Waste Camellia oleifera shell-derived hierarchically porous carbon modified by Fe3O4 nanoparticles for capacitive removal of heavy metal ions

•Fe3O4 nanoparticles-modified hierarchically porous carbon (Fe3O4 NPs/HPC) was synthesized.•Fe3O4 NPs/HPC delivered high performance in capacitive deionization and heavy metal removal.•Cd(II) uptake includes electrosorption and electrodeposition but only electrosorption for Pb(II).•Fe3O4 NPs/HPC exhibited a high selectivity for the removal of Pb(II) from NaCl solution. Capacitive deionization is an efficient technology for water purification and treatment. The electrode material is crucial to improving the performance of capacitive deionization. Herein, Fe3O4 nanoparticles-modified hierarchically porous carbon (Fe3O4 NPs/HPC) was successfully synthesized by the pyrolysis of waste Camellia oleifera shell coupling with the post-modification. Owing to its unique structure and composition, the as-prepared Fe3O4 NPs/HPC delivered great potential in capacitive deionization and heavy metal removal. Under the current density of 0.5 A g-1, the Fe3O4 NPs/HPC electrode showed a high specific capacitance of 134.5 F g−1 in 1 M NaCl solution, much more than the bare HPC electrode of 99.9 F g−1. In addition, the Fe3O4 NPs/HPC electrode exhibited excellent cycle stability with negligible loss of capacitance after 1000 cycles at 2 A g-1. At an operating voltage of 1.2 V, the Fe3O4 NPs/HPC electrode released high uptake capacity of 34.22 and 39.52 mg g−1 for Cd(II) and Pb(II) ions, respectively. XPS spectra and competitive adsorption demonstrated that Cd(II) was mainly removed by the oxygen-containing groups of HPC through an electrosorption coupling with an electrodeposition multilayer process, but Pb(II) was uniformly adsorbed on the active sites including the oxygen-containing groups of HPC and the modified Fe3O4 NPs by a monolayer electrosorption process. The impressive results indicate that the as-prepared Fe3O4 NPs/HPC composites possess potential for the selective removal of heavy metal ions from saline wastewater.