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Introduction to Metal Pollution


Humans have been introducing trace metals into the environment since they first gained knowledge of their many useful properties. Approximately 8000 years ago, in western Asia, almost pure deposits of gold and copper were discovered. Initially, having been attracted to their lustrous beauty, it was subsequently observed that these metals possessed both malleability and strength. Since then, humans have used various means to extract these and other metal compounds from the earth's crust. Despite the benefits that these properties infer, society has had to embrace the harsh consequences of metal pollution. Early on, metal pollution only affected that small portion of the population who were in the local region surrounding the source. During this period, mining, smelting and manufacturing took place in small, isolated facilities. However, at the turn of the 19th century, the start of the Industrial Revolution resulted in a greater demand for products such as coal, iron and steel. Pollutants were no longer restricted to local areas, but instead, were distributed over a wide area, by means of air and water. This has caused visible detrimental effects to the ecosystem and consequences to human health. This paper will discuss various aspects of metals and metal pollution, such as: the properties and uses of metals, the toxic effects of specific metals (mercury, lead, cadmium and arsenic), the effects of speciation on toxicity and the effects of metal binding to intracellular biological ligands.

Metals are found throughout the earth, in rocks, soils and sediments, primarily trapped in some stable form. Yet, through natural processes such as weathering and erosion, small amounts of metals are removed from bedrock and are allowed to circulate in water and air. This is essential because many biochemical processes require a given amount of many of these metals. So, even if a trace metal is toxic at high concentrations, it may be needed in small quantities to maintain life. Therefore, the biogeochemical cycle which exists ensures that the distribution of any given metal within an ecosystem be held relatively constant over time. However, with the increase in the standard of living and in the number of technological advances, as seen in the 20th century, large quantities of various metals have been required to meet the demands imposed. Inevitably, as the usage of metals increased, so did the pollution associated with it. The demand has been so extreme that quantities of most trace metals introduced into the environment by anthropoegenic sources now far outweigh natural sources. Mining of ores, in addition to smelting and other purification practices, result in the metal being released from its stable forms and into the environment. This has produced a situation in which the natural biogeochemical cycle has been overwhelmed.

Metal pollutants are primarily distributed in the atmosphere, water, soil and sediments. Atmospheric metal pollution arises mainly from the mining, smelting and refining of metallic ores, the manufacturing and use of metallic products, and the burning of fossil fuels. Atmospheric pollutants are often the largest source of waterborne metals. For example, it is estimated that 70% of lead in water and over 50% of many of the other trace metals in the Great Lakes are derived from atmospheric transfer. However, in the case of mercury, atmospheric pollutants account for only 30% of the total waterborne amount. Similarly, cadmium is delivered to water via direct industrial discharge into waterbodies. In general, freshwater ecosystems have low natural background metal levels and therefore tend to be sensitive to even small additions of most trace metals. Soils and sediments are the ultimate sink for many pollutants. Pollution in soils is predominantly derived from decomposition of metal-containing products and the disposal of coal. In rural areas, atmospheric deposition of metals plays a major role. In addition, arable soils receive metal burdens from pesticides, fertilizers and animal waste. Metals tend to accumulate in the biologically active regions of the soil, where they can be taken up by crops. For example, high metal concentrations have rendered 9.5% of Japanese rice paddies incapable of producing consumable products. Accumulation of metals in sediments may enter the foodchain through benthic invertebrates or by fish feeding on the sediments. This deposition in the sediments may result in persistent metal concentrations within the aquatic ecosystem.

Some of the major metal pollutants that appear to be of primary concern include lead, mercury, cadmium and arsenic. Lead is a soft, malleable and stable metal which is often used in the manufacturing of storage batteries and as an anti-knocking additive in gasoline (tetraethyl lead). Some European countries, the US and Canada phased out leaded gasoline by 1990, due to research which indicated that 60% of all lead emissions came from automobile exhaust. Other major sources of lead also include mining, smelting and refining of lead, nickel and copper ores. In water, lead tends to accumulate in aquatic organisms through the foodchain and by direct uptake. Accumulation of lead in soils occurs at municipal waste sites because of various electronic components, glass, ceramics and batteries. Lead is believed to cause hypertension, reproductive disorders, neurological and metabolic problems. The critical point concerning chronic toxicity of lead is that, it is chemically similar to calcium, and therefore it is accumulated in the bone matrix. Whenever the body triggers calcium release from bones, lead is also released into the blood stream. This could cause high levels of lead to enter into the bloodstream, possibly long after the body was exposed to the lead.

Mercury is usually collected from the mineral, mercury sulphide. Mercury is commonly used as slimicides and fungicides because it is found to be toxic for many organisms. Atmospheric mercury is usually produced during mining and refining, and incineration of garbage containing electrical equipment. Water pollutants are usually directly discharged by chlor-alkali and pulp and paper industries causing increases in local concentrations of mercury. Toxicity of mercury is greatest when it is in an organic form, produced by biotransformation of ionic mercury to methylmercury. This transformation is provided by bacterial action on the sendiment-bound mercury. Methylmercury dissolves in water and is easily absorbed by organisms. This toxicant is persistent because it is not easily removed from the body, and as a result mercury can remain in the body for up to 20 to 30 years. Toxicity from inorganic mercury is primarily an occupational disease resulting in central nervous system damage and kidney failure.

Cadmium is usually produced from the by-product of various industrial activities, such as smelter operations and the burning of fossil fuels. These sources are the main contributors to atmospheric cadmium levels. Cadmium pollution in water primarily occurs by direct industrial discharges into waterbodies. Toxicity of this metal usually occurs during occupational exposure, resulting in kidney damage and hypertension.

Arsenic is a semi-metal, which although toxic at high concentrations, is essential at very low concentrations for promoting growth. It is generally used as an insecticide and as a hardener in the making of alloys and semi-conductors. Arsenic is discharged into the atmosphere during coal combustion, metal smelting and roasting, and during pesticide use. Use of pesticides containing arsenic results in elevated levels of the metal in food and water pollution by soil runoff. Epidemiological studies have indicated a correlation between low dosage exposure to arsenic and the incidence of cancers.

The major problems associated with excessive release of trace metals into the environment are that metals neither biodegrade, nor are they eliminated by incineration processes. These elements tend to be persistent pollutants, and can accumulate in ecosystems and foodchains. In addition, each metal has a specific chemical form (speciation) which determines its solubility in water, and consequently its ability to incorporate into biological systems. Ionic forms, because of their water solubility, are allowed to enter into biologic processes. They have a tendency to non-discriminately bind to enzymatic, electronegative ligands in the organism. It is believed that in acute toxicity, binding occurs to the first available ligand. This would imply that the route of administration is critical in determining the effects of acute toxicity. During chronic toxicity, the metal distributes itself throughout the body and preferentially binds to the ligand with the highest binding affinity. Cells with high affinity ligands associated with toxicity, are referred to as target cells. Organometallic compounds are able to pass through biological membranes because of their high degree of lipophilicity. Consequently, membranes such as the blood-brain barrier can be permeated and allowed to persist for long periods of time.

The induction of metallothionien and glutathione in response to exposure to increased levels of trace metals are examples of proteins that have evolved to help protect organisms against the problem of high trace metal levels within the body. These proteins contain sulphur, allowing them to bind to metals very tightly, thus reducing their toxicity. In cases of acute poisoning, non-specific ligands, known as chelating agents can be administered to help bind free ionic metal.

Anthropoegenic sources have caused detrimental effects to the natural biogeochemical cycling pattern through excessive processing of mineral ores from the earth's crust. Through several processes, stable mineral ores are being mined, thus producing metals that are capable of interfering with biological activities. These metals, since they are unable to be biodegraded or biotransformed, remain as persistent toxicants within ecosystems and specific foodchains. In conclusion, coordinated efforts should be made to continually reduce the amount of metal pollutants entering into the environment.

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