Stability and ecological risk assessment of nickel (Ni) in phytoremediation-derived biochar

Improper treatment of heavy metal-enriched biomass generated after phytoremediation might cause secondary pollution in soil and water. At present, the pyrolysis process is an effective method for the treatment of phytoremediation residue. In this study, Ni-enriched biomass was prepared using hydroponics method and further pyrolyzed at different temperatures (300–700 °C). At low pyrolysis temperatures (below 500 °C), carbonate precipitation was the main reason of Ni stabilization in biochar. Nevertheless, the formed phosphate and aluminosilicate were important factors for immobilizing Ni in biochar at high pyrolysis temperatures (above 500 °C). Moreover, the oxidizable (F3) and residual (F4) components of Ni in biochar increased with increasing pyrolysis temperature, which indicated that higher pyrolysis temperature could effectively reduce the bioavailability of Ni in biochar. The results of deionized water, acidification, oxidation, and toxic characteristic leaching procedure (TCLP) experiments showed that pyrolysis temperature was the dominant factor for Ni stabilization in biochar. The ecological risk assessments further proved that pyrolyzed Ni-enriched biochar could reduce the environmental toxicity and potential ecological risks of Ni. In the soil simulated experiment, the soil microenvironment gradually promoted the transformation of Ni in BCX Ni from bioavailable fraction to stable fraction. Overall, this study would expose more reasonable reference for the long-term storage of phytoremediation residues.