Laboratory and full-scale studies of a permeable reactive barrier on the dechlorination of chlorinated aliphatic hydrocarbons

The reactivity of zero-valent iron (Fe0) on the dechlorination of chlorinated aliphatic hydrocarbons (CAHs) and the hydraulic behaviors of the Fe0 packed medium were investigated through laboratory-scale flow-through columns and a field-scale permeable reactive barrier (PRB). Full-scale study of the performance of the funnel-and-gate Fe0 PRB installed at the Vapokon site, Denmark indicates that contaminated groundwater flowed preferentially through the PRB at a seepage velocity of 99.5 m/yr probably owing to the clogging of the PRB. An average Peclet number of 7.30 showed the significance of both advection and dispersion on governing contaminant transport inside the PRB. Based on the past 4 years of field data obtained from the Vapokon site, effective CAH dechlorination by the PRB was observed. Although there was continuous decrease in dissolved ions in the groundwater along the PRB, no noticeable deterioration of the Fe0 reactivity was examined. Instead, there was climatic variation of the Fe0 reactivity on CAH dechlorination. Study of the performance of laboratory Fe0 packed column (LFPC) gave a Peclet number of 488, thereby indicating the domination of advection over dispersion on governing the contaminant transport inside. Comparison of the performances of the fullscale PRB and LFPC showed that there was a high likelihood of overestimating the performance of a full-scale PRB on CAH dechlorination if the results from the LFPC are directly used for the design of PRB thickness. In consequence, a correction factor of 2.94 was practically determined for the correction of the disparity in the performance between the LFPC and field PRB. In addition, laboratory-scale investigation of the rate-limiting step of the reductive dechlorination reaction of CAHs by Fe0 indicated an activation energy in a range of 38.6 to 70.3 kJ/mol. As a result, electron transfer rather than mass transfer process is the dominant rate-limiting step for the dechlorination reaction. Keywords: Contaminated groundwater; longitudinal dispersivity; natural gradient tracer test; permeable reactive barrier; preferential flow; reductive dechlorination; zero-valent iro