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.

中文翻译：

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生物炭中碳-硅偶联结构的水相氧化依赖性异质性驱动的持久性自由基的动态形成机制


生物炭 （BC） 中持久性自由基 （PFR） 的形成与 BC 的结构特征和反应性密切相关，对环境具有毒性作用。然而，在 BC 氧化老化过程中通过结构进化驱动 PFRs 形成的机制仍不清楚。在此，我们通过评估碳-硅耦合结构中的氧化依赖性异质性驱动 BC-PFRs 形成的一种新的动力学机制，通过评估它们的异质相关性、顺序响应和协同关系。“外碳-中间硅-内碳”空间层的连续破坏和 BC 连续氧化引起的分子成分的转化有助于形成具有两个浓度峰的 BC-PFR。此外，二维相关光谱结合红外光谱和高分辨率质谱揭示了碳-硅官能团在 BC（Si-O-Si 基团→碳基团的硅封闭结构→碳基团）和 BC 衍生的溶解有机分子（脂质-/脂肪族/碳水化合物类分子→木质素-/单宁类分子→缩合芳香族分子）中碳-硅官能团的顺序响应，导致 BC-PFR 的分阶段形成。高分子量碳层中的木质素/单宁类和缩合芳香族分子有助于 BC-PFR 的形成，而结晶硅组分阻碍了内部芳香族碳的氧化降解和随后的 PFRs 形成。我们的研究结果有助于阐明与 BC-PFR 相关的潜在环境行为和风险。