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.

Automated summary

In this study, solvothermal carbon (STC) with high uranium (U(VI)) adsorption performance was synthesized from cotton stalk using a green and simple solvothermal carbonization method in a deep eutectic solvent (DES) composed of choline chloride and citric acid. The STC exhibited a maximum U(VI) adsorption capacity of 353 mg/g, a high product yield of 81%, and better performance compared to hydrothermal carbon (HTC). The results showed that STCs had higher carbonization degrees and richer carboxyl and phenol groups, which significantly enhanced the U(VI) adsorption. The STCs also exhibited stable U(VI) adsorption under various conditions and could be regenerated for multiple cycles without significant loss of adsorption performance. The study proposed a green and simple method for synthesizing STC from low-cost lignocellulosic biomass as a high-performance and reusable U(VI) adsorbent.