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

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.