Modification of sludge-based biochar using air roasting-oxidation and its performance in adsorption of uranium(VI) from aqueous solutions

[Display omitted] Modification methods for sludge-based biochar are often complex and generally ineffective. In this study, sludge-based biochars were prepared at low cost using a simple air roasting-oxidation modification method and the adsorption performance on U(VI) was investigated. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results together indicated that more carbon–oxygen functional groups were formed on the surface of oxidized biochar (OBC) compared to unoxidized biochar (BC). The adsorption performance of 550-OBC (biochar oxidized at 550 °C) on U(VI) was explored in batch experiments. The maximum adsorption capacity was up to 490.2 mg/g at 25 °C and pH 6, exceeding most of the reported biochars. 550-OBC also showed good adsorption performance at low U(VI) concentration, with 96% removal at pH 6 and an initial U(VI) concentration of 1 mg/L. Density functional theory (DFT) calculations indicated that the H-bond length between the solvated U(VI) and functional groups on the OBC was about 1.7 Å, which forms stronger H-bonds between them compared to that between U(VI) and BC (4.21 Å), and the adsorption energy value for this complex was highly negative −31.82 kcal/mol. In addition, 550-OBC exhibited high selectivity for U(VI) adsorption and excellent regeneration performance, making it a cost-effective and high-performance adsorbent.