Humin and biochar accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol under weak electrical stimulation

The extracellular electron transfer (EET) is regarded as one of the crucial factors that limit the application of the bioelectrochemical system (BES). In this study, two different solid-phase redox mediators (RMs), biochar (1.2 g/L, T-B) and humin (1.2 g/L, T-H) were used for boosting the microorganisms accessing the electrons required for 2,4,6-TCP dechlorination under weak electrical stimulation (−0.278 V vs. Standard hydrogen electrode). BES with dissolved RM anthraquinone-2,6-disulfonate (AQDS 0.5 mmol/L, T-A) was used as a comparison. The results showed that dechlorination of 2,4,6-TCP could be greatly accelerated by biochar (1.78 d−1) and humin (1.50 d−1) than AQDS (0.24 d−1) and no RM control (T-M, 0.27 d−1). Moreover, phenol became the predominant dechlorination product in T-H (78.5 %) and T-B (63.0 %) instead of 4-CP in T-M (67.1 %) and T-A (89.8 %). Pseudomonas, Sulfurospirillum, Desulfuromonas, Dehalobacter, Anaeromyxobacter, and Dechloromonas belonging to Proteobacteria or Firmicutes rather than Chloroflexi might be responsible for the dechlorination activity. Notably, different RMs tended to stimulate distinct electroactive bacteria. Pseudomonas was the most abundant microorganism in T-M (41.92 %) and T-A (17.24 %), while Rhodobacter was most prevalent in T-H (20.04 %) and Azonexus was predominant in T-B (48.48 %). This study is essential in advancing the understanding of EET in BES for microbial degradation of organohalide contaminants under weak electrical stimulation.