Effect of Different Phosphates on Pyrolysis Temperature-Dependent Carbon Sequestration and Phosphorus Release...

Carbon sequestration is the primary function of biochar. Hence, it is necessary to design biochar with high carbon (C) retention and low C loss. In this study, three P compounds, including KH₂PO₄, Ca(H₂PO₄)₂, and NH₄H₂PO₄, were premixed with corn stalk (1:4, w/w), aiming to produce biochars (CSB+K, CSB+Ca, and CSB+N) with high C sequestration and slow release of P at three temperatures (300, 500, and 700 °C). The addition of all P sources obviously increased C retention, with the order of NH₄H₂PO₄ (65.6-83.5%) > Ca(H₂PO₄)₂ (60.4-78.2%) > KH₂PO₄ (50.1-76.1%), compared with the pristine biochar (47.8-73.6%). The addition of Ca(H₂PO₄)₂ and KH₂PO₄ led to an increase in aromaticity and graphitization, as evidenced by H/C, FTIR, Raman and XPS analysis, whereas an opposite result occurred on CSB+N. Furthermore, all three phosphates reduced C loss of biochars with H₂O₂ oxidation, and CSB+Ca showed the best effect. Ca(H₂PO₄)₂ and KH₂PO₄ pretreated biochars had higher resistance to K₂Cr₂O₇ oxidation and thermal treatment. In contrast, the C loss of NH₄H₂PO₄-added biochar at 500 and 700 °C with K₂Cr₂O₇ oxidation was increased by 54% and 36%, respectively. During the pyrolysis process, Ca(H₂PO₄)₂ was transformed into insoluble Ca₂P₂O₇, leading to the lowest P release rate of CSB+Ca. This study indicates that co-pyrolysis of corn stalk and Ca(H₂PO₄)₂ is optimal for increasing C retention, enhancing C stability and improving slow-release performance of P regardless of pyrolysis temperature.