Phosphorus speciation and release kinetics of swine manure biochar under various pyrolysis temperatures

Converting swine manure to biochar is an effective way to recycle valuable nutrients, but there are few reports on its feasibility as a phosphorus (P) source. The objective of this study was to clarify the unique nature, including P speciation, of manure biochar products under various pyrolysis temperatures. We used solution ³¹P nuclear magnetic resonance and P K-edge X-ray adsorption near-edge spectroscopy (P XANES) to characterize P species in swine manure biochar. For every 100 °C increment starting from 300 °C, the P content in manure biochar increased by 2.16 to 3.37 g kg^-1. However, above 400 °C, organic P species did not appear anymore, and only inorganic P, including orthophosphate and pyrophosphate, existed. P K-edge XANES spectra further showed all biochar samples had higher percentages of Ca₃ (PO₄)₂ and NaP₂O₇, and lower percentages of FePO₄, AlPO₄, and inositol hexaphosphate compared to manure. Interestingly, percentages of Ca₃(PO₄)₂, FePO₄, and AlPO₄ in MB400 (indicating manure pyrolysed at 400 °C) were comparable with those in MB700 while the percentage of NaP₂O₇ was higher in MB400. Phosphorus release from MB400 maintained a relatively high level at 0.33 g kg^-1 during the whole 300-h observation period. These results suggest that with a suitable pyrolysis temperature, it was feasible for manure biochar to be a P source alternative. Graphical abstract ᅟ.