Chlorine Dioxide is a small, volatile and very strong molecule consisting of 1 Chlorine atom and 2 oxygen atoms. Abbreviated to ClO2, chlorine dioxide exists as a free radical in dilute solutions • It has a molecular weight of 67.45.
- It is a gas at normal temperatures and pressures.
- It has a melting point of -59oC.
- It has a boiling point of 11oC.
- It is yellowish/green and has an odour similar to that of Chlorine.
- It is denser than air and is water soluble at standard temperatures and pressures up to 5000ppm.
- It is explosive in air at concentrations > 10%
- It is prohibited from road and sea transport in its “free” form, and is normally generated at the point of application using 2 precursor chemicals.
- It will decompose in the presence of UV, high temperatures, and high alkalinity (>pH12)
ClO2 is an oxidizing biocide. It deactivates micro organisms by attacking and penetrating their cell wall, disrupting the transport of nutrients across the cell wall and inhibiting protein synthesis. Since this action occurs regardless of the metabolic state of the organism, oxidizing biocides are effective against dormant organisms and spores (Giardia Cysts and Poliovirus) also.
When the cell wall is penetrated by chlorine dioxide, Organic substances within cells and on the surface of cell membranes react with chlorine dioxide, causing cell metabolism to be disrupted. Chlorine dioxide also reacts directly with amino acids and the RNA in the cell. This reaction is not dependent on reaction time or concentration. Unlike non-oxidizing disinfectants, chlorine dioxide kills microorganisms even when they are inactive. Microorganisms are unable to build up resistance to chlorine dioxide, in practical terms.
Many biocides have particular problems in penetrating this biofilm, due to the polysaccharide “glue” that is secreted by the bacteria to hold the biofilm together. Unlike most biocides, chlorine dioxide can effectively penetrate biofilm to provide complete protection. Chlorine dioxide kills viruses by preventing protein formation. ClO2 reacts with peptone, a water-soluble substance that originates from hydrolysis of proteins to amino acids.
The DBPs of chlorine dioxide reactions are chlorite (ClO2-) and chlorate (ClO3-), and eventually chloride (Cl-). The fate of any disinfection by-products depends largely on the conditions at the time, such as concentration, temperature and the presence of other molecules.
Unlike ozone (O3), chlorine dioxide does not oxidize bromide (Br-) ions into bromate ions (BrO3-). Additionally, chlorine dioxide does not produce large amounts of aldehydes, ketones, or other disinfection by-products that originate from the ozonization of organic substances.
Because chlorine dioxide is a dissolved gas, it does not ionize to form weak acids (as chlorine and bromine do) in aqueous solutions. This allows ClO2 to be effective over a wide pH range from Acidic to Alkaline (4-11).
Although Chlorine Dioxide has the word Chlorine in its name but the two chemicals have completely different chemical structures. The additional oxygen atom radically changes the molecule and creates completely different chemical behaviors and by-products. Their differences are as profound as those between hydrogen, the explosive gas, and hydrogen combined with oxygen, which creates di-hydrogen oxide – commonly called water.
Chlorine dioxide safely oxidizes phenol’s, cyanide’s, aldehydes, and mercaptans, reduced sulfur compounds and some pesticides. It is useful in both wastewater treatment and scrubber systems. Chlorine dioxide is an eco-friendly oxidant that is preferred by many regulating water utilities and supply authorities for final discharge disinfection.
Because of its biocidal characteristics, ClO2 is ideal for water hygiene applications in hospitals and healthcare facilities. It has consistently been shown to be the best molecule for eradicating the causative organism of Legionnaires’ disease. In the UK, the Building Services Research and Information Association (BSRIA) has recommended chlorine dioxide as the best available technology for control of Legionella in hot and cold water systems.