Chlorine dioxide
Template:Short description Template:Distinguish Template:Redirect Template:Use mdy dates Template:Chembox Chlorine dioxide is a chemical compound with the formula ClO2 that exists as yellowish-green gas above 11 °C, a reddish-brown liquid between 11 °C and −59 °C, and as bright orange crystals below −59 °C. It is usually handled as an aqueous solution. It is commonly used as a bleach. More recent developments have extended its applications in food processing and as a disinfectant.
Structure and bondingEdit
The molecule ClO2 has an odd number of valence electrons, and therefore, it is a paramagnetic radical. It is an unusual "example of an odd-electron molecule stable toward dimerization" (nitric oxide being another example).<ref>Template:Greenwood&Earnshaw2nd</ref>
ClO2 crystallizes in the orthorhombic Pbca space group.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
HistoryEdit
Chlorine dioxide was first prepared in 1811 by Sir Humphry Davy.<ref>Aieta, E. Marco, and James D. Berg. "A Review of Chlorine Dioxide in Drinking Water Treatment." Journal (American Water Works Association) 78, no. 6 (1986): 62-72. Accessed April 24, 2021. http://www.jstor.org/stable/41273622</ref>
In 1933, Lawrence O. Brockway, a graduate student of Linus Pauling, proposed a structure that involved a three-electron bond and two single bonds.<ref>Template:Cite journal</ref> However, Pauling in his General Chemistry shows a double bond to one oxygen and a single bond plus a three-electron bond to the other. The valence bond structure would be represented as the resonance hybrid depicted by Pauling.<ref name=Pauling>Template:Cite book</ref> The three-electron bond represents a bond that is weaker than the double bond. In molecular orbital theory this idea is commonplace if the third electron is placed in an anti-bonding orbital. Later work has confirmed that the highest occupied molecular orbital is indeed an incompletely-filled antibonding orbital.<ref>Template:Cite journal</ref>
PreparationEdit
The reaction of chlorine with oxygen under conditions of flash photolysis in the presence of ultraviolet light results in trace amounts of chlorine dioxide formation.<ref>Template:Cite journal</ref>
- Cl2 + 2 O2 <chem>->[\ce{UV}]</chem> 2 ClO2 ↑
Chlorine dioxide can decompose violently when separated from diluting substances. As a result, preparation methods that involve producing solutions of it without going through a gas-phase stage are often preferred.
Oxidation of chloriteEdit
In the laboratory, ClO2 can be prepared by oxidation of sodium chlorite with chlorine:<ref>Template:Cite book</ref> Template:Block indent
Traditionally, chlorine dioxide for disinfection applications has been made from sodium chlorite or the sodium chlorite–hypochlorite method: Template:Block indent or the sodium chlorite–hydrochloric acid method: Template:Block indent or the chlorite–sulfuric acid method: Template:Block indent
All three methods can produce chlorine dioxide with high chlorite conversion yield. Unlike the other processes, the chlorite–sulfuric acid method is completely chlorine-free, although it suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide. Alternatively, hydrogen peroxide may be efficiently used in small-scale applications.<ref name="Vogt, H. 2010" />
Addition of sulfuric acid or any strong acid to chlorate salts produces chlorine dioxide.<ref name=Pauling/>
Reduction of chlorateEdit
In the laboratory, chlorine dioxide can also be prepared by reaction of potassium chlorate with oxalic acid: Template:Block indent
or with oxalic and sulfuric acid: Template:Block indent
Over 95% of the chlorine dioxide produced in the world today is made by reduction of sodium chlorate, for use in pulp bleaching. It is produced with high efficiency in a strong acid solution with a suitable reducing agent such as methanol, hydrogen peroxide, hydrochloric acid or sulfur dioxide.<ref name="Vogt, H. 2010">Template:Ullmann</ref> Modern technologies are based on methanol or hydrogen peroxide, as these chemistries allow the best economy and do not co-produce elemental chlorine. The overall reaction can be written as:<ref>Template:Cite conferenceTemplate:Dead link</ref>
As a typical example, the reaction of sodium chlorate with hydrochloric acid in a single reactor is believed to proceed through the following pathway: Template:Block indent Template:Block indent Template:Block indent
which gives the overall reaction Template:Block indent
The commercially more important production route uses methanol as the reducing agent and sulfuric acid for the acidity. Two advantages of not using the chloride-based processes are that there is no formation of elemental chlorine, and that sodium sulfate, a valuable chemical for the pulp mill, is a side-product. These methanol-based processes provide high efficiency and can be made very safe.<ref name="Vogt, H. 2010"/>
The variant process using sodium chlorate, hydrogen peroxide and sulfuric acid has been increasingly used since 1999 for water treatment and other small-scale disinfection applications, since it produce a chlorine-free product at high efficiency, over 95%.Template:Cn
Other processesEdit
Very pure chlorine dioxide can also be produced by electrolysis of a chlorite solution:<ref name=white>Template:Cite book</ref>
High-purity chlorine dioxide gas (7.7% in air or nitrogen) can be produced by the gas–solid method, which reacts dilute chlorine gas with solid sodium chlorite:<ref name=white/> Template:Block indent
Handling propertiesEdit
Chlorine dioxide is very different from elemental chlorine.<ref name="Vogt, H. 2010"/> One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. Chlorine dioxide does not react with water; it remains a dissolved gas in solution. Chlorine dioxide is approximately 10 times more soluble in water than elemental chlorine<ref name="Vogt, H. 2010" /> but its solubility is very temperature-dependent.
At partial pressures above Template:Convert<ref name="Vogt, H. 2010" /> (or gas-phase concentrations greater than 10% volume in air at STP) of ClO2 may explosively decompose into chlorine and oxygen. The decomposition can be initiated by light, hot spots, chemical reaction, or pressure shock. Thus, chlorine dioxide is never handled as a pure gas, but is almost always handled in an aqueous solution in concentrations between 0.5 and 10 grams per liter. Its solubility increases at lower temperatures, so it is common to use chilled water (5 °C, 41 °F) when storing at concentrations above 3 grams per liter. In many countries, such as the United States, chlorine dioxide may not be transported at any concentration and is instead almost always produced on-site.<ref name="Vogt, H. 2010" /> In some countries,Template:Which chlorine dioxide solutions below 3 grams per liter in concentration may be transported by land, but they are relatively unstable and deteriorate quickly.
UsesEdit
Chlorine dioxide is used for bleaching of wood pulp and for the disinfection (called chlorination) of municipal drinking water,<ref>Template:Cite book</ref><ref name="epa1999">Template:Citation</ref>Template:Rp<ref name="block2001" /> treatment of water in oil and gas applications, disinfection in the food industry, microbiological control in cooling towers, and textile bleaching.<ref name="Simpson">Template:Cite book</ref> As a disinfectant, it is effective even at low concentrations because of its unique qualities.<ref name="Vogt, H. 2010" /><ref name="epa1999"/><ref name="Simpson"/>
BleachingEdit
Chlorine dioxide is sometimes used for bleaching of wood pulp in combination with chlorine, but it is used alone in ECF (elemental chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced.<ref name="eero">Template:Cite book</ref> Chlorine dioxide (ECF technology) currently is the most important bleaching method worldwide. About 95% of all bleached kraft pulp is made using chlorine dioxide in ECF bleaching sequences.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Chlorine dioxide has been used to bleach flour.<ref>Template:Cite journal</ref>
Water treatmentEdit
Template:Further The water treatment plant at Niagara Falls, New York first used chlorine dioxide for drinking water treatment in 1944 for destroying "taste and odor producing phenolic compounds."<ref name="epa1999" />Template:Rp<ref name="block2001" /> Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, when Brussels, Belgium, changed from chlorine to chlorine dioxide.<ref name="block2001" /> Its most common use in water treatment is as a pre-oxidant prior to chlorination of drinking water to destroy natural water impurities that would otherwise produce trihalomethanes upon exposure to free chlorine.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="volk2002">Template:Cite journal</ref> Trihalomethanes are suspected carcinogenic disinfection by-products<ref>Template:Cite journal</ref> associated with chlorination of naturally occurring organics in raw water.<ref name="volk2002" /> Chlorine dioxide also produces 70% fewer halomethanes in the presence of natural organic matter compared to when elemental chlorine or bleach is used.<ref name=":1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Chlorine dioxide is also superior to chlorine when operating above pH 7,<ref name="epa1999" />Template:Rp in the presence of ammonia and amines,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and for the control of biofilms in water distribution systems.<ref name="volk2002" /> Chlorine dioxide is used in many industrial water treatment applications as a biocide, including cooling towers, process water, and food processing.<ref>Template:Cite journal</ref>
Chlorine dioxide is less corrosive than chlorine and superior for the control of Legionella bacteria.<ref name="block2001">Template:Cite book</ref><ref>Template:Cite journal</ref> Chlorine dioxide is superior to some other secondary water disinfection methods, in that chlorine dioxide is not negatively impacted by pH, does not lose efficacy over time, because the bacteria will not grow resistant to it, and is not negatively impacted by silica and phosphates, which are commonly used potable water corrosion inhibitors. In the United States, it is an EPA-registered biocide.
It is more effective as a disinfectant than chlorine in most circumstances against waterborne pathogenic agents such as viruses,<ref>Template:Cite journal</ref> bacteria, and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium.<ref name="epa1999" />Template:Rp
The use of chlorine dioxide in water treatment leads to the formation of the by-product chlorite, which is currently limited to a maximum of 1 part per million in drinking water in the USA.<ref name="epa1999" />Template:Rp This EPA standard limits the use of chlorine dioxide in the US to relatively high-quality water, because this minimizes chlorite concentration, or water that is to be treated with iron-based coagulants, because iron can reduce chlorite to chloride.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The World Health Organization also advises a 1ppm dosification.<ref name=":1" />
Use in public crisesEdit
Chlorine dioxide has many applications as an oxidizer or disinfectant.<ref name="Vogt, H. 2010" /> Chlorine dioxide can be used for air disinfection<ref>Template:Cite journal</ref> and was the principal agent used in the decontamination of buildings in the United States after the 2001 anthrax attacks.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> After the disaster of Hurricane Katrina in New Orleans, Louisiana, and the surrounding Gulf Coast, chlorine dioxide was used to eradicate dangerous mold from houses inundated by the flood water.<ref>Template:Cite journal</ref>
In addressing the COVID-19 pandemic, the U.S. Environmental Protection Agency has posted a list of many disinfectants that meet its criteria for use in environmental measures against the causative coronavirus.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Some are based on sodium chlorite that is activated into chlorine dioxide, though differing formulations are used in each product. Many other products on the EPA list contain sodium hypochlorite, which is similar in name but should not be confused with sodium chlorite because they have very different modes of chemical action.
Other disinfection usesEdit
Chlorine dioxide may be used as a fumigant treatment to "sanitize" fruits such as blueberries, raspberries, and strawberries that develop molds and yeast.<ref>Template:Cite journal</ref>
Chlorine dioxide may be used to disinfect poultry by spraying or immersing it after slaughtering.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Chlorine dioxide may be used for the disinfection of endoscopes, such as under the trade name Tristel.<ref>Template:Cite journal</ref> It is also available in a trio consisting of a preceding pre-clean with surfactant and a succeeding rinse with deionized water and a low-level antioxidant.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Chlorine dioxide may be used for control of zebra and quagga mussels in water intakes.<ref name="epa1999" />Template:Rp
Chlorine dioxide was shown to be effective in bedbug eradication.<ref>Template:Cite journal</ref>
For water purification during camping, disinfecting tablets containing chlorine dioxide are more effective against pathogens than those using household bleach, but typically cost more.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Other usesEdit
Chlorine dioxide is used as an oxidant for destroying phenols in wastewater streams and for odor control in the air scrubbers of animal byproduct (rendering) plants.<ref name="epa1999" />Template:Rp It is also available for use as a deodorant for cars and boats, in chlorine dioxide-generating packages that are activated by water and left in the boat or car overnight.
In dilute concentrations, chlorine dioxide is an ingredient that acts as an antiseptic agent in some mouthwashes.<ref name="pmid32410557">Template:Cite journal</ref><ref name="pmid36634129">Template:Cite journal</ref>
Safety issues in water and supplementsEdit
Potential hazards with chlorine dioxide include poisoning and the risk of spontaneous ignition or explosion on contact with flammable materials.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>
Chlorine dioxide is toxic, and limits on human exposure are required to ensure its safe use. The United States Environmental Protection Agency has set a maximum level of 0.8 mg/L for chlorine dioxide in drinking water.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Occupational Safety and Health Administration (OSHA), an agency of the United States Department of Labor, has set an 8-hour permissible exposure limit of 0.1 ppm in air (0.3 mg/m3) for people working with chlorine dioxide.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Chlorine dioxide has been fraudulently and illegally marketed as an ingestible cure for a wide range of diseases, including childhood autism<ref name=":0">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and coronavirus.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite news</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Children who have been given enemas of chlorine dioxide as a supposed cure for childhood autism have suffered life-threatening ailments.<ref name=":0" /> The U.S. Food and Drug Administration (FDA) has stated that ingestion or other internal use of chlorine dioxide, outside of supervised oral rinsing using dilute concentrations, has no health benefits of any kind, and it should not be used internally for any reason.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="FDA 2019"/>
PseudomedicineEdit
On 30 July and 1 October 2010, the United States Food and Drug Administration warned against the use of the product "Miracle Mineral Supplement", or "MMS", which when prepared according to the instructions produces chlorine dioxide. MMS has been marketed as a treatment for a variety of conditions, including HIV, cancer, autism, acne, and, more recently, COVID-19. Many have complained to the FDA, reporting life-threatening reactions,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and even death.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The FDA has warned consumers that MMS can cause serious harm to health, and stated that it has received numerous reports of nausea, diarrhea, severe vomiting, and life-threatening low blood pressure caused by dehydration.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This warning was repeated for a third time on 12 August 2019, and a fourth on 8 April 2020, stating that ingesting MMS is just as hazardous as ingesting bleach, and urging consumers not to use them or give these products to their children for any reason, as there is no scientific evidence showing that chlorine dioxide has any beneficial medical properties.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="FDA 2019">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
ReferencesEdit
External linksEdit
Template:Chlorine compounds Template:Oxides Template:E number infobox 920-929 Template:Oxygen compounds Template:Authority control