Phosphorus Recovery from Sewage Sludge-Derived Hydrochar: Balancing Phosphorus Recovery, Heavy Metal Concomitant Leaching and Residual Hydrochar Utilization

Hydrothermal carbonization combined with wet chemical methods for treating sewage sludge produce valuable hydrochar and phosphorus. However, further investigation is needed to optimize the balance among phosphorus recovery, heavy metal leaching, residual hydrochar utilization to enhance overall process benefits. In this study, acid leaching on hydrochar was conducted using HCl, H₂SO₄, citric acid at 0.1 – 1.0 M acid concentration for 10 – 1440 min, the liquid-to-solid ratio between 50 to 500 mL/g. Employing single-factor experiments, response surface methodology, various characterization techniques, the impacts of acid leaching on the leaching rates of phosphorus and heavy metals, along with residual rate and properties of the remaining hydrochar, were quantitatively assessed. The results indicate leaching parameters directly impact the phosphorus leaching rate. HCl and H₂SO₄ achieve their corresponding maximum phosphorus leaching rate within 60 min at acid concentration 0.5 M, liquid-to-solid ratio 500 mL/g, while using citric acid requires 360 min. When H₂SO₄ is used as the extractant, the corresponding maximum phosphorus leaching rate can be maintained within a wider leaching time and acid concentration range. A moderate acid concentration, prolonged leaching time, higher liquid-to-solid ratio lead to increased levels of heavy metal concomitant leaching. Compared to HCl and citric acid, using H₂SO₄ as extractant results in lower heavy metal leaching and lower sensitivity to leaching parameters. The residual rates of hydrochar range from 59.3% to 77.3%, with H₂SO₄ and HCl as extractant yielding higher residual rates than citric acid. The properties of the remaining hydrochar were altered by acid leaching, resulting in a decrease in specific surface area and pore number, but an increase in oxygen-containing functional groups and calorific value, which affects its subsequent utilization. Finally, based on the aforementioned results and potential process scenarios, the selection of optimal acid leaching parameters was discussed. The results provide reference for the selection of phosphorus recovery and resource utilization pathway based on sludge-derived hydrochar.