Highly Selective and Efficient Removal of Heavy Metals by Layered Double Hydroxide Intercalated with the MoS₄^2- Ion

The MoS₄^2- ion was intercalated into magnesium-aluminum layered double hydroxide (MgAl - NO₃-LDH) to produce a single phase material of Mg_0.66Al_0.34(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⁺ 蠐 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 (K_d) 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.