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 a luminosilicate mineral surfaces. At pH 7.25, montmorillonite clays were fou nd to be as much as 1000 times less effective than ferric oxides at mediati ng the reaction when expressed on a surface area basis. Reaction rates incr ease dramatically as the pH rises and at pHs above 7.5 approach those previ ously reported for surface bound Fe(II) on ferric oxides at pH 7.22. This i ncrease 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 kinet ics model for the pH dependence of the reaction. The overall reaction is mo deled as the sum of the reactions between 4-Cl-NB and three reductants; FeO H+ 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 surf ace sites and FeOH+ are presented. Although the minerals investigated are m uch less effective at mediating the reaction than ferric oxides, the rates are sufficiently fast to be of importance to environmental processes. At ne utral pHs, half-lives are less than a week and decrease to the scale of hou rs above pH 7.5. This is quite rapid in the context of groundwater systems in which residence times can be months or years.