Preparation of a Mg/Al/Fe layered supramolecular compound and application for removal of Cr(vi) from laboratory wastewater

Abstract

Herein, a Mg/Al/Fe layered supramolecular compound (Mg/Al/Fe-LDH) was fabricated via a co-precipitation method with a Mg/Al/Fe mixed solution and used to remove Cr(VI) in laboratory wastewater. The physicochemical properties of the materials were studied using X-ray diffraction patterns, Fourier transform spectra, N₂ adsorption–desorption isotherms, pore size distribution curves, and scanning electron microscopy and X-ray photoelectron spectroscopy images. Mg/Al/Fe-LDH was prepared with the Mg(II) : Fe(II) : Al(III) molar ratios of 5 : 1 : 2 and aged at 80 °C for 24 h. Structural analysis showed that the Mg/Al/Fe-LDH was a layered supramolecular compound. The optimal removal process of calcined Mg/Al/Fe-LDH (Mg/Al/Fe-CLDH) for Cr(VI) was as follows: the pH value was 9–10 and the adsorption process was carried out at room temperature for 100 min. It was found that the saturated adsorption capacity reached 725.61 mg g⁻¹, much higher than that of the calcined Mg/Al layered supramolecular compound (Mg/Al-CLDH) (199.41 mg g⁻¹). The main reason for this high saturated adsorption capacity of the material was that Cr(VI) was reduced to Cr(III) by Fe(II) in the LDH host layer, and then, Cr(III) combined with OH⁻ to yield Cr(OH)₃. The sorption kinetics of Mg/Al/Fe-CLDH for Cr(VI) were appropriately described by the pseudo-second order, Elovich, and intraparticle diffusion models. Furthermore, Mg/Al/Fe-CLDH exhibited excellent regenerative property. Hence, Mg/Al/Fe-LDH could be potentially used as a structured adsorbent for Cr(VI) in laboratory wastewater.