Ligand-mediated control of calcium fate: Organic acid-assisted extraction and indirect carbonation of food waste biochar

This is the authors' abstract. We don't add key points for this paper.

While indirect carbonation has been extensively investigated using inorganic alkaline residues such as steel slag and fly ash, calcium recovery and carbonation from organic waste-derived biochar remain underexplored. The utilization of food waste biochar (FWB) as a biofuel is limited by its ash-forming minerals, including calcium (Ca) and chloride salts. Utilizing these inorganic fractions for CO 2 storage enhances biofuel value and supports resource circulation. This study investigated a dual valorization approach involving Ca extraction from FWB followed by indirect carbonation. Four organic acids—citric acid (CA), acetic acid (AA), iminodiacetic acid (IDA), and L(+)-ascorbic acid (LAA)—were evaluated at concentrations of 0.01–0.1 M. After 30 min extraction, leachates were subjected to Ca recovery under CO 2 -injected or non-carbonated conditions. The results revealed distinct behaviors during extraction and recovery. Ca release was strongly correlated with final leachate pH (Ca R 2 = 0.89; Mg R 2 = 0.93), indicating that ligand chemistry influenced metal mobilization. During recovery, ligand type and phosphate competition governed Ca precipitation pathways. Strongly coordinating ligands promoted non-carbonate Ca phases, whereas AA enabled selective calcite formation. AA 0.05 M achieved the highest theoretical CO 2 sequestration capacity of 1.34 ± 0.28 mg CO 2 g -1 biochar. These findings demonstrate that maximizing Ca extraction does not necessarily maximize CO 2 sequestration, highlighting the importance of ligand functionality in controlling both Ca extraction and mineralization pathways. Among the extractants, AA showed the best balance between Ca recovery and indirect carbonation through selective calcite formation, supporting FWB upgrading as a biofuel while enabling CO 2 sequestration.