Efficient removal of copper and lead by Mg/Al layered double hydroxides intercalated with organic acid anions: Adsorption kinetics, isotherms, and thermodynamics

Abstract Mg/Al layered double hydroxides (LDH) directly intercalated with organic anions (citrate and malate) were synthesized through a co-precipitation method at low supersaturation, yielding Citrate-LDH and Malate-LDH, respectively. Three obtained LDH samples were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential, and scanning electron microscopy. The results indicated that the organic anions were intercalated into the interlayer region of LDH as a horizontal orientation. The adsorption capacity of the LDH samples toward Cu(II) and Pb(II) ions was strongly affected by the solution’ pH, with the highest adsorption capacity reaching at pH 5.0. As revealed in the kinetic study, equilibrium was rapidly established with adsorption half-lives at 30 °C in the range of 7.38–11.4 min for Cu(II) adsorption and 1.59–3.87 min for Pb(II) adsorption. The maximum Langmuir adsorption capacity of Pb(II) and Cu(II) cations at 30 °C exhibited the following order: Citrate-LDH (166 mg/g and 137 mg/g) > Malate-LDH (141 mg/g and 118 mg/g) > pristine LDH (84.7 mg/g and 59.9 mg/g), respectively. Thermodynamic studies demonstrated that the adsorption process was spontaneous (−ΔG°), endothermic (+ΔH°), and increasingly random (+ΔS°). The results demonstrated that the intercalation of citrate and malate anions into LDH successfully resulted in improving the adsorption capacity and rate of potentially toxic cations.