Spatial heterogeneity of selenium reduction in model soil aggregates

Soils display large variations with respect to their physical, geochemical and biological characteristics at scales ranging from nanometers to kilometers. The impact of small-scale spatial heterogeneity on ecosystem-scale biogeochemical processes is as of yet poorly understood. The aggregate scale (mm-cm) is of particular interest in structured soils, due to the sharp transition in size between inter-aggregate macropores and intra-aggregate micropores. Micropores limit advective transport thus facilitating the formation of aggregate-scale concentration gradients. Selenium (Se) is an essential micronutrient that has recently emerged as an environmental contaminant. A mechanistic understanding of Se reduction and retention within soil aggregates may lead to improved predictions of Se transport and attenuation in the surface layers of contaminated soils. In order to investigate the coupling between physical and biogeochemical processes controlling Se reduction at the aggregate scale, we used flow-through reactor systems, recreating the transition between advection-dominated macropores and diffusion-dominated micropores. Each system consisted of a spherical artificial aggregate (ID 2.5 cm) contained in a flow-through reactor cell (ID 5.1 cm, L 3.7 cm), with inflow solution providing selenate and an electron donor. Aggregates were constructed using either sand or ferrihydrite-coated sand (to investigate the effect of sorption on selenium reduction and retention) homogenously inoculated with Se-reducing bacteria. Oxic and anoxic experiments were compared. Concentrations of selenite and total Se were measured in the outflow solution and in concentric sections of the aggregates' solid phase. A 2D reactive transport model of reactor-aggregate systems was developed. The majority of selenium reduced inside the aggregates was exported in the form of selenite, unless sorption was significant due to presence ferrihydrite. Selenite export rates were enhanced by the absence of oxygen, and by higher selenate or C-source concentrations in the input solution. The reactive transport model shows that observed differences in selenate solid phase concentrations between aggregates were driven by the interplay between intra-aggregate consumption, supply from the surrounding fluid via diffusively dominated transport, and sorption. We found that solid phase selenite concentrations increased linearly towards the core of aggregates under all conditions investigated. Reactive transport modeling confirmed the role of aggregate geometry and diffusively limited transport in creating the observed pattern: slow transport allows for the build-up and retention of reduced selenium products within the core of aggregates. This suggests that aggregate size may have a predictable, first-order impact on the retention of selenium in soils.