Two-dimensional correlation infrared spectroscopy reveals the evolution of functional groups governing biochar oxidation resistance

To investigate the mechanistic pathways and intrinsic stability of biochar functional groups under controlled oxidation, a novel approach integrating Fourier transform infrared two-dimensional correlation spectroscopy (2D-COS) and moving window 2D-COS (MW2D-COS) analysis was employed. Wheat straw (WS) and dairy manure (DM) were pyrolyzed at 500/700°C (WSB500, DMB500, WSB700, DMB700) and oxidized with 5%-30% H₂O₂. Ultimate/proximate analyses, dissolved organic carbon (DOC) measurements, and 2D-COS/MW2D-COS analyses were applied to track compositional and functional group changes. Results showed that pyrolysis temperature was the primary factor influencing oxidation-induced changes. 2D-COS analysis revealed the lower stability of WSB500 and DMB500, attributed to their functional groups' higher oxidation susceptibility. The evolution sequences of carbon-related functional groups were identified as: C-O → CH₂/C=C/aromatic C=C → C=O → C-OH (for WSB500), C-O → aromatic C=C → CH₂/C=C → C-OH/C=O (for DMB500), C=O → =C-H/aromatic C-H (for WSB700), and C=O → aromatic C-H/carbonate → =C-H (for DMB700). MW2D-COS analysis confirmed aromatic C=C/CH₂/C=C degradation and C=O formation in WSB500 and DMB500 during oxidation, aligning with significant alterations in C, O, fixed carbon, DOC, and atomic ratios (p < 0.01). WSB700 demonstrated superior oxidation resistance. These findings provide a mechanistic basis for predicting biochar's long-term carbon sequestration potential and guide the selection of optimal pyrolysis conditions for specific environmental applications.