Process-based modeling of electrokinetic-enhanced bioremediation of chlorinated ethenes

Temperature-dependent dynamics of electrokinetic conservative and reactive transport in porous media: A model-based analysis

Electrokinetic techniques employ direct current electric fields to enhance the transport of amendments in low permeability porous media and have been demonstrated effective for in situ remediation of both organic contaminants and heavy metals. The application of electric potential gradients give rise to coupled chemical, hydraulic and electric fluxes, which are at the basis of the main transport mechanisms: electromigration and electroosmosis. Previous research has highlighted the significant impacts of charge interactions and fluid composition, including temperature-dependent properties such as electrolyte conductivity and density, on these transport phenomena. However, current models of electrokinetic applications often assume isothermal conditions and overlook the production of heat resulting from Joule heating. This study provides a detailed model-based investigation, systematically exploring the effects of temperature on electrokinetic conservative and reactive transport in porous media. By incorporating temperature-dependent material properties and progressively investigating the impact of temperature on each transport mechanism, we analyze the effects of temperature variations in both 1D and 2D systems. The study reveals how temperature dynamically influences the physical, chemical and electrostatic processes controlling electrokinetic transport. A temperature increase results in a higher speed of amendments delivery by both electromigration and electroosmosis and increases the kinetics of degradation reactions. The simulations also reveal a feedback mechanism in which higher aqueous conductivity results in increased Joule heating, leading to a faster temperature rise and, subsequently, to higher electrolyte conductivity. Finally, we estimate the electric energy requirements of the system at varying temperatures and show how these changes impact the rate of contaminant removal.

Electrokinetic-Enhanced Bioremediation of Trichloroethylene-Contaminated Low-Permeability Soils: Mechanistic Insight from Spatio-Temporal Variations of Indigenous Microbial Community and Biodehalogenation Activity

Electrokinetic-enhanced bioremediation (EK-Bio), particularly bioaugmentation with injection of biodehalogenation functional microbes such as Dehalococcoides, has been documented to be effective in treating a low-permeability subsurface matrix contaminated with chlorinated ethenes. However, the spatio-temporal variations of indigenous microbial community and biodehalogenation activity of the background matrix, a fundamental aspect for understanding EK-Bio, remain unclear. To fill this gap, we investigated the variation of trichloroethylene (TCE) biodehalogenation activity in response to indigenous microbial community succession in EK-Bio by both column and batch experiments. For a 195 day EK-Bio column (∼1 V/cm, electrolyte circulation, lactate addition), biodehalogenation activity occurred first near the cathode (<60 days) and then spread to the anode (>90 days), which was controlled by electron acceptor (i.e., Fe(III)) competition and microbe succession. Amplicon sequencing and metagenome analysis revealed that iron-reducing bacteria (Geobacter, Anaeromyxobacter, Geothrix) were enriched within initial 60 d and were gradually replaced by organohalide-respiring bacteria (versatile Geobacter and obligate Dehalobacter) afterward. Iron-reducing bacteria required an initial long time to consume the competitive electron acceptors so that an appropriate reductive condition could be developed for the enrichment of organohalide-respiring bacteria and the enhancement of TCE biodehalogenation activity.

Electrokinetic delivery of permanganate in clay inclusions for targeted contaminant degradation

The use of electrokinetics (EK) has great potential to deliver reactants in impervious porous media, thus overcoming some of the challenges in the remediation of contaminants trapped in low-permeability zones. In this work we experimentally investigate electrokinetic transport in heterogeneous porous media consisting of a sandy matrix with a target clay inclusion. We demonstrate the efficient EK-delivery of permanganate in the target clay zone (transport velocity 0.3-0.5 m day^-1) and its reactivity with Methylene Blue, a positively charged contaminant trapped within the inclusion. The delivery method was optimized using a KH₂PO₄/K₂HPO₄ buffer to attenuate the effect of electrolysis reactions in the electrode chambers, thus mitigating the propagation of pH fronts and preventing the phenomenon of permanganate stalling. The experiments showed that the buffer electrical conductivity greatly impacts the potential gradient in the heterogeneous porous medium with implications on the observed rates of electrokinetic transport (variation up to 40%). The reactive experiments provided direct evidence of the permanganate penetration within the clay and of its capability to degrade the target immobilized contaminant. The experimental results were analyzed using a process-based model, elucidating the governing transport mechanisms and highlighting the effect of different mass transfer processes on conservative and reactive electrokinetic transport.

On the interplay between electromigration and electroosmosis during electrokinetic transport in heterogeneous porous media

Electrokinetic techniques represent a valuable approach to enhance solute transport, reactant delivery and contaminant degradation in complex environmental matrices, such as contaminated soil and groundwater, and have a great potential for the remediation of many organic and inorganic pollutants. This study investigates the complex interplay between the key electrokinetic transport mechanisms, electromigration and electroosmosis, in physically heterogeneous porous media and its impact on tracer distribution, reactant mixing and degradation efficiency. We perform experiments in a multidimensional setup, considering different types of heterogeneities, injected tracers and reactants, as well as background electrolyte pore water with different chemical composition and pH. We show that EK transport is significantly affected by the physical heterogeneities, due to the interaction between electrokinetic and hydraulic processes, and by the pore water chemistry that plays a key role on the magnitude and spatial distribution of electroosmotic fluxes. The latter affect the overall transport of charged and non-charged species, including the migration velocity of injected plumes, their spatial patterns, spreading and mixing with the background groundwater, and the extent of degradation and the spatio-temporal evolution of reactive zones in the heterogeneous porous media. Process-based numerical modeling allowed us to interpret the experimental observations and to disentangle the coupled effects of physical, chemical and electrostatic processes in the multidimensional, heterogeneous setups. Besides elucidating the mechanisms controlling electrokinetic transport, the results of this study have also important implications for practical field implementation of EK approaches in intrinsically heterogeneous subsurface systems.

Impact of solute charge and diffusion coefficient on electromigration and mixing in porous media

The application of electrokinetic techniques in porous media has great potential to enhance mass transfer rates and, thus, to mobilize contaminants and effectively deliver reactants and amendments. However, the transport mechanisms induced by the application of an external electric field are complex and entail the coupling of physical, chemical and electrostatic processes. In this study we focus on electromigration and we provide experimental evidence of the impact of compound-specific properties, such as the aqueous diffusivity and the valence of charged species, on the macroscopic electrokinetic transport. We performed a series of multidimensional experiments considering the displacement of three different tracer plumes (i.e., permanganate, allura red and new coccine) in different background electrolyte solutions. The outcomes of the experiments clearly show that both the compound-specific diffusivity and the charge of the injected and resident ions impact the transport of the selected color tracer plumes, whose evolution was monitored with image analysis. The investigated experimental scenarios led to distinct plume behavior characterized by different mass distribution, average displacement velocities, longitudinal and lateral plume spreading, shape of the invading and receding fronts, as well as dilution of the injected solutes. A numerical simulator, based on the Nernst-Planck-Poisson equations and on aqueous speciation reactions in the pore water, allowed us to quantitatively interpret the experimental results, to capture the observed patterns of plume evolution, and to illuminate the coupling between the governing physico-chemical mechanisms and the controlling role of small scale compound-specific and electrostatic properties. Finally, the model was also extended to a typical configuration of in situ electrokinetic remediation of contaminated groundwater to show the impact of such mechanisms at larger scale.

Electrokinetically-enhanced emplacement of lactate in a chlorinated solvent contaminated clay site to promote bioremediation

Bioremediation through the injection of electron donors and bacterial cultures is effective at treating chlorinated solvent contamination. However, it has had limited application in low permeability zones where amendments cannot be delivered successfully. This field-scale study investigated the application of electrokinetics to enhance the delivery of lactate at a clay site contaminated with chlorinated solvents. Groundwater and soil samples were collected before, during and for 1 year after the 71-day field test and analyzed for a wide suite of chemical and biological parameters. Lactate was successfully delivered to all monitoring locations. Lactate emplacement resulted in the stimulation of bacterial populations, specifically within the phylum Firmicutes, which contains fermenters and strict anaerobes. This likely led to biodegradation, as the field trial resulted in significant decreases in both soil and aqueous phase chlorinated solvent concentrations. Contaminant decreases were also partially attributable to dilution, given evidence of some advective lactate flux. This research provides evidence that electrokinetically-enhanced bioremediation has potential as a treatment strategy for contaminated low permeability strata.

Impact of diffuse layer processes on contaminant forward and back diffusion in heterogeneous sandy-clayey domains

Low-permeability aquitards can significantly affect the transport, distribution, and persistence of contaminant plumes in subsurface systems. Although such low-permeability materials are often charged, the key role of charge-induced electrostatic processes during contaminant transport has not been extensively studied. This work presents a detailed investigation exploring the coupled effects of heterogeneous distribution of physical, chemical and electrostatic properties on reactive contaminant transport in field-scale groundwater systems including spatially distributed clay zones. We performed an extensive series of numerical experiments in three distinct heterogeneous sandy-clayey domains with different levels of complexity. The flow and reactive transport simulations were performed by explicitly resolving the complex velocity fields, the small-scale electrostatic processes, the compound-specific diffusive/dispersive fluxes and the chemical processes utilizing a multi-continua based reactive transport code (MMIT-Clay). In each particular domain, numerical experiments were performed focusing on both the forward and back diffusion through the sandy-clayey interfaces. The results illuminate the control of microscopic electrostatic mechanisms on macroscopic mass transfer. Coulombic interactions in the clay's diffuse layer can significantly accelerate or retard a particular species depending on its charge. Furthermore, the chemical heterogeneity plays a major role in mass storage and release during reactive transport. Neglecting such processes can lead to substantial over- or underestimation of the overall transport behavior, which underlines the need for integrated physical, chemical and electrostatic approaches to accurately describe mass transfer processes in systems including low-permeability inclusions.

Electroosmotic flow: From microfluidics to nanofluidics

Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nanoscale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applications—from biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so‐called electrical double layer (EDL), which forms in the vicinity of a charged solid–liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of electrical double layer models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid–liquid interface. Moreover, we discuss the physicochemical phenomena owing to the interaction of electrical double layer when the characteristic length of the conducting media is decreased from the microscale to the nanoscale. Finally, we highlight the pioneering studies and the most recent works on electro osmotic flow devoted to both theoretical and experimental aspects.

Electrokinetic Delivery of Reactants: Pore Water Chemistry Controls Transport, Mixing, and Degradation

Electrokinetics in porous media entails complex transport processes occurring upon the establishment of electric potential gradients, with a wide spectrum of environmental applications ranging from remediation of contaminated sites to biotechnology. The resulting electric forces cause the movement of pore water ions in opposite directions, leading to charge interactions that can affect the distribution of charged species in the domain. Here, we demonstrate that changes in chemical conditions, such as the concentration of a background electrolyte in the pore water of a saturated porous medium, exert a key control on the macroscopic transport of charged tracers and reactants. The difference in concentration between the background electrolyte and an injected solute can limit or enhance the reactant delivery, cause nonintuitive patterns of concentration distribution, and ultimately control mixing and degradation kinetics. With nonreactive and reactive electrokinetic transport experiments combined with process-based modeling, we show that microscopic charge interactions in the pore water play a crucial role on the transport of injected plumes and on the mechanisms and rate of both physical and chemical processes at larger, macroscopic scales. Our results have important implications on electrokinetic transport in porous media and may greatly impact injection and delivery strategies in a wide range of applications, including in situ remediation of soil and groundwater.