Highly Selective and Efficient Removal of Heavy Metals by Layered Double Hydroxide Intercalated with the MoS42– Ion

The MoS42– ion was intercalated into magnesium–aluminum layered double hydroxide (MgAl—NO3-LDH) to produce a single phase material of Mg0.66Al0.34(OH)2(MoS4)0.17·nH2O (MgAl—MoS4-LDH), which demonstrates highly selective binding and extremely efficient removal of heavy metal ions such as Cu2+, Pb2+, Ag+, and Hg2+. The MoS4-LDH displays a selectivity order of Co2+, Ni2+, Zn2+ < Cd2+ ≪ Pb2+ < Cu2+ < Hg2+ < Ag+ for the metal ions. The enormous capacities for Hg2+ (∼500 mg/g) and Ag+ (450 mg/g) and very high distribution coefficients (Kd) of ∼107 mL/g place the MoS4-LDH at the top of materials known for such removal. Sorption isotherm for Ag+ agrees with the Langmuir model suggesting a monolayer adsorption. It can rapidly lower the concentrations of Cu2+, Pb2+, Hg2+, and Ag+ from ppm levels to trace levels of ≤1 ppb. For the highly toxic Hg2+ (at ∼30 ppm concentration), the adsorption is exceptionally rapid and highly selective, showing a 97.3% removal within 5 min, 99.7% removal within 30 min, and ∼100% removal within 1 h. The sorption kinetics for Cu2+, Ag+, Pb2+, and Hg2+ follows a pseudo-second-order model suggesting a chemisorption with the adsorption mechanism via M—S bonding. X-ray diffraction patterns of the samples after adsorption demonstrate the coordination and intercalation structures depending on the metal ions and their concentration. After the capture of heavy metals, the crystallites of the MoS4-LDH material retain the original hexagonal prismatic shape and are stable at pH ≈ 2–10. The MoS4-LDH material is thus promising for the remediation of heavy metal polluted water.