Polyethylene Encapsulation of Mercury paper

Elemental mercury, contaminated with radionuclides, is a problem throughout the Department of Energy (DOE) complex. This paper describes work conducted at Brookhaven National Laboratory (BNL) supported by the DOE Office of Science and Technology Mixed Waste Focus Area on the development and testing of a process to immobilize elemental mercury, contaminated with radionuclides. The product is a monolithic solid waste form that is non-dispersible, will meet EPA leaching criteria, and has low mercury vapor pressure. The BNL Sulfur Polymer Stabilization/Solidification (SPSS) process (patent pending) mixes elemental mercury with an excess of powdered sulfur polymer cement (SPC) and additives in a heated vessel until all of the mercury is converted into mercuric sulfide (HgS). Additional SPC is then added and melted, resulting in a homogeneous molten liquid which is then poured into a suitable mold where it cools and solidifies. Waste forms containing as much as 33 wt% elemental mercury were formulated which successfully passed Environmental Protection Agency Toxicity Characteristic Leaching Procedure(TCLP) criteria, exhibited extremely low leachability when subjected to long term leaching, and significantly reduced vapor pressure compared with untreated mercury. In addition to technology development, this work resulted in the successful processing of the entire inventory of mixed mercury waste stored at Brookhaven National Laboratory (BNL).

BACKGROUND

DOE has estimated that > 38,000 m³ of mixed low-level and transuranic wastes contain mercury. These wastes are found at virtually every DOE facility throughout the U.S. Approximately 6 m³ of liquid elemental mercury are currently being stored and additional inventories are expected to be generated at planned treatment facilities such as the Defense Waste Processing Facility at the Savannah River Site and the Advanced Mixed Waste Treatment Facility at the Idaho National Environmental Engineering Laboratory.⁽¹⁾ In addition, treatment of other mercury wastes (e.g., soil, debris) through incineration (IMERC) and retort (RMERC) will result in additional volumes of elemental mercury requiring stabilization.

Elemental mercury contaminated with radionuclides, i.e., mixed waste mercury is a particularly challenging waste stream to treat. Conventional techniques such as hydraulic cement are not effective in containing mercury or mercury salts and higher temperature processes such as incineration or vitrification volatilize the mercury, requiring secondary treatment of off gasses.¹ The performance requirements (e.g., leachability, vapor pressure) of treated products are stringent. Mercury is a highly toxic metal, so Environmental Protection Agency (EPA) regulations contained in 40 CFR 261 restrict allowable concentrations of leachable mercury to very low levels (0.2 mg/l). Occupational Safety and Health Administration (OSHA) regulations restrict worker exposure to mercury vapors to a low Threshold Limit Value (TLV) of 0.05 mg/m³.

For liquid elemental mercury, EPA has identified amalgamation (AMLGM) as the appropriate treatment standard.⁽²⁾ This "amalgamation" requires that the mercury be combined with reagents such as copper, gold, or sulfur that result in a solid, non-volatile product. A study by Oak Ridge National Laboratory found mercury stabilization using sulfur to have better leaching properties and lower vapor pressures compared with other reagents.⁽³⁾ . Although referred to as amalgamation, combining mercury with sulfur results in mercuric sulfide, a new compound, not an amalgam or alloy, which is the result of mercury mixing with a metal. Although sulfur treatment produces a chemically stable dry powder, it does not provide any additional barrier to leaching and is susceptible to mechanical dispersion of the radioactive material. These limitations have significant health and safety consequences for the storage, transport, and ultimate disposal of treated mixed waste mercury. The objective of this work, supported by the DOE Mixed Waste Focus Area Quick Win program was to develop an improved treatment process for radioactively contaminated mercury that would address these deficiencies.

TECHNOLOGY DESCRIPTION

Sulfur Polymer Stabilization/Solidification (SPSS) is based on a patented mixed waste treatment technology previously developed at BNL.⁽⁴⁾ Sulfur Polymer Cement (SPC) consists of 95 wt% elemental sulfur reacted with 5 wt% of an organic modifier to enhance mechanical integrity and long-term durability. Previous testing conducted on sulfur polymer waste forms indicate excellent performance under anticipated disposal conditions.⁽⁵⁾^,⁽⁶⁾ SPSS mercury treatment is conducted in two stages. The first step is a reaction between mercury and powdered SPC, forming mercuric sulfide.

Since the BNL SPSS process includes chemical stabilization of the mercury yielding mercury sulfide, it meets EPA requirements for AMLGM. Equal masses of mercury and SPC are mixed in the reaction vessel, assuring nearly a sixfold molar excess of sulfur to mercury and facilitating a faster reaction of the mercury metal with sulfur. Prior to mixing, the reaction vessel is placed under inert gas atmosphere to prevent the formation of mercuric oxide (a water soluble and highly leachable compound) and a small quantity of additive is included to accelerate the reaction. The vessel is heated to ~ 40 C during the stabilization phase to accelerate the sulfide formation reaction and the materials are mixed until the mercury is completely reacted with the sulfur. Once the mercury is chemically stabilized, additional SPC is added and the mixture is heated at about 130 C until a homogeneous molten mixture is formed. It is then poured into a suitable mold where it cools to form a monolithic solid waste form.

PROCESS DEVELOPMENT

Bench-scale process development was conducted in 5-gallon steel drum agitated using a commercially available paint shaker (Red Devil, Minneapolis, MN). Quartz cobble was added to the mixture to enhance agitation of the waste and reagents. The mixing vessel was modified by the addition of electric resistance heating tape, enabling stabilization and solidification processing in a single vessel. This facilitated efficient processing and reduced health and safety risks associated with handling dispersed mixed waste powder. A thermocouple was inserted into the vessel and connected to a digital controller to precisely monitor and control the mixture temperature. The equipment was installed inside a walk-in fume hood, to allow any mercury or sulfur vapors to be safely dispersed through a vent port. Completion of the reaction was determined by periodically removing subsamples of the mixture and centrifuging to monitor for the presence of unreacted mercury. Samples were spun between 7,500 and 10,000 rpm for one hour. When unreacted elemental mercury remained in the mixture, a visible layer of mercury was observed on the surface of the tube. One liter batches of liquid mercury were processed between 4 and 8 hours.

During process development trials, various formulations were attempted. Final waste forms containing 33 wt% Hg, 64 wt% SPC, and 3 wt% additive were formulated which consistently passed EPA TCLP testing. However, due to the limited scope of this project, optimization of waste loadings was not conducted, so that higher waste loadings may be attainable.

WASTE FORM PERFORMANCE

Performance of final waste form products was evaluated by examining short and long-term leaching and mercury vapor pressure. Short term leaching and compliance with RCRA Land Disposal Restrictions was determined by conducting the EPA TCLP.⁽⁷⁾ Leaching was conducted using 50 grams of sample material, rather than the prescribed 100 grams, while maintaining the same relative reagent quantities required for a standard TCLP test. This modified version of the TCLP reduces the volume of waste generated without compromising the assay. The filtered TCLP solution was analyzed using a Liberty 100 Inductively Coupled Plasma (ICP) Spectrometer or, for greater sensitivity, the mercury cold vapor method. The cold vapor analyses (EPA method 7470) were performed using a Perkin Elmer Model 4000 Atomic Absorption spectrometer with a Perkin Elmer Model MHS-10 Mercury/Hydride system. For all TCLP tests performed in this study, the required pre-tests showed that TCLP solution I was required. Rather than destroy microencapsulation of the mercury by grinding final waste forms, TCLP specimens were fabricated as pellets, capable of passing through a 9.5mm sieve. Typical TCLP leach results summarized in Table 1 indicate that SPSS treatment of elemental mercury was successful in reducing Hg leachability by a factor of more than 100 times. Untreated mercury resulted in TCLP concentrations of 2,640 ppb (about 10 times higher than allowable limits), while SPSS treated mercury resulted in TCLP concentrations of 26 ppb (about 10 times lower than allowable limits).

Table 1. Typical Toxicity Characteristic Leaching Procedure Results

Hg Concentration, ppb

Untreated Elemental Hg 2640

SPSS Treated Elemental Hg

(33 wt% Hg, 64 wt% SPC, 3 wt% additive)
26

Maximum permissible TCLP Hg concentration 200

Long-term leachability was evaluated according to the Accelerated Leach Test, ASTM C-1308.⁽⁸⁾ This method is a dynamic leach test in which the distilled water leachant is replaced on a periodic basis. Data is evaluated using a related computer program that calculates incremental and cumulative contaminant fractions released, identifies predominant leaching mechanism(s) and effective diffusion coefficient, and enables prediction of long-term releases if diffusion is the controlling mechanism. Leach results closely match those predicted by the diffusion model, indicating that diffusion is the predominant leaching mechanism. Following 11 days of leaching, a total of only 5.8x10^-4percent of the mercury leached from the waste form. The effective diffusion coefficient was measured to be 4.15 x 10^-18.

Mercury vapor from the process vessel was tested using a commercial mercury vapor detector (Model MV-2 Mercury Vapor Sniffer) and Draeger tubes (Sargent Welch). A maximum concentration of 2 mg/m³ Hg was detected by Draeger tube at the vent port during initial stabilization processing. Following reaction to HgS and during the second stage heating to 130C for solidification, mercury vapor was below detection limits (0.05 mg/m³). To evaluate the effectiveness of treatment for reducing Hg vapor pressure, mercury vapor generated from elemental mercury and from a waste form were compared. Samples were sealed in a 250-mL plastic bottle and the mercury vapor was allowed to reach equilibrium at room temperature. An aliquot (5 mL) of the air from the head space of the bottle was sampled and measured using the mercury cold vapor apparatus of the AA spectrophotometer. Measured Hg vapor was reduced from an average of 103 g/l for untreated mercury to an average of 2.8 g/l for SPSS treated mercury .

TREATMENT OF ACTUAL MIXED WASTE MERCURY

Following process development and evaluation of waste form performance, about 25 kg of actual radioactively contaminated elemental mercury was treated using the SPSS process. This material represented the current inventory of stored mixed waste mercury at BNL. The inventory and characterization data for this waste is given in Table 2. Each batch of treated waste was discharged into about (3) one gallon metal cans for cooling and solidification to a monolithic solid. Five batches of waste were processed, resulting in a total of 15 waste forms. Subsamples were taken from each batch and a TCLP test was conducted on a 50 g composite sample. These wastes were successfully rendered non-hazardous, with a TCLP concentration of 50 ppb, well below the EPA allowable concentration of 200 ppb.

Table 2. Inventory and Characterization of BNL Mixed Waste Mercury

Radionuclide Activity, Ci Mass of Mercury, kg

Germanium, Ge-68 1 2.2

Carbon, C-14 25 < 0.2

Cesium, Cs-137 0.01 16.1

Tritium, H-3 1,000 6

CONCLUSIONS

A unique process (patent pending) to stabilize and solidify mixed waste mercury was successfully developed and tested using bench-scale processing equipment. The process:

reduces mercury solubility to enable compliance with EPA TCLP criteria

lowers mercury vapor pressure during processing and in the final product

eliminates dispersibility of the stabilized product

reduces leachability of radioactive constituents

With support from the Mixed Waste Focus Area Quick Win Program, the entire BNL inventory of mixed waste elemental mercury was successfully treated, allowing disposal of the resulting waste forms as conventional low-level radioactive waste. Current efforts are aimed at determining applicability to other mercury contaminated wastes and testing the process at pilot-scale.

ACKNOWLEDGMENTS

This work was funded by the U.S. Department of Energy Office of Science and Technology, Mixed Waste Focus Area.

REFERENCES

1. MWFA, "Mercury Amalgamation: Mixed Waste Focus Area Technology Development Requirements Document," INEL/EXT-97-00314, LMITCO, Idaho Fall, ID, March 1997.
2. US EPA, "Land Disposal Restrictions, Subpart D- Treatment Standards, " 40 CFR 268.40, Code of Federal Regulations, Office of the Federal Register National Archives and Records Administration, 1996.
3. Gorin, A.H., J. H. Leckey and L.E. Nulf, "Final Disposal Options for Mercury/Uranium Mixed Wastes From The Oak Ridge Reservation", Y/DZ-1106, August 1994.
4. Colombo, P., Kalb, P.D., Heiser, J. H. "Process for the Encapsulation and Stabilization of Radioactive, Hazardous and Mixed Wastes." U.S. Patent 5,678,234, October 14, 1997.
5. Kalb, P.D., J.H. Heiser, R. Pietrzak, and P. Colombo, "Durability of Incinerator Ash Waste Encapsulated in Modified Sulfur Cement," Presented at the 1991 Incineration Conference: Thermal Treatment of Radioactive, Hazardous, Chemical, Mixed, and Medical Wastes, Knoxville, TN, May 1991.
6. Kalb, P.D., J.H. Heiser, and P. Colombo, "Modified Sulfur Cement Encapsulation of Mixed Waste Contaminated Incinerator Fly Ash," Waste Management, Vol. 11, No. 3, pp. 147-153, Pergamon Press, 1991.
7. US EPA, "Toxicity Characteristic Leaching Procedure (TCLP)", 40 CFR 261, Appendix II, U.S. Environmental Protection Agency, Washington DC, September 19, 1994.
8. ASTM, "Accelerated Leach Test for Diffusive Releases from Solidified Waste and a Computer Program to Model Diffusive, Fractional Leaching from Cylindrical Waste Forms," ASTM C-1308-95, American Society of Testing and Materials, West Conshohocken, PA, 1995.

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