Reduction of chlorinated hydrocarbons by nano-sized iron.

摘要

Chlorinated hydrocarbons are common groundwater contaminants in the U.S. treatment of groundwater contaminated with chlorinated hydrocarbons by zero-valent metal is one of the latest innovative technologies for environmental remediation. In the past decade, zero-valent iron has been applied in situ using permeable reactive barriers (PRBs) in which a porous wall of zero-valent iron is constructed in the path of groundwater flow. However, mechanistic details of the reactions have not been fully understood to provide sufficient information to properly design PRBs and to extend the treatment capabilities to wider range of compounds and conditions. The use of nano-sized iron is an attractive alternative to the conventional PRBs technique because of possible advantages in very rapid mass transfer, in-situ facile delivery of particles, and changes in reaction mechanism with very small particle size.;In this work, nano-sized iron was synthesized using chemical borohydride reduction of ferric iron and applied to a total of 18 chlorinated hydrocarbons including chlorinated methanes, ethylenes and ethanes. Parent species disappearance and the formation of reaction intermediates and products were monitored. Various physicochemical factors including pH, metal loading, initial substrate concentration, and hydrogen concentration were examined to elucidate dominant reaction mechanisms.;Results indicate that reduction by nano-sized iron involves different reaction mechanisms specific to each class of compounds. Chlorinated methanes and ethanes reacted with nano-sized iron primarily via a direct reduction mechanism (possibly electron transfer) to give rise to less chlorinated or fully dechlorinated products, whereas chlorinated ethylenes were reduced mostly via a indirect reduction mechanism (possibly catalytic hydrogenation), and transformed to ethylene and ethane with no production of chlorinated intermediates.;Pseudo-first order kinetics adequately described the reaction kinetics of most compounds tested. The reduction of some of chlorinated ethylenes, however, was found to be affected by reaction site saturation, requiring more complicated kinetics models. Modeling with mixed-order kinetics and modified Langmuir-Hinshelwood-Houger-Watson (LHHW) kinetics gave better prediction of experimental data for such cases.;The results obtained in this study regarding reaction pathways and reaction kinetics will help to clarify the reactivity of nano-sized iron and to predict and optimize the performance of treatment systems based on nanoscale particles.