pH dependence on reduction rate of 4-Cl-nitrobenzene by Fe(II)/montmorillonite systems

The pseudo-first-order reduction of 4-Cl-nitrobenzene by Fe(II) in aqueous systems containing montmorillonite clays is investigated over the pH range 6.00-8.00. Silica and alumina is also investigated as simple analogues to aluminosilicate mineral surfaces. At pH 7.25, montmorillonite clays were found to be as much as 1000 times less effective than ferric oxides at mediating the reaction when expressed on a surface area basis. Reaction rates increase dramatically as the pH rises and at pHs above 7.5 approach those previously reported for surface bound Fe(II) on ferric oxides at pH 7.22. This increase in reactivity is attributed to both an increase in concentration of the FeOH+ ion and to the increased sorption of Fe(II) at high pH. Sorption isotherms for Fe(ll) to montmorillonite clays at pH 7.00 are reported. Two surface sites are suggested on clay minerals and incorporated into a kinetics model for the pH dependence of the reaction. The overall reaction is modeled as the sum of the reactions between 4-Cl-NB and three reductants; FeOH+ and Fe(ll) bound to the two surface sites. FeOH+ is found to be the most effective reductant in our systems. intrinsic rate constants for both surface sites and FeOH+ are presented. Although the minerals investigated are much less effective at mediating the reaction than ferric oxides, the rates are sufficiently fast to be of importance to environmental processes. At neutral pHs, half-lives are less than a week and decrease to the scale of hours above pH 7.5. This is quite rapid in the context of groundwater systems in which residence times can be months or years.