Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of coupling anoxic-oxic process which often happens in subsurface soil on contaminant transformation remained 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 alternating system showed faster 2,4-DCP transformation than corresponding control single anoxic and oxic system, therefore, a higher transformation rate (63.4%) was obtained in alternating system relative to control systems (27.9-42.4%). Compared to stable pH in alternating system, control systems presented clear OH^- accumulation, caused by more Fe(II) regeneration in control anoxic system and longer oxygenation in control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in anoxic process and induced –OH oxidation in oxic process, OH^- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited –OH generation via complexing with Fe(II). However, 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.

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soil iron minerals,redox condition,coupling reduction-oxidation,hydroxyl radicals,2,4-DCP transformation