Summary: By using polybromide and iodide resins, dealers can promote an effective water treatment over a wide range of different water chemistries while maintaining safety. Here, we look at key components such as pH that make these halogenated resins so attractive to water treatment professionals.
Can you imagine turning on your tap and worrying about whether the water that fills your glass would make you ill? Having convenient access to safe potable drinking water is something most people in the industrialized world now take for granted. No American today remembers 1908, which heralded a revolution in water treatment that advanced public health and increased life expectancy by 50 percent. That year, chlorine was introduced to U.S. water supplies, effectively reducing the number of deaths caused by waterborne diseases to virtually zero from levels comparable to today’s automobile fatality rates. Even today’s era of 24-hour news that generates alarming headlines out of contaminants du jour and rare catastrophic failures, such as Milwaukee’s Cryptosporidium outbreak, has failed to erode the public’s confidence in municipal water treatment systems.
Today, chlorine remains the disinfectant of choice with over 98 percent of U.S. water treatment systems relying on it to kill various bacteria and viruses. As pervasive as the use of chlorine is, though, it’s just one of several water treatment options available to professionals for the prevention of microbial contamination.
Ultraviolet (UV) light and ozone are often used when it’s impractical or hazardous to store and handle chlorine. Electrical power, however, isn’t always readily available or sufficiently reliable for the continuous powering of UV and ozone equipment. In these cases, disposable cartridges that contain resin impregnated with iodine or bromine may be the answer.
Iodine is a black crystalline solid that sublimes to deep violet vapors at 184 degrees C. It’s element No. 53 on the periodic table and has an atomic weight of 126.9, a melting point of 236.7 degrees F, and a boiling point of 363.9 degrees F. Like other halogens, iodine is manufactured commercially by electrolyzing seawater or processing brine from wells. For years, backpackers have used iodine to disinfect drinking water because it’s not only economical and easy to use, it’s light weight makes it very portable. Iodine also has its drawbacks as a disinfectant. Most people dislike its unpleasant taste; and while it’s an effective disinfectant for bacteria and viruses, iodine–like other halogens–has been proven to be ineffective in killing Cryptosporidium, a protozoan that backpackers encounter in their water supplies on an increasing basis.
Iodine isn’t more widely applied because concerns associated with ingestion of iodine and thyroid disorders led the U.S. Environmental Protection Agency (USEPA) to limit use of iodine in water treatment. The USEPA, however, has approved iodinated resins for emergency use and long-term purposes when employed in conjunction with appropriate scavenger systems to remove iodine residuals that may leach from the resin. As a result, iodine is typically used in temporary purification situations such as disaster relief and military operations.
Since the 1970s, iodine resins have been used on manned space flights to help disinfect and maintain purity of water consumed during missions. In fact, crews of both the space shuttle and new International Space Station rely on iodinated resins to treat their water. On the shuttle, the resin is incorporated into a microbial check valve (MCV) designed into the port from which astronauts draw water for drinking. The resin acts as a “check valve” to prevent microbial contamination from entering the water system from this point. The International Space Station has a more complex water system in which the iodinated resin is positioned at multiple treatment ports–and various other areas for recirculation within the water system–to provide safe water for the astronauts on a continuous basis.
Bromine is another halogen used extensively in water treatment. It’s a dark, reddish-brown element that’s a liquid at ambient temperature and pressures. Bromine is element No. 35 on the periodic table and has an atomic weight of 79.909, a melting point of 19.04 degrees F, a boiling point of 137.8 degrees F, and a liquid density of 3.12 grams per milliliter (g/ml) at 68 degrees F. The liquid is readily soluble in water (3.58 g/100 ml at 68 degrees F), and solubility decreases with increased temperature.
Bromine is six times more soluble in water than chlorine and, therefore, is theoretically easier to dispense and disperse than chlorine. Unfortunately, elemental bromine is extremely corrosive and becomes even moreso when combined with even trace amounts of water, which makes the liquid form of bromine incompatible with most materials and impractical as a disinfectant. Bromine adsorbed onto a resin, however, has been effectively applied in water treatment.
While much has been written regarding use of iodinated resins and applications in which they’re substituted for chlorine, very little has been written on brominated resins, even though both iodinated and brominated resins remain important tools in the spectrum of water treatment solutions.
Resins for marine applications
Solid forms of chlorine are very strong oxidizing agents and have been responsible for causing several shipboard fires when combined with fuel or oil. Fires on a ship or offshore oil platform are especially dangerous because the only place to escape a fire that can’t be extinguished is going overboard. For this reason, the USEPA approved use of brominated resins for general marine purification applications, which is why bromine is commonly used in marine, shipboard and off-shore platform applications where chlorine can be hazardous due to confined spaces and the potential for fires.
As early as 1933, researchers first suggested using bromine to disinfect water. They found little difference in the disinfecting activity of chlorine and bromine. In 1967, Dow Chemical created a halogenated ion exchange resin, which opened up use of bromine in offshore water treatment applications and iodine in emergency and temporary water systems. This work revealed these resins have an unusually high affinity for halogens, especially for iodine. Since then, a number of improvements and modifications were made in the formulations of brominated and iodinated resins to enhance their chemical characteristics and anti-microbiological performance. Initially, these resins were used as a way of metering iodine or bromine into a stream of water going to a retention tank. The halogen was used as a residual to kill microorganisms over time.
A major discovery in 1969 changed how iodinated resins are used. It was discovered that contaminated water, passed through a bed of iodinated resin, was instantaneously purified. This meant a properly designed iodinated resin bed would kill all measurable microbiological contamination. Residual iodine could be removed immediately after passing through the resin bed. Two researchers also demonstrated that merely creating physical contact between the microbiological contamination and the resin kills bacteria even without an iodine residual. This is especially important for people who are sensitive to iodine and need emergency or temporary supplies of water free of iodine residue. Iodinated and brominated resins are ideal for small and medium scale point-of-use/point-of-entry (POU/POE) water systems for marine and short-term emergency and temporary uses. These systems can be very practical because they require:
- No electrical connections (unless recirculation or repressurization is required),
- No direct handling of harmful chemicals,
- Minimal space requirements, and
- Low maintenance.
These resins are made from polystyrene anion exchange resins that have a positively charged ammonium functional group: R-CH2N + (CH3)3, where R is the polystyrene backbone. The positively charged functional site holds the negatively charged polyhalide anion. The resin’s affinity for triiodide is much greater than for chloride. When triiodide resin is made properly, virtually no detectable levels of iodine are found in the effluent of the cartridge. Even in the presence of high concentrations of chloride or sulfate, no triiodide exchange occurs and only low levels of iodide ion (I-) are found in the effluent.
Maintaining proper pH
Water going into a polyiodide resin purification system should be between a pH of 7 and 8. At a pH above 9, iodinated resins release high amounts of iodine that can diminish their efficiency, and shorten their effective life as well as the scavenger system’s capacity. Lowering the pH to less than 5 isn’t recommended because it reduces the resin’s effectiveness on viruses.
Low temperatures can reduce the effectiveness of the resin and lower the amounts of iodine and bromine in the water stream if it’s being used as a residual. Higher temperatures will make the resin more effective in killing waterborne pathogens and increase the amount of halogen released into the water. Water temperature and total dissolved solids (TDS) affect the amount of bromine released from the cartridge. The lower the water temperature, the less bromine is released; and the higher the TDS, the more bromine is released. So whether disinfection relies on the halogen residual or direct contact with the resin, temperature and TDS must be considered in determining its “kill effectiveness.”
Iron and organic material can foul the halogenated resin and make it less effective. Because the iodine resin’s primary kill mechanism relies on direct physical contact between the microorganism and the resin beads or particles, adequate protection against resin fouling or coating of the beads is critical. Most water treatment professionals have experienced effects of resin fouling in simple softening or demineralizing applications. Resin fouling or coating by iron floc or organic material will prevent dissolved ions from contacting or entering the resin beads, effectively interfering with the softening or demineralizing action. This same phenomenon establishes a physical barrier that blocks microorganisms from coming into direct contact with disinfecting action of the iodinated resins. Obviously, potential for resin fouling is an extremely critical factor that must not be overlooked when considering halogenated resins for water disinfection.
Halogens have proven to be ineffective in killing protozoan cysts. Only the pentaiodide (I5-) resins have shown they’re effective against Giardia. Alternate filtration methods such as carbon block should be used to remove cysts before the resin bed, which will also act as a prefilter for sediment and organics that may foul the bed.
Bromine and ammonia
One study demonstrated that bactericidal activity of bromine isn’t significantly affected by the presence of ammonia, while bactericidal properties of chlorine are appreciably affected by the presence of ammonia–requiring much longer times for 99 percent inactivation of E. coli organisms (see Table 1).
Today, polybromide resin is used in the water treatment systems of many U.S. Navy ships, off-shore oil platforms and marine installations. It’s a strong base anion exchange resin impregnated with elemental bromine to the level of 30 percent by weight. Polybromide resin is a dry solid that’s safer to handle than calcium hypochlorite because it’s non-toxic and won’t start fires if combined with petroleum distillates such as paint or fuel. The resin is available in sealed cartridges used in brominating systems making it easy to contain, handle and monitor as bromine is released into the water supply at a predictable rate necessary for disinfection. It’s an effective disinfectant over a wider pH range than chlorine. Chlorine is 50 percent active at a pH of 7.6, whereas bromine is 50 percent active at a pH of 8.7. Bromine is less affected than chlorine by the presence of organics.
Pretreatment of water in special applications is a concern. Shipboard water systems that are brominated typically receive water from a waste heat evaporative distiller or reverse osmosis (RO) system. An emergency or temporary water system could have no prefiltration at all or as much as a high-pressure RO system. It’s important the amount and type of resin is properly calculated for the factors that affect the efficiency of the resin.
Even though these resins have remarkable disinfection capabilities, proper design, thorough testing and prudent application are absolutely necessary to assure maximum reliability of systems in which they’re used. Current water treatment technology is available to provide microbiologically safe water. Through use of polybromide and iodide resins, water treatment can be conveniently and safely accomplished over a wide range of water chemistries and conditions without compromising safety. The next time you’re onboard a ship or plane, in a foreign country or a remote location, ask how they make and purify their water. Hopefully, they use one of the technologies we’ve mentioned here–or you may want to find something else to drink.
- Morris, J.C., et al., “The Chemistry of Disinfectants in Water: Reactions and Products,” National Research Council, NTIS #PB 292776, 1979.
- National Sanitation Foundation, “Survey and Evaluation of Currently Available Water Disinfection Technology Suitable for Passenger Cruise Vessel Use,” Water Disinfection Technology, Contract No. 200-80-0535.
- Mills, J. F., U. S. Patent 3,462,363: “Control of microorgainsms with polyhalide resins,” (assignee) The Dow Chemical Company, Midland , Michigan, August 19, 1969.
- Lambert, J.L. and L.R. Fina, U.S. Patent 3,817,860: “Method of disinfecting water and demand bactericide for use therein,” (assignee) Kansas State University Research Foundation, Manhattan, Kan., June 18, 1974.
- Johanneson, J. K., American Journal Public Health, Vol. 50, pp. 1731-1736, 1960.
- Department of the Navy, “Water Supply Afloat,” Chapter 6, Manual of Naval Preventive Medicine, NAVMED P-5010-6.
About the authors
Gary L. Hatch, Ph.D., is director of research and development for Pentek™–formerly of USFilter–by Pentair Water Treatment, of Sheboygan, Wis. He is responsible for new product R&D including the development of products utilizing halogenated resins. Hatch graduated from Kansas State University with a doctorate degree in analytical-inorganic chemistry and has been actively involved in water treatment for the past 30 years. He can be reached at (920) 451-9353, (920) 451-9384 (fax), email: ghatch@plymouth water.com or website: www.pentekfiltration.Com
Kenneth L. Korslin has served as a technical support specialist with Pentek™ by Pentair Water Treatment for the past five years. He is a CWS-III. He holds a master’s degree in management and organizational behavior from Silver Lake College as well as a bachelor’s degree in environmental science from the University of Wisconsin-Green Bay. He can be reached at (920) 451-9474 or email: kkorslin@ plymouthwater.com