Metal salts dissolve in water and split into ions, which carry an electric charge. The ionic forms of dissolved metals, cations, carry a positive charge. Ions can be modified or removed using ion-exchange.
The first commercial ion-exchange resins removed the common ions found in most water supplies. The prevalent ions in these water supplies are derived from salts of calcium, magnesium and sodium, primarily the bicarbonates, sulfates, chlorides and silica. The following table lists the common ions found in most city waters:
In the last 50 years there has been large growth and development in many industries, including electroplating, semiconductor and printed circuit board. These industries added new heavy metal contaminants to wastewater, such as copper, nickel, lead, chromium and zinc. In the early years, there was neither information on how to use ion-exchange technology to remove these contaminants, nor was there need. Environmental regulations had not been written and resource recovery did not seem an economical or viable option.
Recently, as waste volumes have grown, it has become necessary to remove heavy metals from process effluents. Some of these metals are relatively valuable, making it economical to recover them. During the last 20 years, several new ion-exchange resins have been developed that are selective for heavy metals. Recently, recovery has become widely recognized as a way to reduce waste treatment costs.
Ion-exchange resins have different affinities for different ions. These affinities vary with the ion-exchange resin. Since the first commercially available ion-exchange resins removed all the common ions, they had no special affinity for the small amount of the heavy metal ions in process effluents.
Special purpose ion-exchange resins are available that have extremely high affinities for particular metal ions, including the heavy metals. These resins attract and hold only the heavy metal ions, while ignoring the common ions, even though they may exist at higher concentrations. For example, a waste stream might contain 200 times as many of the common ions (calcium, magnesium and sodium) as it does heavy metal ions such as copper. A nonselective resin would have to remove 201 ions in order to get one ion of copper. A selective resin would ignore the other 200 and only remove the one copper.
|TABLE I - Common Ions|
|Calcium (Ca)||Alkalinity (HCO3)|
|Magnesium (Mg)||Sulfate (S04)|
|Sodium (Na)||Chloride (C1)|
There is a difference in performance of the standard cation resin compared with a resin that is highly selective for copper.
Most ion-exchange resins have ionic capacities that are equivalent to an equal volume of caustic or acid at a concentration of five to 10 pct. Ion-exchange resins can be regenerated and reused many times. The waste volume created by the regeneration is small compared to the volume of water treated by the resin. The concentrated waste can be recovered or batch treated. The net effect is to concentrate the wastes. In some cases, the resins are not regenerated, but are disposed of for economic reasons.
The typical regeneration process requires about 100 gal of water per cu ft of resin (15 bed volumes). The ion-exchange process is optimized when the ratio of regenerated wastewater to the amount of water processed during the service cycle is minimized. In order to get the most out of the ion-exchange process, three steps should be considered:
- Optimize the wastewater generating process itself to take maximum advantage of the ion-exchange resin.
This simply involves some investigative work to determine exactly what is present in the wastewater and where it originated. Most cationic-exchange resins are good at removing most cationic metals; however, the presence of ordinary ions such as calcium or magnesium in the wastewater may compete for exchange sites, causing interference. If calcium or magnesium is present in wastewater, one should find out if it comes from the manufacturing process or the city water. If the calcium or magnesium is from the city water, a simple water softener upstream of the process removes the salts before the water is introduced to the process. The wastewater discharge from the process will only include the contaminating ions and sodium ions from the effluent of the water softener. Sodium ions do not interfere with the removal of most heavy metals by the ordinary cation resins.
- Develop an optimal operating procedure for the performance of the ion-exchange resin for the specific application.
An older plating operation may have a common trench for draining all the rinse tanks. This trench collects the waste rinse water from all plating operations, and, therefore, would have the metals from all of the operations combined. Depending on the total wastewater chemistry, it may be a less expensive process to treat each specific metallic waste separately rather than at once. More important, segregating the waste streams will greatly benefit the recovery effort because of higher purity of the recovered metal.
- Select the best resin for the specific situation.
Resin selection should be based on an accurate wastewater chemistry. For existing wastewater treatment applications, this is simply a matter of submitting samples to a lab. For new applications, calculations must be made to come up with the estimated strength and characteristics of the wastewater discharge.
Deionized water is essentially free of cations such as calcium, magnesium, and sodium as well as anions like sulfate, bicarbonate, chloride and silica. Softened water is essentially free of all calcium and magnesium and usually contains less than 500 mg/liter of sodium.
Standard industrial-grade strong acid cation resins can select most heavy metals over sodium, but cannot select them over calcium or magnesium. This kind of resin, which is the least expensive, has excellent operating capacities for heavy metals when the raw water going into the process is either softened or demineralized. These resins cost less and are easier to regenerate.
Both the pH and wastewater composition play an important role in deciding between various kinds of selective resins. All of these resins have very high affinities for the hydrogen ion. They tend to auto regenerate at a low pH. In some cases the selection of resin type is entirely based on the effect the pH plays in the operating capacity of the deficient types of resins.
Selection of the right resin for the job must account for several factors in itself. First, the ultimate destination of the resin should be considered. Will it be used on a onetime basis and then disposed of, or will it be regenerated? In some waste treatment applications, it makes sense to use resin on a sacrificial basis. The metals are concentrated on the resin, which allows operators to discharge the wastewater stream, but also requires proper disposal of the resin. In regenerable applications the resin will be used repeatedly. Regeneration creates a volume of regenerant wastewater that includes the concentrated metals. The metals can be recovered with electrowinning or the regeneration waste can be treated by chemical flocculation and settling.
Calculating the concentration of metals in a regenerant waste stream is relatively easy. Divide the total amount of water treated with one cu ft of resin by the amount of water used to regenerate that resin to get the concentration factor. Multiply the concentration factor by the influent concentration of the contaminating ion(s) to get the level of that ion in the regenerant wastewater. For example, if one cu ft of resin treated 2,000 gal of water and it took 100 gal of water to regenerate one cubic foot, 2,000 divided by 100 equals 20. The concentration factor is 20. This number is multiplied by the inlet concentration of the metal being removed. If, for example, one assumes that the metal was copper and it was at a level of two ppm, the concentration factor of 20 multiplied by two equals 40 ppm; therefore, the 100 gal of regenerant wastewater would contain 40 ppm of copper.
Closing the loop eliminates any discharge by recycling all of the water. This is common in the plating industry and involves the use of demineralized water for makeup water in rinse tanks. The wastewater from the rinse tanks is recycled back through ion-exchange resins to remove any metals and restore the water to its original demineralized condition. Usually two ion-exchange systems are employed: one demineralizes the influent city water used for makeup of the rinse baths, and the other system treats the wastewater from the rinse baths. The wastewater treatment system would be regenerated separately. The optimum system would include recovery of the metal from the regenerant waste to further improve the economics of the closed-loop system and replaced hazardous waste hauling with the sale of valuable recovered resources.
Ion-exchange resins are ideally suited for the removal of ionic contaminants from dilute waste streams. Some streams, especially mixed wastewaters, can contain high levels of dissolved and suspended solids. These streams may require pretreatment before they can be treated by ion-exchange resins. Ion-exchange resins can foul in the presence of suspended solids, oils, greases and some organics. Cleaning rinses that contain detergents or degreasers should not come in contact with the resins. A good rule of thumb is to consider other treatment processes when the total dissolved solids of the wastewater stream approaches 500 to 1,000 ppm.