Abstract
Chlorinated organic solvents, such as tetrachloroethene (PCE) and trichloroethylene (TCE) are common groundwater contaminants. The removal of chlorinated hydrocarbons by permeable iron barrier has received much attention. However, the long-term effectiveness of permeable iron barriers and the influence of humic acid and metal ions on dechlorination, frequently found compounds at contaminated sites, remain unclear. Therefore, the aim of this study was to understand the reduction of PCE by zero-valent iron in the presence of metal ions, such as Co(II), Cu(II), Ni(II) and Pd(II) and/or humic acid. Results showed that dechlorination rate of PCE could be enhanced by zero-valent iron (ZVI) amended with metal ions. Ethane and ethene were the major end products, showing that b-elimination is the major reaction pathway for PCE dechlorination. XPS results showed that divalent metal ions were reduced to zero-valent metals by ZVI. In the presence of humic acid, it was adsorbed on the surface of the iron particles. Due to this reason, the chemisorption of PCE to ZVI surface was inhibited because of the completion of PCE with humic acid for sorption sites resulting in inhibiting the surface reaction of degradation rate by ZVI. After equilibrium for 24 h between humic acid and ZVI, dechlorination efficiency and rate of PCE can be enhanced at elevated humic acid concentration by Pd/Fe. The aqueous humic acid acted as mediator to accelerate the electron/atomic hydrogen transfer from Fe0 to PCE. Consequently, the rate-limiting step of chlorinated ethylene might shift from the chemisorption (surface touching reaction) to atomic hydrogen reduction.