Dechlorination of Carbon Tetrachloride by Fe(II) Associated with Goethite

Abstract

Carbon tetrachloride (CT) was dechlorinated to chloroform (CF) under anoxic conditions by Fe(II) that was sorbed to the surface of goethite (α-FeOOH). No reaction occurred when Fe(II) was present and goethite was absent. Several abiotic experiments were conducted with goethite at 30 °C in which the total amount of Fe(II) in the system, the amount of sorbed Fe(II), the density of sorbed Fe(II), and the pH were varied. Regeneration of sorbed Fe(II) occurred when dissolved Fe 2+ was available and maintained pseudo-first-order conditions with respect to CT. Analysis of the rates of CT loss for experiments with sorbed-Fe(II) regeneration showed the rate-determining-step to be first order with respect to CT, second order with respect to the volumetric concentration of sorbed Fe(II) (i.e., mmol sorbed Fe(II) L - 1 suspension), and zero order with respect to H + for pH between 4.2 and 7.3. The absolute rate constant for the reaction was determined to be 42 ± 5 M - 2 s - 1 . Normalization of the observed rate constants to account for different goethite concentrations yielded reaction orders of one and zero, respectively, for CT and H +, and a second-order reaction with respect to the density of sorbed Fe(II) (i.e., mmol sorbed Fe(II) g - 1 goethite). On the basis of the kinetic data, the rate-determining step is proposed to be a termolecular two-electron-transfer reaction involving two Fe 2+ ions sorbed to adjacent sites on the goethite surface and a CCl 4 molecule that approaches the surface. The primary role of the goethite surface, thus, is to catalyze the reaction by fixing the position of the two charged reactants in a geometry suitable for reaction with CT. In separate experiments, biogenic Fe(II) formed by the enzymatic reduction of goethite by the Fe(III)-reducing bacterium Shewanella alga, strain BrY, dechlorinated CT. Of the CT degraded by abiotic and biogenic Fe(II) on goethite, 83−90% was converted to chloroform (CF), which accumulated in the reaction vial. These results indicate that dechlorination reactions in Fe(III)-reducing environments may indirectly result from the enzymatic or chemical reduction of Fe(III)-bearing minerals such as goethite.