Preparation of nano-Fe0 modified coal fly-ash composite and its application for U(VI) sequestration

Abstract

The nanoscale zero valent iron (NZVI) modified coal fly ash (CFA) magnetic composites (NZVI/CFA) were synthesized from the waste CFA and applied for the high-efficiency sequestration of U(VI) ions from water solution. The sequestration capacity of U(VI) ions on NZVI/CFA (0.62 mmol·g−1) was larger than that on bare CFA (0.25 mmol·g−1) or NZVI (0.38 mmol·g−1). The sequestration of U(VI) on NZVI and NZVI/CFA was primary controlled by outer-sphere surface complexation at low pH, and by inner-sphere surface complexation at high pH. According to the thermodynamic data and the adsorption isotherms' analysis, the U(VI) sorption process to the NZVI/CFA was spontaneous and endothermic. The main reaction mechanism was that, most of the soluble U(VI) ions were firstly adsorbed to NZVI/CFA and then the surface adsorbed U(VI) was partly reduced to U(VI) on the porous NZVI/CFA composites, and this synergistically affected U(VI) removal from solution to NZVI/CFA. The results of DFT calculation indicated that the NZVI/CFA had a strong sequestration capacity towards U(VI) and U(IV) ions, and the binding energy of U(IV) species was larger than that of U(VI) species, which evidenced the higher sorption of U(IV) than U(VI) on NZVI/CFA. The results showed that NZVI/CFA was an appropriate material for U(VI) sequestration from large volumes of actual effluent in wastewater cleanup.