Effective adsorptive removal of heavy metal ions by aggregated MoS₂ nanoflakes with edge site exposure

Abstract

Recently, metal-sulfide-based adsorbents have garnered a lot of attention with regard to heavy metal recovery due to their intrinsic selectivity to soft metal ions. Molybdenum sulfide (MoS₂), a novel two-dimensional material, was identified as a suitable metal sulfide adsorbent with good acid and air stability. In this study, different MoS₂ materials were prepared by a facile hydrothermal method with different optimization approaches: the doping of metal, adjustment of precursor concentration and incorporation of mesocellular siliceous foams (MCF). The MoS₂ formation was confirmed by Raman spectroscopy and elemental analysis. Metal screening studies revealed that the MoS₂ materials could effectively immobilize Cu²⁺, Hg²⁺, Ag⁺ and Pb²⁺ Cu²⁺ was chosen for further investigation because Cu²⁺ received less interference from potential side reactions and the formation of metal molybdate. All the MoS₂ materials were investigated for Cu²⁺ adsorption performance. Based on the screening data on Cu²⁺ adsorption, MoS₂-1:4 and MoS₂-1:7 were selected for further adsorption studies, including kinetics, isotherm, effect on pH, effect on salt and regeneration and reuse of the material. A kinetic study reveals that the Cu²⁺ adsorption on MoS₂-1:4 and MoS₂-1:7 was completed in a 180-240 minute contact time frame with ~99% removal. The Elovich model is the best model to describe the Cu²⁺ adsorption kinetics of MoS₂-1:4 and MoS₂ The isotherm study demonstrates that the maximum Cu²⁺ adsorption capacities of MoS₂-1:7 and MoS₂-1:4 calculated from Sips isotherm are 201.35 and 226.20 mg/g respectively. Isotherm modelling reveals that both non-linear Dubinin-Radushkevitch and Sips isotherms can best simulate the equilibrium data of Cu²⁺ adsorption on MoS₂-1:4, while the Sips isotherm is the best model for simulating the data of Cu²⁺ adsorption on MoS₂-1:7. In addition, the optimal pH range for the Cu²⁺ adsorption on MoS₂-1:4 and MoS₂-1:7 was between 4 and 6. The zeta potential study suggests that coulombic interaction between the positively-charged Cu²⁺ and the negatively-charged MoS₂ is one of the driving forces for the Cu²⁺ adsorption. In the desorption study, 86% of the adsorption capacity of MoS₂-1:7 remained after three adsorption/desorption cycles using 1.0 M HCl as a desorbing agent. A combination of scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), N₂-adsorption/desorption isotherm measurement and zeta potential measurement, was used to evaluate and characterize the Cu²⁺ adsorption mechanism. The SEM images reveal that MoS₂-1:4 and MoS₂-1:7 are wire-brush like and are composed of aggregated MoS₂ nanoflakes with edge site exposure. The formation of nanosheets in MoS₂-1:4 and MoS2-1:7 are also confirmed by the XRD analysis of the MoS₂ materials. The XPS studies have discovered that the S₂²⁻ species located at the edge sites of MoS₂ nanosheets contributes significantly to the binding of Cu²⁺. This study shows that aggregated MoS₂ nanoflakes with edge site exposure have been successfully synthesized by a facile hydrothermal method. The MoS₂ nanoflakes have demonstrated great potential as a highly effective adsorbent for the removal of Cu²⁺ from wastewater and contaminated water. The role of S₂²⁻ species in the adsorption of Cu²⁺ on the MoS₂ nanosheets was first reported in this study. This can offer an alternative strategy to the precise engineering of MoS₂ adsorbent in the future.