Mercury sequestration in alkaline salt low-level radioactive waste

Liquid low-level radioactive waste at the Savannah River Site contains several species of mercury, including inorganic, elemental, and methylmercury. This waste is solidified and stabilized in a cementitious waste form referred to as saltstone. Soluble mercury is stabilized as β-cinnabar, HgS as the result of reaction between the mercury and sulfur present in blast furnace slag, one of the cementitious reagents. In this investigation, Mersorb®, a commercial granular activated carbon impregnated with sulfur, was evaluated as a pretreatment to remove mercury from the solution prior to cementation. Mersorb® was found to remove more than 96 mass percent of the methylmercury in simulated tank waste solution when the mass ratio of Mersorb® to mercury was above 2.5. Slag sequestered relatively more inorganic mercury than organic mercury in simulated tank waste after 24 h of contact. This is likely due to the mercury-carbon bond being more covalent than the mercury-oxygen bond and therefore more difficult to break and slower to form HgS.