Polyserotonin-Functionalized Graphitic Carbon Nitride Nanosheets as Composite Adsorbents for Uranium (VI) and Thorium (IV) Removal

Abstract Effective management of radioactive nuclear waste is vital for a sustainable energy supply. However, challenges in the synthesis and design of ideal adsorbent materials with desired adsorption properties remain. Here, a novel graphitic carbon nitride-based adsorbent was developed for the removal of uranium(VI) and thorium(IV) from wastewater. For this, sulfur-doped graphitic carbon nitride (SCN) nanosheets were prepared through the calcination of thiourea, and their surfaces were modified with polyserotonin (PS) using a simple enzymatic polymerization process. The modification process enhanced the surface area, functionality, and dispersion stability of the SCN. Batch experiments were performed at various temperatures, contact times, pH levels, and initial solution concentrations to assess adsorption performance, kinetics, and thermodynamics. The experiments showed that SCN had an excellent Th(IV) adsorption capacity of 336.43 mg/g. The PS modification increased its Th(IV) adsorption by 1.5 times (565.83 mg/g) and enhanced U(VI) adsorption performance by more than 2.5 times (from 36.88 to 94.10 mg/g). Moreover, these adsorption performances were achieved at low adsorbent concentrations (1–5 mg) and low radionuclide concentrations (5–50 mg/L). This composite adsorbent (SCN@PS) exhibited relatively good reusability and strong anti-interference properties, highlighting its stability and practical application. The adsorption mechanisms were elucidated by XPS analysis. The findings indicated that U(VI) ions selectively coordinated mainly with the oxygen atoms of the composite material, whereas Th(IV) ions coordinated mainly with sulfur atoms.