Phosphorus recovery from municipal sludge-derived hydrochar: Insights into leaching mechanisms and hydroxyapatite synthesis

Hydrothermal liquefaction has the potential to exploit resources from municipal sewage sludge. It converts most organics into a liquid biofuel (biocrude), concentrates P in the solid residue (hydrochar), and consequently enables its efficient recovery. This study thoroughly evaluated the effects of extraction conditions on P and metal release from hydrochar by nitric acid. Among assessed factors, acid normality (0.02–1 N), liquid-to-solid ratio (5–100 mL/g), and contact time (0–24 h) had positive effects while decreasing eluate pH (0.5–4) improved leaching efficiencies of P and metals. Notably, eluate pH played a dominant role in P leaching and pH < 1.5 was crucial for complete extraction. P and metal leaching from hydrochar have strong interactions and their leaching mechanism was identified as product layer diffusion using the shrinking core model. This suggests that the leaching efficiency is susceptible to agitation and particle size but not temperature. Using 10 mL/g of 0.6 N HNO3 for 2 h was considered the best extraction condition for efficient P leaching (nearly 100%) and minimization of cost and contaminants (heavy metals). Following extraction, adding Ca(OH)2 at a Ca:P molar ratio of 1.7–2 precipitated most P (99–100%) at pH 5–6, while a higher pH (13) synthesized hydroxyapatite. The recovered precipitates had high plant availability (61–100%) of P and satisfactory concentrations of heavy metals as fertilizers in Canada and the US. Overall, this study established reproducible procedures for P recovery from hydrochar and advanced one step closer to wastewater biorefinery.