Competition for sorption and degradation of chlorinated ethenes in batch zero-valent iron systems

The sorption and degradation of the chlorinated ethenes tetrachloroethene (PCE, 5 mg L-1) and trichloroethene (TCE, 10 mg L-1) were investigated in zero-valent iron systems (ZVI, 100 g L-1) in the presence of compounds common to contaminated groundwater with varying physicochemical properties. The potential competitors were chlorinated ethenes, monocyclic aromatic hydrocarbons, and humic acids. The effect of a complex matrix was tested with landfill contaminated groundwater. Nonlinear Freundlich isotherms adequately described chloroethene sorption to ZVI. In the presence of the more hydrophobic PCE (5 mg L-1), TCE sorption and degradation decreased by 33% and 30%, respectively, while TCE (10 mg L-1) decreased PCE degradation by 30%. In the presence of nonreactive hydrophobic hydrocarbons (i.e., benzene, toluene, and m-xylene at 100 mg L-1), TCE and PCE sorption decreased by 73% and 55%, respectively. The presence of the hydrocarbons had no effect on TCE degradation and increased PCE reduction rates by 50%, suggesting that the displacement of the chloroethenes from the sorption sites by the aromatic hydrocarbons enhanced the degradation rates. Humic acids did not interfere significantly with chloroethene sorption or with TCE degradation but lowered PCE degradation kinetics by 36% when present at high concentrations (100 mg L-1). The landfill groundwater with an organic carbon content of 109 mg L-1 C had no effect on chloroethene sorption but inhibited TCE and PCE degradation by 60% and 70%, respectively.