pH Dependence on Reduction Rate of 4-Cl-Nitrobenzene by Fe(II)/Montmorillonite Systems

Abstract

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(II) 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(II) 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.

Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)

“…The association of Fe 2ϩ with solid phases has been shown to significantly increase the reactivity of Fe 2ϩ (Sørensen and Thorling, 1991;Schultz and Grundl, 2000;Pecher et al, 2002;Hofstetter et al, 2003;Elsner et al, 2004). This has been attributed to a decrease in the Fe(III)-Fe(II) redox potential, due to surface complexation of Fe(II) by hydroxyl groups on the mineral's surface stabilising the Fe(III) oxidation state (Wehrli, 1990;Stumm et al, 1990;Stumm, 1997).…”

Heterogeneous oxidation of Fe(II) on iron oxides in aqueous systems: Identification and controls of Fe(III) product formation

“…In particular, understanding coupled oxidant-iron redox reactions is paramount for evaluating the relative importance of iron-mediated reactions compared to other natural attenuation processes; to this end, these prior studies report rates, extents, product distributions, and geochemical influences on redox reactions. Oxidant half-lives can be minutes (e.g., nitrobenzenes (Klausen et al, 1995)) to months (e.g., trichloroethene (Zwank et al, 2005)) and are strongly influenced by Fe(II) concentration (Strathmann and Stone, 2003), solution pH (Klausen et al, 1995;Pecher et al, 2002;Strathmann and Stone, 2003), and mineral phase (Klausen et al, 1995;Buerge and Hug, 1999;Schultz and Grundl, 2000;Strathmann and Stone, 2003;Elsner et al, 2004). Depending on oxidant class, electron transfer to the oxidant molecule may result in reduction of an organic moiety, reduction of the central metal ion, reductive precipitation of a metal, or reductive dechlorination .…”

Effects of Oxide Coating and Selected Cations on Nitrate Reduction by Iron Metal

“…According to Stumm (1992), at pH of ≥7.0, inner‐sphere complexation of Fe 2+ to metal oxides can create a stronger reductant. Hydrolysis of Fe 2+ to FeOH + can also increase reducing power (Schultz and Grundl, 2000). Thus, the nature of the iron (hydr)oxides and their relationships to Fe 0 and Fe 2+ are critical to understanding mechanisms of nitrate reduction by Fe 0…”