PH dependence of carbon tetrachloride reductive dechlorination by magnetite
PH dependence of carbon tetrachloride reductive dechlorination by magnetite
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DOI：
10.1021/es0496874
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发表时间：
2004-09-15
影响因子：
11.4
通讯作者：
Hayes, KF
中科院分区：
文献类型：
--
作者：
Danielsen, KM;Hayes, KF
磁铁矿由异化铁还原菌沉淀或通过零价铁（ZVI）在可渗透反应屏障中的腐蚀形成。在间歇式反应器中考察了合成磁铁矿还原四氯化碳（CCl4）的反应速率和产物分布对pH值的依赖性。这项工作提出了第一个数据，磁铁矿促进四氯化碳脱氯独立添加吸附的Fe（II）或共存的矿物，保持Fe2+以上的磁铁矿溶解度限制。在该体系中，反应速率常数随着pH值在6和10之间的增加而增加。的pH值依赖性解释由两个表面站点之间的酸碱平衡，其中更多的去质子化表现出更大的脱氯反应性。还发现反应产物的分布取决于pH。主要反应产物是一氧化碳（CO），然后是氯仿（CHICl3）CHCl3的生产在pH 6时最低，但通过pH 10增加。两种产品的形成速率常数随pH值的增加而增加，但CHCl3的值以更快的速率增加。提出了一个假设，涉及到CHCl3速率增强的去质子化的表面位点的能力降低，以稳定的三氯碳负离子过渡态复合物。这些数据构成了评估铁还原条件下形成的磁铁矿的自然衰减能力的基础。这一信息渗透屏障技术的应用表明，从长远来看，氧化的ZVI磁铁矿可能伴随着转向更良性的反应产物，以及2个数量级的反应速率常数下降。
Magnetite is precipitated by dissimilatory iron-reducing bacteria or forms through corrosion of zero-valent iron (ZVI) in permeable reactive barriers. Reduction of carbon tetrachloride (CCl4) by synthetic magnetite was examined in batch reactors to evaluate the pH dependence of the reaction rates and product distributions. This work presents the first data where magnetite promotes CCl4 dechlorination independent of added sorbed Fe(II) or coexisting minerals that maintained Fe2+ above the magnetite solubility limit. In this system, reaction rate constants increase with increasing pH values between 6 and 10. The pH dependence is explained by acid-base equilibrium between two surface sites, where the more deprotonated exhibits greater dechlorination reactivity. The distribution of reaction products was also found to depend on pH. The primary reaction product is carbon monoxide (CO) followed by chloroform (CHICl3) CHCl3 production is at a minimum at pH 6 but increases through pH 10. Formation rate constants for both products increase with increasing pH, but the values for CHCl3 increase at a much faster rate. A hypothesis is proposed that relates the CHCl3 rate enhancement to the reduced capacity of deprotonated surface sites to stabilize the trichlorocarbanion transition-state complex. These data form a basis to assess the natural attenuation capacity of magnetite formed under iron reducing conditions. Application of this information to permeable barrier technology suggests that, in the long term, oxidation of ZVI to magnetite may be accompanied by a shift toward more benign reaction products as well as a 2 order of magnitude decrease in reaction rate constants.