Research on the removal of Cr(VI) ions from wastewater by Mg/Al-layered double oxides

Abstract

Cr(VI) is a common pollutant in wastewater that poses a threat to human health and wellbeing. Layered double hydroxides (LDHs) have been identified as an effective physical adsorbent due to their strong anion exchange capacity and memory effect. This work found that calcined Mg/Al-LDHs can effectively remove Cr(VI) ions from wastewater. The adsorption capacity of Mg/Al- layered double oxides (LDOs) for Cr(VI) ions was tested under varying temperature conditions, and the results were found to conform to the Langmuir isotherm model. The maximum adsorption capacity of Mg/Al-LDOs for Cr(VI) ions was 147.39 mg/g at 20 °C. The crystal plane spacing and microscopic morphology of Mg/Al-LDHs before and after the adsorption of Cr(VI) ions were analyzed using a X-ray diffractometer, high-resolution transmission electron microscopy and selected area electron diffraction, and their microstructures were analyzed by Fourier transformation infrared absorption spectroscopy and X-ray photoelectron spectroscopy. The results showed that the adsorption of Cr(VI) ions by Mg/Al-LDOs can be divided into two processes at 20 °C. The first process was the recovery of the single-crystal structure of Mg/Al-LDHs by the adsorption of Cr(VI) ions by Mg/Al-LDOs, and the adsorption capacity of Cr(VI) ions in this process was limited due to the restricted layer spacing in the single-crystal structure. The second process was that the recovered Mg/Al-LDHs single-crystal structure continued to adsorb Cr(VI) ions and grew into a nanoflower-like compound with a polycrystalline structure, and this process led to an increase in the layer spacing of Mg/Al-LDHs, greatly increasing the interlayer Cr(VI) ions adsorption capacity. Additionally, the adsorption of Cr(VI) ions by Mg/Al-LDOs was accompanied by the conversion of Cr(VI) ions to Cr(III) ions at 20 °C, and Cr(III) ions could enter the plate structure to form the Mg/Al/Cr-LDHs structure, which further improved the adsorption capacity and efficiency for interlayer Cr(VI) ions.