Ligand-Enhanced Citric Acid Extractable Phosphorus in Sewage Sludge Hydrochar

High Resolution Image Download MS PowerPoint Slide Hydrothermal treatment (HT) has been proven to be an effective strategy for recovering phosphate (P) from sewage sludge (SS). Current research has focused on improving directly plant-available P in sewage sludge hydrochar (SSH), yet explicitly linking inorganic–organic ligand effects to crystallinity modulation of stable P-apatite P (AP) during hydrothermal treatment. In this study, we found that citric acid-extractable P (CA-P) was mainly extracted from the AP and initial pH influenced the content of CA-P most. Specifically, as the initial pH increased, extraction efficiencies (CA-P/AP) rose from 5 to 67% and the CA-P content increased from 1 to 21 mg/g. Mechanistically, the XRD analysis revealed that the phase crystallinity of SSH decreased with increasing initial pH (OH − as the inorganic ligand), exhibiting a strong negative correlation with the CA-P content. Furthermore, –COOH in SSH as the organic ligand exhibited the best performance of improving CA-P content in neutral conditions. Dynamic force spectroscopy measurements confirmed the pivotal role of COOH at the molecular scale, revealing the strongest binding force (448 pN) to the SSH interface. The findings of this study suggest a possible ligand-mediated mechanism for increasing the CA-extractable fraction of P in SSH, contributing to the understanding of stable P transformation during hydrothermal treatment.