Dynamic formation mechanism of persistent free radicals driven by water-phase oxidation-dependent heterogeneity of the carbon-silicon coupling structure in biochar.

The formation of persistent free radicals (PFRs) in biochar (BC) is closely related to the structural characteristics and reactivity of BC, which have toxic effects on the environment. However, the mechanisms driving PFRs formation through structural evolution during oxidative aging of BC remain unclear. Herein, we propose a novel dynamic mechanism for BC-PFRs formation driven by oxidation-dependent heterogeneity in carbon-silicon coupling structures by evaluating their heterogeneous correlations, sequential responses, and synergistic relations. The sequential destruction of the "outer carbon-middle silicon-inner carbon" spatial layer and the transformation of molecular components caused by continuous oxidation of BC contributed to the formation of BC-PFRs with two concentration peaks. Moreover, two-dimensional correlation spectroscopy combined with infrared spectroscopy and high-resolution mass spectrometry revealed the sequential responses of carbon-silicon functional groups in BC (Si-O-Si groups → silicon enclosed structures → carbon groups) and BC-derived dissolved organic molecules (lipid-/aliphatic-/carbohydrate-like molecules → lignin-/tannin-like molecules → condensed aromatic molecules), leading to the staged formation of BC-PFRs. High molecular-weight lignin-/tannin-like and condensed aromatic molecules in the carbon layer contributed to BC-PFRs formation, whereas crystalline silicon components hindered the oxidative degradation of inner aromatic carbon and subsequent PFRs formation. Our findings help elucidate potential environmental behaviors and risks associated with BC-PFRs.