Flow Cell Experiments Research Articles

Inverse analysis of material parameters in coupled multi-physics biofilm models

In this article we propose an inverse analysis algorithm to find the best fit of multiple material parameters in different coupled multi-physics biofilm models. We use a nonlinear continuum mechanical approach to model biofilm deformation that occurs in flow cell experiments. The objective function is based on a simple geometrical measurement of the distance of the fluid biofilm interface between model and experiments. A Levenberg-Marquardt algorithm based on finite difference approximation is used as an optimizer. The proposed method uses a moderate to low amount of model evaluations. For a first presentation and evaluation the algorithm is applied and tested on different numerical examples based on generated numerical results and the addition of Gaussian noise. Achieved numerical results show that the proposed method serves well for different physical effects investigated and numerical approaches chosen for the model. Presented examples show the inverse analysis for multiple parameters in biofilm models including fluid-solid interaction effects, poroelasticity, heterogeneous material properties and growth.

Efficient removal of trifluoroacetic acid from water using surface-modified activated carbon and electro-assisted desorption

Trifluoroacetic acid (TFA) is a very persistent, very mobile substance (vPvM) with potential toxicity, and causes increasing environmental concerns worldwide. Conventional wastewater treatment strategies are inefficient for selective TFA removal in the presence of inorganic anions. Here we show that surface defunctionalized activated carbon felt (DeACF) carrying anion exchange sites exhibits an outstanding adsorption efficiency towards TFA thanks to introduced electrostatic attraction and enhanced interactions between hydrophobic carbon surface and CF3 moieties (qmax = 30 mg/g, Kd = (840 ± 80) L/kg at cTFA = 3.4 mg/L in tap water). Flow-cell experiments demonstrated a strongly favored TFA uptake by DeACF from tap water over Cl- and SO42- but a remarkable co-adsorption of the inorganic water contaminant NO3-. Electro-assisted TFA desorption using 10 mM Na2SO4 as electrolyte and oxidized ACF as anode showed high recoveries of ≥ 87% at low cell voltages (< 1.1 V). Despite an initial decrease in TFA adsorption capacity (by 33%) caused by partial surface oxidation of DeACF after the 1st ad-/desorption cycle, the system stability was fully maintained over the next 4 cycles. Such electro-assisted ‘trap&release’ approach for TFA removal can be exploited for on-site regenerable adsorption units and as a pre-concentration step combined with degradation technologies.

Microfluidic droplet detection via region-based and single-pass convolutional neural networks with comparison to conventional image analysis methodologies

As the complexity of microfluidic experiments and the associated image data volumes scale, traditional feature extraction approaches begin to struggle at both detection and analysis pipeline throughput. Deep-neural networks trained to detect certain objects are rapidly emerging as data gathering tools that can either match or outperform the analysis capabilities of the conventional methods used in microfluidic emulsion science. We demonstrate that two types of neural-networks, You Only Look Once (YOLOv3, YOLOv5) and Faster R-CNN, can be trained on a dataset which comprises of droplets generated across several microfluidic experiments and systems. The latitude of droplets used for training and validation, produce model weights which are easily transitive to emulsion systems at large, while completely circumventing any necessity of manual feature extraction. In flow cell experiments which comprised of greater than either 10,000 mono- or polydisperse droplets, the models show excellent or superior statistical symmetry to classical implementations of the Hough transform or widely utilized ImageJ plugins. In more complex chip architectures which simulate porous media, the produced image data typically requires heavy pre-processing to extrapolate valid data, where the models were able to handle raw input and produce size distributions with accuracy of ± 2μm for intermediate magnifications. This data harvesting fidelity is extended to foreign datasets not included in the training such as micrograph observation of various emulsified systems. Implementing these neural networks as the sole feature extraction tools in these microfluidic systems not only makes the data pipelining more efficient but opens the door for live detection and development of autonomous microfluidic experimental platforms due to inference times of greater than 100 frames per second.

Impact of Cosolvents on the Interphase Mass Transfer of NAPLs in Porous Media

AbstractCosolvent flushing has evolved as a viable technology for the remediation of groundwater resources contaminated with non‐aqueous phase liquids (NAPLs). This study investigates the effect of cosolvents on the interphase mass transfer of NAPLs into the flushing solutions, a key process that controls the effectiveness of the enhanced dissolution technology. A series of laboratory‐scale flow cell experiments were conducted to test the effect of cosolvent content, mean grain diameter and flushing velocity on the interphase mass transfer of NAPLs present in saturated porous media pooled form. The cosolvent and NAPL selected for this purpose are ethanol and 1,2‐dichlorobenzene, respectively. The experimental results are interpreted using a two‐dimensional numerical model and validated with a one‐dimensional analytical solution. The best fit mass transfer coefficients estimated from the model are used to develop Sherwood correlations expressed in terms of the Peclet and Schmidt numbers and the porous media main grain diameter. It is observed that the presence of the cosolvent can have an important impact on the interphase mass transfer coefficient. Specifically, the Sherwood correlation is found to be inversely proportional to the Schmidt number, expressed as a function of the viscosity and density of the cosolvent flushing solution. The developed Sherwood correlations can potentially be used in modeling studies and field applications involving the enhanced dissolution of entrapped NAPLs.

Iron-Tungsten Redox Flow Battery

Redox flow batteries are hailed as a panacea for grid level storage. Since the successful commercialization of vanadium redox batteries, a lot of efforts are being made to develop alternative chemistries. Most of them are based on metal ions which can exist in multiple oxidation states. One of them being, Fe based batteries developed first by R. F. Savinell.[1] Iron based batteries offer significantly cheaper and abundant alternatives to vanadium chemistry and is crucial from a strategic viewpoint. We present here initial development of an iron-tungsten based redox flow battery. One of the key advantages of the battery system presented here is the determination of state of charge by colorimetry. The iron salt as ferrous sulphate was used as an electrolyte for the positive electrode and the tungsten salt as phosphotungstic acid was used as the negative electrode. Although the Fe reaction involves 1 electron only, the reaction of phosphotungstic acid 4 electrons, thus the battery is expected to provide higher capacity compared to a normal Fe battery. Moreover, the maximum potential that can be achieved by using the highest reduced form of phosphotungstic acid is comparable to a vanadium or an all-iron battery.A flow cell was constructed and assembled in house using normal single cell fuel cell setup complete with gold-plated copper current collectors, graphite plates engraved with a serpentine flow field, porous graphite felt as electrode or reaction surface and nafion 117 as the selective proton conducting membrane. The schematic of the flow cell is presented in Figure 1(a). The electrolyte used for the positive electrode was iron(II) sulphate heptahydrate and for the negative electrode was phosphotungstic acid. Prior to the flow cell tests, CV experiments were conducted in a traditional 3 electrode setup. A usual CV (see Figure 1(b)) for Fe2+ and Fe3+ was obtained while the CV of phosphotungstic acid showed 3 oxidation and reduction waves corresponding to the 3 reactions. At very low negative potential, the hydrogen evolution reaction severely interfered with the 3rd reduction wave. An analysis of the peak potential with scan rate revealed that the rate constant for the Fe reaction to be 3x10-4 cm/s close to the reported value of 3.3x10-5 cm/s.[2] Meanwhile, an analysis of the peak separation of the 3 peaks for the phosphotungstic acid revealed that the reactions are reversible with a peak separation of ~ 59 mV and invariant with respect to scan rate.Meanwhile, for the flow cell experiments, the solution concentrations were similar to that for the CV and initially the battery was charged by applying a potential of 1.3 V for 30 minutes. It may be mentioned here that the volume of each of the electrolytes was same and equal to 100 mL. The flow rate of the electrolyte through the cell was kept constant at 120 mL/min. The discharge was performed by drawing current from 10 mA to 1000 mA for 10 s and the potential was recorded to ensure pseudo steady state. Finally the current and the potential values were rearranged to obtain the IV (current-potential) plot along with the power density curve (see Figure 1(c)). The maximum power obtained was 50 mW/cm2 and the maximum current that could be drawn was 160 mA/cm2, which is slightly low compared to the all-iron battery reported by R. F. Savinell.[1] The results obtained here suggest that the Iron-PTA battery demonstrated here could indeed prove to be a viable alternative to the vanadium flow battery. The electrochemical reactions for the iron-phosphotungstic acid redox flow battery are as follows:Negative:[PW12O40]3- + e - ↔ [PW12O40]4- Eo ' = +0.222V vs. RHE[PW12O40]4- + e - ↔ [PW12O40]5- Eo ' = -0.033V vs. RHE[PW12O40]5- + 2e - + H+ ↔ H[PW12O40]6- Eo ' = -0.348V vs. RHEPositive:Fe2+ ↔ Fe3+ + e - Eo ' = +0.705V vs. RHEOverall:4Fe2+ + [PW12O40]3- ↔ 4Fe3+ + H[PW12O40]6- Eo ' = +1.053V vs. RHEReferences[1] Hruska, L. W., & Savinell, R. F. (1981). Investigation of Factors Affecting Performance of the Iron‐Redox Battery. Journal of The Electrochemical Society, 128(1), 18–25.[2] Tanimoto, S., & Ichimura, A. (2013). Discrimination of Inner- and Outer-Sphere Electrode Reactions by Cyclic Voltammetry Experiments. Journal of Chemical Education, 90(6), 778–781. Figure 1

Separating kinetics and mass transfer in formic acid and formate oxidation on boron doped diamond electrodes

Abstract This paper describes the electrochemical oxidation of formic acid (pH 2) and formate (pH 6 and pH 10) on boron doped diamond electrodes and separates kinetic and mass transfer information using RDE and flow cell experiments. The voltammetry experiments show that the oxidation of formic acid and formate takes place before water oxidation, which indicates that the oxidation takes place via a direct electron transfer (DET) mechanism. The current densities measured in the linear sweep voltammograms were in line with the expected limiting current densities confirming mass transfer limitations. During chrono-amperometric experiments current densities were significantly lower, indicating a decrease in the kinetic rates. This is probably related to a surface modification at the electrode. This modification is reversible as bringing the electrode to low potentials restores the activity. The stable currents observed in the chronoamperometry experiments in the region before water oxidation indicate that there is probably a second DET mechanism, which has a much higher overpotential and a different pH dependency than the first DET mechanism. Koutecky-Levich plots showed clear mixed kinetic and mass transfer control and were used to deduce the Tafel slopes of this second mechanism. The observed high Tafel slopes of 240–300 mV/dec are in line with values reported for other reactions on BDD. Experiments in a parallel plate electrolyser show that the current efficiency is approximately 100% at 2.3 V for all pHs. At higher potentials current efficiency decreases due to the water oxidation. Interestingly at these potentials the formic acid oxidation exceeds the limiting current density, possibly related to the diffusion of ·OH radicals or other oxidation mediators to the bulk solution or the formation of oxygen bubbles on the electrode.

(Invited) Factors Influencing the Performance of Vanadium Flow Batteries: Electrodes and Electrolytes

There is presently great interest in redox flow batteries1-8 (RFBs) for large-scale energy storage9 due to advantages over other electrical energy storage (EES) technologies, and research activities in this area have grown exponentially in recent years. The vanadium flow battery (VFB), also known as the vanadium redox flow battery (VRFB), is commonly regarded as one of the most promising flow batteries.2-3,10-12 Active areas of research on VFBs include electrolytes,13-16 electrodes,5-7 membranes,8 cell design and modelling,8 and performance and state-of-charge (SoC) monitoring.17-18 In this paper, we will review our work in two areas: kinetics on carbon electrodes and thermal stability of electrolytes.The kinetics of the VII-VIII, VIII-VIV and VIV-VV redox couples have been studied for a range of different carbon materials using a variety of techniques, and it is clear that the kinetic rates depend strongly both on the type of carbon used and on the preparation of the electrode surface. We will discuss our investigations5-7 of the effects of anodic and cathodic pretreatments of carbon on electrode kinetics in the VFB. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we showed for four different types of carbon that electrode treatments at negative potentials enhance the kinetics of VIV-VV and inhibit the kinetics of VII-VIII while electrode treatments at positive potentials inhibit the kinetics of VIV-VV and enhance the kinetics of VII-VIII. These observations may explain conflicting reports in the literature. We examined in detail the potentials required for activation and deactivation of electrodes. The results suggest that interchanging the positive and negative electrodes in a vanadium flow battery (VFB) would reduce the overpotential at the negative electrode and so improve the performance. This is supported by flow-cell experiments. Thus, periodic catholyte-anolyte interchange, or equivalent alternatives such as battery overdischarge, show promise of improving the voltage efficiency of VFBs.We have developed13-16 a standard methodology for measuring the thermal stability of vanadium catholytes based on their induction time for precipitation of V2O3. Using this, we have investigated the thermal stability of typical vanadium flow battery (VFB) catholytes at temperatures in the range 30–70°C for VV concentrations of 1.4–2.2 mol dm-3 and sulfate concentrations of 3.6–5.4 mol dm-3. In all cases, V2O5 precipitates after an induction time, which decreases with increasing temperature. Plots of the logarithm of induction time versus the inverse of temperature (equivalent to Arrhenius plots) show excellent linearity and all have similar slopes, yielding a value of 1.791±0.020 eV for the activation energy. The logarithm of induction time also increases linearly with sulfate concentration and decreases linearly with VV concentration. The slopes of these plots give values of concentration coefficients β S and β V5 which quantify the effects of concentration on induction times. Combining these with the Arrhenius slope, we have constructed a model to predict the stability of sulfate-based vanadium catholytes. The model can also be used as a basis for accelerated testing of the thermal stability of electrolytes.We have investigated a range of additives and shown that many suggested additives are ineffective. However, the Group V elements phosphorus and arsenic in the +5 oxidation state have a significant stabilizing effect. We have also quantified their effect in the temperature range 30-70°C. REFERENCES A.Z. Weber et al., J. Appl. Electrochem. 41, 1137 (2011)M. Skyllas-Kazacos et al., J. Electrochem. Soc. 158, R55 (2011)M.J. Watt-Smith et al., J. Chem. Technol. Biotechnol. 88, 126 (2013)S. Roe et al., J. Electrochem. Soc. 163, A5023 (2016)A. Bourke et al., J. Electrochem. Soc. 163, A5097 (2016)A. Bourke et al., J. Electrochem. Soc. 162, A1547 (2015)M.A. Miller et al., J. Electrochem. Soc. 163, A2095 (2016)D. Reed et al., J. Electrochem. Soc. 163, A5211 (2016)R. M. Darling et al., Energy Environ. Sci ., 7, 3459 (2014)R. M. Darling et al., J. Electrochem. Soc .,163, A5014 (2016)A. K. Manohar et al., J. Electrochem. Soc., 163, A5118 (2016)A. M. Pezeshki et al., J. Electrochem. Soc., 163, A5202 (2016)D. Oboroceanu et al., J. Electrochem. Soc ., 163, A2919 (2016)D. Oboroceanu et al., J. Electrochem. Society, 164, A2101 (2017)D. N. Buckley et al., J. Electrochem. Society, 165, A3263 (2018) Oboroceanu et al., J. Electrochem. Soc ., 166, A2270 (2019)C. Petchsingh et al., J. Electrochem. Soc. 163, A5068 (2016)D. N. Buckley et al., J. Electrochem. Soc., 161 A524 (2014)

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate

Isoprene has the highest emission into Earth's atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO2 and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Plutonium Desorption from Nuclear Melt Glass-Derived Colloids and Implications for Migration at the Nevada National Security Site, USA.

The migration of low levels of plutonium has been observed at the Nevada National Security Site (NNSS) and attributed to colloids. To better understand the mechanism(s) of colloid-facilitated transport at this site, we performed flow cell desorption experiments with mineral colloid suspensions produced by hydrothermal alteration of NNSS nuclear melt glass, residual material left behind from nuclear testing. Three different colloid suspensions were used: (1) colloidal material from hydrothermal alteration of nuclear melt glass at 140 °C; (2) at 200 °C; and (3) plutonium sorbed to SWy-1 montmorillonite at room temperature. The 140 °C sample contained only montmorillonite, while zeolite and other phases were present in the 200 °C sample. Overall, more plutonium was desorbed from the 140 °C colloids (ca. 9-16%) than from the 200 °C colloids (ca. 4-8%). Furthermore, at the end of the 4.5 day flow cell experiments, the desorption rates for the 140 °C colloids and the Pu-montmorillonite colloids were similar while the desorption rates from the 200 °C colloids were up to an order of magnitude lower. We posit that the formation of zeolites and clays hydrothermally altered at 200 °C may lead to a more stable association of plutonium with colloids, resulting in lower desorption rates. This may give rise to more extensive colloid-facilitated transport and help explain why trace levels of plutonium are found downgradient from their original source decades after a nuclear detonation. Interestingly, in the case of cesium (a co-contaminant of plutonium), no difference was observed between the 140 and 200 °C colloids. This reflects intrinsic differences between cesium and plutonium sorption/desorption behavior (charge, cation size) and suggests that the Cs sorption mechanism (cation exchange) is not similarly affected by colloid formation temperature.

Mechanistic Investigations for Electrocatalytic Oxidation of Furfural Using Density Functional Theory

Mechanistic Investigations for Electrocatalytic Oxidation of Furfural Using Density Functional Theory Naveen Agrawal,a Li Gong,a,b Alex Roman,c Lesli Mark, c Will Medlin,c Adam Holewinski,c Michael J. Janika aDepartment of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA. b College of Environment, Zhejiang University of Technology, Hangzhou 310014, China. cDepartment of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA. Furfural is a promising biomass-derived synthetic platform species with numerous chemical outlets[1]. Furfural oxidation could provide an alternative to fossil feedstock production of furanones, furoic acid, maleic anhydride (MAN), maleic acid (MA) and succinic acid (SA) [2], but scalable production routes remain elusive due to poor selectivity and energy efficiency. Most previous studies of furfural partial oxidation to acid anhydrides and dicarboxylic acids utilized either oxygen or hydrogen peroxide as the oxidant together with heterogeneous catalysts. Driving reactions with electric potential, such as the electro-oxidation of furfural, could be a promising green method using electricity derived from solar, wind and geothermal sources. Electro-oxidation may offer different reaction paths and selectivity control not offered in chemical oxidation, motivating our mechanistic analysis. We have applied density functional theory (DFT) to investigate the electrocatalytic oxidation mechanism on the Pt (111) surface, and validated the mechanistic insights with comparison to both low-pressure TPD and HREELS studies as well as electrochemical flow cell experiments. The potential-dependent reaction free energy profiles for furfural electrocatalytic oxidation to furoic acid, succinic acid, maleic acid, and maleic anhydride are reported. After comparing several possible furfural oxidation paths, we conclude that the electro-oxidation of furfural preferentially proceeds to furoic acid, with further oxidation slowed by difficult C-C bond dissociation. HREELS studies confirmed the presence of furoic acid like intermediate (furoate) during furfuryl alcohol oxidation on an oxygen covered Pt (111) surface, which might represent the Pt (111) surface under oxidative conditions [3]. Oxidation beyond furoic acid can proceed to succinic acid via 2(3H)-furanone as an intermediate and to maleic acid and maleic anhydride via 2(5H)-furanone as an intermediate. The rate of these processes is likely limited by the decarboxylation of furoic acid. DFT analysis of elementary step thermodynamics and kinetics suggest that the selectivity between furoic acid, succinic acid, maleic acid, or other oxidized products is tunable by varying the electrode potential. Initial electrochemical flow cell experimental results show furoic acid as the most significant product (> 80 % selectivity) at 0.9 V-RHE on a Pt electrode, in agreement with DFT results. These results broaden our fundamental understanding into electrocatalytic oxidation of furfural, which is applicable in upgrading renewable biomass derivatives.

Determination of the Effective Viscosity of Non-newtonian Fluids Flowing Through Porous Media

When non-Newtonian fluids flow through porous media, the topology of the pore space leads to a broad range of flow velocities and shear rates. Consequently, the local viscosity of the fluid also varies in space with a non-linear dependence on the Darcy velocity. Therefore, an effective viscosity is usually used to describe the flow at the Darcy scale. For most non-Newtonian flows the rheology of the fluid can be described by a (non linear) function of the shear rate. Current approaches estimate the effective viscosity by first calculating an effective shear rate mainly by adopting a power-law model for the rheology and including an empirical correction factor. In a second step this averaged shear rate is used together with the rheology of the fluid to calculate. In this work, we derive a semi-analytical expression for the local viscosity profile using a Carreau type fluid, which is a more broadly applicable model than the power-law model. By solving the flow equations in a circular cross section of a capillary we are able to calculate the average viscous resistance directly as a spatial average of the local viscosity. This approach circumvents the use of classical capillary bundle models and allows to upscale the viscosity distribution in a pore with a mean pore size to the Darcy scale. Different from commonly used capillary bundle models, the presented approach does neither require tortuosity nor permeability as input parameters. Consequently, our model only uses the characteristic length scale of the porous media and does not require empirical coefficients. The comparison of the proposed model with flow cell experiments conducted in a packed bed of monodisperse spherical beads shows, that our approach performs well by only using the physical rheology of the fluid, the porosity and the estimated mean pore size, without the need to determine an effective shear rate. The good agreement of our model with flow experiments and existing models suggests that mean viscosity is a good estimate for the effective Darcy viscosity providing physical insight into upscaling of non-Newtonian flows in porous media.

Direct electrochemical determination of methotrexate using functionalized carbon nanotube paste electrode as biosensor for in-vitro analysis of urine and dilute serum samples

Chemotherapy is a widely used effective method for the treatment of cancer in humans. Severity of dose levels and treatment duration necessitate frequent continuous monitoring of active drug concentration in human serum and urine. Development of an electrochemical biosensor for direct determination of methotrexate (MTX), a chemotherapy agent, from physiological fluids has been investigated. Functionalized multi-walled carbon nanotube paste electrodes (f-CNTPE) have been fabricated for simple, cost-effective and reusable electrochemical sensors. Scanning electron microscopy studies revealed that the fabricated electrode surface is comprised of nano-streaks of independent segregated CNTs with high porosity. Cyclic voltammetric analysis showed that f-CNTPE had an enhanced electrocatalytic activity with sensitivity as much as three times and less over potentials by 70 mV compared with a conventional carbon paste electrode. Under optimized experimental parameters, differential pulse voltammograms (DPVs) exhibited a gradual increase in peak current with concentration, and the plot of peak current against MTX concentration was linear in the range of 0.4–5.5 μM. Square-wave voltammetry analysis of methotrexate exhibited a linear determination range from 0.01 to 1.5 μM, and the low-detection-limit was 2.9 × 10−9 M. Steady-state current–time analysis mimic to hydrodynamic flow-cell experiments demonstrated a low-detection-limit of 10 nM (S/N ratio ~8) and an analysis time of mere 10 s. Selective determination of MTX at micromolar concentrations in the presence of a mixture of possible electroactive interferents, ascorbic acid, dopamine, etc. (1.5 μM each) is successfully demonstrated. The electrochemical biosensor allows the detection of MTX in-vitro directly from pharmaceutical drug, human serum and undiluted urine samples at very low concentration limits of as low as 5.0 × 10−7 M with recovery error limits of 4.5% at maximum.

Contributions of biotic and abiotic pathways to anaerobic trichloroethene transformation in low permeability source zones

Low permeability source zones sustain long-term trichloroethene (TCE) groundwater contamination. In anaerobic environments, TCE is transformed by both biological reductive dechlorination and abiotic reactions with reactive minerals. Little is known about the relative contribution of these two pathways as TCE diffuses from low permeability zones (LPZs) into high permeability zones (HPZs). This study combines a flow cell experiment, batch experiments, and a diffusion-reaction model to evaluate the contributions of biotic and abiotic TCE transformation in LPZs. Natural clay (LPZ) and sand (HPZ) from a former Air Force base were used in all experiments. In batch, the LPZ material transformed TCE and cis-1,2-dichloroethene (cis-DCE) to acetylene with pseudo first-order rate constants of 8.57 × 10−6 day−1 and 1.02 × 10−6 day−1, respectively. Biotic and abiotic pathways were then evaluated together in a bench-scale flow cell (16.5 cm × 2 cm × 16.5 cm) that contained a LPZ layer, with a source of TCE at the base, overlain by a HPZ continuously purged with lactate-amended groundwater. Diffusion controlled mass transfer in the LPZ, while advection controlled migration in the HPZ. The mass discharge rate of TCE and its biotic (cis-DCE and vinyl chloride) and abiotic (acetylene) transformation products were measured over 180 days in the flow cell effluent. Depth profiles of these compounds through the LPZ were determined after terminating the experiment. A one-dimensional diffusion-reaction model was used to interpret the effluent and depth profile data and constrain reaction parameters. Abiotic transformation rate constants for TCE to acetylene, normalized to in situ solids loading, were approximately 13 times greater in batch than in the flow cell. Slower transformation rates in the flow cell indicate elevated TCE concentration and/or further degradation of acetylene to other reduced gas compounds in the flow cell LPZ (thereby partially masking TCE abiotic transformation). Biotic and abiotic parameters used to interpret the flow cell data were then used to simulate a field site with a 300 cm thick LPZ. Abiotic processes contributed to a 2% reduction in TCE flux after 730 days. When abiotic rate constants were changed to that observed in batch, or to rate constants previously reported for a pyrite rich mudstone, the TCE flux reduction was 21% and 53%, respectively, after 730 days. Though biotic processes dominated TCE transformation in the flow cell experiment, the simulations indicate that abiotic processes have potential to significantly contribute to TCE attenuation in electron donor limited environments provided suitable reactive minerals are present.

Rationally Selecting Manganese Oxide Electrodes for Salinity Gradient Energy Production

Electricity can be generated from the controlled mixing of salinity gradients (i.e., two waters with different salt concentrations). Capturing salinity gradient energy using existing technologies such as Pressure Retarded Osmosis and Reverse Electrodialysis are limited by membrane fouling and costs. Capacitive electrode technologies, like capacitive mixing, may be used to overcome the limitations of membrane-based technologies by taking advantage of capacitive double layer expansion (CDLE) at the electrode surface to generate power. However, due to intrinsically low charge storage capacities, researchers have begun to explore pseudo-capacitive electrode materials, such manganese oxides. The charge-storage capabilities of pseudo-capacitive materials differs from purely capacitive materials in that that they can store charge via faradaic, redox reactions between the cations and the oxide crystal structure (MnOO + Na+ + e- = MnOONa). Presently, the exact charging mechanisms involved when manganese oxide electrodes harness salinity gradient energy are poorly understood. The goal of this study is to optimize the performance of a salinity gradient flow cell using MnO2 electrodes by determining the charge-discharge mechanism of manganese oxide structures, which will facilitate rational material selection in the future. Material characterization tests, cyclic voltammetry analysis, and galvannostatic charge-discharge tests were used to characterize and compare a suite of manganese oxide powders that varied by crystal structure (amorphous, δ, γ, α, and β) and morphology (nanorod, sphere, urchin, and plate-like). Subsequently, flow cell experiments were performed with varying solution chemistries to obtain information on power production, stability, and cyclability. Preliminary flow cell experiments show that open circuit voltage window and power production for MnO2 structures decreases in the order of amorphous, α, β, which follows the trend for BET surface area (amorphous > α > β). Additionally, electrochemical tests indicate the material is stable over multiple cycles and CV deconstruction shows that charge storage capabilities of MnO2 may be largely attributed to surface reactions; suggesting that BET surface area may potentially be used as an indicator of performance for harnessing salinity gradient energy.

The Role of Capillary Hysteresis and Pore‐Scale Heterogeneity in Limiting the Migration of Buoyant Immiscible Fluids in Porous Media

AbstractUnderstanding the main mechanisms affecting long‐term migration and redistribution of injected CO2 in geological carbon storage is needed for developing predictive models to assess environmental risks and designing monitoring schemes. Preparation of a postinjection site care plan is required for CO2 injection wells, including monitoring of pressure changes and injected CO2 plume. Knowledge gaps exist regarding assessment of postinjection monitoring timeframes for the CO2 plume because the processes driving long‐term CO2 plume migration and trapping are not fully understood. In the postinjection stage of geological carbon storage, redistribution of CO2 occurs mainly due to buoyancy and capillary forces. This work presents experimental and modeling studies to investigate processes contributing to postinjection plume distribution and stabilization. We conducted a flow cell experiments (0.5 m × 0.05 m × 0.01 m) with two immiscible fluid phases in a glass bead porous medium to study postinjection plume behavior. We employed a hysteretic macroscopic two‐phase flow model to interpret the experimental results and to understand main processes leading to plume stabilization. Our findings show that capillary pressure hysteresis explains the experimentally observed plume shape and redistribution at early postinjection stages; however, the long‐term plume migration and eventual plume stabilization can only be represented when in addition microscale heterogeneity is accounted for. Results also show that plume stabilization can be extremely slow and that the migration of the plume front can occur through multiple intermittent bursts over long times. Further studies are needed to understand implications of the results for more realistic porous media and large‐scale storage reservoirs.

Transport of silver nanoparticles in single fractured sandstone

Silver nanoparticles (Ag-NP) are used in various consumer products and are one of the most prevalent metallic nanoparticle in commodities and are released into the environment. Transport behavior of Ag-NP in groundwater is one important aspect for the assessment of environmental impact and protection of drinking water resources in particular. Ag-NP transport processes in saturated single-fractured sandstones using triaxial flow cell experiments with different kind of sandstones is investigated. Ag-NP concentration and size are analyzed using flow field-flow fractionation and coupled SEM-EDX analysis. Results indicate that Ag-NP are more mobile and show generally lower attachment on rock surface compared to experiments in undisturbed sandstone matrix and partially fractured sandstones. Ag-NP transport is controlled by the characteristics of matrix porosity, time depending blocking of attachment sites and solute chemistry. Where Ag-NP attachment occur, it is heterogeneously distributed on the fracture surface.

Seeing through porous media: An experimental study for unveiling interstitial flows

AbstractWe describe a novel inexpensive method, utilizing particle image velocimetry (PIV) and refractive index‐matching (RIM) for visualizing and quantifying the flow field within bio‐amended porous media. To date, this technique has been limited to idealized particles, whose refractive index does not match that of fresh water and thus requires specialized and often toxic or hazardous fluids. Here, we use irregularly shaped grains made of hydrogel as the solid matrix and water as the fluid. The advantage of using water is that it provides, for the first time, the opportunity to study both hydraulic and biological processes, which typically occur in soils and streambeds. By using RIM coupled with PIV (RIM‐PIV), we measured the interstitial flow field within a cell packed with granular material consisting of hydrogel grains in a size range of 1–8 mm, both in the presence and in the absence of Sinorhizobium meliloti bacteria (strain Rm8530). We also performed experiments with fluorescent tracer (fluorescein) and fluorescent microbes (Shewanella GPF MR‐1) to test the capability of visualizing solute transport and microbial movements. Results showed that the RIM‐PIV can measure the flow field for both biofilm‐free and biofilm‐covered hydrogel grains. The fluorescent tracer injection showed the ability to visualize both physical (concave surfaces and eddies) and biological (biofilms) transient storage zones, whereas the fluorescent microbe treatment showed the ability to track microbial movements within fluids. We conclude that the proposed methodology is a promising tool to visualize and quantify biofilm attachment, growth, and detachment in a system closer to natural conditions than a 2D flow cell experiment.

A design of experiment aided stochastic parameterization method for modeling aquifer NAPL contamination

Numerical models have been widely applied in simulating subsurface Non-aqueous Phase Liquid (NAPL) contamination processes. In order to examine modeling uncertainties and improve simulation performance, a new hybrid stochastic - design of experiment (DOE) aided parameterization method was developed by using a coupled experimental and modeling approach. In a case study, an existing commercial groundwater modeling tool BioF&T 3D was applied to conduct numerical simulations of subsurface contamination processes based on flow cell experiments. Parameterization results indicated that porosity, distribution coefficient, and Henry's constant were the most significant parameters. The result also revealed their interactions. The DOE predicted responses were found reasonably close to the actual ones from the models' simulations. Monte Carlo simulation was applied to conduct uncertainty analysis within the narrowed parameters ranges, which were generated by centralizing the DOE optimized values, and the combinations of parameters were further updated when better responses were found. After parameterization, R2 valued 0.80, 0.91, 0.89, and 0.90 for benzene, toluene, ethylbenzene, and xylene (BTEX), respectively. A good consistency (R2 = 0.76 to 0.90 for BTEX) was also achieved during the model verification, which confirmed that after the parameterization processes, the simulation model can potentially be used for predictions under similar conditions.

In situ stabilization of NAPL contaminant source-zones as a remediation technique to reduce mass discharge and flux to groundwater

Widely used flushing and in-situ destruction based remediation techniques (i.e. pump-and treat, enhanced-solubilization, and chemical oxidation/reduction) for sites contaminated by nonaqueous phase liquid (NAPL) contaminant sources have been shown to be ineffective at complete mass removal and reducing aqueous-phase contaminant of concern (COC) concentrations to levels suitable for site closure. A remediation method was developed to reduce the aqueous solubility and mass-flux of COCs within NAPL through the in-situ creation of a NAPL mixture source-zone. In contrast to remediation techniques that rely on the rapid removal of contaminant mass, this technique relies on the stabilization of difficult-to-access NAPL sources to reduce COC mass flux to groundwater. A specific amount (volume) of relatively insoluble n-hexadecane (HEXDEC) or vegetable oil (VO) was injected into a trichloroethene (TCE) contaminant source-zone through a bench-scale flow cell port (i.e. well) to form a NAPL mixture of targeted mole fraction (TCE:HEXDEC or TCE:VO). NAPL-aqueous phase batch tests were conducted prior to the flow-cell experiments to evaluate the effects of various NAPL mixture ratios on equilibrium aqueous-phase concentrations of TCE to design optimal NAPL (HEXDEC or VO) injection volumes for the flow-cell experiments. The NAPL-stabilization flow-cell experiments initiated and sustained significant reductions in COC concentration and mass flux due to a combination of both reduced relative permeability (increased NAPL-saturation) and via modification of NAPL composition (decreased TCE mole fraction). Variations in remediation performance (i.e. impacts on TCE concentration and mass flux reduction) between the different HEXDEC injection volumes were relatively minor, and therefore inconsistent with Raoult's Law predictions. This phenomenon likely resulted from non-uniform mixing of the injected HEXDEC with TCE in the source-zone. VO injection caused TCE concentrations and mass-flux to decrease more rapidly than with HEXDEC injections. This phenomenon occurred because the injected VO was observed to mix more uniformly with TCE in the source-zone due to a lower mobilization potential. The relative lower density differences (buoyancy effects) between VO and the flushing solution (water) was the primary factor contributing to the lower mobilization potential for VO. Overall, this study indicated that the delivery of HEXDEC or VO into the toxic TCE source-zone was effective in significantly reducing contaminant aqueous-phase concentration and mass-flux. However, the effectiveness of this in-situ NAPL stabilization technique depends on source delivery, uniform mixing of amendment, and that the amendment remains immobilized within and around the NAPL contaminant source.

Selection of Colloidal Silica Grouts with Respect to Gelling and Erosion Behaviour

Cembinder, Eka EXP36, and MEYCO MP320 are three colloidal silica materials that have been proposed for post-excavation grouting of deep tunnels in a radioactive waste repository. In this study, samples of these colloidal silicas were tested for their particle size distribution, gel induction time (tG), gel time (TG), and physical erosion, under mildly saline groundwater flow conditions. In order to achieve a desired gel time range, from 15 to 50 min, it is recommended that the colloidal silica is mixed with a NaCl accelerator at a 5:1 volume ratio. At 20 °C, the concentration range for the NaCl solution should be 1.5 to 1.7 M for MEYCO, 1.23 to 1.38 M for Eka EXP36, and 1.3 to 1.47 M for Cembinder. The physical erosion of the set silicas remained steady during a 10 h flow cell experiment, when grouts were subjected to 0.05 M NaCl at a superficial velocity of 2.2 × 10−5 m/s. For these test conditions, the results show that MEYCO has the highest average erosion rate (0.85 mg/h) of the three grout materials, as well as the greatest variability in this rate. Cembinder performed best with the lowest silica removal rate. Extrapolation of the measured erosion rates suggests that grout fracture dilation would not be significant under natural quiescent groundwater flow conditions, but would be high if there was hydraulic communication between the geosphere and the repository.