Treatment of Cr(VI) in aqueous solution by Ni-Al and Co-Al layered double hydroxides: Equilibrium and kinetic studies

Ni-Al layered double hydroxide (Ni-Al LDH) and Co-Al LDH were prepared using a coprecipitation method. Ni-Al LDH and Co-Al LDH were found to remove Cr(VI) from aqueous solution through anion exchange of Cr2O72- with Cl (-) intercalated in the interlayer of the LDHs. Ni-Al LDH and Co-Al LDH were both found to be superior to Mg-Al LDH for removal of Cr(VI). This excellent behavior is attributed to the buffering effect of Ni-Al LDH and Co-Al LDH that results in a lower amount of OH- in solution and allows Cr2O72- to be more easily intercalated in the LDH interlayer. The Cr(VI) removal process was consistent with a Langmuir-type adsorption, indicating that the removal of Cr(VI) proceeded via chemical adsorption involving the anion exchange of Cr2O72- with intercalated Cl-. The maximum adsorption and equilibrium adsorption constant values were 2.0 mmol g(-1) and 2.4, respectively, for Ni-Al LDH, and 1.9 mmol g(-1) and 1.5, respectively, for Co-Al LDH. The kinetics of Cr(VI) removal by Ni-Al LDH and Co-Al LDH were consistent with a pseudo-second-order reaction, and the rate-determining step for Cr(VI) removal was found to be the anion exchange. The apparent rate constants at 10, 30, and 60 degrees C for Ni-Al LDH were 2.2 x 10(-3), 2.4 x 10(-3), and 3.2 x 10(-3) gmmol(-1) min(-1), respectively. The apparent rate constants at 10, 30, and 60 degrees C for Co-Al LDH were 1.2 x 10(-3), 1.6 x 10(-3), and 1.9 x 10(-3)g mmol(-1) min(-1), respectively. The Arrhenius plot of the rate constants yielded apparent activation energies of 6.1 and 6.6 kJ mol(-1) for Ni-Al LDH and Co-Al LDH, respectively. (C) 2014 Elsevier Ltd. All rights reserved.