Importance of flow and particle-scale heterogeneity on CoII/IIIEDTA reactive transport

Szecsody, James E.; Zachara, John M.; Chilakapati, Ashokkumar; Jardine, P. M.; Ferrency, Alan S.

doi:10.1016/s0022-1694(98)00114-0

Cited by 56 publications

(30 citation statements)

References 50 publications

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“…Because acetylene is the main reaction product observed in previous studies, the combination of reactions (Equations 2.6, 2.10, and 2.14) described the major TCE degradation pathway: describes the TCE degradation rate (decreasing) as a function of each constituent concentration to each respective stoichiometric coefficient, and when integrated describes mass flux as a function of time. A set of differential equations for all redox reactions can be numerically solved (55 mixed equilibrium and kinetic reactions with 71 species described in Szecsody et al 1998;Yeh et al 1998), but this type of 2.8 detailed modeling is useful only if extensive knowledge of the reaction parameters exists. In the case of TCE degradation, not enough information is known about the reaction pathways and reaction parameters to justify this approach.…”

Section: Tce Dechlorination Rate In Reduced Sedimentmentioning

confidence: 99%

Treatability Study of In Situ Technologies for Remediation of Hexavalent Chromium in Groundwater at the Puchack Well Field Superfund Site, New Jersey

Vermeul¹,

Szecsody²,

Truex³

et al. 2006

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SummaryThis treatability study was conducted by Pacific Northwest National Laboratory (PNNL), at the request of the U. S. Environmental Protection Agency (EPA) Region 2, to evaluate the feasibility of using in situ treatment technologies for chromate reduction and immobilization at the Puchack Well Field Superfund site in Pennsauken Township, New Jersey. In addition to in situ reductive treatments, which included the evaluation of both abiotic and biotic reduction of Puchack aquifer sediments, natural attenuation mechanisms were evaluated (i.e., chromate adsorption and reduction). Chromate exhibited typical anionic adsorption behavior, with greater adsorption at lower pH, at lower chromate concentration, and at lower concentrations of other competing anions. Competitive anions, sulfate in particular (at 50 mg/L), suppressed chromate adsorption by up to 50%. Chromate adsorption was not influenced by inorganic colloids.Results from long-term experiments indicate that natural chromate reduction occurs in Puchack sediments. Sediment composites of seven different Puchack aquifer units showed either no reduction or very slow chromate reduction (0.3 to 3.1 year half-life). However, three selected sediments showed much more rapid reduction (half lives: 49 h to ~3000 h). Although the spatial variability in natural reductive capacity appears to be relatively large, experimental results indicate that chromate reduction can remain viable even in the presence of dissolved oxygen. Most sediments that showed some natural reduction of chromate had very small reductive capacities (average 2.76 ± 1.02 µmol/g), but one sediment had a large reductive capacity (636 µmol/g) with considerable amorphous ferrous iron and 2.25% organic carbon (i.e., contained clays, iron oxides, and possibly lignite). There was wide variability in the iron oxide content of Puchack sediments.Results from chemical reduction experiments on Puchack sediments showed a considerable increase in the reductive capacity (average 60.2 ± 25.6 µmol/g, maximum of 390 µmol/g), indicating that reductive treatment may be an effective approach when deployed at the field scale. The average measured reductive capacity, assuming typical site groundwater chemistry conditions, would result in a treatment zone longevity of 360 pore volumes or 39 years of treatment for a uniform treatment zone 40 ft wide and an average groundwater velocity of 1 ft/d. It was also shown that reductive capacity is a function of pH. The capacity of the sediment column to consume 2.0 mg/L chromate and 8.4 mg/L dissolved oxygen (saturated conditions) was estimated to be 310 pore volumes at pH 4.5 and 480 pore volumes at pH 7.0. Chromate reduction/immobilization was shown to be effective in four long-term column studies at pH 4.5 to 7.0. Chromate was removed in all columns initially, then partial chromate breakthrough occurred by 170 pore volumes for pH values of 4.5 and 5.3, 280 pore volumes at pH 6.2, and 340 pore volumes at pH 7.0. There was no evidence of mobile Cr(III) species at any pH. Iron (II)...

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Section: Tce Dechlorination Rate In Reduced Sedimentmentioning

confidence: 99%

Treatability Study of In Situ Technologies for Remediation of Hexavalent Chromium in Groundwater at the Puchack Well Field Superfund Site, New Jersey

Vermeul¹,

Szecsody²,

Truex³

et al. 2006

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“…An alternative approach couples chemical speciation calculations to transport equations. Such models of reactive transport have been developed and demonstrated by a number of researchers including Parkhurst (1995), Lichtner (1996); Bethke (1997); Szecsody et al (1998), Yeh et al (1995Yeh et al ( , 2002, and others reviewed in Lichtner et al (1996), Steefel and Van Cappellen (1998), and Browning and Murphy (2003). Uses of such models to simulate radionuclide transport of uranium in 1-D column experiments are illustrated by Sims et al (1996) and Kohler et al (1996).…”

Section: Representation Of Sorption Of Radionuclides Under Natural Comentioning

confidence: 99%

Environmental Geochemistry of Radioactive Contamination

Siegel

Bryan

2003

Treatise on Geochemistry

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This report attempts to describe the geochemical foundations of the behavior of radionuclides in the environment. The information is obtained and applied in three interacting spheres of inquiry and analysis: 1) experimental studies and theoretical calculations, 2) field studies of contaminated and natural analog sites and 3) model predictions of radionuclide behavior in remediation and waste disposal. Analyses of the risks from radioactive contamination require estimation of the rates of release and dispersion of the radionuclides through potential exposure pathways. These processes are controlled by solubility, speciation, sorption, and colloidal transport, which are strong functions of the compositions of the groundwater and geomedia as well as the atomic structure of the radionuclides.The chemistry of the fission products is relatively simple compared to the actinides. Because of their relatively short half-lives, fission products account for a large fraction of the radioactivity in nuclear waste for the first several hundred years but do not represent a long-term hazard in the environment. The chemistry of the longer-lived actinides is complex; however, some trends in their behavior can be described. Actinide elements of a given oxidation state have either similar or systematically varying chemical properties due to similarities in ionic size, coordination number, valence, and electron structure. In dilute aqueous systems at neutral to basic pH, the dominant actinide species are hydroxy-and carbonato-complexes, and the solubility-limiting solid phases are commonly oxides, hydroxides or carbonates. In general, actinide sorption will decrease in the presence of ligands that complex with the radionuclide; sorption of the (IV) species of actinides (Np, Pu, U) is generally greater than of the (V) species.The geochemistry of key radionuclides in three different environments is described in this report. These include: 1) low ionic strength reducing waters from crystalline rocks at nuclear waste research 4 sites in Sweden; 2) oxic water from the J-13 well at Yucca Mountain, Nevada, the site of a proposed repository for high level nuclear waste (HLW) in tuffaceous rocks; and 3) reference brines associated with the Waste Isolation Pilot Plant (WIPP). The transport behaviors of radionuclides associated with the Chernobyl reactor accident and the Oklo Natural Reactor are described. These examples span wide temporal and spatial scales and include the rapid geochemical and physical processes important to nuclear reactor accidents or industrial discharges as well as the slower processes important to the geologic disposal of nuclear waste.Application of geochemical information to remediating or assessing the risk posed by radioactive contamination is the final subject of this report. After radioactive source terms have been removed, large volumes of soil and water with low but potentially hazardous levels of contamination may remain. For poorly-sorbing radionuclides, capture of contaminated water and removal of radi...

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“…The set of differential equations can be numerically solved (55 mix-ed equilibrium and kinetic reactions with 71 species described in Szecsody et al [1995Szecsody et al [ , 1998aSzecsody et al [ , 1998b), but this type of detailed modeling is useful only if extensive knowledge of the reaction parameters exists. In the case of TCE degradation, not enough information is known about the reaction pathways and reaction parameters to justify this approach.…”

Section: Tce Degradationmentioning

confidence: 99%

In situ redox manipulation of subsurface sediments from Fort Lewis, Washington: Iron reduction and TCE dechlorination mechanisms

Szecsody¹,

Fruchter²,

Sklarew³

et al. 2000

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Portions of this document may be illegible in electronic image products.Images are produced from the best available original document. SummaryThe feasibility of chemically treating sediments from the Ft. Lewis, Washington, Logistics Center .to develop a permeable barrier for dechlorination of trichloroethylene (TCE) was investigated in a series of laboratory experiments.The proposed remediation technology uses a chemical treatment to reduce existing iron in sediments, then relies on the ability of the ferrous iron to act as an electron .donor to dechlorinate organic contaminants. The effects of temperature, partial iron reduction, and flow on these redox reactions were also studied to ascertain how to achieve viable TCE dechlorination rates at the field scale. The fraction of reducible iron in Ft. Lewis sediments would create a reduced zone that would remain anoxic for -300 pore volumes. Because the kinetics of the reduction reaction are third-order, significant amounts of iron are reduced early in the reduction period. The reduction is slower at later times. Because the slower disproportionation reaction destroys the remaining dithionite, specific sedimentisolution contact times (32 h at 25°C, 100 h at 12°C) are needed to efficiently reduce 80% of the iron in the sediment.When the pH buffer concentration was less than four times the dithionite concentration, there was a significant loss in reduction efficiency along with a significant pH decrease and increased iron mobility. The long contact times needed for reduction at ambient aquifer temperature coupled with density effects of the solution at the field scale indicate that heated injections (with high concentration of pH buffer) can efficiently reduce the sediment zones of interest. Dithionite-reducedFt. Lewis sediments were shown to degrade TCE in Ft. Lewis groundwater at sufficiently fast rates (1.2 h to 19 h) during static and transport experiments to create a permeable barrier at the field scale. The TCE degradation rate can be calculated for all sediments from the product of the intrinsic degradation rate (0.0034/h pmol) and the mass of reduced iron (range of 12 pmol/g to 126 pmol/g; averaged = 63 pmol/g). Products of TCE dechlorination clearly show that 99.5% to 100% is occurring via reductive elimination, producing acetylene, ethylene, and chloroacetylene. The TCE degradation rate decreased up to 3 orders of magnitude in partially reduced sediment. This departure on fraction of reduced iron has significant implications, because uniform full sediment reduction is not possible at the field scale. Although minimally reduced sediment had nearly no TCE reactivity, X070 reduced sediment resulted in TCE reduction rates that were viable at the field scale (<65 h). The second-order dependence of the TCE dechlorination rate on the fraction of reduced iron demonstrates the significant role of the iron oxide surface (as a catalyst or for surface coordination) in addition to Fe**as the electron donor for TCE dechlorination to proceed. Reduced sediment barrier longe...

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Importance of flow and particle-scale heterogeneity on CoII/IIIEDTA reactive transport

Cited by 56 publications

References 50 publications

Treatability Study of In Situ Technologies for Remediation of Hexavalent Chromium in Groundwater at the Puchack Well Field Superfund Site, New Jersey

Treatability Study of In Situ Technologies for Remediation of Hexavalent Chromium in Groundwater at the Puchack Well Field Superfund Site, New Jersey

Environmental Geochemistry of Radioactive Contamination

In situ redox manipulation of subsurface sediments from Fort Lewis, Washington: Iron reduction and TCE dechlorination mechanisms

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