Diffusion Layer Thickness Research Articles

Halate electroreduction from acidic solution at rotating disk electrode: Theoretical study of the steady-state convective-migration-diffusion transport for comparable concentrations of halate ions and protons

Theoretical analysis of electrochemically inactive halate-anion (XO3−, X = halogen) electroreduction at rotating disk electrode (RDE) under steady-state conditions via autocatalytic mediator cycle based on X2/X− redox couple in acidic aqueous medium has been performed in relation to prospects to use this reaction as cathodic process of high energy density flow batteries. For the first time, full set of coupled diffusion-migration-convection transport equations for the components of the system, including kinetic terms due to the comproportionation reaction between solute species, has been solved via numerical and analytical tools. All principal characteristics of the system have been found: distributions of concentrations of all components inside the diffusion layer as well as of induced electric field and of the chemical reaction rate, diffusion-layer thicknesses for all components as functions of the principal dimensionless parameters: passing current, J, ratios of the diffusion-limited currents for species H (protons) and A (XO3−), JHA, or for species C (X2) and A, JCA, ratio of the diffusion and kinetic layer thicknesses for species A, xdkA = zdA / zk, as well as ratios of the diffusion coefficients of components, DiA = Di / DA (i = other components). The strongest dimensionless current, Jmax, which can pass across the system, can also be calculated for any set of particular values of the dimensionless parameters: JHA, JCA, xdkA and DiA. General features of the dependence, Jmax vs. xdkA, have been analyzed for various values of JHA (i.e. of the ratio of their bulk-solution concentrations, Ho/Ao) while the value of JCA (and consequently of the concentrations' ratio, Co/Ao) is very small. It has been demonstrated that within the range of sufficiently low rotation frequencies the maximal dimensional current density, jmax, is comparable with the combined diffusion-limited current of species A (XO3−) and H (H+), even for extremely low bulk-solution concentration of catalytic species C (X2), Co, i.e. passing current can reach enormous values, despite the electrochemical inactivity of species A and H. This feature which is highly beneficial for the application of this reaction in power sources is a direct consequence of the autocatalytic character of the redox-mediator cycle based on reactions of species XO3−, X− and X2 in acidic medium.

Characterization of Quasi-Static/Dynamic Contact Mechanical Properties of Mo Surface-Modified TC4

By using the double glow plasma surface alloying technique, a Mo surface-modified layer is prepared on Ti6Al4V(TC4). The element concentration and microstructure are characterized with a glow-discharge optical emission spectroscope and a scanning electron microscope. The results indicate that the Mo modified layer is compact in structure and gradation in composition, which consists of a pure Mo deposition layer and a thick Mo diffusion layer. Nanoindentation test results indicate that the surface hardness of TC4 is significantly improved after surface modification. Therefore, the initial part of the Mo diffusion layer has higher hardness than the Mo deposition layer. The impact tests for 10,000 cycles at different loads demonstrate that impact load 100 N only causes small plastic deformation, while impact loads 300 N and 500 N could result in cracks. Combining nanoindentation test with finite element reverse analysis, plastic parameters of the Mo modified layer are quantitatively determined. By using the impact test and finite element forward analysis, the dynamic contact mechanical properties of Mo surface-modified TC4 are characterized. When the impact cycles are fixed, ring cracks firstly occur and then radial cracks occur with the impact load increase on the graded layer. The ring cracks are mainly caused by impact stretch fatigue and corresponding cyclic stress is between 3.53 and 2.62 GPa. The radial cracks are mainly related to the tension-compression fatigue and corresponding cyclic stress is between 3.92 and −0.97 GPa.

Diffusion Bonding of Al-Mg-Si Alloy and 301L Stainless Steel by Friction Stir Lap Welding Using a Zn Interlayer.

Friction stir lap welding (FSLW) is expected to join the hybrid structure of aluminum alloy and steel. In this study, the Al-Mg-Si aluminum alloy and 301L stainless steel were diffusion bonded by FSLDW with the addition of 0.1 mm thick pure Zn interlayer, when the tool pin did not penetrate the upper aluminum sheet. The characteristics of lap interface and mechanical properties of the joint were analyzed. Under the addition of Zn interlayer, the diffusion layer structure at lap interface changed from continuous to uneven and segmented. The components of the diffusion layer were more complex, including Fe-Al intermetallic compounds (IMCs), Fe-Zn IMCs and Al-Zn eutectic. The largely changed composition and thickness of uneven and segmented diffusion layer at the lap interface played a significant role in the joint strength. The tensile shear load of Zn-added joint was 6.26 kN, increasing by 41.3% than that of Zn-not-added joint. These two joints exhibited interfacial shear fracture, while the Zn interlayer enhanced the strength of diffusion bonding by extending the propagation path of cracks.

Effects of microbially induced carbonate precipitation on diffuse double layer and particle fabric of oil sands fine tailings

Microbially induced carbonate precipitation (MICP) is a sustainable biological process that catalyzes carbonate mineral precipitation within geomaterials. This study evaluates the performance and mechanisms of the MICP treatment for flocculating the oil sands fine tailings (FT). Column tests showed that the untreated FT did not decant during the 31 days. However, the MICP technique shortened the dewatering process. To elucidate the mechanisms of the MICP-induced flocculation of the FT, the diffuse double layer (DDL) thickness and microstructure of the specimens were evaluated. Three chemical equilibrium scenarios that gradually considered the MICP-biochemical reactions were explored to analyze the change of the DDL thickness. The results showed that increasing of ionic strength by urea hydrolysis decreased the DDL thickness. The fabric observation indicated that the specimens with the most calcium carbonate precipitation had the densest fabric. In summary, the MICP technique densified the fabric of FT via the ureolysis process and precipitating minerals.

Study on Interface Microstructure and Bonding Properties of Hot-Rolled Nickel-Based Composite Plate

In this paper, the interface microstructure, elements’ diffusion features at the interface, and bonding properties in nickel-based alloy/carbon steel clad composite prepared by vacuum hot-roll bonding were investigated, comprehensively. The influence of element distribution on the interface bonding strength was revealed as well. The results showed that there was a 13 μm thick diffusion layer at the interface of nickel-based alloy/carbon steel composite plate, which was beneficial to a strong bond between nickel-based alloy and carbon steel, as well as the stable transition of mechanical properties in the thickness direction. Kirkendall voids and fine-grained structure (the grain size is about 41.5 nm) were observable by peeling off the nickel-based alloy cladding, which greatly promoted element diffusion and enhanced the interfacial bonding strength of the nickel-based alloy/carbon steel composite plate. The diffusion coefficient of Ni at the interface was about 2 orders of magnitude larger than that of nanocrystalline Fe. The shear strength reached up to 453 MPa, which was much higher than the minimum of 140 MPa defined in ASTM A-264 specifications. Furthermore, in the shear test, the fracture occurred on the X52 carbon steel side at the contact rather than at the composite plate interface.

The transition to turbulence in rarefaction-driven Rayleigh–Taylor mixing: Effects of diffuse interface

Effects of interface diffusion on the transition to turbulence in rarefaction-driven flows are numerically investigated via Implicit Large-Eddy simulation. Three-dimensional, multimode perturbations are imposed on the diffuse interface between Air and SF6, with various diffusion layer thicknesses. A non-constant acceleration ranging from 103g0 to 104g0, where g0 is the acceleration due to gravity, is generated by the interaction between the interface and a rarefaction wave. Evolution of first- and second-order statistics, instantaneous flow structures, and the power spectrum of turbulent kinetic energy as well as spatial distributions of energy budget are evaluated, in order to confirm the accuracy and robustness of the mixed mass transition criterion proposed here. Meanwhile, it turns out that transitional behaviors are mainly governed by Reynolds normal stresses in the plane perpendicular to the streamwise direction. Furthermore, as interface diffuses, the decrease in peak values of pressure and advection components dominated in the laminar regimes, particularly at the bubble tips, eventually leads to transition delay.

Multi-Variate Optimization of Polymer Electrolyte Membrane Fuel Cells in Consideration of Effects of GDL Compression and Intrusion

This study applies, tests, and compares comprehensive surrogate-based optimization techniques to optimize the performance of polymer electrolyte membrane fuel cells (PEMFCs). Moreover, parametric cases considering four important design variables, i.e., gas diffusion layer thickness (), channel depth (), channel width (), and land width (), are defined using the latin hypercube sampling technique under reasonable constraint conditions. Multidimensional, two-phase PEMFC model simulations are performed to generate the training and test data under these design conditions. Three famous surrogate models, i.e., response surface approximation (RSA), radial basis neural network (RBNN), and kriging (KRG), are employed to construct objective functions for the PEMFC cell voltages operating in the galvanostatic mode, and their accuracies are tested and compared using root mean square error and adjusted R-square. The surrogates are then linked to stochastic optimization algorithms, i.e., genetic algorithm and particle swarm optimization, to determine the optimal design points. A comparative study of these surrogates reveals that the KRG model outperforms the RBNN and RSA models in terms of prediction capability. Furthermore, the PEMFC model simulations at the optimal design points clearly demonstrate that performance improvements of around 56–69 mV at 2.0 A cm−2 are achieved with the optimum design values compared to typical PEMFC design conditions.

High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi2–Al2O3 composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ∼2 μm thick Mo5Si3 diffusion layer, a ∼65 μm thick MoSi2 inner layer and a ∼36 μm thick outer layer of mixture of MoSi2 and Al2O3. After oxidation at 1600 °C for 5 h, all MoSi2 phases were completely converted to intermediate silicide Mo5Si3 by solid-state diffusion, and the formed Mo5Si3 phase would be transformed into Mo3Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO2-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi2–Al2O3 coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi2–Al2O3 coating is due to the formation of multi-layer structure containing a SiO2-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.

Transient chronoamperometric current at rotating disc electrode for second-order ECE reactions

The approximate transient chronoamperometric limiting current generated from the electrochemical reaction in a rotating disc electrode for second-order ECE reactions is derived when the chemical step is irreversible. This model is based on nonlinear reaction–diffusion equations containing a nonlinear term related to homogeneous reaction. By solving the nonlinear equations applying a new approach of homotopy perturbation methods, simple and closed-form expressions for the reactant concentration and the current response as a function of the rotation rate and diffusion coefficients are obtained. The impact of various physical parameters on concentration and current has been analyzed and graphically displayed. Sensitivity analysis of parameters such as rate constant, the ratio of bulk concentration, the thickness of diffusion layer on current is discussed. In addition, the concentration/current expressions match well with the numerical and limiting case results.

Determination of diffusion coefficients in agarose and polyacrylamide gels for 112 organic chemicals for passive sampling by organic Diffusive Gradients in Thin films (o-DGT).

The diffusive gradient in thin film technique was recently adapted to organic compounds. The diffusional coefficient (D) is a key parameter needed to calculate the time-weighted average concentration. In this study, two methods are used for D measurement in two gels (agarose and polyacrylamide): the diffusion cell method (Dcell) and the slice stacking method (Dstack). Thus, D were discussed and compared for 112 organic compounds, including pesticides, hormones, and pharmaceuticals. Dstack tends to be higher than Dcell. It could be explained by the presence of a non-negligible diffusive boundary layer thickness in diffusion cell. Consequently, the use of sampling rates (RS) should be more adequate to determine water concentration, for a given bulk flow velocity. Dstack also corresponds to the diffusion in gel only, allowing the determination of the maximal RS, and would be considered as a reference value that can be adjusted to in situ conditions, by applying the appropriate DBL thickness. The range and variability of D values found in the literature and obtained in this work were discussed. Relationships between D and compound physicochemical properties (molecular mass, log Dow, polar surface area, van der Waals volume) were investigated. We did not find clear and robust correlation between D and any single physicochemical property, for the set of compounds tested.

Insight into Debye H\xfcckel length (\u03ba\u22121): smart gravimetric and swelling techniques reveals discrepancy of diffuse double layer theory at high ionic concentrations

Smart gravimetric and swelling techniques were utilized in this work to examine the validity of the Debye Hückel length (κ−1) equation when shale interacts with highly concentrated salt solutions. The swelling and shrinkage behavior of two different shales, when exposed to monovalent and divalent ionic solutions (NaCl, KCl and CaCl2) at concentrations ranging from 2 to 22%w/w was observed and measured. Shale swelling and shrinkage results show that Debye Hückel length (κ−1) equation seems to work adequately at low ionic concentrations where osmotic water flow out of shale plays a major role in decreasing the diffuse double layer thickness by withdrawing water out and thereby shrinking κ−1. At high ionic concentration levels, the flow of associated water into the diffuse double layer negates the withdrawal of osmotic water out of the diffuse double layer which could maintain κ−1 or possibly increase it. Data on measured ionic uptake into shale suggests that excessive ionic diffusion into shale, especially at high concentrations, leads to higher electrical repulsion between alike ions in the diffuse layer which could lead to the expansion of the diffuse double layer thickness. Furthermore, swelling and shrinkage data analysis for shale suggests the existence of a ‘critical concentration’ below which the Debye Hückel length equation works. Above the critical concentration, the validity of the Debye Hückel length equation might be in question. The critical concentration is different for all ions and depends on ionic valence, hydrated ion diameter, and clay type.

Improving the mechanical behaviour of clay contaminated with glycerol and anthracene using lime and Portland cement

ABSTRACT The specimens of contaminated clay improved with Portland cement and lime were cured for seven days and were subjected to uniaxial compressive strength (UCS) testing. The results for anthracene-contaminated specimens improved with three percent Portland cement (by weight of clay) were approximately equivalent to those improved with 10% lime. The results for glycerol-contaminated specimens improved with nine percent Portland cement was approximately equivalent to that improved with 10% lime. According to the results, both Portland cement and lime were able to improve the clay soil contaminated with anthracene and glycerol, but Portland cement had a greater effect than lime on the anthracene-contaminated specimens because of the non-polar behaviour of anthracene and the grain-like (flocculated) behaviour of the contaminated soil. These factors were the result of a decrease in the diffuse double layer thickness by anthracene, which has a lower dielectric constant relative to water. The lime had a greater effect than cement on the specimens contaminated with glycerol because of the polar behaviour of glycerol, the increase in the required moisture content for hydration, and because the dielectric constant of glycerol is close to that of water. These results were evaluated and confirmed by scanning electron microscopy.

Influence of ultraviolet radiation on kinetic characteristics in erythrocytes

Erythrocytes were irradiated with an ultraviolet lamp of the VMT type (λ = 253.7 nm). The kinetic characteristics in irradiated and non-irradiated blood samples were compared by the acid (chemical) erythrogram method and by the counter transport method. Obtained:
 - by the method of acidic erythrograms it was found that in ultraviolet-irradiated blood samples there is a decrease in the time of half hemolysis.
 - ultraviolet radiation increases the speed of counter movement of ions through erythrocyte membranes.
 - a decrease in the time of half hemolysis and an increase in the rate of counter transfer of ions, irradiated blood samples, is due to a decrease in the "effective" thickness of the near-membrane diffusion layer - an immiscible layer of water adjacent to the erythrocyte membrane.
 A decrease in the "effective thickness" of the near-membrane water layer changes the rate of metabolic processes in the "cell - intercellular medium" system, changing the mode of cell functioning. The altered mode of functioning is a biological response to light radiation. Red blood cells with an altered mode of functioning are stimulus signals that force the body to mobilize resources to fight pathology. These circumstances can predict the creation of a universal phototherapeutic equipment for extracorporeal blood irradiation based on LEDs with certain exposure parameters. Figs.: 4. Tabl.: 3. Refs.: 12 titles.

Long-term effect of water chemistry on the swelling pressure and hydraulic conductivity of compacted claystone/bentonite mixture with technological gaps

A compacted mixture of Callovo-Oxfordian (COx) claystone/MX80 bentonite is a promising sealing material in the French Cigéo project for radioactive waste disposal at great depth. Over time, the sealing material will be saturated by the site water originating from the surrounding rock and the water chemistry can affect the performance of the sealing material. In this study, the effects of water chemistry and technological gaps on the hydro-mechanical property were investigated by performing hydration tests on the compacted claystone/bentonite mixture with technological gaps using deionised water and synthetic site solution for 10 and 590 days. After the tests, the dry density, water content and microstructure distributions were also determined to complement the program. Results show that the synthetic site solution did not affect the swelling pressure and hydraulic conductivity after a short term of 10 days. With the increase of hydration time, the specimens with technological gaps tended to be homogeneous, resulting in a reduction of hydraulic conductivity. Because of the cation exchanges and the decreased diffuse double layer thickness, the synthetic site solution reduced the swelling pressure and increased the hydraulic conductivity in long term, especially for the specimens with lower global dry densities.

Electrochemistry Under Microscope: Measuring the Thickness of Diffusion Layer in Thin Layer Electrochemical Cells with Optical Microscope

Electrochemical reactions are heterogeneous in nature, and an electrochemical reaction involves mass transfer from the bulk solution to the electrode surface, heterogeneous electron transfer at the electrode surface, and mass transfer back to the bulk solution. There are three modes of mass transport to the working electrode surface: diffusion, migration, and convection. Among these modes, diffusion is the one that involved in all electrochemical processes and sometimes is the sole mode of mass transfer. Thus, the key concept underpinning electrochemical science are that of the diffusion and diffusing layer, the region in the vicinity of an electrode layer that affected by electrode reaction. The thickness of diffusion layer is an important parameter in understanding chronoamperometry, cyclic and hydrodynamic voltammetry. The thickness of this zone can be measured either by simulation or analysis of the electrochemical responses. But interpretation of the concentration profiles and related current time responses require understanding the Fick’s law and its related mathematical representations. Although these basic are crucial in understanding electrochemistry but their mathematical representation is challenging specially for the students or scientists who are new to the field. The thickness of diffusion layer for a typical electroanalytical experiment varies between 20 to 200 micrometers, depends on the time of experiment. The size is not visible to the human eye easily but doesn’t required expensive research-grade microscopes and can be seen using even kids grade microscopes with 100 times magnification. Here we have developed a thin-layer electrochemical cell compatible with low-cost microscopes to demonstrate a microscopy activity in which students measure the thickness of micrometer-sized diffusion layer. The electrode processes involve the reactions of redox indicators with sharp color changes or acid base indicators with sharp color change in response to electrochemically generated proton or hydroxide, from water electrolysis. This Imaging tools advance electrochemistry education by enabling students to see the electrode processes that are not visible to the human eye without any research grade instrumentation. This device also aids in the explicit visualization and interpretation of a typical concentration profiles near the working electrode surface for chronoamperometry, cyclic and hydrodynamic (e.g., rotated disk) voltammetry experiments.

Effect of Temperature and Cl- Concentration on the Catalytic Activity of a Stainless-Steel Oxide

Galvanic couples between high-strength, noble metal fasteners, such as those made out of titanium or stainless-steel, can cause extensive damage in aluminum alloy airframes. These galvanic couples can drive higher rates of oxidation than would be expected for general corrosion and the corrosion damage can become a stress concentration point for the nucleation of cracks. Understanding the factors that govern the catalytic activity for the oxygen reduction reaction on stainless-steel oxides is important for enabling predictions of the amount of corrosion that can occur for a given set of environmental conditions. We have focused on measurements of the diffusion limited current density for oxygen reduction on UNS S13800 stainless-steel in solutions of varying NaCl concentrations from 0.1 M to 4.6 M and air temperatures ranging from 10oC to 40oC. Estimates of the oxygen diffusion limited current were obtained from potentiodynamic polarization measurements under the various electrolyte concentrations and environmental conditions. The experimental data was then compared with calculated ilim values for the same conditions and assuming a constant diffusion layer thickness. Regression and physics-based models for the effect of environmental conditions on the experimental data have been developed and are used to estimate the effect of the electrolyte on the stainless-steel oxide.The U.S. Naval Research Laboratory sponsored this work under work unit 1L66. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, or the U.S. government.

A microbubble-assisted rotary tubular titanium cathode for boosting Fenton’s reagents in the electro-Fenton process

To improve cathodic H2O2 accumulation and Fe3+ reduction synchronously in the electro-Fenton (EF) process, a microbubble-assisted rotary tubular titanium cathode (MRTTC) was designed for the first time. By utilizing this MRTTC, H2O2 accumulation improved by 4.05-fold, along with a 200% enhancement in iron reduction compared to the conventional EF process. This promotion is mainly attributed to a considerably higher oxygen mass transfer, which reduces the thickness of the adhered diffusion layer. The oxygen mass transfer coefficient (KLa) also improved from 0.0073 s−1 to 0.012 s−1 at a rotational speed of 300 rpm. In addition, the microbubble-assisted cathode further improved the KLa to 0.047 s−1. The synergistic effect between the rotating and microbubble-assisted cathodes further intensified H2O2 accumulation in MRTTC. Apart from H2O2 promotion, the iron reduction rate was elevated because the newly formed O2-• provided an additional reduction pathway for Fe3+ reduction in addition to the cathodic path. The effectiveness of MRTTC was confirmed by treating a benchmark organic pollutant, sulfamerazine (SMR), where approximately 100% SMR decay was obtained in 3 h. The results show that MRTTC is a novel and promising design in EF for antibiotic wastewater treatment.

Impedance scaling for gold and platinum microelectrodes

Objective. Electrical measurement of the activity of individual neurons is a primary goal for many invasive neural electrodes. Making these ‘single unit’ measurements requires that we fabricate electrodes small enough so that only a few neurons contribute to the signal, but not so small that the impedance of the electrode creates overwhelming noise or signal attenuation. Thus, neuroelectrode design often must strike a balance between electrode size and electrode impedance, where the impedance is often assumed to scale linearly with electrode area. Approach and main results. Here we study how impedance scales with neural electrode area and find that the 1 kHz impedance of Pt electrodes (but not Au electrodes) transitions from scaling with area (r −2) to scaling with perimeter (r −1) when the electrode radius falls below 10 µm. This effect can be explained by the transition from planar to spherical diffusion behavior previously reported for electrochemical microelectrodes. Significance. These results provide important intuition for designing small, single unit recording electrodes. Specifically, for materials where the impedance is dominated by a pseudo-capacitance that is associated with a diffusion limited process, the total impedance will scale with perimeter rather than area when the electrode size becomes comparable with the diffusion layer thickness. For Pt electrodes this transition occurs around 10 µm radius electrodes. At even lower frequencies (1 Hz) impedance approaches a constant. This transition to r −1 scaling implies that electrodes with a pseudo-capacitance can be made smaller than one might expect before thermal noise or voltage division limits the ability to acquire high-quality single-unit recordings.

Facile co-removal of As(V) and Sb(V) from aqueous solution using Fe-Cu binary oxides: Structural modification and self-driven force field of copper oxides

Currently, the environmental and ecological damage caused by As(V) and Sb(V) co-contamination has attracted widespread attention worldwide. Due to the similar intrinsic structure configuration and electrostatic repulsion of As(V) and Sb(V), the long-standing issue of their low co-removal capacity remains unresolved. In this study, novel Fe-Cu (FC) binary materials with varied Fe/Cu proportions were synthesized via a simple co-precipitation method to co-eliminate aquatic As(V) and Sb(V). A 2/1 ratio of Fe/Cu was determined to be a suitable proportion with a higher co-adsorption capacity, specifically 70.9 mg·g−1 for As(V) and 94.3 mg·g−1 for Sb(V). Detailed morphological and structural analyses indicated that the FC material gradually changed from microscale aggregates to nanoscale spheres with increasing Cu content, accompanied by an increasing crystalline degree and higher surface area. Additionally, the transformation of amorphous ferrihydrite (FO) into FeO(OH) was suppressed by Fe-Cu complexion during the co-adsorption process, in which ferrihydrite (FO) had more adsorption sites than FeO(OH). In addition, the addition of Cu promoted the pHpzc of FC materials from the acidic range into the neutral or alkaline range. The increased potential difference of FC materials accelerated the As(V) and Sb(V) diffusion rate and effectively offset native electrostatic repulsion, which exhibited a considerable effect than the adsorption sites. Through detailed kinetic data analysis, it was determined that the proportion of the diffusion layer thickness around Sb(V) was suppressed to the As(V) level, and the adsorption kinetics of the two species were both promoted by the self-driven force field. All the results indicated that the co-adsorption capacity depended on the coupling contribution of Fe and Cu, where Fe oxide acted as the major adsorption potential and Cu provided a self-driven force for As(V) and Sb(V) diffusion. This study may provide a novel prospective for homogeneous metal ion co-removal.

Impact of bedding plane direction and type of plastic microparticles on stiffness of inherently anisotropic gap-graded soils: Index, wave propagation and micromechanical-based interpretations

Sand-clay mixtures are often encountered in natural deposits and compacted soils in earth structures in the form of gap-graded soils. They commonly display anisotropic fabric which markedly influences their wave propagation and mechanical properties. In this study, a comprehensive experimental program is carried out to thoroughly examine the influence of the inclined bedding plane and clay type on the small strain shear modulus (Gmax) of granular soils mixed with various portions of low and high plasticity fines, controlling in this way the double diffuse layer of the microparticles. To this end, cylindrical samples containing different fines contents are prepared with their corresponding optimum water contents and maximum dry densities in a standard proctor mold. Undisturbed specimens are then extracted by the continuous coring method and wave propagation tests with bender elements are applied at different angles with respect to the bedding plane (α-direction), evaluating in this way shear wave velocity and Gmax. The test results suggested that α-direction has a significant influence on Gmax of sand-clay mixtures and this influence is amplified at lower fines contents and higher confining pressures. Mixtures of sand with high plasticity clay are observed to have lower degree of anisotropy compared to those containing low plasticity clay, specifically at lower confining pressures. The contribution of fines in the load carrying structure of the solid skeleton decreases for high plasticity clays; the phenomenon which is primarily attributed to the thick diffuse double layer of the microparticles. This trend becomes more pronounced at higher contents of fines inclusion where the sand-in-fine structure dominates. A previously proposed expression for sands is rectified and extended so as to develop a Gmax model of sand-cohesive clay mixtures taking into account the bedding plane direction. Micromechanical-based interpretations are further elaborated considering the intervention of the microparticles on the contact response of sand grains altering their normal and tangential contact behavior, so as to supplement the findings, from the wave propagation tests, with multi-scale insights.