Batch Simulation of In Situ Uranium Recovery and Subsequent Groundwater Restoration with An Iron-Sulfide Amendment

In-situ recovery (ISR) used for uranium extraction employs a leaching solution (lixiviant) containing an oxidant and carbonate to solubilize reduced uranium minerals by oxidative dissolution and complexation of uranium, respectively.  Current restoration techniques following ISR involve a groundwater sweep whereby multiple pore volumes of groundwater are pumped from mining-affected areas to flush out residual dissolved uranium.   In this study, the in-situ uranium recovery process was simulated in a batch-leaching experiment using 50 g of solids from a uranium roll front deposit in 250 mL of a synthetic lixiviant (deionized water, hydrogen peroxide and sodium carbonate).  Another stage of the experiment examined the use of mackinawite, a nanoparticulate reduced iron monosulfide (FeS), as an amendment following groundwater sweep to promote the sequestration of uranium.  Results show that the synthetic lixiviant leaches significant amounts of uranium from the solid phase and reaches steady state concentrations within 4 hours.  The simulated groundwater sweep, which involved the replacement of lixiviant solution containing uranium with fresh deionized water, was effective at lowering residual concentrations of uranium.  However, the concentrations remained nearly 100 times the Environmental Protection Agency’s (EPA’s) maximum contaminant level (MCL) of 30 ug/L.  The addition of mackinawite to the batch reactor during the restoration simulation promoted the precipitation of a reduced uranium (IV) solid tentatively identified as uraninite by x-ray absorption spectroscopy, resulting in concentrations of uranium in solution consistent with the EPA’s MCL.  This research provides a basis for further examination of the feasibility of using mackinawite as a remediation medium for groundwater restoration following ISR mining and in-situ remediation of uranium contaminated groundwater.