Evaluation on phosphorus extraction potential in hydrochar obtained from hydrothermal liquefaction of sewage sludge

Hydrochar is the main solid product after hydrothermal treatment of sewage sludge (SS) and can serve as a valuable secondary source of phosphorus (P). This work aims to identify the ideal acid extractant by weighing P extraction efficiency and metal leaching, and to elucidate the P extraction mechanism based on kinetic models. H₂SO₄, HCl, citric acid and acetic acid were selected as acid extractants for P extraction experiments under various acid concentrations, liquid to solid (L/S) ratios, and leaching times. It also explored the co-dissolution behaviors of key metals (Ca, Al, Fe, Mg) and heavy metals (Zn, Cu, Cr, Cd, Ni, Pb, Mn), and mineral compositions and micro-morphologies of acid leaching residues. Increasing the H⁺ concentration in the acid solution within a certain range can effectively improve the P extraction efficiency. The results show that the highest P extraction efficiency (98.68%) was achieved using 0.3 M H₂SO₄, 50 mL kg⁻¹ of L/S ratio and 2 h of leaching time, meanwhile the lowest concentration of total heavy metals (30.06 mg L⁻¹) was obtained compared to HCl and citric acid. Kinetic studies show that P extraction from hydrochar by acid is equivalent to a chemisorption process, with intra-particle diffusion being the primary rate-limiting step. The Ca/P molar ratios in the leachate further suggest that the P extraction mechanism is dominated by the reactions of Ca–P minerals with acid, along with the dissolution of Al, Fe, and heavy metals, etc. This information provides valuable guidance for the P recovery from hydrochar derived from SS thermal conversion.