Inter- and intraspecies competitive effects in reactions of chlorinated ethylenes with zero-valent iron in column reactors

Reaction kinetics are a key component of process models used to design Fe(0) permeable barriers employed to treat groundwater contaminated with chlorinated solvents. In this work, columns packed with a mixture of zero-valent iron and glass microbeads were used to assess whether the competitive effects observed in batch systems also occur in column reactors. As with the batch studies previously conducted, both inter- and intraspecies competition were observed, in qualitative agreement with predictions obtained from a modified Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model in which species compete for a limited number of reactive sites. The observed changes in pseudo first-order rate 'constant' at different initial concentrations or in the presence of competitors could not be attributed to alterations in the transport properties of the column packing or to 'aging' of the iron. Numerical simulations were used to explore the consequences of applying a pseudo first-order kinetic model instead of the LHHW model. The results demonstrate that intraspecies competitive effects are sufficiently important to merit explicit consideration in permeable barrier design.