Temperature-Dependent Phosphorus Speciation and Release from Magnesium-Rich Biochars

Thermal conversion of biomasses into biochar-based slow-release fertilizers presents an attractive approach to recover and reuse phosphorus (P) in agriculture, and the presence of magnesium (Mg) can stabilize the P into poorly leachable yet plant-available forms. Nevertheless, the effect of pyrolysis temperature on P speciation and release from Mg-rich biochars is poorly understood. To understand this, spent green tea leaves were treated with magnesium oxide (MgO) and phosphoric acid (H3PO4) and pyrolyzed at 300-700 degrees C. By using X-ray diffraction (XRD) and FTIR spectroscopy, three types of Mg phosphate forms were identified in the Mg/P-modified biochars: (i) crystalline Mg pyrophosphate (Mg2P2O7) at 400-700 degrees C, (ii) crystalline Mg orthophosphate (Mg3(PO4)2) at 700 degrees C, and (iii) an amorphous ortho-/condensed phosphate phase. Sequential (Hedley) extraction confirmed that increasing the pyrolysis temperature from 300 to 500 degrees C decreased the orthophosphate (Pi) extractability and increased that of polyphosphate (Px), and the readily available P (in H2O and 0.5 M NaHCO3) was high at 300-500 degrees C. Biochars produced at 600 and especially 700 degrees C contained primarily long-term available P species (extractable in 1 M HCl). This temperature-dependent extractability was ascribed to phosphate phase transformations and increased crystallinity at higher temperatures. Similar trends in P release with pyrolysis temperature were found in kinetic release in water, and post-kinetics characterization confirmed that struvite or struvite-K and Mg2P2O7 were rate-limiting phases for phosphate dissolution. These results provide mechanistic insights into the P transformations for the design of more efficient Mg-rich biochar-based slow-release fertilizers with minimal P risk losses. The increased scarcity of phosphorus reserves and poor efficiency of phosphorus fertilizers necessitate the development of more efficient phosphorus fertilizers from renewable sources. Biochar-based slow-release fertilizers prepared by pyrolysis of (phosphorus-rich) biowastes can contribute to closing the phosphorus cycle. Whereas the phosphorus in most biochars is bound to calcium, this study focused on the unique role of magnesium. We provide insight into the phosphorus transformations in magnesium-rich biochars as a function of temperature and demonstrate how the pyrolysis temperature and phosphate species affect the phosphorus release and extractability. Compared to calcium, magnesium keeps the phosphorus in a more phytoavailable form with minimum risk losses. These results can contribute towards the design of more effective biochar-based fertilizers. Magnesium/phosphorus-modified biochars were prepared at 300-700 degrees C.Phosphorus species were monitored with XRD, FTIR, sequential extraction and kinetic release.Three P phases were identified: Mg2P2O7, Mg3(PO4)2, and amorphous phosphates.Total P extractability was high at 300-500 degrees C but decreased at 600 and 700 degrees C.Increasing temperature converted readily available P to long-term available P.