Phosphorus recovery from microbial biofuel residual using microwave peroxide digestion and anion exchange

• 90% of phosphorus remains in residuals after lipid processing for biofuel.
• Advanced oxidation converts organic phosphorus to useful orthophosphate.
• Anion exchange captures orthophosphate and releases it for reuse upon regeneration.
• Recovery can reduce phosphorus demand of microalgae for biofuel by over 50%.

Sustainable production of microalgae for biofuel requires efficient phosphorus (P) utilization, which is a limited resource and vital for global food security. This research tracks the fate of P through biofuel production and investigates P recovery from the biomass using the cyanobacterium Synechocystis sp. PCC 6803. Our results show that Synechocystis contained 1.4% P dry weight. After crude lipids were extracted (e.g., for biofuel processing), 92% of the intracellular P remained in the residual biomass, indicating phospholipids comprised only a small percentage of cellular P. We estimate a majority of the P is primarily associated with nucleic acids. Advanced oxidation using hydrogen peroxide and microwave heating released 92% of the cellular P into orthophosphate. We then recovered the orthophosphate from the digestion matrix using two different types of anion exchange resins. One resin impregnated with iron nanoparticles adsorbed 98% of the influent P through 20 bed volumes, but only released 23% during regeneration. A strong-base anion exchange resin adsorbed 87% of the influent P through 20 bed volumes and released 50% of it upon regeneration. This recovered P subsequently supported growth of Synechocystis. This proof-of-concept recovery process reduced P demand of biofuel microalgae by 54%.

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