Removal of hexavalent chromium from water by modified sponge iron particles and insights into mechanism

Sponge iron particles modified with expanded graphite and Cu were used to purify solutions contaminated with aqueous Cr(VI). A removal mechanism that involved physical adsorption and a redox reaction is proposed. The reaction, which consisted of rapid adsorption, a desorption stage, and an adsorption–desorption equilibrium stage, corresponded to a first-order kinetic model. The properties of the adsorption materials before and after use were investigated by X-ray diffraction, scanning electron microscopy–energy-dispersive spectroscopy, Fourier-transform infrared spectroscopy, energy-dispersive X-ray fluorescence spectroscopy, and surface area measurements. Changes in the surface properties, e.g., attachment of material to the surface and filling of pores with Cr, were clearly observed. The Langmuir model best described Cr(VI) adsorption on the sponge iron and its modified particles. Removal efficiencies of 98.7, 98.8, and 100% were achieved in 7 h at a Cr(VI) dosage of 10 mg/L. Sponge iron particles are therefore potential adsorbents and after modification give good removal of Cr(VI) ions from contaminated water.