Formation of Chlorate, Bromate, and Perchlorate on Site and by Sodium Hypochlorite
A critical step in addressing the research objectives outlined within this thesis is identifying and understanding the process by which chlorate, bromate, and perchlorate occur; consisting of the formation, break down, and interactions of each with sodium hypochlorite. Additionally, analysis and examination of the typical observations of these formations, both on-site and by sodium hypochlorite is necessary in understanding how these compounds form. The chemistry and mechanics behind the formation of these compounds, as well as typical observations, will allow this researcher to gain valuable insights that can be used to effectively respond to the research question. As such, the following paragraphs will provide a detailed discussion into these significant areas, which will include extensive technical scientific data designed to strengthen the findings and conclusions drawn as a result of this literature review.
The majority of water systems present within the United States that “disinfect drinking water use some type of chlorine-based process, either alone or in combination with other disinfectants;...(as of) 2002, may utilities have started using sodium hypochlorite in place of chlorine gas” and these agencies are now finding elevated levels of chlorate, perchlorate and bromate present within the resulting treated water, in spite of the fact that these contaminants were not present when chlorine gas was being used. The reason for this is the process by which these contaminants are formed within sodium hypochlorite. Sodium hypochlorite, typically referred to as bleach, is used by a large percentage of water treatment facilities to ensure both the cleanliness and potability of drinking water. Oxyhalide species known as perchlorate, chlorate, and bromate are compounds that form in sodium hypochlorite during and after the manufacturing process; with the majority of the contaminants being formed long after the manufacturing process is complete, as a result of temperature variants, shelf life, and other contributing factors. Similar to most chemical byproducts that result from manufacturing procedures, oxyhalides such as perchlorate, chlorate, and bromate all have the distinct potential to contaminate drinking water, given the difficulties that are present with removing these contaminants from the water itself. Sodium hypochlorite, or bleach, loses strength through decomposition; decomposition results in increased chlorate ion levels present within the remaining solution. During the time period in which liquid bleach is being stored, the hypochlorite ion will decompose, forming the chlorate ion; this occurs “at pH > 10, rate = l [POC-] 2”.
A perchlorate ion is made up of one chlorine atom surrounded by four oxygen atoms in a tetrahedral structure. Perchlorate is highly soluble in water, kinetically inert, and has little tendency to adsorb either mineral or organic surfaces, which is the reason that it persists as a contaminant in groundwater. Perchlorate is found in initial concentrations of 120000 ug/L in concentrated sodium hypochlorite solutions. Over time, the levels of perchlorate will increase as the sodium hypochlorite solution ages; increasing steadily the longer the sodium hypochlorite is stored, resulting in increased levels present within the water leaving the treatment plants. There are several factors that directly impact perchlorate formation in sodium hypochlorite including the concentration of hypochlorite ions, the chlorate ion concentration, ionic strength, temperature of the stored solution, and the pH level of the stored solution; indirect factors include transition metal ions and the presence of bromide. Factors that have no bearing on the formation of perchlorate include noble metal ions and the concentration of bromate present within the sodium hypochlorite. The formation of perchlorate is occurs as a result of disproportionate amounts of ClO2, driven by formation of HClO: 2 ClO2 + H2O -> HClO4 + HClO /\G=-91.7 k~mol.
Chlorate forms as the sodium hypochlorite solution decomposes with age, and it is thermal decomposition that primarily sources the formation of chlorate within the sodium hypochlorite solution. In other words, storage at temperatures other than what is recommended, or storage in unregulated temperatures will increase the formation of chlorate within the sodium hypochlorite solution. The chlorate ion is present in concentrations ranging from 0.1 to 0.5 mg/L (ppm) in treated water. These compounds are formed by the electrolysis of brine utilizing the following formula: NaCl +3H2O+ 6 e- -> NaClO3 + 3H2. Sodium hypochlorite is most likely to decompose as a result of heat, ultraviolet light, or contaminants present within the bleach itself, and there are two different decomposition pathways that it may follow. The primary pathway for decomposition is 3NaOCl = 2NaCl + NaClO3 (Chlorate). Chlorate may be formed in two different ways; the first may occur if the reaction of chlorine and caustic solution, during the production of sodium hypochlorite, occurs in a low pH region (below 10pH) hypochlorous acid will form, resulting in chlorate formation later. The second method of chlorate formation is a result of decomposition due to the initial strength of the sodium hypochlorite, pH levels of the sodium hypochlorite, temperature at which it is stored, exposure to sunlight, and contaminants. The standard rate of sodium hypochlorite decomposition without the presence of sunlight, contaminants, or heavy metals, and with a pH of 11-13 is expressed as Rate = K2(OCl-)2.
Bromate results from the oxidation of bromide present within the sodium hypochlorite solution; it may form from oxybromide, which may break down into bromate, though the chance of the degeneration of oxybromide is very small. Bromate may also be produced by on-site generation as a result of the electrolysis process of the brine solution that is used in on-site generation of sodium hypochlorite, the concentration of which will vary based on the type of salt source used in this process. The presence of bromates in finished hypochlorite solutions may exceed health based drinking water guidelines if care is not taken during the process of manufacturing and storing hypochlorite solutions. It is, in essence, a disinfection byproduct formed by the reaction of ozone and bromine present within the disinfected drinking water. The ozone reacts directly with bromide in order to form oxygen and hypobromite (O3 +Br- ? O2 + OBr-), and then the ozone reacts with the hypobromite to form bromate and oxygen (2O3 + OBr- ? 2 O2 +BrO3) (Thompson & McGonnell, 2013). According to EPA standards for drinking water, only 50% of the maximum contaminant level for bromate (10ppb) may occur as a result of the use of sodium hypochlorite. Initial concentrations of bromate in raw water were 56.3 ug/L.
According to Stanford et al. (2011) the risk of contaminants present within sodium hypochlorite can increase significantly when inadequate control measures are employed by manufacturers, control measures that are designed to effectively minimize the formation of these dangerous compounds during the manufacturing, shipping, and storage processes of hypochlorite solutions. Due to the risk that perchlorate, chlorite, and bromate can pose to finished drinking water, as well as their connection to the hypochlorite manufacturing process, it is important to fully analyze and understand the specific conditions by which these hazardous compounds form.
Further research has found that the presence of such oxyhalides in drinking water has become a major issue of concern for the water industry. In fact, perchlorate has been identified as a predominant contaminant of concern in recent years, which has caused this contaminant of hypochlorite solution to garner major attention by the scientific and water purification community, and additional regulatory steps are in the process of being pushed through in order to address the risks presented by the presence of these contaminants in what is otherwise considered to be potable drinking water. Understanding that these contaminants, perchlorate, chlorate, and bromate, result from inadequate manufacturing, storage, shipping, and handling procedures of sodium hypochlorite makes it possible to expand the research further in order to determine exactly how these compounds form in hypochlorite solutions in the first place. This will serve to provide key insights into the reasons why the aforementioned specifications for sodium hypochlorite have been recommended, and why it is necessary that these recommendations be followed stringently.