Utilization of Citrullus lanatus L. seeds to synthesize a novel MnFe2O4-biochar adsorbent for the removal of U(VI) from wastewater: Insights and comparison between modified and raw biochar

Abstract Uranium (U) is a radioactive and highly toxic metal. Its excessive concentrations in the aqueous environments may result in severe and irreversible damage. To fight this hazard, a raw biochar was prepared from Citrullus lanatus L. seeds, then characterized and compared with a MnFe2O4 modified biochar, both tested for U(VI) adsorption from wastewater, which was assayed for the first time in this study. The characterization of the adsorbent materials was performed by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) with elemental mapping, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) techniques. The effects of solution pH, concentration of sorbate and sorbents, temperature, time and ionic strength were assessed as regards their influence on U(VI) adsorption. The experimental adsorption data showed good fit to a pseudo-second-order kinetic model (reaching a value of qe = 15.12 mg g−1, R2 = 0.96 at equilibrium), and to the Langmuir isotherm (achieving a maximum score of qmax = 27.61 mg g−1, R2 = 0.96). The maximum adsorption capacity was found at 318 K. The results of the study indicate that the binding of negatively charged functional groups (carbonyls, hydroxyls, and some carboxylic groups) with MnFe2O4 significantly enhanced U(VI) adsorption. In view of the overall results, it can be concluded that the MnFe2O4 modification of the Citrullus lanatus L. seeds biochar could give an efficient alternative adsorbent for U(VI) removal in a variety of environmental conditions, simultaneously promoting resource utilization and good sustainable management of the materials studied, aiding to protect the environment and human health.