Heavy metals
Heavy metals is a controversial and ambiguous term for metallic elements with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context, and arguably, the term "heavy metal" should be avoided. A heavy metal may be defined on the basis of density, atomic number, or chemical behaviour. More specific definitions have been published, none of which has been widely accepted. The definitions surveyed in this article encompass up to 96 of the 118 known chemical elements; only mercury, lead, and bismuth meet all of them. Despite this lack of agreement, the term is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.
The earliest known metals—common metals such as iron, copper, and tin, and precious metals such as silver, gold, and platinum—are heavy metals. From 1809 onward, light metals, such as magnesium, aluminium, and titanium, were discovered, as well as less well-known heavy metals, including gallium, thallium, and hafnium.
Some heavy metals are either essential nutrients, or relatively harmless, but can be toxic in larger amounts or certain forms. Other heavy metals, such as arsenic, cadmium, mercury, and lead, are highly poisonous. Potential sources of heavy-metal poisoning include mining, tailings, smelting, industrial waste, agricultural runoff, occupational exposure, paints, and treated timber.
Physical and chemical characterisations of heavy metals need to be treated with caution, as the metals involved are not always consistently defined. Heavy metals, as well as being relatively dense, tend to be less reactive than lighter metals, and have far fewer soluble sulfides and hydroxides. While distinguishing a heavy metal such as tungsten from a lighter metal such as sodium is relatively easy, a few heavy metals, such as zinc, mercury, and lead, have some of the characteristics of lighter metals, and lighter metals, such as beryllium, scandium, and titanium, have some of the characteristics of heavier metals.
Heavy metals are relatively rare in the Earth's crust, but are present in many aspects of modern life. They are used in, for example, golf clubs, cars, antiseptics, self-cleaning ovens, plastics, solar panels, mobile phones, and particle accelerators.
Definitions
Controversial terminology
The International Union of Pure and Applied Chemistry, which standardizes nomenclature, says "the term 'heavy metals' is both meaningless and misleading". The IUPAC report focuses on the legal and toxicological implications of describing "heavy metals" as toxins when no scientific evidence supports a connection. The density implied by the adjective "heavy" has almost no biological consequences, and pure metals are rarely the biologically active form.This characterization has been echoed by numerous reviews. The most widely used toxicology textbook, Casarett and Doull’s Toxicology uses "toxic metal", not "heavy metal". Nevertheless, there are scientific and science related articles which continue to use "heavy metal" as a term for toxic substances. To be an acceptable term in scientific papers, a strict definition has been encouraged.
Use outside toxicology
Even in applications other than toxicity, no widely agreed criterion-based definition of a heavy metal exists. Reviews have recommended that it not be used. Different meanings may be attached to the term, depending on the context. For example, a heavy metal may be defined on the basis of density, and the distinguishing criterion might be atomic number or chemical behaviour.Density criteria range from above 3.5 g/cm3 to above 7 g/cm3. Atomic weight definitions can range from greater than sodium ; greater than 40 ; or more than 200, i.e. from mercury onwards. Atomic numbers are sometimes capped at 92. Definitions based on atomic number have been criticised for including metals with low densities. For example, rubidium in group 1 of the periodic table has an atomic number of 37, but a density of only 1.532 g/cm3, which is below the threshold figure used by other authors. The same problem may occur with definitions which are based on atomic weight.
The United States Pharmacopeia includes a test for heavy metals that involves precipitating metallic impurities as their coloured sulfides. On the basis of this type of chemical test, the group would include the transition metals and post-transition metals.
A different chemistry-based approach advocates replacing the term "heavy metal" with two groups of metals and a gray area. Class A metal ions prefer oxygen donors; class B ions prefer nitrogen or sulfur donors; and borderline or ambivalent ions show either class A or B characteristics, depending on the circumstances. The distinction between the class A metals and the other two categories is sharp. The class A and class B terminology is analogous to the "hard acid" and "soft base" terminology sometimes used to refer to the behaviour of metal ions in inorganic systems. The system groups the elements by where is the metal ion electronegativity and is its ionic radius. This index gauges the importance of covalent interactions vs ionic interactions for a given metal ion. This scheme has been applied to analyze biologically active metals in sea water for example, but it has not been widely adopted.
Origins and use of the term
The heaviness of naturally occurring metals such as gold, copper, and iron may have been noticed in prehistory and, in light of their malleability, led to the first attempts to craft metal ornaments, tools, and weapons.In 1817, German chemist Leopold Gmelin divided the elements into nonmetals, light metals, and heavy metals. Light metals had densities of 0.860–5.0 g/cm3; heavy metals 5.308–22.000. The term heavy metal is sometimes used interchangeably with the term "heavy element". For example, in discussing the history of nuclear chemistry, Magee noted that the actinides were once thought to represent a new heavy-element transition group, whereas Seaborg and co-workers "favoured ... a heavy metal rare-earth like series ...".
The counterparts to the heavy metals, the light metals, are defined by the Minerals, Metals and Materials Society as including "the traditional and emerging light metals "
Biological role
Trace amounts of some heavy metals, mostly in period 4, are required for certain biological processes. These are iron and copper ; cobalt ; vanadium and manganese ; chromium ; nickel ; arsenic and selenium. Periods 5 and 6 contain fewer essential heavy metals, consistent with the general pattern that heavier elements tend to be less abundant and that scarcer elements are less likely to be nutritionally essential. In period 5, molybdenum is required for the catalysis of redox reactions; cadmium is used by some marine diatoms for the same purpose; and tin may be required for growth in a few species. In period 6, tungsten is required by some archaea and bacteria for metabolic processes. A deficiency of any of these period 4–6 essential heavy metals may increase susceptibility to heavy metal poisoning.An average human body is about 0.01% heavy metals, 2% light metals and nearly 98% nonmetals.
A few non-essential heavy metals have been observed to have biological effects. Gallium, germanium, indium, and most lanthanides can stimulate metabolism, and titanium promotes growth in plants.
Toxicity
Heavy metals are often assumed to be highly toxic or damaging to the environment and while some are, certain others are toxic only when taken in excess or encountered in certain forms. Inhalation of certain metals, either as fine dust or most commonly as fumes, can also result in a condition called metal fume fever.Environmental heavy metals
Chromium, arsenic, cadmium, mercury, and lead have the greatest potential to cause harm on account of their extensive use, the toxicity of some of their combined or elemental forms, and their widespread distribution in the environment. Hexavalent chromium, for example, is highly toxic as are mercury vapour and many mercury compounds. These five elements have a strong affinity for sulfur; in the human body they usually bind, via thiol groups, to enzymes responsible for controlling the speed of metabolic reactions. The resulting sulfur-metal bonds inhibit the proper functioning of the enzymes involved; human health deteriorates, sometimes fatally. Chromium and arsenic are carcinogens; cadmium causes a degenerative bone disease; and mercury and lead damage the central nervous system.Lead is the most prevalent heavy metal contaminant. Levels in the aquatic environments of industrialised societies have been estimated to be two to three times those of pre-industrial levels. As a component of tetraethyl lead,, it was used extensively in gasoline from the 1930s until the 1970s. Although the use of leaded gasoline was largely phased out in North America by 1996, soils next to roads built before this time retain high lead concentrations. Later research demonstrated a statistically significant correlation between the usage rate of leaded gasoline and violent crime in the United States; taking into account a 22-year time lag, the violent crime curve virtually tracked the lead exposure curve.
Other heavy metals noted for their potentially hazardous nature, usually as toxic environmental pollutants, include manganese ; cobalt and nickel ; copper, zinc, selenium and silver ; tin, as organotin ; antimony ; and thallium.