Mechanisms and Products of Surface-Mediated Reductive Dehalogenation of Carbon Tetrachloride by Fe(II) on Goethite
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- 作者：Martin Elsner ; Stefan B. Haderlein ; Thomas Kellerhals ; Samuel Luzi ; Luc Zwank ; Werner Angst ; and René ; P. Schwarzenbach
- 刊名：Environmental Science & Technology
- 出版年：2004
- 出版时间：April 1, 2004
- 年：2004
- 卷：38
- 期：7
- 页码：2058 - 2066
- 全文大小：191K
- 年卷期：v.38,no.7(April 1, 2004)
- ISSN：1520-5851
文摘
Natural attenuation processes of chlorinated solvents insoils and groundwaters are increasingly considered asoptions to manage contaminated sites. Under anoxicconditions, reactions with ferrous iron sorbed at iron(hyro)xides may dominate the overall transformation of carbontetrachloride (CCl4) and other chlorinated aliphatichydrocarbons. We investigated mechanisms and productformation of CCl4 reduction by Fe(II) sorbed to goethite, whichmay lead to completely dehalogenated products or tochloroform (CHCl3), a toxic product which is fairly persistentunder anoxic conditions. A simultaneous transfer of twoelectrons and cleavage of two C-Cl bonds of CCl4 wouldcompletely circumvent chloroform production. To distinguishbetween initial one- or two-bond cleavage, 13C-isotopefractionation of CCl4 was studied for reactions with Fe(II)/goethite (isotopic enrichment factor 
= -26.5”) andwith model systems for one C-Cl bond cleavage and eithersingle-electron transfer (Fe(II) porphyrin, = -26.1”) orpartial two-electron transfer (polysulfide, = -22.2”). These values differ significantly from calculations for simultaneouscleavage of two C-Cl bonds ( 
-50”), indicatingthat only one C-Cl bond is broken in the critical first stepof the reaction. At pH 7, reduction of CCl4 by Fe(II)/goethite produced ~33% CHCl3, 20% carbon monoxide(CO), and up to 40% formate (HCOO-). Addition of 2-propanol-d8 resulted in 33% CDCl3 and only 4% CO, indicating thatboth products were generated from trichloromethyl radicals(
CCl3), chloroform by reaction with hydrogen radicaldonors and CO by an alternative pathway likely to involvesurface-bound intermediates. Hydrolysis of CO to HCOO-was surface-catalyzed by goethite but was too slow to accountfor the measured formate concentrations. Chloroformyields slightly increased with pH at constant Fe(II) sorptiondensity, suggesting that pH-dependent surface processesdirect product branching ratios. Surface-stabilizedintermediates may thus facilitate abiotic mineralization ofCCl4, whereas the presence of H radical donors, such asnatural organic matter, enhances formation of toxicCHCl3.
= -26.5”) andwith model systems for one C-Cl bond cleavage and eithersingle-electron transfer (Fe(II) porphyrin, = -26.1”) orpartial two-electron transfer (polysulfide, = -22.2”). These values differ significantly from calculations for simultaneouscleavage of two C-Cl bonds ( -50”), indicatingthat only one C-Cl bond is broken in the critical first stepof the reaction. At pH 7, reduction of CCl4 by Fe(II)/goethite produced ~33% CHCl3, 20% carbon monoxide(CO), and up to 40% formate (HCOO-). Addition of 2-propanol-d8 resulted in 33% CDCl3 and only 4% CO, indicating thatboth products were generated from trichloromethyl radicals(CCl3), chloroform by reaction with hydrogen radicaldonors and CO by an alternative pathway likely to involvesurface-bound intermediates. Hydrolysis of CO to HCOO-was surface-catalyzed by goethite but was too slow to accountfor the measured formate concentrations. Chloroformyields slightly increased with pH at constant Fe(II) sorptiondensity, suggesting that pH-dependent surface processesdirect product branching ratios. Surface-stabilizedintermediates may thus facilitate abiotic mineralization ofCCl4, whereas the presence of H radical donors, such asnatural organic matter, enhances formation of toxicCHCl3.