U(VI) adsorption to Fe3O4 nanoparticles coated with lignite humic acid: Experimental measurements and surface complexation modeling

Abstract Lignite humic acid-coated Fe3O4 nanoparticles (LHA-coated Fe3O4 NPs) with different extents of LHA coating were synthesized and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and potentiometric titration. The results of characterization revealed that LHA-coated Fe3O4 NPs contained plentiful oxygen-containing groups. Batch adsorption measurements show that U(VI) adsorption is pH dependent, with an optimum pH range of 5.0 to 8.0. The removal kinetics of U(VI) can be well fitted by the pseudo-second-order model. The maximum adsorption capacities at 298 K were 42.5, 55.6, and 68.7 mg g-1 on 0.5, 1.5, and 2.5 LHA-coated Fe3O4 NPs, respectively. The regeneration experiment showed that LHA-coated Fe3O4 NPs exhibited good recoverability for U(VI) adsorption. The adsorption mechanism of U(VI) on LHA-coated Fe3O4 NPs revealed by XPS and surface complexation model. XPS results indicate that both > Fe surface sites and oxygen-containing groups are involved in U(VI) adsorption. Moreover, no significant U(VI) reduction occurs on the surfaces of the LHA-coated Fe3O4 NPs, possibly because of the insulating properties of the LHA coating. Surface complexation modeling further demonstrated that U(VI) adsorption onto the Fe3O4 NPs with the highest extents of LHA coating is dominated by binding onto the LHA-associated binding sites (e.g., carboxyl sites), whereas > Fe surface sites are also involved in U(VI) adsorption onto Fe3O4 NPs with the lowest extent of LHA coating. LHA-coated Fe3O4 NPs represents a cost-efficient and good stability adsorbent for the effective removal of U(VI) from aqueous solutions, and our results provide the means for modeling the adsorption behavior as a function of the pH and extent of LHA coating.