RESUMO
Two-dimensional MoS2 nanosheets have shown great potential in heavy metal remediation due to their unique properties. MoS2 has two primary phases: 1T and 2H. Each has different physiochemical properties, but the impact of these differences on the overall material's heavy metal removal performance and associated mechanisms is rarely reported. In this study, we synthesized morphologically similar but phase-distinct MoS2 samples via hydrothermal synthesis, which comprised dominantly either a metallic 1T phase or a semiconducting 2H phase. 1T-MoS2 samples exhibited higher removal capacities for Ag+ and Pb2+ cations relative to 2H-MoS2. In particular, an eight-fold increase in the Pb2+ adsorption capacity was observed in the 1T-MoS2 samples (i.e. â¼632.9 mg g-1) compared to the 2H-MoS2 samples (â¼81.6 mg g-1). The mechanisms driving the enhanced performance of 1T-MoS2 were investigated through detailed characterization of metal-laden MoS2 samples and DFT modelling. We found that 1T-MoS2 intrinsically had a larger interlayer spacing than 2H-MoS2 because water molecules were retained between the hydrophilic 1T nanosheets during hydrothermal synthesis. The widened interlayer spacing in 1T-MoS2 allowed the diffusion of heavy metal ions into the nanochannels, increasing the number of adsorption sites and total removal capacities. On the other hand, DFT modelling revealed the energy-favorable adsorption complex of Ag+ and Pb2+ for 1T-MoS2, in which each metal atom was bonded with three S atoms leading to much higher adsorption energies relative to 2H-MoS2 for Ag+ and Pb2+. This study unravels the underlying mechanisms of phase-dependent heavy metal remediation by MoS2 nanosheets, providing an important guide for the use of 2D nanomaterials in environmental applications which include heavy metal removal, contaminant sensing, and membrane separation.
RESUMO
Understanding the behavior of heavy metals in wastewater is critical for the development of metal removal and detection techniques. In this study, we characterize the dynamic and evolving size and partitioning behavior of lead (Pb), cadmium (Cd), and arsenite (As(III)) throughout the wastewater treatment train (WWTT). Metal concentrations were determined in three size fractions (>0.45 µm, 0.45 µm - 5 kDa, and <5 kDa), and the partitioning/complexation of the metals was quantified for the <0.45 µm fraction. Cd was found to be highly mobile, with the fraction of dissolved Cd gradually increasing throughout the WWTT. As(III) was also highly mobile, with its size distribution and partitioning remaining largely steady, except when FeCl3 was used as a flocculation agent, which led to the formation of arsenic/iron complexes. However, Pb was found primarily in complex forms or adsorbed onto inorganic particulates. The WWTT had little impact on the size and partitioning of Pb, except that the formation of the Pb/iron complex occurred after flocculation with FeCl3. An increase of water hardness slightly increased the metals in the dissolved fraction. Overall, this study provides insight into the evolution of metals throughout the WWTT, offering guidance to users and researchers regarding their treatment and detection.
RESUMO
Point-of-use (POU) devices with satisfactory lead (Pb2+) removal performance are urgently needed in response to recent outbreaks of lead contamination in drinking water. This study experimentally demonstrated the excellent lead removal capability of two-dimensional (2D) MoS2 nanosheets in aqueous form and as part of a layer-stacked membrane. Among all materials ever reported in the literature, MoS2 nanosheets exhibit the highest adsorption capacity (740 mg/g), and the strongest selectivity/affinity toward Pb2+ with a distribution coefficient Kd that is orders of magnitude higher than that of other lead adsorption materials (5.2 × 107 mL/g). Density functional theory (DFT) simulation was performed to complement experimental measurements and to help understand the adsorption mechanisms. The results confirmed that the cation selectivity of MoS2 follows the order Pb2+ > Cu2+ â« Cd2+ > Zn2+, Ni2+ > Mg2+, K+, Ca2+. The membrane formed with layer-stacked MoS2 nanosheets exhibited a high water flux (145 L/m2/h/bar), while effectively decreasing Pb2+ concentration in drinking water from a few mg/L to less than 10 µg/L. The removal capacity of the MoS2 membrane is a few orders of magnitude higher than that of other literature-reported membrane filters. Therefore, the layer-stacked MoS2 membrane has great potential for POU removal of lead from drinking water.
