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

TLDR

The MoS4(2–) ion was intercalated into MgAl‑NO3‑LDH to form a single‑phase Mg0.66Al0.34(OH)2(MoS4)0.17·nH2O that selectively and efficiently binds Cu2+, Pb2+, Ag+, and Hg2+. The resulting MgAl‑MoS4‑LDH exhibits a selectivity hierarchy (Co2+ < Ni2+ < Zn2+ < Cd2+ ≪ Pb2+ < Cu2+ < Hg2+ < Ag+), extraordinary adsorption capacities (~500 mg g–1 for Hg2+ and 450 mg g–1 for Ag+), very high distribution coefficients (~10^7 mL g–1), Langmuir‑type monolayer adsorption, rapid removal to ≤1 ppb within minutes, chemisorption via M–S bonding, structural stability across pH 2–10, and overall promise for heavy‑metal remediation.

Abstract

The MoS4(2-) 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 Cu(2+), Pb(2+), Ag(+), and Hg(2+). The MoS4-LDH displays a selectivity order of Co(2+), Ni(2+), Zn(2+) < Cd(2+) ≪ Pb(2+) < Cu(2+) < Hg(2+) < Ag(+) for the metal ions. The enormous capacities for Hg(2+) (∼500 mg/g) and Ag(+) (450 mg/g) and very high distribution coefficients (Kd) of ∼10(7) 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 Cu(2+), Pb(2+), Hg(2+), and Ag(+) from ppm levels to trace levels of ≤1 ppb. For the highly toxic Hg(2+) (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 Cu(2+), Ag(+), Pb(2+), and Hg(2+) 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.