Diffuse Double Layer Research Articles

Research on Measuring Pure Membrane Electrical Resistance under the Effects of Salinity Gradients and Diffusion Boundary Layer and Double Layer Resistances.

Forward osmosis membranes are an emerging technology with great potential applicability in energy-efficient wastewater treatments and the differentiation between two solutions. Such solutions often differ in their concentrations or compositions. In this study, the membrane electrical resistances of three different membranes, including cation or anion-exchange membranes and forward osmosis membranes, were analyzed by Luggin capillary coupled with AC impedance spectroscopy (EIS) so as to obtain the real membrane and ion transfer impedance values near the membrane interface. The results reveal that the membrane impedance obtained by both the DC and AC approaches decreased as the lowest external solution concentration increased. Furthermore, the relationship between the membrane conductivity and the internal salt solution concentration was also investigated. It can be seen that the external ion concentration is directly proportional to the free ion concentration in the membrane, and the free ion concentration in the membrane is closely related to the membrane electrical resistance.

Experimental study and surface complexation modeling of non-monotonic wettability behavior due to change in brine salinity/composition: Insight into anhydrite impurity in carbonates

Wettability alteration as the main mechanism of improved oil recovery in carbonates during low salinity/engineered water flooding (LS/EWF) is a complex phenomenon due to high heterogeneity of rock. During LS/EWF, wettability changes when electrochemical interactions at carbonate-brine interface happen. Anhydrite impurity in carbonates is one of the most important parameters affecting the electrochemical interactions at the rock-brine interface and the wettability alteration process. Therefore, the success of LS/EWF in carbonate reservoirs lies in perceiving the role of impurities such as anhydrite, from a geochemical and dissolution point of view.Modified flotation tests (MFT) were performed on two rock samples (calcite and a mixture of calcite and 10 wt% of anhydrite) to assess the wettability alteration which showed a non-monotonic wettability effect as the presence of anhydrite lowered the optimum salinity from 10-times diluted seawater (SW/10) to SW/25. Mineral dissolution and zeta potential measurement showed no comprehensive correlation with the wettability state of particles obtained from MFTs. To resolve this challenge, a diffuse double layer (DDL) based surface complexation model (SCM) was developed using PHREEQC geochemical package to shed light on the anhydrite role during the desorption of oil acidic components from the rock surface imposed on various brines. The model was validated using measured zeta potential. The DDL-based SCM proposed an anhydrite dominant window in which the presence of anhydrite, its dissolution, and in-situ supply of SO42-, lead to altering the wettability to more water wetness.This work is significant to explain the effect of anhydrite impurity on the performance of low salinity water injection using experimental data and DDL-based SCM, while the simple techniques of rock dissolution and zeta potential are not able to justify the non-monotonic behavior of wettability of the carbonates. Also, it introduces a new quantitative parameter of the wettability state of carbonate surface by considering the adsorbed carboxylic acid components on the calcite surface reported by SCM.

Numerical Simulation of Electrochemical Systems by Coupling Analytical Calculation of the Diffuse Double Layer and Finite Element Description of Solution Bulk

AbstractNumerical modelling of electrochemical systems covers length scales from the nanometers up to the macroscopic scale. With finite element methods, the mesh must be extremely fine to describe the diffuse double layer, thus increasing the needed computational resources. We propose a method to describe the diffuse double layer by analytical equations, expressed as boundary conditions for the partial differential equations describing the solution bulk. We apply the method to a one‐dimensional system, i. e. to a cell with plane parallel electrodes, in the presence of a redox couple and a supporting electrolyte. We provide evidence of the precision of the method.

Square-Wave Electrogravimetry Combined with Voltammetry Reveals Reversible Submonolayer Adsorption of Redox-Active Ions.

Square-wave voltammetry on electrolytes containing reversible redox pairs in solution was complemented by acoustic microgravimetry, where multifrequency lock-in amplification provides for a time resolution of 2.5 ms and a frequency resolution after accumulation of 12 mHz. The instrument operates similar to a quartz crystal microbalance with dissipation monitoring (QCM-D). The use of square-waves rather than linear ramps makes the analysis more transparent because it reduces the contribution of non-Faraday currents. Also, square-wave electrogravimetry determines the rates of mass transfer with much better sensitivity than its counterpart based on linear voltage ramps. The shifts of frequency and bandwidth are in agreement with the Sauerbrey prediction, meaning that the overtone-normalized frequency shifts, Δf/n, are similar on the different overtones and that the shifts in half bandwidth, ΔΓ, are smaller than the shifts in frequency. Small deviations from the Sauerbrey prediction presumably result from the softness of the adsorbed layer. Because the response time of the QCM signals is much longer than the RC time of double layer recharging as determined with electrochemical impedance spectroscopy (EIS), interpretation in terms of adsorption and desorption is more plausible than interpretation in terms of changed viscosity in the diffuse double layer. Ions of methyl viologen (MV) were found to adsorb to the electrode surface more strongly in the state with a single charge than in the fully oxidized state carrying two charges. The difference in apparent thickness between the oxidized and the reduced state was up to 2 nm, depending on concentration. The gravimetric results obtained on flavin adenine dinucleotide (FAD) depended on pH. At neutral pH, adsorption was largest close to the redox potential. Presumably, the adsorbed molecules are semiquinones, that is, are the intermediates of the underlying two-electron process.

On the efficiency of convective mixing in a Y-shaped channel

Continuous-flow microfluidic devices are widely used in microbiology, fine organic synthesis, pharmaceuticals, biomedicine, etc. Most applications require rapid mixing of the fluids that pass through the microfluidic chip. The mechanism of natural diffusion is not always efficient due to limitations on the length of the channel. In this work, we numerically study the efficiency of using various mechanisms of natural convection for the mixing of fluids entering the microfluidic chip. Solutions typically differ in buoyancy and diffusion rates of dissolved components, making them sensitive to gravity-dependent instabilities such as Rayleigh-Taylor convection, double diffusion and diffusion layer convection. We consider a Y-shaped microchannel, which is, on the one hand, the simplest, and, on the other hand, a typical element of a microfluidic network. We assume that two miscible solutions independently flow into a common channel where they come into contact. For each type of instability, we numerically estimated the characteristic channel length, after which complete mixing of the solutions occurs. The simulations were performed in the framework of both 2D and 3D models. Finally, we compare the numerical results with the experimental data obtained recently.

Effect of seepage conditions on the microstructural evolution of loess across north-west China

SummaryLoess features metastable microstructure and is deemed susceptible to chemical contaminant permeation. However, studies on the loess permeability evolution under water and chemical environments are remarkably limited. In this study, the response of the loess to the water and sodium sulfate seepages was analyzed using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM) tests. The permeability evolution characteristics were identified, and its underlying mechanisms were revealed from aspects of the diffuse double layer (DDL) theory and physiochemical actions. The discharge of Mg2+ and precipitation of calcium carbonate, referred also to as the dedolomitization, degraded the macro permeability when subjected to the water seepage test. The salt-induced swelling, induced by the intrusion of Na+ into the DDL, caused an increase in the micropore fraction under the sodium sulfate seepage test, thereby increasing the macro permeability.

Mechanisms leading to the formation of double-diffusive layers during unidirectional solidification of aqueous NH4Cl solution

Double diffusive layers (DDLs) are one of the flow patterns which arise due to the coupled effects of thermal and solutal gradients and find relevance in large-scale applications. We report observations of DDLs using unidirectional solidification of a binary mixture, which naturally introduces thermal and solutal buoyant flows. The convection rolls at the opposite sidewalls, having an opposite sense of rotation, ultimately merge to form the DDLs that expand throughout the breadth of the test chamber. Our research on exploring the mechanisms of DDLs uses a combination of nonintrusive techniques capturing full-field measurements in real-time to obtain insights on their evolution.

Consolidation Properties of Soil/Modified Bentonite Backfill in Salt Solution

The consolidation properties of soil-bentonite (SB) backfills containing 20% of the weight of sodium-hexametaphosphate-modified calcium bentonite (SHMP-20CaB) and prepared with 0 mM to 1000 mM calcium chloride (CaCl2) solution were evaluated. The results indicated that both consolidation stress and CaCl2 concentration had significant effects on the consolidation behaviors of the SHMP-20CaB backfill. In general, an increase in consolidation stress resulted in a decrease in the void ratio (e) and in the coefficient of volume change (mv), alongside an increase in the coefficient of consolidation (cv). The increased CaCl2 concentration yielded a dropped void ratio, and a nonlinear decreased compression index (Cc) and rebound index (Cs), respectively, from 0.18 to 0.13 and from 0.022 to 0.010, and a nonlinear increase cv. A threshold CaCl2 concentration of 100 mM was observed at the inflection points of the Cc, Cs, and cv. In contrast, the mv was insensitive to the CaCl2 solutions. The deterioration in e, Cc, and Cs and the increase in cv were the result of a compressed diffuse double layer of the bentonite by the CaCl2 solution, which thus exerted certain negative effects on the consolidation behaviors of the SHMP-20CaB backfill.

Effect of pH, ionic strength, and temperature on the adsorption behavior of Acid Blue 113 onto mesoporous carbon

Mesoporous carbon (MC) derived from cassava starch was used to remove Acid Blue 113 azo dye from aqueous solutions. The influence of temperature, pH, ionic strength, and the adsorbent dose was investigated in a set of batch experiments. Experimental data showed that Acid Blue 113 adsorption was higher in the acid pH range than in the alkaline one, that dye adsorption increases when the ionic strength and temperature increase, and that adsorption results presented a good correlation with the Langmuir isotherm model. The adsorption capacity of MC was 295mgg-1, at pH = 7.0 and 298K, respectively. Zeta potential (ζ) showed the compression of the diffuse double layer of adsorbent with an increase in temperature and ionic strength, promoting the decrease of electrostatic repulsion between the negatively charged surface of the carbon particles and the anionic dye. Thermodynamic results demonstrate that the adsorption process was spontaneous and endothermic. Moreover, for the first time, this work has demonstrated that the pH, temperature, and ionic strength of the aqueous medium are also able to change the surface charge of carbon-based adsorbents and surely influence the adsorption capacity. Finally, the regeneration of the adsorbent by the photo-Fenton reaction regenerated the adsorption capacity of the adsorbent without generating secondary pollution to the environment.

A modified Kozeny–Carman equation for predicting saturated hydraulic conductivity of compacted bentonite in confined condition

Kozeny–Carman (KC) equation is a well-known relation between hydraulic conductivity and pore properties in porous material. The applications of KC equation to predicting saturated hydraulic conductivities of sands and non-expansive soils are well documented. However, KC equation is incapable of predicting saturated hydraulic conductivity of expansive soil (e.g. bentonite) well. Based on a new dual-pore system, this study modified KC equation for improving the prediction of saturated hydraulic conductivities of bentonites. In this study, an assumption that inter-layer space (micropore) has limited effect on fluid flow performance of compacted bentonite was adopted. The critical parameters including total porosity and total tortuosity in conventional KC equation were replaced by macroporosity and tortuosity of macropore, respectively. Macroporosity and microporosity were calculated by basal spacing of compacted bentonite, which was estimated by assuming that specific surface area is changeable during saturation process. A comprehensive comparison of bentonite's saturated hydraulic conductivity predictions, including modified KC equation proposed in this study, conventional KC equation, and prediction method based on diffuse double layer (DDL) theory, was carried out. It was found that the predicted saturated hydraulic conductivity of bentonites calculated using modified KC equation fitted the experimental data better than others to a certain extent.

DENSE: DiElectric Novel Soil Evaluation System to Electrochemically Profile Soil Matrices

Soil health and quality monitoring in the near past has been highly qualitative and speculative with more recent advancements still trying to fill the void of a holistic soil profile. In this work, an electrochemical approach has been proposed to build a unique quantitative model—“DENSE” that probes the soil diffuse double layer (DDL) dielectric to evaluate a thorough interfacial profile of the soil matrix. Room Temperature Ionic Liquid (RTIL) thin film was employed to modify the electrode probe to leverage as an effective transducer for a system driven by electrostatic interactions and charged diffusive behavior. The soil is surveyed based on fundamental understandings of electrochemistry and thereby obtaining equivalent metrics using chronoamperometry, electrochemical impedance spectroscopy and chronocoulometry modalities to determine interfacial characteristics at the soil-water double layer. Capacitive dominant nature is exhibited in more coarse-drier soils similar to a case of non-faradaic electrochemical characteristics while the presence of water in the composite system seems to induce a slightly more charge transfer behavior indicative of mobile electrochemically active species. This gives a fundamental discernibility between the soil physico-chemical state corresponding to the output that is modelled using an impedemetric circuit fit and serves as a soil dielectric probing mechanism.

Hydraulic conductivity of novel geosynthetic clay liner to bauxite liquor from China: Modified fluid loss test evaluation

A modified sodium bentonite geosynthetic clay liner (GCL) designed for acid-and-alkaline resistance was evaluated for its potential application in the containment of bauxite residue leachate. A modified fluid loss test was employed to quickly evaluate the hydraulic conductivity (k) of the GCL using distilled water, tap water, and four bauxite liquors (BLs, leachate from bauxite residue reservoirs). The effects of swelling capacity of bentonite, prehydration, hydraulic gradient (i), ionic strength (I), and relative abundance of monovalent and multivalent cations (RMD) on the hydraulic conductivity of the GCL were analyzed. The results indicated that the BLs significantly decreased free swell index of the bentonite. As compared to increasing i, prehydration obviously enhanced hydraulic performance of the GCL. The four BLs increased k of the GCL by a factor of 4–12 relative to the tap water permeation condition, and the resultant k exceeded upper limit of 5.0 × 10−11 m/s for GCLs. The increase in k was attributed to compression in diffuse double layer of the bentonite and dissolution in clay minerals in ion-rich and hyperalkaline BLs, manifesting that further modification on the GCL is needed. The I was found a better indicator than the RMD on correlation with chemical compatibility of the GCL.

Nonlinear electric response of the diffuse double layer to an abrupt charge displacement inside a biological membrane

In order to elucidate the old, still unsolved problem of how the diffuse electric double layer responds to an abrupt, intramolecular charge displacement inside a biological membrane, we investigated the fastest components of the light-induced electric signals of bacteriorhodopsin and its mutants, in numerous ionic and buffer solutions. The obtained data for temperature and solute concentration dependence were interpreted as a consequence of changes in the capacity of the diffuse double layer surrounding the purple membrane. The possible physiological consequences of this so far not demonstrated phenomenon are discussed.

Effect of water chemistry on the hydro-mechanical behaviour of compacted mixtures of claystone and Na/Ca-bentonites for deep geological repositories

In the French deep geological disposal for radioactive wastes, compacted bentonite/claystone mixtures have been considered as possible sealing materials. After emplacement in place, such mixtures are hydrated by the site solution as well as the cement solution produced by the degradation of concrete. In this study, the effects of synthetic site solution and cement solution on the hydro-mechanical behaviour of compacted mixtures of claystone and two types of bentonites (MX80 Na-bentonite and Sardinia Ca-bentonite) were investigated by carrying out a series of swelling pressure, hydraulic conductivity and mercury intrusion porosimetry (MIP) tests. It was found that for the MX80 bentonite/claystone mixture hydrated with synthetic site solution, the swelling capacity was reduced compared to the case with deionised water owing to the transformation of Na-montmorillonite to multi-cation dominant montmorillonite by cation exchanges. For the Sardinia bentonite/claystone mixture, the similar increasing rate of swelling pressure was observed during the crystalline swelling process for different solutions, suggesting insignificant cation exchanges. Additionally, the cations in the synthetic site solution could reduce the thickness of diffuse double layer and the osmotic swelling for both MX80 bentonite/claystone and Sardinia bentonite/claystone mixtures. The large-pore volume increased consequently and enhanced water flow. In the cement solution, the hydroxide could also dissolve the montmorillonite, reducing the swelling pressure, and increase the large-pore volume, facilitating the water flow. Furthermore, the decrease of swelling pressure and the increase of hydraulic conductivity were more significant in the case of low dry density because of more intensive interaction between montmorillonite and hydroxide due to the high permeability.

Oil-in-water emulsion separation: Fouling of alumina membranes with and without a silicon carbide deposition in constant flux filtration mode

Ceramic membranes have drawn increasing attention in oily wastewater treatment as an alternative to their traditional polymeric counterparts, yet persistent membrane fouling is still one of the largest challenges. Particularly, little is known about ceramic membrane fouling by oil-in-water (O/W) emulsions in constant flux filtration modes. In this study, the effects of emulsion chemistry (surfactant concentration, pH, salinity and Ca2+) and operation parameters (permeate flux and filtration time) were comparatively evaluated for alumina and silicon carbide (SiC) deposited ceramic membranes, with different physicochemical surface properties. The original membranes were made of 100% alumina, while the same membranes were also deposited with a SiC layer to change the surface charge and hydrophilicity. The SiC-deposited membrane showed a lower reversible and irreversible fouling when permeate flux was below 110 L m−2 h−1. In addition, it exhibited a higher permeance recovery after physical and chemical cleaning, as compared to the alumina membranes. Increasing sodium dodecyl sulfate (SDS) concentration in the feed decreased the fouling of both membranes, but to a higher extent in the alumina membranes. The fouling of both membranes could be reduced with increasing the pH of the emulsion due to the enhanced electrostatic repulsion between oil droplets and membrane surface. Because of the screening of surface charge in a high salinity solution (100 mM NaCl), only a small difference in irreversible fouling was observed for alumina and SiC-deposited membranes under these conditions. The presence of Ca2+ in the emulsion led to high irreversible fouling of both membranes, because of the compression of diffusion double layer and the interactions between Ca2+ and SDS. The low fouling tendency and/or high cleaning efficiency of the SiC-deposited membranes indicated their potential for oily wastewater treatment.

Effect of sodium montmorillonite clay on the kinetics of CH4 hydrate - implication for energy recovery

Silty-clayey host sediments have been widely identified in naturally-occurring hydrate-bearing sediments in the South China Sea. However, the effect of clay minerals on the kinetics of CH4 hydrate (MH) formation and dissociation remains unknown and warrants further investigation. The objective of this paper is to elucidate the effect of montmorillonite (MMT) clay particles on the formation and dissociation kinetics of MH. A series of experiments involving kinetics and morphological observations were designed to investigate the kinetics of MH in MMT suspensions with mass fractions ranging from 0.1 wt% to 9.0 wt%. The experimental results reveal that MMT promotes nucleation but delays the growth kinetics of MH. The diffuse double layer (DDL) on clay minerals is quantified to explain the observed kinetic behavior. The electric field induced on the clay surface resulted in a shorter induction time. The morphology observed indicated that MH initially forms at the gas–liquid interface and develops upward along the reactor surface, resulting in the hydrate-clay stratification behavior. The upward migration of water due to hydrate growth promotes the agglomeration of MMT clay and reduces the conversion of free water to hydrate. The findings of this study are essential in understanding the mechanisms of the naturally occurring negatively-charged clay particles on the kinetics of MH. Moreover, the results have implications for the occurrence of MH in clay-rich sediments in nature and the development of energy-efficient recovery strategies from silty-clayey hydrate-bearing sediments.

Enzyme-assisted dewatering of oil sands tailings: Significance of water chemistry and biological activity

Remediation of oil sands tailings is a crucial environmental aspect of the Canadian oil sands industry. Because of the poor released water quality and slow consolidation of tailings, engineered non-natural strategies to improve the consolidation of tailings are warranted. Here, we investigated enzymatic treatment (using cellulase, protease, and lysozyme) to accelerate the dewatering of fluid fine tailings (FFT). Our findings illustrated that lysozyme (0.5% and 1%) significantly improved FFT dewatering by increasing the water recovery (WR) up to 20% compared with the other enzymes (up to 12%) or the control (2%). Moreover, lysozyme treatment resulted in the highest increase in ionic strength (0.038 to 0.1 mol L−1), decrease in diffuse double layer (DDL) thickness (1.54 × 10−7 to 9.40 × 10−8 cm), and increase in zeta potential (−34.7 to −14.8 mV). Increased methane production was observed for cellulase (0.5% and 1%), lysozyme (0.5% and 1%), and protease (0.5%). The enhanced dewatering could be linked to the ebullition of methane gas resulting from the methanogenic activity, which created pathways for the trapped water release. In addition, the dissolution of carbonate minerals during the release of methane gas increased ionic strength and decreased the DDL of the FFT. Lysozyme 1% treatment was also the most effective in reducing naphthenic acid fractions (1934.6 to 243 ng mL−1); however, the released water had high toxicity toward Vibrio fischeri and had a slight decrease in microbial populations. This study provides a fundamental insight into enzymatic treatment for oil sand tailings.

Evaluation of the Hydraulic Conductivity of Geosynthetic Clay Liners

Abstract The hydraulic conductivity of geosynthetic clay liners (GCLs) is not fully understood and certain gaps in knowledge are still present, such as the effect of coupled mechanical and chemical processes. The current study aimed to develop a simplified mathematical model to predict the hydraulic conductivity of GCLs, particularly regarding the coupled effects of mechanical and chemical processes. Based on Darcy's Law and Poiseuille’s Law, the method combines diffuse double layer (DDL) theory and fractal theory. External factors such as confining pressure and the concentration of the permeating solution, and inherent properties such as exchangeable cations, ionic radius, montmorillonite surface fractal dimension, the distance between two montmorillonite layers (m) after swelling at the exchangeable cation i (i denotes the primary exchangeable cations, such as Na+, Ca2+, K+, and Mg2+ in bentonite), density, and coefficient of viscosity of interlayer water between two montmorillonite layers, were considered. The proposed theoretical model gave relatively accurate predictions. A practical estimate of GCL hydraulic conductivity was also derived. The predictions were compared with experimental results and good qualitative agreement was found. From the experimental results, the proposed prediction model has a maximum deviation of ~1:10–10:1, and the empirical model has a mean deviation of ~1:15–15:1.

Liquid limit of marine soft clay encountering desalination percolation process

The diffuse double layer (DDL) theory indicated that the liquid limit (LL) of clays decreases based on pore-water salinity during salinization tests conducted in a laboratory. However, LL also decreased based on pore-water salinity when authors conducted geological investigations in the regression depositional zone in Lianyungang city, contradicting the DDL theory. This contradictory finding is inferred due to the coupled effect of salinity leaching and erosion of the colloid during a desalination percolation process. To confirm this hypothesis, 3 model tests of distilled water percolation and seven accelerated percolation tests were designed in this study, and the soil/water characteristics after desalination percolation were determined. The results reveal that the pore-water salinity leaches are combined with the colloidal erosion during the desalination percolation process, leading to the gradual decrease in LL based on the reduction of salinity and suggesting that the dominant effect is the colloidal erosion. At the initial stage of the desalination process, the colloids easily erode from the clay matrix due to the weakened salinity inhibition on the colloid release, leading to the seepage channel expansion, and hence the increase in colloid dimension and concentration in leachates throughout percolation. Based on the further decrease in pore-water salinity, the colloids in the clay matrix continuously release and inversely clog the inner channels of the soils, decreasing the colloid dimension and concentration in leachates. When the percolation process is sufficiently long, all salinity would be leached out from the clay matrix, and only the erosion of the colloid would be identified.

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.