Effects of EDTA and Bicarbonate on U(VI) Reduction by Reduced Nontronite

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.