Beyond Enhancement: Stabilizing Iron-Based Materials in Anaerobic Digestion and Unveiling Their Transformation and...

Iron-based materials have garnered increasing attention for enhancing anaerobic digestion (AD), but their mechanistic roles, stability, and environmental fate in AD remain unclear. This study evaluated the transformation pathways and fate of representative exogenous iron-based materials through batch and continuous experiments. The nanoscale zero-valent iron (nZVI), nanomagnetite (nFe₃O₄), nanohematite (nα-Fe₂O₃), and microscale zero-valent iron (mZVI) were used in batch experiments. nZVI showed the highest iron utilization (70.3%) and primarily transformed into Fe₃O₄ and FeCO₃, with 79% utilized iron derived from Fe⁰ and 72% residual Fe⁰ remaining unreacted. nZVI also provided the highest bioavailable iron (6.45 mg g^-1 TS). mZVI displayed limited reactivity, leaving 82% of Fe⁰ unreacted. nFe₃O₄ and nα-Fe₂O₃ maintained extreme structural stability and exhibited minimal phase transformation that nFe₃O₄ maintained 88% Fe₃O₄ retention while nα-Fe₂O₃ maintained 96% Fe₂O₃ retention. Due to the superior performance of nFe₃O₄ and nZVI in batch AD, subsequent continuous experiments were conducted to further elucidate their transformation and fate in long-term AD. nFe₃O₄ demonstrated superior long-term stability. A significant portion of iron was retained in AD system, with nFe₃O₄ exhibiting lower iron loss than nZVI (estimated 6.40% and 17.49% over 365 days, respectively). These findings provide insights into the underlying mechanisms of iron-enhanced AD and offer a theoretical basis that may inform the future development and optimization of iron-based materials for potential applications in AD systems.