3.3. P releasing kinetics
The P, mainly inorganic orthophosphate and pyrophosphate, in biochar is generally combined with K, Ca, and Mg (Chirone et al., 2000). The P release is just a dissolution reaction. As a result, the Pdouble bond; length as m-dashO peak in FTIR spectra before and after P release remains almost unchanged (Fig. S1).
In the experiment of P releasing kinetics, the pH of the solution after 8 h is among 8–9 (Fig. S2). At this pH range the main species of orthophosphate is View the MathML source, which is more than 85% of the total orthophosphate; and the main species of pyrophosphate are View the MathML source (takes up 50∼92% of total pyrophosphate) and View the MathML source (takes up 2.3∼44%) (Fig. S3a and b). When the ionic products of Ca2+/Mg2+ and phosphate are higher than their solubility product constant (Ksp), precipitation should occur, and the concentration of P in solution would decrease. The concentrations of Ca2+ and Mg2+ in the solution and the ionic products of Ca2+/Mg2+ and View the MathML source/View the MathML source at different times are shown in Table S1. All ionic products of Ca2+/Mg2+ and View the MathML source are lower than their Ksp, indicating that the precipitates of CaHPO4 and MgHPO4 cannot be occurred in the P release process (Fig. 2a). While the ionic products of Ca2+/Mg2+ and View the MathML source, are larger than their Ksp (Fig. 2a), which indicates that it is the gradual precipitation of Ca2P2O7 and Mg2P2O7 on biochar surface caused a decrease in the release of pyrophosphate (Fig. 2b).