Colloid Mediated Transport of Uranium(IV) in Sulfide-Rich Aquifers: An Overlooked Pathway for Uranium Remobilization under the Reductive Conditions

Abstract

Immobilization of uranium via reductive precipitation is considered for remediation of uranium contaminated groundwater, however, the potential risk deriving from the migration of colloidal-state uranium challenges the remediation effectiveness. In this work, interaction and cotransport of uranium with bentonite colloids (BC) and humic acid (HA) were investigated under reductive conditions through sets of pyrite doped quartz sand columns. Uranium showed considerable mobility as the tetravalent state in the reductive media, both BC and HA significantly affected the transport of uranium, which were regulated by hydrochemical conditions. High resolution transmission electron microscope revealed that the crystalline particles of tetravalent uranium distributing between 6-12 nm tended to attach onto BC particles, BC provided an efficient ride for U(IV) granule. Extended X-ray absorption fine structure revealed that the migration products of uranium existed as the form of non-crystalline U(IV) and crystalline UO_2+x. The reduction of uranium was inhibited in the presence of HA since the chelating and enwrapping of HA toward uranyl blocked the electron transfer process. Therefore, the boosted uranium transport in reductive media arising from perturbation of BC and HA should be taken into consideration when evaluating the potential migration risk and optimizing the reductive immobilization remediation for uranium contaminated aquifer.