Chlorine dioxide

Chlorine dioxide is a chemical compound with the formula ClO₂. This greenish-yellow gas crystallizes as orange crystals at −59 °C. As one of several oxides of chlorine, it is a potent and useful oxidizing agent used in water treatment and in bleaching.

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Uses

Chlorine dioxide is used primarily (>95%) for bleaching of wood pulp, but is also used for the bleaching of flour and for the disinfection of water. The Niagara Falls, New York water treatment plant first used chlorine dioxide for drinking water treatment in 1944 for phenol destruction. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, when Brussels, Belgium, changed from chlorine to chlorine dioxide. Its most common use in water treatment is as a pre-oxidant prior to chlorination of drinking water to reduce trihalomethanes which are a carcinogenic disinfection by-product associated with chlorination of naturally occurring organics in the raw water. Chlorine dioxide is also used in conjunction with ozone disinfection of water to reduce the formation of bromates which are regulated carcinogens. Chlorine dioxide is also superior to chlorine when operating above neutral pH, when ammonia is present and for the control of biofilms. Chlorine dioxide is used in many industrial water treatment applications as a biocide including cooling towers, process water and food processing. Chlorine dioxide is less corrosive than chlorine and superior for the control of legionella bacteria.

It is more effective than chlorine against viruses, bacteria and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium.

Protective effect of low-concentration chlorine dioxide gas against influenza A virus infection Ogata N, Shibata T. J Gen Virol 89 (2008), 60-67; DOI 10.1099/vir.0.83393-0 http://vir.sgmjournals.org/cgi/content/abstract/89/1/60

It can also be used for air disinfection, and was the principal agent used in the decontamination of buildings in the United States after the 2001 anthrax attacks. Recently, after the disaster of Hurricane Katrina in New Orleans, Louisiana and the surrounding Gulf Coast, chlorine dioxide has been used to eradicate dangerous mold from houses inundated by water from massive flooding.

Chlorine dioxide is used as an oxidant for phenol destruction in waste water streams, control of zebra mussels in water intakes and for odor control in the air scrubbers of animal byproduct (rendering) plants.

Stabilized chlorine dioxide can also be used in an oral rinse to treat oral disease and malodor.^[1]

Preparation

In the laboratory, ClO₂ is prepared by oxidation of sodium chlorite:^[2]

2NaClO₂ + Cl₂ → 2ClO₂ + 2 NaCl

Over 95% of the chlorine dioxide produced in the world today is made from sodium chlorate and is used for pulp bleaching. It is produced with high efficiency by reducing sodium chlorate in a strong acid solution with a suitable reducing agent such as hydrochloric acid and sulfur dioxide. The reaction of sodium chlorate with hydrochloric acid proceeds in one reactor via the following pathway:

HClO₃ + HCl → HClO₂ + HOCl

HClO₃ + HClO₂ → 2ClO₂ + Cl₂ + 2H₂O

HOCl + HCl → Cl₂ + H₂O

A much smaller but important market for chlorine dioxide is for use as a disinfectant. Since 1999 a growing proportion of the chlorine dioxide made globally for water treatment and other small scale applications has been made using the chlorate, hydrogen peroxide and sulfuric acid method which can produce a chlorine free product at high efficiency. Traditionally, chlorine dioxide for disinfection applications has been made by one of three methods using sodium chlorite or the sodium chlorite - hypochlorite method:

2NaClO₂ + 2HCl + NaOCl → 2ClO₂ + 3NaCl + H₂O

or the sodium chlorite - hydrochloric acid method:

5NaClO₂ + 4HCl → 5NaCl + 4ClO₂

All three sodium chlorite chemistries can produce chlorine dioxide with high chlorite conversion yield, but the chlorite-HCl method suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide.

Catalytic chlorine dioxide generators produce extremely high conversion yields (>98.5%). With these systems sodium chlorite solution is passed through an ion exchange column. The process of ion exchange yields chlorous acid, which is then passed through a catalyst column which assists in the conversion to chlorine dioxide. The advantage of these systems is that low concentrations of chlorine dioxide can be produced directly at the point of application.

Chlorine dioxide can also be produced by electrolysis of a chlorite solution:

2NaClO₂ + 2H₂O   →   2ClO₂ + 2NaOH + H₂

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.

2NaClO₂ + Cl₂   →   2ClO₂ + 2NaCl

These processes and several slight variations have been reviewed.^[3]

Handling properties

At concentrations greater than 15% volume in air at STP, ClO₂ explosively decomposes into chlorine and oxygen. The decomposition is initiated by light. Thus, it is never handled in concentrated form, but is almost always used as a dissolved gas in water in a concentration range of 0.5 to 10 grams per liter. Its solubility increases at lower temperatures: it is thus common to use chilled water (5 °C or 41 °F) when storing at concentrations above 3 grams per liter. In many countries, such as the USA, chlorine dioxide gas may not be transported at any concentration and is almost always produced at the application site using a chlorine dioxide generator. In some countries, chlorine dioxide solution below 3 grams per liter in concentration may be transported by land, but are relatively unstable and deteriorate quickly.

A number of products are marketed as "stabilized chlorine dioxide" (SCD). These solutions do not actually contain chlorine dioxide but consist of solutions of buffered sodium chlorite. A weak acid can be added to SCD to "activate" it and make chlorine dioxide in-situ without a chlorine dioxide generator. The use of SCD is effective when the demand for chlorine dioxide is low and when impurities, such as small amounts of sodium, can be tolerated. For application requiring above 5 kg day⁻¹ ClO₂, chlorine dioxide produced by a generator with either sodium chlorite or sodium chlorate is typically more economical.

References

1. ^ US patent 4689215
2. ^ Derby, R. I.; Hutchinson, W. S. "Chlorine(IV) Oxide" Inorganic Syntheses, 1953, IV, 152-158.
3. ^ White, G. C. "Handbook of Chlorination and Alternative Disinfectants", 4th Edition (Wiley, 1999).

• National Library of Medicine (US)
• WebElements
• NIST Standard Reference Database
• European Chemicals Bureau