For many years, mercury was used at DOE sites as shielding for radioactive materials, in instrumentation, and in processes for concentrating certain isotopes in weapons production. Much of this mercury was lost to the environment or to process sludge and debris. Today, DOE is responsible for thousands of cubic meters of mercury waste that will require treatment to meet EPA regulations before it can be accepted into landfills.

What’s the baseline?
EPA distinguishes between mercury waste treatments on the basis of the concentration of mercury in the waste to be treated. For waste with mercury concentrations below 260 ppm, EPA specifies stabilization before disposal. For waste with mercury concentrations above 260 ppm, EPA specifies retorting, a process for separating elemental mercury from its matrix for the recovery and recycle or reuse of the elemental mercury. However, EPA’s treatment standards for mercury were set with mercury not radiologically contaminated in mind. Separation of mercury from mixed waste results in mercury that is often still radioactive, which precludes its recycle or reuse and requires more treatment for disposal. For these reasons, EPA has agreed to consider stabilization as a treatment standard for waste that is contaminated with mercury at concentrations above 260 ppm, as well as below that threshold.

MWFA sponsors a bakeoff
While great desserts won’t be created, this kind of bakeoff will produce waste forms to provide a basis of comparison among different processes for stabilizing mercury concentrations in excess of 260 ppm. Up to three mercury stabilization processes from commercial vendors and the sulfur polymer stabilization/solidification (SPSS) process developed by Brookhaven National Laboratory will be compared. Raduce, Inc. will demonstrate its vacuum retort system, which is the baseline for recovering mercury for reuse. (For a description of this technology see “Spotlight on the Mixed Waste Focus Area”). To get the commercial vendors on board, the Mixed Waste Focus Area (MWFA) is negotiating with International Technologies, Allied Technology Group, and Nuclear Fuel Services.

The waste forms produced during the bakeoff will be evaluated on the basis of leachability testing specified by EPA’s Toxic Characteristic Leach Procedure (TCLP) and waste form evaluation protocols developed by Rutgers University that may eventually replace or augment the TCLP analysis. To be eligible for disposition in Subtitle C landfills, stabilized mercury waste must meet EPA’s current Phase III Land Disposal Requirement—leachability must be less than 0.2 mg/L (or 200 ppb) to pass. EPA is proposing a stricter standard—the Phase IV Universal Treatment Standard, or UTS—that would set leachability of mercury from a stabilized waste form at 0.025 mg/L (or 25 ppb). By meeting this proposed standard, DOE waste could be disposed of at a Subtitle D landfill as a nonhazardous low-level radioactive waste—a cheaper alternative than disposal in Subtitle C landfills.

Waste from Brookhaven pits
All competitors in the bakeoff will demonstrate their stabilization processes on similar contaminated soil and debris that BNL excavated during the summer of 1997 from 55 waste pits collectively known as the Animal/Chemical Pits and Glass Holes. With assistance from BNL’s Environmental Restoration Division (EM-40), waste was stored, segregated, and characterized. Two B-25 boxes were then filled with approximately 4,990 kg of radioactive mercury–contaminated soil. Average total mercury concentrations of the two boxes were 6,750 mg/kg and 18,000 mg/kg. TCLP tests on representative samples from each box yielded mercury concentrations of 3.56 mg/L and 0.26 mg/L—both measurements above EPA’s proposed UTS of 25 ppb, as well as the current Land Disposal Restriction limits of 200 ppb.

The two boxes also differed in isotopic mixture and concentrations. One contained relatively high concentrations of americum-241, and the other primarily europium-152 and radium-226. In preparation for the bakeoff, lab technicians subdivided each box of soil into seven 55-gallon drums: drums containing americium were labeled A1 through A7; drums containing europium were labeled E1 through E7. During repackaging, the contents among each set of seven drums were homogenized by manually shoveling small scoops into each drum in turn.

SPSS is up first
Brookhaven’s sulfur polymer stabilization/solidification (SPSS) process, whose development MWFA helped fund, is the first technology to show what it can do at the bakeoff. SPSS is an application of Brookhaven’s patented sulfur polymer cement (SPC) encapsulation process. SPC consists of 95 weight percent elemental sulfur reacted with 5 weight percent of an organic modifier to enhance mechanical integrity and long-term durability.

SPSS mercury treatment is conducted in two steps:

Stabilization. In the first step, mercury and powdered SPC react and form mercuric sulfide. The reaction vessel is placed under an inert gas atmosphere to prevent the formation of mercuric oxide, a water soluble and highly leachable compound. The reaction vessel is heated to about 40°C to accelerate the reaction, and the materials are mixed until the mercury is completely reacted with the sulfur.

Solidification. When the mercury is chemically stabilized, additional SPC is added, and the mixture is heated to 130°C until a homogeneous molten mixture is formed. It is then poured into a suitable mold, where it cools to form a solid waste form.

During the bakeoff, Brookhaven personnel used an off-the-shelf, pilot-scale vertical cone blender to mix the soil and SPC. They mixed 12 batches of soil from two drums (A-4 and E-1) and found that the viscosity of the mixture limited waste loading to 60 weight percent soil. At 70 weight percent soil, the mixture stuck to the walls of the blender, requiring the technicians to manually scrape the mixture into the collection container. These results suggest that modifications in the mixer design to include a wiper blade and a rotating mixing auger would help the SPSS process achieve increased waste loadings. At 60 weight percent soil, the volume of the waste form was the same as the volume of untreated waste; i.e., there was no increase in volume.

Analysis of SPSS results
An analysis of the bakeoff data reveals that SPSS-processed soils passed the current EPA LDR standard for mercury. While the six batches of treated soil originating from Drum E-1 obtained a composite TCLP of 147 ppb, the six batches from Drum A-4 and a another batch (consisting of residuals from both E-1 and A-4) benefited from an optimized formulation. This second run obtained a composite TCLP of 0.5 ppb, well below the UTS level for mercury of 25 ppb.

Mass balance measurements confirmed that the processed mercury was captured in the final waste form and not volatilized and captured in the off-gas treatment system. Analysis of samples taken from the liquid condensate, the cryogenic trap, and the carbon filter of the off-gas system revealed that only 0.33 percent of the processed mercury was released and captured in the off gas—a capture efficiency of 99.7 percent in the final waste form.

SPSS gets down to work
It remains to be seen how SPSS’s performance in the bakeoff will stack up against other stabilization processes yet to be demonstrated. But SPSS is already a winner in one sense—it’s been used twice to stabilize DOE mercury-contaminated waste streams. Following development and testing of the SPSS process at BNL, the process treated about 25 kg of actual radioactively contaminated elemental mercury, which was BNL’s current inventory of stored mixed waste mercury. This deployment of an OST-funded technology was supported under MWFA’s Quick Win Program.

The laboratory’s Environmental Restoration Division (EM-40) funded the second deployment. The 1997 excavation of the Animal/Chemical Pits and Glass Holes yielded about 65 kg (144 pounds) of elemental mercury mixed waste that SPSS readied for disposition. Pilot-scale processing was completed in March.

Remember, when…
The bakeoff isn’t the first time MWFA has supported demonstrations of different vendors’ mercury stabilization processes. In FY98, Allied Technology Group, International Technologies Corporation, Nuclear Fuel Services, and GTS Duratek demonstrated their processes on surrogate or actual waste with mercury concentrations of less than 260 ppm. The demonstrations gave assurance to MWFA that commercial processes are available that meet EPA Phase IV Universal Treatment Standards for toxic metals and the waste acceptance criteria for waste disposal at Envirocare of Utah’s site. The vendors stabilized surrogate waste spiked with a variety of mercury species and actual mixed wastes. All the demonstrated processes required bench-scale preliminary tests to adjust the chemistry to the specific waste stream—a complication that adds to treatment costs.