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Mineralogical Characteristics and Transformations during Long-Term Operation of a Zerovalent Iron Reactive Barrier

Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831

View larger version (75K): [in a new window]   Fig. 1. (a) Location of the Fe0 permeable reactive barrier (PRB) at the study site; (b) aerial view of the PRB showing the sampling locations of the cores and ground water wells; and (c) side view of the PRB showing the angled cores through the barrier material.

View larger version (125K): [in a new window]   Fig. 2. Minerals present on the surface of a Fe0 filing from the shallow portion of Core 2 at 5.5 to 5.8 m collected after 30 mo of operation: (a) scanning electron microscopy (SEM) photomicrograph showing hexagonal-shaped goethite crystals and elongated aragonite crystals, (b) energy dispersive X-ray (EDX) analysis of the goethite from (a); (c) EDX analysis of the aragonite from (a); (d) SEM photomicrograph showing the distribution of rounded formations of FeS in relation to the goethite and aragonite shown in (a); and (e) EDX analysis of the FeS from (d). Arrows indicate the location of the EDX analysis areas.

View larger version (62K): [in a new window]   Fig. 3. X-ray diffractograms showing the minerals present in Core 1 (6.4–8.2 m) from a reduced zone at the up-gradient interface to a highly cemented oxidized zone at the down-gradient interface collected 30 mo after operation. Ak, akaganeite; Ar, aragonite; C, calcite; Ca Fd, calcium feldspar; Fe, zerovalent iron; IHC, iron hydroxy carbonate; G, goethite; L, lepidocrocite; Mg, maghemite and/or magnetite; Mk, mackinawite; Q, quartz; and S, siderite.

View larger version (108K): [in a new window]   Fig. 4. Carbonate mineral precipitates from the Fe0 permeable reactive barrier (PRB) sampled 30 mo after operation: (a) scanning electron microscopy (SEM) photomicrograph of clusters of needle-shaped aragonite crystals from the deep down-gradient portion of Core 1 at 5.8 to 6.1 m; (b) energy dispersive X-ray (EDX) analysis of the aragonite from (a); (c) SEM photomicrograph of cube-shaped siderite crystals from the shallow cemented interface from Core 2 at the up-gradient interface at 4.6 to 4.9 m; (d) EDX of the siderite from (c); (e) SEM photomicrograph of the euhedral hexagonal-shaped carbonate form of green rusts from the deep down-gradient interface from Core 2 at 7.9 to 8.2 m; and (f) EDX analysis of the green rusts from (e).

View larger version (149K): [in a new window]   Fig. 5. Polished sections of (a) a fresh Fe0 filings, (b) loose Fe0 filing from the shallow up-gradient Core A collected after the first 15 mo of operation showing thin Fe (oxy)hydroxides corrosion rinds and aragonite crystals on the filing surface; (c) loose Fe0 filing from the shallow midsection of Core 1 collected 30 mo after installation showing thick Fe (oxy)hydroxide corrosion rinds from the depletion of the underlying Fe0 filing; (d) loose Fe0 filing corroding into smaller pieces from the shallow midsection of Core 1 collected 30 mo after installation; (e) scanning electron microscopy (SEM)–backscatter electron (BSE) analysis of cemented Fe0 filings from the up-gradient shallow Core A collected after the first 15 mo of operation; and (f) SEM–BSE of cemented Fe0 filings from the up-gradient shallow Core 1 collected after 30 mo of operation. [Note the smaller Fe0 filings and greater disintegration of the sample collected after 30 mo of operation (f) compared with the sample collected after the first 15 mo of operation (e).]

View larger version (100K): [in a new window]   Fig. 6. Cemented areas of the permeable reactive barrier (PRB): (a) backscatter electron (BSE) analysis of an undisturbed sample collected after the first 15 mo of operation at the shallow up-gradient interface of Core A showing Fe0 filings cemented together by Fe (oxy)hydroxides, Fe2O3, aragonite, and FeS; (b) map showing the distribution of the different minerals in (a); (c) BSE of an undisturbed sample collected 30 mo after installation at the shallow up-gradient interface from Core 1 showing the buildup of precipitation on an Fe0 surface; and (d) map showing the different minerals in (c). (Note the layering of mineral sequences, the transformation of calcite from aragonite, and the goethite crystals on the surface.)