Effects of carbonate precipitates on long-term performance of granular iron for reductive dechlorination of TCE

Long-term column experiments were conducted to evaluate the effects of secondary carbonate minerals on permeability and reactivity of commercial granular iron treating trichloroethene (TCE). The results showed that carbonate precipitates caused a decrease in reactivity of the iron, and spatially and temporally varying reactivity loss resulted in migration of mineral precipitation fronts, as well as profiles of TCE, pH, alkalinity, calcium, and dissolved iron. In the columns receiving solutions of dissolved calcium carbonate, porosity gradually decreased in proportion to the source concentrations, as carbonate minerals accumulated. However, the rate of porosity loss slowed over time because of the declining reactivity of the iron. Thus, secondary minerals are not likely to accumulate to the extent that there is a substantial reduction in hydraulic conductivity. The reactivity of the iron was found to decrease as an exponential function of the carbonate mineral volume fraction. This changing reactivity of iron should be incorporated into predictive models for improved designs of iron permeable reactive barriers (PRBs).