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°C to 500°C before increasing again at 600°C. 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°C–550°C and then declined, with protein-like C3 peaking within this temperature range. FT-ICR MS revealed a significant reduction in lignin content at 550°C. These results indicate that 500°C−550°C represents a key inflection point in dissolved organic matter (DOM) transformation and an optimal pyrolysis range for producing environmentally favorable biochar.