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

Photosynthetic biogas upgrading offers a sustainable approach for CO₂ 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, CO₂ removal efficiencies remained above 96% across all stages, while CH₄ 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.