Natural Attenuation of 2,4-Dichlorophenol in Fe-Rich Soil during Redox Oscillations: Anoxic–Oxic Coupling Mechanism

Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of the coupling anoxic-oxic process, which often happens in subsurface soil, on contaminant transformation remains poorly understood. Here, we investigated 2,4-dichlorophenol (2,4-DCP) transformation in Fe-rich soil under anoxic-oxic alternation. The anoxic and oxic periods in the alternating system showed faster 2,4-DCP transformation than the corresponding control single anoxic and oxic systems; therefore, a higher transformation rate (63.4%) was obtained in the alternating system relative to control systems (27.9-42.4%). Compared to stable pH in the alternating system, the control systems presented clear OH^- accumulation, caused by more Fe(II) regeneration in the control anoxic system and longer oxygenation in the control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in the anoxic process and induced ^·OH oxidation in the oxic process, OH^- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited ^•OH generation via complexing with Fe(II). However, the alternating system exhibited OH^--buffering capacity via anoxic-oxic coupling processes because the subsequent oxic periods intercepted Fe(II) regeneration in anoxic periods, while shorter exposure to O₂ in oxic periods avoided excessive OH^- generation. These findings highlight the significant role of anoxic-oxic alternation in contaminant attenuation persistently.