Unlocking phosphorus recovery from microalgae biomass: The enhanced transformation and release of phosphorus species

The intertwined challenges of harmful algae blooms and the phosphorus (P) resource crisis have necessitated the recovery of P from algae biomass. For the first time, a co-pyrolysis strategy that incorporates NaHCO₃ into the pyrolysis process of chlorella to efficiently recover P in the form of vivianite was proposed. The findings demonstrated that the addition of 20 wt.% NaHCO₃ during pyrolysis significantly enhanced P extraction from biochar, increasing the extraction efficiency from 2.8 % to 94.37 %. A complementary array of techniques including chemical extraction, nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), as well as two-dimensional correlation spectroscopy (2D-COS), was employed to elucidate the transformation of hard-to-extract P in chlorella to easy-to-extract P during pyrolysis. It was observed that organophosphorus (OP), pyrophosphate (pyro-P), and polyphosphates (poly-P) reacted with NaHCO₃ at 700 °C, undergoing depolymerization and hydrolysis, which led to the formation of orthophosphate (ortho-P) species (e.g., Na₃PO₄, NaCa(PO₄)₃, (Fe₂(PO₄)₃), accounting for 98.88 % of the P species in biochar product. High-purity vivianite (∼98.13 %) was subsequently obtained without the need for impurity removal, as indicated by chemical equilibrium simulations, due to the minimal ions and dissolved organic matter (DOM) present in the leaching solution, a consequence of the simple and pure structure of microalgae biomass. The estimated economic profit of this strategy is $1.51 per kilogram of dry chlorella. Additionally, the resulting biochar exhibited a high surface area (518.40 m²/g) and a well-developed pore structure, make it a promising material for adsorption and catalytic applications. This study provides a novel perspective for addressing the P crisis while effectively mitigating harmful algal blooms.