Oxyhalogen Chemistry

Oxyhalogen chemistry accounts for disinfection of nearly all public water supplies, as well as treatment of significant quantities of process water, cooling water, waste water and recreational water. Oxychlorine species used to treat drinking water are nominally few:  

Chlorine gas (molecular chlorine) is a commercially important, highly reactive oxidant and disinfectant. Generally, it is manufactured in chloralkali plants by electrolysis of sodium chloride (salt). Co-products are hydrogen and sodium hydroxide (caustic). The manufactured chlorine typically is liquefied and shipped to the point of use in railcars, trucks or cylinders. In the early 20th Century, Dr. Abel Wolman and colleague Linn Enslow standardized the methods for chlorinating drinking water. Chlorine is now used to treat most public water supplies.

Chlorine reacts with water (hydrolyzes). The anti-microbial efficacy of chlorine is highly dependent on pH. Chlorine often functions by substitution; in reactions with organic materials, this can create toxic chlorinated organic disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs). Regulation of chlorinated DBPs has created a need for alternative disinfectants, such as chlorine dioxide, ozone and monochloramine.

Risks associated with accidental release of chlorine gas are driving some drinking water utilities to transition from transported chlorine gas to alternatives, such as sodium hypochlorite (bleach).

Sodium hypochlorite solution, commonly known as bleach, is frequently used as a disinfectant. It can be centrally manufactured by passing chlorine gas into cold, dilute sodium hydroxide solution. It also can be produced on site by electrolysis of sodium chloride (salt).

Sodium hypochlorite in aqueous solution exhibits much the same chemical behavior as chlorine (gas); its use for drinking water disinfection does not resolve the problem of chlorinated disinfection byproducts associated with chlorination.

In addition to chlorinated disinfection byproducts (THMs, HAAs), contaminants of concern associated with sodium hypochlorite include chlorate ion, perchlorate ion and bromate ion. These may be present in appreciable amounts in both purchased and on-site generated bleach, due to a number of factors including feedstock (salt) composition, process parameters and procedures for storage and handling.

Chlorine dioxide is a relatively small, volatile and highly energetic molecule. It typically is generated as needed, at the point of use by a number of methods. The generation process used and the operating conditions can have a profound affect on the purity of the chlorine dioxide produced.

Chlorine dioxide is highly soluble in water but, unlike chlorine, chlorine dioxide does not react with water (hydrolyze). It exists in aqueous solution as a dissolved gas. Chlorine dioxide functions as a highly selective oxidant owing to unique, one-electron transfer mechanisms. One-electron transfer does not produce meaningful amounts of problematic chlorinated organic byproducts (THMs, HAAs) associated with the use of chlorine.

Chlorine dioxide is a relatively powerful, fast-acting disinfectant, which rapidly inactivates bacteria, viruses, and parasites. The combination of power and selectivity enable chlorine dioxide to penetrate biofilm and destroy the resident (sessile) microorganisms.

Monochloramine, a relatively weak oxidant and disinfectant, is highly unstable in concentrated form. For drinking water, it is produced as a dilute aqueous solution in the treatment process, by adding ammonia to water to which chlorine (gas or bleach) has already been introduced. When used to treat public drinking water supplies, it typically is used as a “secondary disinfectant” in place of chlorine, to maintain a relatively weak but persistent disinfectant residual throughout a distribution system. Monochloramine reacts with organics at a much slower rate than chlorine; it is therefore often part of a strategy for minimizing formation of regulated chlorinated disinfection byproducts (THMs, HAAs). Recently, concerns have been raised over the possible contribution of monochloramine use to the formation of certain carcinogens (e.g., nitrosamines) that have been detected in drinking water supplies.

Ozone is a powerful, highly reactive oxidizer and disinfectant. It is slightly soluble in water. It is unstable at high concentrations, and almost always produced on site, as needed by a number of commercially available methods, all of which require substantial amounts of electric power. Ozone is a primary irritant, affecting especially the eyes and respiratory systems. Ozonation of drinking water does not form chlorinated organic disinfection byproducts (THMs, HAAs) associated with chlorination. Ozone does not persist in the water and therefore does not provide a disinfecting residual. Ozonation of drinking water containing bromide ion can produce bromate ion, a regulated disinfection byproduct. The amount of bromate ion produced is a function of a number of inter-related factors.

 

Oxyhalogen chemistry is much more complex than the relatively small number of simple chemical formulas used to represent these compounds, or the equations we write to describe the overall reactions that occur.

The activity of oxyhalogen compounds in water involves a host of reactions. The extent and rate of the reactions are affected by many factors, including the absolute and relative concentrations of various oxyhalogen species, pH, temperature, light and other chemical constituents present in the water.

Even slight differences in the chemical composition of oxyhalogen compounds can have profound consequences for chemical reactivity (including analytical chemistry), disinfection efficacy and compatibility with materials. Not only are the rates of reaction affected, but different chemical reaction mechanisms can come into play.

News

May 1, 2012. The US Environmental Protection Agency (EPA) has published a list of contaminants  that public water systems will monitor as part of the agency’s unregulated contaminant monitoring program. The chemical contaminants listed include chlorate ion, which is associated with drinking water disinfectants chlorine dioxide and hypochlorite (bleach). 

January 23, 2012. 
 American Water Works Association (AWWA) Water Quality & Technology Division names Best Paper 2012.   Stanford, B. et al. "Perchlorate, Bromate, and Chlorate in Hypochlorite Solutions: Guidelines for Utilities" (Journal AWWA, Vol. 103, Issue 6) describes recent work to develop a predictive model for quantifying the by-product formation in hypochlorite solutions during storage.  Dr. Gilbert Gordon, a co-author of the paper, is Distinguished Research Professor (Emeritus) of Chemistry at Miami University of Ohio and a Principal of Gordon & Rosenblatt, LLC. 

September 18, 2011.  The International Ozone Association (IOA) has announced that Gilbert Gordon, a Past President of IOA, will be awarded the Morton J. Klein Medal of Excellence. The inscription on the medal reads: "An award of excellence to recognise contributions of the highest order to the International Ozone Association in commemoration of the clear vision, diplomatic proficiency, and unparalleled leadership which characterised the life of Morton J. Klein."  Dr. Gordon is Distinguished Research Professor (Emeritus) of Chemistry at Miami University of Ohio and a Principal of Gordon & Rosenblatt, LLC.