Spatial Patterns and Modeling of Reductive Ferrihydrite Transformation Observed in Artificial Soil Aggregates

Abstract

Within soils, biogeochemical processes controlling elemental cycling are heterogeneously distributed owing, in large part, to the physical complexity of the media. Here we quantify how diffusive mass-transfer limitation at the soil aggregate scale controls the biogeochemical processes governing ferrihydrite reductive dissolution and secondary iron mineral formation. Artificial cm-scale aggregates made of ferrihydrite-coated sand inoculated with iron-reducing bacteria were placed in flow-through reactors, mimicking macro- and microporous soil environments. A reactive transport model was developed to delineate diffusively and advectively controlled regions, identify reaction zones and estimate kinetic parameters. Simulated iron (Fe) breakthrough-curves show good agreement with experimental results for a wide-range of flow rates and input lactate concentrations, with only a limited amount (< or =12%) of Fe lost in the reactor outflow over a 31 day period. Model simulations show substantial intra-aggregate, mm-scale radial variations in the secondary iron phase distributions, reproducing the trends observed experimentally where only limited transformation of ferrihydrite was found near the aggregate surface, whereas extensive formation of goethite/lepidocrocite and minor amounts of magnetite and/or siderite were observed toward the aggregate center. Our study highlights the important control of variations in transport intensities on microbially induced iron transformation at the soil aggregate scale.

Aggregate-scale spatial heterogeneity in reductive transformation of ferrihydrite resulting from coupled biogeochemical and physical processes

“…In fact, solute transport inside the aggregates is dominated by diffusion and hence is very slow. This could result in a local build-up of Fe(II) and bicarbonate in the aggregate innermost sections, which was confirmed by reactive transport modeling (Pallud et al, 2010). Those microenvironments high in Fe(II) and bicarbonate may promote siderite supersaturation and precipitation as postulated by Mortimer and Coleman (1997).…”

“…Since Fe(II) and bicarbonate are produced inside the aggregate, the solution flowing around the aggregate will act as a sink for these reaction reduction products, resulting in gradual concentration gradients that extend to near the center of the aggregates. In fact, reactive transport modeling of these systems indicate that Fe(II) in the aggregate exterior can be as low as 0.7 lM (Pallud et al, 2010) -three orders of magnitude lower than concentrations in the aggregate interior after 31 d of reaction with an input lactate concentration of 3 mM. Conversely, the solids in the aggregate mid-section and interior were more dynamic than that of the exterior.…”

“…Flow rates used (0.35-1.7 mL h À1 ) correspond to velocities in the free fluid ranging from 7 Â 10 À8 to 3.5 Â 10 À7 m s À1 . For aggregate permeability on the order of 5.5 Â 10 À10 m 2 , this results in diffusion-dominated solute transport within the aggregate (Pallud et al, 2010). Conversely, solute transport is dominated by advection in the surrounding (macropore) region.…”

“…Reactive transport modeling of the no-flow control aggregates showed that the absence of advective flow around the aggregates promotes the establishment of a largely homogenous Fe(II) concentration field (Pallud et al, 2010), mainly because Fe(II) builds-up in the surrounding flow field rather than being continuously removed. As a consequence, a lack of heterogeneity of the solid phase results within the aggregates, highlighting the importance of the interface between advective and diffusive domains.…”

“…Due to diffusion-dominated mass transfer in our aggregate systems, chemical gradients establish that influence local biogeochemical conditions and thus the reductive transformation of ferrihydrite. Simulations of intra-aggregate aqueous chemistry, resulting from reactive transport modeling, reveal that steep (and negative) Fe(II) gradients exist from the interior to the exterior by the termination of experiment and increase with reaction period (Pallud et al, 2010). Fe(II) reaches concentrations as high as 1.1 mM in the interior section of aggregate reacted with 3 mM input lactate concentrations, but only 0.28 mM in the interior section of aggregate run with 0.3 mM lactate.…”