Gradual Dissolved Oxygen Variations Enhance 2,4-Dichlorophenol Attenuation by Promoting Hydroxyl Radical Generation: Experimental Demonstration and a Kinetic-Thermodynamic Perspective

Oxygen perturbation in subsurface environments often induces redox fluctuations. Generally, the dissolved oxygen (DO) levels manifest as sharp oscillations or gradual variations depending on the mass transfer rate. Unlike sharp DO oscillations, the impact of gradual DO variations remains unclear. This study demonstrated that gradual DO variations (47.3%) enhanced 2,4-dichlorophenol (2,4-DCP) attenuation in iron-rich soils compared with sharp DO oscillations (31.2%). Hydroxyl radicals (^•OH) derived from surface-bound Fe(II) oxygenation were the primary oxidants in both systems. However, the gradual DO variation promoted surface-bound Fe(III)/Fe(II) cycling, favoring steady hydrogen peroxide (H₂O₂) generation and its selective decomposition to ^•OH in kinetics, while the sharp DO oscillations showed declined H₂O₂ generation and increased termination reactions producing nonradical species (e.g., Fe(IV)). Thermodynamic calculations confirmed that Fe(III)/Fe(II) cycling sustained the surface-bound Fe(II) reduction capacity by suppressing amorphous Fe(III) oxide formation under gradual DO variations, thereby providing an efficient thermodynamic force to maintain favorable reaction kinetics for ^•OH generation. These findings broaden the scope of natural contaminant attenuation, extending its relevance beyond narrow, sharp redox oscillation zones to wider regions governed by gradual DO fluctuations in the subsurface soils.