Dual Roles of Phenolic Groups in Pyrogenic Carbon as Mediators and Quenchers for Hydroxyl Radical Production

Hydroxyl radical (•OH) production from ferrous iron (Fe(II)) oxidation is ubiquitous in redox interfaces and is crucial for element cycling and contaminant remediation. Pyrogenic carbon (PyC) has been widely known to mediate electron transfer, yet a quantitative relationship between the active functional groups within PyC and •OH production remains lacking. Here, we quantitatively assessed •OH production from Fe(II) oxidation with activated carbon (AC) containing varying oxygen-containing functional groups. The amount of phenolic groups (Ar–OH) was the key parameter determining •OH production, as evidenced by a strong nonlinear relationship between Ar–OH content and •OH production. During 8 h of oxygenation, •OH accumulation increased from 76.84 to 103.45 μM in the presence of 0.19 to 0.31 mmol Ar–OH/g C and decreased from 103.45 to 65.30 μM with 0.31 to 0.38 mmol Ar–OH/g C. The promoting effects were attributed to the increased electron-donating capacity (EDC), which enhanced electron-mediating ability from Fe(II) to O₂. The inhibitory effects were due to the addition reactions between •OH and Ar–OH, as evidenced by the measured accumulation of •OH being lower than the calculated net production. These findings are essential for precisely predicting •OH accumulation and contaminant degradation in PyC-applied cultivated lands.