In-Depth Identification of the Sources and Features of Organic Fluorescent Structures in the Sewage Sludge Hydrothermal Aqueous Phase

Abstract

Fluorophores in natural water are usually considered to be humic acid-like and protein-like compounds, while the fluorophores in the hydrothermal carbonization aqueous phase (HTC-AP) prepared under intense hydrothermal conditions (160−280 °C and 2−10 MPa) may not be these compounds. For that, we employed hydrogen nuclear magnetic resonance, Raman spectra, and gas chromatography-mass spectrometry analysis to recognize fluorophores. The results indicate that the fluorescent structures in HTC-AP are mainly rigid planar structures with large conjugated systems resulting from hydrothermal condensation and cross-linking reactions, including aromatics, N-heterocyclics, and alicycles. Specifically, nitrogencontaining heterocycles may be the fluorescence sources of C355 (Em/Ex = 355/275 nm), C405 (Em/Ex = 405/260, 345 nm), and C445 (Em/Ex = 445/ 250, 360 nm) compounds in HTC-AP. In comparison, the fluorescence sources of C300 (Em/Ex = 300/270 nm) and C380 (Em/Ex = 380/315 nm) include alicyclic compounds and aromatic organics, and aromatic organics are the only fluorescence source of C400 (Em/Ex = 400/250, 300 nm). The C300 and C380 component fluorescences are mainly provided by low-molecular-weight organics and hydrophobic acids, respectively. The source of the fluorescent organics was verified using the Fe(II)-based advanced oxidation system. This study can provide a reference to facilitate the simple identification of organics in HTC-AP.