Dioxin
Dioxins are chemical compounds that are produced from combustion processes involving chlorine. The term is applied interchangeably to polychlorodibenzofurans (PCDFs) and polychlorodibenzodioxins (PCDDs).
Dioxins are a group of chemical compounds that become persistent environmental pollutants. Dioxins are found in the environment throughout the world and due to their persistence they accumulate throughout the food chain, mainly in the adipose tissue of animals due to their solubility in fats. Animal products are the largest contributors to human intake of dioxins.,
Origin and presence
In food they are present in the form of traces, that is, in quantities of the order of nanograms and picograms per kilogram,
Dioxins have become well known in recent years because of concern about their presence in the environment since they are found in many places, albeit in low concentrations, and some of them are extremely toxic. Together with dioxins, they are often find furans which are similar chemical compounds.
Dioxins are primarily by-products of industrial processes, but they can also be produced in natural processes such as volcanic eruptions and forest fires. Dioxins are unwanted by-products of many manufacturing processes such as smelting, chlorine bleaching of pulp, or the manufacture of some herbicides and pesticides. Regarding the release of dioxins into the environment, the uncontrolled incineration of waste (solid and hospital) is usually the most serious cause, since combustion is incomplete. There is technology that allows the controlled incineration of waste with low emissions.
Dioxins are not deliberately manufactured, except in small amounts for research purposes.
There are several hundred dioxins and furans. A mere 6-millionth of a gram dose of the deadliest dioxin, 2,3,7,8-TCDD, can kill a rat [citation needed]. It is still not well known how these substances affect humans -specifically-. It has been possible to observe the action of these compounds when someone has been accidentally exposed to them, but in these cases it is only possible to know the dose that they have received very approximately. For this reason, it is risky to pronounce on the effects produced by the different doses, especially when we talk about contact with these substances for long periods of time.
Dioxin 2,4,5-T is one of the components of Agent Orange, a chemical weapon used by the United States in the Vietnam War.
Effects on humans and the environment
Dioxins are highly toxic and can cause reproductive and developmental problems, affect the immune system, interfere with hormones, and thereby cause cancer. When some people have been exposed to high concentrations of 2,3,7,8-TCDD have had chloracne ([#Víktor Yushchenko]), with effects that can take decades to eliminate.
Recently, dioxins have been found to be associated with the genesis of endometriosis, a gynecological disease characterized by the growth of endometrial tissue outside the uterine cavity and which can cause pelvic pain, dysmenorrhea or menstrual pain, and infertility.
Many researchers [citation needed] are more concerned about the long-term effects that can occur in people exposed to very low doses, which do not cause appreciable short-term effects term. The problem with this type of substances is that they are not easily eliminated (they take five years to halve or degrade and, therefore, they accumulate in the tissues. In laboratory experiments with animals [ citation needed] Non-lethal doses have been shown to cause cancer, birth defects, reduced fertility, and changes in the immune system.
Carcinogenic effects
Multiple studies carried out with people exposed to these products have associated exposure to dioxins with an increased probability of cancer. On the other hand, there are studies carried out and exposed in the Occupational & Environmental Medicine which they attribute, after a 23-year follow-up after the closure of a chemical plant in Hamburg, concluding that the mortality of its workers from cancer has been due to exposure to PCDD/F compounds. Abnormally high proportions of a rare type of cancer among people who worked with herbicides containing very small amounts of 2,3,7,8-TCDD.
Effects on sexual reproduction
Dioxins and furans also reduce reproductive success in laboratory animals by causing low birth weights, smaller litter sizes, and premature abortions[citation needed]. The problems only occur when the mother is exposed to 2,3,7,8-TCDD, never when it is the male, which shows that there is no DNA alteration, but alterations in the embryo formation process [citation required].
Many studies [citation needed] have been done on birth defects among women exposed to 2,3,7,8-TCDD. Some have found a higher than normal number of birth defects, but most investigations [citation needed] have found no evidence of birth defects or reproductive problems for this reason.
From what we know so far, with careful and detailed studies [citation needed], people who have received abnormally high doses of these substances maintain normal health[citation required]. Everything indicates [citation required] that man supports these substances much better than most laboratory animals[citation required] . It is also clear that traces (very low, almost negligible concentrations) of these substances have been found in tissues and in the breast milk of people in many countries; but nothing can be said with certainty [citation needed] about the long-term effects that this contamination can cause until new and more detailed studies are carried out.
Training mechanisms
Dioxins come mainly[citation needed] from emissions from incinerators and other sources of combustion. Its formation occurs as a consequence of chemical processes that occur during combustion, mainly from related chemical compounds such as chlorobenzenes, chlorophenols and biphenyl polychlorides. Due to the high toxicity of dioxins, numerous studies have been carried out on their formation mechanisms, since this knowledge is of vital importance when studying possible processes for their degradation. A gas phase mechanism has been proposed that occurs through a series of radical reactions:
1. P → P• + H
2. P + OH → P• + H2O
3. P• → Pr
4. P + P• → PD + Cl
5. PD → D + HCl
6. PD + OH → D + H2O
7. P• + R → P+ R•
8. P• + OH → Pr
9. D → Pr
10. D + OH → Pr
11. P• + O2 → Pr
12. R + OH → R• + H2O
13. R → Pr
Where P are polychlorinated phenols, P• are polychlorinated phenoxy radicals, PD are polychlorinated 2-phenoxyphenols (dioxin precursors), D are PCDDs, R is some component of the organic fuel, R• is a fuel molecule without a hydrogen atom and Pr are unspecified products.
The formation of dioxins in the gas phase would only explain a part of the total content of these compounds found in the emissions from combustion processes, so a mechanism based on heterogeneous catalysis has also been proposed. It is suggested that a Langmuir-Hinshelwood mechanism, which involves surface radical-radical reactions, and an Eley-Rideal mechanism, which involves reactions between a gas-phase molecule and an adsorbed one, are responsible for the formation of PCDFs and PCDDs, respectively, on surfaces. The main differences of this mechanism with the gas phase mechanism is the formation of the chlorinated phenoxy radical by the chemisorption impact of polychlorinated phenol on the catalytic surface (usually metal oxides such as CuO) and the steric hindrance of oxygen-centered radicals adsorbed on the gas phase. the surface, which inhibit radical-radical reactions that lead to the formation of dibenzo-p-dioxin.
These types of reactions are very complex and difficult to study, since the radicals involved in them are extremely reactive. In addition, the variability of the incinerated organic material and the wide range of combustion technologies make its study even more complex. Therefore, the precise mechanism of dioxin formation is still not entirely clear, although there are several developing theories.
It is important to mention that studies can also be found on possible mechanisms for the inhibition of these reactions, which suggest that the presence of some basic compounds such as ammonia, calcium oxide or sodium hydroxide inhibit the formation of PCDDs and PCDFs from of chlorinated phenols and benzenes with efficiencies up to 99%.
Degradation mechanisms
There is evidence[citation needed] that dioxins are susceptible to biodegradation in the environment as part of the natural chlorine cycle. Low-chlorinated dioxins can be broken down by aerobic bacteria of the genus Sphingomonas, Pseudomonas and Burkholderia[citation needed]. The degradation is normally initiated by angular dioxygenases that attack the ring adjacent to the oxygen of the ether, finally obtaining the chlorinated phenols[citation needed]. These dioxins can also be metabolically attacked under aerobic conditions by fungi using extracellular lignin peroxidases. Highly chlorinated dioxins can be reductively dechlorinated in anaerobic sediments by bacteria of the genus Dehalococcoides[citation needed]. These studies[citation needed] indicate that biodegradation can contribute to the natural attenuation of dioxin levels in soils, surface waters or sediments, but this degradation is very slow, with half-life times ranging from 2 to 170 years depending on the type of dioxin, and for some of them the observed degradation is nil. For this reason, numerous studies have been carried out[citation needed] to increase the effectiveness of this degradation by unnatural pathways.
The degradation of PCDDs in concentrations of 10 ng/L becomes complete[citation needed] with Fe(II)/H2O2/UV systems in aqueous solution at times between 20 and 300 minutes under optimal conditions. The rate of this photodegradation decreases with the number of chlorine atoms in the dioxin. This process appears to be initiated by an oxidative reaction produced by the attack of OH• radicals on the four carbon atoms adjacent to the oxygen atoms, finally producing chlorinated phenols. The production of these radicals would be photochemically induced from iron hydroxide. Other studies[citation needed] suggest that the speed of this process can be increased by adding ultrasound to the system that favors the formation of OH• radicals. Based on these results, these systems could be a useful technology for the treatment of wastewater containing these contaminants.
A process by which dioxins can be dechlorinated in organic solvents such as ethanol, n-nonane and toluene has also been found[citation needed]. It is based on a radiolytic degradation with γ rays of a Co isotope. The reactive species produced by the irradiation of the solvents are hydrogen atoms, solvent radicals and free or solvated electrons. The degradation is mainly attributed to dechlorination by electrons and, in part, by radicals of the solvent. The proposed mechanism[citation needed] for the degradation of octachloro dibenzo-p-dioxin (OCDD) in ethanol:
1. OCDD + e- → hepta-CDD• + Cl-
2. hepta-CDD• + CH3CH2OH → hepta-CDD + CH3C•HOH
3. OCDD + CH3C•HOH → hepta-CDD• + HCl + CH3CHO
4. hepta-CDD• + CH3C•HOH → hepta-CDD-CH3CHOH(adduct)
Similarly, hepta-CDD is reduced to hexa-CDD[citation needed], and through sequential dechlorination dibenzo-p-dioxin is produced. This study[citation needed] demonstrates that the addition of ethanol to liquid waste produces the degradation of more than 90% of the dioxins.
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