Molecular transformation pathway and bioavailability of organic phosphorus in sewage sludge under hydrothermal treatment: Importance of biopolymers interactions

Non-renewable phosphorus (P) largely converges into waste activated sludge (WAS) during sewage treatment, thus P reclamation from WAS is essential for sustainable utilization of P resource. Hydrothermal (HT) process is a promising and widely used sanitation technique for improving WAS recycling efficiency, whilst the transformation and bioavailability of dissolved organic P (DOP) in WAS following HT treatment are unclear. Therefore, this work unveiled the transformation pathway of DOP during HT process using Fourier transform-ion cyclotron resonance mass spectrometry coupled with molecular reaction network analysis, and its bioavailability was determined by enzymatic hydrolysis. In summary, DOP underwent a two-stage transformation following HT treatment. In stage I (50 °C–120 °C), adenosine triphosphate and ribonucleic acid were initially solubilized at 50 °C, followed by P-diesters (ribonucleic and deoxyribonucleic acid) and P-monoesters (mononucleotides, phospholipids) at 70 °C. Meanwhile, the release of P-monoesters is highly parallel to the solubilization of protein (R2 = 0.81–0.96, p < 0.05). Then, these DOP compounds were decomposed into DOP hydrolysates with high bioavailability including phosphoenolpyruvic acid, dihydroxyacetone phosphate, and 1-phosphono butanedioate, etc. DOP enzymatic hydrolysis test also revealed a maximum bioactive concentration of 41.69 mg L−1 at 120 °C. During stage II (120 °C–200 °C), the DOP hydrolysates reacted with nitrogenous heterocyclic compounds (e.g., pyrimidine, pyridine, and pyrazine) generated in Maillard reactions, and caused formation of CNON compounds with low bioavailability (8.23 mg L−1 at 200 °C). In addition, reduction, decarbonization, decarboxylation, and nitration played significant roles in DOP transformation. And toxic phosphonates were dissolved into aqueous phase above 160 °C and progressively increased with temperature. This study provided a theoretic basis for sludge P recovery and efficient utilization based on HT treatment.