Microwave-assisted pyrolysis of rape stalk to prepare biochar for heavy metal wastewater removal

Cd(II) adsorption performance and energy storage potential of biochar produced from microwave pyrolysis of rape stalk are investigated. Adsorption experiment indicates that Cd(II) adsorption data fits well with the Langmuir model with the adsorption capacity of 53.17 mg/g. The effective Cd(II) wastewater volume is 700 mL in the column adsorption experiment. The adsorption mechanism analysis indicates that pi-electron is the primary adsorption mechanism for Cd(II) removal with the contribution percentage of 56.22 %. The density functional theory analysis result shows that Cd(II) is easier to bind with the hydroxyl groups compared to carboxyl group. The coexisted ions have little influenced on Cd(II) adsorption. Biochar still has large Cd(II) adsorption capacity after three times adsorption-desorption experiment. Biochar is used as the lithium ion battery anode material for energy storage with the specific capacity of 195 mA g-1. Besides, pyrolysis gas is recycled to prepare bio-gas with large heating value. Our findings provide a successful example of rape stalk waste converting into the highquality biochar for Cd(II) removal/energy storage and bio-gas.

Automated summary

This study investigates the Cd(II) adsorption performance and energy storage potential of biochar produced from microwave pyrolysis of rape stalk. The adsorption experiments reveal that Cd(II) adsorption fits well with the Langmuir model, with a capacity of 53.17 mg/g. The primary adsorption mechanism for Cd(II) removal is pi-electron, accounting for 56.22% contribution. The density functional theory analysis shows that Cd(II) prefers binding with hydroxyl groups rather than carboxyl groups. Coexisting ions have minimal influence on Cd(II) adsorption. After multiple adsorption-desorption cycles, biochar still exhibits a significant Cd(II) adsorption capacity. It is also used as an anode material for energy storage in lithium-ion batteries with a specific capacity of 195 mA g-1. In addition, pyrolysis gas is recycled to produce bio-gas with high heating value. This research provides a successful example of converting rape stalk waste into high-quality biochar for Cd(II) removal/energy storage and bio-gas production.