Single/synchronous adsorption of Cu(II), Cd(II) and Cr(VI) in water by layered double hydroxides doped with different divalent metals

Layered double hydroxides (LDHs) are a class of excellent adsorbents for the simultaneous removal of anionic and cationic pollutants in water. In this paper, four types of LDHs doped with differ-ent divalent metals were prepared by the co-precipitation method, namely MgAl-LDH, ZnAl-LDH, CaAl-LDH and CoAl-LDH, to explore their adsorption performance for Cu(II), Cd(II) and Cr(VI) respectively. The removal efficiencies of Cu(II) and Cd(II) by MgAl-LDH and CaAl-LDH were higher, and the removal efficiencies of Cr(VI) by MgAl-LDH and CoAl-LDH were higher. In comparison, MgAl-LDH showed the best all-around adsorption performance, and its corresponding optimal dos-age was 0.50 g/L. Effects of pH, initial pollutant concentration, coexisting heavy metals and coex-isting anions on adsorption by MgAl-LDH were studied. The adsorption behavior of MgAl-LDH was consistent with the pseudo-second-order kinetic model and the Freundlich isotherm model. In the synchronous adsorption system, the removal efficiencies of MgAl-LDH for Cu(II), Cd(II) and Cr(VI) reached 99.50%, 70.28% and 22.50%, respectively. Combined with scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction and Brunauer-Emmett-Teller characterization results, the main removal mechanisms of Cu(II), Cd(II) and Cr(VI) were deduced.