Freezing-Induced Redistribution of Fe(II) Species within Clay Minerals for Nonlinear Variations in Hydroxyl Radical Yield and Contaminant Degradation

Hydroxyl radical (center dot OH) formation from Fe(II)-bearing clay mineral oxygenation in the shallow subsurface has been well documented under moderate environmental conditions. However, the impact of freezing processes on the center dot OH production capability of Fe(II)-bearing clay minerals for organic contaminant degradation, particularly in seasonally frozen soils, remains unclear. In this study, we investigated the influence of pre-freezing durations on the mineral proprieties, center dot OH production, and phenol degradation during the oxygenation of reduced Fe-rich nontronite (rNAu-2) and Fe-poor montmorillonite (rSWy-3). During the freezing process of reduced clay minerals (1 mM Fe (II)), the content of edge surface Fe and Fe(II) decreased by up to 46% and 58%, respectively, followed by a slight increased as clay mineral particles aggregated and subsequently partially disaggregated. As the edge surface Fe(II) is effective in O2 activation but less effective in the transformation of H2O2 to center dot OH, the redistribution of edge surface Fe(II) leads to that center dot OH production and phenol degradation increased initially and then decreased with pre-freezing durations ranging from 0 to 20 days. Moreover, the rate constants of phenol degradation for both the rapid and slow reaction phases also first increase and then decrease with freezing time. However, pre-freezing significantly influenced the rapid phase of phenol degradation by rNAu-2 but affected the slow phase by rSWy-3 due to the much higher edge-surface Fe(II) content in rNAu-2. Overall, these findings provide novel insights into the mechanism of center dot OH production and contaminant degradation during the freeze-thaw processes in clay-rich soils.