The History Of CLO2
Chlorine Dioxide is becoming an increasingly popular choice for biocidal applications in industries around the world.
Although lower chemical costs and better generation equipment have increased its market profile significantly over recent years, the core chemistry is well established and far from new. Here we cover what is CLO2, key facts about the chemical, and its development and use over time as a powerful disinfectant.
CLO2 Fact File
What is Chlorine Dioxide?
Chlorine Dioxide is a small, volatile and very strong molecule consisting of 1 Chlorine atom and 2 oxygen atoms. Abbreviated to CLO2, Chlorine Dioxide exists as a free radical in dilute solutions
- Molecular weight of 67.45.
- It is a gas at normal temperatures and pressures.
- Melting point of -59oC.
- Boiling point of 11oC.
- Yellowish/green colour and has an odour similar to that of Chlorine.
- Denser than air and is water soluble at standard temperatures and pressures up to 2500ppm.
- Explosive in air at concentrations >10%. It is therefore normally generated in-situ within an aqueous solution at <0.2%
- Some pre-mixed solutions are sold on the market at concentrations of 0.6%, however these are hazardous to transport and costly. Most CLO2 is applied using on site dosing and generation equipment from companies like ChloroKlean.
- It will decompose in the presence of UV, high temperatures >70oC, and high alkalinity (>pH12).
CLO2 Timeline
- 1811 first discovered by Sir Humphrey Davey
- 1944 First commercial application. Used as a biocide/taste and odour control agent in domestic water at Niagara Falls in the USA.
- 1977 Three thousand municipal water systems achieving biological control using Chlorine Dioxide.
- 1980’s Chlorine Dioxide gradually replacing Chlorine in many industries.
- Pulp and Paper industry – used as a bleaching agent.
- Industrial water treatment – used as a biocide and as an odour control agent.
- Food processing – used as a sanitiser.
- 1990’s Increasingly used for the secondary disinfection of potable water.
- 2005 – New generation equipment and control technology introduces CLO2 as a practical alternative to many industrial applications.
More Effective, Less Cost And Kind To The Environment
CLO2 Advantages Over Chlorine
Higher yield & greater cost efficiencies – Chlorine Dioxide has a higher oxidation capacity, and a lower oxidation strength than most species of chlorine, making it at least 2.6 times more powerful per ppm according to WHO CT values.
No carcinogenic by-products & no bad taste occurrences in water – Chlorine Dioxide acts only by oxidation and does not combine with organic compounds to form environmentally hazardous by-products such as Trihalomethane and other chlorinated organic compounds that have been listed as potentially carcinogenic.
Less corrosive – Chlorine Dioxide has a lower oxidation potential and does not hydrolyse to form an acid, and therefore is less corrosive.
Works over a wide pH – The effectiveness of chlorine is very pH dependent, and is almost ineffective above pH8. Chlorine Dioxide is effective at all pH’s below 12.
Effective against complex organisms – Chlorine Dioxide has been found to be effective against complex organisms such as cysts and protozoa including Cryptosporidium, Giardia and amoeba. Chlorine is not.
Destroys biofilm completely at source by penetrating polysaccharide layers to destroy pseudomonas and other base bacteria within the biofilm. Chlorine does not.
CLO2 – The Safe, Broad Spectrum Biocide
Oxidising biocides such as ozone, hydrogen peroxide and peracetic acid are known for their instability and difficulty in handling and applying safely.
Whilst Chlorine Dioxide belongs to the same family of biocides as the oxidising biocides, sharing more in common with them than its “chlorine” namesake, Chlorine Dioxide has a number of advantages over other oxidising biocides that make it more suitable for use in a number of water treatment applications.
When compared with other oxidising biocides, Chlorine Dioxide has a significantly lower oxidation strength – this means that it reacts with fewer compounds, such as organic compounds and ammonia, yet is strong enough to attack the disulphide bonds found in the membranes of bacteria and other biological material.
This process of “selective oxidation” allows the Chlorine Dioxide biocide to be targeted where it is needed most, disinfecting areas quickly and at lower dose rates, leading to greater cost efficiencies.
As an example, where Hydrogen Peroxide based products have been promoted for use in water treatment, this is often at dose rates 10-30 times greater than CLO2, leading to difficulties in handling peroxide based products >15% that are now listed as “Explosives Precursors” under EU Regulations.
The Powerful Bactericide And Virucide
The oxidising properties and the radical nature of Chlorine Dioxide make it an excellent virucidal and bactericidal agent in a large pH range.
In 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.
Compounds within the cells and on the surface of cell membranes that contain oxidisable material react with chlorine dioxide, causing cell metabolism to be disrupted. Chlorine dioxide also reacts directly with disulphide bonds in the amino acids and the RNA in the cell. Unlike non-oxidizing disinfectants, chlorine dioxide kills microorganisms even when they are inactive. The oxidative load placed on the cells by the action of chlorine dioxide mean that most microorganisms are unable to build up resistance to chlorine dioxide.
In practical terms however, few bacteria live alone, and they are most often found in water and on surfaces in the form of a “biofilm” which is a close association of many millions of bacteria. Many biocides have particular problems in penetrating this biofilm, due to the polysaccharide “glue” that is secreted by bacteria such as Pseudomonas to hold the biofilm together. Unlike most biocides, chlorine dioxide can effectively penetrate the polysaccharide layer of biofilm without being used up in reacting with the inert sugars. This allows the CLO2 to act on the bacteria themselves, destroying the biofilm.
Chlorine dioxide is one of a number of disinfectants that are effective against Giardia Lambia and Cryptosporidium oocysts, which cause diseases such as cryptosporidiosis in public drinking water supplies. A number of public water works are now utilising chlorine dioxide generation systems alongside UV systems in order to provide complete protection from Cryptosporidium.
The Key Differences
While chlorine dioxide has “chlorine” in its name, its chemistry is radically different from that of chlorine.
As we all learned in high school chemistry, we can mix two compounds and create a third that bears little resemblance to its parents. For instance, by mixing two parts of hydrogen gas with one of oxygen – liquid water is the formed. We should not be misled by the fact that chlorine and chlorine dioxide share a word in common. The chemistries of the two compounds are completely different.
Chlorine and chlorine dioxide are both oxidising agents (electron receivers). However, chlorine has the capacity to take in two electrons, whereas chlorine dioxide can absorb five. This means that, mole for mole, CLO2 is 2.6 times more effective than chlorine.
If equal, if not greater importance is the fact that chlorine dioxide will not react with many organic compounds, and as a result CLO2 does not produce environmentally dangerous chlorinated organics. For example; aromatic compounds have carbon atoms arranged in rings and they may have other atoms, such as chlorine, attached to these rings, to form a chlorinated aromatic – a highly toxic compound that persists in the environment long after it is produced.
Chlorine dioxide’s behaviour as an oxidising agent is quite dissimilar. 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). This means that chlorinated organic compounds such as THMs and HAAs are not produced as a result of disinfection using chlorine dioxide.
CLO₂ Structure And Its Reactions
Because of its radical structure, Chlorine Dioxide has a particular reactivity – totally different from that of chlorine or ozone.
The electrophilic nature of chlorine or hypochlorous acid can lead, through reaction of addition or substitution, to the formation of organic species while the radical reactivity of chlorine dioxide mainly results in oxycarbonyls.
Generally Chlorine Dioxide (CLO2) rapidly oxidises phenol type compounds, secondary and tertiary amines, organic sulphides and certain hydrocarbon polycyclic aromatics such as benzopyrene, anthracene and benzoathracene.
In general, Chlorine Dioxide will not react on double carbon bonds, aromatic cores, quinionic and carboxylic structures as well as primary amines and urea.
Commercial applications have shown that Chlorine Dioxide can effectively oxidise many compounds considered to be waste and water pollutants. The table below lists a selection of pollutants found in various industries from our files, and demonstrates the wide range of possible applications for Chlorine Dioxide. ChloroKlean possess over 25 years application expertise in chlorine dioxide technology in challenging applications.
Aldehydes
In general, Chlorine Dioxide can oxidise an aldehyde to its corresponding carboxylic acid. Aldehydes are produced by a number of common industrial processes. Their treatment is a common problem, especially so in the photographic industry. Formaldehyde is a major component in the formulations used in photo processing. Chlorine Dioxide oxidises formaldehyde to formic acid and finally to carbon dioxide. Para formaldehyde can be depolymerised and eliminated completely by oxidation with Chlorine Dioxide.
Amines and Mercaptans
The major sources of odorous substances such as mercaptans and substituted amines include the chemical and petroleum industries, cooking and sanitary processes, animal feedlots and rendering plants.
Between pH 5 & 9, 4.5 parts by weight of Chlorine Dioxide instantaneously oxidises 1 part by weight of mercaptan (expressed as sulphur) to the respective sulphonic acid or sulphonate compound, thus destroying the mercaptan odour. Similarly, chlorine dioxide reacts with organic sulphides and disulphides destroying the original odour.
Secondary and tertiary amines are also present in many waste water’s, causing their own unique odour problems. The oxidation of amines with Chlorine Dioxide depends on the pH of the reaction mixture and the degree of substitution of the amine.
Between pH 5 and 9, an average of 10 parts by weight of Chlorine Dioxide oxidises 1 part by weight of a secondary aliphatic amine (expressed as nitrogen) removing all traces of amine odour. The higher the pH of the reaction mixture (chlorine dioxide and tertiary and/or secondary aliphatic amines) the more rapidly oxidation proceeds.
THM Precursors
The key to understanding why Chlorine Dioxide is so effective can be found in the differences in the reactions of chlorine dioxide and chlorine with Tri-halomethane (THM) precursors such as humic and fulvic acids.
Chlorine reacts with THM precursors by oxidation and electrophilic substitution to yield both volatile and non-volatile chlorinated organic substances (THMs).
Chlorine Dioxide, however reacts with THM precursors primarily by oxidation to make them non-reactive or unavailable for THM production. This means that pre-treatment with chlorine dioxide has an inhibiting effect on THM formation when chlorine is subsequently used.
Pesticides
Chlorine Dioxide can oxidise toxic materials to less toxic materials. Specifically, Methylchlor (DMDT) and Adrian react with CLO2. With parathion, the reaction is slow near to pH 7; however, when pH is above 8, less biodegradable herbicides such as paraquat and diquat are eliminated within a few minutes.
Algae/Slime
Chlorine Dioxide has been sown to be effective in controlling algae growth. In one study, CLO2 was found to be more effective than copper sulphate, at comparable treatment costs. Chlorine Dioxide is believed to attach the pyrolle ring of the chlorophyll. This leaves the ring and leaves the chlorophyll inactive. Since algae cannot function without chlorophyll metabolism, they are destroyed. The reaction of Chlorine Dioxide with algae and their essential oils forms tasteless, odourless substances.
Algae control is carried out by adding chlorine dioxide to the reservoir at night (To prevent photolytic decomposition of CLO2) The algae killing action is fast enough to be effective before the sun rises. A dosage of 1 mg/litre has been reported to control algae populations
Sulphides
Between pH 5 and 9, an average of 5.2 parts by weight of Chlorine Dioxide instantaneously oxidises 1 part by weight of hydrogen sulphide (expressed as sulphide ion) to the sulphate ion.
Many industrial processes produce sulphide-containing gases and waste products. These are generated, for example, during petroleum refining, coal coking, black liquor evaporation in kraft pulping, viscose rayon manufacture and natural gas purification. These gases and wastes are frequently scrubbed with alkaline solutions and require treatment before discharge.
Nitrogen Compounds
Nitrogen oxides are dangerous and corrosive. Nitrous Oxide (NO) and nitrogen dioxide (NO2) are industrial effluents which result from fuel combustion, nitric acid manufacture and use, and from metal finishing operations which use nitrates, nitrites or nitric acid. Other sources include chemical processes in which nitrogen compounds are used as reagents.
Chlorine Dioxide has been used to scrub these contaminants. Nitric Oxide contained in gas discharges from coke kilns may be eliminated by oxidation by Chlorine Dioxide. This process is particularly convenient for continuous operation.
Cyanides
Cyanide compounds originate from processes such as metal plating, steel case hardening, pickle liquor neutralising, gold and silver ore refining and blast furnace stack gas scrubbing. Chlorine Dioxide oxidises simple cyanide to cyanate (a less toxic substance) and/or carbon dioxide and nitrogen. The end products depend on reaction conditions.
In neutral and alkaline solutions below pH 10, an average of 2.5 parts by weight of chlorine dioxide oxidises 1 part by weight of cyanide ion to cyanate. Above pH 10, an average of 5.5 parts by weight of Chlorine Dioxide oxidises 1 part by weight of cyanide ion to carbon dioxide and nitrogen. Chlorine Dioxide does not react with cyanate ion, nor has it been observed to form cyanogen chloride during the oxidation of cyanide.
Chlorine Dioxide also oxidises thiocyanate to sulphate and cyanate. In neutral solutions, an average of 3.5 parts by weight of chlorine dioxide oxidises 1 part by weight of thiocyanate ion.
For more interesting facts on Chlorine Dioxide see these pages on our ChloroKlean brand website:
What is Chlorine Dioxide?
Advantages of CLO2 Over Chlorine
Advantages Over Other Oxidising Biocides
No Harmful By-Products
Environmental safety is a key advantage of chlorine dioxide when comparing with alternative biocides such as chlorine.
Unlike chlorine, CLO2 will react to form mainly inorganic disinfection by products, the predominant species being chlorite. Chlorite will subsequently reduce to form harmless chloride. The speed of this reaction depends upon a number of factors, however within saltwater conditions this can be as low as 5 minutes.
Poorly designed or tuned chlorine dioxide generation equipment can lead to the production of chlorate as a disinfection by product. ChloroKlean systems are extensively tested to minimise the production of chlorate, since this reduces the biocidal efficiency of the process.
Generally, it is the concentration of chlorite residuals that is the “monitored” DBP of chlorine dioxide. Modern generation systems such as those produced by ChloroKlean are able to monitor the downstream residual DBP and adjust the dose rate to ensure that environmental limits are not breached. In special cases, downstream reactions can be used to remove excess chlorite residual from the water stream.
It is important to note that the disinfection by products of chlorine dioxide are easily managed with the correct experience and advice, and do not present nearly the same scale of problem as found with other biocides with a higher oxidation potential. Unlike ozone (O3), chlorine dioxide does not oxidise bromide (Br-) ions into bromate ions (BrO3-) which have been identified as carcinogenic. Additionally, chlorine dioxide does not produce large amounts of aldehydes, ketones, or other disinfection by products that originate from the ozonisation of organic substances.
For more information on this, visit our ChloroKlean brand website.
