| Abstract
| - This letter addresses how iron redox cycling and the hydration properties of the exchangeable cation influence theBrønsted basicity of adsorbed water in 2:1 phyllosilicates. The probe pentachloroethane undergoes faciledehydrochlorination to tetrachloroethene, attributed to increases in the Brønsted basicity of near-surface hydratingwater molecules following the reduction of structural Fe(III) to Fe(II). This dehydrochlorination process is studiedin the presence of Na+- or K+-saturated Upton montmorillonite [(Na0.82(Si7.84Al0.16)(Al3.10Fe3+0.3Mg0.66) O20(OH)4]or ferruginous smectite [(Na0.87(Si7.38Al0.62)(Al1.08Fe3+2.67Fe2+0.01Mg0.23)O20(OH)4 ]. The effect of iron redox cyclingon pentachloroethane dehydrochlorination is studied using reduced or reduced and reoxidized smectite samplessaturated with Na+ (fully expanded clay) or K+ (fully collapsed clay). Variations in the clay Brønsted basicity followingNa+-for-K+ exchange are explained by cationic charge compensation or interlayer hydration/expansion imposed bythe nature of the exchangeable cation. Inverse relations between K+ fixation and clay water content as well as trendsin pentachloroethane transformation indicate that increases in the Brønsted basicity result from increases in the clayhydrophilicity and shifts in the local activity of distorted clay water. Potassium fixation causes partially collapsedsmectites bearing low amounts of structural Fe(II) to have a similar reactivity to that of fully expanded smectites (Na+form) bearing higher amounts of structural Fe(II). In particular, the conversion of up to 80% of the pentachloroethaneto tetrachloroethane by K+-saturated, reoxidized Upton was explained because the fixation of K+ causes nonreversibleexpansion and incomplete reoxidation of structural Fe(II), which contributes to the stabilization of charge density nearsites bearing Fe(II). Higher pentachloroethane conversions by Upton montmorillonite over ferruginous smectite,however, suggest that charge dispersion rather than site specificity contributes predominantly to clay reactivity. Thus,clay interlayer hydration/expansion imposed by the nature of the exchangeable cation alters water dissociation andproton exchange in Fe(II)−Fe(III) phyllosilicates susceptible to iron redox cycling.
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