Efficient U(VI) removal from aerobic solution by synergistic interaction of nano zero-valent iron with g-C3N4 and assessment of toxicity to microorganism

Abstract

The graphitic carbon nitride (g-C3N4) supported nano zero-valent iron (nZVI) composites (g-Fe) were synthesized to assess the performance of U(VI) removal and bacterial inactivation. Under aerobic conditions at pH 6.0, 97.0 % of U(VI) was removed within 60 min, while only 7.2 % or 22.6 % of U(VI) were removed by pristine g-C3N4 or nZVI. Effects of operating parameters (pH, temperature, initial U(VI) concentration, and g-Fe dosage) on U(VI) removal were also studied under aerobic conditions. The U(VI) adsorption followed a pseudo-second-order kinetic model, and the adsorption process was in accordance with the Langmuir adsorption isotherm model. This indicated that the removal of U(VI) by g-Fe was a chemisorption and homogeneous monolayer adsorption process. The maximum removal capacity of U(VI) was 952.38 mg/g at pH = 6.0. The absorbed uranium on the g-Fe composite were confirmed to be solely U(VI) under aerobic conditions, while both U(VI) and U(IV) were observed under anaerobic conditions. Hence, adsorption rather than reduction serves as the primary mechanism for U(VI) removal in aerobic environments. Escherichia coli or Bacillus subtilis could be inactivated by nZVI and g-Fe composite, but the biotoxicity of g-Fe was much lower than nZVI. Physical isolation reduced the biotoxicity of nZVI, and the residual biotoxicity of nZVI after excluding physical contact should be ascribed to oxidative stress or free reactive oxidative species (ROS). The biotoxicity of g-Fe was irrelevant to physical isolation, proving that oxidative stress or free ROS might dominate the inactivation mechanisms. The g-Fe composite, an eco-friendly, facile and sustainable material, can be developed for U(VI) removal from radioactive wastewater.