Single-component Systems Research Articles

Simultaneous and efficient removal of Cr(VI) and methyl orange on LDHs decorated porous carbons

Layered double hydroxides (LDHs) are currently attracting intense research interests as pollutant adsorbent due to its low cost, non-toxicity, structure amenability and anion exchange capability. Herein, 3-dimensional (3D) flower-like Ni/Al LDH were prepared to decorate H3PO4 activated biomass-derived porous carbons (PAB) via a facile and green hydrothermal method. The as-synthesized nanocomposites (Ni/Al@PAB) displayed favorable removal performance towards both Cr(VI) and methyl orange (MO) from aqueous solution due to the inherited excellent Cr(VI) removal ability from PAB and MO removal ability from Ni/Al LDH. In single-component removal system, the maximum adsorption capacity of 271.5 and 412.8 mg/g for Cr(VI) and MO were achieved on Ni/Al@PAB, respectively. Remarkably, the removal capacity of Cr(VI) and MO both increased in the Cr(VI)MO binary system due to the synergistic effect including, (i) additional adsorption sites of N-containing groups from MO, (ii) electrostatic interactions by the reduced Cr(III), and (iii) mutual thermal compensation in the binary system. This work paved the way for designing novel LDHs-decorated porous carbon adsorbents and revealed that the acicular Ni/Al LDH decorated PAB could be applied as efficient and simultaneous adsorbent for coexisting toxic pollutants situations.

A one-dimensional transport model for multi-component solute in saturated soil

A modified multi-component solute diffusion equation described with diffusion flux was derived in detail based on the classical Maxwell-Stefan diffusion theory. The friction between the solute species and the soil skeleton wall, which is proportional to the relative velocity between the solute species and the soil skeleton, is introduced. The chemical potential gradient is considered the driving force. A one-dimensional model for transport of multi-component solute in saturated soil was developed based on the modified diffusion equation and the modified competitive Langmuir adsorption equation. Numerical calculation of a case of two heavy metal ion species, which was chosen as an example, was carried out using the finite element software COMSOL Multiphysics. A comparative analysis was performed between the multi-component solute transport model developed in this study and the convection-diffusion transport model of single-component solute based on Fick's law. Simulation results show that the transport behavior of each species in a multi-component solute system is different from that in a single-component system, and the friction characteristics considered in the developed model contribute to obstructing the movement of each solute component. At the same time, the influence of modified competitive Langmuir adsorption on solute transport was investigated. These research results can provide strong theoretical support for the design of antifouling barriers in landfills and the maintenance of operation stability.

Errors in nanoparticle growth rates inferred from measurements in chemically reacting aerosol systems

Abstract. In systems in which aerosols are being formed by chemical transformations, individual particles grow due to the addition of molecular species. Efforts to improve our understanding of particle growth often focus on attempts to reconcile observed growth rates with values calculated from models. However, because it is typically not possible to measure the growth rates of individual particles in chemically reacting systems, they must be inferred from measurements of aerosol properties such as size distributions, particle number concentrations, etc. This work discusses errors in growth rates obtained using methods that are commonly employed for analyzing atmospheric data. We analyze “data” obtained by simulating the formation of aerosols in a system in which a single chemical species is formed at a constant rate, R. We show that the maximum overestimation error in measured growth rates occurs for collision-controlled nucleation in a single-component system in the absence of a preexisting aerosol, wall losses, evaporation or dilution, as this leads to the highest concentrations of nucleated particles. Those high concentrations lead to high coagulation rates that cause the nucleation mode to grow faster than would be caused by vapor condensation alone. We also show that preexisting particles, when coupled with evaporation, can significantly decrease the concentration of nucleated particles. This can lead to decreased discrepancies between measured growth rate and true growth rate by reducing coagulation among nucleated particles. However, as particle sink processes become stronger, measured growth rates can potentially be lower than true particle growth rates. We briefly discuss nucleation scenarios in which the observed growth rate approaches zero while the true growth rate does not.

Chiral Plasmonic Fields Probe Structural Order of Biointerfaces.

The structural order of biopolymers, such as proteins, at interfaces defines the physical and chemical interactions of biological systems with their surroundings and is hence a critical parameter in a range of biological problems. Known spectroscopic methods for routine rapid monitoring of structural order in biolayers are generally only applied to model single-component systems that possess a spectral fingerprint which is highly sensitive to orientation. This spectroscopic behavior is not a generic property and may require the addition of a label. Importantly, such techniques cannot readily be applied to real multicomponent biolayers, have ill-defined or unknown compositions, and have complex spectroscopic signatures with many overlapping bands. Here, we demonstrate the sensitivity of plasmonic fields with enhanced chirality, a property referred to as superchirality, to global orientational order within both simple model and “real” complex protein layers. The sensitivity to structural order is derived from the capability of superchiral fields to detect the anisotropic nature of electric dipole–magnetic dipole response of the layer; this is validated by numerical simulations. As a model study, the evolution of orientational order with increasing surface density in layers of the antibody immunoglobulin G was monitored. As an exemplar of greater complexity, superchiral fields are demonstrated, without knowledge of exact composition, to be able to monitor how qualitative changes in composition alter the structural order of protein layers formed from blood serum, thereby establishing the efficacy of the phenomenon as a tool for studying complex biological interfaces.

Inspection-Based Policy Considering Human Errors for Three-Stage Delay Time Degradation Systems

A periodic inspection policy for a single component system based on a three-stage failure process is proposed, and two different kinds of failures covering “hard” and “human” are considered in the proposed policy. The system is periodically inspected and inspections are perfect so that they can identify the intended defect. If the severe defect is detected by an inspection, an immediate repair is needed. However, once the system is identified to be in the minor defective state, there are two options. The first is to do nothing till the arrival of identifying the severe defect or hard failure, and the second is to repair immediately. Repair for any defect can renew the system with a limited probability such that the system may fail after repair due to human errors, which is common in many industrial applications. Two models are constructed by minimizing the expected cost per unit time and compared. We provide a numerical example to demonstrate the proposed model.

Molecular dynamics simulation of the solid-liquid interface migration in terbium.

We developed a Tb embedded atom method potential which properly reproduces the liquid structure obtained from the ab initio molecular dynamics simulation, the hexagonal close packed (hcp)-body-centered cubic (bcc) phase transformation, and melting temperatures. At least three crystal phases [hcp, face-centered cubic (fcc), and bcc] described by this potential can coexist with the liquid phase. Thus, the developed potential provides an excellent test bed for studies of the completive phase nucleation and growth in a single component system. The molecular dynamics simulation showed that all crystal phases can grow from the liquid phase close to their melting temperatures. However, in the cases of the hcp and fcc growth from the liquid phase at very large supercoolings, the bcc phase forms at the solid-liquid interface in the close packed orientations in spite of the fact that both hcp and fcc phases are more stable than the bcc phase at these temperatures. This bcc phase closes the hcp and fcc phase from the liquid such that the remaining liquid solidifies into the bcc phase. The initial hcp phase then slowly continues growing in expense of the bcc phase.

CHEMICAL POTENTIALS OF HARD-CORE MOLECULES BY A STEPWISE INSERTION METHOD

ABSTRACT A molecular simulation algorithm was implemented to calculate chemical potentials of hard-core molecular systems at high densities. The method is based on the Widom particle insertion method and the step-function character of free energy variations. The algorithm was evaluated for hard-sphere mixtures at infinite dilution approximation by varying the solute/solvent diameter ratio, for systems with reduced densities from 0.1 to 0.8. The proposed methodology was verified by comparing simulations of trimers diluted in spheres and of single-component dimer systems with results from the literature. Then, the method was applied to mixtures of hard-spheres and dimers at several conditions regarding composition, reduced density, and bond-length/diameter ratio. The results were used to validate equations of state from the literature. The proposed approach was able to obtain accurate chemical potentials for different hard-core molecular mixtures. Lower uncertainties were obtained when comparing with traditional methods, especially at high densities.

Fixed bed column modeling of lead(II) and cadmium(II) ions biosorption on sugarcane bagasse

In this paper the results of the biosorption of lead(II) and cadmium(II) with sugarcane bagasse in fixed bed columns are presented. Experimental data were fitted to several models describing the rupture curve for single-component and two-component systems. The percentages of removal of lead and cadmium in single-component systems are 91% and 90%, respectively. In lead-cadmium bicomponent systems the percentage of elimination of lead was 90% and cadmium 92%. In single-component systems, Yoon-Nelson and Thomas models successfully reproduce the rupture curves. In two-component system, the Dose-Response model was the best one reproducing the experimental rupture curves in the entire measured range.

On the stability and finite-size effects of a columnar phase in single-component systems of hard-rod-like particles

ABSTRACTColloidal rod-like particles self-assemble into a variety of liquid crystal phases. In contrast to the formation of the nematic and smectic phases for which it is well understood that it can be driven by entropy, the stabilisation mechanism of a prolate columnar phase (), observed for example in fd-virus suspensions, is still unclear. Here, we investigate whether or not a phase can exist in a purely entropy-driven single-component system. We perform computer simulations of hard particles with different shapes: spherocylinders, top-shaped rods, cuboidal particles, and crooked rods. We show that the phases observed in previous simulation studies are mere artefacts due to either finite-size effects or simulation boxes that are incommensurate with the stable thermodynamic phase. In particular, we observe that the characteristic layering of the stable smectic or crystal phase disappears when the dimension of the simulation box along the direction of the layers is too small. Such a system-size effect depends both on particle shape and the competing phases, and appears to be more pronounced for less anisotropic particles.

Stable Metallic State of a Neutral-Radical Single-Component Conductor at Ambient Pressure.

Molecular metals have been essentially obtained with tetrathiafulvalene (TTF)-based precursors, either with multicomponent ionic materials or, in a few instances, with single-component systems. In that respect, gold bis(dithiolene) complexes, in their neutral radical state, provide a prototype platform toward such single-component conductors. Herein we report the first single-component molecular metal under ambient pressure derived from such Au complexes without any TTF backbone. This complex exhibits a conductivity of 750 S·cm-1 at 300 K up to 3800 S·cm-1 at 4 K. First-principles electronic structure calculations show that the striking stability of the metallic state finds its origin in sizable internal electron transfer from the SOMO-1 to the SOMO of the complex as well as in substantial interstack and interlayer interactions.

A novel approach for synthesis of exfoliated biopolymeric-LDH hybrid nanocomposites via in-stiu coprecipitation with gum Arabic: Application towards REEs recovery

Delamination and exfoliation of layered double hydroxides (LDH) is an interesting way for the synthesis of novel nanocomposites. Herein, we report the synthesis of various exfoliated biopolymeric-LDH nanocomposites via in-situ coprecipitation method with gum Arabic (GA). The influence of various divalent ions on the morphological characteristics of GAnXA (n = wt% of GA and X = Mg, Ca, Ba, Sr) was explored Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy (SEM). X-ray diffraction pattern was used for the determination of phase composition of synthesized nanocomposites. The surface functional groups of GAnXA were investigated by Fourier Transform Spectroscopy (FTIR). Furthermore, the surface charge characteristics was explored by zeta potential analysis. Specific surface area of GAnXA was determined by BET analysis. The overall adsorption of REEs decreased with an increase in ionic size of divalent ions and the noticeable difference in the morphologies of exfoliated GAnXA was observed. In a single component system, the REEs adsorption capacities followed the order: Sc > Y > Nd > Ce > Eu > La, whereas, in multicomponent system, adsorption seems to be competitive and presence of competing ions affect the overall REEs removal. Moreover, HREEs removal superseded over LREEs and the nanocomposites (GA5CA, GA5SA, GA5BA) were highly selective for Sc recovery. The post-adsorption FTIR and SEM results revealed the importance of surface hydroxyl and carboxyl functional groups enacting as the principal REE binding sites. Overall, ability to extract REEs at pH 4 (slightly acidic) presented a facile route for the recovery and separation of REEs from aqueous medium and enhances the possibility of its use in many industrial applications.

Comparison of coagulation and magnetic chitosan nanoparticle adsorption on the removals of organic compound and coexisting humic acid: A case study with salicylic acid

Removals of salicylic acid (SA) and humic acid (HA) in individual and bi-component systems by coagulation employing aluminum sulfate (alum) and adsorption using magnetic chitosan (MCS), were comparatively investigated in this study. In single-component system, alum at sufficient dose (12.0 mg/L), although efficient in removing HA, was not desirable in removal of SA. Conversely, higher SA removal rate in the adsorption process was achieved due to its superior adsorption tendencies and size-exclusion effects than that of HA. Moreover, individual SA/HA by MCS adsorption fit with Freundlich isotherm model, indicating a multilayer adsorption process. In the bi-component system, the SA and HA removal rates were observed to decrease to varying extents. Dynamic floc sizes during coagulation processes were studied using a laser diffraction particle sizing device. The results demonstrated high molecular weight (MW) HA-Al(III) complex could grow into enough large size and consequently tended to separate in water; while more coagulants (>14.0 mg/L) were required in the bi-component system compared with individual HA water when the system emerged obvious flocs, which was associated with the discrepancy of charge neutralization and competition of adsorption sites. For sole adsorption, the reaction between HA and MCS was faster than that of SA, and parts of adsorption sites were preoccupied by HA and thereby result in the low SA removal efficiency. Furthermore, combination of alum and MCS could significantly enhance organic matter removal, and alum with low dose of 8.0 mg/L coupled with MCS could evidently improve flocs settlement performance and increase the floc sizes.

The effect of ball mass on the mechanochemical transformation of a single-component organic system: anhydrous caffeine

AbstarctMechanochemical methodologies, particularly ball milling, have become commonplace in many laboratories. In the present work, we examine the effects of milling ball mass on the polymorphic conversion of anhydrous caffeine. By investigating a single-phase system, the rate-limiting step of particle–particle contact formation is eliminated. It is found that larger milling balls lead to considerably faster conversion rates. Modelling of the transformation rate suggests that a single, time-independent rate constant is insufficient to describe the transformation. Instead, a convolution of at least two rate-determining processes is required to correctly describe the transformation. This suggests that the early stages of the transformation are governed only by the number of particle–ball collisions. As the reaction proceeds, these collisions less frequently involve reactant, and the rate becomes limited by mass transport, or mixing, even in originally single-phase systems, which become multi-phase as the product is formed. Larger milling balls are less hindered by poorly mixed material. This likely results from a combination of higher impact energies and higher surface areas associated with the larger milling balls. Such insight is important for the selective and targeted design of mechanochemical processes.

Phase field benchmark problems for dendritic growth and linear elasticity

We present the second set of benchmark problems for phase field models that are being jointly developed by the Center for Hierarchical Materials Design (CHiMaD) and the National Institute of Standards and Technology (NIST) along with input from other members in the phase field community. As the integrated computational materials engineering (ICME) approach to materials design has gained traction, there is an increasing need for quantitative phase field results. New algorithms and numerical implementations increase computational capabilities, necessitating standard problems to evaluate their impact on simulated microstructure evolution as well as their computational performance. We propose one benchmark problem for solidification and dendritic growth in a single-component system, and one problem for linear elasticity via the shape evolution of an elastically constrained precipitate. We demonstrate the utility and sensitivity of the benchmark problems by comparing the results of (1) dendritic growth simulations performed with different time integrators and (2) elastically constrained precipitate simulations with different precipitate sizes, initial conditions, and elastic moduli. These numerical benchmark problems will provide a consistent basis for evaluating different algorithms, both existing and those to be developed in the future, for accuracy and computational efficiency when applied to simulate physics often incorporated in phase field models.

Sorption of Hg(II) and Pb(II) Ions on Chitosan-Iron(III) from Aqueous Solutions: Single and Binary Systems.

The present work describes the study of mercury Hg(II) and lead Pb(II) removal in single and binary component systems into easily prepared chitosan-iron(III) bio-composite beads. Scanning electron microscopy and energy-dispersive X-ray (SEM-EDX) analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and point of zero charge (pHpzc) analysis were carried out. The experimental set covered pH study, single and competitive equilibrium, kinetics, chloride and sulfate effects as well as sorption–desorption cycles. In single systems, the Langmuir nonlinear model fitted the experimental data better than the Freundlich and Sips equations. The sorbent material has more affinity to Hg(II) rather than Pb(II) ions, the maximum sorption capacities were 1.8 mmol·g−1 and 0.56 mmol·g−1 for Hg(II) and Pb(II), respectively. The binary systems data were adjusted with competitive Langmuir isotherm model. The presence of sulfate ions in the multicomponent system [Hg(II)-Pb(II)] had a lesser impact on the sorption efficiency than did chloride ions, however, the presence of chloride ions improves the selectivity towards Hg(II) ions. The bio-based material showed good recovery performance of metal ions along three sorption–desorption cycles.

Nonequilibrium two-dimensional Ising model with stationary uphill diffusion.

Usually, in a nonequilibrium setting, a current brings mass from the highest density regions to the lowest density ones. Although rare, the opposite phenomenon (known as "uphill diffusion") has also been observed in multicomponent systems, where it appears as an artificial effect of the interaction among components. We show here that uphill diffusion can be a substantial effect, i.e., it may occur even in single component systems as a consequence of some external work. To this aim we consider the two-dimensional ferromagnetic Ising model in contact with two reservoirs that fix, at the left and the right boundaries, magnetizations of the same magnitude but of opposite signs.We provide numerical evidence that a class of nonequilibrium steady states exists in which, by tuning the reservoir magnetizations, the current in the system changes from "downhill" to "uphill". Moreover, we also show that, in such nonequilibrium setup, the current vanishes when the reservoir magnetization attains a value approaching, in the large volume limit, the magnetization of the equilibrium dynamics, thus establishing a relation between equilibrium and nonequilibrium properties.

Inverse design of multicomponent assemblies.

Inverse design can be a useful strategy for discovering interactions that drive particles to spontaneously self-assemble into a desired structure. Here, we extend an inverse design methodology-relative entropy optimization-to determine isotropic interactions that promote assembly of targeted multicomponent phases, and we apply this extension to design interactions for a variety of binary crystals ranging from compact triangular and square architectures to highly open structures with dodecagonal and octadecagonal motifs. We compare the resulting optimized (self- and cross) interactions for the binary assemblies to those obtained from optimization of analogous single-component systems. This comparison reveals that self-interactions act as a "primer" to position particles at approximately correct coordination shell distances, while cross interactions act as the "binder" that refines and locks the system into the desired configuration. For simpler binary targets, it is possible to successfully design self-assembling systems while restricting one of these interaction types to be a hard-core-like potential. However, optimization of both self- and cross interaction types appears necessary to design for assembly of more complex or open structures.

Creating Stiff, Tough, and Functional Hydrogel Composites with Low-Melting-Point Alloys.

Reinforcing hydrogels with a rigid scaffold is a promising method to greatly expand the mechanical and physical properties of hydrogels. One of the challenges of creating hydrogel composites is the significant stress that occurs due to swelling mismatch between the water-swollen hydrogel matrix and the rigid skeleton in aqueous media. This stress can cause physical deformation (wrinkling, buckling, or fracture), preventing the fabrication of robust composites. Here, a simple yet versatile method is introduced to create "macroscale" hydrogel composites, by utilizing a rigid reinforcing phase that can relieve stress-induced deformation. A low-melting-point alloy that can transform from a load-bearing solid state to a free-deformable liquid state at relatively low temperature is used as a reinforcing skeleton, which enables the release of any swelling mismatch, regardless of the matrix swelling degree in liquid media. This design can generally provide hydrogels with hybridized functions, including excellent mechanical properties, shape memory, and thermal healing, which are often difficult or impossible to achieve with single-component hydrogel systems. Furthermore, this technique enables controlled electrochemical reactions and channel-structure templating in hydrogel matrices. This work may play an important role in the future design of soft robots, wearable electronics, and biocompatible functional materials.

MIL-53 (Fe)-based photo-sensitive composite for degradation of organochlorinated herbicide and enhanced reduction of Cr(VI)

Abstract Highly robust composite photocatalyst (WO3/MIL-53(Fe)) was synthesized and characterised. WO3/MIL-53(Fe) exhibits remarkable photocatalytic efficiency for reduction of Cr(VI) and oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) organochlorinated herbicide. After 240 min sunlight irradiation, only ∼30% reduction ratio of Cr(VI) was recorded in the presence of WO3. However, about 70% and 94% Cr(VI) reduction were achieved in the presence of MIL-53(Fe) and WO3/MIL-53(Fe), respectively. The removal efficiency proceeds more rapidly in the binary mixture of 2,4-D and Cr(VI) than in the single component system, indicating synergetic effect between the photooxidation and reduction reactions. O2 − played a key role in the reduction of Cr (VI) and h+ is confirmed to be the dominant active species in the degradation of 2,4-D.

Effect of a core-softened O–O interatomic interaction on the shock compression of fused silica

Isotropic soft-core potentials have attracted considerable attention due to their ability to reproduce thermodynamic, dynamic, and structural anomalies observed in tetrahedral network-forming compounds such as water and silica. The aim of the present work is to assess the relevance of effective core-softening pertinent to the oxygen-oxygen interaction in silica to the thermodynamics and phase change mechanisms that occur in shock compressed fused silica. We utilize the MD simulation method with a recently published numerical interatomic potential derived from an ab initio MD simulation of liquid silica via force-matching. The resulting potential indicates an effective shoulder-like core-softening of the oxygen-oxygen repulsion. To better understand the role of the core-softening we analyze two derivative force-matching potentials in which the soft-core is replaced with a repulsive core either in the three-body potential term or in all the potential terms. Our analysis is further augmented by a comparison with several popular empirical models for silica that lack an explicit core-softening. The first outstanding feature of shock compressed glass reproduced with the soft-core models but not with the other models is that the shock compression values at pressures above 20 GPa are larger than those observed under hydrostatic compression (an anomalous shock Hugoniot densification). Our calculations indicate the occurrence of a phase transformation along the shock Hugoniot that we link to the O–O repulsion core-softening. The phase transformation is associated with a Hugoniot temperature reversal similar to that observed experimentally. With the soft-core models, the phase change is an isostructural transformation between amorphous polymorphs with no associated melting event. We further examine the nature of the structural transformation by comparing it to the Hugoniot calculations for stishovite. For stishovite, the Hugoniot exhibits temperature reversal and associated phase transformation, which is a transition to a disordered phase (liquid or dense amorphous), regardless of whether or not the model accounts for core-softening. The onset pressures of the transformation predicted by different models show a wide scatter within 60-110 GPa; for potentials without core-softening, the onset pressure is much higher than 110 GPa. Our results show that the core-softening of the interaction in the oxygen subsystem of silica is the key mechanism for the structural transformation and thermodynamics in shock compressed silica. These results may provide an important contribution to a unified picture of anomalous response to shock compression observed in other network-forming oxides and single-component systems with core-softening of effective interactions.