Sustainable Flame-Retardant Poly Lactic Acid Biocomposites Reinforced with Polyphosphate-Enriched Microalgae: Unlocking the Potential of Hyper-Compensation

Abstract

This study examines the dual benefits of microalgae cultivation for wastewater treatment and the enhancement of polylactic acid-based biocomposites. Using Desmodesmus sp. in a photobioreactor, both batch and continuous operations achieve total nitrogen (TN) and total phosphorus (TP) removal rates of up to 99.9%, maintaining TN and TP levels below 0.02 mg L⁻¹ in the effluent, aligning with European discharge standards. Continuous cultivation increases biomass productivity from 0.102 to 0.43 g L⁻¹ day⁻¹, a 322% improvement over batch operations. Nutrient starvation followed by reintroduction to nutrient-rich wastewater induces hyper-compensation luxury uptake, with P-enriched cells accumulating 1.33% intracellular P within six hours — 21% higher than natural accumulation. The results reveal that luxury phosphorus uptake in microalgae follows a triphasic system of uptake and storage, challenging the previously suggested biphasic model. When incorporated into Poly lactic acid (PLA), the biomass enhances versatility, offering potential replacement of inorganic P in industrial applications, particularly flame retardants. Pyrolysis and cone calorimetry confirm the thermal and fire-retardant benefits, with a 20% reduction in peak heat release rate and increased char yield. This work highlights microalgae's role in sustainable biocomposites, supporting wastewater treatment, nutrient recovery, and CO₂ sequestration.