Pyrolysis temperature-dependent changes in dissolved phosphorus speciation of plant and manure biochars

Pyrolysis of plant and animal wastes produces a complex mixture of phosphorus species in amorphous, semicrystalline, and crystalline inorganic phases, organic (char) components, and within organo-mineral complexes. To understand the solubility of different phosphorus species, plant (cottonseed hull) and manure (broiler litter) wastes were pyrolyzed at 350, 500, 650, and 800 C and exposed to increasingly more rigorous extraction procedures: water (16 h), Mehlich 3 (1 mM EDTA at pH 2.5 for 5 min), oxalate (200 mM oxalate at pH 3.5 for 4 h), NaOH-EDTA (250 mM NaOH + 5 mM EDTA for 16 h), and total by microwave digestion (concentrated HNO₃/HCl + 30% H₂O ₂). Relative to the total (microwave digestible) P, the percentage of extractable P increased in the following order: M3 < oxalate ≈ water < NaOH-EDTA for plant biochars and water < M3 < NaOH-EDTA < oxalate for manure biochars. Solution phase ³¹P NMR analysis of NaOH-EDTA extracts showed the conversion of phytate to inorganic P by pyrolysis of manure and plant wastes at 350 C. Inorganic orthophosphate (PO₄ ^3-) became the sole species of ≥500 C manure biochars, whereas pyrophosphate (P₂O₇^4-) persisted in plant biochars up to 650 C. These observations suggested the predominance of (i) amorphous (rather than crystalline) calcium phosphate in manure biochars, especially at ≥650 C, and (ii) strongly complexed pyrophosphate in plant biochars (especially at 350-500 C). Correlation (Pearson's) was observed (i) between electric conductivity and ash content of biochars with the amount of inorganic P species and (ii) between total organic carbon and volatile matter contents with the organic P species.