Many remediation technologies have been developed to treat extensive subsurface contamination by chlorinated hydrocarbons (CHCs). One such technology is a permeable reactive barrier (PRB), which contains reactive media that acts as an electron donor in order to reduce CHCs. Extensive effort has been placed on finding the most suitable reactive media in PRBs, with zero-valent iron (ZVI) being the most commonly utilized media. However, zero-valent zinc (ZVZ) is a promising replacement for ZVI in PRBs as it will more readily donate electrons, resulting in more rapid degradation of CHCs. In addition, amending a secondary/catalytic metal to primary metal surface can result in increased degradation kinetics and yield of dechlorinated byproducts.
In this investigation, chlorinated methanes (CF and DCM), ethanes (1,1,2,2-TeCA, 1,1,2-TCA, 1,1,1-TCA, and 1,2-DCA), ethenes (PCE, TCE, cis-DCE, trans-DCE, and VC) and propanes (1,2,3-TCP and 1,2-DCP) were reduced by ZVZ and Zn bimetallic reductants. Results indicated that Cu/Zn was a more potent bimetallic reductant than Pd/Zn and Ni/Zn due to impressive CF degradation kinetics and increased yield of methane formed via direct transformation of CF to methane. Cu/Zn was used in reduction of all other compounds and did not significantly change byproduct distributions when compared to ZVZ.
Reductive β-elimination was an important degradation pathway observed for chlorinated ethanes and ethenes, with reduction of chlorinated ethenes via this pathway resulting in the formation of highly reactive chlorinated acetylene intermediates, thus preventing the formation of VC. It appears that ZVI will reduce chlorinated ethenes via reductive β-elimination more readily than Zn reductants, suggesting an advantage of ZVI in the reduction of chlorinated ethenes. However, Zn reductants appear to be well suited for PRB applications at sites contaminated with 1,2,3-TCP, as reduction with Zn reductants yields fully dechlorinated byproducts via a reductive β-elimination to hydrogenolysis sequence, unlike ZVI.
Cu/Zn increased degradation kinetics compared to ZVZ for all CHCs investigated, but only slight enhancement in kinetics was observed for chlorinated propanes. Increased chlorinated methane, ethane, and propane kinetics have been reported with ZVZ when compared to ZVI, thus suggesting the need to further study Cu/Zn as a potential replacement for ZVI in PRB applications.