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

item Uchimiya, Sophie
item HIRADATE, SHYUNTARO - National Institute For Agro-Environmental Sciences

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2014
Publication Date: 2/4/2014
Citation: Uchimiya, M., Hiradate, S. 2014. Pyrolysis temperature-dependent changes in dissolved phosphorus speciation of plant and manure biochars. Journal of Agricultural and Food Chemistry. 62(8):1802-1809.

Interpretive Summary: Agricultural wastes contain various forms of phosphorus that can be used as the organic fertilizer. When agricultural wastes are heated in the absence of air (a process called pyrolysis), new phosphorus species are formed that are often less water-soluble than the original phosphorus forms. This study utilized different solution chemistry and advanced spectroscopic analyses to identify the soluble phosphorus species of agricultural wastes before and after pyrolysis to produce charcoal. Charcoals prepared from plant and animal wastes had distinctively different phosphorus forms. Most notably, phosphorus in plant-derived charcoals were much less soluble and more stable towards degradation than phosphorus in manure-derived charcoals. These differences in phosphorus forms can be used to prepare desirable organic phosphorus fertilizers for sustainable agriculture.

Technical Abstract: Pyrolysis of plant and animal wastes produces a complex mixture of phosphorus species in amorphous, semi-crystalline, and crystalline inorganic phases, organic (char) components, and within organo-mineral complexes. In order 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 (conc. HNO3/HCl + 30% H2O2). Relative to the total (microwave digestible) P, % extractable P increased in the following order: M3 < oxalate ˜ water < NaOH-EDTA for plant biochars; 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 (PO43-) became the sole species of =500 °C manure biochars, whereas pyrophosphate (P2O74-) 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 between (i) electric conductivity and ash content of biochars with the amount of inorganic P species, and (ii) total organic carbon and volatile matter contents with the organic P species.