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Permeable reactive barriers (PRBs) have gained popularity in recent years as a low-cost method for ground water remediation. However, their cost advantage usually requires that these barriers remain maintenance free for a number of years after installation. In this study, sediment cores were retrieved from a pilot-scale PRB consisting of a sand and wood particle (sawdust) mixture that has been in continuous operation for 15 years treating nitrate from a septic system plume in southern Ontario (Long Point site). Reaction rates for the 15-year-old media were measured in dynamic flow column tests and were compared to rates measured in year 1 using the same reactive mixture. Nitrate removal rates in the 15-year-old media varied, as expected, with temperature in the range of 0.22 to 1.1 mg N/L/d at 6°C to 10°C to 3.5 to 6.0 mg N/L/d at 20°C to 22°C. The latter rates remained within about 50% of the year 1 rates (10.2 6 2.7 mg N/L/d at 22°C). Near the end of the year 15 column test, media particles >0.5 mm in diameter, containing most of the wood particles, were removed from the reactive media by sieving. Nitrate removal subsequently declined by about 80%, indicating that the wood particles were the principal energy source for denitrification. This example shows that some denitrifying PRBs can remain maintenance free and be adequately reactive for decades.

Nanosized Fe0 exhibits markedly different anaerobic corrosion rates in water compared to that disseminated in moist quartz sand. In water, hydrogen production from corrosion exhibits an autocatalytic style, attaining a maximum rate of 1.9 mol kg(-1) d(-1) within 2 d of reaction. The rate then drops sharply over the next 20 d and enters a period of uniformly decreasing rate, represented equally well by first-order or diffusion-controlled kinetic expressions. In quartz sand, hydrogen production exhibits a double maximum over the first 20 d, similar to the hydration reaction of Portland cement, and the highest rate attained is less than 0.5 mol kg(-1) d(-1). We ascribe this difference in early time corrosion behavior to the ability of the released hydrogen gas to convect both water and iron particles in an iron/water system and to its inability to do so when the iron particles are disseminated in sand. By 30 d, the hydrogen production rate of iron in quartz sand exhibits a uniform decrease as in the iron/water system, which also can be described by first-order or diffusion-controlled kinetic expressions. However, the corrosion resistance of the iron in moist sand is 4 times greater than in pure water (viz. t1/2 of 365 d vs 78 d, respectively). The lower rate for iron in sand is likely due to the effect of dissolved silica sorbing onto iron reaction sites and acting as an anodic inhibitor, which reduces the iron's susceptibility to oxidation by water. This study indicates that short-term laboratory corrosion tests of nanosized Fe0/water slurries will substantially underestimate both the material's longevity as an electron source and its potential as a long-term source of hydrogen gas in groundwater remediation applications.

A pilot-scale zero valent iron (ZVI) Permeable Reactive Barrier (PRB) was installed using an azimuth-controlled vertical hydrofracturing at an industrial facility to treat a chlorinated Volatile Organic Compound (VOC) plume. Following ZVI injection, no significant reduction in concentration was observed to occur with the exception of some multilevel monitoring wells, which also showed high levels of total organic carbon (TOC). These patterns suggested that the zero valent iron was not well distributed in the PRB creating leaky conditions. The geochemical data indicated reducing conditions in these areas where VOC reduction was observed, suggesting that biotic processes, associated to the guar used in the injection of the iron, could be a major mechanism of VOC degradation. Compound-Specific Isotope Analysis (CSIA) using both carbon and chlorine stable isotopes were used as a complementary tool for evaluating the contribution of abiotic and biotic processes to VOC trends in the vicinity of the PRB. The isotopic data showed enriched isotope values around the PRB compared to the isotope composition of the VOC source confirming that VOC degradation is occurring along the PRB. A batch experiment using site groundwater collected near the VOC source and the ZVI used in the PRB was performed to evaluate the site-specific abiotic isotopic fractionation patterns. Field isotopic trends, typical of biodegradations were observed at the site and were different from those obtained during the batch abiotic experiment. These differences in isotopic trends combined with changes in VOC concentrations and redox parameters suggested that biotic processes are the predominant pathways involved in the degradation of VOCs in the vicinity of the PRB.

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Robert Focht is a project manager with EnviroMetal Tecbnologies Inc. (ETI).