Deep eutectic solvent-mediated preparation of solvothermal carbon with rich carboxyl and phenol groups from crop straw for high-efficient uranium adsorption

In this study, solvothermal carbon (STC) with high uranium (U(VI)) adsorption performance was successfully synthesized from cotton stalk by a green and simple solvothermal carbonization in deep eutectic solvent (DES) composed of choline chloride and citric acid at a molar ratio of 1:1, and the maximum U(VI) adsorption capacity of STC prepared at 180 degrees C for 3 h reached 353 mg/g with a high product yield of 81 %, which was much higher than the conventional hydrothermal carbon (HTC). The characterization results showed that STCs had higher carbonization degrees, and much richer contents of carboxyl (4.0 - 4.4 mmol/g) and phenol (3.6 - 6.7 mmol/g) groups than the conventional HTCs, which played a crucial role in remarkably enhancing the U(VI) adsorption. Moreover, the STCs showed stable U(VI) adsorptions at pH 4.0 - 8.0, with various competing cations, and after 5 successive regeneration cycles. The used DES was also recyclable for the successive preparation of STC without obvious loss of adsorption performance after 5 cycles. The adsorption mechanism analyses suggested that electrostatic attraction, surface complexation and chemical reduction were involved in U(VI) adsorption onto STC. Finally, this study proposed a green and simple method for the synthesis of STC with enriched surface carboxyl and phenol groups from low-cost lignocellulosic biomass as the high-performance and reusable U(VI) adsorbent.