Chlorine Dioxide Chemistry
ClO2 is a powerful disinfectant that reacts rapidly via oxidation to provide effective microbiocidal impact. It delivers broad spectrum performance against bacteria, fungi, algae, viruses, and parasitic microorganisms. ClO2 kills vegetative microorganisms and effectively deactivates sporulated species.
Vegetative bacteria are killed quickly and at low effective dose levels, reducing the time required to achieve the contact time (CT) values for inactivation. ClO2 rapidly inactivates waterborne viruses like Rotavirus and can also be used to kill both Giardia cysts and Cryptosporidium oocysts.
ClO2 disinfects via two separate mechanisms. As a permeable gas, it migrates through the cell membrane and reacts selectively with cellular components (amino acids: cystine, tryptophan and tyrosine) and viral capsid proteins. It also inhibits critical cell physiological functions, including the disruption of protein synthesis via the oxidation of the disulfide bonds in amino acids and alteration of the permeability of the outer cell membrane.
The collective damage that results from this multi-faceted cellular attack makes ClO2 both broad spectrum and efficacious at very low dose rates. When used for bacterial control, the concentration can be similar to that required for free chlorine. However, when used for control of Giardia and viruses, ClO2 is considerably more efficacious than either chlorine or monochloramine. Its efficacy is also less impacted by water temperature (see Table I).
In wastewater treatment applications,ClO2 is used to disinfect treated water prior to discharge, in an effort to minimize chlorinated organic formation.ClO2 is also used to oxidize hydrogen sulfide odors and minimize potential corrosion concerns at the head works, sludge press and collection systems. For oil and gas applications,ClO2 reduces bio-fouling and provides superior water disinfection properties to allow producers and well operators a simple solution to control aerobic and anaerobic bacteria.ClO2 also oxidizes both hydrogen sulfide and iron sulfide in unconventional hydraulic fracturing and produced water treatment systems.ClO2 is also used in hospital and healthcare environments as a sanitization/sterilization agent and for Legionella control.
ClO2 chemistry is well established technology that is currently being used in a wide variety of industrial, oil and gas, food and municipal applications all around the globe. As the use of traditional chlorine or bleach continues to be subject to more stringent regulatory compliance requirements,ClO2 is likely being specified as a replacement for chlorine in cutting-edge applications.
The following is a list of the major applications whereClO2 excels:
- Ammonia plants
- Aquifer storage and recovery (ASR)
- Brewing and beverage
- Cooling tower
- Ethanol fermentation
- Flume water
- Fruit and vegetable processing
- Hard surface sanitizer
- Industrial process waters
- Leather bleaching
- Legionella control
- Mollusk control
- NOx oxidation
- Odor abatement/control
- Oil and gas hydraulic fracturing
- Paper slimicide
- Potable water disinfections
- Power utilities
- Process waters microbial control
- Pulp bleaching
- Rendering odor control
- RO membranes (RO/NF/UF/MF)
- SOx oxidation
- Wastewater deodorization
- Wastewater oxidation
- Zebra mussel control
Microorganisms are known to alter their structure to develop immunity to conventional microbiocides, requiring shock treatments with higher dosages or alternate microbiocidal chemistries. This is not known to occur with ClO2 treatment.
ClO2 reacts with the basic cell structure, and shuts down the metabolism of microorganisms, rendering them incapable of modifying their cell structure and, therefore, making the use of alternating chemistries unnecessary.
ClO2 effectively reduces biofilm formation and provides excellent ongoing control of biofilm growth, particularly in water systems. Biofilms in water systems are known to reduce flow and impede heat transfer efficiency in industrial piping and heat exchangers.
Some water treatment processes, such as ozone oxidation, are known to enhance biofilm formation, unless steps are taken to minimize the assimilable organic carbon (AOC) produced via the fractionation of organics. It is important to note that AOC is a food source for biofilm formation.
Because ClO2 is a true dissolved gas in solution, it can rapidly diffuse and penetrate the polysaccharide-based biofilm substrate, killing microbes both throughout and beneath the biofilm (see Figure 1).
ClO2 oxidation disrupts the physical biofilm, inducing a sloughing of the biofilm. Routine re-application of ClO2 can be used to inhibit the regrowth of new biofilm. In contrast, because alternative chlorine-based chemistries are inefficient at penetrating biofilm substrate, they have limited remedial benefit.
No. Although chlorine is part of the chemical structure of ClO2, a key chemical attribute of ClO2 reaction chemistry is that it does not produce halogenated DBPs from the oxidized compound. ClO2 only reacts with substances that give up electrons in true redox reactions. Therefore, ClO2 functions as a highly selective oxidant when it reacts with organic compounds, because it only attacks electron-rich bonds in the organic compounds.
In contrast, because pH impacts the reaction chemistry of chlorine, it can attach a chlorine atom or substitute a chlorine atom into an organic compound, which leads to the formation of undesirable chlorinated organics, such as chloroform, bromoform, dioxins or THMs. These halogenated DBPs are coming under increasing regulatory pressure around the globe.
The predictable redox reaction chemistry of ClO2 with organics is also important, because it significantly reduces the formation potential of halo acetic acid precursors, such as aldehydes, ketones and ketoacids, which then lowers the potential for THM formation in finished treated drinking water.
ClO2 gas is highly soluble in water and does not hydrolyze into ions. This enables it to maintain its oxidative and biocidal properties over a broad pH range—from 4 to 10. This characteristic is contrary to traditional oxidizing biocides (chlorine and bromine), which have a narrow pH efficacy range and are relatively ineffective antimicrobials in alkaline pH environments (see Figure 2).
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).
As shown in Table 2, 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 ClO2molecule 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 microbiocide, 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 microbiocides, 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.
ClO2, chlorate (ClO3-) and chlorite (ClO2-) can all eventually decompose into chloride ion and typically be re-associated with earth mineral cations to form simple salts (NaCl). In the case of ClO2, the first reaction takes up an electron and reduces to chlorite ion:
ClO2 + R(e-) -> ClO2–
The chlorite ion is then available for further redox reactions, including environmental reducing agents, photodecomposition and biochemical uptake by bacterium, becoming a chloride ion:
ClO2– + 4 R(e-) -> Cl- +O2
These reactions illustrate that ClO2 can be reduced to chloride ion, and, that during this reaction process, it accepts five electrons. Chloride ion is the most ubiquitous ion in the earth’s environment. A similar amount of chloride ion will be returned to the environment from the ClO2 dose applied.
Sir Humphrey Davy is credited with the discovery of ClO2 in 1814. In 1944, ClO2 was first used commercially in the United States—for taste and odor control in the city of Niagara Falls, NY drinking water supply system. Twelve years later, the first large-scale ClO2 use in Europe was in the drinking water supply for the city of Brussels, Belgium.
In most industrial and municipal applications, ClO2 is generated on-site at the point of use, because it cannot be safely compressed like chlorine gas—making it difficult to transport. The majority of this ClO2 is produced in specially designed generation equipment systems, which safely combine sodium chlorite (NaClO2)—the primary chemical precursor for ClO2—in combination with one or two additional chemical precursors to produce ClO2 on-site.
ClO2 chemistry has been rapidly gaining acceptance as the best alternative to traditional disinfecting chemistries, especially when regulatory compliance or broad spectrum microbial performance is a challenge. As a result, there are now thousands of ClO2 applications to treat water throughout the world.
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