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% H2O2). 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 (31)P 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.
Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 1100 Robert E. Lee Boulevard, New Orleans, Louisiana 70124, United States.
Minori
M
Uchimiya
Syuntaro
S
Hiradate
D000818:Animals; D002417:Cattle; D002606:Charcoal / Q000737:chemistry*; D003369:Cottonseed Oil / Q000737:chemistry*; D003368:Gossypium / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008372:Manure / Q000032:analysis*; D010758:Phosphorus / Q000737:chemistry*; D012995:Solubility; D013696:Temperature
D003369:Cottonseed Oil; D008372:Manure; C540010:biochar; D002606:Charcoal; D010758:Phosphorus