Abstract
Lignin crucially influences the methanogenesis of lignocellulose. However, the specific mechanism of lignin that affects anaerobic digestion remains unclear. Accordingly, this study investigated the effects of the structural monomers of lignin on anaerobic digestion using batch fermentation and model compounds. The results reveal that at low concentrations (0.02–0.1 g/L) phenol and guaiacol promoted biogas production, whereas higher concentrations inhibited methanogenesis and delayed peak biogas production. Complete inhibition of biogas production occurred at phenol concentrations ≥ 3.0 g/L or guaiacol concentrations ≥ 8.0 g/L. Furthermore, 2,6-dimethoxyphenol had no significant effect on digestion at 0.02–0.2 g/L, but increased both the delay period and overall biogas production at 1.0–4.0 g/L. The inhibitory effects are ascribed primarily to acid production being hindered, particularly acetic acid for phenol and guaiacol, whereas 2,6-dimethoxyphenol caused acidification. As found, the inhibitory effect of syringyl lignin was lower than those of guaiacyl lignin and p-hydroxyphenyl lignin. These findings underscore the complex interactions between lignin monomers and anaerobic digestion for methane production.