Comprehensive study on the removal of chromate from aqueous solution by synthesized kaolin supported nanoscale zero-valent iron

A removal mechanism of chromate (Cr) by synthesized kaolin supported nanoscale zero-valent iron (K-nZVI) from aqueous solution is demonstrated. Parameters which potentially influenced the functioning of K-nZVI have been investigated as well. Based on the scanning electron microscopy, Fourier transform infrared spectroscopes, X-ray crystal powder diffraction and X-ray photoelectron spectroscopy identifications, we confirm that amorphous Fe-0 core/FexOy shell nZVI can be successfully loaded into the pores and cracks, and onto the surface of kaolin. Removal efficiency of Cr by K-nZVI decreased with increasing initial pH and Cr(VI) concentration, but increased while K-nZVI dosage increased. Humic acid and phosphate had similar dual impacts on chromium removal by K-nZVI, and the inhibitory effect was obvious at high concentrations in spite of their different reaction mechanisms. In contrast, high concentrations of sulfate and nitrate could advance the chromium removal. Adsorption isotherms indicate that the removal processes are endothermic. The data obtained can be better explained with Langmuir than Freundlich model. At the conditions of 318K and optimized pH 4.0, the maximum adsorption capacity was 33.39mgg(-1) illustrating that K-nZVI was effective for the removal of total Cr. The removal mechanism is proposed to divide into four phases, including: (1) aqueous Cr(VI) ions are captured on the surface of K-nZVI; (2) the captured Cr(VI) are partly reduced to Cr(III) accompanied by Fe-0 oxidizing to Fe2+; (3) part of oxidized Fe2+ continues to reduce Cr(VI); and (4) produced Cr(III), Fe2+, and Fe3+ are formed passivation layers on the K-nZVI surface which prevent further removal of chromium and result in redundant Fe-0.