Synthesis of zeolite-supported microscale zero-valent iron for the removal of Cr(6+) and Cd(2+) from aqueous solution

Zeolite-supported microscale zero-valent iron (Z-mZVI) was synthesized and used to remove heavy metal cation (Cd(2+)) and anion (Cr(6+)) from aqueous solution. Transmission electron microscope (TEM) confirmed that mZVI (100-200 nm) has been successfully loaded and efficiently dispersed on zeolite. Atomic absorption Spectroscopy (AAS) revealed the amount of stabilized mZVI was about 1.3 wt.%. The synthesized Z-mZVI has much higher reduction ability and adsorption capacity for Cr(6+) and Cd(2+) compared to bare nanoscale zero-valent iron (nZVI) and zeolite. Above 77% Cr(6+) and 99% Cd(2+) were removed by Z-mZVI, while only 45% Cr(6+) and 9% Cd(2+) were removed by the same amount iron of nZVI, and 1% Cr(6+) and 39% Cd(2+) were removed by zeolite alone with an initial concentration of 20 mg/L Cr(6+) and 200 mg/L Cd(2+). The removal of Cr(6+) by Z-mZVI follows the pseudo first-order kinetics model, and X-ray photoelectron spectroscopy (XPS) analysis confirmed that Cr(6+) was reduced to Cr(3+) and immobilized on the surface of Z-mZVI. The removal mechanisms for Cr(6+) include reduction, adsorption of Cr(3+) hydroxides and/or mixed Fe(3+)/Cr(3+) (oxy)hydroxides. The pseudo-second-order kinetic model indicated that chemical sorption might be rate-limiting in the sorption of Cd(2+) by Z-mZVI. This synthesized Z-mZVI has shown the potential as an efficient and promising reactive material for removing various heavy metals from wastewater or polluted groundwater.