Fe-MoS₄: An Effective and Stable LDH-Based Adsorbent for Selective Removal of Heavy Metals

It has always been a serious challenge to design efficient, selective, and stable absorbents for heavy-metal removal. Herein, we design layered double hydroxide (LDH)-based Fe-MoS 4, a highly efficient adsorbent, for selective removal of heavy metals. We initially synthesized FeMgAl-LDH and then enriched its protective layers with MoS 4 2– anions as efficient binding sites for heavy metals. Various characterization tools, such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray, X-ray photoelectron spectroscopy (XPS), CHN analysis, and inductively coupled plasma analysis, were applied to confirm structural and compositional changes during the synthesis of Fe-MoS 4 as final product. The prepared Fe-MoS 4 offered excellent attraction for heavy metals, such as Hg 2+, Ag +, Pb 2+, and Cu 2+, and displayed selectivity in the order Hg 2+ ∼ Ag + > Pb 2+ > Cu 2+ > Cr 6+ > As 3+ > Ni 2+ ∼ Zn 2+ ∼ Co 2+ . The immense capacities of Hg 2+, Ag +, and Pb 2+ (583, 565, and 346 mg/g, respectively), high distribution coefficient ( K d ∼ 10 7 –10 8 ), and fast kinetics place Fe-MoS 4 on the top of materials list known for removal of such metals. The sorption kinetics and isothermal studies conducted on Hg 2+, Ag +, Pb 2+, and Cu 2+ suit well pseudo-second-order kinetics and Langmuir model, suggesting monolayer chemisorption mechanism through M–S linkages. XRD and FTIR studies suggested that adsorbed metals could result as coordinated complexes in LDH interlayer region. More interestingly, LDH structure offers protective space for MoS 4 2– anions to avoid oxidation under ambient environments, as confirmed by XPS studies. These features provide Fe-MoS 4 with enormous capacity, good reusability, and excellent selectivity even in the presence of huge concentration of common cations.