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 KH(2)PO(4), Ca(H(2)PO(4))(2), and NH(4)H(2)PO(4), 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 NH(4)H(2)PO(4) (65.6–83.5%) > Ca(H(2)PO(4))(2) (60.4–78.2%) > KH(2)PO(4) (50.1–76.1%), compared with the pristine biochar (47.8–73.6%). The addition of Ca(H(2)PO(4))(2) and KH(2)PO(4) 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 H(2)O(2) oxidation, and CSB+Ca showed the best effect. Ca(H(2)PO(4))(2) and KH(2)PO(4) pretreated biochars had higher resistance to K(2)Cr(2)O(7) oxidation and thermal treatment. In contrast, the C loss of NH(4)H(2)PO(4)-added biochar at 500 and 700 °C with K(2)Cr(2)O(7) oxidation was increased by 54% and 36%, respectively. During the pyrolysis process, Ca(H(2)PO(4))(2) was transformed into insoluble Ca(2)P(2)O(7), leading to the lowest P release rate of CSB+Ca. This study indicates that co-pyrolysis of corn stalk and Ca(H(2)PO(4))(2) is optimal for increasing C retention, enhancing C stability and improving slow-release performance of P regardless of pyrolysis temperature.