Effects of 8-hydroxyquinoline on the anaerobic digestion performance and action mechanism of Chinese cabbage waste

Anaerobic digestion is one of the most important processes to realize the conversion of vegetable waste into carbon-based materials. This study aims to investigate the effects of 8-hydroxyquinoline on methane production, intermediate metabolite conversion, microbial community structure, and electrochemical properties during anaerobic digestion of Chinese cabbage wastes. The low and high concentrations of 8-hydroxyquinoline were also selected to clarify the microbial and electrochemical mechanisms during methane production. Batch anaerobic digestion experiments were conducted with three treatments: a control without 8-hydroxyquinoline, a low-dose treatment containing 50 mg/L 8-hydroxyquinoline, and a high-dose treatment containing 150 mg/L 8-hydroxyquinoline. Each treatment was performed in triplicate at 35 °C. Biogas production, methane production, volatile fatty acids, and methane-production kinetics were determined to verify the concentration regulation for risk assessment. Microbial community composition was characterized by high-throughput sequencing of the 16S ribosomal RNA gene. Co-occurrence network analysis was used to evaluate potential correlations among bacterial and archaeal taxa. Cyclic voltammetry and electrochemical impedance spectroscopy were applied at the initial, acidification, and peak methane-production stages to assess redox activity and interfacial electron-transfer characteristics. The results show that there was a strong effect of 8-hydroxyquinoline concentration. Compared with the control, the 50 mg/L treatment increased cumulative biogas production from 1 935.00 mL to 2 134.00 mL, and cumulative methane production from 568.80 mL to 693.09 mL, with increases of 10.28% and 21.85%, respectively. In contrast, the 150 mg/L treatment reduced cumulative biogas and methane production to 1 162.00 mL and 211.32 mL, indicating decreases of 39.95% and 62.85%, respectively. Modified Gompertz fitting showed that the predicted methane-production potential increased from 552.03 mL in the control to 673.03 mL in the 50 mg/L treatment, whereas there was a decrease of 198.54 mL in the 150 mg/L treatment. The maximum methane-production rate decreased by 84.41% in the high-dose treatment, compared with the control. The peak of total volatile fatty acids shifted from day 25 in the control to day 28 in both high and low 8-hydroxyquinoline treatments. The proportion reached 30.12% in the 150 mg/L treatment on day 5, while acetate accumulated during the later digestion stage. The high-dose exposure impaired the conversion of acidification products into methane. Microbial analysis showed that the relative abundance of Chloroflexota decreased from 57.16% in the control to 44.13% in the 150 mg/L treatment, while unidentified Anaerolineaceae decreased from 56.65% to 43.47%. These taxa were associated with primary fermentation and degradation of complex organic matter. Although Methanocorpusculum increased from 76% in the control to 83% in the high-dose treatment, the methane yield remained markedly lower, indicating that methanogen abundance failed to directly represent methanogenic activity. Co-occurrence networks showed that there were more nodes and edges in the bacterial network than in the archaeal network. Positive correlations predominated, indicating that substrate hydrolysis, acidification, and syntrophic fatty-acid conversion depended on coordinated interactions among multiple bacterial groups. Electrochemical responses varied among digestion stages. During the initial stage, the charge-transfer resistance values of the control, 50 mg/L, and 150 mg/L treatments were 311.9, 307.9, and 367.8 Ω, respectively. The high-dose treatment therefore increased the initial charge-transfer resistance by 17.9%, compared with the control. During the acidification stage, the differences among treatments were below 1.9%. At the peak methane-production stage, low charge-transfer resistance in the 150 mg/L treatment failed to obtain high methane production, indicating that electrochemical resistance itself does not represent the overall methanogenic performance. Low-dose 8-hydroxyquinoline promoted methane recovery to maintain the coordinated relationship among substrate degradation, volatile fatty acid conversion, and methane generation. High-dose 8-hydroxyquinoline inhibited methane production to increase the initial electron-transfer resistance. Acidification products were accumulated to reduce key fermentative taxa, and then disrupt metabolic cooperation between fermentative bacteria, syntrophic microorganisms, and methanogenic archaea. These findings can provide a mechanistic basis for risk assessment and regulation in the anaerobic treatment of vegetable wastes with 8-hydroxyquinoline residues.