Preparation of Fe(II)–Al layered double hydroxides: Application to the adsorption/reduction of chromium

Chromium is highly toxic and readily soluble in water. Herein, Fe(II)–Al layered double hydroxides (LDH) with Fe:Al ratios of 2:1 (LDH-2) and 3:1 (LDH-3) were synthesized by a hydrothermal synthesis method. The resulted products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) measurements, scanning electron microscopy (SEM), thermogravimetric analysis coupled with differential scanning calorimetry (TG/DSC), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Highly crystalline forms with sheet structure were revealed by XRD and TEM results, while the BET surface area of the LDH-2 and LDH-3 materials were 46.85 and 53.22 m2/g, respectively. In addition, both products showed good performances in the removal of Cr(VI), although the removal efficiency of LDH-2 was higher than that of LDH-3. Based on the characterization results, two different removal mechanism involving both adsorption and reduction processes were proposed. The Fe2+ ions in LDH were involved in reducing Cr(VI) to Cr(III) while simultaneously being oxidized to Fe3+. pH was found to influence the adsorption of Cr(VI), with higher pH values being beneficial to the removal process. Common anions present in the environment such as SO42−, NO3−, and CO32− showed minor influence in the adsorption performance of the LDH materials. A competitive adsorption between Cr(VI) and humic acid (HA) was observed. The crystalline structure of LDH-2 collapsed after adsorption of very high concentrations of Cr(VI), thereby demonstrating once again that Fe involved in the lamination of LDH-2 was dissolved out to a large extent promoting the Cr(VI) to Cr(III) process.