Reforming solid-liquid interfacial chemical compositions of anaerobic digestate liquor through rapid subcritical...

Dewatering is essential for improving the handling efficiency of anaerobic digestate. However, the separated liquid fraction, anaerobic digestate liquor (ADL), always contains high concentrations of fine colloidal particles (D₅₀ < 50 μm, 2000-3000 mg/L), which hinder subsequent biological processes and membrane-based treatments. This study proposed a strategy to enhance colloidal particle removal by briefly exposing ADL to subcritical conditions through rapid heating and quenching, a process that modulates water polarity, weakens solid-liquid interface polar affinity, and minimizes undesirable side reactions. Under optimized conditions (350 °C, 15 min heating, followed by 3000 g centrifugation for 10 min), >90 % of suspended solids were removed from ADL with limited solubilization (soluble COD increase <30 %). Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) further revealed molecular transformations of dissolved organic matter (DOM) during the above process, including the breakdown of aromatic and unsaturated CHON compounds and the formation of aliphatic, saturated structures, indicating partial degradation of humic-like substances and improved biodegradability. Notably, the stable relative abundances of amine-N and pyridine-N suggested that Maillard-derived compounds preferentially partitioned into the solid phase, underscoring the advantage of the newly proposed approach in controlling refractory byproducts of hydrothermal process. The mechanism of colloidal particle destabilization was investigated through Raman and sum frequency generation (SFG) spectroscopy, focusing on the conformational transformation of interfacial water molecules during rapid exposure to subcritical conditions. A transition from ordered to disordered interfacial water caused by surface restructuring and carbonization was identified. This disordering reduced hydrogen-bond saturation in the hydration layer, facilitated molecular exchange with the bulk water phase, and lowered the interparticle energy barrier imposed by hydration films. Consequently, hydration repulsion was weakened, promoting aggregation of colloidal particles. Overall, these findings advance the mechanistic understanding of colloid stability and demonstrate a physical strategy to substantially reduce the pollution load of ADL by modifying the molecular conformation of interfacial water while minimizing secondary pollution.