Evolving reactivity of iron, resulting from precipitation of secondary minerals within iron permeable reactive barriers (PRBs), was included in a reactive transport model for trichloroethene (TCE) treatment. The accumulation of secondary minerals and reactivity loss were coupled using an empirically derived relationship that was incorporated into an existing multicomponent reactive transport code (MIN3P) by modifying the kinetic expressions. The simulation results were compared to the observations from long-term column experiments, which were designed to assess the effects of carbonate mineral formation on the performance of iron for TCE treatment. The model successfully reproduced the evolution of iron reactivity and the dynamic changes in geochemical conditions and contaminant treatment. Predictions under various hydrogeochemical conditions showed that TCE would be treated effectively for an extended period of time without a significant loss of permeability. Although there are improvements yet to be made, this study provides a significant advance in our ability to predict long-term performance of iron PRBs.

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).

Mineralogical examination, using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and optical microscopy, was conducted on the Fe0-bearing reactive materials derived from long-term column experiments undertaken to assess the treatment capacity of Fe0 under different geochemical conditions. The columns received either deionized water or solutions of differing dissolved calcium carbonate concentrations, together either with trichloroethene (TCE) or hexavalent chromium (Cr(VI)). The major reaction product in the columns receiving deionized water was magnetite-maghemite, and for the columns receiving dissolved calcium carbonate, the main products were iron hydroxy carbonate and aragonite. Replacement of Fe0 by reaction products occurred mainly at the edges of the Fe0 particles, and penetrative replacement was focused along cracks and along and around graphitic inclusions. Fibrous or flake-shaped iron hydroxy carbonate mostly replaced the edges of the Fe0 particles. Aragonite had needle-shaped morphology, and some occurred as clusters of crystals. Aragonite was deposited on iron hydroxy carbonate, thus providing at least a partial armoring effect. The mineral was also observed to cement groups of Fe0 particles into compact aggregates. The Cr was present mostly as Cr(III) in Cr(III)-Fe(III) (oxy)hydroxides and in trace amounts in iron hydroxy carbonate.

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This study compares saturated hydraulic conductivities (K s ) of three sandy soils such as coarse, medium and fine sand. The K s was obtained using three different methods: empirical methods based on the grain size analysis, the relative effective porosity model (REPM), and breakthrough curve analyses based on tracer tests. Column drainage tests were performed to characterize the water retention properties of the samples, which are required in the REPM. Bench scale tracer tests with various conditions were conducted to obtain reasonable linear velocities of the samples using breakthrough curve analyses and then K s was estimated using Darcy's law. For the REPM, the differences of K s for the coarse and fine sand soils were less than one order of magnitude; however, the difference of K s values between the empirical methods and the breakthrough curve analyses was larger than one order of magnitude. The comparison results suggest that the REPM can be a reliable method for estimating K s for soil grains, and is cost effective due to its experimental simplicity.

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In granular iron permeable reactive barriers (PRBs), hydrogen gas formation, entrapment and release of gas bubbles, and secondary mineral precipitation have been known to affect the permeability and reactivity. The multicomponent reactive transport model MIN3P was enhanced to couple gas formation and release, secondary mineral precipitation, and the effects of these processes on hydraulic properties and iron reactivity. The enhanced model was applied to a granular iron column, which was studied for the treatment of trichloroethene (TCE) in the presence of dissolved CaCO(3). The simulation reasonably reproduced trends in gas formation, secondary mineral precipitation, permeability changes, and reactivity changes observed over time. The simulation showed that the accumulation of secondary minerals reduced the reactivity of the granular iron over time, which in turn decreased the rate of mineral accumulation, and also resulted in a gradual decrease in gas formation over time. This study provides a quantitative assessment of the evolving nature of geochemistry and permeability, resulting from coupled processes of gas formation and mineral precipitation, which leads to a better understanding of the processes controlling the granular iron reactivity, and represents an improved method for incorporating these factors into the design of granular iron PRBs.

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