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

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• 作者：James E. Amonette ; Darla J. Workman ; David W. Kennedy ; Jonathan S. Fruchter ; and Yuri A. Gorby
• 刊名：Environmental Science & Technology
• 出版年：2000
• 出版时间：November 1, 2000
• 年：2000
• 卷：34
• 期：21
• 页码：4606 - 4613
• 全文大小：112K
• 年卷期：v.34,no.21(November 1, 2000)
• ISSN：1520-5851

文摘

Carbon tetrachloride (CT) was dechlorinated to chloroform(CF) under anoxic conditions by Fe(II) that was sorbedto the surface of goethite (-FeOOH). No reaction occurredwhen Fe(II) was present and goethite was absent.Several abiotic experiments were conducted with goethiteat 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) occurredwhen dissolved Fe²⁺ was available and maintained pseudo-first-order conditions with respect to CT. Analysis ofthe rates of CT loss for experiments with sorbed-Fe(II)regeneration showed the rate-determining-step to be firstorder with respect to CT, second order with respect tothe volumetric concentration of sorbed Fe(II) (i.e., mmolsorbed Fe(II) L^-1 suspension), and zero order with respectto H⁺ for pH between 4.2 and 7.3. The absolute rateconstant for the reaction was determined to be 42 ± 5M^-2 s^-1. Normalization of the observed rate constants toaccount for different goethite concentrations yielded reactionorders of one and zero, respectively, for CT and H⁺, anda second-order reaction with respect to the density of sorbedFe(II) (i.e., mmol sorbed Fe(II) g^-1 goethite). On the basisof the kinetic data, the rate-determining step is proposed tobe a termolecular two-electron-transfer reaction involvingtwo Fe²⁺ ions sorbed to adjacent sites on the goethitesurface and a CCl₄ molecule that approaches the surface.The primary role of the goethite surface, thus, is tocatalyze the reaction by fixing the position of the twocharged reactants in a geometry suitable for reaction withCT. In separate experiments, biogenic Fe(II) formed bythe enzymatic reduction of goethite by the Fe(III)-reducingbacterium 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), whichaccumulated in the reaction vial. These results indicatethat dechlorination reactions in Fe(III)-reducing environmentsmay indirectly result from the enzymatic or chemicalreduction of Fe(III)-bearing minerals such as goethite.