Tank 50 Simulant Grout and Toxicity Characteristic Leaching Procedure (TCLP) Results for Methyl Mercury Waste Acceptance Criteria (WAC) Limit

Abstract

Preliminary non-baseline studies were recently performed by the Savannah River National Laboratory (SRNL) to investigate the effects of various concentrations of mercury in saltstone on the Toxicity Characteristic Leaching Procedure (TCLP). Tank 50 salt solution simulants were prepared containing both chromium and ionic mercury (Hg(II)) in addition to elevated concentrations of methylmercury (MeHg). These simulants were used to make saltstone samples that cured for 28 days and were evaluated using the TCLP. Saltstone samples containing the full premix constituents of slag, fly ash and cement at the mass ratios of 45,45,10 wt% were prepared with total Hg concentrations between 147 mgHg/L to 372 mgHg/L containing primarily MeHg. Total Hg mercury concentrations in the corresponding TCLP leachates ranged from 0.035 mg/L to 0.095 mg/L and were compared to the reference Total Hg leachate TCLP limit of 0.2 mg/L. Consequently, at these leached mercury concentrations, saltstone did not exhibit the characteristic of mercury toxicity.Cement-free saltstone formulation containing 60 wt% slag and 40 wt% fly ash prepared with similar total Hg concentrations between 147 mg/L to 372 mg/L resulted in TCLP total Hg leachate values for this formulation that were 9% to 18% lower than the total Hg leachate values shown above for the full premix samples. An explanation for this result is that sulfide in slag is responsible for stabilizing mercury as mercury sulfide, which has very low solubility. Additionally, replacing the cement with slag could influence the overall saltstone reaction products, hydration gel chemistry and pore structure, all of which might impact Hg retention via encapsulation and adsorption.The saltstone particle size used in these TCLP tests ranged from 4.760 mm (0.1874 inches) to 9.525 mm (0.375 inches), which is the same size range used for quarterly Tank 50 saltstone regulatory testing. Test results presented in this report on nonradioactive saltstone suggest that the current waste acceptance criteria (WAC) limits for total Hg (325 mg/L) and MeHg (350 mg/L) will support production of saltstone that does not exhibit the characteristics of mercury toxicity (TCLP leachate for total Hg limit of less than 0.2 mg/L). These results could be confirmed for saltstone made with Tank 50 solution spiked with these higher concentrations of MeHg in proposed future studies.These simulant exploratory tests suggest that the current WAC limits on total Hg (325 mg/L) and MeHg (350 mg/L) are adequate for protecting the TCLP Hg limit of 0.2 mg/L. Further testing with more replicates at current and higher concentrations of Hg spikes, as well as similar testing with Hg-spiked radioactive Tank 50 solutions could provide further insight into the Hg behavior/retention in saltstone samples. Initial tests for the direct analysis of total Hg in saltstone yield results similar to those expected from the calculated Hg in saltstone based on the simulant Hg analysis and the batching amounts used to prepare the saltstone.