Enhanced removal of uranium(VI) by nanoscale zerovalent iron supported on Na-bentonite and an investigation of mechanism.

The reductive removal of U(VI) by nanoscale zerovalent iron (NZVI) was enhanced by using Na(+)-saturated bentonite (Na-bent) as the support, and the mechanism for the enhanced removal were investigated comprehensively. Under the same experimental conditions, NZVI supported on the negatively charged Na-bent showed much higher removal efficiency (99.2%) of cationic U(VI) than either bare NZVI (48.3%) or NZVI supported on the positively charged bentonite (Al-bent) did. Subsequent experimental investigations revealed the unique roles of bentonite on enhancing the reactivity and reusability of NZVI. First, Na-bent can buffer the pH in reaction media, besides preventing NZVI from aggregation. Second, Na-bent promoted the mass transfer of U(VI) from solution to NZVI surface, leading to the enhanced removal efficiency. Third, the bentonite may transfer some insoluble reduction products away from the iron surface according to X-ray absorption fine structure (XAFS) study. Finally, Na-bent as the adsorbent to Fe(II) makes it more reactive with U(VI), which enhanced stoichiometrically the reduction capacity of NZVI besides accelerating the reaction rate.