Influence of iron oxide inclusion shape on Co(II/III)EDTA reactive transport through spatially heterogeneous sediment

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 (Co(II)EDTA) to Fe oxides, followed by two competing surface reactions: oxidation forming Co(III)EDTA and Fe dissolution forming Fe(III)EDTA and Co2+. 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.