No abstract

The working lifetime of permeable reactive barriers (PRBs) using Fe 0 as the reactive media is limited by precipitation of secondary minerals, due to reaction of groundwater with Fe 0 . Since PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of their formation, can vary widely. Using plausible phases obtained from field PRBs, the study shows that chemical equilibrium modeling can correctly predict the amounts of precipitates formed, based on the thermodynamic properties of Fe 0 and groundwater constituents. These predictions were compared to the results from the solid phase analysis from a field column experiment and from a field-installed PRB at Y-12 Plant, Oak Ridge, TN. Using the column chemical data molar distributions of the precipitates along the flow path were modeled. The maximum precipitation at the Fe 0 -sand interface at the influent end was predicted, where pore water showed high saturation index (SI) with respect to calcite and iron (oxyhydr)oxide. In the absence of flow information, the field sampling data were used to construct an SI-pH diagram, from which the extent of reaction with Fe 0 , the potential for precipitate buildup, and relative residence time for the pore water were identified. Kinetic and heterogeneous flow effects were also discussed. To illustrate the application of chemical equilibrium modeling to the design and planning phase of PRBs, groundwater data from four PRB sites were analyzed. The analysis shows that up to 0.63 cm 3 /L solid could form in pore water using an average Fe 0 dissolution rate, leading to severe clogging of Fe 0 medium over a 10-yr period of operation.

Thermogravimetric analysis (TGA) combined with X-ray diffraction (XRD) was used to identify mineral phases and determine corrosion rates of granular iron samples from a 2-yr field column study. Similar to other studies, goethite, magnetite, aragonite, and calcite were found to be the major precipitated minerals, with Fe2(OH)2CO3 and green rust as minor phases. Based on TGA-mass spectrometry (MS) analysis, Fe0 corrodes at rates of 0.5-6.1 mmol kg(-1) d(-1) in the high NO3- (up to 13.5 mM) groundwater; this rate is significantly higher than previously reported. Porosity reduction was 40.6%-45.1% for the inlet sand/Fe0 interface and 7.4%-25.6% for effluent samples of two test columns. Normalized for treatment volumes, porosity loss values are consistent with studies that use high levels of SO4(2-) but are higher than those using low levels of corrosive species. Aqueous mass balance calculations yield corrosion rates similar to the TGA-MS method, providing an alternative to coring and mineralogical analysis. A severely corroded iron sample from the column simulating a 17-yr treatment throughput showed >75% porosity loss. Extensive porosity loss due to high levels of corrosive species in groundwater will have significant impact on long-term performance of permeable reactive barriers.