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Soda blasting used to remove PCB's and radioactive particles

 Remember when you were a kid and your family ran out of toothpaste? You might have used baking soda, also known as sodium bicarbonate, to put an extra sparkle in your smile. That same sodium bicarbonate was used by O'Brien & Gere Technical Services, Inc. during a demonstration of soda blasting to decontaminate U.S. Department of Energy facilities and equipment at the Oak Ridge K-25 Site in Tennessee. The soda blasting technique removed greater than 95 percent of radioactive and polychlorinated biphenyl (PCB) surface contamination to a level below DOE's unrestricted use release limits. And after treatment of the blasting residual; aqueous radionuclides, heavy metals, and PCBs were removed below DOE and U.S. Environmental Protection Agency treatment objectives.

O'Brien & Gere won a Program Research and Development Announcement for a pilot-scale demonstration of the effectiveness of soda blasting to remove uranium isotopes, technetium-99, heavy metals, and PCBs on both concrete structures and metal equipment. The soda blasting system aimed abrasive sodium bicarbonate under pressure and at high speed at contaminated surfaces to mechanically remove surface contamination while leaving structures and equipment intact for further use.

The demonstration was conducted in building K-29, where a gaseous diffusion process separated uranium-235 from uranium ore for use in atomic weapons and commercial reactors. The project, conducted between December 1993 and September 1994, also included cleanup of the wastewater from the blasting operations.

Soda blasting test
To meet health and safety concerns during the soda blast, a blasting enclosure was built to contain hazardous and radioactive waste. Blasting personnel were fitted with special protective clothing and supplied-air gear. The test was designed to evaluate the effects of six variables on the operation of the blasting system.

• air pressure,
• water pressure,
• nozzle hole diameter,
• nozzle hole design,
• media type, and
• media flow rate.

A concrete floor at K-29 was gridded into ten one-meter squares. Major contaminants for each square were recorded. After scouring with sodium bicarbonate, the contaminants in the squares were measured again to test the effectiveness of the soda blast. The soda blast was also used on such metal surfaces as an aluminum circuit breaker handle, steel trash can, and compressor blade.

For surfaces tested using selected blasting variables, test results showed contaminant removal averaged between 95 and 100 percent for beta/gamma radioactivity and between 0 and 100 percent for alpha radioactivity. The lower percent removal for alpha was caused by pre-blast readings approaching background levels. In each test, the post-blast alpha readings were below the release limit of 5,000 disintegrations per minute (dpm).

After each blasting test, the tested surfaces were hosed down with tap water to remove the loosened waste. The surfaces were then vacuumed with a wet/dry vacuum and the resulting slurry (water, used sodium bicarbonate, and hazardous and radioactive waste) was transferred to 55 gallon drums in preparation for treatment.

Pretreatment of blasting residuals
Several physical methods were used to remove uranium, other heavy metals, and PCBs from the slurry. The first step was the addition of a 70 percent nitric acid solution to 450 gallons of blasting residual. This nitric acid addition lowered the pH value and facilitated the removal of aqueous phase carbon dioxide following two hours of aeration. The pH level was then elevated by adding sodium hydroxide, which facilitated the formation of insoluble uranium and other heavy metal hydroxides, which settled to the bottom of the holding tank and were removed. Unsettled particulates were further treated by filtration. The last phase of the pretreatment removed dissolved PCBs with an activated carbon system.

The pilot pretreatment system effectively removed more than 97 percent of uranium and more than 99 percent of PCBs and lead. Also, 61 percent removal of technetium was achieved during pretreatment.

Technetium removal
Two technetium removal methods were tested. The first test involved the setup of an ion exchange system employing commercially available resins from Dow Chemicals and Reilly Industries arranged in column beds. During testing, 50 gallons of pretreated blasting residual solution were processed through each column. Column effluents were sampled at 30-minute time intervals. Analysis of the samples demonstrated technetium removal below the 100 pCi/1 treatment objective for both types of resins. However, the Dow resins were more efficient in meeting the standard than were the Reilly resins.

Removing technetium using a chemical reduction method did not prove effective.

Benefits
Soda blasting and the associated waste treatment system hold promise as a surface decontamination method. Sodium bicarbonate is non-toxic and water soluble. Its dissolution in water means it is easily removed from contaminants, thus reducing waste volume. Soda blasting is cost effective and energy efficient. The average amount of sodium bicarbonate used was 1.39 pound per square foot blasted. It is estimated approximately 1 pound per square foot would be used during a full-scale decontamination. The volume of waste generated during the demonstration project averaged 11.9 gallons per test grid. It is estimated the volume of waste generated during full-scale decontamination would be approximately 1.0 gallon per square foot. Treatment of the residual waste achieved a significant 71 percent waste volume reduction.

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Last modified: 11/08/08