Mechanism of Eu(III), La(III), Nd(III), and Th(IV) Removal by G-C3n4 Based on Spectroscopic Analyses and DFT Theoretical Calculations

Spectroscopy and density functional theory (DFT) were employed to study the mechanism of Eu(III), La(III), Nd(III), and Th(IV) ions adsorption on graphitic carbon nitrides (g-C3N4) from aqueous solutions. Two structures of g-C3N4 were obtained using DFT calculation, triazine (C3N4a) and tris-triazine (C3N4b) units. The g-C3N4 layer structure possessed a large amount of = N–, –NH–, –NH2, C–N, and C = N, groups. The basic triazine units and surface functional groups remained after the reaction with ions and generated a three-coordinated (C3N4a-Eu/La/Nd/Th) structure with ions bound above the C3N4a plane and six-coordinated (C3N4b-Eu/La/Nd/Th) structure where the ion was trapped within the C3N4b plane possessing higher sorption energy and coordination number. Moreover, after reaction with Eu and Nd, the three-coordinated structure derived two structures (C3N4a-Eu/Nd-A, C3N4a-Eu/Nd-B) due to the lower sorption energy, which indicated the stability of the six coordination structure was higher than that of three coordination structure.