Pyrolysis temperature-dependent variations in spectral and molecular characteristics of water- and alkali-extractable organic matter derived from biochar produced by municipal sludge

Biochar-derived dissolved organic matter (BDOM) plays a crucial role in environmental processes. This study investigated the characteristics of water-extractable organic matter (WEOM) and alkali-extractable organic matter (AEOM) from sludge-derived biochar produced across different pyrolysis temperatures, combining spectroscopic analysis with Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Dissolved organic carbon (DOC) concentrations in WEOM and AEOM declined significantly (from 5.72 mg/g to 1.95 mg/g and from 5.93 mg/g to 2.36 mg/g, respectively) with increases in temperature from 400℃ to 500℃ before increasing again at 600℃. AEOM generally exhibited higher DOC concentrations than WEOM. Three fluorescent components were identified: a humic-like component C1 with two peaks (one at an excitation wavelength (λ_Ex) of 230 nm and an emission wavelength (λ_Em) of 400 nm and the other at λ_Ex = 310 nm and λ_Em = 400 nm), an ultraviolet C (UVC) humic-like component C2 with a single peak (λ_Ex = 260 nm and λ_Em = 460 nm), and a protein-like component C3 with two peaks (one at λ_Ex = 220 nm and λ_Em = 290 nm and the other at λ_Ex = 285 nm and λ_Em = 290 nm). AEOM was predominantly composed of polycyclic aromatic compounds, aromatic formulas, lignin, and nitrogen-containing aliphatic compounds, which altogether accounted for more than 75% of its composition. With increasing pyrolysis temperature, the relative abundance of most aromatic structures decreased, while N-aliphatic compounds became more prominent. The humification degree and molecular weight of BDOM increased with temperature rises to 500℃−550℃ and then declined, with protein-like C3 peaking within this temperature range. FT-ICR MS revealed a significant reduction in lignin content at 550℃. These results indicate that 500℃−550℃ represents a key inflection point in dissolved organic matter (DOM) transformation and an optimal pyrolysis range for producing environmentally favorable biochar.