Heavy metal (chemistry)

A 25-foot (7.6 m) wall of coal fly ash contaminated with heavy metals, resulting from the release of 5.4 million cubic yards of coal fly ash slurry into the Emory River, Tennessee, and nearby land and water features, in December 2008.^[1] Testing showed significantly elevated levels of arsenic, copper, barium, cadmium, chromium, lead, mercury, nickel, and thallium in samples of slurry and river water.^[2] Cleanup costs may exceed $1.2 billion.^[3]

Commonly encountered heavy metals

A heavy metal is generally regarded as any relatively dense metal or metalloid of environmental concern.^[4] The term originated^[5] with reference to the harmful effects of metals like cadmium, mercury and lead, all of which are denser than iron. Commonly encountered heavy metals are chromium, cobalt, nickel, copper, zinc, arsenic, selenium, silver, cadmium, antimony, mercury, thallium and lead. More specific definitions of a heavy metal have been proposed; none have obtained widespread acceptance.^[6]

Definitions

There is no widely agreed criteria-based definition of a heavy metal. Criteria used to define heavy metals have included density, atomic weight, atomic number, aqueous chemistry or periodic table position.^[7] Density criteria range from above 3.5 g/cm³ to above 7 g/cm³. Atomic weight definitions start at greater than sodium (22.98) to greater than 40.^{[n 1]} Atomic numbers of heavy metals are generally given as greater than 20; sometimes this is capped at 92 (uranium). The United States Pharmacopeia includes a test for heavy metals, which is described as a test for "metallic impurities that are colored by sulfide ion."^[9] Hawkes, writing in 1997, and in the context of fifty years of experience with the term, said it referred to "metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water and whose complexes are usually colored." He suggested referring to heavy metals as "all the metals in Groups 3 to 16 that are in periods 4 and greater" or, in other words, the transition metals and post-transition metals, and commented that this definition "should serve the needs of most chemists and some others who use the term."^[10] In contrast, and writing in 2002, Duffus concluded that "over the 60 years or so in which it has been used in chemistry, it has been given such a wide range of meanings by different authors that it is effectively meaningless."^[7] Despite these inconsistencies, references to "heavy metals" appear regularly in the chemistry and chemistry-related scientific literature.

An early use of the term dates from 1817, when Gmelin divided the elements into nonmetals, light metals and heavy metals.^[11] Light metals had densities of 0.860–5.0 gm/cm³; heavy metals 5.308–22.000.^[12] In 1868, Wanklyn and Chapman speculated on the adverse effects of the heavy metals "arsenic, lead, copper, zinc, iron and manganese" in drinking water. They noted an "absence of investigation" and were reduced to "the necessity of pleading for the collection of data."^[13] In 1884, Blake described a supposed connection between toxicity and the atomic weight of an element.^[14]

Beryllium and aluminium, although light metals,^[15] are sometimes counted as heavy metals in view of their toxicity.^[16][17] Beryllium exposure can result in lung and heart disorders, and possibly death;^[18] aluminium is a major inhibitor of crop growth in acid soils.^[19]

Contamination sources

Tetraethyl lead (CH₃CH₂)₄Pb is probably the most significant heavy metal contaminant in recent use.^[20]

Heavy metals are found naturally in the earth, and become concentrated as a result of human caused activities. Common sources are from mining and industrial wastes; vehicle emissions; lead-acid batteries; fertilisers; paints; treated woods; and aging water supply infrastructure.^[21] Lead is the most prevalent heavy metal contaminant.^[22] As a component of tetra-ethyl lead it was used extensively in gasoline during the 1930s-1970s.^[23] Lead levels in the aquatic environments of industrialised societies have been estimated to be two to three times those of pre-industrial levels.^[24] 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. Lead (from lead azide or lead styphnate used in firearms) gradually accumulates at firearms training grounds, contaminating the local environment and exposing range employees to a risk of lead poisoning.^[25]

Entry routes

Heavy metals enter plant, animal and human tissues via air inhalation, diet and manual handling. Motor vehicle emissions are a major source of airborne contaminants including arsenic, cadmium, cobalt, nickel, lead, antimony, vanadium, zinc, platinum, palladium and rhodium.^[26] Water sources (groundwater, lakes, streams and rivers) can be polluted by heavy metals leaching from industrial and consumer waste; acid rain can exacerbate this process by releasing heavy metals trapped in soils.^[27] Plants are exposed to heavy metals through the uptake of water; animals eat these plants; ingestion of plant- and animal-based foods are the largest sources of heavy metals in humans.^[28] Absorption through skin contact, for example from contact with soil, is another potential source of heavy metal contamination.^[29] Heavy metals can bioaccumulate in organisms as they are hard to metabolize (process and eliminate).^[30]

Detrimental effects

Heavy metals "can bind to vital cellular components, such as structural proteins, enzymes, and nucleic acids, and interfere with their functioning."^[31] Symptoms and effects can vary according to the metal or metal compound, and the dose involved. Broadly, long-term exposure to heavy metals can have carcinogenic, central and peripheral nervous system and circulatory effects. For humans, typical presentations associated with exposure to any of the "classical"^[32] heavy metals, or chromium (another heavy metal) or arsenic (a metalloid), are shown in the table.^[33]

Historical reports (examples)

Cadmium

99.999% purity cadmium bar and 1 cm³ cube.

Cadmium exposure is a phenomenon of the early 20th century, and onwards. In Japan in 1910, the Mitsui Mining and Smelting Company began discharging cadmium into the Jinzugawa river, as a byproduct of mining operations. Residents in the surrounding area subsequently consumed rice grown in cadmium contaminated irrigation water. They experienced softening of the bones and kidney failure. The origin of these symptoms was not clear; possibilities raised at the time included "a regional or bacterial disease or lead poisoning."^[34] In 1955, cadmium was identified as the likely cause and in 1961 the source was directly linked to mining operations in the area.^[35] In February 2010, cadmium was found in Wal-Mart exclusive Miley Cyrus jewelry. Wal-Mart continued to sell the jewelry until May, when covert testing organised by Associated Press confirmed the original results.^[36] In June 2010 cadmium was detected in the paint used on promotional drinking glasses for the movie Shrek Forever After, sold by McDonald's Restaurants, triggering a recall of 12 million glasses.^[37]

Mercury

Saint Isaac's Cathedral, in Saint Petersburg, Russia. The gold-mercury amalgam used to gild its dome caused numerous casualties among the workers involved.

The first emperor of unified China, Qin Shi Huang, it is reported, died of ingesting mercury pills that were intended to give him eternal life.^[38] The phrase "mad as a hatter" is likely a reference to mercury poisoning among milliners (so-called "mad hatter disease"), as mercury-based compounds were once used in the manufacture of felt hats in the 18th and 19th century.^[39] Historically, gold amalgam (an alloy with mercury) was widely used in gilding, leading to numerous casualties among the workers. It is estimated that during the construction of Saint Isaac's Cathedral alone, 60 workers died from the gilding of the main dome.^[40] Outbreaks of methylmercury poisoning occurred in several places in Japan during the 1950s due to industrial discharges of mercury into rivers and coastal waters. The best-known instances were in Minamata and Niigata. In Minamata alone, more than 600 people died due to what became known as Minamata disease. More than 21,000 people filed claims with the Japanese government, of which almost 3000 became certified as having the disease. In 22 documented cases, pregnant women who consumed contaminated fish showed mild or no symptoms but gave birth to infants with severe developmental disabilities.^[41] Since the industrial Revolution, mercury levels have tripled in many near-surface seawaters, especially around Iceland and Antarctica.^[42]

Lead

Dutch Boy white lead paint advertisement, 1912.

The adverse effects of lead were known to the ancients. In the 2nd century BC the Greek botanist Nicander described the colic and paralysis seen in lead-poisoned people.^[43] Dioscorides, a Greek physician who is thought to have lived in the 1st century CE,^[44] wrote that lead "makes the mind give way". Lead was used extensively in Roman aqueducts from about 500 BC to 300 AD.^[45] Julius Caesar's engineer, Vitruvius, reported, "water is much more wholesome from earthenware pipes than from lead pipes. For it seems to be made injurious by lead, because white lead is produced by it, and this is said to be harmful to the human body."^[46] During the 17th and 18th centuries, people in Devon were afflicted by a condition referred to as Devon colic; this was discovered to be due to the imbibing of lead-contaminated cider. In 2013, the World Health Organization estimated that lead poisoning resulted in 143,000 deaths, and "contribute[d] to 600,000 new cases of children with intellectual disabilities", each year.^[47] In 2015, drinking water lead levels in north-eastern Tasmania, Australia, were reported to reach over 50 times national drinking water guidelines. The source of the contamination was attributed to “a combination of dilapidated drinking water infrastructure, including lead jointed pipelines, end-of-life polyvinyl chloride pipes and household plumbing.”^[48]

Chromium

Potassium chromate, a carcinogen, is used in the dyeing of fabrics, and as a tanning agent to produce leather.

Chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known since at least the late 19th century.^[49] In 1890, Newman described the elevated cancer risk of workers in a chromate dye company.^[50] Chromate-induced dermatitis was reported in aircraft workers during World War II.^[51] In 1963, an outbreak of dermatitis, ranging from erythema to exudative eczema, occurred amongst 60 automobile factory workers in England. The workers had been wet-sanding chromate-based primer paint that had been applied to car bodies.^[52] In Australia, chromium was released from the Newcastle Orica explosives plant on August 8, 2011. Up to 20 workers at the plant were exposed as were 70 nearby homes in Stockton. The town was only notified three days after the release and the accident sparked a major public controversy, with Orica criticised for playing down the extent and possible risks of the leak, and the state Government attacked for their slow response to the incident.^[53]

Arsenic

Orpiment, a toxic arsenic mineral used in the tanning industry to remove hair from hides.

Arsenic, as realgar (As₄S₄) and orpiment (As₂S₃), was known in ancient times. Strabo (64–50 BCE – c. AD 24?), a Greek geographer and historian,^[54] wrote that only slaves were employed in realgar and orpiment mines since they would inevitably die from the toxic effects of the fumes given off from the ores. Arsenic contaminated beer poisoned over 6,000 people in the Manchester area of England in 1900, and is thought to have killed at least 70 victims.^[55] Clare Luce, American ambassador to Italy from 1953 to 1956, suffered from arsenic poisoning. Its source was traced to flaking arsenic-laden paint on the ceiling of her bedroom. She may also have eaten food contaminated by arsenic in flaking ceiling paint in the embassy dining room.^[56] Ground water contaminated by arsenic, as of 2014, "is still poisoning millions of people in Asia."^[57]

Remediation

A metal EDTA anion. Pb displaces Ca in Na₂[CaEDTA] to give Na₂[PbEDTA], which is passed out of the body in urine.^[58]

In humans, heavy metal poisoning is generally treated by the administration of chelating agents.^[59] These are chemical compounds, such as CaNa₂ EDTA (calcium disodium ethylenediaminetetraacetate) that convert heavy metals to chemically inert forms that can be excreted without further interaction with the body. Chelates are not without side effects and can also remove beneficial metals from the body. Vitamin and mineral supplements are sometimes co-administered for this reason.^[60]

Soils contaminated by heavy metals can be re-mediated by one or more of the following technologies: isolation; immobilization; toxicity reduction; physical separation; or extraction. Isolation involves the use of caps, membranes or below-ground barriers in an attempt to quarantine the contaminated soil. Immobilization aims to alter the properties of the soil so as to hinder the mobility of the heavy contaminants. Toxicity reduction attempts to oxidise or reduce the heavy metal ions, via chemical or biological means into less toxic or mobile forms. Physical separation involves the removal of the contaminated soil and the separation of the metal contaminants by mechanical means. Extraction is an on or off-site process that uses chemicals, high-temperature volatization, or electrolysis to extract contaminants from soils. The process or processes used will vary according to contaminant and the characteristics of the site.^[61]

Benefits

Some elements regarded as heavy metals are essential, in small quantities, for human health. These elements include vanadium, manganese, iron, cobalt, copper, zinc, selenium, strontium and molybdenum.^[62] A deficiency of these essential metals may increase susceptibility to heavy metal poisoning.^[63]

See also

• Heavy metal detoxification
• Kingston Fossil Plant coal fly ash slurry spill
• Light metal
• Metal toxicity

Notes

Citations

References