Immobilization of Calcined Layered Double Hydroxide into Alginate Hydrogel Beads for PNP and PAP Removal: Kinetics, Isotherms, Thermodynamics, and Mechanism

The calcined layered double hydroxide (CLDH)-alginate hydrogel beads were synthesized by embedding CLDH into alginate hydrogel beads. The beads were used to remove p-nitrophenol (PNP) and p-aminophenol (PAP) from aqueous solution. The structure and composition of CLDH-alginate hydrogel beads were characterized by scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), elemental mapping, transmission electron microscope (TEM), X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, BET, and X-ray photoelectron spectroscopy (XPS). The adsorption kinetic, isothermal, thermodynamic properties and adsorption mechanism of PNP and PAP on CLDH-alginate hydrogel beads were studied. The results demonstrated that the adsorption kinetic data fitted the pseudo-second-order model well with the correlation coefficients (R-2) of 0.9975 and 0.9995 for PNP and PAP, respectively. The isothermal data followed the Freundlich equation, and the values of R-2 for PNP and PAP were 0.9728 and 0.9946, respectively. The adsorption processes were feasible, spontaneous, and endothermic. The adsorption mechanism was predominated by anionic exchange and hydrogen bonding for PNP and hydrogen bonding for PAP, which was evidenced by the results of adsorption experiments, characterization of FTIR and XPS, and theoretical calculation. Furthermore, the CLDH-alginate hydrogel beads can be separated easily due to their larger particle size. This will provide convenience in practice application.