Enhanced anaerobic degradation of phenolic and nitrogenous heterocyclic pollutants by iron-based additives: Insights into refractory pollutants removal and microbiome function

Phenolics and nitrogenous heterocyclic (NHC) compounds are the primary recalcitrant pollutants in coal chemical wastewater (CCW), posing challenges to conventional biological treatment processes. This study systematically evaluated the enhanced anaerobic removal of five representative phenolic and NHC compounds (phenol, o-cresol, hydroquinone, quinoline, and pyridine) using zero-valent iron (ZVI), cementite (Fe 3 C), magnetite (Fe 3 O 4 ), hematite (Fe 2 O 3 ), and ferrous chloride in semi-continuous reactors. The results showed that ZVI and Fe 3 C provided the strongest enhancement, increasing the chemical oxygen demand (COD) degradation capacity of anaerobic sludge by 33.9 % and 28.8 %, respectively. Phenol degradation was improved by 21.5 % and 18.3 %, o-cresol by 46.3 % and 33.3 %, and hydroquinone by 40.1 % and 46.5 %, respectively. In contrast, Fe 3 O 4 supplementation moderately improved COD, phenol, and hydroquinone degradation, whereas Fe 2 O 3 provided a slight but statistically significant improvement in COD removal. High-throughput 16S rRNA gene sequencing revealed that ZVI and Fe 3 C increased species diversity, richness, and evenness within the sludge community. They also raised the relative abundance of aromatic degraders such as Syntrophus , Sphingomonas , Bacillus , Chryseobacterium , and Thermomonas , while concurrently increasing the predicted abundance of functional genes associated with xenobiotic biodegradation and metabolic pathways. In contrast, Fe 3 O 4 and Fe 2 O 3 favored different degraders like Longilinea , Comamonas, and Ottowia. Conductive Fe 3 O 4 also stimulated electroactive genera such as Brooklawnia and Geobacter . This study elucidates how iron additives influence microbial structure and function. ZVI and Fe 3 C are identified as optimal for enhancing anaerobic CCW treatment by enriching key degraders and functional metabolic pathways.