Phosphorus speciation and transformation during anaerobic digestion of organic solid wastes: From molecular characterization to process-controlled valorization.

Phosphorus (P) is an essential but finite resource, and global sustainability is challenged by the depletion of phosphate rock reserves and P-driven water eutrophication. Organic solid wastes (OSWs) represent potential urban P mines. Anaerobic digestion (AD) is a central platform for OSW stabilization and energy recovery, and the accompanying shifts in P speciation largely determine downstream P recovery potential. However, the complex biochemical transformation mechanisms governing P redistribution during AD remain insufficiently resolved, limiting efficient P recovery. This review systematically explored state-of-the-art characterization methods and current knowledge of P occurrence in OSWs, deciphering molecular-level P speciation in highly complex matrices. The P transformation pathways were comprehensively elucidated, highlighting enzymatic hydrolysis/mineralization, redox-mediated phase transfer, and dissolution-precipitation processes. Furthermore, by deciphering the intrinsic structural constraints, feedstock compositional characteristics, and specific bioactive components, this review subsequently evaluated how extrinsic factors regulate P transformation pathways. Notably, coexisting ions and pH were identified as dominant controls on solid-liquid P partitioning, while the degradation of organic P and the formation of phosphine remain insufficiently understood. Finally, we proposed forward-looking perspectives, emphasizing the necessity of integrating multi-element (C-N-P-S-metal) synergistic mechanisms and advancing molecular identification techniques. By aligning substrate-specific P transformation characteristics with recovery strategies, this review provides a scientific basis for overcoming current bottlenecks in P recovery, offering guidance for advancing precision P valorization and sustainable management of OSWs.