ClO2 functions as a highly selective oxidant with less environmental impact than other biocides. It only reacts with substances that surrender electrons. The available energy (strength) of a given chemical to act as an oxidizer (electron receiver) or as a reducer (electron donor) is commonly called the oxidation-reduction potential (ORP). This property is measured in volts (V) and millivolts (mV).
ClO2 has 0.95 V of oxidation potential, which is a mild oxidizer compared to many other common disinfectants used in water treatment applications. However, a single ClO2 molecule can accommodate up to five electrons, which gives it 2.6 times the oxidative capacity of chlorine. This makes ClO2 a very efficient disinfectant.
In typical industrial applications, ClO2 is used to oxidize compounds that have substituted carbon bonds, such as phenols, or active reducing compounds, such as sulfides, cyanides, iron and manganese compounds at very low dosages compared to alternative oxidative chemistries.
One growing application for ClO2 is in bromide-rich source waters, where oxidizers with higher ORP values will react with the bromide in the source water to form bromate, a highly toxic and regulated DBP. ClO2 has a lower ORP value than bromine, and, therefore, does not oxidize bromide to form bromate.
ClO2 is an effective biocide, because it selectively oxidizes sulfur-containing compounds, complex amines and organic compounds used in biochemical protein synthesis. Because it has a lower oxidation potential than many other oxidizing biocides, it does not react with as wide a variety of organic and inorganic materials, which reduces the required effective dosage and minimizes the potential for the formation of unwanted DBPs in general.
As shown in Figure 3, at comparable dosage concentrations in heavily polluted waters, the residual concentration of ClO2 is normally much higher than the residual concentration of chlorine. A good example of this significant advantage is in ammonia-contaminated water. Specifically, ClO2 does not react with ammonia to form chloramines, a much weaker oxidant and disinfectant than chlorine.