Mass Diffusion Research Articles

Constructing 3D magnetic MXenes/chitosan composites to reinforce U(VI) and Cr(VI) trapping: Excavating from performance to mechanism

Herein, we proposed a novel chitosan-induced self-assembly strategy to construct three-dimensional magnetic MXenes/chitosan composites to eliminate U(VI)/Cr(VI). Spectroscopic analysis elucidated that the magnetic composites exhibited a three-dimensional network structure, and the abundant –OH and –NH2 groups played a pivotal role in boosting elimination performance of the composites. Batch experiment demonstrated that the synergistic effect of the amino and hydroxyl groups contributed greatly to adsorption, and the maximum adsorption capacity of U(VI) and Cr(VI) was up to 106.2 and 83.1 mg/g, respectively. Meanwhile, the kinetic studies showed that an obvious transition between diffusion control and mass transfer control, and the adsorption behavior was endothermic and spontaneous. Furthermore, a comprehensive synergistic adsorption mechanism was proposed based on the systematic spectra analysis. Namely, the target contaminant was rapidly immobilized on the 3D composites via electrostatic interaction, then the U(VI) was complexed with the amino groups, while a part of Cr(VI) was reduced to Cr(III) by the hydroxyl groups. Thus, this study not only paved the way for mitigating the increasing heavy metal/radionuclide contamination, but also promoted the application of MXexes-based materials in the field of environmental remediation.

Spatiotemporal diffusion metamaterials: Theories and applications

Diffusion metamaterials with artificial spatial structures have significant potential in controlling energy and mass transfer. Those static structures may lead to functionality and tunability constraints, impeding the application scope of diffusion metamaterials. Dynamic structures, adding the temporal dimension, have recently provided a new possibility for electric charge and heat diffusion regulation. This perspective introduces the fundamental theories and practical constructions of spatiotemporal diffusion metamaterials for achieving nonreciprocal, topological, or tunable properties. Compared with traditional static design, spatiotemporal modulation is promising to manipulate diffusion processes dynamically, with applications of real-time thermal coding and programming. Existing spatiotemporal diffusion explorations are primarily at macroscopic systems, and we may envision extending these results to microscale and other physical domains like thermal radiation and mass diffusion shortly.

Effect of thermodiffusion on compositional grading in hydrocarbon reservoirs: Insights from field cases and simulations

Heterogeneity of fluid composition in vertical and lateral directions is observed in many reservoirs, known as compositional grading. It is necessary to evaluate the diffusion forces for modeling this phenomenon. In addition to the chemical and gravity forces, the temperature gradient causes mass diffusion. In the present investigation, the accuracy and reliability of the Haase theory and the Shukla-Firoozabadi method in thermodiffusion modeling are studied comprehensively using field samples from light to heavy-oil reservoirs. On the one hand, the Shukla-Firoozabadi model results in compositional grading reduction by thermodiffusion; on the other hand, the Haase theory reveals enhancement and attenuation effects of thermodiffusion depending on reference enthalpy quantification. A new approach, which employs the reference enthalpies of pseudocomponents as tuning parameters, is proposed to improve the results of the Haase theory. The proposed procedure is also validated using field cases. In addition, the experimental field information compared with isothermal and nonisothermal models are studied to answer the question of "how" and to "what extent" thermodiffusion contributes to compositional grading. Results demonstrated the limited attenuation effect of thermodiffusion in light oils, while this phenomenon leads to severe enhancement of compositional grading in the medium and heavy-oil reservoirs. In heavy oils, the thermodiffusion effect is more pronounced on heavy pseudocomponents rather than light paraffinic species. Finally, the practice of compositional gradient model optimization, which is of particular interest to reduce the uncertainty in fluid initialization and to address reservoir compartmentalization, is presented.

Effects of molecular interactions on the release of small molecules from silicone rubber

AbstractSilicone rubber is a good candidate as a carrier of small molecules in such applications as drug, fragrance and semiochemical delivery. When one matrix is loaded, the molecular interactions between the molecules and the matrix may affect their release rates. For this reason, it is important to investigate the effects of molecular interactions on release to better develop delivery systems with desirable release rates. In this study, the mass transfer and diffusion coefficients of three model molecules (i.e., octanol, octyl acetate, and octyl butyrate) were determined by monitoring their release from silicone rubber sheets loaded using headspace gas chromatography–mass spectrometry. Differential scanning calorimetry was used to characterize the plasticizing effects and Fourier transform infrared spectroscopy was used to characterize the molecular interactions. It was found that the release of octanol conformed to the Fickian diffusion pattern at 1 wt% initial concentration but deviated as the concentration increased due to hydrogen bonding between octanol and the silanol group of the silica filler. When two small molecules were released simultaneously, the effects of one molecule on the diffusion coefficient of the other differed, a result explained by the competing effects of plasticizing and hydrogen bonding.

Effect of Micro-Cracks on Chloride Ion Diffusion in Concrete Based on Stochastic Aggregate Approach

For concrete structures in offshore areas, chloride ion erosion is one of the main factors affecting durability. It is crucial to evaluate the chloride ion permeability resistance of concrete structures. In this paper, a finite element simulation of the chloride ion diffusion process in concrete is conducted. A mass diffusion finite element model based on a random aggregate approach is established to investigate the influences of an aggregate, the interface transition zone, and micro-cracks on the chloride ion diffusion coefficients in concrete. The results show that the mass diffusion finite element analysis based on the exponential function model and the power function model can effectively simulate the chloride ion diffusion process in concrete. In addition, the data reveal that volume fraction and distribution aggregates considerably affect chloride ion diffusivity in concrete. Also, the interface transition zone significantly accelerates chloride ion diffusion in concrete. Moreover, this acceleration effect exceeds the barrier effect of an aggregate.

Unsteady MHD oblique stagnation point slip flow of maxwell fluid over an oscillating–stretching cylinder with Cattaneo–Christov double diffusion model

The unsteady oblique stagnation point slip flow of a Maxwell fluid over an oscillating–stretching cylinder with convective heat transfer is investigated. The evaluation of this phenomenon is facilitated by employing the Cattaneo–Christov double diffusion model. According to the oblique stagnation point flow characteristics, an ordinary differential equation about pressure is derived, and pressure terms can be corrected by solving this equation. The equations are subjected to the similarity transformation, resulting in a set of dimensionless equations. The homotopy analysis method (HAM) is subsequently utilized to obtain their solutions. The effects of various parameters on the velocity, temperature, and concentration are analyzed. It is observed that slip boundary conditions accelerate fluid flow by reducing resistance. Elevating the Schmidt number means a reduction in the mass diffusion, leading to a diminished concentration. The temperature distributions of different thermal relaxation time parameters intersect at a thermal equilibrium point. Moreover, three-dimensional plots depicting the evolution of velocity and temperature over time demonstrate excellent agreement with the flow behavior and boundary conditions. They contribute to the comprehension and prediction of time-dependent flow and heat transfer phenomena.

New insights into hydration shells in boosting marine uranium adsorption kinetics

Identifying mass transfer is essential for boosting adsorption kinetics. The internal and external diffusions serve as rate-limiting steps. The liquid film diffusion is commonly identified as external diffusion, while the mass diffusion across the ubiquitous hydration shell is generally ignored. Charged functional groups can bind water molecules strongly via electrostatically induced hydration. Herein, three representative polar polymers, including cation polymer (PDMAPAA), anion polymer (PSS) and zwitterion polymer (PSBMA), are crosslinked with polyamioxime (PAO) to synthesize radially aligned PAO-PDMAPAA, PAO-PSS, PAO-PSBMA, respectively, by a bidirectional freeze-casting. We focus on the effect of hydration structure and hydration dynamics on marine uranium extraction performance. It is found that the external diffusion in PAO-polar polymer adsorbents can be facilitated due to fast water exchange in dynamic hydration shell, leading to a reduced adsorption equilibrium time compared with the pristine PAO. However, the high hydration degree causes strong viscous shear and high dehydration barrier, which impede the complexation of UO22+ with binding sites of PAO. Although the hydration degree follows the order of PAO-PSBMA > PAO-PDMAPAA > PAO-PSS, PAO-PDMAPAA delivers the shortest equilibrium time required. In addition, the increasing temperature would weaken viscous shear in static hydration shell, which equivalently reduces the apparent friction of hydrated uranyl ions. Consequently, light-irradiated GO@PAO-PDMAPAA adsorbent, in which GO acts as a photothermal mediator, yields a shorter equilibrium time (∼18 days) than PAO-PDMAPAA together with a high uranium adsorption capacity of 12.24 mg-U/g-Ads in natural seawater. These findings provide valuable guidance for enhancing uranium adsorption kinetics associated with hydration shell.

A computational analysis on two phase Eyring-Powell (non-Newtonian) dust particles flow for heat and mass transfer phenomenon with variable thermal features

In this instigation, the heat and mass transfer pattern on Eyring-Powell nanofluid with suspension of dust particles is focused. For heat transfer phenomenon, the Eyring-Powell nanofluid capturing the variable thermal conductivity. Furthermore, the variable and mass diffusivity assumptions are entertained for summarizing the observations chemical reaction phenomenon. The flow is endorsed to magnetic force impact. The additional impact of Brownian motion and thermophoresis consequences have been adopted. The numerical computations with shooting technique are performed. The physical aspects of parameters have been observed graphically. Based on depicted observations, it is examined that the fluid velocity reduces due to interaction velocity parameter which reverse trend is noted for dust particles velocity. The temperature profile for dusty fluid particles enhanced due to interaction parameter for temperature. Furthermore, the concentration profile reduces due to concentration interaction parameter. Current results present applications in solar energy, thermal processes, heat transform devices cooling systems, plasma, manufacturing processes, energy systems, etc.

Control of Interface Migration in Nonequilibrium Crystallization of Li2SiO3 from Li2O-SiO2 Melt by Spatiotemporal Temperature and Concentration Fields.

During liquid-solid transformation, bulk mass and thermal diffusion, along with the evolved interfacial latent heat, work in tandem to generate interfacial thermodynamic and kinetic forces, the interplay of which decides the solidification velocity and consequently the solidified phase attributes. Hence, access to interface dynamics information in dependence of bulk transfer processes is pivotal to tailor the desired quantity of solid phases of unique compositions. It finds particular application for engineering concentrated Lithium (Li) phases out of Li-ion battery slags, thus generating a high value-added product from a conventional waste process stream. However, considerable challenge exists to predict the impact of the diverse external cooling rates on the evolving internal transfer processes and thus tuning solidification routes for achieving phases of interest. Hence, in this work, a thermodynamically consistent nonequilibrium model, by considering spatiotemporal temperature and concentration fields, is developed and applied to study solidification of Li2SiO3 from a Li2O-SiO2 melt that constitutes an important subsystem of the Li containing battery-recycling slags. The approach treats the sharp solid/liquid interface as a moving heat source. In the presence of different heat extraction profiles, it evaluates the spatial temperature heterogeneity and its implicit correlation to internal material fluxes resulting from maximization of dissipation and consequently the interrelation to interface velocities. Model calculations revealed that irrespective of the external cooling rate, for an initial short time duration, the magnitude of which increased with decreasing cooling rates, the interface velocities show a reducing trajectory directly relatable to the reducing thermodynamic forces due to localized interfacial temperature rise from the generated latent heat of fusion from the initial solidification. This is followed by a thermodynamically controlled regime, whereby for each cooling rate, the interface velocities increase until a maxima, the magnitude of which decreases with decreasing cooling rates. Finally, the interface propagation speeds decrease as controlled by the kinetic regime.

On the inaccuracies of point-particle approach for char conversion modeling

Char conversion is a complex phenomenon that involves not only heterogeneous reactions but also external and internal heat and mass transfer. Reactor-scale simulations often use a point-particle approach (PP approach) as sub-models for char conversion because of its low computational cost. Despite a number of simplifications involved in the PP approach, there are very few studies that systematically investigate the inaccuracies of the PP approach. This study aims to compare and identify when and why the PP approach deviates from resolved-particle simulations (RP approach). Simulations have been carried out for CO2 gasification of a char particle under zone II conditions (i.e., pore diffusion control) using both PP and RP approaches. Results showed significant deviations between the two approaches for the effectiveness factor, gas compositions, particle temperature, and particle diameter. The most significant sources of inaccuracies in the PP approach are negligence of the non-uniform temperature inside the particle and the inability to accurately model external heat transfer. Under the conditions with low effectiveness factors, the errors of intra-particle processes were dominant while the errors of external processes became dominant when effectiveness factors were close to unity. Because it assumes uniform internal temperature, the models applying the PP approach always predict higher effectiveness factors than the RP approach, despite its accurate estimation of intra-particle mass diffusion effects. As a consequence, the PP approach failed to predict the particle size changes accurately. Meanwhile, no conventional term for external heat transfer could explain the inaccuracy, indicating the importance of other sources of errors such as 2D/3D asymmetry or penetration of external flows inside the particles.

Mechanism of stress evolution in environmental barrier coatings considering the presence of crack channels

The roles of crack channel transitions and thermally grown oxides (TGO) in the failure of environmental barrier coatings (EBCs) should be clarified. Hence, in this study, the laws of penetration and delamination crack evolution were investigated using a virtual crack extension method for multilayer materials to gain insights into the transformation conditions with a framework for a coupled thermal/mechanical/chemical numerical model of TGO containing channel cracks developed based on this law. The framework integrates diffusion, TGO growth, and phase transformation of the oxides. A theoretical model for high-temperature gas corrosion of EBCs coupled with deformation, mass diffusion, and chemical reactions was proposed, with the results indicating that the cracks transitioned from vertical to exfoliation cracks in the ytterbium dibasic silicate (YbDS) layer. The coupling effect of cracks and stresses led to non-uniform growth of the TGO by affecting the oxygen diffusion and inflow rates. The y-direction stresses at the YbDS/TGO interface near the crack channel changed from compressive to tensile stresses after cooling, making the first location to undergo delamination failure. The proposed model can provide better guidance for designing EBCs systems and supporting the life prediction of EBCs in service environments.

Variation of (001) surface structures of strontium titanate single crystals caused by flash event under direct/alternative current electric fields

Flash events, which are electric power spikes occurring under an electric field and temperature above the threshold, can significantly accelerate mass diffusion over short periods of time. This study investigated the surface structure of the (001) surface of strontium titanate single-crystal treated by flash events caused by direct current (DC) and alternating current (AC) electric fields from a viewpoint of surface-related mass diffusion. The surface structuring caused by flash events was significantly different among DC and AC electric fields. The DC-flash event destabilized surface steps, causing them to wander and form surface mounds with a height of several tens of nanometers. In addition, numerous two-dimensional (2D) nuclei with a size of several nanometers are formed on the terraces. In contrast, the AC-flash event stabilized surface steps without the formation of surface mounds, causing them nearly straight with uniform terrace width at approximately 500 nm. 2D nuclei with a size of a few 10 nm were formed at the step edges with round shapes. Both methods were confirmed as evidenced by the formation of two-dimensional nuclei on the surface after the flash events, to enhance surface-related mass diffusion.

Study on two-phase displacement law considering mass transfer and diffusion

The influence of channeling on viscous fingering instability of CO2 miscible displacement is studied in the paper. Due to the viscous fingering not easily observed in porous media, channeling is used to simplify the viscous fingering instability. We adopted nonlinear simulation to investigate the development of viscous fingering instability during the displacement of Newtonian fluids in a channel by miscible fluids, and the influence of different Pe and different viscosity ratio R was studied. Under homogeneous conditions, when R is the same, the larger the Pe is, the more obvious the convection in the process of CO2 displacement is, the earlier the viscous fingering occurs, and the shorter the time for the finger to break through to the right boundary is. When Pe is the same, the larger the R is, the more unstable the contact area of miscible displacement becomes. More finger structures appear, and more complex fingering phenomena occur. The larger the R is, the earlier the fingering phenomenon appears, and the earlier it breaks through to the right boundary. In addition, we studied the change in Relative Mixing Length (RML) during the diffusion process quantitatively. Finally, our investigation delved into the impact of heterogeneity on viscous fingering. We observed that under significant heterogeneity, viscous fingering tends to manifest preferentially in the direction of increasing permeability.

Physics-based modeling of the effective gas transport properties of single jersey knitted fabrics based on images

Physics-based modeling of the effective gas transport properties of single jersey knitted fabrics based on images

A novel multiphase and multicomponent model for simulating molecular diffusion in shale oil reservoirs with complex fracture networks

Molecular diffusion is critical for enhanced oil recovery (EOR) in shale oil reservoirs with complex fracture networks. Understanding the influence of fractures on diffusive mass transfer is crucial for predicting oil recovery and remaining oil distribution. Diffusive mass transfer between fractures and matrix is critical in comprehensively and effectively simulating molecular diffusion. Resolution of matrix cells significantly affects diffusion accuracy at the fracture–matrix interface. Low resolution results in multiple fractures in the same matrix cell, leading to decreased precision in calculating mass transfer by conventional methods. To address this, a novel multiphase and multicomponent model is proposed. The new model integrating the consideration of fracture spacing modifies molecular diffusion transmissibility between fracture and matrix in an embedded discrete fracture model. The discretization employs the two-point flux approximation in the finite-volume method. Validation compares the coarser mesh to the finest grid as a reliable reference. Results show the proposed model accurately captures diffusive mass transfer in a coarser mesh. Modified models study molecular diffusion's effects on EOR in shale oil reservoirs with complex fracture networks by CO2 huff and puff. Results indicate that increasing injection rates cannot improve oil recovery under extremely low porosity and permeability. Molecular diffusion facilitates CO2 penetration into the formation. This expands the swept CO2 volume and increases both volume expansion and formation energy. In addition, the light and heavy components of the crude oil are diffused into the fractures and eventually produced, which reduces gas production in the case of diffusion.

Unravelling the synergistic effects in GdCe composite oxides supported biochar catalysts for formaldehyde elimination: Elevated performance and SO2 toleration

A chain of GdCe oxides boosted biochars derived from maize straw and sewage sludge (GdyCe1-y/MPBs) were fabricated for formaldehyde (HCHO) catalytic decomposition. The ingenerate relationship between the abatement performance and corresponding structural feature was comprehensively evaluated by XPS, in situ DRIFTS, BET, XRD, SEM and H2-TPR. Meanwhile, 10%Gd0.25Ce0.75/MPB exhibited excellent performance, favorable SO2 and moisture toleration over a broad temperature range from 160 to 320 ℃, where it achieved 96.8% removal efficiency with 90.5 % selectivity at 200 ℃. The single or united effects of O2, SO2, H2O on HCHO abatement over 10 %Gd0.25Ce0.75/MPB were tested, and the findings demonstrated that the suppressive effects of SO2 and H2O outweighed the promoting influence of O2 within a specific range. Gd and Ce co-modified MPB revealed superior HCHO removal capability in contrast to that of Gd or Ce severally modified MPB, ascribing to the synergistic effect of GdOx and CeOx and benefitting from the augmentation of surface area and total pore volume, the aggrandizement of surface active oxygen species, the promotion of redox ability and the inhibition crystallization of CeOx. According to in situ DRIFTS, a series of intermediates including formate species and dioxymethylene (DOM) were produced, which would eventually decompose into H2O and CO2. In addition, the mass transfer and diffusion of the reactants along with the accessibility of the catalytic sites were enlarged by the hierarchical porous structure of the support, which were also answerable for its distinguished catalytic performance. Furthermore, 10%Gd0.25Ce0.75/MPB possessed remarkable potential for industrial applications.

Mechanical insights into desulfurization by peroxymonosulfate oxidation via a non-reactive oxygen species pathway

Air pollution by sulfur dioxide (SO2) remains a pressing concern for both the environment and human health. Desulfurization enhanced by persulfate based advanced oxidation processes (PS-AOPs) has been proven to be a feasible method. However, the inherent contradiction between the rapid diffusion mass transfer of SO2 in the “gas-liquid-gas” phase and the limited lifespan of reactive oxygen species (ROS) can not be ignored. Excessive investment in PS is required to sustainably generate ROS to achieve continuous desulfurization performance, which may lead to excessive PS consumption. To address this issue, whether PS can achieve the oxidation absorption of SO2 via a non-reactive oxygen species pathway was investigated. Experimental and computational results demonstrated that peroxymonosulfate (PMS) instead of peroxydisulfate (PDS) had a great SO2 removal performance, the utilization of PS could be effectively achieved by maintaining a 1:1 molar ratio of PMS and removed SO2. The presence of HOO bonds in the PMS introduced a partial positive charge to the oxygen atom, making the PMS polar and more susceptible to be attacked by the nucleophile HSO3-. So SO2 underwent a series of processes including dissolution, dissociation, one-oxygen atom transfer, and ionization before ultimately being converted into SO42- ions, effectively achieving its removal from flue gas. This study may presents a novel approach for achieving high-efficiency flue gas desulfurization.

Thermal analysis of the water-based micropolar hybrid nanofluid flow comprising diamond and copper nanomaterials past an extending surface

This research demonstrates that hybrid nanofluids are superior to traditional fluids in terms of their capacity to transmit heat. An intensive investigation of hybrid nanofluids is needed to fill gaps in knowledge of heat transmission improvement methods. Therefore, scientists and researchers are combining different solid particles in various base fluids to boost the material's thermal conductivity. That's why the author has combined diamond and copper nanoparticles in a water solvent to examine the two-dimensional, steady, and incompressible magnetohydrodynamic (MHD) hybrid micropolar nanofluid flow over a permeable extending surface through the porous media. The Cattaneo-Christov heat and mass flux model is used to assess the heat and mass diffusion occurrences in temperature and concentration distributions. The similarity transformations are used to convert the PDEs along with their related boundary constraints into a set of nonlinear ODEs. A semi-analytical method known as HAM is used with the help of MATHEMATICA software to perform an analytical analysis on the set of nonlinear ODEs. The impacts of many important factors on the velocity, microrotation, thermal, and concentration functions are demonstrated through graphs and discussed in detail. The study results demonstrate that velocity distribution is a declining function of the magnetic field and porosity parameter. The magnetic field, Brownian motion, heat source, thermophoresis, and radiation parameters have positively influenced the thermal profile, while the thermal relaxation parameter has a negative influence on it. Nanoparticle volume fraction improves the Nusselt and Sherwood numbers. Heat transfer and skin friction are both amplified by the magnetic field. The skin-friction coefficient improves with an increase in the permeability parameter and the volume percentage of nanoparticles. Also, nanoparticles volume fraction enhances the mass transfer rate. Hybrid nanofluids have superior thermal characteristics over regular fluids.

Study on uranium leaching from uranium purification residue with ammonium hydrogen fluoride

Residues generated from the uranium purification process, characterized by a high uranium content, pose a significant challenge for recovery through leaching and present a considerable environmental threat. After using XRD and SEM-mapping characterization analysis combined with the BCR continuous graded extraction test to analyze the content of different states of uranium, it was found that the main reason why the uranium in the residue was difficult to leach because it was encapsulated by SiO2 crystals. Using NH4HF2 as a leaching agent, a leaching study of uranium in the residue was carried out, and the results showed that the H+ and F− produced by NH4HF2could react with SiO2, destroying the crystal lattice of SiO2 and causing the encapsulated uranium to come into contact with the leaching agent, facilitating the leaching of uranium in the residue. The optimum conditions for uranium leaching were 10% mass fraction of NH4HF2, a liquid-solid ratio of 30:1, a reaction temperature of 30 °C and a reaction time of 120 min, and the leaching efficiency of uranium from the residue was as high as 98.95%. The leaching kinetics of uranium by NH4HF2 were consistent with the mixed controlled model in the shrinking core models, indicating that the surface chemical reaction and mass diffusion dominated both uranium leaching processes. This may provide a viable method for resource recovery and the treatment of uranium purification residues.

Controlled synthesis of Au@TiO2 mesoporous hollow nanofibers by one-step polyethylenimine-regulated electrospinning method for efficiently photocatalytic oxidation of CO

TiO2 mesoporous hollow nanofibers with excellent mass diffusion is an attractive photocatalytic material to remove toxic gaseous pollutants like CO, while it is highly desired to improve the photogenerated charge separation and O2 activation. Herein, the Au nanoparticles inner-loaded TiO2 mesoporous hollow nanofibers (Au@TO MPHNFs) have been controllably fabricated by one-step coaxial electrospinning, in which it is crucial to use polyethyleneimine as porogen and stabilizer for mesoporous and TiO2 in nanofibers shell. Concurrently, a stepwise phase transfer technique is employed to realize the uniform and stable dispersion of the Au in the paraffin oil as core precursor. The Au@TO MPHNFs exhibits 2.7 and 7.4-fold CO photooxidation enhancement compared with commercial P25 and TO MPHNFs, respectively. The enhanced photoactivity can be attributed to the creation of the mesoporous hollow structure for facilitating better mass transportation, and the incorporation of Au nanoparticles for efficient charge separation and O2 activation. Meanwhile, the photothermal effect of Au expedites intermediaries transformation and product desorption.