Enhanced hexavalent chromium removal by activated carbon modified with micro-sized goethite using a facile impregnation method.

In this study, activated carbon (AC) was modified with micro-sized geothite (mFeOOH) using a facile and cost-effective impregnation method for enhanced Cr(VI) removal from aqueous solutions. X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis showed that FeOOH particles with a diameter of 0.1-1 µm were dispersed homogeneously on the surfaces and pores of the AC. Fourier transform infrared spectrum (FTIR) and X-ray photoelectron spectra (XPS) analysis indicated that Cr(VI) was easily adsorbed onto the mFeOOH and reduced to Cr(III) by the AC, eventually deposited as Cr(III)-Fe(III) hydroxides (e.g., (CrxFe1-x)(OH)3). Hence, the mFeOOH@AC achieved a significantly higher Cr(VI) removal efficiency of 90.4%, 4.5 times of that the AC. The adsorption of Cr(VI) onto the mFeOOH@AC agreed well with the Langmuir adsorption model, demonstrating that the adsorption process was controlled by monolayer adsorption. This adsorption process also followed the pseudo second-order kinetics and the adsorption rate constant K2 was determined to be 0.013 g/mg·min. The Cr(VI) removal efficiency decreased with pH values as the adsorption process was highly pH-dependent. After the desorption-adsorption process by 0.1 M HCl solution for 4 cycles, the removal efficiency of Cr(VI) was still kept up to 75.1%, indicating that the mFeOOH@AC has a good stability and can be easily regenerated. In addition, the mFeOOH@AC also exhibited a promising potential for reutilization since a Cr(VI) removal efficiency of 85.4% was achieved after stripping all the mFeOOH and Cr(III)-Fe(III) hydroxides by 1 M HCl solution and regeneration with mFeOOH. We demonstrate that the modified AC with micro-sized goethite can remarkably enhance its ability for Cr(VI) removal in water treatment.