Acetylene

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Acetylene

Acetylene

Names
Preferred IUPAC name Acetylene
Systematic IUPAC name Ethyne
Identifiers
CAS Number	74-86-2
3D model (JSmol)	Interactive image
Beilstein Reference	906677
ChEBI	CHEBI:27518
ChEMBL	ChEMBL116336
ChemSpider	6086
ECHA InfoCard	100.000.743
EC Number	200-816-9
Gmelin Reference	210
KEGG	C01548
PubChem CID	6326
RTECS number	AO9600000
UNII	OC7TV75O83
UN number	1001 (dissolved) 3138 (in mixture with ethylene and propylene)
CompTox Dashboard (EPA)	DTXSID6026379

Properties
Chemical formula	C2H2
Molar mass	26.038 g·mol−1
Appearance	Colorless gas
Odor	Odorless
Density	1.1772 g/L = 1.1772 kg/m3 (0 °C, 101.3 kPa)
Melting point	−80.8 °C (−113.4 °F; 192.3 K) Triple point at 1.27 atm
Sublimation conditions	−84 °C; −119 °F; 189 K (1 atm)
Solubility in water	slightly soluble
Vapor pressure	44.2 atm (20 °C)
Acidity (pKa)	25
Conjugate acid	Ethynium
Magnetic susceptibility (χ)	−12.5×10−6 cm3/mol
Structure
Molecular shape	Linear
Thermochemistry
Std molar entropy (So298)	201 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	+226.88 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H220, H336
Precautionary statements	P202, P210, P233, P261, P271, P304, P312, P340, P377, P381, P403, P405, P501
NFPA 704 (fire diamond)	1 4 3
Autoignition temperature	300 °C (572 °F; 573 K)
NIOSH (US health exposure limits):
PEL (Permissible)	none
REL (Recommended)	C 2500 ppm (2662 mg/m3)
IDLH (Immediate danger)	N.D.
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Acetylene (systematic name: ethyne) is the chemical compound with the formula C₂H₂. It is a hydrocarbon and the simplest alkyne. This colorless gas (lower hydrocarbons are generally gaseous in nature) is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities such as divinyl sulfide and phosphine.

As an alkyne, acetylene is unsaturated because its two carbon atoms are bonded together in a triple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°.

Discovery

Acetylene was discovered in 1836 by Edmund Davy, who identified it as a "new carburet of hydrogen". It was an accidental discovery while attempting to isolate potassium metal. By heating potassium carbonate with carbon at very high temperatures, he produced a residue of what is now known as potassium carbide, (K₂C₂), which reacted with water to release the new gas. It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name acétylène. Berthelot's empirical formula for acetylene (C₄H₂), as well as the alternative name "quadricarbure d'hydrogène" (hydrogen quadricarbide), were incorrect because chemists at that time used the wrong atomic mass for carbon (6 instead of 12). Berthelot was able to prepare this gas by passing vapours of organic compounds (methanol, ethanol, etc.) through a red hot tube and collecting the effluent. He also found that acetylene was formed by sparking electricity through mixed cyanogen and hydrogen gases. Berthelot later obtained acetylene directly by passing hydrogen between the poles of a carbon arc.

Preparation

Since the 1950s, acetylene has mainly been manufactured by the partial combustion of methane. It is a recovered side product in production of ethylene by cracking of hydrocarbons. Approximately 400,000 tonnes were produced by this method in 1983. Its presence in ethylene is usually undesirable because of its explosive character and its ability to poison Ziegler–Natta catalysts. It is selectively hydrogenated into ethylene, usually using Pd–Ag catalysts.

Acetylene factory with annual capacity of 90,000 tons, commissioned in 2020 by BASF.

Until the 1950s, when oil supplanted coal as the chief source of reduced carbon, acetylene (and the aromatic fraction from coal tar) was the main source of organic chemicals in the chemical industry. It was prepared by the hydrolysis of calcium carbide, a reaction discovered by Friedrich Wöhler in 1862 and still familiar to students:

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H_2↑

Calcium carbide production requires extremely high temperatures, ~2000 °C, necessitating the use of an electric arc furnace. In the US, this process was an important part of the late-19th century revolution in chemistry enabled by the massive hydroelectric power project at Niagara Falls.

Bonding

In terms of valence bond theory, in each carbon atom the 2s orbital hybridizes with one 2p orbital thus forming an sp hybrid. The other two 2p orbitals remain unhybridized. The two ends of the two sp hybrid orbital overlap to form a strong σ valence bond between the carbons, while on each of the other two ends hydrogen atoms attach also by σ bonds. The two unchanged 2p orbitals form a pair of weaker π bonds.

Since acetylene is a linear symmetrical molecule, it possesses the D_∞h point group.

Physical properties

Changes of state

At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The triple point on the phase diagram corresponds to the melting point (−80.8 °C) at the minimal pressure at which liquid acetylene can exist (1.27 atm). At temperatures below the triple point, solid acetylene can change directly to the vapour (gas) by sublimation. The sublimation point at atmospheric pressure is −84.0 °C.

Other

At room temperature, the solubility of acetylene in acetone is 27.9 g per kg. For the same amount of dimethylformamide (DMF), the solubility is 51 g. At 20.26 bar, the solubility increases to 689.0 and 628.0 g for acetone and DMF, respectively. These solvents are used in pressurized gas cylinders.

Applications

Welding

Approximately 20% of acetylene is supplied by the industrial gases industry for oxyacetylene gas welding and cutting due to the high temperature of the flame. Combustion of acetylene with oxygen produces a flame of over 3,600 K (3,330 °C; 6,020 °F), releasing 11.8 kJ/g. Oxyacetylene is the hottest burning common fuel gas. Acetylene is the third-hottest natural chemical flame after dicyanoacetylene's 5,260 K (4,990 °C; 9,010 °F) and cyanogen at 4,798 K (4,525 °C; 8,177 °F). Oxy-acetylene welding was a popular welding process in previous decades. The development and advantages of arc-based welding processes have made oxy-fuel welding nearly extinct for many applications. Acetylene usage for welding has dropped significantly. On the other hand, oxy-acetylene welding equipment is quite versatile – not only because the torch is preferred for some sorts of iron or steel welding (as in certain artistic applications), but also because it lends itself easily to brazing, braze-welding, metal heating (for annealing or tempering, bending or forming), the loosening of corroded nuts and bolts, and other applications. Bell Canada cable-repair technicians still use portable acetylene-fuelled torch kits as a soldering tool for sealing lead sleeve splices in manholes and in some aerial locations. Oxyacetylene welding may also be used in areas where electricity is not readily accessible. Oxyacetylene cutting is used in many metal fabrication shops. For use in welding and cutting, the working pressures must be controlled by a regulator, since above 15 psi (100 kPa), if subjected to a shockwave (caused, for example, by a flashback), acetylene decomposes explosively into hydrogen and carbon.

Acetylene fuel container/burner as used in the island of Bali

Portable lighting

Calcium carbide was used to generate acetylene used in the lamps for portable or remote applications. It was used for miners and cavers before the widespread use of incandescent lighting; or many years later low-power/high-lumen LED lighting; and is still used by mining industries in some nations without workplace safety laws. Carbide lamps were also used extensively as headlights in early motor vehicles and as an early light source for lighthouses.

Plastics and acrylic acid derivatives

Except in China, use of acetylene as a chemical feedstock has declined by 70% from 1965 to 2007 owing to cost and environmental considerations. Acetylene can be semihydrogenated to ethylene, providing a feedstock for a variety of polyethylene plastics. Another major application of acetylene, especially in China is its conversion to acrylic acid derivatives. These derivatives form products such as acrylic fibers, glasses, paints, resins, and polymers.

Niche applications

In 1881, the Russian chemist Mikhail Kucherov described the hydration of acetylene to acetaldehyde using catalysts such as mercury(II) bromide. Before the advent of the Wacker process, this reaction was conducted on an industrial scale.

The polymerization of acetylene with Ziegler–Natta catalysts produces polyacetylene films. Polyacetylene, a chain of CH centres with alternating single and double bonds, was one of the first discovered organic semiconductors. Its reaction with iodine produces a highly electrically conducting material. Although such materials are not useful, these discoveries led to the developments of organic semiconductors, as recognized by the Nobel Prize in Chemistry in 2000 to Alan J. Heeger, Alan G MacDiarmid, and Hideki Shirakawa.

In the early 20th century acetylene was widely used for illumination, including street lighting in some towns. Most early automobiles used carbide lamps before the adoption of electric headlights.

In the 1920s, pure acetylene was experimentally used as an inhalation anesthetic.

Acetylene is sometimes used for carburization (that is, hardening) of steel when the object is too large to fit into a furnace.

Acetylene is used to volatilize carbon in radiocarbon dating. The carbonaceous material in an archeological sample is treated with lithium metal in a small specialized research furnace to form lithium carbide (also known as lithium acetylide). The carbide can then be reacted with water, as usual, to form acetylene gas to feed into a mass spectrometer to measure the isotopic ratio of carbon-14 to carbon-12.

Natural occurrence

The energy richness of the C≡C triple bond and the rather high solubility of acetylene in water make it a suitable substrate for bacteria, provided an adequate source is available. A number of bacteria living on acetylene have been identified. The enzyme acetylene hydratase catalyzes the hydration of acetylene to give acetaldehyde:

C₂H₂ + H₂O → CH₃CHO

Acetylene is a moderately common chemical in the universe, often associated with the atmospheres of gas giants. One curious discovery of acetylene is on Enceladus, a moon of Saturn. Natural acetylene is believed to form from catalytic decomposition of long-chain hydrocarbons at temperatures of 1,700 K (1,430 °C; 2,600 °F) and above. Since such temperatures are highly unlikely on such a small distant body, this discovery is potentially suggestive of catalytic reactions within that moon, making it a promising site to search for prebiotic chemistry.

Reactions

Vinylation: hydration, hydrohalogenation, and related reactions

In vinylation reactions, H-X compounds add across the triple bond. Alcohols and phenols add to acetylene to give vinyl ethers. Thiols give vinyl thioethers. Similarly, vinylpyrrolidone and vinylcarbazole are produced industrially by vinylation of 2-pyrrolidone and carbazole.

The hydration of acetylene is a vinylation reaction, but the resulting vinyl alcohol isomerizes to acetaldehyde. The reaction is catalyzed by mercury salts. This reaction once was the dominant technology for acetaldehyde production, but it has been displaced by the Wacker process, which affords acetaldehyde by oxidation of ethylene, a cheaper feedstock. A similar situation applies to the conversion of acetylene to the valuable vinyl chloride by hydrochlorination vs the oxychlorination of ethylene.

Addition to formaldehyde

Acetylene adds to ketones and aldehydes in the presence of base catalysts. With carbonyl groups to give α-ethynyl alcohols in ethynylation reactions: Formaldehyde gives sequentially propargyl alcohol and butynediol. 1,4-Butynediol is produced industrially in this way from formaldehyde and acetylene.

Carbonylation

Walter Reppe discovered that in the presence of catalysts, acetylene react to give a wide range of industrially significant chemicals.

With carbon monoxide, acetylene reacts to give acrylic acid, or acrylic esters, which can be used to produce acrylic glass:

Organometallic chemistry

Acetylene and its derivatives (2-butyne, diphenylacetylene, etc.) form complexes with transition metals. Its bonding to the metal is somewhat similar to that of ethylene complexes. These complexes are intermediates in many catalytic reactions such as alkyne trimerisation to benzene, tetramerization to cyclooctatetraene, and carbonylation to hydroquinone:

Fe(CO)₅ + 4 C₂H₂ + 2 H₂O → 2 C₆H₄(OH)₂ + FeCO₃ at basic conditions( 50-80 °C, 20-25 atm).

In the presence of certain transition metals, alkynes undergo alkyne metathesis.

Metal acetylides, species of the formula L_nM-C₂R, are also common. Copper(I) acetylide and silver acetylide can be formed in aqueous solutions with ease due to a poor solubility equilibrium.

Acid-base reactions

Main article: Acetylide § Preparation

Acetylene has a pK_a of 25, acetylene can be deprotonated by a superbase to form an acetylide:

HC≡CH + RM → RH + HC≡CM

Various organometallic and inorganic reagents are effective.

Safety and handling

Acetylene is not especially toxic, but when generated from calcium carbide, it can contain toxic impurities such as traces of phosphine and arsine, which give it a distinct garlic-like smell. It is also highly flammable, as are most light hydrocarbons, hence its use in welding. Its most singular hazard is associated with its intrinsic instability, especially when it is pressurized: under certain conditions acetylene can react in an exothermic addition-type reaction to form a number of products, typically benzene and/or vinylacetylene, possibly in addition to carbon and hydrogen. Consequently, acetylene, if initiated by intense heat or a shockwave, can decompose explosively if the absolute pressure of the gas exceeds about 200 kilopascals (29 psi). Most regulators and pressure gauges on equipment report gauge pressure, and the safe limit for acetylene therefore is 101 kPa_gage, or 15 psig. It is therefore supplied and stored dissolved in acetone or dimethylformamide (DMF), contained in a gas cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling. Acetylene cylinders should be used in the upright position to avoid withdrawing acetone during use.

Information on safe storage of acetylene in upright cylinders is provided by the OSHA, Compressed Gas Association, United States Mine Safety and Health Administration (MSHA), EIGA, and other agencies.

Copper catalyses the decomposition of acetylene, and as a result acetylene should not be transported in copper pipes.

Cylinders should be stored in an area segregated from oxidizers to avoid exacerbated reaction in case of fire/leakage. Acetylene cylinders should not be stored in confined spaces, enclosed vehicles, garages, and buildings, to avoid unintended leakage leading to explosive atmosphere. In the US, National Electric Code (NEC) requires consideration for hazardous areas including those where acetylene may be released during accidents or leaks. Consideration may include electrical classification and use of listed Group A electrical components in USA. Further information on determining the areas requiring special consideration is in NFPA 497. In Europe, ATEX also requires consideration for hazardous areas where flammable gases may be released during accidents or leaks.

Organophosphate

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Organophosphate 

 

This article is about the organic derivatives of phosphate. For the inorganic ion, see phosphate.

General chemical structure of the organophosphate functional group

Organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure O=P(OR)_3, a central phosphate molecule with alkyl or aromatic substituents. They can be considered as esters of phosphoric acid. Like most functional groups organophosphates occur in a diverse range of forms, with important examples including key biomolecules such as DNA, RNA and ATP, as well as many insecticides, herbicides, nerve agents and flame retardants. OPEs have been widely used in various products as flame retardants, plasticizers, and performance additives to engine oil. The popularity of OPEs as flame retardants came as a substitution for the highly regulated brominated flame retardants. The low cost of production and compatibility to different polymers made OPEs to be widely used in different industries including textile, furniture, electronics as plasticizers and flame retardants. These compounds are added to the final product physically rather than by chemical bond. Due to this, OPEs leak into the environment more readily through volatilization, leaching, and abrasion. OPEs have been detected in different environmental compartments such as air, dust, water, sediment, soil and biota samples at higher frequency and concentration.

Chemistry

Synthesis

Various routes exist for the synthesis of organophosphates

Esterification of phosphoric acid

OP(OH)₃ + ROH → OP(OH)₂(OR) + H₂O

OP(OH)₂(OR) + R'OH → OP(OH)(OR)(OR') + H₂O

OP(OH)(OR)(OR') + R"OH → OP(OR)(OR')(OR") + H₂O

Alcohols can be detached from phosphate esters by hydrolysis, which is the reverse of the above reactions. For this reason, phosphate esters are common carriers of organic groups in biosynthesis.

Oxidation of phosphite esters

Organophosphites can be readily oxidised to give organophosphates

P(OR)₃ + [O] → OP(OR)₃

Alcoholysis of POCl₃

Phosphorus oxychloride reacts readily with alcohols to give organophosphates

O=PCl₃ + 3 ROH → O=P(OR)₃ + 3 HCl

Properties

The phosphate esters bearing OH groups are acidic and partially deprotonated in aqueous solution. For example, DNA and RNA are polymers of the type [PO₂(OR)(OR')⁻]_n. Polyphosphates also form esters; an important example of an ester of a polyphosphate is ATP, which is the monoester of triphosphoric acid (H₅P₃O₁₀).

OPEs have a central phosphate molecular group. In the case of organophosphate (OP) triesters, these are three ester bonds with alkyl or aromatic substituents. However, OP diesters are different from the triesters as one of the alkyl ester groups is replaced by a hydroxyl group, making OP diesters phosphoric acids. The wide variety of substitutes used in organophosphate esters results in great variations in physicochemical properties, varying from highly polar to very hydrolysis resistant characteristics. OPEs exhibit a wide range of octanol water partitioning coefficient where log Kow values range from -0.98 up to 10.6. The predominant OPEs used as flame retardants and plasticizers have a positive log Kow values ranging between 1.44 and 9.49 signifying hydrophobicity. Thus, owing to this hydrophobicity OPEs are presumptively bioaccumulated and biomagnified in aquatic ecosystems. Laboratory experiments had shown that the non-halogenated OPEs are prone to photolysis, while the chlorinated OPEs such as TCEP and TCPP, however, seemed to be resistant to degradation by sunlight.

In nature

Guanitoxin

Guanitoxin is a naturally occurring organophosphate produced by cyanobacteria.

The detection of OPEs in the air as far away as Antarctica at concentrations around 1 ng/m³ suggests their persistence in air, and their potential for long-range transport. OPEs were measured in high frequency in air and water and widely distributed in northern hemisphere. The chlorinated OPEs (TCEP, TCIPP, TDCIPP) in urban sampling sites and non-halogenated like TBOEP in rural areas respectively were frequently measured in the environment across multiple sites. In the Laurentian Great Lakes total OPEs concentrations were found to be 2–3 orders of magnitude higher than concentrations of brominated flame retardants measured in similar air. Waters from rivers in Germany, Austria, and Spain have been consistently recorded for TBOEP and TCIPP at highest concentrations. From these studies, it is clear that OPE concentrations in both air and water samples are often orders of magnitude higher than other flame retardants, and that concentrations are largely dependent on sampling location, with higher concentrations in more urban, polluted locations.

Pesticides

Today, organophosphates make up about 50% of the killing agents in chemical pesticides.

Organophosphate pesticides (OPPs), like some nerve agents, inhibit acetylcholinesterase (IRAC mode of action 1b), which is broadly essential for normal function in insects, but also in humans and many other animals. OPPs affect this enzyme in varied ways, a principal one being through irreversible covalent inhibition, and so create potentials for poisoning that vary in degree. The brain sends out neurotransmitters to the nerve endings in the body; organophosphates disrupt this process from occurring. Organophosphate works by disrupting the enzyme acetylcholinesterase. Acetylcholinesterase break down the acetylcholine neurotransmitter, which sends out signals to other nerve endings in the body.

For instance, parathion, one of the first OPPs commercialized, is many times more potent than malathion, an insecticide used in combating the Mediterranean fruit fly (Med-fly) and West Nile virus-transmitting mosquitoes. Human and animal exposure to them can be through ingestion of foods containing them, or via absorption through the skin or lungs.

The human and animal toxicity of OPPs make them a societal health and environmental concern; the EPA banned most residential uses of organophosphates in 2001, but their agricultural use, as pesticides on fruits and vegetables, is still permitted, as is their use in mosquito abatement in public spaces such as parks. For instance, the most commonly used OPP in the U.S., malathion, sees wide application in agriculture, residential landscaping, and pest control programs (including mosquito control in public recreation areas). As of 2010, forty such OPPs were registered for use in the U.S., with at least 73 million pounds used in one time period in agricultural and residential settings. Commonly used organophosphates have included:

• parathion
• malathion
• methyl parathion
• chlorpyrifos
• diazinon
• dichlorvos
• phosmet
• fenitrothion
• tetrachlorvinphos
• azamethiphos
• azinphos-methyl
• terbufos

Studies have shown that prolonged exposure to OPPs—e.g., in the case of farm workers—can lead to health problems, including increased risks for cardiovascular and respiratory disease, and cancer. In the case of pregnant women, exposure can result in premature births. In addition, permanent damage to the brain's chemical make-up, and changes in human behavior and emotion can occur to the fetus in pregnant women.

Organophosphate pesticides degrade rapidly by hydrolysis on exposure to sunlight, air, and soil, although small amounts can be detected in food and drinking water. Organophosphates contaminate drinking water by moving through the soil to the ground water. When the pesticide degrades, it is broken down into several chemicals. Organophosphates degrade faster than the organochlorides. The greater acute toxicity of OPPs results in the elevated risk associated with this class of compounds (see the Toxicity section below).

Nerve agents

Main article: Nerve agents

History

Early pioneers in the field include Jean Louis Lassaigne (early 19th century) and Philippe de Clermont (1854). In 1932, German chemist Willy Lange and his graduate student, Gerde von Krueger, first described the cholinergic nervous system effects of organophosphates, noting a choking sensation and a dimming of vision after exposure on themselves, which they attributed to the esters themselves. This discovery later inspired German chemist Gerhard Schrader at company IG Farben in the 1930s to experiment with these compounds as insecticides. Their potential use as chemical warfare agents soon became apparent, and the Nazi government put Schrader in charge of developing organophosphate (in the broader sense of the word) nerve gases. Schrader's laboratory discovered the G series of weapons, which included Sarin, Tabun, and Soman. The Nazis produced large quantities of these compounds, though did not use them during World War II. British scientists experimented with a cholinergic organophosphate of their own, called diisopropylfluorophosphate, during the war. The British later produced VX nerve agent, which was many times more potent than the G series, in the early 1950s, almost 20 years after the Germans had discovered the G series.

After World War II, American companies gained access to some information from Schrader's laboratory, and began synthesizing organophosphate pesticides in large quantities. Parathion was among the first marketed, followed by malathion and azinphosmethyl. The popularity of these insecticides increased after many of the organochlorine insecticides such as DDT, dieldrin, and heptachlor were banned in the 1970s.

Structural features

Effective organophosphates have the following structural features:

• A terminal oxygen connected to phosphorus by a double bond, i.e. a phosphoryl group
• Two lipophilic groups bonded to the phosphorus
• A leaving group bonded to the phosphorus, often a halide

Fine tuning

Within these requirements, a large number of different lipophilic and leaving groups have been used. The variation of these groups is one means of fine tuning the toxicity of the compound. A good example of this chemistry are the P-thiocyanate compounds which use an aryl (or alkyl) group and an alkylamino group as the lipophilic groups. The thiocyanate is the leaving group.

Flame retardants

Flame retardants (FRs) are chemicals that have been used on different consumer materials to prevent combustion and to delay the spread of fire after ignition. The increased demand to satisfy fire safety standards for flammability of plastic materials used in devices and appliances along with the strict regulation of brominated flame retardants has driven the high volume of production and consumption of OPEs. Most flame retardants used are halogenated OPEs, and the effectiveness of the flame retardant increases with the increased number of halogenated substituents.

OPEs are utilized as additive flame retardants which means the concentration of these flame retardants decreases with time as they readily leak into the environment. There are several mechanisms flame retardants use to prevent fire, however the most effective ones are the gas phase and the solid phase reactions. In the solid phase, halogenated flame retardants produce a char layer on burning materials suffocating the combustion, as well as in the gas phase they remove H⁺ and OH⁻ radicals from the flammable gasses, by reaction with the Br and Cl atoms to further slow down the burning process. Non-halogenated OPEs are effective mainly in the solid phase of burning materials. Upon exposure to heat the phosphorus compounds react to form a polymeric form of phosphorous acid. The acid causes a char layer that covers the burning material, blocking it from contact with oxygen, which in turn slows down the combustion reaction.

Health effects

Poisoning

Main article: Organophosphate poisoning

See also: Organophosphate-induced delayed neuropathy

Many "organophosphates" are potent nerve agents, functioning by inhibiting the action of acetylcholinesterase (AChE) in nerve cells. They are one of the most common causes of poisoning worldwide, and are frequently intentionally used in suicides in agricultural areas. Organophosphosphate pesticides can be absorbed by all routes, including inhalation, ingestion, and dermal absorption. Their inhibitory effects on the acetylcholinesterase enzyme lead to a pathological excess of acetylcholine in the body. Their toxicity is not limited to the acute phase, however, and chronic effects have long been noted. Neurotransmitters such as acetylcholine (which is affected by organophosphate pesticides) are profoundly important in the brain's development, and many organophosphates have neurotoxic effects on developing organisms, even from low levels of exposure. Other organophosphates are not toxic, yet their main metabolites, such as their oxons, are. Treatment includes both a pralidoxime binder and an anticholinergic such as atropine.

Chronic toxicity

Repeated or prolonged exposure to organophosphates may result in the same effects as acute exposure including the delayed symptoms. Other effects reported in workers repeatedly exposed include impaired memory and concentration, disorientation, severe depression, irritability, confusion, headache, speech difficulties, delayed reaction times, nightmares, sleepwalking, drowsiness, or insomnia. An influenza-like condition with headache, nausea, weakness, loss of appetite, and malaise has also been reported.

A recent study done by Madurai Kamaraj University in India have shown a direct correlation between usage of organophosphates and diabetes among the Indian agricultural population.

The physiological difference of OPEs in size and polarity greatly influences the physical and biochemical toxicity of the compound group. The chemical structures of OP triesters used as flame retardants and plasticizers are essentially similar to those of OP insecticides that target the nervous system of insects. Multiple toxicological studies have shown that OPEs such as TBOEP, TCIPP, TDCIPP, triethyl phosphate (TEP), and tris(methylphenyl)phosphate (TMPP) elicit effects on embryonic development, mRNA expression, thyroid hormones, circulating bile acid concentrations, and the neurological system in fish, birds, rodents, and/or humans.

Low-level exposure

Even at relatively low levels, organophosphates may be hazardous to human health. These pesticides act on acetylcholinesterase, an enzyme found in the brain. Thus, fetuses and young children, whose brain development depends on a strict sequence of biological events, may be most at risk. They can be absorbed through the lungs or skin or by eating them on food. According to a 2008 report from the U.S. Department of Agriculture, ″detectable″ traces of organophosphate were found in a representative sample of produce tested by the agency, 28% of frozen blueberries, 20% of celery, 27% of green beans, 17% of peaches, 8% of broccoli, and 25% of strawberries.

Cancer

The United States Environmental Protection Agency lists parathion as a possible human carcinogen. The International Agency for Research on Cancer (IARC), found that some organophosphates may increase cancer risk. Tetrachlorvinphos and parathion were classified as "possibly carcinogenic", whereas malathion and diazinon were classified as probably carcinogenic to humans.

Health effects on children

A 2013 review of 27 studies on prenatal and early childhood exposures to organophosphate pesticides found all but one showed negative neurodevelopmental outcomes. In the ten studies that assessed prenatal exposure, "cognitive deficits (related to working memory) were found in children at age 7 years, behavioral deficits (related to attention) seen mainly in toddlers, and motor deficits (abnormal reflexes), seen mainly in neonates."

A systematic review of neurodevelopmental effects of prenatal and postnatal organophosphate pesticide exposure was done in 2014. The review found that "Most of the studies evaluating prenatal exposure observed a negative effect on mental development and an increase in attention problems in preschool and school children."

In the US the organophosphate phosmet was banned for use on household fruit trees, ornamentals, and domestic pets in 2001 as other pesticides became available for these uses. Many other uses of phosmet were still allowed, especially commercial ones.

Affected populations

According to the EPA, organophosphate use in 2004 accounts for 40% of all insecticide products used in the United States. Out of concerns for potential hazards of organophosphate exposure to child development, the EPA began phasing out forms of organophosphates used indoors in 2001. While it is used in forestry, urban, and public health spraying (mosquito abatement programs, etc.) as well, the general population has been observed to have low exposure. Thus, the primary affected population that faces exposure to organophosphates are farmworkers, especially those in countries that have fewer restrictions on its usage, such as in India.

Farmworkers in the United States

In the United States, migrant and seasonal farmworkers are the most susceptible to organophosphate exposure. Of the U.S. farmworker population, there are about 4.2 million seasonal or migrant men, women, and even children, 70% of which are born in Mexico and an overwhelming 90% majority of these are Latino. This almost homogenous racial aspect of employment in farm work in the United States highly suggests social, economic, and political factors undercurrents that would explain their vulnerability. Half of the farmworker population in the United States do not have legal documentation and two thirds live in poverty, making it difficult to fully understand and document the characteristics of this population with relative certainty. Furthermore, the group faces linguistic barriers, with about 70% of the migrant seasonal farmworker population reporting that they cannot speak English well. 

In the United States, poverty and lack of documentation status puts migrant farmworkers in housing situations that make them far more likely to contract infectious or parasitic diseases and to suffer from chemical related ailments than the general U.S. population. Field workers who are exposed to pesticides continue to further expose their families in their residences, especially through contaminated clothing in which the residue settles as house dust. Increased rates of a full range of adverse birth outcomes result from high exposure to pesticides in a study of 500,000 births among farm workers in California's San Joaquin Valley.

Economic, social, racial, and political barriers make passing policy and creating protective measures less likely to occur; in the context of their jobs, migrant seasonal farm workers are structurally vulnerable to exploitation and working conditions that are Occupational Factors not up to health standards if they are unable to find the necessary physical and social resources to protect themselves.    

The nature of their job may require constant exposure to toxins and pesticides and subjects them to increasingly extreme weather as climate change progresses. Thus, migrant farm work has been ranked conservatively as possibly the second most dangerous job in the country.

Regulatory efforts

Organophosphates (OPs) were among the most widely used insecticides until the 21st century. And until the mid 1990s, general pesticide regulation was dependent on the Federal Food, Drug and Cosmetic Act (FFDCA) and the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) passed in 1938 and 1947, respectively. In 1993, the Environmental Protection Agency (EPA) was bound by a pledge made to Congress to significantly reduce the amount of pesticides used in the United States, and the U.S. Department of Agriculture, along with the Food and Drug Administration, joined the EPA in this commitment. Then, in 1996, The Food Quality Protection Act (FQPA) was signed into law to strengthen the regulation of pesticide in food and make regulation practices more consistent. One way that this strengthening was accomplished was through mandating aggregate and cumulative exposure risk assessments in derivative food tolerance levels. The EPA selected OPs as the first class of pesticides for assessing food tolerances because of their specific toxicity behavior as acetylcholinesterase inhibitors.

Between 1996 and 1999, the use of OPs actually increased (despite the passing of the FQPA) from 75 million to 91 million pounds per year. However, this is mainly due to the cotton boll weevil eradication program through the U.S Department of Agriculture and the use of OPs eventually decreased to 46 million pounds per year by 2004. The residential use of OP pesticides may have declined more quickly, when compared to commercial use, largely due to the voluntary cancellation of chlorpyrifos and diazinon as approved pesticides for home use. The phaseout of both chlorpyrifos and diazinon for most residential uses was complete in 2005.

Parathion (Ethyl) use is banned or restricted in 23 countries and its import is illegal in a total of 50 countries. Its use was banned in the U.S. in 2000 and it has not been used since 2003.

In 2001, the EPA placed new restrictions on the use of the organophosphates phosmet and azinphos-methyl to increase protection of agricultural workers. The crop uses reported at that time as being phased out in four years included those for almonds, tart cherries, cotton, cranberries, peaches, pistachios, and walnuts. The crops with time-limited registration included apples/crab apples, blueberries, sweet cherries, pears, pine seed orchards, brussels sprouts, cane berries, and the use of azinphos-methyl by nurseries for quarantine requirements. The labeled uses of phosmet include alfalfa, orchard crops (e.g. almonds, walnuts, apples, cherries), blueberries, citrus, grapes, ornamental trees (not for use in residential, park, or recreational areas) and nonbearing fruit trees, Christmas trees and conifers (tree farms), potatoes, and peas. Azinphos-methyl has been banned in Europe since 2006.

In May 2006, the Environmental Protection Agency (EPA) reviewed the use of dichlorvos and proposed its continued sale, despite concerns over its safety and considerable evidence suggesting it is carcinogenic and harmful to the brain and nervous system, especially in children. Environmentalists charge that the latest decision was the product of backroom deals with industry and political interference.

As of 2013, thirty-six types of organophosphates were registered for use in the United States. Organophosphates are currently used in a variety of environments (e.g. agriculture, gardens and veterinary practices), however, several notable OPs have been discontinued for use. This includes parathion, which is no longer registered for any use, and chlorpyrifos (as mentioned previously), which is no longer registered for home use. And again, other than for agricultural use, the OP diazinon has been banned in the U.S.

Phosphorus cycle

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Phosphorus_cycle 

 

Phosphorus cycle

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs in only specialized, local conditions. Therefore, the phosphorus cycle should be viewed from whole Earth system and then specifically focused on the cycle in terrestrial and aquatic systems.

Living organisms require phosphorous for their proper functioning, for instance plants assimilate phosphorous as phosphate and incorporate it into organic compounds and in animals, phosphorous is the main component of DNA, RNA, bones, teeth, etc.On the land, phosphorus gradually becomes less available to plants over thousands of years, since it is slowly lost in runoff. Low concentration of phosphorus in soils reduces plant growth and slows soil microbial growth, as shown in studies of soil microbial biomass. Soil microorganisms act as both sinks and sources of available phosphorus in the biogeochemical cycle. Short-term transformation of phosphorus is chemical, biological, or microbiological. In the long-term global cycle, however, the major transfer is driven by tectonic movement over geologic time.

Humans have caused major changes to the global phosphorus cycle through shipping of phosphorus minerals, and use of phosphorus fertilizer, and also the shipping of food from farms to cities, where it is lost as effluent.

Phosphorus in the environment

Phosphorus cycle on land

The aquatic phosphorus cycle

Ecological function

Phosphorus is an essential nutrient for plants and animals. Phosphorus is a limiting nutrient for aquatic organisms. Phosphorus forms parts of important life-sustaining molecules that are very common in the biosphere. Phosphorus does enter the atmosphere in very small amounts when the dust is dissolved in rainwater and seaspray but remains mostly on land and in rock and soil minerals. Eighty percent of the mined phosphorus is used to make fertilizers. Phosphates from fertilizers, sewage and detergents can cause pollution in lakes and streams. Over-enrichment of phosphate in both fresh and inshore marine waters can lead to massive algae blooms. In fresh water, the death and decay of these blooms leads to eutrophication. An example of this is the Canadian Experimental Lakes Area.

These freshwater algal blooms should not be confused with those in saltwater environments. Recent research suggests that the predominant pollutant responsible for algal blooms in saltwater estuaries and coastal marine habitats is nitrogen.

Phosphorus occurs most abundantly in nature as part of the orthophosphate ion (PO₄)³⁻, consisting of a P atom and 4 oxygen atoms. On land most phosphorus is found in rocks and minerals. Phosphorus-rich deposits have generally formed in the ocean or from guano, and over time, geologic processes bring ocean sediments to land. Weathering of rocks and minerals release phosphorus in a soluble form where it is taken up by plants, and it is transformed into organic compounds. The plants may then be consumed by herbivores and the phosphorus is either incorporated into their tissues or excreted. After death, the animal or plant decays, and phosphorus is returned to the soil where a large part of the phosphorus is transformed into insoluble compounds. Runoff may carry a small part of the phosphorus back to the ocean. Generally with time (thousands of years) soils become deficient in phosphorus leading to ecosystem retrogression.

Major pools in aquatic systems

There are four major pools of phosphorus in freshwater ecosystems: inorganic phosphorus , dissolved organic phosphorus (DOP), particulate organic phosphorus (POP), and particulate inorganic phosphorus (PIP). Dissolved material is defined as substances that pass through a 0.45 μm filter. DIP consists mainly of orthophosphate (PO₄^3-) and polyphosphate, while DOP consists of DNA and phosphoproteins. Particulate matter are the substances that get caught on a 0.45 μm filter and do not pass through. POP consists of both living and dead organisms, while PIP mainly consists of hydroxyapatite, Ca₅(PO₄)₃OH . The inorganic phosphorous is orthophosphate (readily soluble). Particulate organic phosphorous occurs in suspension in living and dead protoplasm and is insoluble. Dissolved organic phosphorous is derived from the particulate organic phosphorous by excretion and decomposition (soluble).

Biological function

The primary biological importance of phosphates is as a component of nucleotides, which serve as energy storage within cells (ATP) or when linked together, form the nucleic acids DNA and RNA. The double helix of our DNA is only possible because of the phosphate ester bridge that binds the helix. Besides making biomolecules, phosphorus is also found in bone and the enamel of mammalian teeth, whose strength is derived from calcium phosphate in the form of hydroxyapatite. It is also found in the exoskeleton of insects, and phospholipids (found in all biological membranes). It also functions as a buffering agent in maintaining acid base homeostasis in the human body.

Phosphorus cycling

Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.

The global phosphorus cycle includes four major processes:

(i) tectonic uplift and exposure of phosphorus-bearing rocks such as apatite to surface weathering;

(ii) physical erosion, and chemical and biological weathering of phosphorus-bearing rocks to provide dissolved and particulate phosphorus to soils, lakes and rivers;

(iii) riverine and subsurface transportation of phosphorus to various lakes and run-off to the ocean;

(iv) sedimentation of particulate phosphorus (e.g., phosphorus associated with organic matter and oxide/carbonate minerals) and eventually burial in marine sediments (this process can also occur in lakes and rivers).

In terrestrial systems, bioavailable P (‘reactive P’) mainly comes from weathering of phosphorus-containing rocks. The most abundant primary phosphorus-mineral in the crust is apatite, which can be dissolved by natural acids generated by soil microbes and fungi, or by other chemical weathering reactions and physical erosion. The dissolved phosphorus is bioavailable to terrestrial organisms and plants and is returned to the soil after their decay. Phosphorus retention by soil minerals (e.g., adsorption onto iron and aluminum oxyhydroxides in acidic soils and precipitation onto calcite in neutral-to-calcareous soils) is usually viewed as the most important process in controlling terrestrial P-bioavailability in the mineral soil. This process can lead to the low level of dissolved phosphorus concentrations in soil solution. Various physiological strategies are used by plants and microorganisms for obtaining phosphorus from this low level of phosphorus concentration.

Soil phosphorus is usually transported to rivers and lakes and can then either be buried in lake sediments or transported to the ocean via river runoff. Atmospheric phosphorus deposition is another important marine phosphorus source to the ocean. In surface seawater, dissolved inorganic phosphorus, mainly orthophosphate (PO₄^3-), is assimilated by phytoplankton and transformed into organic phosphorus compounds. Phytoplankton cell lysis releases cellular dissolved inorganic and organic phosphorus to the surrounding environment. Some of the organic phosphorus compounds can be hydrolyzed by enzymes synthesized by bacteria and phytoplankton and subsequently assimilated. The vast majority of phosphorus is remineralized within the water column, and approximately 1% of associated phosphorus carried to the deep sea by the falling particles is removed from the ocean reservoir by burial in sediments. A series of diagenetic processes act to enrich sediment pore water phosphorus concentrations, resulting in an appreciable benthic return flux of phosphorus to overlying bottom waters. These processes include

(i) microbial respiration of organic matter in sediments,

(ii) microbial reduction and dissolution of iron and manganese (oxyhydr)oxides with subsequent release of associated phosphorus, which connects the phosphorus cycle to the iron cycle, and

(iii) abiotic reduction of iron (oxyhydr)oxides by hydrogen sulfide and liberation of iron-associated phosphorus.

Additionally,

(iv) phosphate associated with calcium carbonate and

(v) transformation of iron oxide-bound phosphorus to vivianite play critical roles in phosphorus burial in marine sediments.

These processes are similar to phosphorus cycling in lakes and rivers.

Although orthophosphate (PO₄^3-), the dominant inorganic P species in nature, is oxidation state (P5+), certain microorganisms can use phosphonate and phosphite (P3+ oxidation state) as a P source by oxidizing it to orthophosphate. Recently, rapid production and release of reduced phosphorus compounds has provided new clues about the role of reduced P as a missing link in oceanic phosphorus.

Phosphatic minerals

The availability of phosphorus in an ecosystem is restricted by the rate of release of this element during weathering. The release of phosphorus from apatite dissolution is a key control on ecosystem productivity. The primary mineral with significant phosphorus content, apatite [Ca₅(PO₄)₃OH] undergoes carbonation.

Little of this released phosphorus is taken up by biota (organic form), whereas a larger proportion reacts with other soil minerals. This leads to precipitation into unavailable forms in the later stage of weathering and soil development. Available phosphorus is found in a biogeochemical cycle in the upper soil profile, while phosphorus found at lower depths is primarily involved in geochemical reactions with secondary minerals. Plant growth depends on the rapid root uptake of phosphorus released from dead organic matter in the biochemical cycle. Phosphorus is limited in supply for plant growth. Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.

Low-molecular-weight (LMW) organic acids are found in soils. They originate from the activities of various microorganisms in soils or may be exuded from the roots of living plants. Several of those organic acids are capable of forming stable organo-metal complexes with various metal ions found in soil solutions. As a result, these processes may lead to the release of inorganic phosphorus associated with aluminum, iron, and calcium in soil minerals. The production and release of oxalic acid by mycorrhizal fungi explain their importance in maintaining and supplying phosphorus to plants.

The availability of organic phosphorus to support microbial, plant and animal growth depends on the rate of their degradation to generate free phosphate. There are various enzymes such as phosphatases, nucleases and phytase involved for the degradation. Some of the abiotic pathways in the environment studied are hydrolytic reactions and photolytic reactions. Enzymatic hydrolysis of organic phosphorus is an essential step in the biogeochemical phosphorus cycle, including the phosphorus nutrition of plants and microorganisms and the transfer of organic phosphorus from soil to bodies of water. Many organisms rely on the soil derived phosphorus for their phosphorus nutrition.

Eutrophication

Nitrogen and phosphorus cycles in a wetland

Eutrophication is an enrichment of water by nutrient that lead to structural changes to the aquatic ecosystem such as algae bloom, deoxygenation, reduction of fish species. The primary source that contributes to the eutrophication is considered as nitrogen and phosphorus. When these two elements exceed the capacity of the water body, eutrophication occurs. Phosphorus that enters lakes will accumulate in the sediments and the biosphere, it also can be recycled from the sediments and the water system. Drainage water from agricultural land also carries phosphorus and nitrogen. Since a large amount of phosphorus is in the soil contents, so the overuse of fertilizers and over-enrichment with nutrients will lead to increasing the amount of phosphorus concentration in agricultural runoff. When eroded soil enters the lake, both phosphorus and the nitrogen in the soil contribute to eutrophication, and erosion caused by deforestation which also results from uncontrolled planning and urbanization.

Wetland

Wetlands are frequently applied to solve the issue of eutrophication. Nitrate is transformed in wetlands to free nitrogen and discharged to the air.  Phosphorus is adsorbed by wetland soils which are taken up by the plants. Therefore, wetlands could help to reduce the concentration of nitrogen and phosphorus to remit and solve the eutrophication. However, wetland soils can only hold a limited amount of phosphorus. To remove phosphorus continually, it is necessary to add more new soils within the wetland from remnant plant stems, leaves, root debris, and undecomposable parts of dead algae, bacteria, fungi, and invertebrates.

Human influences

Phosphorus fertilizer application

 

Phosphorus in manure production

Nutrients are important to the growth and survival of living organisms, and hence, are essential for development and maintenance of healthy ecosystems. Humans have greatly influenced the phosphorus cycle by mining phosphorus, converting it to fertilizer, and by shipping fertilizer and products around the globe. Transporting phosphorus in food from farms to cities has made a major change in the global Phosphorus cycle. However, excessive amounts of nutrients, particularly phosphorus and nitrogen, are detrimental to aquatic ecosystems. Waters are enriched in phosphorus from farms' run-off, and from effluent that is inadequately treated before it is discharged to waters. The input of P in agricultural runoff can accelerate the eutrophication of P-sensitive surface waters. Natural eutrophication is a process by which lakes gradually age and become more productive and may take thousands of years to progress. Cultural or anthropogenic eutrophication, however, is water pollution caused by excessive plant nutrients; this results in excessive growth in the algal population; when this algae dies its putrefaction depletes the water of oxygen. Such eutrophication may also give rise to toxic algal bloom. Both these effects cause animal and plant death rates to increase as the plants take in poisonous water while the animals drink the poisoned water. Surface and subsurface runoff and erosion from high-phosphorus soils may be major contributing factors to this fresh water eutrophication. The processes controlling soil Phosphorus release to surface runoff and to subsurface flow are a complex interaction between the type of phosphorus input, soil type and management, and transport processes depending on hydrological conditions.

Repeated application of liquid hog manure in excess to crop needs can have detrimental effects on soil phosphorus status. Also, application of biosolids may increase available phosphorus in soil. In poorly drained soils or in areas where snowmelt can cause periodic waterlogging, reducing conditions can be attained in 7–10 days. This causes a sharp increase in phosphorus concentration in solution and phosphorus can be leached. In addition, reduction of the soil causes a shift in phosphorus from resilient to more labile forms. This could eventually increase the potential for phosphorus loss. This is of particular concern for the environmentally sound management of such areas, where disposal of agricultural wastes has already become a problem. It is suggested that the water regime of soils that are to be used for organic wastes disposal is taken into account in the preparation of waste management regulations.

Human interference in the phosphorus cycle occurs by overuse or careless use of phosphorus fertilizers. This results in increased amounts of phosphorus as pollutants in bodies of water resulting in eutrophication. Eutrophication devastates water ecosystems by inducing anoxic conditions.