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Iron Hydroxy Carbonate Formation in Zerovalent Iron Permeable Reactive Barriers: Characterization and Evaluation of Phase Stability
Citation:
LEE, T. R. AND R. T. WILKIN. Iron Hydroxy Carbonate Formation in Zerovalent Iron Permeable Reactive Barriers: Characterization and Evaluation of Phase Stability. JOURNAL OF CONTAMINANT HYDROLOGY. Elsevier Science Ltd, New York, NY, 116(1-4):47-57, (2010).
Impact/Purpose:
the purpose is to provide new data that will aid in developing better predictive models of mineral accumulation in zerovalent iron PRBs.
Description:
Predicting the long-term potential of permeable reactive barriers for treating contaminated groundwater relies on understanding the endpoints of biogeochemical reactions between influent groundwater and the reactive medium. Iron hydroxy carbonate (chukanovite) is frequently observed as a secondary mineral precipitate in granular iron PRBs. Carbonate minerals, in particular, are recognized as pervasive mineral products that impact reactivity and hydraulic conductivity of in situ systems that utilize zerovalent iron. Mineralogical characterization was carried out using X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and X-ray absorption spectroscopy on materials collected from three field-based PRBs in the US (East Helena, MT; Elizabeth City, NC; Denver Federal Center, CO). These PRBs were installed to treat a range of contaminant types, including chlorinated organics, hexavalent chromium, and arsenic. Results obtained indicate that chukanovite is a prevalent secondary precipitate in each of the studied PRBs, with a diagnostic signature detectable with the analytical tools used in this study. Laboratory experiments on high-purity chukanovite separates were carried out to constrain the room-temperature solubility for this mineral, which is not presently available in the literature. An estimated Gibbs energy of formation (ΔfG°) for chukanovite is -1174.4 ± 6 kJ/mol. A mineral stability diagram constructed with this new thermodynamic value for chukanovite is consistent with observations from the field. Water chemistry from the three reactive barriers falls inside the predicted stability field for chukanovite, at inorganic carbon concentrations intermediate to the stability fields of siderite and ferrous hydroxide. These new data will aid in developing better predictive models of mineral accumulation in zerovalent iron PRBs.
