Removal of U(VI) from Aqueous Solution by Amino Functionalized Flake Graphite Prepared by Plasma Treatment

Flake graphite (FG) with high uranium(VI) entrapment efficiency was successfully fabricated via a simple and efficient nonthermal plasma treatment method. FG was modified with −NH₂ functional groups through nonthermal plasma with different treatment times under vacuum conditions. The modified FG samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FT-IR spectra), thermogravimetric analysis (TGA), BET surface area measurements, and zeta potential. FG samples with different treatment times are used as contrast adsorbents for U(VI) entrapment. The adsorption experiments show that the modified FG has higher high U(VI) entrapment efficiency than others, and longer treatment time results in higher efficiency, demonstrating that the plasma treatment can greatly increase the active sites of FG samples and lead to the successful grafting of −NH₂ on FG surface. The −NH₂ modified FG with 2 h treatment time shows the highest adsorption capacity with 140.68 mg·g^-1 among the five samples at 333.15 K. Thermodynamic studies reveal that the U(VI) entrapment process is spontaneous and entropy-driven endothermic. XPS studies reveal that the adsorption mechanism for U(VI) entrapment is achieved through the complexation of U(VI) with both −NH₂ and phenolic hydroxyl group on the surface of modified FG. Moreover, desorption studies exhibit that PTFG-4 can be used repeatedly and adsorption capacity only shows slight decrease after five cycles. Thus, it can be concluded that the nonthermal plasma treatment can be used as an effective method for the fabrication of adsorbents with great adsorption performance for heavy metals entrapment.