The interhalogen compounds are the bromine- and iodine-base materials. It is the larger, more positive halogen that is the reactive portion of the interhalogen molecule during the disinfection process. Although only used on a limited basis at present, there are members of this class that show great promise as environmentally safe disinfectants.
PROPERTIES OF BROMINE AND BROMIDES
Bromine (from the Greek word bromos, meaning stench) has an atomic weight 79.909, atomic number 35, melting point - 7.2" C, and boiling point 58.78" C. As a gas it has a density of 7.59 g/l and as a liquid 3.12 g/l(20" C). The element was discovered by Balard in 1826 but not prepared in quantity until 1860. It is a member of the halogen group of elements. Bromine is found mainly in the bromide form, widely distributed and in relatively small proportions. Extractable bromides occur in the ocean and salt lakes, brines, or saline deposits left after these waters evaporated during earlier geological periods. The average bromide content of ocean water is 65 ppm by weight (about 308,000 tons of bromine per cubic mile of sea water). The Dead Sea is one of the richest commercial sources of bromine in the world (containing nearly 0.4 percent at the surface and up to 0.6 percent at deeper levels). In the United States, major sources of bromine are the brine wells in Arkansas, Ohio, and Michigan (bromide contents range from 0.2 percent to 0.4 percent).
Bromine is considered a moderate fire hazard. As liquid or vapor, it
can enter spontaneous chemical reactions with reducing materials.
It is a very powerful oxidizer.
Bromine is considered a highly dangerous material. Upon being heated, it emits highly toxic fumes. It will react with water or
steam to produce toxic and corrosive fumes.
Bromine is the only liquid nonmetallic element. It is a heavy, mobile, reddish-brown liquid that readily volatilizes at room temperature to a red vapor having a strong pungent odor. Its disagreeable odor strongly resembles chlorine and has a very irritating effect on the eyes and throat.
Bromine is readily soluble in water or carbon disulfide, forming a red solution. It is less active than chlorine but more so than iodine. Bromine unites readily with many elements and has a bleaching action.
The toxic action of bromine is similar to that of chlorine and can cause physiological damage to humans through inhalation and oral routes. It is an irritant to the mucous membranes of the eyes and upper respiratory tract. Severe exposures may result in pulmonary edema. Chronic exposure is similar to therapeutic ingestion of excessive bromides.
WATER STEREEATION TECHNOLOGIES 477
The most common inorganic bromides are sodium, potassium, ammonium, calcium, and magnesium bromides. Methyl and ethyl bromides are among the most common organic bromides. The inorganic bromides produce a number of toxic effects in humans: depression, emaciation, and in severe cases, psychoses and mental deterioration. Bromide rashes (called bromoderma) can occur especially on the facial area and resemble acne and furunculosis. This often occurs when bromide inhalation or administration is prolonged. Organic bromides such as methyl bromide and ethyl bromide are volatile liquids of relatively high toxicity. When any of the bromides are strongly heated, they emit highly toxic fumes.
INTERHALOGEN COMPOUNDS AND THEIR PROPERTIES
Interhalogen compounds are formed from two different halogens. These compounds resemble the halogens themselves in both their physical and chemical properties.
Principal differences show up in their electronegativities. This is clearly shown by the polar compound ICl, which has a boiling point almost 40" C above that of bromine, although both have the same molecular weights. Interhalogens have bond energies that are lower than halogens and therefore in most cases they are more reactive. These properties impart special germicidal characteristics to these compounds. The principal germicidal compound of this group is bromine chloride.
At equilibrium, BrCl is a fuming dark red liquid below 5" C. It exists as a solid only at relatively low temperatures. Liquid BrCl can be vaporized and metered as a vapor in equipment similar to that used for chlorine.
BrCl is prepared by the addition of equivalent amounts of chlorine to bromine until the solution has increased in weight by 44.3 percent: The reaction is as follows:
Br, + C1, + 2BrCl
BrCl can be prepared by the reaction in the gas phase or in aqueous hydrochloric acid solution. In the laboratory, BrCl is prepared by oxidizing bromide salt in a solution containing hydrochloric acid.
kBrO, + 2kBr + 6HCI + 3BrCI + 3kC1 + 3H,o
BrCl exists in equilibrium with bromine and chlorine in both gas and liquid phases.
Table 5 lists various physical properties of BrC1. Due to the polarity of BrC1, it shows greater solubility than bromine in polar solvents. In water, it has a solubility of 8.5 gms per 100 gms of water at 20, C (that is, 2.5 times the solubility of bromine; 11 times that of chlorine). Bromine chloride's solubility in water is increased greatly by adding chloride ions to form the complex chlorobromate ion, BrC1,.
478 WATER AND WASTEWATER TREATMENT TECHNOLOGIES
rable 5 . Physical Properties of BrCl.
CHEMISTRY OF BROMINE CHLORIDE
Various organic and inorganic species that act as reducing agents react with and destroy free halogen residuals during interaction with microorganisms (see Figure 2.13 for examples of competitive reactions). Competitive reactions depend on the reactivity of the chemical species, temperature, contact time, and pH. The quality of the effluent and the method of adding the disinfectant also help determine the specific reaction pathways. Bromine chloride is about 40 percent dissociated into bromine and chlorine in most solvents. Because of its high reactivity and fast equilibrium, BrCl often generates products that result almost entirely from it. This is illustrated by the disinfectant products shown in Figure 5 . The major portion of the BrCl is eventually reduced to inorganic bromides and chlorides, with the exception of addition and substitution reactions with organic constituents.
CI -
BACTERIA,VIRUS DISINFECTION I
Figure 5. Reactions in wastewater disinfection.
WATER STERILIZATION TECHNOLOGIES 479
It should be noted that although BrCl is mainly a brominating agent that is competitive with bromine, its chemical reactivity makes its action similar to that of chlorine (that is, disinfection, oxidation, and a bleaching agent). BrCl hydrolyzes exclusively to hypobromous acid, and if any hydrobromic acid (HBr) is formed by hydrolysis of the dissociated bromine, it quickly oxidizes to hydrobromous acid via hypochlorous acid.
Since hypohalous acid is a much more active disinfectant than the hypohalite ion, the effect of pH on ionization becomes important. Hypobromous acid has a lower ionization value than hypochlorous acid and this contributes to the higher disinfectant activity of BrCl compared with chlorine.
Bromine chloride also undergoes very specific reactions with ammonia and with organics. Monobromamine and dibromamine are the major products formed from reactions between BrCl and ammonia. These are unstable compounds in most conventional wastewater treatment plant effluents. In comparing the activities of bromarnine versus chloramines, the effects of ammonia and high pH tend to improve the bromarnine performance whereas the chloramine activity is reduced significantly. The reaction of ammonia with either BrCl or chlorine to form the halamine is very fast &d generally goes to completion. As such, the presence of ammonia is essential to the disinfectant properties. Most sewage effluents typically have high ammonia concentrations in the range of 5 - 20 ppm. For such samples, the predominant bromine species (pH at 7 to 8) monobromamine and dibromamine are approximately equally distributed.
There are a large number of organics that undergo disinfection during the purification process. There are unfortunately a number of undesirable byproducts and side reactions which occur with some of them. One is the reaction between chlorine and phenol, producing chlorophenols, which are suspected carcinogens.
Chlorophenols have obnoxious tastes and are toxic to aquatic life even at very low concentrations. Brominated phenolic products which are formed in the chlorobromination of wastewater are generally more readily degraded and often less offensive than their chlorinated counterparts.
The major organic reactions of BrCl consist of electrophilic brominations of aromatic compounds. Many aromatic compounds do not react in aqueous solution unless the reaction involves activated aromatic compounds (an example being phenol). Bromine chloride undergoes free-radical reactions more readily than bromine.
Metal ions in their reduced state also undergo reactions with BrC1. Examples include iron and manganese.
Fe+* + BrCl + Fe+3 + Br- + Cr
Mn + BrCl + Mn'' + Br- + C1'
Wastewater occasionally contains hydrogen sulfide and nitrites. These contribute to higher halogen demands. Many of these reactions reduce halogens to halide salts.
480 WATER AND WASTEWATER TREATMENT TECHNOLOGIES
Bromine chloride's reactivity with metals is not as great as that of bromine;
however, it is comparable to chlorine. Dry BrCl is typically two orders of magnitude less reactive with metals than dry bromine. Most BrCI is less corrosive than bromine. Like chlorine, BrCl is stored and shipped in steel containers. Also, Kynar and Viton plastics and Teflon@ are preferred over polyvinyl chloride (PVC) when BrCl is in the liquid or vapor states.
DISINFECTION WITH BROMINE CHLORINE
In chlorination, chlorine's reaction with ammonia forms chloramines, greatly reducing its bactericidal and virucidal effectiveness. The biocidal activity of monochloramine is only 0.02 - 0.01 times as great as that of free chlorine. Typical
ammonia concentrations found in secondary sewage range from 5 - 20 ppm, which is about an order of magnitude greater than the amount needed to form monochloramine from normal chlorination dosages (which requires about 5 - 10 ppm). Therefore, monochloramine is the major active chlorine constituent in chlorinated sewage plant effluents. In contrast, BrCl ammonia reactions produce the major product bromamines. Bromamines have disinfectant characteristics which are significantly different than chloramines.
TOXICITY OF AQUATIC LIFE
Bromamines are considerably less stable than chloramines in receiving waters.
Bromamines tend to break down into relatively harmless constituents typically in under 60 minutes. Consequently, BrCl is less damaging to marine life than chlorine. Chloramines at concentrations below 0.1 ppm have resulted in fish kills.
There are also indirect effects from chloramine contamination. For example, fish populations tend to avoid toxic regions, even at very low levels of concentrations.
Consequently, large areas of receiving waters can become unavailable to many species of fish and even cause blockage of upstream migrations during the spawning season. It should be noted that although chlorine efficiency is increased by nitrification, BrCl performance is not. Because of the high biocidal activity of bromamines, it is not necessary to utilize high concentrations and breakpoint conditions to achieve active halogen residuals, as is the case in chlorination. The breakpoint reaction with BrCl is achieved almost immediately in the presence of even slight excess amounts of bromine at pH levels of 7 to 8. There is, however, no need to reach the breakpoint to achieve good disinfectant properties with BrCI.
In contrast, with chlorine it is necessary to add amounts in excess of the breakpoint to obtain sterilizing characteristics,
PROPERTIES OF IODINE
Iodine (from the Greek, iodines, meaning violet) has an atomic weight of 126.9044, atomic number 53, melting point 113.5" C, and boiling point 184.35' C. As a gas,
WATER STERnIZATION TECHNOLOGIES 481
its density is 11.27 g/l and as a solid its specific gravity is 4.93 (20° C). This halogen was discovered by Courtois in 1811. It occurs sparingly in the form of iodides in sea water from which it is assimilated by seaweeds, in Chilean saltpeter and nitrate-bearing soil, in brines from ancient sea deposits, and in brackish waters derived from oil and salt wells. Pure grades of iodine can be obtained from the reaction of potassium iodide with copper sulfate. Iodine is a grayish-black, lustrous solid that volatilizes at ordinary temperatures to a blue-violet gas. It forms compounds with many elements. However, it is less active than many of the other halogens which displace it from iodides. Iodine dissolves readily in chloroform, carbon tetrachloride, and carbon disulfide. It is only slightly soluble in water.
Iodine is highly irritating to the skin, eyes, and mucous membranes. Its effect on the human body is similar to that of bromine and chlorine. However, it is more irritating to the lungs.
DISINFECTION WITH IODINE COMPOUNDS
Two interhalogens having strong disinfecting properties are iodine monochloride (IC1) and iodine bromide (IJ3r). Iodine monochloride has found use as a topical antiseptic. It may be complexed with nonionic or anionic detergents to yield bactericides and fungicides that can be used in cleansing or sanitizing formulations.
These generally have a polymer structure which establishes its great stability, increased solubility, and lower volatility. By reducing the free halogen concentration in solution, polymers reduce both the chemical and bactericidal activity. Complexes of IC1 are useful disinfectants which compromise lower bactericidal activity with increased stability. Iodine monochloride is itself a highly reactive compound, reacting with many metals to produce metal chlorides. Under normal conditions it will not react with tantalum, chromium, molybdenum, zirconium, tungsten, or platinum. With organic compounds, reactions cause iodination, chlorination, decomposition, or the generation of halogen addition compounds. In water, IC1 hydrolyzes to hypoiodous and hydrochloric acids. In the absence of excess chloride ions, hypoiodous acid will disproportionate into iodic acid and iodine. Iodine bromide has a chemistry similar to IC1. Iodine bromide reacts with aromatic compounds to produce iodination in polar solvents and bromination in nonpolar solvents. It has complex chemical properties, as its solubility is increased more effectively by bromide than by chlorided ions. Primary hydrolysis takes place in the presence of hydrobromic acid. As a disinfectant, IBr is used in its complexed or stabilized forms. Unfortunately, it undergoes hydrolysis and dissociation reactions in aqueous solutions, both reactions being major limitations. Its disinfecting properties are similar to IC1 and as in the case of IC1, germicidal activity should not be reduced by haloamine formation since bromamines are highly reactive and iodoamines are not generated. Upon application of prepared solutions to control microorganisms, the complex releases Il3r gradually. This process forms free iodine during the decomposition of Il3r (the decomposition takes place as fast as the IBr is released).
482 WATER AND WASIEWATER TREATMENT TECHNOLOGIES
DISINFECTION WITH HALOGEN MIXTURES
Two approaches that have been investigated recently for disinfection are mixtures of bromine and chlorine, and mixtures containing bromide or iodide salts. Some evidence exists that mixtures of bromine and chlorine have superior germicidal properties than either halogen alone. It is believed that the increased bacterial activity of these mixtures can be attributed to the attacks by bromine on sites other than those affected by chlorine. The oxidation of bromide or iodide salts can be used to prepare interhalogen compounds or the hypollalous acid in accordance with the following reaction:
HOC1 + NaBr + HOBr + HCl
It has been reported that the rate of bacterial sterilization by chlorine in the presence of ammonia is accelerated with small amounts of bromides. As little as 0.25 ppm of bromamines can be significant under some conditions. However, if chloramines are produced prior to contact with bromide ions, the reaction and subsequent effect are reduced. Improved germicidal activity has also been shown for mixtures containing bromides and iodides with various chlorine releasing compounds.
Bromide improves the disinfecting properties of dichloroisocyanuric acid and hypochlorite against several bacteria. Brominecontaining compounds are useful for their combined bleaching and disinfectant properties. There has been the concern that the use of interhalogen compounds in wastewater disinfection could produce unknown organic and inorganic halogen-containing substances. In the case of iodine, concern has been expressed over the physiological aspects in water supplies.
Extensive studies have been reported on the role played by iodine and iodides in the thyroid glands of animals and man. Information on acute inhibition of hormone formation by excessive amounts of iodine is well known. Despite the fact that no strong evidence exists that iodine is harmful as a water disinfectant, only limited use has been attempted.Chronic bromide intoxication from continuous exposure to dosages above 3 - 5 g is called bromism. Typical symptoms are skin rash, glandular excretions, gastrointestinal disturbances, and neurological disturbances. Bromide can be absorbed from the intestinal tract and contaminate the body in a manner very similar to that for chloride. Brominated drinking water does not, however, significantly increase the amount of bromine admitted internally. The amount of additional bromine in chlorobrominated waters will not significantly increase human bromine concentrations nor result in bromism.