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-MoS₄, a highly efficient adsorbent, for selective removal of heavy metals. We initially synthesized FeMgAl-LDH and then enriched its protective layers with MoS₄^2- 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-MoS₄ as final product. The prepared Fe-MoS₄ offered excellent attraction for heavy metals, such as Hg²⁺, Ag⁺, Pb²⁺, and Cu²⁺, and displayed selectivity in the order Hg²⁺ ∼ Ag⁺ > Pb²⁺ > Cu²⁺ > Cr⁶⁺ > As³⁺ > Ni²⁺ ∼ Zn²⁺ ∼ Co²⁺. The immense capacities of Hg²⁺, Ag⁺, and Pb²⁺ (583, 565, and 346 mg/g, respectively), high distribution coefficient (K_d ∼ 10⁷-10⁸), and fast kinetics place Fe-MoS₄ on the top of materials list known for removal of such metals. The sorption kinetics and isothermal studies conducted on Hg²⁺, Ag⁺, Pb²⁺, and Cu²⁺ 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 MoS₄^2- anions to avoid oxidation under ambient environments, as confirmed by XPS studies. These features provide Fe-MoS₄ with enormous capacity, good reusability, and excellent selectivity even in the presence of huge concentration of common cations.