What Is Chlorine Dioxide? A Detailed Guide to a Wastewater Treatment Term
Chlorine dioxide (ClO₂) is a chlorine-oxygen compound whose molecule contains two oxygen atoms. It has strong oxidising properties and is widely used for disinfection, sterilisation, bleaching and the removal of organic pollutants. In the gaseous state it is yellow-green with a pungent odour, and at ambient temperature and pressure it is relatively unstable, readily decomposing into chlorine and oxygen. It differs from conventional chlorine gas in that its decomposition products are more environmentally benign and do not form toxic halogenated by-products.
The chlorine atom (Cl) has seven valence electrons and takes part in covalent bonding with the oxygen atoms. Because oxygen is strongly electronegative, the chlorine atom carries a partial positive charge. The oxygen atom (O) has six valence electrons and bonds covalently with chlorine; since chlorine is less electronegative, the shared electrons are drawn toward oxygen, giving oxygen a partial negative charge. The two oxygen atoms are not directly bonded to each other but are linked through the chlorine atom, so their interaction is maintained by pi-pi interaction or van der Waals forces, which makes the chlorine dioxide molecule relatively unstable.
The history of chlorine dioxide goes back to Carl Wilhelm Scheele, who in 1774 first isolated chlorine gas and found that chlorine reacting with oxidising agents could produce chlorine oxide. This discovery laid the foundation for later research on chlorine dioxide. Although Scheele did not discover chlorine dioxide directly, he established the theoretical basis for the study of chlorine oxides. In 1811, Joseph Proust and other chemists began to study the reaction of chlorates such as sodium chlorate with acids. They found that chlorates (for example sodium chlorate reacting with hydrochloric acid) could produce a yellow gas, which later research showed to be chlorine dioxide.
In 1834 the French chemist Henri Vauquelin identified and named chlorine dioxide (ClO₂) and carried out related experiments. Through experiment he confirmed the existence of the gas and showed that its properties differed from those of chlorine.
In 1853 Joseph Proust and other scientists studied the properties of chlorine dioxide further and determined its molecular structure. They used gas discharge methods to study the composition and chemical properties of the gas and confirmed that it is an oxygen-containing chlorine compound.
At the end of the 19th century, scientists explored the chemical reactivity of chlorine dioxide further through reactions with chlorates. Experiments showed that chlorine dioxide is a strong oxidant that reacts with many organic and inorganic substances, and its potential in water treatment and disinfection gradually attracted attention.
In the early 20th century Erman Bauer and others carried out a systematic quantitative analysis of chlorine dioxide, using gas chromatography to study its reactivity and confirming its effectiveness as a disinfectant.
In the 1950s, with advances in water treatment technology, the bactericidal and disinfecting effects of chlorine dioxide came to be recognised and applied in industry. In that decade American scientists and engineers began to study its use in drinking water treatment. Chlorine dioxide became an important disinfectant in water purification, performing particularly well in removing organic matter, algae and microorganisms.
In the 1960s industrial production and application of chlorine dioxide spread gradually. Companies in the United States and Europe began to produce chlorine dioxide by chlorate reduction for large-scale water disinfection and industrial bleaching. As technology advanced, production became more efficient and economical, and chlorine dioxide gradually became a common chemical in water and wastewater treatment.
In the 1970s, with growing concern for environmental protection, chlorine dioxide came to be regarded as more environmentally friendly than conventional chlorine gas. It does not produce toxic trihalomethane by-products, so its environmental impact during disinfection is small, and it became the preferred chemical in many industries.
From the 1980s to the present, modern technology has broadened the application of chlorine dioxide further. It is used not only in water treatment and the food industry but has also entered pharmaceuticals, medical disinfection, papermaking, textiles and many other sectors. Because of its strong oxidising power, its bactericidal efficiency in water treatment has been widely adopted, and it has earned a place among green, environmentally friendly chemicals.
On this basis, industry experts have also carried out extensive research and improvement.
Contribution: by discovering chlorine gas and studying the reactions of chlorates he laid the foundation for subsequent research on chlorine dioxide. He was one of the discoverers of chlorine gas and, although he did not study chlorine dioxide directly, his work inspired later chemists to investigate chlorine-oxygen compounds.
Contribution: in 1834 he named and confirmed the existence of chlorine dioxide, making an important contribution to the study of chlorine-oxygen compounds. He studied experimentally the reaction of chlorates with acids to produce chlorine dioxide and was the first to demonstrate its oxidising properties.
Contribution: further investigation of the molecular structure and chemical characteristics of chlorine dioxide, confirming its molecular composition and chemical behaviour and providing important support for the chemical theory of chlorine dioxide.
In the 20th century scientists used modern analytical techniques such as gas chromatography and infrared spectroscopy to reveal further the chemical properties, molecular structure and applications of chlorine dioxide. Industrial application developed rapidly, especially in water treatment and environmental protection, where its strong oxidising power is widely used to remove harmful microorganisms, algae and organic matter from water.
The chemical formula of chlorine dioxide is ClO2 and its molecular mass is 67.452. At room temperature it is a yellow-green to orange gas with a boiling point of 11°C and a melting point of -59°C. When the chlorine dioxide content reaches 14-17 ppm most people can detect its presence, and at 45 ppm it causes irritating damage to the sense of smell. Chlorine dioxide is strongly oxidising; compared with other oxidants its standard potential is about 1.50 V. The four main redox reactions of chlorine dioxide in water involve an electrically neutral compound. In drinking water disinfection, chlorite (ClO2−) is the main reaction product, accounting for about 70% of the chlorine dioxide converted, with the remainder being chlorate (ClO3−) and chloride (Cl−). The bactericidal effect of chlorine dioxide on microorganisms is not affected by high pH, ammonia, nitrogen-containing compounds or organic compounds, and it does not give rise to bromide (hypobromous acid) or form organic bromine compounds.
It is worth noting that the technology and standards in this field are also continuously developing and improving.
Because chlorine dioxide is produced by chemical synthesis it has no separate natural resource source. Production depends on sodium chlorate (NaClO₃) or calcium chlorate (Ca(ClO₃)₂) as raw materials; these are widely available, and sodium chlorate and calcium chlorate can be obtained by electrolysis of sodium chloride or by chemical reaction.
On this basis, industry experts have also carried out extensive research and improvement.
The preparation of chlorine dioxide usually relies on chemical reduction of chlorates such as sodium chlorate and calcium chlorate to chlorine dioxide. A common sodium chlorate reduction method for preparing chlorine dioxide in the laboratory is described in detail below.
Chlorine dioxide (ClO₂) can be prepared by reducing sodium chlorate (NaClO₃). In this reaction sodium chlorate reacts with a strong reducing agent, usually hydrochloric acid (HCl), to produce chlorine dioxide gas, chlorine gas (Cl₂) and water.
On this basis, industry experts have also carried out extensive research and improvement.
Condenser: used to cool and collect the reaction products.
In addition, this technology has wide application and practice in related fields.
Prepare the apparatus: make sure the reaction flask, gas collection device and other equipment are clean and correctly connected. Connect the condenser to the reaction flask and prepare a gas collection bottle or gas bag.
At the same time, the associated processes and equipment are being continuously optimised and upgraded.
Start the reaction: once hydrochloric acid is added the reaction begins immediately and chlorine dioxide gas is evolved with the bubbling. A moderate temperature must be maintained to ensure the reaction goes to completion.
End of reaction: during the reaction the sodium chlorate solution is gradually consumed and the rate of chlorine dioxide generation slows. When the experiment is finished, close the gas collection device.
Product handling: the collected chlorine dioxide solution or gas can be converted into other chlorine-oxygen compounds, such as sodium chloride and sodium chlorate solutions, by further dilution or appropriate measures.
Avoid high concentrations of chlorine dioxide: at high concentration it is extremely unstable and readily decomposes into chlorine and oxygen, so the generation of high-concentration chlorine dioxide must be avoided.
Strictly control the temperature: at too high a reaction temperature chlorine dioxide decomposes unstably, so excessive temperature must be avoided and the reaction is usually kept in the 30-40°C range.
In addition, many factors must be considered in practical engineering applications.
Solution: control the amount of chlorine dioxide generated during the reaction so that it does not accumulate in the apparatus, and maintain appropriate temperature and concentration.
At the same time, the associated processes and equipment are being continuously optimised and upgraded.
Chlorine oxides are compounds formed by the reaction of chlorine with oxygen, and chlorine dioxide (ClO₂) is an important chlorine oxide. Besides chlorine dioxide there are chloric acid (HClO₃), dichlorine monoxide (Cl₂O), dichlorine dioxide (Cl₂O₂) and others.
Chlorine dioxide is itself one of the chlorine oxides. It is a strong oxidant commonly used for disinfection, bleaching and as an oxidant in water treatment. Its oxidising power is stronger than that of chlorine gas (Cl₂), and it does not readily form hydrogen chloride (HCl).
It is worth noting that the technology and standards in this field are also continuously developing and improving.
Chlorine nitrides are compounds of chlorine and nitrogen, the most important being nitrogen trichloride (NCl₃). Nitrogen trichloride and chlorine dioxide are related in some reactions, especially where chlorine gas (Cl₂) is present.
On this basis, industry experts have also carried out extensive research and improvement.
Chlorine dioxide (ClO₂) and chlorine oxides such as chloric acid and dichlorine monoxide can be interconverted. Through reduction reactions chlorine dioxide can be converted into chloric acid, dichlorine monoxide and the like, and conversely chloric acid and dichlorine monoxide can be reduced to chlorine dioxide.
There is a reversible relationship between chlorine dioxide and chloric acid. Under suitable conditions chlorine dioxide can be reduced to chloric acid, or conversely chloric acid can decompose to chlorine dioxide.
It is worth noting that the technology and standards in this field are also continuously developing and improving.
Chlorine dioxide (ClO₂) is a strong oxidant with a wide range of applications, particularly in water treatment, bleaching, disinfection, wastewater treatment and certain industrial processes. Thanks to its high oxidising efficiency and environmentally friendly character, its use is growing, especially as a replacement for conventional disinfectants.
Fewer by-products: disinfection with chlorine dioxide produces relatively few by-products and no harmful trihalomethanes (THMs) or other toxic by-products.
Broad-spectrum disinfection: chlorine dioxide is highly effective against bacteria, viruses, fungi and algae. It effectively removes pathogenic microorganisms from water, including bacteria, viruses and protozoa.
Urban tap water plants in the United States, Europe, Japan and other regions commonly use chlorine dioxide as a replacement for chlorine gas, especially where organic pollutants in the water must be controlled.
Chlorine dioxide is also used for industrial wastewater treatment, particularly in the petrochemical, papermaking and textile industries. It effectively degrades organic matter in wastewater and reduces the concentration of toxic chemicals.
In addition, many factors must be considered in practical engineering applications.
In the United States, Europe and other countries chlorine dioxide is approved as a food disinfectant and is used in agriculture, especially in the post-treatment of fruit and vegetables.
At the same time, the associated processes and equipment are being continuously optimised and upgraded.
Chlorine dioxide is also used in air purification, effectively removing bacteria, moulds and viruses from the air, especially in poorly ventilated or high-risk environments.
It is worth noting that the technology and standards in this field are also continuously developing and improving.
Hazard of chlorine gas: the reaction producing chlorine dioxide also generates chlorine gas (Cl₂), a toxic gas, so experiments must be carried out with good ventilation, preferably in a fume hood, to avoid chlorine leakage.
As a strong oxidant, chlorine dioxide is also used for waste gas treatment, especially for emissions containing organic gases and odours. It effectively removes organic matter, odour and other harmful substances from the gas stream.
In addition, from an industry perspective, market demand is also driving technological progress.
Chlorine dioxide also has potential applications in environmental remediation, degrading harmful organic pollutants in soil and water bodies, particularly in the clean-up of oil contamination.
Chlorine dioxide is also used as a disinfectant in oral care. For example, it is used in toothpastes and mouthwashes to help clean the mouth, remove odours and provide a degree of antibacterial effect.
In wastewater reuse systems, chlorine dioxide is used to remove organic pollutants so that the treated water can be safely used for agricultural irrigation or industrial cooling.
Skin contact: chlorine dioxide can cause severe irritation of the skin and eyes, and contact may cause burns or blisters. At higher concentrations its corrosiveness to skin is strong.
In addition, many factors must be considered in practical engineering applications.
Storage: chlorine dioxide should be stored in a cool, ventilated, dry place, away from high temperature and ultraviolet light, and away from reducing agents, organic matter and other chemicals that may trigger reactions. Sealed containers should be used to prevent gas leakage.
On this basis, industry experts have also carried out extensive research and improvement.
Oxidative pollution: the oxidising power of chlorine dioxide means it readily reacts with organic matter in water, producing some oxidation by-products. Although these by-products are of low environmental concern, overdosing or incomplete reaction can lower the dissolved oxygen level in the water and affect water quality.
If chlorine dioxide leaks into soil it may adversely affect the soil microbial community and plant growth. Because it is a strong oxidant it can destroy organic matter in the soil, changing its physical and chemical properties and affecting plant growth.
Microbial impact: the soil microbial community may be inhibited by the oxidising action of chlorine dioxide, affecting biological activity and the natural purification function of the soil.
In addition, many factors must be considered in practical engineering applications.
Waste gas may be generated during the production, transport and use of chlorine dioxide. Such gas usually contains small amounts of unreacted chlorine dioxide or its by-products such as chlorine, and must be purified by suitable methods before discharge to the environment.
Chemical absorption: the waste gas can be absorbed in a solution, for example sodium hydroxide or potassium hydroxide solution, reacting to form sodium chloride or potassium chloride and thereby removing chlorine dioxide.
Chlorine dioxide wastewater comes mainly from industrial wastewater treatment, from reaction and generation processes, and from equipment cleaning. Such wastewater may contain chlorine dioxide, chlorate (ClO₃⁻) or other chlorides.
Reduction: chlorine dioxide in wastewater can be treated by reduction, usually with sulphite or nitrite reducing agents that reduce chlorine dioxide to chloride (such as sodium chloride or potassium chloride) and remove the oxidising power from the water.
Neutralisation: chlorate in wastewater can be converted into harmless chloride by a neutralisation reaction. Reducing chemicals such as sodium thiosulphate and hydrogen sulphide can effectively remove chlorate from the water.
Physical adsorption: some organic pollutants in chlorine dioxide wastewater may need to be adsorbed with activated carbon or other adsorbents to reduce the harmful components in the water.
In addition, this technology has wide application and practice in related fields.
High-temperature incineration: some chlorine dioxide-containing wastes can be incinerated at high temperature and converted into harmless gases such as hydrogen chloride and chlorides, but the temperature and gas emissions during incineration must be strictly controlled to avoid forming harmful substances.
In China the use of chlorine dioxide is regulated by the national environmental protection and market regulation authorities, mainly in the fields of water treatment, food disinfection and medical disinfection.
The use of chlorine dioxide as a drinking water disinfectant in China is governed by the Sanitary Standard for Drinking Water (GB5749-2006) and the Measures for the Administration of Drinking Water Treatment Chemicals.
Maximum residual: according to the Sanitary Standard for Drinking Water, the maximum residual chlorine dioxide in drinking water is 0.8 mg/L, a limit similar to the standards of the United States and the European Union.
Chlorine dioxide is also used for surface disinfection and preservation of food. Under the Standard for the Use of Food Additives (GB2760-2014), the concentration of chlorine dioxide used as a food disinfectant is explicitly limited.
In addition, many factors must be considered in practical engineering applications.
China has strict rules on the industrial use of chlorine dioxide and on wastewater treatment, particularly regarding discharge standards. The regulations require strict control of chlorine dioxide emissions during use to avoid environmental pollution.
