Ion Exchange—A Workhorse for Drinking Water Treatment
Have you ever wondered how we would clean up our drinking water if modern ion exchange resins had not been invented? The simple answer is that someone would have had to invent them. There are so many ways in which ion exchange resins are used on a daily basis to make our water safer that many people consider them the undisputed workhorses for drinking water treatment.
We use them routinely for aesthetic reasons such as removal of hardness, iron and manganese to prevent the buildup of unsightly scales and deposits on fixtures in our homes. Even more importantly, we find them almost indispensable for removal of a host of harmful contaminants that may be present in our drinking water.
Take nitrate, which causes blue baby syndrome in infants, as an example. The regulated maximum contaminant level (MCL) for nitrate in the U.S. is 45 parts per million, as nitrate. Removal technology at the municipal or household level relies almost exclusively on the use of resins such as Purolite A300E, a Type II strong base resin, or on more nitrate selective resins, such as Purolite A520E. Alternative treatment approaches include biological and reverse osmosis (RO) processes. Biological reduction of nitrate, while feasible for municipal supplies, is not used in the U.S. due to consumer concerns, and of course is impractical at the household level. RO technology, in order to remove nitrate, must essentially remove all other dissolved salts as well, using what is referred to as the “shotgun approach,” or non-specific targeting. Ion exchange technology, on the other hand, uses the “rifle approach,” capable of targeting and removing specific contaminants in the water, making it the most preferred cost-effective treatment option.
Another good example is perchlorate. High selectivity strong base anion (SBA) resins are now routinely used for removal of perchlorate at potable installations across the nation. Perchlorate was once considered harmless and was routinely dumped where it would seep into groundwater systems. It dissolves in water as a very stable ClO4- anion, meaning that it will be around for a long time to come. California Department of Health Services has set an action limit of 6 parts per billion (ppb), Massachusetts has set its MCL at 1 ppb and the U.S. Environmental Protection Agency (EPA) is considering setting a nationwide MCL. Resins specifically designed with a hydrophobic (water hating) matrix, such as Purolite A530E, are extremely effective in discriminating between the hydrophobic ClO4- anion and competing anions such as nitrate (NO3-) and sulfate (SO4-). This is especially amazing because nitrate and sulfate are generally present at concentrations 1,000 times higher than perchlorate concentration and compete for the same ion exchange sites on the resin. Despite this, selective resins are capable of reducing perchlorate in the treated water to concentrations less than 1 ppb.
The Economical Choice
It is interesting that these highly selective resins can generally be used for many months, even years, before they reach their full removal capacity for perchlorate. With such long operating times, it is extremely cost-effective to remove the resin when it is saturated with perchlorate, simply dispose of it by incineration and replace it with fresh resin. Economically speaking, no other technology comes close to ion exchange for this application. Alternatives such as RO make sense only for small household applications, while biological reduction is only viable for higher concentrations of perchlorate. Because most remediation sites contain only part per billion levels of the chemical, biological treatment is ruled out.
Disinfection byproducts (DBP) control is another interesting example where using ion exchange resins makes better sense than the popular method. When disinfectants are added to drinking water and organic matter (measured as total organic carbon or TOC) is present, the potential exists for formation of cancer-causing DBPs, such as trihalomethanes and haloacetic acids. Trihalomethanes and haloacetic acids are regulated at maximum concentrations of 80 ppb and 40 ppb respectively. Therefore, reducing the incoming TOC in advance of disinfecting the water will reduce the formation potential of these DBPs and help keep them from exceeding the MCL targets.
The first thought when it comes to removing TOC from water is to use activated carbon because it is a tried and proven adsorbent. Carbon does a great job of adsorbing aromatic and hydrophobic organic compounds such as benzene, but only a meager job when it comes to low molecular weight aliphatic or hydrophilic compounds. In addition, it cannot be regenerated on site and must be taken off site for thermal reactivation, making it viable only for the very large treatment plants. With specially developed acrylic and styrenic strong base anion resins, TOC removal is easy. These macroporous resins are manufactured with large internal pores for accommodating large organic molecules. A number of installations around the country have been successfully using these resins for many years now. It is a simple but important pretreatment step for protecting our health. Operating cost is nominal, with a standard brine solution used to strip the TOC from the resins, allowing repeated reuse of the resins for many years.
Resin technology also can be used to replace existing and troublesome technology for the removal of oxyanions such as arsenic. Granular iron-based media, such as GFO and GFH, have been popular choices for arsenic removal because of the strong affinity of iron oxide for the arsenate anion, H2AsO4-, but easy breakup and dusting of the granular iron media requires long rinse times before the water cleans up enough for service. During service, more breakup of the media occur, generating fines and plugging the bed. Therefore, regular backwashes are needed during service to remove these fines to avoid greater pressure losses and consequently lower water flowrate. Unfortunately, the fines that are discharged with the backwash water are loaded with arsenic and great care must be taken to prevent these from getting back into the environment. After unloading spent granular media, caution must also be used to ensure the dusty media do not dry out and become windblown.
Over the last few years, a new patent-pending iron-impregnated anion resin has become available. Nanoparticles of iron are embedded deep within the matrix of the resin beads. The resin does a similar job to the granular iron media in adsorbing arsenic, but that’s where the similarity ends. The iron particles are locked in place and will not migrate out of the resin beads to generate fines. Using spherical resin beads to host nanoparticles of iron oxide provides far better hydraulics and kinetics, so pressure drop is minimal and the speed at which arsenic can be adsorbed is significantly faster than granular iron. Because of the friability of granular media, they cannot be regenerated and therefore must be used only once before being disposed of. In contrast, iron-impregnated resin can be repeatedly regenerated and reused, giving large municipal installations, in particular, far better use economics than available before.
A Sensible Option
The examples above serve to demonstrate why it makes good sense to consider ion exchange technology for the removal of a long list of contaminants that are now regulated by the EPA. There are more than 30 inorganic and 50 organic contaminants that are either already regulated or soon will be. Most of the 30-plus inorganic contaminants are particularly suitable for removal by ion exchange resins. Barium, uranium, radium, lead and copper are some of the more prominent contaminants that are widely removed by ion exchange.
Weak base resins in combination with pH reduction are increasingly used to remove contaminants such as chromium, mercury and other heavy metals. In all these cases, superior hydraulics, faster kinetics and ease of design and operation make ion exchange resins a natural and compelling choice.