BACKGROUND: Pyrolysis of slaughterhouse waste could promote more sustainable phosphorus (P) usage through the development of alternative P fertilizers. This study investigated how pyrolysis temperature (220, 350, 550 and 750°C), rendering before pyrolysis, and wood or corn biomass additions affect P chemistry in bone char, plant availability, and its potential as P fertilizer.

RESULTS: Linear combination fitting of synchrotron-based X-ray Absorption Near Edge Structure (XANES) spectra demonstrated that higher pyrolysis temperatures decreased the fit with organic P references, but increased the fit with a hydroxyapatite (HA) reference, used as indicator of high calcium phosphate (CaP) crystallinity. The fit to the HA reference increased approximately from 0 to 69% in bone with meat residue and from 20 to 95% in rendered bone. Biomass additions to the bone with meat residue reduced the fit to the HA reference by 83% for wood and 95% for corn, and additions to rendered bone by 37% for wood. No detectable aromatic P forms were generated by pyrolysis. High CaP crystallinity was correlated with low water-extractable P, but high formic acid-extractable P indicative of high plant availability. Bone char supplied available P which was only 24% lower than Triple Superphosphate fertilizer and two- to five-fold higher than rock phosphate.

CONCLUSION: Pyrolysis temperature and biomass additions can be used to design phosphorus fertilizer characteristics of bone char through changing CaP crystallinity that optimize P availability to plants.