Properties

Basic Properties of ClO₂

High purity ClO₂ has a number of physical and chemical properties that make it the ideal product for addressing liquid, surface and/or airborne microbial contamination in a safe and effective manner. Among the properties that make ClO₂ the agent of choice for addressing microbial contamination are the following:

• ClO₂ has a long history of effective use as a biocide in a variety of liquid and surface treatment applications including drinking water disinifection, wastewater disinfection, food plant sanitation, pharmaceutical and medical device sterilization, bio-medical waste treatment, and cooling water system treatment. The effectiveness of gaseous ClO₂ has also been well documented in numerous large-scale fumigation applications.
• ClO₂ remains a "true" gas when placed into water or other solution. This means that ClO₂ retains its distinct chemical structure and properties after being dissolved, just like carbon dioxide does when dissolved in a can of soda pop. This property has important implications for both treatment efficacy and safety. First, with regard to efficacy, ClO₂ remains just as potent a biocide when dissolved in solution as it was in its pure gaseous form. In contrast, chlorine is an acid gas that dissociates to form hypochlorous acid and hydrochloric acid when placed in water. These two chlorine species are both more corrosive and less effective as biocides than the original chlorine gas. With respect to safety, ClO₂ does not chlorinate organic material to form trihalomethanes and other potentially carcinogenic substances the way chlorine does. In addition, the existence of ClO₂ as a true gas means that it can be generated in a water solution and kept safely in that solution until it reaches its point of use, either as a liquid treatment agent, or as a fumigant gas after being stripped back out of the water solution.
• ClO₂ is highly soluble in water as well as in a variety of organic materials. Because it is so soluble, ClO₂ penetrates through materials that protect microorganisms from other biocides. For example, in water systems, bacteria can be protected by a polysaccharide film. Chlorine has difficulty penetrating this barrier because of its ionic nature in water. ClO₂ readily penetrates through this layer to kill underlying organisms. This quality makes ClO₂ highly effective in controlling cooling tower biofilms. Another example of its extreme liquid solubility and penetrability occurs in the petroleum production environment where ClO₂ in used to penetrate through a hydrocarbon layer to oxidize contaminants and kill bacteria being shielded by the hydrocarbon material.
• ClO₂ has more than twice the oxidative capacity of chlorine, but has lower oxidation strength. Because ClO₂ has lower oxidation strength, it is more selective in its reactions. Typically, ClO₂ will react with compounds that have activated carbon bonds such as phenols, or with other active compounds like sulfides, cyanides, and reduced iron and manganese compounds. Because chlorine is a more powerful oxidizer that ClO₂, it will react with a wider variety of chemicals, including ammonia. This property limits the overall effectiveness of chlorine as a biocide.
• ClO₂ exists as a gas under standard temperature and pressure (STP) conditions. Many potential fumigants, such as hydrogen peroxide and formaldehyde, are vapors, meaning they are in a liquid state at STP conditions and must be continually heated to keep them in an effective gaseous form. This property greatly limits their fumigation utility to very small areas where the required high temperature conditions can be maintained. Conversely, ClO₂ can be used to fumigate structures of any size with a single application, and has been proven effective in buildings as large as 14 million ft3.
• ClO₂ has been shown to be a highly penetrating biocide in its gaseous state. Because it is so penetrating, contaminated facilities can be fumigating with ClO₂ without first removing all porous substances. Through placement of surrogate spore strips, it has been proven that ClO₂ gas can readily penetrate through cloth, fabrics, paper, etc. and inactivate high concentrations of spores (e.g. 106 log spore strips). It has also been shown that ClO₂ will even penetrate beneath coffee cups, bottles, etc. that are sitting on hard surfaces and inactivate underlying microorganisms. By way of contrast, vaporized hydrogen peroxide has little, if any, penetrating ability. As such, all porous materials within a treatment zone must first be removed before fumigating with this agent, thereby leading to potential delays and unnecessary treatment and/or disposal costs.
• ClO₂ has an attractive toxicological profile relative to other compounds that could be used to fumigate contaminated structures. The potential adverse health effects associated with ClO₂ are largely acute in nature and generally related to its irritant properties. For example, ClO₂ exposure has been associated with irritation of the eyes, nose, and throat; coughing; wheezing; shortness of breath, etc. In contrast, many other potential fumigants, such as formaldehyde, ethylene oxide, and methyl bromide, have all been associated with serious acute and chronic adverse health effects, including potential carcinogenicity.
• ClO₂ doesn't need to be removed from surfaces or air spaces following treatment. Because it is a "free radical" molecule, ClO₂ decays naturally on its own, whether applied in liquid or gaseous form. For example, a gaseous ClO₂ concentration in the range of 750 ppmv will normally "disappear" completely without any action being taken in a period of only a few hours. In the event that quicker removal of ClO₂ gas is desired, this can also be readily accomplished by converting application emitters into gas strippers.
• ClO₂ doesn't leave any visible or harmful residues on surfaces following treatment. Any ClO₂ that is applied as part of either a liquid or gaseous treatment decays quickly to harmless concentrations of various sodium salts including chlorite, chlorate, and chloride. Animal testing has been conducted using a sensitive animal test species and concentrations of these salts several orders of magnitude higher than those found on surfaces following gaseous fumigation. The results of these tests demonstrate that these "microscopic" salt levels are completely non-irritating.

This combination of attractive properties, when considered together, formed the basis of a decision by the USEPA in October of 2001 that ClO₂ should be used to eliminate Bacillus anthracis (i.e. anthax) contamination from buildings affected by the 2001 anthrax attacks.