Recycling of heavy metals and modification of biochar derived from Napier grass using HNO3

The disposal of biomass enriched with heavy metals (HMs) limits the application of phytoextraction. This study investigated the feasibility of obtaining K-rich fertilizer with low risk of HMs and biochar with good application prospect by extracting Napier grass biochar using 15% HNO 3 and separating HMs from the filtrate using 40% KOH. In this study, Napier grass biochar produced at 500 °C showed better potential for utilization owing to its relatively low HM contents, high nutrient contents, and high yield. In fact, 61.26% Cd, 84.22% Zn, and more K were extracted from biochar when the pH was adjusted to 1 using 15% HNO 3 . Then, Cd and Zn could be almost separated from the filtrate by adjusting the pH to 10 or more by adding 40% KOH. The Cd content in the biochar was reduced from a low risk level to a no-risk level, and the Zn content in the biochar was reduced from a medium risk level to a low risk level when the pH was adjusted to 1 and 2 by adding 15% HNO 3 . The adsorption capacity of biochar to dyes was enhanced when the pH was adjusted to 1 using 15% HNO 3 . The cation exchange mechanism endows the biochar with better potential for reuse (for methylene blue). This work provides a safe, efficient, and maneuverable resource allocation method. • Napier grass biochar produced at 500 °C has good potential in resource utilization. • ●HNO 3 can realize simultaneous extraction and modification of Napier grass biochar. • ●KOH can precipitate heavy metals in the leach liquor and supplement K element. • ● q m of MB and KNR on modified biochar reached 101.13 and 13.23 mg g −1 , respectively. • ●The cation exchange mechanism endows the biochar with better potential for reuse.