Self-Tunable, Exfoliated Oxygen-Rich Flower-like MoS₂ Nanosheets for Arsenic Removal: Investigations on Substitution, Stability, and Sustainability (3S) for Maxi-Sorption

In this study, we synthesized La-incorporated O-rich defective MoS₂ nanosheets by a simple, inexpensive, in situ hydrothermal reaction to self-exfoliate the bulky MoS₂ layers themselves so that they can readily trap hard base anions, arsenic (arsenite and arsenate), from water. Attempting to modify MoS₂ surfaces by incorporating O allows for more active sites, which is confirmed by powder XRD patterns where the exfoliated layers have a d-spacing of 0.63 nm, while the spacing for the bulky layers is 0.60 nm. The substitution of La at different equivalent ratios on the interlayer/surface improves the adsorption properties of arsenite and arsenate in simple solutions, as shown by the Langmuir adsorption density values of 0.7760 and 1.4363 mmol g^-1, respectively. When the O-rich MoS₂ layers were loaded with La, the adsorption densities improved, with La1.0 equiv showing the best values among the materials studied. The presence of O and S was more responsible for the removal of arsenite ions, and La and O, together with a small amount of N, were able to remove arsenate ions from water according to the well-known Pearson’s Lewis acid−base principle. The stability of the materials was characterized after the experiments, and it was found that there was no leaching of the materials by ICP-OES and the stability was maintained after 6 regeneration cycles. With the exception of phosphate, which behaves chemically similar to arsenic, the adsorption densities were not significantly affected by the mono- and divalent anions, indicating the selectivity of the prepared materials. The synthesis cost of MoO_xS_2-x was 2 times lower than that of bulky MoS₂, and its adsorption properties were 10 times higher than those of the latter. The results suggest that La-substituted O-rich MoS₂ is a potential candidate for the removal of soft and hard base metals from water.