An iron core and minerals
Mineral Precipitates and Their Occurrence
Iron corrosion in groundwater results in the formation of ferrous or ferric ions (when dissolved O2 is present), which ultimately form iron oxyhydroxide mineral precipitates because of their low solubility. It is not surprising, therefore, that many investigators observed iron oxyhydroxides to be the predominant minerals found in the iron reactive barriers (Gu et al., 1999; Phillips et al., 2000; Pratt et al., 1997; Roh et al., 2000) . Core samples were taken ~1.2 and 2.5 years after the Fe0 barrier was installed, and X-ray diffraction (XRD) analysis revealed akaganeite (b-FeOOH) as the major iron mineral precipitant throughout the cores, while goethite (a-FeOOH) was present to a lesser extent (Phillips et al., 2000) . Although they were not detected by the XRD analysis, amorphous iron oxyhydroxide deposits were also observed throughout the iron core materials by means of scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopic analyses. Presumably, these amorphous iron oxyhydroxides gradually transform to crystalline akaganeite and goethite within the barrier. The formation of akaganeite may be related to a relatively high concentration of chloride in groundwater entering the trench, because, in laboratory studies, akaganeite is commonly observed as the dominant mineral phase from precipitation of ferric chloride (Schwertmann and Cornell, 1991) . The presence of goethite within the Fe0 barrier instead of lepidocrocite, which has been reported in laboratory column studies (Gu et al., 1999) , could result from relatively high dissolved O2 and bicarbonate contents of the groundwater. It has been reported that the formation of goethite is favored over the formation of lepidocrocite when carbonates or CO2 are present in the system (Schwertmann and Taylor, 1977) .
Although to a lesser extent, green rusts were also observed as corrosion products in the Fe0 barrier, and similar observations have been reported previously (Gu et al., 1999; Phillips et al., 2000; Roh et al., 2000) . However, green rusts are not stable and can transform into crystalline iron minerals quickly when exposed to the air or subjected to oven drying. Therefore, care must be taken in sample preservation and preparation in order to observe green rusts in the iron barrier material (Phillips et al., 2001) .
In addition to iron oxyhydroxide minerals, analysis of Fe0 core materials indicated the presence of abundant calcium carbonates such as aragonite (CaCO3) and siderite (FeCO3) (Figures above). These results are consistent with decreased concentrations of calcium and carbonates in groundwater within and downgradient of the Fe0 barrier. Crystalline aragonite was observed throughout the core materials of the Fe0 barrier, and its structure and forms were identified by both SEM and EDX analyses . As indicated previously, relatively high concentrations of Ca2+ and carbonates, coupled with a relatively high pH within the Fe0 barrier, may be largely responsible for the precipitation of CaCO3 minerals (Phillips et al., 2000) .
The formation of ferrous carbonate (i.e., siderite) offers another mechanism for a decreased carbonate or bicarbonate concentration in the Fe0 barrier. Ferrous iron is one of the major byproducts of Fe0 corrosion in groundwater; it is thus conceivable that the formation of siderite can be a favorable reaction when high amounts of carbonate are present in groundwater, particularly at a relatively high pH condition (Mackenzie et al., 1999) . The bicarbonate contents from monitoring wells such as TMW-9 were particularly low and could be attributed largely to its precipitation with both Ca2+ and Fe2+ to form carbonate minerals. However, siderite precipitation was much less extensive than aragonite precipitation. The presence of siderite was detected only in patches in some of the iron core samples. Several factors may contribute to these observations. A relatively high pH and high carbonate but low Ca2+ concentrations favor the formation of siderite (Phillips et al., 2000) . On the other hand, a relatively low pH (about neutral) and a low carbonate concentration shift the chemical equilibrium in favor of Fe(OH)2 precipitation. High Ca2+ concentrations in groundwater may compete with Fe2+ for carbonate and form CaCO3 minerals as described above.
of amorphous ferrous sulfide (
Implications for Long-Term Performance
occurrence of a suite of mineral precipitates could have serious
implications for the long-term performance of Fe0 reactive
barriers. Specifically, these mineral precipitates commonly exist as
coating and cementing materials on Fe0 surfaces.
They not only reduce the reactivity of Fe0 and thus its
capacity to degrade or retain target contaminants of concern, but also
cause the cementation and clogging of the reactive Fe0 filings.
Ultimately, they may result in reduced hydraulic conductivity or
the diversion of groundwater through the barrier.
However, site groundwater geochemistry and contaminant
concentration may determine the rate and forms of mineral precipitant
formation and thus the life span of the Fe0 permeable reactive
barriers. For groundwater of
relatively low ionic strength, McMahon et al. (1999) estimated a 0.35% yearly loss of total porosity in the
iron-reactive media at the
more extensive cementation and clogging of iron-reactive media were found
in the iron core materials taken ~2.5 years after the installation of the
Fe0 barrier, particularly at the soil/barrier interfaces where
groundwater enters the Fe0 barrier. The cemented iron cores
appeared to be hard to break (Figure 2), and a close examination (by SEM)
revealed an extensive iron corrosion and subsequent mineral precipitation
on Fe0 surfaces (Figure 18b).
The SEM-EDX analysis of a polished cross-section of the cemented
iron filings indicated that iron oxyhydroxides were the
primary mineral precipitates accumulated on or between individual iron
particles as a thick rind (Phillips
et al., 2000)
. These iron
oxyhydroxides may therefore be largely responsible for the cementation of
Fe0 particles in the barrier.
Similarly, Mackenzie et al.
(1999) reported the portion of an iron column clogged with iron
oxyhydroxides to be a hardened solid mass that greatly decreased hydraulic
the precipitation and formation of aragonite and
Based on an average Fe0 filing thickness of ~0.5 to 1.25 mm, Phillips et al. (2000) estimated that these Fe0 filings could be completely corroded within ~5 to <10 years under the specific site geochemical conditions. This estimated life span of an iron reactive barrier is substantially shorter than the life spans that have been estimated previously, ~15 to 30 years (Gillham et al., 1994; Liang et al., 2000; McMahon et al., 1999) , and may be explained by the fact that the site groundwater contains relatively high levels of NO3- and HCO3-, both of which are known to accelerate the corrosion of Fe0 (Davies and Burstein, 1980; Gu et al., 1999; Huang et al., 1998) . It is also important to note that mineral precipitation and iron cementation appeared to occur progressively with time. Within ~1.5 years, spotted cementations of Fe0 filings were observed mostly at the interface where groundwater enters the Fe0 barrier. Cementation extended and further developed downgradient of the Fe0 barrier, as observed in the second coring event (~2.5 years after the barrier was installed). An important implication of these observations is that such an uneven distribution of iron corrosion and mineral precipitation could potentially result in early system clogging at the interface regions and therefore shorten the functional lifetime of in situ Fe0 barriers. Therefore, close attention should be given to areas in the barrier that seem more vulnerable to corrosion, mineral precipitation, and subsequent cementation (e.g., where groundwater first enters the barriers). Particular attention should also be given to the geochemical composition and concentration in groundwater, which may largely determine the corrosion rate and thus the life span of the Fe0 reactive barriers.