Reduction and potential remediation of U(VI) by dithionite at an in-situ recovery mine: Insights gained by δ238U

Restoration Insights Gained from a Field Deployment of Dithionite and Acetate at a Uranium In Situ Recovery Mine

“…As a prelude to the work reported in this paper, a small-scale study was performed at the SRH mine, where sodium dithionite (Na 2 S 2 O 4 ) was used to reduce uranium following ISR mining [22,23]. In this study, two single-well push-pull field experiments were conducted, in which dithionite was injected into an ore zone that had not undergone any restorative treatment, such as groundwater sweep or reverse osmosis.…”

“…After the dithionite was deployed, the wells were "shut in" for ~3 days to allow it to react with ore-zone sediments, and then ore-zone water was drawn back into the wells until uranium concentrations returned to pre-test levels. This procedure enabled estimates of volumes of ore-zone water treated (i.e., volumes from which U had been removed) and moles of U reduced in each test [22,23]. Uranium reduction was confirmed by uranium isotope ratio measurements [22,23].…”

“…This procedure enabled estimates of volumes of ore-zone water treated (i.e., volumes from which U had been removed) and moles of U reduced in each test [22,23]. Uranium reduction was confirmed by uranium isotope ratio measurements [22,23]. Interestingly, the two single-well dithionite deployments effectively treated similar volumes of ore-zone water per mole of dithionite introduced (460 and 530 L/mole dithionite), but the amounts of uranium reduced per mole of dithionite were significantly different (0.023 and 0.044 moles U/mole dithionite, with these values being approximately proportional to the U concentrations in the waters drawn into the two wells).…”

“…In the amendment test, 8400 kg of nominally 90% purity sodium dithionite, or about 43,400 moles of dithionite, was used. Assuming about 500 L of water treated per mole of dithionite (estimated from the single-well dithionite tests [22,23]), this was enough dithionite to treat about 21,700 m 3 of water, which was about 3.1 times the 6900 m 3 estimated to be accessed by the six well patterns in the tracer test. The margin of a factor of around 3 was to allow for uncertainties in going from single-well tests to a multi-well configuration, and to compensate for some additional volume accessed in the aquifer because the dithionite injection was planned as a 10% over-injection into the injection wells (i.e., the combined injection flow rate into all injection wells would exceed the combined production flow rate from all production wells by 10%).…”

“…The 2σ precision of the ratio measurements was 0.11‰. Details of the isotope work, including descriptions of the sample purification procedure, spikes, and quality control, can be found in [23].…”