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 trans-formation 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 degrees C-120 degrees C), adenosine triphosphate and ribonucleic acid were initially solubilized at 50 degrees C, followed by P-diesters (ribonucleic and deoxyribonucleic acid) and P-monoesters (mononucleotides, phospholipids) at 70 degrees 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 butane-dioate, etc. DOP enzymatic hydrolysis test also revealed a maximum bioactive concentration of 41.69 mg L-1 at 120 degrees C. During stage II (120 degrees C-200 degrees 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 degrees C). In addition, reduction, decarbonization, decar-boxylation, and nitration played significant roles in DOP transformation. And toxic phosphonates were dissolved into aqueous phase above 160 degrees C and progressively increased with temperature. This study provided a theoretic basis for sludge P recovery and efficient utilization based on HT treatment.