Nitrate transformation enables selective Fe-As decoupling from iron tailings through pyrrolic-N-associated electron transfer.

Strong iron-arsenic (Fe-As) coupling in iron tailings hinders selective As separation during Fe recovery, posing risks of secondary contamination and limiting tailings valorization. This study developed a nitrate-biomass hydrothermal treatment that enables selective Fe-As decoupling from iron tailings through pH evolution and As(V) reduction. The proposed strategy achieved ∼90% Fe recovery in the solid phase for valorization and ∼90% As separation to the liquid phase for safe management. During hydrothermal treatment, nitrate reduction induced pH rebound, which promoted Fe recovery. Notably, nitrate addition markedly enhanced the electron-donating capacity of the hydrothermal system, resulting in a six-fold increase in As(V) reduction efficiency. The reduction of As(V) weakened the association between As and Fe-bearing phases, thereby promoting Fe-As separation. Nitrogen transformation analysis indicated that nitrate participated in the formation of pyrrolic-N structures within hydrochar through interactions with biomass-derived intermediates. Combined experimental evidence and density functional theory (DFT) calculations suggested that pyrrolic-N contributed to As(V) reduction by enhancing electron transfer from carbon matrices to As(V) and lowering the energy requirement for reduction. Overall, this study establishes a nitrate-driven electron transfer system for selective Fe-As decoupling, providing mechanistic insights into nitrogen-mediated reductive transformation and a sustainable strategy for resource recovery from complex solid wastes.