Straw biochar enhanced removal of heavy metal by ferrate

This study demonstrated that As(III) was appreciably removed by ferrate in the presence of straw biochar. Removal efficiency of As in ferrate/biochar system was over 91%, increased by 34% compared with ferrate alone ([biochar]₀ = 10 mg/L, [ferrate]₀ = 6 mg/L, [As(III)]₀ = 200 μg/L). In the reaction process, As(III) was oxidized to As(V) mainly by ferrate, while ferrate was reduced into ferric (hydr)oxides and coated on the biochar. Biochar was oxidized in the reaction and its surface area, pore volume and the amount of Lewis acid functional groups were substantially improved, which provided interaction sites for As adsorption. Analysis of hydrodynamic diameter and zeta potential revealed that biochar interacted with the ferrate resulted ferric oxides and enlarged the Fe-C-As particle/floc, which promoted their settlement and thus the liquid-solid separation of As. As(V) was adsorbed on the surface of biochar and ferric (hydr)oxides through hydrogen bond, electrostatic attraction and As-(OFe) bond. Ferrate/biochar was not only effective for As removal, but removed 73.31% of As, 50.38% of Cd, and 75.27% of Tl when these hazardous species synchronously existed in polluted water (initial content: As, 100 μg/L; Cd, 50 μg/L; Tl, 1 μg/L). The combination of ferrate with biochar has potential for the remediation of hazardous species polluted water.