Improving the efficiency of in situ uranium leaching (remediation) with periodic injection and extraction: Insight from reactive transport modeling and field test

[Display omitted] •Physiochemical processes of in situ uranium leaching under varying pumping rates are revealed.•Interaction between chemical reactions and fluid dynamics is fully considered.•Periodic pumping promotes the spreading of agent but reduces chemical reaction rate.•Efficiency of in situ leaching varies non-monotonically with the period and phase shift of pumping rates. In situ leaching (and remediation) techniques are widely used to recover the economic minerals and remove toxic contaminants by injecting reactive agents into aquifers. However, the efficiency of in situ leaching (ISL) is often low under constant pumping rates attributed to the laminar characteristics of water flow in the porous aquifer. This study investigates the efficiency of ISL under periodic pumping by reactive transport modeling and field test. It is found that periodic but asynchronous injection and extraction can maximize the lateral spreading of reactive agents and reduce the “dead corner”, driven by the imbalance between the instantaneous injection and extraction rates. However, the agent concentration (controlling the reaction rate) is reduced with the spreading due to the dilution. In the illustrated uranium leaching aquifer, the increase in the spreading area and decrease in the chemical reaction rate play reverse roles on the efficiency of ISL. Moreover, the existence of calcite reacts with the agent to form gypsum blockage. Periodic pumping can alleviate the blockage because more agent reacts with uraninite instead of calcite. Despite the periodic pumping improving the efficiency of ISL, period and phase shift of pumping rates should be carefully optimized, regarding the interaction of chemical reactions and fluid dynamics.