Widespread Fe-bearing clay minerals are potential materials capable of reducing and immobilizing U(VI). However, the kinetics of this process and the impact of environmental factors remain unclear. Herein, we investigated U(VI) reduction by chemically reduced nontronite (rNAu-2) in the presence of EDTA and bicarbonate. U(VI) was completely reduced within 192 h by rNAu-2 alone, and higher Fe(II) in rNAu-2 resulted in a higher U(VI) reduction rate. However, the presence of EDTA and NaHCO3 initially inhibited U(VI) reduction by forming stable U(VI)-EDTA/carbonato complexes and thus preventing U(VI) from adsorbing onto the rNAu-2 surface. However, over time, EDTA facilitated the dissolution of rNAu-2, releasing Fe(II) into solution. Released Fe(II) competed with U(VI) to form Fe(II)-EDTA complexes, thus freeing U(VI) from negatively charged U(VI)-EDTA complexes to form positively charged U(VI)-OH complexes, which ultimately promoted U(VI) adsorption and triggered its reduction. In the NaHCO3 system, U(VI) complexed with carbonate to form U(VI)-carbonato complexes, which partially inhibited adsorption to the rNAu-2 surface and subsequent reduction. The reduced U(IV) largely formed uraninite nanoparticles, with a fraction present in the rNAu-2 interlayer. Our results demonstrate the important impacts of clay minerals, organic matter, and bicarbonate on U(VI) reduction, providing crucial insights into the uranium biogeochemistry in the subsurface environment and remediation strategies for uranium-contaminated environments.
Keywords: U(VI) reduction; U(VI) species transformation; U(VI)−EDTA complex; U(VI)−carbonato complexes; reduced nontronite rNAu-2.