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

• Published in: Biomass & bioenergy Vol. 182; p. 107121
• Main Authors: Xu, Mingxin, Wang, Yu, Liu, Tonggui, Yang, Lijie, Liu, Huanteng, Xu, Donghai
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2024
• Subjects: Acid extractants; Hydrochar; Mechanism; Phosphorus extraction; Sewage sludge; Acid extractants; Hydrochar; Phosphorus extraction; Sewage sludge; Mechanism
• ISSN: 0961-9534; 1873-2909
• DOI: 10.1016/j.biombioe.2024.107121

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. [Display omitted] •H2SO4 had the best performance in extracting P (98.7%) than HCl (96.8%), citric acid (90.5%), and acetic acid (29.6%).•H2SO4 extraction led to the lowest heavy metal capacity (0.1504 mg) than HCl (0.1903 mg) and citric acid (0.1609 mg).•Calcium and magnesium were consistently extracted more efficiently than iron and aluminum in each acid leaching system.•A new insight on the P extraction mechanism from hydrochar was described by a chemical adsorption process.