Interfacial iron redistribution and extracellular matrix adjustment accompany community assembly during recovery from non-additive co-contaminant stress in anaerobic wastewater treatment.

Industrial wastewater often contains mixtures of organic pollutants. Unlike single-pollutant exposure, their antagonistic or synergistic interactions redirect microbial community assembly trajectories and destabilize anaerobic conversion. Iron amendment strategies such as nanoscale zero-valent iron (nZVI) may promote process recovery under such stress. How granule-surface iron transformation, extracellular matrix remodeling, and microbial community assembly are coordinated during recovery remains unclear. Here we conducted 2,4-dichlorophenol (2,4-DCP)/N,N-dimethylformamide (DMF) batch experiments under iron amendment and provided directional cross-checking in expanded granular sludge bed (EGSB) reactors treating real industrial wastewater. Batch experiments followed a disturbance-adaptation-recovery trajectory. Over an observation window comparable to that of the batch experiments, nZVI increased mean chemical oxygen demand (COD), 2,4-DCP, and DMF removal in EGSB by 17.13%, 22.72%, and 17.32%, respectively. Community Bureau of Reference (BCR) fractionation and extracellular polymeric substance (EPS) analysis showed reducible-iron enrichment at the granule interface coinciding with a shift from an acute polysaccharide (PS) response to protein (PN)-dominated tightly bound EPS during recovery. Using the infer Community Assembly Mechanisms by Phylogenetic-bin-based null model analysis (iCAMP) framework and molecular ecological network analysis, we identified enrichment of Chloroflexi and Actinobacteriota and a more centralized and specialized network structure. Community assembly shifted from homogeneous selection toward a more balanced mix of deterministic and stochastic processes. These interfacial iron-extracellular matrix associations and assembly shifts accompanied recovery within the same time series and were directionally consistent under complex influent, providing microecological insights into microbial community reorganization during anaerobic industrial wastewater treatment.