Competing TCE and cis-DCE degradation kinetics by zero-valent iron-experimental results and numerical simulation

The successful dechlorination of mixtures of chlorinated hydrocarbons with zero-valent metals requires information concerning the kinetics of simultaneous degradation of different contaminants. This includes intraspecies competitive effects (loading of the reactive iron surface by a single contaminant) as well as interspecies competition of several contaminants for the reactive sites available. In columns packed with zero-valent iron, the degradation behaviour of trichloroethylene (TCE), cis-dichloroethylene (DCE) and mixtures of both was measured in order to investigate interspecies competition. Although a decreasing rate of dechlorination is to be expected, when several degradable substances compete for the reactive sites on the iron surface, TCE degradation is nearly unaffected by the presence of cis-DCE. In contrast, cis-DCE degradation rates decrease significantly when TCE is added. A new modelling approach is developed in order to identify and quantify the observed competitive effects. The numerical model TBC (Transport, Biochemistry and Chemistry, Schäfer et al., 1998a) is used to describe adsorption, desorption and dechlorination in a mechanistic way. Adsorption and degradation of a contaminant based on a limited number of reactive sites leads to a combined zero- and first-order degradation kinetics for high and low concentrations, respectively. The adsorption of several contaminants with different sorption parameters to a limited reactive surface causes interspecies competition. The reaction scheme and the parameters required are successfully transferred from Arnold and Roberts (2000b) to the model TBC. The degradation behaviour of the mixed contamination observed in the column experiments can be related to the adsorption properties of TCE and cis-DCE. By predicting the degradation of the single substances TCE and cis-DCE as well as mixtures of both, the calibrated model is used to investigate the effects of interspecies competition on the design of permeable reactive iron barriers. Even if TCE is present in only small concentrations (>3% of molar cis-DCE concentration) it is the contaminant limiting the residence time and the required thickness of the iron barrier.