Effects of Mixing Granular Iron with Sand on the Kinetics of Trichloroethylene Reduction

A substantial cost of granular iron permeable reactive barriers is that of the granular iron itself. Cutting the iron with sand can reduce costs, but several performance issues arise. In particular, reaction rates are expected to decline as the percentage of iron in the blend is diminished. This might occur simply as a function of iron content, or mass transfer effects may play a role in a much less predictable fashion. Column experiments were conducted to investigate the performance consequences of mixing Connelly granular iron with sand using the reduction kinetics of trichloroethylene (TCE) to quantify the changes. Five mixing ratios (i.e., 100%, 85%, 75%, 50%, and 25% of iron by weight) were studied. The experimental data showed that there is a noticeable decrease in the reaction rate when the content of sand is 25% by weight (iron mass to pore volume ratio, Fe/V-p = 3548 g/L) or greater. An analysis of the reaction kinetics, using the Langmuir-Hinshelwood rate equation, indicated that mass transfer became an apparent cause of rate loss when the iron content fell below 50% by weight (Fe/V-p = 2223 g/L). Paradoxically, there were tentative indications that TCE removal rates were higher in a 15% sand + 85% iron mixture (Fe/V-p = 4416 g/L) than they were in 100% iron (Fe/V-p = 4577 g/L). This subtle improvement in performance might be due to an increase of iron surface available for contact with TCE, due to grain packing in the sand-iron mixture.