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.

水热液化具有从城市污水污泥中开发资源的潜力。它将大多数有机物转化为液体生物燃料（生物原油），将磷浓缩在固体残渣（水力炭）中，从而实现其有效回收。本研究彻底评估了萃取条件对硝酸从水焦中释放 P 和金属的影响。在评估的因素中，酸当量 (0.02–1 N)、液固比 (5–100 mL/g) 和接触时间 (0–24 小时) 具有积极影响，同时降低洗脱液 pH (0.5–4) 可改善P和金属的浸出效率。值得注意的是，洗脱液 pH 在磷浸出中起主导作用，pH < 1.5 对于完全提取至关重要。水炭中的 P 和金属浸出具有很强的相互作用，使用收缩核心模型将其浸出机制确定为产物层扩散。这表明浸出效率易受搅拌和粒度的影响，但不受温度的影响。使用 10 mL/g 0.6 N HNO ₃持续 2 小时被认为是有效磷浸出（接近 100%）以及成本和污染物（重金属）最小化的最佳提取条件。萃取后，以1.7-2 的 Ca:P 摩尔比添加 Ca( OH) _{2，在 pH 5-6 时沉淀大部分 P (99-100%)，而较高的 pH (13) 则合成羟基磷灰石。}回收的沉淀物具有较高的植物利用率（61-100%）的磷和令人满意的重金属浓度，在加拿大和美国用作肥料。总体而言，这项研究建立了从水热炭中回收磷的可重复程序，并向废水生物精炼厂迈进了一步。