Fe-C Micro-Electrolysis Driven Dyeing Wastewater Purification in Constructed Wetlands

Textile dyeing wastewater poses significant environmental threats due to high COD, persistent organic compounds, and intense color. To investigate the effects of Fe-C micro-electrolysis on wastewater treatment, greenhouse gas emissions, and microbial communities, four intermittently aerated vertical-flow constructed wetlands (CWs) were established: gravel-based (GCW), biochar-amended (BCW), zero-valent iron (ZVI) amended (ZCW), and a combined biochar-ZVI system (ICW). Results showed that all CW configurations effectively removed dyes (94%), COD (81%), and NO₃ ^--N (94%). The ICW demonstrated superior and stable performance, maintaining > 85% NH₄ ⁺-N removal under high dye loading and achieving exceptional TP removal (79.75%-96.46%, 2-3 times higher than GCW and BCW). Crucially, ICW also exhibited the lowest integrated GWP, achieving 62.75% reduction compared to GCW, with a further 31.2% reduction at higher dye concentrations. This significant greenhouse gas (GHG) mitigation originated from suppressed CH₄ emissions (indicated by a 26-fold higher pmoA/mcrA ratio, reflecting enhanced methanotrophy and suppressed methanogenesis) and reduced N₂O emission, linked to an increased nosZ/(nirK+nirS) ratio (0.16 in ICW vs. 0.14 in GCW), signifying more complete denitrification (N₂O to N₂). Mechanistic analysis revealed that ICW fostered the highest microbial richness (Sobs = 1818 OTUs) and diversity, significantly enriching key functional phyla (Proteobacteria, Bacteroidota, Chloroflexi, Desulfobacterota) and genera (Flavobacterium for dyes degradation; Denitratisoma, and VadinHA17 for denitrification; Desulfomonile and Geothrix for electrogenesis). In this system, biochar promoted ammonia oxidation dominated by ammonia-oxidizing archaea (AOA) over ammonia-oxidizing bacteria (AOB), and served as an electron shuttle and carbon source, while ZVI supplied electrons for redox reactions. The synergistic Fe-C micro-electrolysis in ICW optimally integrated these functions, modulating the microbial community and enhancing key biochemical pathways. Thus, Fe-C micro-electrolysis offers dual advantages of efficient dyeing wastewater treatment and significant GHG mitigation, providing a novel low-carbon solution for the remediation of complex industrial wastewater.