Near-complete phosphorus valorization from sewage sludge: Cost-effective co-production of vivianite and bioavailable detoxified hydrochar via a recyclable citric acid-mediated hydrothermal process.

Efficient phosphorus recovery from sewage sludge is bottlenecked by stable phosphorus-binding within complex organic-inorganic matrices. Conventional hydrothermal treatments suffer from limited P dissolution, inevitable re-fixation, poor hydrochar P-bioavailability, and heavy metal (HM) risks. This work proposes a novel, recyclable citric acid (CA)-mediated hydrothermal process. Relying on strong CA-mediated coordination and its synergistic effects with hydrothermal conditions and sludge components, this process achieves efficient phosphorus release and targeted speciation control while mitigating environmental risks of hydrochar. Optimally, the liquid-phase phosphorus release rate reaches 70.5%, while CA is efficiently recycled via stepwise ligand regeneration, with a unit consumption as low as 0.06 g/g TS per treatment cycle. Bioavailable phosphorus in the hydrochar peaks at 92.1%, alongside 90.9% of HMs removed or stabilized. Precise hydrothermal regulation enables a controllable balance between vivianite recovery and bioavailable phosphorus retention, achieving an integrated phosphorus valorization rate of up to 92.5%. Mechanistic investigations reveal a four-step cascading pathway governing phosphorus release and targeted fixation: (i) sequential disruption of extracellular polymeric substances, cell structures, and protein secondary layers to expose phosphorus-binding sites; (ii) preferential complexation of Fe/Al ions to cleave metal-phosphorus bonds and thermodynamically suppress phosphorus re-fixation; (iii) the sludge components-catalyzed decomposition of CA into active derivatives that amplify phosphorus dissolution via enhanced acidity and ligand-to-metal charge transfer-induced Fe3+ reduction; and (iv) recombination of released phosphate with Ca/Fe ions within the confined hydrochar matrix to form novel, highly bioavailable phases. Techno-economic analysis (TEA) confirms commercial viability with a 27.8% internal rate of return, offering an efficient pathway for near-complete phosphorus recovery from sewage sludge.