Arsenic

Arsenic is a chemical element on the periodic table that belongs to the group of metalloids, also called semimetals; It can be found in various forms, although it is rarely found in a solid state.

It has been known since ancient times and is recognized as extremely toxic. At atmospheric pressure, arsenic sublimes at 613 °C.

It is an essential element for life and its deficiency can give rise to various complications; however, the biological function is not precisely known. The daily intake of 12 to 15 μg can be consumed without problems in the daily diet of meat, fish, vegetables and cereals; fish and crustaceans being the ones with the highest arsenic content.

Arsenic (from the Persian zarnikh, 'yellow orpiment' or from the Greek arsenikón, 'masculine') is a chemical element on the periodic table whose symbol is As and the atomic number is 33. In the periodic table of elements it is in the fifth main group. Arsenic rarely occurs as a solid, mainly in the form of sulphides. It belongs to the metalloids, since it shows intermediate properties between transition metals and nonmetals.

Arsenic compounds have been known since antiquity, being extremely toxic, although they are used as components in some medicines. Arsenic is used for the manufacture of semiconductors and as a component of III-V semiconductors as gallium arsenide.

Arsenic is very common in Earth's atmosphere, in rocks and soils, in the hydrosphere and biosphere. It is delivered to the environment through a combination of processes such as:

• Naturals such as meteorization, biological activity, volcanic emissions
• Anthropogenics such as mining, use of fossil fuels, use of pesticides, herbicides, etc.

Main features

Pure metallic grey arsenic.

Arsenic occurs in three allotropic states, gray or metallic, yellow and black. Metallic gray arsenic (α form) is the stable form under normal conditions and has a rhombohedral structure, it is a good conductor of heat but a poor conductor electric, its density is 5.73 g/cm³, it is brittle and loses its metallic luster when exposed to air.

“Yellow” arsenic (γ form) is obtained when arsenic vapor is rapidly cooled. It is extremely volatile and more reactive than metallic arsenic and phosphorescent at room temperature. The gas is made up of tetrahedral As₄ molecules in a similar way to phosphorus and the solid formed by the condensation of the gas has a cubic structure, a soapy texture and an approximate density of 1.97 g/ cm³. Exposed to light or heat it reverts to the stable (gray) form. Orpiment, a mineral of arsenic trisulfide, is also called yellow arsenic.

A third allotropic form, “black” arsenic (β form) with a hexagonal structure and a density of 4.7 g/cm³, has intermediate properties between the allotropic forms described and is obtained by thermal decomposition of arsine or by cooling slowly the arsenic vapor.

All allotropes except gray lack metallic luster and have very low electrical conductivity, so the element will behave as a metal or a nonmetal based basically on its aggregation state. See also heavy metal.

At atmospheric pressure, arsenic sublimes at 613 °C, and at 400 °C it burns with a white flame forming the sesquioxide As₄O₆. It reacts violently with chlorine and combines, on heating, with most metals to form the corresponding arsenide and with sulfur. It does not react with hydrochloric acid in the absence of oxygen, but it does react with hot nitric acid, whether diluted or concentrated, and other oxidants such as hydrogen peroxide, perchloric acid, etc. It is insoluble in water but many of its compounds are.

It is an essential chemical element for life although both arsenic and its compounds are extremely poisonous.

It is in the 2nd analytical group of cations; precipitates with yellow H2S.

Applications

In use

• Preservative of wood (lease and chromium senate), which represents, according to some estimates, about 70% of global arsenic consumption.
• Gallium arseniide is an important semiconductor material used in faster, more expensive integrated circuits than silicon. It is also used in laser and LED diode construction.
• Additive in lead alloys and brass.
• Insecticide (lease sterling), herbicides (sodium seedling) and poisons: at the beginning of the centuryXX. Inorganic compounds were used, but their use has disappeared almost for the benefit of organic compounds (methyl derivatives). However, those applications are declining.
• Arsenic dysfunction is used as a pigment and pyrotechnic.
• Decolorant in the manufacture of glass (arsenic dioxide).

Deprecated

• Historically, arsenic has been used for therapeutic purposes practically abandoned by Western medicine, although the interest for its use has recently been renewed, as evidenced by the case of arsenic trioxide for the treatment of patients with acute promylocytic leukemia.
• It has been used to make dyes for fabrics and inks to print wallpapers. This applications resulted in severe poisoning in the centuryXIX.
• As a fertilizing element in the form of rich primary mineral, for agriculture.
• Throughout history the arsenic and its compounds have been used for murderous purposes, mainly in the form of arsenic anhydride (white, insipid and toilet called the king of poisons).
• Development of insecticides, herbicides, raticides, fungicides, etc., although less and less are used for these purposes.

Biological function

Although arsenic is associated with death, it is an essential element for life and its deficiency can lead to various complications. Daily intake of 12 to 15 μg can be obtained without problems with the daily diet of meats, fish, vegetables and cereals, with fish and crustaceans having the highest arsenic content, generally in the form of arsenobetaine, less toxic than inorganic arsenic.

Arsenobetaine.

In 2010, Dr. Felisa Wolfe-Simon of the NASA Astrobiology Institute announced the discovery in the toxic and brackish waters of Mono Lake, California, of a bacterium from the Halomonadaceae family that could replace phosphorus (which to date considered essential for life) with arsenic, to the point of incorporating this element into their deoxyribonucleic acid (DNA). This announcement had great repercussions in the media because, if confirmed, it would have opened the door to the search for new forms of life on planets that do not contain phosphorus in their atmosphere. However, in several studies conducted as of 2012, most of the claims supporting the study have been refuted. The bacterium is apparently resistant to arsenic but cannot completely replace phosphorus.

Although the above information was published in the prestigious scientific journal Science, to date the results have been and continue to be strongly questioned by numerous scientists who have tried to reproduce the same experimental design without positive results., as a result of which they have postulated that the GFAJ-1 bacterium could survive in the synthetic culture medium used for experimentation thanks to the traces of phosphorus present in it.

History

Arsenic alchemical symbol.

Arsenic (from the Greek άρσενιχόν, orpiment) has been known since ancient times, as have some of its compounds, especially sulfides. Dioscorides and Pliny the Elder (Greeks of the I century) knew about the properties of orpiment and realgar and Celsus Aureliano (Roman of the I) and Galen (century II) were aware of its irritant, toxic, corrosive and parasiticide effects and observed its virtues against persistent coughs, voice disorders and dyspnea.

Arab physicians also used arsenic compounds in fumigations, pills, and potions as well as external applications. During the Middle Ages, arsenical compounds fell into oblivion, being relegated to the healers who prescribed them against scrofula and hydrocele.

Roger Bacon and Albert the Great stopped in his study—the latter is believed to have been the first to isolate the element in the year 1250—and Paracelsus made a panacea out of it. Leonardo da Vinci used it through endotherapy by applying it to apple trees to control fruit thieves.

The first to study it in detail was Brandt in 1633 and Schroeder obtained it in 1649 by the action of carbon on arsenic acid. Berzeliuss is responsible for the first investigations into the composition of arsenic compounds.

In the 18th century, arsenicals achieved a leading position in therapeutics until they were replaced by sulfonamides and the antibiotics.

Abundance and obtaining

Natural arsenic.

It is the 52nd most abundant element in the Earth's crust at 2 ppm (5 10⁻⁴ %) and is one of only 22 elements known to consist of a single stable nucleus. Arsenic is found natively and primarily in the sulfide form in a wide variety of ores containing copper, lead, iron (arsenopyrite or mispickel), nickel, cobalt, and other metals.

In the fusion of copper, lead, cobalt and gold ores, arsenic trioxide is obtained, which volatilizes in the process and is carried away by the gases from the chimney that can contain more than 30% arsenic trioxide. The gases from the chimney are subsequently refined by mixing them with small amounts of galena or pyrite to avoid the formation of arsenites and by roasting arsenic trioxide between 90 and 95% purity is obtained, by successive sublimations it can be obtained with a purity of 99%.

Reducing the oxide with carbon gives the metalloid, however most arsenic is sold as oxide. Virtually all of the world's production of metallic arsenic is from China, which is also the world's largest producer of arsenic trioxide.

According to data from the US Geological Survey (U.S. Geological Survey), copper and lead mines contain approximately 11 million tons of arsenic, especially in Peru and the Philippines, and the metalloid is associated with copper-gold deposits in Chile and gold in Canada.

It is also a component of tobacco and is highly toxic.

Precautions

Arsenic and its compounds are extremely toxic, especially inorganic arsenic. In Bangladesh there has been a massive poisoning, the largest in history, due to the construction of countless water wells promoted by Western NGOs that have turned out to be contaminated, affecting a population of hundreds of thousands of people. Also other regions Geographical regions, Spain included, have been affected by this problem.

Arsenic in the air

The presence of arsenic in the air may affect the prevalence of lung cancer. In foundries this element is very common in the air. Environmental regulations indicate that the maximum allowable is 10 μg/m³.

Arsenic in water

The presence of arsenic in drinking water may be the result of the dissolution of the mineral present in river basins near volcanoes and naturally in the soil through which the water flows before it is collected for human use; or, through anthropogenic means due to industrial contamination or pesticides. Arsenic occurs as As³⁺ (arsenite) and As⁵⁺ (arsenate, abundant), of which arsenite is the most toxic to humans and the most Difficult to remove from bodies of water. The FAO/WHO standard notes that the maximum allowable level has been reduced to 0.01 ppm o in water (previously it was 0.05 ppm).

Ingesting small amounts of arsenic can cause chronic effects by bioaccumulating in the body. Serious poisonings can occur when the amount taken is 100 mg. Arsenic has been attributed to diseases of carcinogenic prevalence to the skin, lungs and bladder.

Some studies of arsenic toxicity indicate that many of the current standards based on World Health Organization (WHO) guidelines indicate very high concentrations and suggest the need to reassess limit values based on epidemiological studies.

Water regulations

One way to ingest arsenic is through water. The aquifers of many Communities are affected, in these cases the arsenic generally comes from deep wells, where there is pyrite or arsenopyrite (a combination of iron, sulfur and arsenic).

The World Health Organization (WHO) standard allows a maximum concentration of arsenic in water intended for human consumption of 10 µg/L. although it is recommended not to exceed 0.05 mg/L in drinking water.

Removal of arsenic from water

Conventional drinking water treatment is aimed at removing color, turbidity and microorganisms. This removal is achieved through an appropriate combination of processes: coagulation, flocculation, sedimentation, filtration and disinfection. But when you want to remove chemical elements from water, such as arsenic, it is sometimes necessary to resort to more complex methods.

The technologies generally used to remove arsenic, in addition to coagulation and flocculation, are: adsorption-coprecipitation using iron and aluminum salts, adsorption on activated alumina, reverse osmosis, ion exchange, and oxidation followed by filtration. Other developed technologies, instead of using salts, use an electrochemical process that involves brass electrodes (Cu2+ and Zn2+) and a power source that provides energy, thus reducing the concentration of diluted arsenic below the detection level of the spectrophotometer. of atomic absorption.

In water treatment plants, As⁺⁵ can be effectively removed by aluminum or iron sulfate coagulation and lime softening processes. The indicated coagulants hydrolyze forming hydroxides, on which As⁺⁵ is absorbed and coprecipitates with other metal ions.

Groundwater contamination risk maps

About a third of the world's population obtains drinking water from groundwater reserves. It is estimated that about 10 percent of the world's population—around 300 million people—get their water from underground reservoirs contaminated with arsenic and fluoride. Contamination by these trace elements is generally of natural origin and is produced by the release of pollutants into the aqueous environment through mechanisms of alteration and/or desorption of minerals contained in both rocks and sediments.

In 2008, the Swiss Institute for Water Research (Eawag) presented a new method for establishing risk maps for toxic substances of geological origin in groundwater.

Links and original posts

• Amini, M.; Mueller, K.; Abbaspour, K.C.; Rosenberg, T.; Afyuni, M.; Møller, M.; Sarr, M.; Johnson, C.A. (2008) «Statistical modeling of global geogenic fluoride contamination in groundwaters. » Environmental Science and Technology, 42(10), 3662-3668, doi:10.1021/es071958y
• Amini, M.; Abbaspour, K. C.; Berg, M.; Winkel, L.; Hug, S.J.; Hoehn, E.; Yang, H.; Johnson, C. A. (2008). «Statistical modeling of global geogenic arsenic contamination in groundwater. » Environmental Science and Technology 42 (10), 3669-3675. doi:10.1021/es702859e
• Winkel, L.; Berg, M.; Amini, M.; Hug, S.J.; Johnson, C.A. «Predicting groundwater arsenic contamination in Southeast Asia from surface parameters. » Nature Geoscience, 1, 536–542 (2008). doi:10.1038/ngeo254
• Rodríguez-Lado, L.; Sun, G.; Berg, M.; Zhang, Q.; Xue, H.; Zheng, Q.; Johnson, C. A. (2013) «Groundwater arsenic contamination throughout China. » Science, 341(6148), 866-868, doi:10.1126/science.1237484 The main advantage of this approach is that it allows to establish, for each extraction area, the likelihood that water is or is not contaminated, which facilitates sampling work and the identification of new potentially contaminated areas.
• In 2016, this group of researchers has made available the knowledge acquired through the Groundwater Assessment Platform GAP (GAP) platformwww.gapmaps.org). This platform allows experts from around the world to use and visualize own analytical data, in order to develop risk maps for a particular area of interest. The GAP platform operates at the same time as a discussion forum for the exchange of knowledge, in order to further develop and refine the methods for the elimination of harmful substances from water for human consumption.

Arsenic in the soil

Arsenic is found naturally on earth in small concentrations, both in soil and in minerals, but it can also get into the air and much more easily into water, through dust storms and runoff waters.

It is a component that is difficult to convert into water-soluble or volatile products. It is a very mobile element, referring to the fact that large concentrations of it do not appear in a specific place, this has positive but also negative aspects, and that is why arsenic contamination is widespread due to its high mobility. and displacement of it.

When it is immobile it cannot be easily mobilized, but due to human activities (mining and smelting of metals) this immobile arsenic is mobilized, which means that it can be found in places where it does not exist naturally.

Once it reaches the environment, it cannot be destroyed, so the amount increases and spreads, causing effects on the health of humans and animals.

Effects on plants

Plants easily absorb it, which means that a wide range of concentrations can be present in food (when As is absorbed by plants for human consumption).

Table I. Arsenic content for 50% growth reduction

Effects on animals

Fish can see their genetic material affected, this is due to the presence of inorganic arsenic that we mentioned before. This is mainly caused by the accumulation of arsenic in freshwater plant-eating organisms.

Birds are also affected, especially those that eat fish with high amounts of arsenic. They die as a result of arsenic poisoning from the decomposition of the fish in their own bodies.

Effects on humans

In humans, exposure to As is higher for those who work in companies where they use it in their industrial processes, for people who live in houses that contain wood preservatives, and people who live on farms where pesticides and herbicides have been applied. with As, for people who use aquifers for their water supply that contain high amounts of As, as occurs almost universally in some South Asian countries (India, Thailand, etc).

The toxic effects of As in humans will depend on the mode and duration of exposure, the source and type of arsenic will also be important.

According to some studies, the lethal dose of As in adults will be 1-4 mg As/kg^{[citation needed]} and its compounds such as AsH₃, As₂O₃, As₂O₅ the dose will vary between 1.5 mg/kg and 500 mg/kg of body mass.^{[citation needed]}

The main route of exposure will be by ingestion or inhalation, in this way it enters the body and reaches the epithelial surfaces of the digestive tract, the respiratory system or the skin where it is absorbed, entering the bloodstream and being transported to other organs, where it can cause permanent damage.

After 24 hours, As can be found in liver, kidney, lungs, spleen and skin; in the skin it accumulates due to the easy reaction with proteins.

When intake is greater than excretion, it accumulates in hair and nails. The normal level of As in urine is 5-40 µg/day, in hair 80-250 µg/day and in nails 430-1080 µg/day.

Toxic effects vary depending on various factors such as genetics, diet, metabolism, nutrition, among other things. Those with the greatest risk are those with low arsenic methylation, the most affected will also be children due to their greater cell division because they are developing and will not metabolize As like an adult.

Urine is the best biomarker for measuring absorbed inorganic arsenic, it can be measured up to the tenth day after exposure. In the hair or nails can be measured between six to twelve months after exposure.

Types of poisoning

Acute poisoning

Arsenic in large quantities affects the digestive tract, presenting itself as a gastrointestinal painting with abdominal pains, vomiting, diarrhea and dehydration. Loss of sensitivity in the peripheral nervous system is the most common neurological effect; it appears one to two weeks after large exposures. Symptoms of acute poisoning may appear in minutes or many hours after ingestion of between 100 and 300 mg of AA, although it is also possible to inhale AA powder or cutaneous absorption.

Chronic poisoning

Ingestion of As for a long time and repetitive doses will appear symptoms such as fatigue, gastroenteritis, leucopenia, anemia, hypertension, skin disorders. In most cases the symptoms of chronic arsenic poisoning are related to the general symptomatology of some common diseases, because of this it is necessary to follow the origin of the arsenic-contaminated source and a series of medical tests that quantify the concentration of this in the organism.

Related diseases

• Lung, bladder, kidney, prostate.
• Mees lines.
• Spontaneous abortions.
• Congenital malformations.

Arsenic in food

Arsenic is ubiquitous in food, since minimal amounts of it are incorporated through contamination.

Arsenic can be present in food for various reasons, these are shown in Table II.

Table II.

• Main causes of arsenic presence in food

Waste from industrial uses

• Production of special steels.
• Manufacture of paints, glass and glazes.

Waste from agricultural uses and related areas

• Herbicides, fungicides.
• Insecticides, Surroundings.
• Phosphates containing arsenic.

Mineral supplementation of feeds

Use of arsanilic acid in pig and poultry feed to promote growth.

Waste from pharmaceutical uses

Used in the treatment of parasitic diseases.

Waste emitted directly into the atmosphere

Coal combustion gases and industrial gases.

Food that reaches the consumer is the product of a long chain of production, preparation and processing, during which it can be contaminated by metallic elements, such as arsenic. These elements are present throughout the biosphere, earth's crust, waters, soils, atmosphere.

The amount of arsenic ingested by humans depends on what food they eat and in what quantity, with maximum contents of up to 40 µg/g being able to be reached.