[en] Herein, we used biochar pyrolyzed from rice straw to adsorb uranium (U) from aqueous solutions. The adsorption of U(VI) on biochar was strongly dependent on pH but independent on ionic strength. HA/FA enhanced the sorption at pH <6.8 while inhibited the sorption at pH >6.8. The sorption reached equilibrium within 3 h, which was not mediated by pH. The adsorption process was spontaneous and endothermic, and enhanced at higher temperature. However, the influence of temperature was negligible at low initial U(VI) concentrations. Therefore, biochar derived from rice straw may be a promising adsorbent for the removal of U(VI). (author)

[en] Highlights: • GO-COOH with increased carboxyl group content and expanded interface distance was prepared from GO via a facile method. • GO-COOH showed obviously higher adsorption capacity of U(VI) than GO. • The adsorption process for U(VI) was mainly dominated by chemisorption and monolayer adsorption. -- Abstract: In this study, carboxylated graphene oxide (GO-COOH) was prepared via a simple and feasible method to enhance the adsorption of U(VI). GO-COOH were characterized by FT-IR, XRD and XPS technique, and then applied as the adsorbent for U(VI) ions removal from water. The effects of conditions for U(VI) adsorption, including initial pH of aqueous solution, adsorbent dose, contact time, and initial concentration of U(VI) solution were investigated. The result indicated that the maximum adsorption of U(VI) on GO-COOH was 315 mg/g at the optimum adsorption condition (C_a = 0.2 g/L, pH = 6.0, t = 6 h, C₀ = 100 mg/L and T = 25 °C), which was significantly higher than that on GO (190 mg/g) in the same condition. Compared to GO, the maximum U(VI) adsorption capacity of GO-COOH increases by 65.8%. The adsorption process was described well with pseudo-second-order kinetics and the Langmuir isotherm model, indicating the chemisorption process and monolayer adsorption process.