Release, migration, and transformation of phosphorus from sludge during citric acid‐enhanced hydrothermal carbonization

Abstract BACKGROUND To address the problem of depleting phosphorus resources, citric acid‐enhanced sludge hydrothermal carbonization was employed to achieve a one‐step phosphorus recovery from sludge. RESULTS The results showed that, under the temperature, reaction time and citric acid dosage of 210 °C, 4 h, and 20 mmol/L, respectively, the rate of release of phosphorus could reach 87.0% of the total phosphorus (TP) of sludge, whereas the concentrations of TP and PO 4 3− in hydrothermal solution could reach 227.7 mg/L and 213.4 mg/L, respectively. CONCLUSIONS During the hydrothermal carbonization of sludge, citric acid could improve the disintegration of sludge by chelating with polyvalent cations and acid hydrolysis, thereby promoting the release of phosphorus and metal ions. Moreover, citric acid chelated with the released metal ions, hindering their combination with phosphate to form metal phosphate precipitates that could be re‐enriched in the sludge hydrothermal carbon. Furthermore, citric acid decreased the proportion of inorganic phosphorus (IP) in the sludge hydrothermal carbon and increased the proportion of organic phosphorus (OP) in the sludge hydrothermal carbon, while it also promoted the transformation of apatite inorganic phosphorus (AIP) to non‐apatite inorganic phosphorus (NAIP). Citric acid‐enhanced hydrothermal carbonization reduced the content of ash in the sludge hydrothermal carbon and increased its calorific value. The work proposes a novel method for the simultaneous recovery of phosphorus and carbon resources from sludge and realizes the high‐value utilization of sludge. © 2026 Society of Chemical Industry (SCI).