| Abstract
| - The aqueous-phase H2 concentration ([H2](aq)) and thepresence of H2-utilizing competitive solutes affect TCEdechlorination efficiency in Pd-based in-well treatmentreactors. The effect of [H2](aq) and H2-utilizing competingsolutes (cis-DCE, trans-DCE, 1,1-DCE, dissolved oxygen(DO), nitrite, nitrate) on the TCE transformation rate andproduct distribution were evaluated using 100 mg/L of apowdered Pd-on-Al2O3 catalysts in batch reactors or 1.0 gof a 1.6-mm Pd-on-γ-Al2O3 catalyst in column reactors.The TCE dechlorination rate constant decreased by 55%from 0.034 ± 0.006 to 0.015 ± 0.001 min-1 when the [H2](aq)decreased from 1000 to 100 μM and decreased sharplyto 0.0007 ± 0.0003 min-1 when the [H2](aq) decreased from100 to 10 μM. Production of reactive chlorinatedintermediates and C4−C6 radical coupling products increasedwith decreasing [H2](aq). At an [H2](aq) of 10 μM (P/Po= 0.01), DCE isomers and vinyl chloride accounted for asmuch as 9.8% of the TCE transformed at their maximumbut disappeared thereafter, and C4−C6 radical couplingproducts accounted for as much as 18% of TCE transformed.The TCE transformation rate was unaffected by thepresence of cis-DCE (202 μM), trans-DCE (89 μM), and 1,1-DCE (91 μM), indicating that these compounds do notcompete with TCE for catalyst active sites. DO is twice asreactive as TCE but had no effect on TCE conversion inthe column below a concentration of 370 μM (11.8 mg/L),indicating that DO and TCE will not compete for activecatalyst sites at typical groundwater DO concentrations.TCE conversion in the column was reduced by as much asa factor of 10 at influent DO levels greater than 450 mM(14.3 mg/L) because the [H2](aq) fell below 100 μM due toH2 utilized in DO conversion. Nitrite reacts 2−5 timesslower than TCE and reduced TCE conversion by less than4% at a concentration of 6630 μM (305 mg/L). Nitratewas not reactive and did not effect TCE conversion at aconcentration of 1290 μM (80 mg/L).
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