Molecular and microbial mechanisms of biochar-facilitated anaerobic detoxification of 3,5-xylenol in phenolic landfill leachate

Detoxifying phenolic leachates containing 3,5-xylenol (3,5-dimethylphenol, DMP) is challenging due to limited electron transfer and microbial inhibition. A biochar–microbe coupling strategy was introduced to improve DMP removal and methane production. As the loading rate increased from 12.2 ± 4.7 to 83.8 ± 10.8 g/m³/day, methane yield (MY) increased from 0.27 ± 0.09 to 0.32 ± 0.08 L/gCOD_removed/day but dropped to 0.19 ± 0.01 at 179.3 ± 10.0 g/m³/day because of volatile fatty acid buildup and methanogenic inhibition. Sawdust biochar (10 %, w/w) restored performance at 373.2 ± 19.9 g/m³/day, reaching 73.5 ± 4.0 % DMP removal and 0.34 ± 0.10 L/gCOD_removed/day. Biochar improved enzyme–substrate affinity, promoted direct interspecies electron transfer (DIET), and increased the abundance of syntrophic degraders (Acinetobacter, Pseudomonas, Rhodoplanes, Syntrophorhabdus) and methanogens (Methanosaeta, Methanobacterium, Methanosarcina, Methanospirillum). Biochar mitigated phenolic toxicity by adsorbing inhibitors, enhancing enzymatic catalysis, and stabilizing bacterial–archaeal syntrophy, thereby enabling efficient detoxification and methane recovery.