Phosphorus Bioavailability in Hydrochars Derived from Different Solid Biowastes: Role of Humic Acid and Metal Species

Hydrothermal carbonization is a promising technology for the recovery of phosphorus (P) from solid biowastes. The poor P bioavailability of hydrochar limits its application as a fertilizer. This study assessed the P bioavailability of hydrochars derived from various solid biowastes, with a focus on the role of humic acid (HA) and metal species. The results demonstrated that Ca-related P minerals predominated in pig-manure-derived hydrochar (Hy-PM). Specifically, apatite P accounted for 57.6–96.8% of total P, and HCl-extractable P represented 17.7–93.2% of total P. In contrast, Fe/Al-related P minerals were dominant in sewage-sludge-derived hydrochar (Hy-SS). Within Hy-SS, nonapatite inorganic P constituted 38.2–48.7% of total P, while NaOH-extractable P accounted for 54.7–76.7% of total P. Additionally, Hy-PM exhibited a relatively higher HA content (61.8–122.4 mg/g C) compared to Hy-SS (28.8–116.1 mg/g C) at the same temperature. P bioavailability evaluation indicated that the highest concentrations of total P (19.0 mg/g), Ca (90.3 mg/L), and HA (352.2 mg/L C) were noted in the liquid phase of the mixture after phosphate-solubilizing microorganism incubation with Hy-PM produced at 100 °C. Further kinetic studies indicated that the P–Ca–HA complex exhibited considerable stability (−8.9 to −10.5 eV) with Ca at the central core, and Ca bound preferentially with P–OH, while HA reacted preferentially with P–O. The binding affinity of HA toward Ca2+ (−1.1 to −2.7 eV) was higher than that toward HPO42‐ (−1.0 to −1.5 eV). The formation of a P–Ca–HA complex enhanced the content of bioavailable P. As such, feedstock rich in Ca (e.g., PM) and promoting HA formation are recommended to achieve high P bioavailability of hydrochar during the HTC process.