Removal and recovery of uranium (VI) from aqueous solutions by residual sludge and its biochars.

The removal and recovery of uranium (VI) from water solutions are critical for energy and environmental security. In this study, hydrochar at 100, 150, and 190 °C (HC100, HC150, and HC190) and pyrochar at 250 °C (BC250) were prepared from residual sludge (RS). The uranium (VI) adsorption behavior, recovery, and heavy metal risk of RS and its biochars were assessed. The sorption distribution coefficient of RS was higher than those of its biochars within the tested concentration range. The maximum adsorption capacity of uranium (VI) by HC190 was 121.26 mg/g at acidic pH (pH 4.5), which was higher than those of other tested biochars, previously reported unmodified biochars, and activated carbon. The zeta potential, FTIR, and XPS results implied that the adsorption of uranium (VI) by RS and its biochars was regulated by electrostatic attraction and the complexation with oxygen- and phosphorus-containing functional groups. Besides, partial reduction of uranium (VI) into uranium (IV) happened during the process of adsorption. More than 86% of the adsorbed uranium (VI) was recovered by 0.01 M hydrochloric acid and 100% by 0.01 M sodium carbonate. The leaching amount of heavy metals was greatly reduced after the sludge was converted to biochar, indicating that hydrothermal carbonization and pyrolysis can promote the stabilization of heavy metals. This work demonstrates that RS and its biochars can be implemented as low-cost, environment-friendly, and high-efficient materials for the purification of uranium (VI)-containing solutions by means of adsorption and desorption.