Effects of mineral precipitation on long-term performance of granular iron permeable reactive barriers: Column experiments and numerical simulation

Granular iron permeable reactive barrier (PRB) technology has received considerable acceptance because of its efficient removal of a wide range of contaminants and low maintenance cost. However, the long-term performance of an iron PRB can be Limited if secondary mineral accumulation increases over time. Accumulation of secondary minerals may affect system hydraulics through a decrease in porosity and hydraulic conductivity. Furthermore, a decrease in reactivity of the iron may result in breakthrough or incomplete treatment of contaminants, rendering the PRB ineffective. Laboratory column experiments and numerical simulations were therefore conducted to evaluate the effects of secondary minerals on permeability and reactivity of iron PRBs. In the column experiments, the extent of carbonate mineral formation under differing dissolved carbonate concentrations was assessed by supplying simulated groundwater to several columns containing commercial iron materials. TCE and Cr were introduced as target contaminants, and organic and inorganic constituents were analyzed. Mineralogical studies were also performed. Observations from the performance of the column experiments were used to establish a conceptual model, and an existing reactive transport model was modified to incorporate the change in reactivity of iron. Finally, comparisons between the laboratory and simulation results were used to evaluate the predictive capability of the model and further to estimate Longevity of iron PRBs.; The results from the column experiments showed that mineral precipitation caused a decrease in reactivity of the iron, and that spatially- and temporally-varying reactivity loss resulted in migration of the mineral precipitation fronts, as well as other profiles such as TCE, Cr, alkalinity, calcium, and dissolved iron. The loss of reactivity was most likely a consequence of the accumulation of carbonate precipitates formed on the iron surfaces. The major carbonate minerals, identified using SEM, XRD, and optical microscopy, were aragonite and iron hydroxy carbonate. For the columns treating Cr, the sequestration of Cr6+ was likely because of substitution of Cr3+ for Fe3+ in magnetite-maghemite and/or in iron hydroxy carbonate, and the formation of iron(III)-chromium(III) (oxy)hydroxides resulted in great decrease in reactivity. The columns receiving de-ionized water showed relatively constant hydraulic conductivities during the period of operation. In contrast, in the columns receiving solutions of dissolved calcium carbonate, hydraulic conductivities gradually decreased over time, as carbonate minerals accumulated. However, as the rate of carbonate precipitation slowed, there was little further decrease in hydraulic conductivity. Overall, it was concluded that the reactivity of iron is likely to be a more limiting factor than the permeability of iron regarding long-term performance.; Coupling the reactivity loss of iron and the accumulation of secondary minerals was the most essential part of the modeling, and the change in reactivity was proposed as an exponential function of the accumulated mineral volume fraction, as determined from the laboratory results. This reactivity change was incorporated into kinetic expressions of an existing multi-component reactive transport model (MIN3P). The simulation results reproduced the observations from the column experiments reasonably well, supporting the predictive capability of the model. Further predictions under various hydrogeochemical conditions showed that TCE and Cr would be treated effectively for an extended period of time, possibly in excess of forty years, and that porosity would be maintained without causing severe clogging problems. Though there are improvements yet to be made, the results indicate that the model could be a valuable tool for predicting long-term performance of iron PRBs.