Sites where 2-chlorobutadiene-1,3 (chloroprene) and 2,3-dichlorobutadiene-1,3 (DCBD) are synthesized for use in chlorobutyl rubber have the potential to release a mixture of at least five chlorinated butenes and butadienes including trans-1,4-dichlorobutene-2 (1,4-DCB-2), 3,4-dichlorobutene-1 (3,4-DCB-1), 2,3,4-trichlorobutene-1 (2,3,4-TCB-1), chloroprene and DCBD into the groundwater environment. Granular iron has been shown to be effective in the remediation of groundwater contaminated with chlorinated organic compounds by reductive dechlorination. To evaluate the possibility of using granular iron in the remediation of the above contaminants a series of batch and column experiments were conducted at the laboratory scale. Chlorine mass balance calculations showed that each compound, with the exception of DCBD, could be fully dechlorinated by the use of granular iron. Kinetic data and proposed reaction pathways, however, suggest that DCBD can also be fully dechlorinated by granular iron. Normalization of observed pseudo-first-order reaction half-lives indicated that compounds were degrading much slower in batch experiments than in column experiments. This, along with the observation that temperature did not affect degradation in batch experiments, led to the conclusion that mass transport to the iron surfaces was limiting degradation rates in batch experiments. Results showed that the three chlorinated butenes degraded much faster (normalized column half-lives ranged from 1.6 to 5.2 min) than the two chlorinated butadienes (normalized column half-lives ranged from 115 to 197 min). Chlorinated and non-chlorinated intermediates were identified. It was determined that all contaminants degrade to 1,3-butadiene as a reaction intermediate which then degraded to a mixture of non-harmful end products consisting of 1-butene, cis-2-butene, trans-2-butene and n-butane. The reaction pathway from 1,4-DCB-2 to 1,3-butadiene was proposed to be a reductive elimination similar to reductive β-elimination. 3,4-DCB-1 and 2,3,4-TCB-1 were proposed to undergo reductive β-elimination reactions resulting in 1,3-butadiene and chloroprene intermediates, respectively. Degradation of chloroprene and DCBD occurred via hydrogenolysis pathways while 1,3-butadiene underwent catalytic hydrogenation resulting in the observed end products. The results suggest that granular iron may be an effective treatment for groundwater contaminated with these compounds.

Sites where 2-chlorobutadiene-1,3 (chloroprene) and 2,3-dichlorobutadiene-1,3 (DCBD) are synthesized for use in chlorobutyl rubber have the potential to release a mixture of at least five chlorinated butenes and butadienes including trans-1,4-dichlorobutene-2 (1,4-DCB-2), 3,4-dichlorobutene-1 (3,4-DCB-1), 2,3,4-trichlorobutene-1 (2,3,4-TCB-1), chloroprene and DCBD into the groundwater environment. Granular iron has been shown to be effective in the remediation of groundwater contaminated with chlorinated organic compounds by reductive dechlorination. To evaluate the possibility of using granular iron in the remediation of the above contaminants a series of batch and column experiments were conducted at the laboratory scale. Chlorine mass balance calculations showed that each compound, with the exception of DCBD, could be fully dechlorinated by the use of granular iron. Kinetic data and proposed reaction pathways, however, suggest that DCBD can also be fully dechlorinated by granular iron. Normalization of observed pseudo-first-order reaction half-lives indicated that compounds were degrading much slower in batch experiments than in column experiments. This, along with the observation that temperature did not affect degradation in batch experiments, led to the conclusion that mass transport to the iron surfaces was limiting degradation rates in batch experiments. Results showed that the three chlorinated butenes degraded much faster (normalized column half-lives ranged from 1.6 to 5.2 min) than the two chlorinated butadienes (normalized column half-lives ranged from 115 to 197 min). Chlorinated and non-chlorinated intermediates were identified. It was determined that all contaminants degrade to 1,3-butadiene as a reaction intermediate which then degraded to a mixture of non-harmful end products consisting of 1-butene, cis-2-butene, trans-2-butene and n-butane. The reaction pathway from 1,4-DCB-2 to 1,3-butadiene was proposed to be a reductive elimination similar to reductive β-elimination. 3,4-DCB-1 and 2,3,4-TCB-1 were proposed to undergo reductive β-elimination reactions resulting in 1,3-butadiene and chloroprene intermediates, respectively. Degradation of chloroprene and DCBD occurred via hydrogenolysis pathways while 1,3-butadiene underwent catalytic hydrogenation resulting in the observed end products. The results suggest that granular iron may be an effective treatment for groundwater contaminated with these compounds.

Hydrogen gas production occurs in permeable reactive iron barriers (PRBs) due to the anaerobic corrosion of granular iron. Once produced, this hydrogen gas can have detrimental physical effects on PRB performance. Corrosion-produced hydrogen may accumulate in pore spaces within the PRB, thereby reducing the porosity and permeability. It may also escape the PRB system, representing a lost electron resource that may otherwise be used in reductive remediation reactions. In addition to these physical effects of hydrogen on PRB performance, chemical interactions between hydrogen and iron also occur. Hydrogen may become absorbed by the iron and stored as an electron resource within lattice imperfections. It may also interact with iron surfaces to influence the corrosion rate of the iron. These chemical interactions between hydrogen and iron may impact the reactivity of the iron granules and therefore affect PRB performance. Currently, the chemical effects of hydrogen on PRB performance remain largely unexplored. In this study, the effect of hydrogen on iron reactivity was investigated by considering hydrogen absorption into iron and hydrogen induced changes to iron corrosion rates. Hydrogen absorption by iron creates a stored electron resource within the iron granules. Release of this stored hydrogen from trapping sites represents an additional electron resource that may be used in contaminant degradation reactions. Therefore, improved hydrogen absorption may contribute to increased iron reactivity. Hydrogen absorption by granular irons has been largely unexplored in PRB performance investigations and the effect of hydrogen absorption on contaminant remediation remains unknown. In this study, an investigation of the factors governing hydrogen absorption by three granular irons, H2Omet56, H2Omet58 and H2Omet86 was conducted. The results demonstrated that rapidly corroding H2Omet86 absorbed hydrogen at a higher rate than the other more slowly corroding irons. The presence of an oxide film on H2Omet56 appeared to improve the proportion of hydrogen absorption compared to the bare irons. Ultrasonic treatment was explored as potential method of release of trapped hydrogen for improved iron reactivity. Ultrasonic treatment appeared to be unsuccessful at releasing stored hydrogen from trapping sites, but further investigations into different ultrasound conditions as well as other methods of hydrogen release could prove useful. Hydrogen gas may also influence iron reactivity by interacting with iron surfaces to alter the corrosion rate of the iron. This may occur by processes such as hydrogen enhanced anodic dissolution, hydrogen induced cracking, enhanced pitting susceptibility and reduction of iron oxides by hydrogen gas. In this study, the effect of hydrogen on iron corrosion rates was assessed by treating two iron materials (H2Omet56 and Connelly) under high pressures of hydrogen for 14 d, then comparing the post-treatment corrosion rates of hydrogen treated irons to the post-treatment corrosion rates of corresponding irons treated under low hydrogen pressures for the same period. The results demonstrated that the post-treatment corrosion rate of high hydrogen treated H2Omet56 iron was lower than the post-treatment corrosion rate of low hydrogen treated H2Omet56 iron. Hydrogen treatment did not appear to affect the post-treatment corrosion rates of Connelly iron. The effect of hydrogen on the corrosion rate of H2Omet56 iron may be a result of hydrogen enhanced anodic dissolution. The presence of a continuous oxide film on Connelly iron appeared to inhibit the effect of hydrogen enhanced anodic dissolution on Connelly iron corrosion rates. The effects of iron oxide reduction by hydrogen and hydrogen induced pitting corrosion were also considered.