Role of initial pH in modulating sulfur cycle dynamics in sludge anaerobic fermentation

The initial pH plays a crucial role in sludge anaerobic fermentation (AF), which affects short-chain fatty acids production. However, its effect on toxic hydrogen sulfide (H₂S) gas has been neglected. The results show that when the initial pH changes from 7 to 4 (9), the cumulative H₂S production increases (decreases) by 142 % (45.4 %). The initial acidic pH and alkaline pH both promoted sludge disintegration, directly increased dissolved sulfide and organic sulfurs release. The acidic initial pH further decreased the ratio of α-helix/(β-fold + random coil) and destroyed the released organic sulfurs structure, which was conducive to H₂S production. However, the initial acidic pH damaged cell membranes integrity and inhibited H₂S producer activity. Moreover, the initial acidic pH changes the sulfide balance and promotes H₂S gas release, while the initial alkaline pH promotes metal sulfide formation. These deepen the understanding of the link between pH and the anaerobic sulfur cycle.