Feedwater pH affects phosphorus transformation during hydrothermal carbonization of sewage sludge

•HTC of SS was performed with feedwater pH (3–11) at temperature of 200–260°C.•HTC significantly accumulated P in the hydrochar.•Different feedwater pH stimulated the transformation of various P forms.•Some metal cations and feedwater pH have much effect on P transformation. In this study, the effect of feedwater pH (3–11) on phosphorus (P) transformation during the hydrothermal carbonization (HTC) of sewage sludge (SS) was investigated at a temperature range of 200–260°C. The HTC significantly accumulated P in the hydrochar. Different feedwater pH stimulated the transformation of various forms of P. An acidic feedwater pH promoted the transformation of apatite phosphorus (AP) to non-apatite inorganic phosphorus (NAIP), and of organic P (OP) to inorganic P (IP). The NAIP tended to transformation to AP and a small part of the IP was transformed to OP when the SS was treated in a basic environment. The combination of three P analysis methods (chemical extractive fractionation, X-ray powder diffraction (XRD) and energy dispersive spectroscopy (EDS)) showed that metal cations (e.g. Al and Ca) and the pH played important roles in the transformation of different forms of P during the HTC of the SS.