Step-wise co-removal and solidification of U and Mo contaminants in mine drainage with ferric salt and tailings

The mining of strategic U-Mo deposits invariably leads to the generation of wastewater with significant risks of radioactive and heavy metal pollution. This study explores a novel process for the simultaneous precipitation-adsorption of U and Mo from mine drainage, with subsequent solidification of the radioactive Fe(OH)3 sediments using tailings. The results demonstrate that, under specific conditions, such as 33.6 mg/L Fe3+ and pH 6.0–7.0, more than 97 % of U and Mo can be efficiently co-removed, and the treated water exhibits a gross α radioactivity of less than 0.2 Bq/L. The U-bearing Fe(OH)3 sediments are transformed into general solid wastes, exhibiting low α radioactivity of less than 1.0 Bq/kg for 238U, 232Th or 226Ra, after being blended with tailings in a mass ratio of 1:50. The reaction, adsorption and solidification mechanisms are further elucidated. The U and Mo are primarily precipitated and adsorbed in the forms of UO2(OH)2 and Fe2(MoO4)3 respectively. The U-bearing Fe(OH)3 particles are firmly encapsulated by negatively charged particles of phlogopite, quartz, and orthoclase, and agglomerate into large particles within the solidification product, thereby reducing U dissolution in simulated acid rain. Ultimately, a sustainable treatment flowsheet is established for the management of U-Mo ore mine drainage and tailings.