Simultaneous removal of Cu（Ⅱ） and Cr（Ⅵ） by Mg-AI-Cl layered double hydroxide and mechanism insight

Mg-A1-C1 layered double hydroxide （C1-LDH） was prepared to simultaneously remove Cu（II） and Cr（VI） from aqueous solution. The coexisting Cu（II） （20 mg/L） and Cr（VI） （40 mg/L） were completely removed within 30 min by C1-LDH in a dosage of 2.0 g/L; the removal rate of Cu（II） was accelerated in the presence of cr（VI）. Moreover, compared with the adsorption of single Cu（II） or Cr（VI）, the adsorption capacities of C1-LDH for Cu（II） and Cr（VI） can be improved by 81.05% and 49.56%, respectively, in the case of coexisting Cu（II） （200 mg/L） and Cr（VI） （400 mg/L）. The affecting factors （such as solution initial pH, adsorbent dosage, and contact time） have been systematically investigated. Besides, the changes of pH values and the concentrations of Mg2＋ and A13＋ in relevant solutions were monitored. To get the underlying mechanism, the C1-LDH samples before and after adsorption were thoroughly characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. On the basis of these analyses, a possible mechanism was proposed. The coadsorption process involves anion exchange of Cr（VI） with C1- in C1-LDH interlayer, isomorphic substitution of Mg2＋ with Cu2＋, formation of Cu2CI（OH）3 precipitation, and the adsorption of Cr（VI） by Cu2CI（OH）3. This work provides a new insight into simultaneous removal of heavy metal cations and anions from wastewater by CI-LDH.