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1. Numerical and Experimental Investigation of Active and Passive Spreading for Groundwater Remediation

   https://doi.org/10.1061/9780784482964.010  

   Neupauer, R. M.; Sather, L. J.; Roth, E. J.; Mays, D. C.; Crimaldi, J. P. (January 2020, 2020 World Environmental and Water Resources Congress)

   During in situ remediation of contaminated groundwater, a chemical or biological amendment is introduced into a contaminant plume to react with and degrade the contaminant. Since the degradation reactions only occur where the amendment and the contaminant are sufficiently close, the success of in situ remediation is controlled by the degree to which the amendment spreads into the contaminant plume. Spreading is defined as the reconfiguration of the plume geometry, which occurs as a result of spatially and temporally varying flow fields. Spreading can occur passively due to spatially varying velocity caused by aquifer heterogeneity. Spreading can also occur actively by inducing spatially and temporally varying flow fields through injections and extractions of clean water in wells surrounding the contaminated site. We used coupled numerical investigations and laboratory experiments to explore the effects of active spreading and passive spreading on contaminant degradation. We report here on the effects of passive spreading on contaminant degradation. We analyze the features of the flow field and plume geometry that encourage spreading contaminant degradation, so that the results from the numerical investigation and experiments can be used to design active spreading pumping sequences for other aquifers with other heterogeneity patterns. 

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2. Contributions of Pore‐Scale Mixing and Mechanical Dispersion to Reaction During Active Spreading by Radial Groundwater Flow

   https://doi.org/10.1029/2019WR026276  

   Neupauer, Roseanna M.; Sather, Lauren J.; Mays, David C.; Crimaldi, John P.; Roth, Eric J. (July 2020, Water Resources Research)

   Abstract Spreading and mixing are complementary processes that promote reaction of two reactive aqueous solutes present in contiguous plumes in groundwater. Spreading reconfigures the plume geometry, elongating the interface between the plumes, while mixing increases the volume of aquifer occupied by each plume, bringing the solute molecules together to react. Since reaction only occurs where the two solute plumes are in contact with each other, local mechanisms that drive flow and transport near the interface between the plumes control the amount of reaction. This work uses local characteristics of the plumes and the flow field near the plume interface to analyze the relative contributions of pore‐scale mixing and mechanical dispersion to instantaneous, irreversible, bimolecular reaction in a homogeneous aquifer with active spreading caused by radial flow from a well. Two solutes are introduced in sequence at the well, creating concentric circular plumes. We allow for incomplete mixing of the solutes in the pore space, by modeling the pore space as a segregated compartment and a mixed compartment with first‐order mass transfer between the two compartments. We develop semi‐analytical expressions for concentrations of the solutes in both compartments. We found that the relative contribution of mechanical dispersion to reaction increases over time and also increases due to increases in the Peclet number, in the relative source concentration of the chasing solute, and in the mass transfer rate from the segregated compartment to the mixed compartment of the pore space. 

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3. Effect of nonreactive kaolinite on 4-chloronitrobenzene reduction by Fe( ii ) in goethite–kaolinite heterogeneous suspensions

   https://doi.org/10.1039/C6EN00469E  

   Strehlau, Jennifer H.; Schultz, Jonathan D.; Vindedahl, Amanda M.; Arnold, William A.; Penn, R. Lee (January 2017, Environmental Science: Nano)

   The kinetics of model contaminant 4-chloronitrobenzene (4-ClNB) reduction by Fe( ii ) in aqueous suspensions containing either or both goethite (α-FeOOH) nanoparticles and kaolinite (Al 2 Si 2 O 5 (OH) 4 ) were quantified to elucidate the effects of nonreactive clay minerals on the attenuation of nitroaromatic groundwater contaminants by iron oxide nanoparticles. Increasing the amount of kaolinite in the presence of goethite decreased the reduction rate of 4-ClNB and competitive Fe( ii ) adsorption on kaolinite occurred. Cryogenic transmission and scanning electron microscopy (cryo-TEM and cryo-SEM) images did not reveal significant loss of accessible reactive surface area as a result of heteroaggregation. Sequential-spike batch reactors revealed that in the presence of kaolinite, 4-ClNB reduction rate decreased by more than a factor of three with extended reaction as a result of kaolinite dissolution and subsequent incorporation of Al and Si in goethite or on the goethite surface. The reactive sites residing on the {110} faces were comparatively more reactive in the presence of a large loading of kaolinite, resulting in shorter and wider goethite particles after reaction. These results elucidate the mechanisms by which nonreactive clays affect the reactions of Fe( ii )/iron oxides in groundwater systems and indicate that nonreactive clays are not passive components. 

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4. Hydrogen Peroxide Interference in Chemical Oxygen Demand Assessments of Plasma Treated Waters

   https://doi.org/10.3390/plasma2030021  

   Groele, Joseph; Foster, John (September 2019, Plasma)

   Plasma-driven advanced oxidation represents a potential technology to safely re-use waters polluted with recalcitrant contaminants by mineralizing organics via reactions with hydroxyl radicals, thus relieving freshwater stress. The process results in some residual hydrogen peroxide, which can interfere with the standard method for assessing contaminant removal. In this work, methylene blue is used as a model contaminant to present a case in which this interference can impact the measured chemical oxygen demand of samples. Next, the magnitude of this interference is investigated by dosing de-ionized water with hydrogen peroxide via dielectric barrier discharge plasma jet and by solution. The chemical oxygen demand increases with increasing concentration of residual hydrogen peroxide. The interference factor should be considered when assessing the effectiveness of plasma to treat various wastewaters. 

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5. Endothermic and exothermic chemically reacting plumes

   https://doi.org/10.1017/S0022112008002917  

   CONROY, DEVIN T.; SMITH, STEFAN G. (October 2008, Journal of Fluid Mechanics)

   null (Ed.)

   We develop a model for a turbulent plume in an unbounded ambient that takes into account a general exothermic or endothermic chemical reaction. These reactions can have an important effect on the plume dynamics since the entrainment rate, which scales with the vertical velocity, will be a function of the heat release or absorption. Specifically, we examine a second-order non-reversible reaction, where one species is present in the plume from a pure source and the other is in the environment. For uniform ambient density and species fields the reaction has an important effect on the deviation from pure plume behaviour as defined by the source parameter Γ. In the case of an exothermic reaction the density difference between the plume and the reference density increases and the plume is ‘lazy’, whereas for an endothermic reaction this difference decreases and the plume is more jet-like. Furthermore, for chemical and density-stratified environments, the reaction will have an important effect on the buoyancy flux because the entrainment rate will not necessarily decrease with distance from the source, as in traditional models. As a result, the maximum rise height of the plume for exothermic reactions may actually decrease with reaction rate if this occurs in a region of high ambient density. In addition, we investigate non-Boussinesq effects, which are important when the heat of reaction is large enough. 

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