Remarkable reusability of magnetic Fe3O4-encapsulated C3N3S3 polymer/reduced graphene oxide composite: A highly effective adsorbent for Pb and Hg ions

Highlights: • Preparation of 2D magnetic poly(C₃N₃S₃)/rGO nanocomposite • Encapsulation of Fe₃O₄ nanoparticles into rGO-poly(C₃N₃S₃) matrix • High adsorption capacities for Pb (270.3 mg/g) and Hg (400.0 mg/g) • Excellent reusability in 15 consecutive adsorption-desorption cycles -- Abstract: Magnetic Fe₃O₄-encapsulated C₃N₃S₃ polymer/reduced graphene oxide composite (rGO-poly(C₃N₃S₃)/Fe₃O₄) was synthesized to remove Pb(II) and Hg(II) from aqueous solutions. This material was characterized by X-ray Photoelectron Spectroscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray powder diffraction, N₂ adsorption, etc. The results suggest that final composite exhibits two-dimensional (2D) nanosheet structure, in which Fe₃O₄ nanoparticles or clusters are encapsulated between the layers of rGO-poly(C₃N₃S₃) matrix, preventing composite aggregation and nanoparticle detachment. The results of adsorption tests suggested high metal removal and short residence time to reach equilibrium. The adsorption kinetics data were well fitted by pseudo-second-order equation. The effect of metal concentration on adsorption was illustrated by Langmuir isotherm equation. Maximum metal-uptake capacities for Pb(II) and Hg(II) ions were 270.3 and 400.0 mg/g, respectively. High-resolution XPS spectra clearly illustrate the adsorption mechanism, in that Hg(II) preferentially binds to sulphur functional groups and Pb(II) tends to be adsorbed by nitrogen groups in poly(C₃N₃S₃) matrix. Recycling performance of this composite was investigated in 15 consecutive adsorption-desorption cycles, after which the adsorption capacities for Pb(II), and Hg(II) ions remain stable thanks to Fe₃O₄ encapsulation into the rGO-poly(C₃N₃S₃) matrix.