Refractory Calcium Phosphate-Derived Phosphorus Fertilizer Based on Hydroxyapatite Nanoparticles for Nutrient Delivery

Phosphorus as an essential nutrient for crops is widely applied in agriculture by a chemical phosphorus fertilizer (CPF), while, globally, phosphate rocks available for CPF production are limited and rapidly being depleted. Moreover, the low phosphorus use efficiency (PUE) of CPF entailing severely environmental burdens such as eutrophication is persistently addressed in agronomic pollution protection. Mimicking nanostructured hydroxyapatite (HAP)-based drug delivery systems to deliver P nutrient to plants and adopting alternative precursors to synthesize HAP can promisingly reconcile these problems. Here an alkali-enhanced hydrothermal process with biomass added is developed to achieve the synthesis of HAP-based nanostructured P fertilizer (NPF) translated from refractory calcium phosphate, that is, calcium hydrogen phosphate (CHP) and calcium pyrophosphate (CPP), via controlling the associated dissolution–precipitation processes for HAP nanocrystals self-assembly. Optimal conditions of this process are explored to synthesize the desirable NPF with uniform size, clear grain boundary, and efficient substitution of PO₄^3– by CO₃^2– in the HAP nanocrystals. The different supply rates of Ca²⁺ and PO₄^3– from refractory calcium phosphate dissolution–precipitation govern HAP nanocrystals to be self-assembled with a rod-like and a hexagonal morphology in the CHP-derived and CPP-derived NPF, respectively. Plant cultivation tests of the NPF indicate an increased PUE for the CHP-derived NPF (45.87%) and CPP-derived NPF (46.21%), compared to that of the CPF (23.44%). Mass balance analysis of P applied with the NPF demonstrates an efficient delivery of HAP nanoparticles to root zones of the plant, showing the great potential of the synthesized NPF as an alternative to a traditional CPF.