Anaerobic Digestion of Phytoremediation Biomass: Biogas Production and Cadmium Stabilization from Sedum alfredii

Phytoremediation is a promising technique for remediation of cadmium (Cd)-contaminated soils, while the derived Cd-rich phytoremediation biomass should be appropriately disposed of. In this work, anaerobic digestion was employed as a synergistic strategy to simultaneously address these challenges by converting Cd-rich Sedum alfredii biomass into biogas energy and stabilized digestate without external chemical amendments. The process maintained robust stability, achieving a methane yield of 237 mL/g VS with kinetics well described by the modified Gompertz model (R2 > 0.99). Microbial community succession revealed that Chloroflexota and Pseudomonadota dominated hydrolysis and acidogenesis, while Desulfobacterota enriched in the mid-to-late phase drove Cd immobilization via sulfide precipitation, and acetoclastic Methanothrix became the dominant methanogen. Over 83% of total Cd was retained in the solid digestate, where the exchangeable fraction dropped sharply from 80% to 22%, and Cd was transformed into stable Fe-Mn oxide-bound, organic-bound, and residual forms. Leachates from both TCLP and SPLP tests were far below U.S. Environmental Protection Agency regulatory limits, and the biogas slurry met China’s fertilizer standards. This study establishes a sustainable anaerobic digestion-based strategy for the concurrent recovery of bioenergy and heavy metal stabilization from phytoremediation biomass, offering a feasible pathway toward safe resource utilization and environmental risk mitigation of phytoremediation residues.