Nanoparticle-assisted photosynthetic biogas upgrading and phycocyanin co-production with Arthrospira platensis

Photosynthetic biogas upgrading offers a sustainable approach for CO 2 valorization from biogas, enabling the production of pure biomethane alongside high-value microalgal compounds. In this study, the combined effect of carbon-coated zero-valent iron nanoparticle supplementation and liquid-to-biogas ratio modification for biomethane enrichment and phycocyanin co-production was investigated in a high-rate algal pond cultivating Arthrospira platensis , interconnected to a bubble column treating raw biogas. Results revealed that the addition of nanoparticles stimulated photosynthetic activity, promoted biomass growth up to 3.8 g VSS·L −1 , and significantly enhanced phycocyanin accumulation, stabilizing close to 30.0 mg·g −1 dry biomass following the reduction of liquid-to-gas ratio. Concurrently, CO 2 removal efficiencies remained above 96% across all stages, while CH 4 content exceeded 96.5%, with maximum values above 98%, fulfilling European biomethane quality standards for grid injection. These findings highlight that carbon-coated iron nanoparticles represent a promising strategy for simultaneously improving biomethane quality and high-value pigment recovery in photosynthetic biogas upgrading systems, advancing the concept of an algal biorefinery within a circular bioeconomy framework. • Fe-based carbon coated nanoparticles improved A. platensis CO 2 uptake in HRAP. • Phycocyanin increased up to 33.9 mg·g −1 dry biomass under L/G optimization. • CO 2 remained below 2% in biomethane, with removals above 96% throughout operation. • Decreasing L/G ratio minimized N 2 and O 2 content, increasing CH 4 purity to 96%.