Hydrothermal carbonization of fish sludge: Effect of FeCl3 on the hydrochar properties and phosphorus and heavy metal transformation

Recycling phosphorus (P) from P-enriched wastes is of great significance in addressing the global phosphorus resource crisis. However, very few studies have explored the phosphorus recovery potential of fish sludge (FS), a sediment from aquaculture systems. In this study, a distinct FS valorization method was proposed using hydrothermal carbonization (HTC) with the FeCl₃ as an additive, which shows significant effect on the formation of slow-release Fe-bound P and inhibits heavy metal pollution risks in the hydrochar (HC) product. The mechanisms of P and heavy metal transformation, as well as the evolution of HC properties, were comprehensively studied. Key reactions include the dissolution of Ca-bound P and CaCO₃, and the re-precipitation of free PO₄^3- with Ca²⁺ and Fe³⁺. Optimal results were achieved with 0.3 M FeCl₃ solution, showing the highest Fe-bound P proportion of 40.58 % and an HC surface area of 17.76 m²/g. The addition of FeCl₃ as a Lewis acid promoted organic matter degradation and aromatic condensation, consequently increasing the porosity and stability of the HC. While HTC treatment increased the total heavy metal in the HC product, it reduced their DTPA-extractable fractions. FeCl₃ facilitated the transfer of heavy metals from the solid product into the liquid phase, with the final HC product meeting organic fertilizer standards for Cd (2.01 mg/kg), Pb (37.78 mg/kg), Zn (304.89 mg/kg), and As (4.59 mg/kg). These findings demonstrated that HTC assisted by FeCl₃ is a sustainable strategy for the resource recovery and utilization of FS.