Electrochemical Enhancement on Utilizing Zero-Valent Iron to Degrade Trichloroethylene

Abstract

The objective of this research is to enhance the degrading ability of zero-valent iron (ZVI) on trichloroethylene (TCE) by using electrochemical technique. ZVI has been shown to effectively degrade TCE by reductive dechlorination, a process in which the ZVI, acting as a reducing agent, causes the chlorine to separate from TCE sequentially. The efficiency of this technique could be enhanced by lowering the redox potential of ZVI using electrochemical methods. In this research the lowering of redox potential was achieved by filling granular ZVI into a cathode compartment in a reactor. The ZVI was mixed with granular graphite to increase the overall electrical conductivity before being filled into the cathode. The anode and cathode compartments were separated with a Nafion membrane. The loss of TCE due to adsorption by the granular graphite during the experiments was evaluated by conducting batch adsorption tests. The electrochemical experiments were conducted by applying a direct current using a potentiostat. The TCE concentration as well as possible degradation products was analyzed with gas chromatography equipped with a mass detector. Chloride concentration was measured with ion chromatography. The results of adsorption tests of TCE by granular graphite fits a linear isotherm with a Kd value of 2.3 L/kg, which is comparable to values reported in the literature. Results from the TCE degradation experiments leads to two major conclusions: (1) degradation of TCE was facilitated by the applied current even when the cathode was filled with only granular graphite and (2) degradation rate of TCE by ZVI was increased by the applied current. The degradation rate of TCE could be fitted with a pseudo-first-order kinetics; the reaction rate constant, kh, increases from 0.017 at no current to 0.064 h-1 at a current of 100 mA. No degradation product was observed in the liquid phase. In the gaseous phase, however, cis-1,2-dichloroethylene was observed. The applied current also caused the increasing rate of pH to decrease. The pH of the solution after 8h of processing time increased from 5.7 to 8.6 at no current, whereas it increased from 5.7 to 5.9 with the same processing time. This suggests the applied current caused the decrease of the corrosion rate of ZVI. The results of this study show the electrochemical technique not only increased the TCE degradation rate by ZVI but also decreased the consumption of ZVI by corrosion. Thus, the technique can be applied to prolong the life of ZVI installed in the field for TCE degradation.