Andrew D. Henderson

Impact of Solids Formation and Gas Production on the Permeability of ZVI PRBs

The permeability of zero-valent iron permeable reactive barriers (ZVI PRBs) may be reduced by the production of gas and solid precipitates. The reduction in permeability was examined using column experiments, which showed that permeability loss was correlated with influent oxidant concentration. The column containing 100  mg/L nitrate experienced the greatest loss, approximately two orders of magnitude over the course of 200 pore volumes. However, the permeability loss owing to precipitated solids was largely independent of oxidant concentration, accounting for only 24% of the observed loss in the 100  mg/L nitrate column, suggesting that the majority of loss was attributable to gas, not precipitates. Geochemical modeling corroborated these findings, indicating that precipitation of solids in the 100  mg/L nitrate system does not account for more than a 10% permeability reduction. These findings suggest that in field PRBs in which a high reduction in permeability is observed, gas production may be implica...

Assessing the importance of spatial variability versus model choices in life cycle impact assessment: The case of freshwater eutrophication in europe

In Life Cycle Impact Assessment (LCIA) both spatial variability and model choices may be influential. In the case of the effect model, the effect factors differ with respect to their assumption of linear/nonlinear responses to increases in environmental stressor levels, and whether or not they account for the current stressor levels in the environment. Here, we derived spatially explicit characterization factors of phosphorus emissions causing eutrophication based on three different effect models (depicted by marginal, linear, and average effect factors) and two freshwater types (lakes and streams) and we performed an analysis of variance (ANOVA) to investigate how the selection of the effect models and the freshwater types influence the impacts of phosphorus emissions to freshwater on heterotrophic species. We found that 56% of the variability of ecological impacts per unit of phosphorus emission was explained, primarily, by the difference between freshwater types and, to a lesser extent, by the difference between effect models. The remaining variability was attributed to the spatial variation between river basins, mainly due to the variability in fate factors. Our study demonstrates the particular importance of accounting for spatial variability and model choices in LCIA.

Permeability of iron sulfide (FeS)-based materials for groundwater remediation

Iron sulfide (FeS) has been extensively assessed as a reactive medium to remove both metals and halogenated organics from groundwater. However, to address its suitability as a material for permeable reactive barriers (PRBs), its propensity for solids and gas production, which result in reduced permeability, must be evaluated. The reduction in permeability for sands coated with FeS (as mackinawite), under the anoxic conditions often encountered at contaminated groundwater sites, was examined through column experiments and geochemical modeling under conditions of high calcium and nitrate, which have been previously shown to cause significant permeability reduction in zero-valent iron (ZVI) systems. The column experiments showed negligible production of both solids and gases. The geochemical modeling predicted a maximum reduction in permeability of 1% due to solids and about 30% due to gas formation under conditions for which a complete loss of permeability was predicted for ZVI systems. This difference in permeability reduction is driven by the differences in thermodynamic stability of ZVI and FeS in aqueous solutions. The results suggest that geochemical conditions that result in high permeability losses for ZVI systems will likely not be problematic for FeS-based reactive materials.

Academic Engagement in Public and Political Discourse: Proceedings of the Michigan Meeting, May 2015

Horace H. Rackham School of Graduate Studies at the University of Michigan, the Erb Institute, the Graham Institute, the Michigan Energy Institute, and the Risk Science Center.