Chlorine vs. Chlorine Dioxide
It is generally agreed upon that chlorine and chlorine dioxide are two of the most commonly utilized secondary disinfectants, but are they interchangeable?
While chlorine dioxide has chlorine in its name, its chemistry is radically different from that of chlorine. The difference has to do with the way electrons interact with one another as we learned early on in chemistry that the product of two compounds can create a third that bears little resemblance. For instance, mixing two parts of explosive hydrogen gas with one of oxygen will form liquid water.
Chlorine has been the primary oxidant for years relative to secondary disinfection. Hypochlorite is an effective disinfectant when utilized within the right parameters. However, water quality, pH, and temperature can diminish the effectiveness of hypochlorite, especially where municipalities use monochloramines for disinfection purposes in which ammonia is present. Chlorine is a powerful oxidant capable of attacking most micro-organisms present in the water supply. A residual of 0.3 to 0.5 ppm is sufficient to minimize the organic growths that could foul building potable water. All forms of chlorine produce hypochlorous acid (HOCL) and hypochlorite ion (OCL¯) when added to water. The hypochlorous acid and hypochlorite ion, together, are called free available chlorine (FAC). Hypochlorous acid is 60 to 100 times more effective than hypochlorite ion at killing or oxidizing microorganisms and as such is the active killing form of chlorine in water. As pH drops, hydrogen ions attach to OCL¯ (the inactive or reserved part of free chlorine) to create HOCL. The dissociation of HOCI to the less microbicidal form depends on pH and the disinfecting efficacy of chlorine decreases with an increase in pH. Also, as HOCL gets used up by oxidizing and sanitizing, hydrogen ions instantly attach to OCL¯ to maintain the same equilibrium, as determined by pH. Other disadvantages of hypochlorite include corrosiveness to metals in high concentrations, inactivation by organic matter, discoloring or “bleaching” of fabrics, the release of toxic chlorine gas when mixed with ammonia or acid, and relative stability. Chlorine is coming under pressure from various environmental groups and its use may be curtailed as a result of this pressure. As well as providing disinfection, chlorine reacts with natural organic material present in the water to form products ranging from chloramines to trihalomethanes or THMs (e.g. chloroform). Some of these are considered as being carcinogenic.
Chlorine dioxide's behavior as an oxidizing agent is quite dissimilar to that of sodium hypochlorite. Like ozone, the predominant oxidation reaction mechanism for chlorine dioxide proceeds through a process known as free radical electrophilic (i.e. electron-attracting) abstraction rather than by oxidative substitution or addition (as in chlorinating agents such as chlorine or hypochlorite). Instead of combining with the aromatic rings, chlorine dioxide breaks these rings apart. In addition, as the use of chlorine dioxide increases, the generation of chlorinated organic byproducts fall dramatically. Chlorine dioxide's chemistry also explains why it is such an effective oxidant, as it is 2.5 times more powerful than chlorine gas and much more selective. In secondary disinfection applications, chlorine dioxide has broad spectrum activity over a wide range of micro-organisms and has the ability to penetrate biofilms and other heavily contaminated areas where chlorine treatment is simply not effective. Because of its increased efficiency, far less chlorine dioxide product needs to be introduced, eliminating waste and reducing handling costs and risk. Chlorine dioxide has been shown to be more powerful, easier to use, and more environmentally friendly than equivalent chlorine treatments.
Chlorine dioxide and chlorine, because of their fundamentally different chemistries, react in distinct ways with organic compounds, and as a result, generate very different by-products. As mentioned earlier, both chlorine and chlorine dioxide are oxidizing agents. Chlorine has the capacity to take in two electrons, whereas chlorine dioxide can absorb five. This property, along with the ways chlorine combines with certain organic materials to form chlorinated organics, explains the superiority of chlorine dioxide-based products. The advantage of chlorine dioxide over chlorine and chloramines is based on the reduction of taste and the prevention of trihalomethanes and other chlorinated disinfection by-product (DBP) formation. Unlike chlorine, it is easy to generate on-site although sodium chlorite in its dry form is highly unstable and dangerous. It does produce two DBPs of its own, chlorite and chlorate. Also, where free chlorine is released during chlorine dioxide generation, there is a risk that halogen-substitute DBPs may be formed. In general, chlorine dioxide continues to be the preferred method of secondary disinfection over chlorine for the reasons outlined above. Even more true is the statement that chlorine and chlorine dioxide are not interchangeable.