Uranium Binding on Landoltia punctata as a Result of Formation of Insoluble Nano-U (VI) and U (IV) Phosphate Minerals

This work investigated the binding mechanism of uranium by an indigenous Landoltia punctata (La. punctata) in the wastewater at a uranium mine. The results showed that the removal capacity of the living (healthy fronds) and the dead (dried powder) La. punctata toward U (VI) were 40 and 132 mg/g after 2 h at pH 5, respectively. The U (VI) removal mechanisms of La. punctata were dependent on the pH of wastewater. SEM images and spectroscopic analysis indicated that U (VI) was immobilized as lamellar crystal insoluble nano-U (VI) and U (IV) phosphate minerals such as chernikovite by the living La. punctata at acidic pH after 30 min, which might have resulted from the binding with phosphate groups that were likely released from the organophosphorus of the living cells. In a contrast, U (VI) mainly existed as amorphous on the dead La. punctata via the complexation with amino and hydroxyl groups. Chernikovite was reduced into UO2 after hydrothermal treatment, while the main phase of uranium was transformed into other U (VI) and U (IV) phosphate minerals after ashing treatment. This simple process of biomineralization and reduction provides a potential method for the treatment of uranium-contaminated wastewater using the living La. punctata.