Chlorine dioxide is a powerful oxidising biocide and has been successfully used as a water treatment disinfectant for several decades in many countries. Rapid progress has been made in the technology for generation of the product and knowledge of its reactivity has increased with improved analytical techniques. Chlorine dioxide is a relatively stable radical molecule. It is highly soluble in water, has a boiling point of 110C, absorbs light and breaks down into ClO3- and Cl-. Because of its oxidising properties chlorine dioxide acts on Fe2, Mn2+ and NO2- but does not act on Cl, NH4+ and Br- when not exposed to light. These ions are generally part of the chemical composition of natural water.
Because of its radical structure, chlorine dioxide has a particular reactivity - totally different from that of chlorine or ozone. The latter behave as electron acceptors or are electrophilic, while chlorine dioxide has a free electron for a homopolar bond based on one of its oxygens. The electrophilic nature of chlorine or hypochloric acid can lead, through a reaction of addition or substitution, to the formation of organic by-products, while the radical reactivity of chlorine dioxide mainly results in oxycarbonyls. Generally, chlorine dioxide rapidly oxidises phenol type compounds, secondary and tertiary amines, organic sulphides and certain hydrocarbon polycylic aromatics such as benzopyrene, anthracene and benzoanthracene. The reaction is slow or non-existent on double carbon bonds, aromatic cores, quinionic and carboxylic structures, as well as primary amines and urea.
The oxidising properties and the radical nature of chlorine dioxide make it an excellent virucidal and bactericidal agent in a large pH range. The most probable explanation is that in the alkaline media the permeability of living cell walls to gaseous chlorine dioxide radicals seems to be increased allowing an easier access to vital molecules. The reaction of chlorine dioxide with vital amino acids is one of the dominant processes of its action on bacteria and viruses.
Chlorine dioxide is efficient against viruses, bacteria and protozoan clumps usually found in raw water. A rise in pH level further increases its action against f2 bacteriophages, amoebic clumps, polioviruses and enterovirus. It is efficient against Giardia and has an excellent biocidal effect against Cryptosporidia, which are resistant to chlorine and chloramines. The reduction of bad tastes and odour with ClO2 is the result of the elimination of algae and on the negligible formation of organo-chlorinated derivatives.
Chlorine Dioxide’s effectiveness against Legionella in both hot and cold systems has enabled building services managers to provide safe water, even in systems which have difficulty maintaining the 60oC level recommended. Dosing the system with chlorine dioxide can also prevent the pipe corrosion caused by the co-existence of two quite different types of bacteria in the biofilm. The upper layers of biofilm are populated by aerobic bacteria, and the lower layers (lacking in oxygen) by anaerobic bacteria. This co-existence creates a corrosion potential between the oxidising and reducing bacteria, resulting in the removal of pipe metal by natural electrolysis. Biofilm develops faster in plastic pipes than in metal pipes, as the biofilm will find nutrients in the organic content of the pipe.
Apart from its effectiveness in penetrating biofilm and killing the actual bacteria, chlorine dioxide solutions have a long residual level of disinfection. It is less reactive than other biocides such as chlorine or ozone, which stay in the water for a significantly shorter period of time. By using solutions based on Stabilised Chlorine Dioxide, protective levels of biocide can be maintained in water systems for 14 days or more and so provide protection during "Shut downs". The need to flush out systems or carry out chlorinations prior to reopening can be eliminated.