CaCO3 coated nanoscale zero-valent iron (nZVI) for the removal of chromium(VI) in aqueous solution

Abstract In this study, calcium carbonate (CaCO3) coated nanoscale zero-valent iron (referred to as C-nZVI) was successfully synthesized and used for the removal of Cr(VI) in aqueous solution. The obtained C-nZVI particles were observed to be chain-like structure of spherical particles with a smooth surface by Transmission Electron Microscope (TEM). The magnetic hysteresis loop manifested that C-nZVI (CaCO3/Fe=1:4, mass ratio) was ferromagnetic and the saturation magnetization (Ms) was 115.48 emu/g, revealing that C-nZVI had favorable properties of separation, recovery and reuse. Regarding the Cr(VI) removal, no positive correlation between the removal efficiencies and CaCO3/Fe mass ratio increasing from CaCO3/Fe=0.5:4 to 4:4. Meanwhile, complete removal of Cr(VI) was observed at initial pH=3.0 and initial Cr(VI) concentration of 5 mg/L and the removal efficiency gradually decreased with increasing the initial pH and initial Cr(VI) concentration. The water chemistry had a more or less influence on the reactivity of C-nZVI. Specifically, the presence of Ca2+ and Mg2+ facilitated the Cr(VI) removal by C-nZVI, but the PO43- and humic acid (HA) impeded the reactivity of C-nZVI. NO3- is generally considered to have an adverse effluence on nZVI, but it displayed a positive effect on C-nZVI, which should be ascribed to the promotion of C-nZVI corrosion. Additionally, C-nZVI aged for 2 d and 30 d in air had a similar removal efficiency (83.2% and 82.6%) of Cr(VI) as fresh C-nZVI (85%), whereas the Cr(VI) removal efficiency by C-nZVI aged for same aging times in aqueous solution were approximate 65.9% and 46.6%, respectively.