Pd-catalyzed TCE dechlorination in water: Effect of [H-2](aq) and H-2-utilizing competitive solutes on the TCE dechlorination rate and product distribution

The aqueous-phase H-2 concentration ([H-2](aq)) and the presence of H-2-uti lizing competitive solutes affect TCE dechlorination efficiency in Pd-based in-well treatment reactors. The effect of [H-2](aq) and H-2-utilizing comp eting solutes (cis-DCE, trans-DCE, 1,1-DCE, dissolved oxygen (DO), nitrite, nitrate) on the TCE transformation rate and product distribution were eval uated using 100 mg/L of a powdered Pd-on-Al2O3 catalysts in batch reactors or 1.0 g of a 1.6-mm Pd-on-gamma -Al2O3 catalyst in column reactors. The TC E dechlorination rate constant decreased by 55% from 0.034 +/- 0.006 to 0.0 15 +/- 0.001 min(-1) when the [H-2](aq) decreased from 1000 to 100 muM and decreased sharply to 0.0007 +/- 0.0003 min(-1) when the [H-2](aq) decreased from 100 to 10 muM. Production of reactive chlorinated intermediates and C 4-C6 radical coupling products increased with decreasing [H-2](aq). At an [ H-2](aq) of 10 muM (P/P-o = 0.01), DCE isomers and vinyl chloride accounted for as much as 9.8% of the TCE transformed at their maximum but disappeare d thereafter, and C4-C6 radical coupling products accounted for as much as 18% of TCE transformed. The TCE transformation rate was unaffected by the p resence of cis-DCE (202 muM), trans-DCE (89 muM), and 1,1-DCE (91 muM), ind icating that these compounds do not compete with TCE for catalyst active si tes. DO is twice as reactive as TCE but had no effect on TCE conversion in the column below a concentration of 370 muM (11.8 mg/L), indicating that DO and TCE will not compete for active catalyst sites at typical groundwater DO concentrations. TCE conversion in the column was reduced by as much as a factor of 10 at influent DO levels greater than 450 mM (14.3 mg/L) because the [H-2](aq) fell below 100 muM due to H-2 utilized in DO conversion. Nit rite reacts 2-5 times slower than TCE and reduced TCE conversion by less th an 4% at a concentration of 6630 muM (305 mg/L). Nitrate was not reactive a nd did not effect TCE conversion at a concentration of 1290 muM (80 mg/L).