Inherent Persistent Free Radicals in Nonextractable Contaminated Soil Residue Driven Nontarget Reactions and Phenolic Polymerization in H₂O₂-Based <i>in Situ</i> Remediation

The mechanistic role of inherent persistent free radicals (PFRs) within nonextractable residues of contaminated soils during H₂O₂-based <i>in situ</i> chemical oxidation (ISCO) remains poorly understood. Here, we employed sequential chemical extraction and electron paramagnetic resonance (EPR) to discriminate the speciation and nontarget reaction reactivity of soil-bound PFRs from a coking plant toward remediation agents. Carbon- (C-PFRs) and oxygen-centered (O-PFRs) radicals, primarily derived from aldehydes (35.7%) and quinones (30.8%), were identified. These PFRs drive significant nonproductive H₂O₂ consumption (>15.9%) via interfacial electron shuttling, accompanied by radical interconversion. Nevertheless, •OH is continuously generated and persists across multiple reaction cycles, accumulating to over 209.16 μM. Electrochemical analyses revealed quinoid PFRs exhibit higher electron-donating capacity than aliphatic analogues (<i>p</i> < 0.05), inversely correlated with radical delocalization (<i>R</i>² = 0.984). Using phenol as a common cocontaminant in coking sites, we demonstrate that these inherent PFRs redirect the oxidation process of coexisting phenol from mineralization to polymerization, forming soil mineral-bound polymers and water-soluble oligomers. Our findings reveal the cascade mechanism of PFRs-mediated H₂O₂ decomposition and contaminant transformation, highlighting the need to identify and modulate the PFR-driven nontarget reaction for efficient and sustainable soil remediation.