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 °C for Ni–Al LDH were 2.2 × 10−3, 2.4 × 10−3, and 3.2 × 10−3 g mmol−1 min−1, respectively. The apparent rate constants at 10, 30, and 60 °C for Co–Al LDH were 1.2 × 10−3, 1.6 × 10−3, and 1.9 × 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.