Understanding pH effects on trichloroethylene and perchloroethylene adsorption to iron in permeable reactive barriers for groundwater remediation

Metallic iron filings have been increasingly used in permeable reactive barriers for remediating groundwater contaminated by chlorinated solvents. Understanding solution pH effects on rates of reductive dechlorination in permeable reactive barriers is essential for designing remediation systems that can meet treatment objectives under conditions of varying groundwater properties. The objective of this research was to investigate how the solution pH value affects adsorption of trichloroethylene (TCE) and perchloroethylene (PCE) on metallic iron surfaces. Because adsorption is first required before reductive dechlorination can occur, pH effects on halocarbon adsorption energies may explain pH effects on dechlorination rates. Adsorption energies for trichloroethylene and perchloroethylene were calculated via molecular mechanics simulations using the Universal force field and a self-consistent reaction field charge equilibration scheme. A range in solution pH values was simulated by varying the amount of atomic hydrogen adsorbed on the iron. The potential energies associated trichloroethylene and perchloroethylene complexes were dominated by electrostatic interactions, and complex formation with the surface was found to result in significant electron transfer from the iron to the adsorbed halocarbons. Adsorbed atomic hydrogen was found to lower the energies of trichloroethylene complexes more than those for perchloroethylene. Attractions between atomic hydrogen and iron atoms were more favorable when trichloroethylene versus perchloroethylene was adsorbed to the iron surface. These two findings are consistent with the experimental observation that changes in solution pH affect trichloroethylene reaction rates more than those for perchloroethylene.