Microalgae realizes self N-doped biochar for heavy metal polluted sediment remediation

Heavy metal contamination in sediment has become a significant global environmental challenge. Numerous studies have demonstrated the effectiveness of modified biochar to solve heavy metal contamination in sediment. However, the modification process with complex methods and expensive modifiers prevented its large-scale application. In this study, an N self-doped biochar was obtained by pyrolysis of Spirulina sp. (SBC). Meanwhile, the K₂CO₃ impregnation method was utilized to prepare Spirulina sp. biochar (KSBC), which demonstrated a higher specific surface area (874 m²/g) and richer O, N functional groups. The adsorption capacity of KSBC550-120 for Cu (Ⅱ), Zn (Ⅱ), and Cd (Ⅱ) was 57.9 ± 0.3 mg/g, 43.6 ± 0.7 mg/g, and 63.9 ± 0.6 mg/g, respectively. The adsorption process is primarily governed by chemical processes, mainly through ion exchange, surface complexation, dissolution-precipitation, electrostatic interactions, adsorption-reduction, and cation-π interactions. Moreover, utilizing KSBC550-120 for mixing or capping effectively reduced heavy metal concentrations in both the overlying and pore water of the sediments. 1.0 wt% KSBC550-120 with capping treatment significantly reduced the release of heavy metals from the sediment by 80.3-91.9%. This study provides effective theoretical support for re-utilizing waste algal residues and remediation of the heavy metal-contaminated river and lake sediments using microalgae biochar.