Events & Media

The Bren School of Environmental Science & Management
at the University of California, Santa Barbara



"Soil aggregate-scale heterogeneity in iron reductive transformation and selenium retention
resulting from coupled biogeochemical and physical processes"

Celine Pallud
Assistant Professor
Department of Environmental Science, Policy and Management
UC Berkeley

Tuesday, Oct. 16, 2012
3:00 - 4:00 p.m.
Bren Hall 1414

"Professor Pallud researches at the intersection of soil physics, soil microbiology, and soil geochemistry to discover how coupled processes affect chemical bioavailability, mobility, and transformation."Patrcia Holden, faculty host

Biogeochemical processes controlling elemental cycling in soils are heterogeneously distributed owing partly to the physical complexity of the media. Soils display large variation with respect to their physical, geochemical and biological characteristics at scales ranging from nanometers to kilometers. Structured soils are composed of individual aggregates that form a network of interconnected microenvironments. This aggregate scale (mm-cm) is of particular interest due to the sharp transition in pore size between the aggregates themselves and the macropores surrounding them. Solutes move rapidly (by advection) through active macropores and slowly (by diffusion) into intra-aggregate micropores, which can lead to mass-transfer limitations in solute transport and to the build up of significant chemical gradients over short distances.

We will present a combined experimental and modelling study on single artificial soil aggregates assessing the biogeochemical processes governing transformations of redox sensitive elements (iron and selenium) in a complex, but controlled, setting representative of natural systems. The objective of this study was to investigate the coupling of physical (transport) and biogeochemical processes that affect small-scale iron transformations and control selenium sequestration within soils. Circumventing byproduct accumulation and substrate exhaustion common in batch systems and avoiding the poor physical analogy to aggregated soils of homogenously packed columns, our novel experiments mimic soils using constructed cm-scale aggregates in flow-through reactors, which results in diffusively and advectively controlled regions. A newly developed reactive transport model is used to delineate transport regimes, identify reaction zones, and estimate kinetic parameters and reaction rates at the aggregate scale.

Overall, our findings demonstrate significant aggregate-scale variations in biogeochemical processes and consequent distribution patterns of solid phases within soils. We show that those chemical gradients are mainly controlled by diffusive mass-transfer limitations of both solute delivery to the aggregates and metabolite removal from the aggregates. This highlights the importance of appreciating the spatial connection between reaction and transport fronts and of obtaining information on transport-limited, intra-aggregate biogeochemical dynamics to better understand reactive transport of redox-sensitive species in structured soils.

Born and raised in France, CĂ©line Pallud received her PhD in Environmental Sciences and Fluid Mechanics from Grenoble University (France), before doing a postdoc in geochemistry at Utrecht University (The Netherlands) and a second postdoc in soil biogeochemistry at Stanford University. Her research integrates microbiological, biogeochemical and reactive transport modeling techniques and focuses on the analysis and prediction of the fate and transport of chemicals that are of importance to the functioning, quality and remediation of soils, littoral sediments and water.

NOTE: Research colloquia are hosted by Bren faculty members and are generally high-level talks about research in a particular area of environmental science and management.


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