Phosphate modified hydrochars produced via phytic acid-assisted hydrothermal carbonization for efficient removal of U(VI), Pb(II) and Cd(II)

Phosphate species can complex with nuclides and heavy metals from aqueous solutions strongly. The introduction of phosphate groups onto the surface of biochar sorbents (mostly <1.0 at.% of P) is highly desired. In this study, phosphate modified hydrochars (HTBs) were prepared through the hydrothermal carbonization of bamboo sawdust with various duration (2, 12, 24 h) in the presence of phytic acid (0-70 wt%). The results showed that the addition of PA with a low concentration of 10 wt% carbonized at 2 h generated hydrogen protons to etch the pristine sawdust, inducing the granulation of surfaces and a 5.5-fold enhancement of surface area. While HTBs carbonized with increasing PA concentrations (30-70 wt%) and longer duration (12 and 24 h) presented more carbonaceous particles with rising sizes from <100 nm to 2.5 μm, which should be due to the cross-linkage of dehydrated phosphate-containing organic carbon components to the matrix, enabling the resultful surface modification (maximum of 2.1 wt% of P). The uptake of U(VI), Pb(II) and Cd(II) on HTBs was investigated given various geochemical conditions including contact time, pH, ionic strength, humic acid and temperature. HTBs could capture U(VI), Pb(II) and Cd(II) efficiently from the ideal and simulated wastewaters, and be reused well after six recycles. This work opened a new strategy for the potential of phosphate-hydrochars in the aqueous remediation.