Influence of iron oxide inclusion shape on Co^II/IIIEDTA reactive transport through spatially heterogeneous sediment

Abstract

Abstract. Multisolute reactive transport was investigated in chemically heterogeneous systems to determine the influence of the shapes of the reactive heterogeneities (iron oxide inclusions) by comparison of two-dimensional heterogeneous experiments with spatially averaged models that had differing inclusion characterization. Eleven reactions were considered in this system, starting with adsorption of the initial solute (conEDTA) to Fe oxides, followed by two competing surface reactions: oxidation forming conIEDTA and Fe dissolution forming FenIEDTA and Co 2+. Spatial moments of the eight mobile species were compared between data and models. One spatially averaged model (homogeneous equivalent), which incorporated inclusion mass only, significantly under predicted oxidation (up to 74%), the influence of reaction kinetics, and species retardation. In contrast, the ensemble average model (incorporating inclusion mass and length) well predicted speciation, retardation, and skewness. This large difference in prediction between two spatially averaging models was caused by the lack of incorporation of contact time of solutes with iron oxides in the homogeneous equivalent model and the importance of the contact time with the differing timescales of reactions. Experimental and modeling results also showed that the uncertainty in prediction of specific species increased as the inclusions varied from more ideal (fixed-length) to more natural (variable-length) shape of inclusions.

In Situ Redox Manipulation Proof-of-Principle Test at the Fort Lewis Logistics Center: Final Report

“…describes mass fluxes as a function of each constituent concentration to each respective stoichiometric coefficient. The set of differential equations can be numerically solved (55 mixed 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 is known about the reaction pathways and reaction parameters to justify this approach.…”

Heterogeneities in Glaciofluvial Deposits Using an Example from New Hampshire

“…These grain coatings are often concentrated in spatially distinct zones that appear as long narrow bands and/or irregular-shaped patches in outcrops. Field observations, laboratory experiments, and numerical modeling studies indicate that heterogeneities in (hydr)oxide grain coatings can strongly influence contaminant fate and transport (Friedly et al 1995;Tompson et al 1996;Szecsody et al 1998). However, grain coating architecture and its geologic origins are poorly understood.…”

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

“…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.…”

“…However, complex geochemical rates have been shown to occur ,at somewhat different rates in columns relative to batch systems, due in part to a significantly higher sediment-to-water ratio and slower access to surface sites by mobile constituents (Szecsody et al 1998a and. Therefore, degradation rate information from column experiments is generally considered more applicable to reactive transport at the field scale.…”