipes, and other metals derived from occupational or accidental exposure.

See also Aquatic chemistry; Ashio, Japan; Atmospheric pollutants; Biomagnification; Biological methylation; Contaminated soil; Ducktown, Tennessee; Hazardous material; Heavy metals precipitation; Itai-Itai disease; Methylmercury seed dressings; Minamata disease; Smelters; Sudbury, Ontario; Xenobiotic

[Frank M. D'Itri ]

RESOURCES

BOOKS

Craig, P. J. "Metal Cycles and Biological Methylation." The Handbook of Environmental Chemistry. Vol. 1, Part A, edited by O. H. Hutzinger. Berlin: Springer Verlag, 1980.

Förstner, U., and G. T. W. Wittmann. Metal Pollution in the Aquatic Environment. 2nd ed. Berlin: Springer Verlag, 1981.

Kramer, J. R., and H. E. Allen, eds. Metal Speciation: Theory, Analysis and Application. Chelsea, MI: Lewis, 1988.