Iron availability as a time-evolving descriptor linking syntrophic metabolic responses and process performance in nano-iron-amended anaerobic systems.

Chlorinated aromatic pollutant are highly toxic and persistent, making iron-mediated metabolic support particularly critical in anerobic systems. Nano-iron is a dynamic iron reservoir whose structure rapidly evolves during corrosion under biogeochemical processes. However, it remains unclear how the transform of iron reservoir is associated with pollutant conversion and microbial adaption. Here, we utilized iron availability as a time-evolving descriptor linking iron transformation to 2,4-dichlorophenol (2,4-DCP) removal, extracellular protective responses, and microbial metabolic adaptation. Iron availability was dominated by dissolved Fe2+ and exchangeable iron (F1) during the early adaption stage, and shifted toward the predominance of F1 and reducible iron (F2) during the later adaptation stage. Elevated available iron fractions were associated with 2,4-DCP removal efficiency, and promoted enrichment of syntrophic oxidizers and aromatic-compound-degrading bacteria. Nano-iron materials with higher iron availability in the early stage rapidly promoted 2,4-DCP removal (averaging 64.59% removal), and induced greater early polysaccharide (PS) secretion to resist acute toxicity. The rapid early response was accompanied by reduced accumulation of VFA and cytotoxic metabolites (e.g., terpene derivatives, complex steroid compounds). Nano-iron with higher late-stage F1 and F2 exhibited a high level of protein (PN) secretion, resulting in the lowest accumulation of metabolic intermediates and a more stable syntrophic oxidation microbial community under prolonged pollutant stress. These finding identify iron availability as a functional descriptor linking iron transformation with system response, and reveal that the differences among nano-iron materials are ultimately expressed through the distinct evolving iron availability that support pollutant conversion and metabolic adaptation over time.