Pomelo peel derived phosphorus-doped biochar for efficient disposal of uranium-containing nuclear wastewater: Experimental and theoretical perspectives

The phosphoric acid activated biochar adsorbent is recognized as a good candidate for application in efficient disposal of uranium-containing nuclear wastewaters. However, the production of high-efficiency biochars with typical characteristics of large specific surface area, hierarchically porous structure and abundant adsorption group remains a challenge. In this work, we report a new hydrogelation-carbonization technique to facilely produce phosphorus-doped biochar from pomelo peel and meantime provide new sights into the interaction mechanism of phosphoric acid groups with uranium ion. An interesting hydrogelation phenomenon is found when simply mixing the phosphoric acid solution and pomelo peel powder. The resulted hydrogel readily undergo carbonization to produce desired biochar. The biochar exhibits good uranium removal efficiency (up to 99 %) with a maximum adsorption capacity of 603 mg/g at 313 K. The distribution coefficient of uranium (VI) attains 10.8 L/g. The biochar undergoes five desorption-adsorption cycles with desorption and adsorption efficiencies above 95 %. It can be applied in fix-bed column for dynamic adsorption of uranium from uranium-containing solutions and real nuclear wastewater with an adsorption capacity over 422 mg/g. Rather than direct interaction with uranium ion, our experimental and computational results reveal that the surface phosphoric acid groups initially undergo in-situ ionization to produce phosphonates and the complexation of phosphonate with uranium ion results in effective uranium adsorption. This work demonstrates the high efficiency of pomelo peel biochar for disposal of uranium-containing nuclear wastewater, and offers new insights into the mechanism of uranium adsorption on the adsorbents containing phosphoric acid and phosphonate groups.