Be sure to wash thoroughly after handling and follow good industrial hygiene practices. The materials should be stored at temperatures below 140ºF.

A recent trial of SAMMS (self-assembled monolayers on mesoporous supports) at the Idaho National Laboratory on the residual liquid wastes from a V-9 tank achieved complete success in reducing mercury well below the EPA TCLP limit of 0.20 mg/L. The agency and the Idaho Department of Environmental Quality (DEQ) approved a variance for the stabilization of the high levels of mercury (greater than 260 mg/kg) in this waste using SAMMS absorbent.

This variance is the first granted by these agencies for treatment of high mercury waste that typically require treatment via roasting, retorting or incineration with mercury recovery. Treatment of the V-9 tank waste with SAMMS resulted in reducing mercury concentrations from 1.6-percent to a leachable concentration of 0.02-0.03 mg/L using the EPA's Toxicity Characteristic Leaching Test (TCLP).

This was about one tenth of the treatment standard established in the variance. The success of the system in stabilizing mercury-laden wastes provides proof of a more direct and effective method for managing mercury-contaminated liquid radioactive waste materials.

Department of Energy legacy wastes

Enlarge this picture Table 1. Mercury concentration for untreated waste in V-9 tank.

Mercury was used in many chemical processes involved in nuclear fuel processing conducted since World War II at various federal facilities within the Department of Energy complex. These operations generated substantial quantities of mixed hazardous and radioactive waste, containing high levels of mercury (greater than 260 mg/kg).

These legacy wastes presented special challenges for remediation due to the combination of radioactivity, mercury and organic contamination, which precluded most normal treatment processes. The inability to separate these disparate hazards eliminated the potential for reclaiming or recycling the mercury via either the roasting or retorting with mercury recovery (RMERC) or incineration with mercury recovery (IMERC) processes.

The highly mercury-contaminated V-9 waste is one example of mercury-laden legacy waste where the EPA's regulatory-prescribed treatment process is not appropriate, and alternative treatment approaches such as use of the SAMMS should be evaluated.

The V-9 tank

Enlarge this picture Table 2. Organic concentrations for untreated waste in V-9 tank.*F001-listed contaminants

The V-9 storage tank^[1] contained significant quantities of mixed organic solvents, mercury and radioactive isotopes. The radioactive isotopes in this waste were primarily the result of the examination and destructive testing of spent nuclear fuel.

Tables 1, 2 and 3 record the hazardous inorganic and organic concentrations that were present in the residual V-9 waste. Mercury (at approximately 1.6 percent by weight) may have existed as several species, such as elemental mercury, organic mercury or ionic water-soluble mercury. In addition, there were substantial quantities of listed organic solvents – namely trichloroethylene (TCE), tetrachloroethylene (PCE) and trichloroethane (TCA) – in the waste. As a result, the V-9 waste was classified under EPA regulations as both an F001 and D009 listed hazardous waste.

Land disposal of F001 and D009 waste must meet certain strict limits on leaching due to environmental effects, such as rain runoff, and acidic or basic conditions created by soil.

F001 and D009 wastes

Enlarge this picture Table 3. Radionuclide concentration for untreated waste.

The F001 category^[2] is applied to wastes containing any of the spent halogenated (volatile organic) solvents used in degreasing (identified with asterisks in Table 2). Treatment of these solvents, to a concentration of 6 mg/kg each, was required before they could be disposed of at an appropriate disposal facility within the Idaho National Laboratory.

The D009 category means the waste exhibits a TCLP mercury leachability of greater than 0.20 mg/L. Treatment using standard mercury absorbents is generally accepted, provided the total mercury concentration within this waste is below 260 mg/L. If mercury concentrations are above 260 mg/L, however, EPA regulations currently specify that mercury must also be removed from the waste, using retorting or incineration to support mercury recycling. The waste in the V-9 tank was far above this limit of 260 mg/kg.

Methods of treatment

An artist construction of the chemical structure of the materials.

A 2003 EPA notice of data availability concluded that adsorbent methods such as amalgamation and sulfur stabilization methods all release high levels of mercury in certain environmental conditions likely to be found in landfills. Therefore, these processes were not deemed acceptable for general use at that time, leaving only incineration or retorting available as acceptable processes.^[3]

While the existing LDR treatment requirement for radioactively contaminated D009 waste containing greater than 260 mg/kg of total mercury was RMERC or IMERC, the EPA also stated that neither technology was appropriate to apply to radioactively contaminated waste like the V-9 wastes. The agency's reasoning was that the recovered mercury still would be radioactively contaminated.

Since there is no known use for the recovered radioactively contaminated mercury, these technologies were considered inappropriate for this type of waste. As a result, the EPA and the DEQ allowed the application of an alternative method of stabilizing the mercury in the V-9 wastes using SAMMS, a thiol-SAMMS product, for this specific waste stream. The requirement for stabilization was that the final stabilized product would pass the TCLP.

The EPA's TCLP test method 1311^[4] was designed to determine the mobility of organic and inorganic constituents subject to the Resource Conservation and Recovery Act (RCRA) standards. The variance approved by the EPA and the DEQ required that the V-9 waste must meet the TCLP target of less than 0.2 mg/L of leachable mercury before it can be considered acceptable for disposal.

Preliminary study

Enlarge this picture Table 4. The TCLP results for the test samples after solidification (<0.2 mg/kg).

Due to the hazardous nature of the liquid in the tank, the initial bench experiment used a 25-ml sample from the V-9 tank, diluted with 65 ml of water and tested by TCLP. The adsorption isotherm was used to estimate the required mass of SAMMS to adsorb the mercury in solution to acceptable levels to pass TCLP after stabilization. Testing was conducted on five samples with the following mass loadings of SAMMS: no SAMMS, 0.64-, 1.6-, 4- and 10 times the predicted amount as estimated by the company that would be required to adsorb the mercury. Due to the complex nature of the waste matrix, a conservative experiment evaluated higher adsorbent mass loadings than the equilibrium predictions would normally specify.

Samples were vigorously mixed using a magnetic stirrer for 48 hours. Samples were obtained for TCLP and the remaining solution was solidified using Water Works SP-400 (10 percent by weight). After waiting one day for solidification, the mass was tested for TCLP.

The laboratory procedure incorporated SW-846 method 7471A requirements for solid samples on a Perkin Elmer FIMS CVAA instrument. Table 4 records that the laboratory was able to show that the sample results meet the TCLP limits for mercury provided the level of addition was greater than the amount predicted to absorb all of the mercury.

Final V-9 tank treatment

Enlarge this picture Table 5. RCRA TCLP test for mercury.

Using the measured concentration of mercury (1.6 percent), the laboratory estimated that 13.5 pounds of mercury was in the V-9 tank, and determined to add 1.4 to 1.6 times the estimated mass of adsorbent predicted by the equilibrium adsorption isotherm after the organics were sparged. According to Table 5, this ratio should yield a TCLP value of about 0.037 mg/L, well below the limit of 0.20 mg/L. The materials were was added with an aqueous mixture of isopropyl alcohol, to increase its wetting potential in the aqueous V-9 waste.

Table 5 summarizes the result of the V-9 treatment. All five samples easily met the treatment standard. Upon demonstration that the VOCs had also met their treatment standard through sparging, the waste was solidified for disposal at the Idaho CERCLA disposal facility. PE