Co-pyrolysis of biomass with phosphate additives: linking phosphorus transformations to nutrient release for tailor-made biochar-based fertilizers

The co-pyrolysis of biomass with phosphates is an emerging strategy for producing advanced biochar-based fertilizers. The properties of these fertilizers, particularly the speciation and release of phosphorus (P), are highly complex and dependent on synthesis parameters. This article provides a comprehensive review on the co-pyrolysis of biomass with well-defined phosphates, including phosphoric acid (H 3 PO 4 ) and salts of ammonium (e.g., NH 4 H 2 PO 4 , (NH 4 ) 2 HPO 4 ), sodium (e.g., NaH 2 PO 4 , Na 2 HPO 4 , Na 3 PO 4 ), potassium (e.g., KH 2 PO 4 , K 2 HPO 4 , K 3 PO 4 ), magnesium (e.g., from H 3 PO 4 with MgO), and calcium (e.g., Ca(H 2 PO 4 ) 2 ). Supported by the thermal transformation pathways of these pure phosphates, we identify the key factors governing P transformations during co-pyrolysis: higher pyrolysis temperatures and higher degrees of protonation favor the condensation of orthophosphates into pyro- and polyphosphates. Furthermore, P release kinetics are primarily dictated by cation valency and biomass composition; monovalent cations (e.g., K + ) typically generate highly soluble P forms, whereas divalent cations like Ca 2+ (either from the additive or present in the biomass feedstock) promote the formation of poorly soluble P species. The review also discusses specific limitations of the current literature and outlines future research directions. The synthesis of this information provides design principles for engineering biochar-based P fertilizers with targeted release profiles to improve sustainable use of P in agriculture.