Simultaneous adsorption of Cr(VI) and fluoride from aqueous over multiple-metal layer double hydroxides: performance and mechanism

A series of multiple-metal layer double hydroxides: MgAl-LDH, CoMgAl-LDH and CoZnMgAl-LDH were prepared for single/simultaneous adsorption of Cr(VI) and fluoride from aqueous solution. X-ray powder diffraction, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller, scanning electron microscope and energy-dispersive X-ray analyses techniques were employed to characterize the as-prepared LDHs. The CoMgAl-LDH displayed the best Cr(VI) and fluoride adsorption performance compared with the other LDHs. In single pollutant removal system, the maximum adsorption capacities of 52.63 and 29.41 mg/g for Cr(VI) and fluoride were calculated from the Langmuir isotherm with an initial pH value of 7. Competitive adsorption between Cr(VI) and fluoride was observed during simultaneous adsorption, and the affinity of Cr(VI) on the LDHs was stronger than that of fluoride. The presence of Co was found to be responsible for the enhanced defluorination of CoMgAl-LDH. The pseudo-second order equation and the Langmuir adsorption isotherm fitted well to the adsorption data of Cr(VI) and fluoride. Ion exchange was the main way to remove Cr(VI) and fluoride by LDHs.