Towards negative-carbon biohydrogen: A dual-function wollastonite strategy for enhanced fermentation and CO2 sequestration

Dark fermentation for biohydrogen requires continuous alkaline dosing to control process acidification and expensive separation units to remove co-produced CO 2 , resulting in high operational costs and energy consumption. Here, we introduce wollastonite as a dual-function agent to simultaneously enhance H 2 yield and capture CO 2 . An optimal dosage of 10 g/L was identified, which shortened the lag phase from 23.13 to 12.38 h and increased the hydrogen yield from 158.11 ± 3.44 mL/g glucose-consumed to 210.75 ± 15.87 mL/g glucose-consumed. Mechanistically, wollastonite buffered the system pH, steering metabolic flux away from lactate towards acetate synthesis by enriching Clostridium and suppressing Lactobacillus . Wollastonite also enabled in-situ CO 2 sequestration by precipitating it as CaCO 3 . However, maximal CO 2 capture was achieved at a higher dosage (≥15 g/L), which passively reached the required neutral pH but compromised the hydrogen yield. This created a conflict with the optimal 10 g/L dosage for hydrogenesis. To prioritize the primary goal of green hydrogen production, a two-stage strategy was developed. This approach first uses the optimal 10 g/L dosage for maximal fermentation, followed by a post-fermentation pH adjustment from 6.54 ± 0.18 to 7.0 to induce carbonation. The optimized process successfully sequestered 0.49 ± 0.05 of CO 2 per liter of medium, thereby increasing the H 2 content in the final biogas to a high of 58.2 ± 1.1%. Finally, a life cycle assessment (LCA) validated the environmental superiority of this strategy, confirming a significantly lower Global Warming Potential (GWP) for the entire process. This work thus provides a proof-of-concept for a pragmatic strategy that co-optimizes hydrogen production and carbon capture in a single, sustainable biorefinery process.