Exploring Optimal Pretreatment Approaches for Enhancing Biohydrogen and Biochar Production from Azolla filiculoides Biomass

Mitigating the irreversible consequences of climate change necessitates the application of sustainable energy resources. Hereby, we investigated the biological anaerobic fermentation of Azolla filiculoides biomass for biohydrogen production as a clean renewable energy source. Azolla filiculoides is a widely growing aquatic plant in polluted freshwater streams. However, the high non-biodegradable organic matter content in Azolla filiculoides biomass remains challenging in efficiently producing renewable energy, especially when it is being used as the sole donor substrate. In order to overcome this challenge, different pretreatment scenarios (namely, alkali, autoclaving, and ultrasonication) have been employed for enhancing the hydrolysis of Azolla filiculoides biomass to maximize the anaerobic fermentation and, consequently, the biohydrogen production potential. The biohydrogen production potential was 250.5, 398, 414.5, and 439.5 mL-H2, giving a hydrogen yield of 60.1, 89.6, 92.9, and 107.9 mL-H2/g-VS, respectively. Gompertz kinetics were applied to estimate the growth parameters of the process, which revealed a good fit with R2 ranging from 0.96 to 0.98. The produced digestate was valorized for biochar production, a material that could be applied for water treatment purposes. The produced biochar was characterized using different physical analyses, including FTIR, SEM, EDX, and TEM. The physicochemical characterizations of biochar demonstrate a successful formation of biochar with a highly porous structure and a rough surface, as evidenced by the emergence of significant functional groups (e.g., O-H, C-H, C=C, and C=O) existing on the surface of the biochar. In conclusion, this study harnesses a sustainable approach for the treatment of organic waste streams, which represents a circular economy model by transforming waste materials into valuable products and reducing the reliance on non-renewable resources.