Mitigating nitrobenzene toxicity in anaerobic digestion using hollow carbon spheres: Enhanced methane production via direct interspecies electron transfer

Abstract

Nitrobenzene, a representative aromatic pollutant, is widely present in industrial wastewaters such as those from the dye, pharmaceutical, and petrochemical industries. Its production and subsequent wastewater generation have raised significant environmental concerns due to its high toxicity and persistence in water bodies. This study investigates the inhibitory effects of nitrobenzene on anaerobic digestion (AD) and assesses the potential of hollow carbon spheres (HCSs) in mitigating these adverse impacts. Nitrobenzene significantly suppressed hydrogenotrophic methanogenesis and acidogenesis, reducing methane production by up to 47.9 % at 50 mg/L. The introduction of HCSs effectively enhanced direct interspecies electron transfer (DIET), resulting in improved methane yields and pH stability under high nitrobenzene concentrations. In reactors with HCSs, biogas production increased by 32.8 %, even at 400 mg/L nitrobenzene, compared to the control. Microbial community analysis revealed that HCSs enriched DIET-related microorganisms, such as Methanothrix and Brooklawnia, while enhancing metabolic activities of key pathways including glycolysis, acetogenesis, and methanogenesis. Notably, HCSs stimulated the expression of genes associated with electron transfer and volatile fatty acid degradation, thereby alleviating nitrobenzene-induced inhibition. The findings demonstrate that HCSs not only counteract the toxic impacts of nitrobenzene but also promote microbial resilience and metabolic efficiency. This work highlights the potential of HCSs as a robust strategy to enhance AD performance in the presence of inhibitory aromatic compounds, offering a promising application for the treatment of industrial wastewater containing nitrobenzene.