Sequential bioethanol and biogas production coupled with heavy metal removal using dry seaweeds: Towards enhanced economic feasibility

Seaweeds have been used in many biotechnological applications including animal feed, human food, cosmetics, fertilizers, and bioremediation. With respect to bioremediation, seaweeds contaminated with heavy metals have limited applications and most suited for biofuel production. The present study aimed to evaluate the potential of seaweeds for dual use in heavy metal biosorption and biofuel production. Three dominant seaweed genera were collected representing the three macroalgal phyla, namely Ulva spp. (Chlorophyte), Gracilaria spp. (Rhodophyte), and Sargassum spp. (Phaeophyte). The later showed the highest cumulative copper (Cu2+) biosorption with 80% removal efficiency, which increased to 94.6% after process optimization. Comparative fermentation of raw biomass (RB) and Cu-sorbed biomass (BHM) showed the highest bioethanol yield of 289.2 mg g−1 for RB at 72 h fermentation, which was 24.3% higher than that of BHM. In addition, the presence of Cu2+ in the BHM showed significant reduction in biogas and biomethane yields by 18.4% and 5.2%, respectively, with respect to RB. However, fermented BHM showed higher biogas and biomethane yields than the fermented RB. Due to dual bioethanol and biogas production, sequential fermentation and anaerobic digestion of Cu-sorbed biomass showed the highest energy output of 1597.3 GJ year−1. The present study suggests a novel approach that provides an integrated method for efficient utilization of seaweeds biomass in wastewater treatment and sustainable energy recovery.