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Superfund site cleanup of chromate-contaminated groundwater
The Boomsnub site in the state of Washington was listed as a Superfund site in 1995. The site consists of two parcels of land, which previously contained two unrelated businesses that contributed separately to contamination of soil and groundwater.
The Boomsnub Metal Plating facility operated on about 0.5 acres, from 1967–1994. This facility was responsible for releases of chromium-contaminated wastes that resulted in contamination of soil and groundwater by hexivalent chrome.
Across the street from the Boomsnub site, on approximately 4 acres, is an active compressed gas production facility. Various organic releases at this location have resulted in volatile organic compounds (VOCs) contamination of both soil and ground-water. Migration of the groundwater has resulted in a merged plume containing both chromium and VOCs.
Both locations have been combined into one site for purposes of environmental investigation and remediation.
For the chrome contamination, ion exchange was selected as the cleanup technology, and a standard weak base anion (WBA) resin was selected to remove the dissolved hexivalent chromate, which is an anion. The state of Washington installed an anion exchange system in 1992 to handle groundwater contaminated by chromate. Also at this time, the building housing the plating business was demolished and removed from the site along with 6,000 tons of contaminated soil.
The initial ion exchange process for chromate removal used a WBA resin. This system, sized to treat 50 gpm flowrate, while effective, was found to be costly to operate due to limited resin capacity and expensive regeneration. Equations 1 and 2 show the ion-exchange and regeneration reactions for the chromate ion–WBA resin system.
2 R OH + CrO4-2 R2CrO4 + 2 OH- Equation 2:
R2CrO4 + 2 NaOH2 R OH + 2 Na+ + CrO4-2
As can be seen by Equation 1, which represents the service cycle, the resin removes hexivalent chrome from the contaminated groundwater and releases a hydroxyl ion in return. This is classic ion-exchange of anions. A WBA resin was selected for its relatively high capacity and reasonable selectivity for chromate.
Equation 2 represents the regeneration cycle in which the hydroxyl ion is put back on the resin using relatively high concentrations of caustic, releasing the chromate from the resin.
One concern with regenerating ion exchange resins in this type of application is that the spent regeneration solution contains high concentrations of the heavy metals, in this case, chromium. Care must be taken to handle and dispose of this waste stream in an environmentally acceptable way. Commercial regeneration facilities licensed to regenerate such resin are available, but the service price is expensive, as can be seen in Table 1.
The initial WBA exchange system consisted of a single vessel containing 25 CF of resin and was operated at a flowrate of 50 gpm as shown in Figure 1. Typical feed concentrations for chrome were 2 to 6 ppm. The maximum contaminant level (MCL) for chrome in drinking water has been established by the EPA at 100 ppb. The loading obtained with this system was about 2 lb Cr/CF of resin. This corresponded to a treated volume of 2.5 million gal. Regeneration frequency was 30 to 40 days.
The initial 50 gpm system was operated for about two years. When it was proposed that a specialty, chromium selective resin be evaluated for this project, ResinTech, Inc. was contacted and a chrome selective resin, SIR-700, was recommended.
The chrome selective resin is a specialty anion resin with a proprietary functionality group, which is very selective for chromate. The capacity of the specialty resin for chromate is more than three times that of the standard WBA resin. This means that instead of having a capacity of 2.5 million gal to exhaustion, the same volume of the chrome selective resin can treat approximately 7.5 million gal before capacity is achieved. Run lengths are increased from 30 to 40 days, to more than 90 days.
Although the chrome selective resin is more expensive than a standard WBA resin, the significantly higher capacity of the SIR-700 allows the system to operate in a one-use or throw-away mode. In other words, the resin is used once and disposed of along with soil removed from the site to a landfill for hazardous waste. This avoids the expense of regenerating a resin containing a heavy metal such as chrome. The chrome is bound to the solid resin beads and cannot be leached out with water, thereby immobilizing the chrome when placed in a secured landfill.
As can be seen in Table 1, the one-use operation of the chrome selective resin is about 15% less than using a standard WBA resin with regeneration. Also, the capacity of the standard WBA resin is too low to consider one-time use. It would be about 50% more expensive to use the standard WBA resin on a one-time use basis than to regenerate this resin.
The SIR-700 chrome selective resin was recommended, and initial testing was very encouraging. Based on these tests, a new larger system was designed and built.
A new three-bed system was designed to treat a flowrate of 100 gpm, as shown in Figure 2. Each bed contains 50 CF of chrome selective resin. The three-bed system was built in 1994. The first bed is the working bed. This bed completes the initial removal of chromate. The effluent quality from the first bed is monitored, and when the bed is no longer removing chrome, it is removed from service. The second bed is the polishing bed, which begins to remove chrome when the first bed becomes exhausted. The third bed is the guard bed. When the first bed is removed from service, the second bed becomes the working bed, and the guard bed becomes the polisher. After removing the resin from the first bed and installing new resin, it is returned to service as the new guard bed.
This way, the beds and the resin are cycled through the plant to achieve extremely high capacity for chrome removal. This system has been in successful operation for five years. The system is very effective in removing chromate from the contaminated groundwater. The treated water from this system is discharged to the city of Vancouver Publicly Owned Treatment Works.
In August 1999, the EPA proposed a plan to upgrade the existing treatment system. The EPA evaluated seven alternatives for improving the cleanup of the Boomsnub site. The preferred and recommended alternative is to continue to use the ion-exchange technology with the highly selective chrome resin and to increase the treatment capacity of the system to 200 gpm. This proposal will involve drilling more wells to remove contaminated groundwater at a faster rate to ensure that the contaminant plume does not spread beyond the existing boundaries and to improve treatment in areas of highest concentration.
The treatment system can be easily upgraded by installing three new tanks and the associated piping. The two systems with three tanks each can be operated in parallel. It is anticipated that this system will be installed within a year.
Specialty ion exchange resins for removal of heavy metals are an economical treatment technique for control of chromate contamination at the Boomsnub Superfund site. This technology would be equally effective for other heavy metals including mercury, nickel and lead. Specialty resins are also available for nitrate and several other specific contaminants.