Formation of active zero-valent iron by simple co-grinding with CaCO3 to protect fresh active surface for efficient removal of hexavalent chromium

A novel composite material consisting of zero-valent iron (ZVI) and CaCO3 was successfully prepared through a simple grinding operation. The as-synthesized ZVI/CaCO3 composites were applied to the reduction precipitation of hexavalent chromium (Cr(VI)) and a tremendous improvement in reaction capacity was obtained compared with raw ZVI and ground ZVI sample without CaCO3 addition. Many factors of the grinding operation and ZVI/CaCO3 mass ratios were investigated to evaluate material properties and to study reaction mechanism. A high removal efficiency of 99.68% within just 30 min on Cr (VI) (10 mg/L) was obtained by the ZVI/CaCO3 composite with ZVI mass ratio at 30 wt%. X-ray diffraction (XRD), Fourier transformed infrared (FTIR), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS) were used to investigate the chemical constitution and physicochemical properties of composites before and after the reaction. Addition of CaCO3 played very important role in both grinding iron particles into fine powders and protecting the fresh surface of active ZVI to enable the superior performance as zero-valent iron. Besides the environment-friendliness of the process itself without any pollutant emission, waste solid materials containing iron particles may serve as the starting sample to decrease the product cost.