Interlayer Adsorption Research Articles

Experimental and theoretical studies of spherical hydroxylamine hydrochloride intercalated molybdenum disulfide for the removal of U(VI), Eu(III), and Cr(VI)

The current environmental problem is the coexistence of multiple pollutants rather than a single pollutant. In this study, U(VI), Eu(III), and Cr(VI) are selected as representatives of the actinides, lanthanide elements, and heavy metal elements for removal study. The hydroxylamine hydrochloride intercalated molybdenum disulfide (HAH/MoS2) was prepared to remove these contaminants. The insertion of hydroxylamine hydrochloride increased layer spacing, which was conducive to the pollutant molecules entering the molybdenum disulfide layer. HAH/MoS2 revealed a spherical shape with a rough surface and relatively high anti-interference. The maximum adsorption capacities of HAH/MoS2 for U(VI), Eu(III), and Cr(VI) reached 104.9 mg/g, 72.9 mg/g, and 81.4 mg/g, respectively. The adsorption mechanism of U(VI) was interlayer adsorption at pH < 6.2 and surface complexation at pH > 6.2. Similarly, the removal of Eu(III) was interlayer adsorption at pH < 5.0, interlayer adsorption and surface complexation at pH 5.0–7.7, and forming precipitation Eu(OH)3(s) at pH > 7.7. The removal of Cr(VI) depended on surface complexation at pH < 4.0 and interlayer adsorption at pH > 4.0. These ions were more likely to be adsorbed between layers instead of at the surface. Compared to U(VI) and Cr(VI), Eu(III) was more easily adsorbed at the interlamination of HAH/MoS2. From the point of view of charge transfer, U(VI) and Eu(III) tended to give away electrons, and Cr(VI) tended to gain electrons in the removal process. This work can offer a new perspective for the design and application of two-dimensional materials for multiple pollutants removal.

Nano-laminated clay-essential oil composite formulations: Key mechanistic antibacterial processes and in vitro antibiofilm activity

This study involves fabrication of nano-clay-essential oil hybrid nanostructures using acoustic cavitation-based probe ultrasonication for controlled release antibacterial effect. Both the pristine and composites were characterized and showed no structural (shape and size) changes in clay nanoparticles (NPs) after adsorption of essential oil as examined by Transmission electron microscope. Adsorption of essential oil in the interlayer space of nano-clay particles was confirmed by increase in d-spacing value for both nano-bentonite (3.31 to 3.44 nm) and nano-MMT (3.36 to 3.45 nm) composites. The FTIR (Fourier transform infrared) spectroscopy showed disappearance or weakening of bands at 3500-3600 cm-1 (hydroxyl group) wavenumber and appearance of new bands at 1700 cm-1 (C-H bending) suggesting interlayer adsorption of essential oil within the nano-clay. Thermogravimetric analysis (TGA) revealed three step mass loss in nano-clay-essential oil composites with the lowest mass loss (81 %) reported in Nano-bentonite-Basil essential oil composite. The GC-MS analysis confirmed the presence of similar components in both pure and composite formulations but with varied compositions. The developed nanocomposites (nano-clay-essential oil) exhibited enhanced antibacterial and anti-biofilm activity against Staphylococcus aureus and Escherichia coli as compared to pure EOs. This work is a first report for elucidating the antimicrobial mechanism of nano-clay-essential oil composite as identified by determining cell membrane damaging effect and level of reactive oxygen species (ROS). The present study provides insights on the potential role of nano-clay as adsorbent/nanocarrier and nano-clay-essential oil formulation for effective drug delivery.

Zn-Al layered double hydroxide supported on waste cow dung-derived biochar as a highly efficient adsorbent for anionic dye removal from contaminated water.

In this study, Zn-Al-SO42- LDH-functionalized biochar was fabricated using the co-precipitation method. The biochar was synthesized from waste cow dung using a low-temperature pyrolysis process (300 °C). The materials were fully characterized by TGA, FTIR, EDS, SEM, and XRD analysis. Then, a comparative study was performed to investigate the adsorption capacity of the materials against an anionic dye (i.e., methyl orange (MO)). The LDH-functionalized biochar demonstrated high adsorption capacity (400 mg/g in 120 min, at pH 5) compared to the raw biochar (212 mg/g in 120 min, at pH 5). The effect of various adsorption parameters (e.g., pH of the dye solution, temperature, initial concentration, adsorbent dosage, and contact time) was investigated. The adsorption of MO on LDH-functionalized biochar followed the Freundlich isotherm and pseudo-second-order kinetics, while the raw biochar followed the Langmuir isotherm and pseudo-second-order kinetics. The thermodynamic data indicated the endothermic nature of adsorption and an increase in the degree of randomness during adsorption. The enhanced adsorption capacity of the Zn-Al LDH-functionalized char was attributed to the synergistic effect of the surface adsorption into the porous biochar matrix, interlayer adsorption, and ion exchange capacity of the LDHs. Therefore, modification of waste cow dung-derived biochar with Zn-Al LDH can be a promising approach to fabricate a highly efficient adsorbent for toxic dyes from wastewater.

Enhanced colorimetric analysis substrate based on graphene oxide open-close actuator

Traditional colorimetric sensors often suffer from low color resolution and insufficient sensitivity. Therefore, for colorimetric detection to serve as a reliable quantitative analysis technique, an enhanced substrate with high sensitivity and repeatability is required. This strategy employs an interfacial network constructed between two-dimensional graphene oxide (GO) lamellar structures to create a substrate for colorimetric analysis. Under normal conditions, the newly synthesized GO inhibits oxidative activity due to van der Waals forces between the lamellar interfaces, resulting in the GO actuator being in the closed state. During Ag+ colorimetric analysis, the GO interlayer adsorption of silver nanoparticles (AgNPs) disrupts the interlayer van der Waals forces, releasing its oxidase activity and significantly enhancing Ag+ oxidation. The resulting actuator exhibits a faster driving speed (0.045 μM/s), excellent sensitivity, and stability. Additionally, the reducing properties of dopamine (DA) can be utilized to encapsulate the GO actuator with polydopamine, closing the GO actuator. Similarly, the reducing property of ascorbic acid (AA) can be utilized to restore the GO actuator to a closed state. These distinct reduction mechanisms allow for an effective visual distinction between AA and DA. With these advantages, the actuator can be developed into simple analytical tools (such as Ag+ colorimetric test strips and DA and AA test kits), demonstrating significant potential for practical application.

Hollow microspheres of layered double hydroxides through surfactant-free method enhanced heavy metals removal from soil

A three-dimensional MgAl layered double hydroxides hollow microsphere (HMS-LDH) is fabricated by surfactant-free method. The microstructure and the formation mechanism of HMS-LDH are investigated by XRD (X-ray diffraction), SEM (Scanning Electron Microscope), EDS (Energy dispersive Spectrometer) and XPS (X-ray photoelectron spectrometer). The morphology of LDHs could be varied by changing the ratio of metal cations to urea. The adsorption properties of HMS-LDH for heavy metals from soil are investigated. The results of experiments demonstrates that the adsorption kinetics of Cu(Ⅱ), Pb(Ⅱ), As(Ⅴ), Cr(Ⅵ) are in good agreement with the pseudo-second-order kinetic and intraparticle diffusion model. The adsorption isotherm data of Cu(Ⅱ)/Pb(Ⅱ) is accordance with the Langmuir-Freundlich model, while the equilibrium adsorption of As(Ⅴ)/Cr(Ⅵ) follow the Generalized-Langmuir isotherm model well. Site energy distribution theory denotes the hollow microsphere structure increases the adsorption sites of heavy metals, which is conducive to the adsorption process. The different mechanisms of precipitation for Cu(Ⅱ)/Pb(Ⅱ) removal and interlayer adsorption for As(Ⅴ)/Cr(Ⅵ) removal are proposed. The effects of coexisting cations and column leaching experiments on the performance evaluation are also investigated, which indicate most the ions have no effect on the adsorption of Cu (Ⅱ)/Pb(Ⅱ) and As(Ⅴ)/Cr(Ⅵ). This work proposes a simple approach to prepare LDHs hollow microspheres and demonstrates their excellent properties for the remediation of heavy metal polluted soil.

Unraveling polycarboxylate superplasticizer (PCE) compatibility in muscovite-blended cement paste through aggregation mechanisms

Muscovite is a common harmful mineral in manufactured sand, and has always led to the poor compatibility of superplasticizers in fresh concrete. Currently, the failure mechanism is usually explained by competitive adsorption and interlayer adsorption. In addition to the adsorption point of view, as a kind of surfactant, the self-aggregation conformation of superplasticizers in solutions also requires attention. This study explored the compatibility of polycarboxylate superplasticizer (PCE) in cement paste containing muscovite and explained the failure mechanism from the PCE aggregation point of view. Results showed that at the saturated dosage of PCE (0.14 wt%), the introduction of 6 wt% muscovite resulted in a complete loss of fluidity. In the pore solution of cement paste blended with muscovite, the PCE exhibited a lower critical micelle concentration (CMC) and larger aggregation size. The aggregation of PCE is due to the high ionic concentration in cement paste with muscovite, which increases the hydrophobic interaction between PCE and counter ion.

Effects of calcium-to-silicon ratio on the properties of fly ash-based tobermorite and its removal performance of Zn2+ and Mn2.

The effects of calcium-to-silicon ratio on the properties of fly ash (FA)-based tobermorite and its removal performance of Zn2+ and Mn2+ were studied. The calcium-to-silicon ratio had a significant effect on the structural properties of the tobermorite samples. The specific surface area, pore volume, and average pore size of mesoporous tobermorite samples with different calcium-to-silicon ratios (0.8TOB, 1.2TOB, and 1.6TOB) were much larger than those of FA, and those of 1.2TOB were the largest, which were 53.29 m2/g, 0.448 cm3/g, and 30.50 nm, respectively. The removal efficiencies of Zn2+ and Mn2+ by 1.2TOB were 84.19% and 47.67%, respectively, which were much higher than those of 0.8TOB (60.62% and 42.41%), 1.6TOB (46.69% and 24.31%), and FA (4.13% and 6.95%). The adsorption of Zn2+ and Mn2+ by 0.8TOB, 1.2TOB, and 1.6TOB was corresponding to the pseudo-second-order kinetic model and Langmuir isotherm model. Particularly, 1.2 TOB showed the highest maximum adsorption capacities of Zn2+ and Mn2+ calculated from the Langmuir model, which were 129.70 mg/g and 82.09 mg/g, respectively. Moreover, the adsorption mechanisms might be due to the combination with -OH and the interlayer adsorption of the samples. This research provides new insight into the fly ash-based adsorbents towards Zn2+ and Mn2+ in wastewater.

Preferential interlayer adsorption sites in phyllosilicate clay edge models by molecular dynamics simulation

Phyllosilicate clay minerals have been proposed as a possible buffer material to be used in deep geological repositories containing high-level waste and used nuclear fuel. This work precisely characterizes ion interactions with two types of adsorption sites present in these clays: MgAl’ substitutions and undercoordinated edge surface atoms. A number of unique structural models were considered to represent the diverse local environments that ions in these systems are likely to encounter. Using molecular dynamics simulation with the CLAYFF potential, the spatial distribution, interlayer composition, and residence times of Na+ and Cl− ions as radionuclide analogs in pyrophyllite and montmorillonite clay models were investigated to identify the most favorable conditions for sequestration. The most significant factor impacting ion adsorption was found to be the localization of charge density at substitution sites. In a montmorillonite system in which substitution sites were distributed evenly to produce a low charge density, sequestration performance was found to be comparable to pyrophyllite.

Binary hetero-structured PVDF/TiO2@MXene composite membranes for the removal of antibiotics from wastewater

The presence of pharmaceuticals in hospital wastewaters has generated significant interest among researchers. Current membrane technologies face limitations in effectively removing these emerging contaminants, which has led to continuous advancements in the field. Two-dimensional (2D) nanomaterial-based membranes have proved their ability in providing high water permeability without compromising separation performance. In this study, a binary hetero-structured PVDF/TiO2@MXene composite 2D membrane was prepared and utilized for the rejection of antibiotics, namely tetracycline and meropenem. Vacuum-assisted filtration was employed to fabricate the photocatalytic and highly selective multi-layer TiO2@MXene stacked sheets on hydrophilic PVDF microfilters. The attachment of TiO2 nanoparticles in the MXene structure has significantly improved the wettability and permeation of the PVDF/TiO2@MXene membrane, with a water contact angle of 19.5° and a permeation capacity of 293 L.m−2.h−1.bar−1. In terms of antibiotic rejection, the developed membrane demonstrated a rejection of 87.8% and 60.7% for tetracycline and meropenem solutions, respectively, through interlayer sieving, adsorption, and photocatalysis. The membrane also performed ideally in neutral pH conditions often present in hospital wastewaters, when compared to acidic and alkaline conditions, aiding in the membrane's adsorptive interactions under UV irradiation. When studied in antibiotic mixtures and spiked real hospital wastewater feed solutions, the membrane exhibited a rejection rate of 77.4% and 70.1% for tetracycline, respectively. This research represents a promising approach to addressing the challenges posed by pharmaceuticals in hospital wastewaters through the synergistic utilization of MXene and anatase TiO2.

Spatially thermal confinement nanoreactors for efficient photocatalytic microinterface reactions and application prospect evaluation under real weather and actual water matrices

Photothermal photocatalysis has been a breakthrough in the photocatalysis field, yet the wasted radicals, insufficient contact of pollutant with catalyst, and low utilization efficiency of heat energy are the major barriers for practical application. In this work, a novel full-spectrum adsorption catalyst (MoS2-MoO3/C3N4) with interlayer confinement space (OMS-CNS) was fabricated as “spatially thermal confinement nanoreactor”, which can enrichment pollutants, free radicals and photothermal effect to improve the degradation efficiency of ampicillin (AMP) by 2.06 times. The synergistic of confinement effects and photothermal effects concentrated heat in the interlayer nanoreactor and simultaneously promoted the generation of •O2− and 1O2, further contributed to a more harmless degradation pathway for AMP compared with conventional photothermal photocatalytic systems. Furthermore, the enlarged interlayer spacing and high interlayer adsorption energy (Eads) enabled AMP to enter the nanoreactor for quick reaction with newly generated active radicals under locally high temperature, avoiding heat loss and the diffusion limitation of free radicals. Furthermore, the excellent degradation efficiencies of AMP were also maintained even in the actual wastewater matrices or at outdoor sunlight irradiation due to the construction of confinement space with high temperature environment and enriched ROS. The combination of the confinement catalysis and photothermal effects provides promising insight into efficient photocatalysis performance.

Investigation on high performance and stabilized surface of AlF3-coated V2O5 micro-flower for zinc energy storage

Vanadium oxide (V2O5) cathode material, possessing abundant reserves and high theoretical capacity, has been extensively researched in aqueous zinc ion batteries (ZIBs). However, its electrochemical performance is significantly hindered by insufficient electronic conductivity, vanadium dissolution into electrolyte, and slow diffusion kinetics. In this study, the V2O5 micro-flower was first synthesized by a solvothermal method, and then modified by coating with AlF3 passivation layer, forming AlF3@V2O5 hybrid material. Impressively, AlF3@V2O5 exhibits excellent electrochemical and kinetic properties due to the special structure of V2O5 and the interfacial protection of AlF3. An exceptional rate property (325 mAh g−1 at 0.1 A g−1, 188 mAh g−1 at 15 A g−1) as well as extraordinary cycling stability with 89.3% capacity retention after 6000 cycles at 10 A g−1 can be attained with 2 wt% AlF3 addition (that is AF-2@V2O5). The computation based on density functional theory (DFT) suggests that AlF3 coating can reduce the interlayer electrostatic interaction and adsorption energy barrier between the intercalated Zn2+ and V2O5 host, thereby enabling fast redox kinetics. This study showcases the efficacy of interfacial modification of passivated coatings and presents an influential approach for material design in ZIBs.

Molecular insights into histidine adsorption on Na-montmorillonite, anatase, and goethite

The adsorption of amino acids (AAs) on minerals plays a role in the geochemical processes that determine the availability of nutrients in the environment. Histidine can be used as typical basic AA because it is frequently detected in natural waters, soils, and sediments. Herein, the pH effects and time factor were analyzed to reveal the molecular-level mechanisms of histidine adsorption on Na-montmorillonite, anatase, and goethite. Attenuated total reflectance Fourier transform infrared (ATR–FTIR) flow-cell measurements, X-ray diffraction (XRD), and density functional theory (DFT) calculations were used to determine the adsorbed species structures and the time-dependent adsorption/desorption processes. The structures of adsorbed histidine were governed by pH conditions, but were not affected by time variations. The histidine adsorbed on the minerals was in the cationic (pH 4), neutral zwitterionic (pH 7.5), and anionic (pH 10.5) forms, which was consistent with those dissolved in solution at the same pH. Adsorption on the clay mineral montmorillonite was favorable due to the dominant interlayer adsorption mechanism at pH 4 and hydrophobic attraction (external basal surface) at pH 4–10.5. Histidine adsorption on the two metal (hydr)oxides was dominated by inner-sphere surface complexation, which involved one or two O atoms of the COO− group. The adsorbed histidine was in the monodentate form on anatase and in the bidentate form on goethite.

Phosphorus controls on the formation of vivianite versus green rust under anoxic conditions

The formation of green rust (GR; a mixed ferric/ferrous hydroxide) and vivianite (ferrous phosphate) are likely to have exerted a major control on phosphorus (P) cycling in ancient anoxic oceans. However, the factors that influence the formation of these minerals under different chemical conditions are poorly constrained, which limits understanding of the pathways that ultimately result in P drawdown and retention in anoxic sediments. This, in turn, limits understanding of P cycling in anoxic oceans and hence potential productivity feedbacks. Here we explore the effect of dissolved P concentration on the formation of sulfate GR (FeIII2FeII3(OH)12SO4) versus vivianite (FeII3(PO4)2·8H2O) under anoxic conditions. Our results show that at low dissolved P concentrations and with P:Fe(II) molar ratios <1:30, P drawdown is effectively controlled by interlayer anion exchange and adsorption onto GR species, via the formation of amorphous Fe-P precursors. Such precursors may delay the precipitation of crystalline GR, but vivianite was not detected under these conditions. At higher dissolved P concentrations and P:Fe(II) ratios, GR also forms. However, the GR formed under these conditions rapidly dissolves, likely forming amorphous ferric hydroxides together with dissolved Fe(II) and phosphate, with the dissolved species subsequently reacting to form crystalline vivianite. Our observations agree with studies showing the water column formation of GR in modern oligotrophic, anoxic Fe-rich (ferruginous) settings, and provide support for a major role for GR in controlling P cycling in ancient oligotrophic ferruginous oceans. By contrast, in more productive ancient anoxic settings, enhanced redox-controlled P recycling and/or increased weathering inputs would have led to higher dissolved P concentrations in the water column and sediments. Our observations show that such conditions ultimately promote the formation of vivianite, which would have exerted a limiting control on the extent of P recycling in ancient, more productive settings, via the long-term fixation of P in the sediments.

Lysine-functionalized layered double hydroxides for the antibiotics’ efficient removal: Controllable fabrication via BBD model and removing mechanism

Lysine-functionalized layered double hydroxides (LDHs) controllable fabrication based on Box-Behnken Design (BBD) model was investigated in this study. Ly @ FeZn presented stable and efficient removal performance for Ciprofloxacin (CIP) removal. Characterizations analysis and batch tests results showed that Ly @ FeZn had significantly different adsorption performance towards CIP and Ofloxacin (OFL). The highest adsorption capacity of CIP was 193.83 mg/g, while that of OFL was 62.12 mg/g due to the bulky structure of OFL, which posed steric hindrance for its interaction with the adsorbent. Furthermore, adsorption kinetics of CIP were better simulated with the pseudo-first-order kinetic model, while that of OFL was better described by the pseudo-second-order. The adsorption thermodynamics of Ly @ FeZn for CIP indicated that the adsorption processes were exothermal, feasible and spontaneous. It was worth noting that the adsorption mechanism of Ly @ FeZn for CIP were the synergistic reaction of surface complexation and interlayer adsorption. Also, this study provided a promising adsorbent for efficient antibiotics removal in practical water remediation.

Nanoporous multilayer graphene oxide membrane for forward osmosis metal ion recovery from spent Li-ion batteries

The recovery of Li and rare metals from the spent electrodes of Li-ion batteries (LIBs) is becoming increasingly important. Herein, nanoporous multilayer graphene oxide (NMG) membranes were fabricated via the confined thermal annealing method to achieve Li-ion selectivity. A graphene oxide (GO) membrane was prepared by coating the membrane with an aqueous layer of GO liquid crystal via the scalable bar coating method, followed by hot-pressing. The influence of the GO interlayer spacing and nanopore presence on the ion separation performance was investigated. The NMG membrane shows different ion permeance phenomena depending on ion concentration. The permeation rate of divalent ions (Co, Ni, Mn) through the NMG membrane was faster than that of Li ions in solution with low ionic strength, whereas the membrane was Li-ion selective in mixed ionic solutions or at high ionic strength. This result indicates that electrostatic interaction by oxygen groups is important at low ionic strength, but size exclusion by interlayer spacing and adsorption by nanopore is critical at high ionic strength. The ion permeation mechanisms were further examined based on molecular calculations, which revealed that the binding energies between the divalent ions and nanopores or basal plane of graphene were high, resulting in slow ion permeation. When the NMG membrane was combined in a forward osmosis system, Li-ion can be separated continuously from the mixture solution simulated from the spent Li-battery electrode. This approach shows the high potential of nanoporous multilayer graphene membrane for Li extraction in particular with dense pore structure and around 7 Å of d-spacing.

The effect of layer charge origin and density, type of interlayer cations, and occupancy of the octahedral sheet of smectites on the adsorption of pyocyanin

Inactivation of pyocyanin, a virulence compound produced by Pseudomonas aeruginosa, has been proposed as a novel alternative approach to combat the multi-drug resistant Pseudomonas aeruginosa infections. It has been shown that montmorillonite was able to bind pyocyanin. Yet, the adsorption mechanism and the influence of the structural properties of smectites — type of interlayer cation, charge density, source of charge, and occupancy of the type of octahedral sheet — on the adsorption of pyocyanin is not clear. Batch adsorption experiments, variable-temperature X-ray diffraction, UV spectroscopic measurements, and electron dispersive spectroscopy analysis were conducted to investigate the role of several smectite structural properties on pyocyanin adsorption — affinity (K), capacity (Qmax), the adsorption mechanism, and the transformation of pyocyanin on smectite surface. The results indicated that interlayer adsorption of pyocyanin in smectite occurred by two types of adsorption mechanisms. 1) protonation of zwitterionic pyocyanin (PYO) and ion-exchange of PYOH+ for interlayer cations. This mechanism was dominant in alkali and alkaline-earth metal smectites. 2) direct coordination of zwitterionic PYO with interlayer cations. This mechanism was dominant for transition/heavy metal smectite. Among the structural factors investigated, 1) the type of interlayer cation showed the greatest effects on pyocyanin adsorption. Except for Cu2+, Qmax was similar for all the cation-saturated smectites (∼0.30–0.56 mol/kg) while the affinity K was higher in monovalent cations compared to the divalent cations. The Cu-smectite recorded the highest Qmax of ∼0.70 mol/kg. 2) smectites with different layer charge densities showed similar Qmax but the K was higher in low charge density smectite suggesting the importance of hydrophobic domain for pyocyanin adsorption. 3) It appeared that the layer charge source or the types of octahedral occupation did not show distinct effects on pyocyanin adsorption, but the presence of accessory minerals may mask the effects.

Edge Coordination of Ni Single Atoms on Hard Carbon Promotes the Potassium Storage and Reversibility.

Hard carbon is the most promising anode for potassium-ion batteries (PIBs) due to its low cost and abundance, but its limited storage capacity remains a major challenge. Herein, edge coordination of metal single atoms is proved to be an effective strategy for promoting potassium storage in hard carbon for the first time, taking B, N co-doped hard carbon nanotubes anchored by edge Ni-N4 -B atomic sites (Ni@BNHC) as an example. It is revealed that edge Ni-N4 -B can provide active sites for interlayer adsorption of K+ and that Ni atoms can facilitate the reversibility of K+ storage on N and B atoms. Furthermore, an unprecedentedly reversible K+ storage capacity of 694mAhg-1 at 0.05Ag-1 is realized by introducing commercial carbon nanotubes. This work provides a new perspective for the application of single-atom engineering and the design of high-performance carbon anodes for PIBs.

Interlayer adsorption of cationic dye on cationic surfactant-modified and unmodified montmorillonite.

Water pollution by toxic organic dyes is one of the most critical health and environmental problems worldwide. By means of molecular dynamics method, the present work aims to evaluate the applicability of montmorillonite (Mt) modified by hexadecyltrimethylammonium cations (HDTMA+) compared to unmodified Na-Mt for the adsorption of cationic methylene blue (MB) dye. The results showed that the adsorption energy of MB on both HDTMA-Mt and Na-Mt absorbent ranged from -100 to -250kJ/mol, indicating the effectiveness of two types of adsorbents in dye water treatment. The highest adsorption energy was found at w =50% in each adsorbent system. Adsorption mechanisms of MB depend on molecular orientations, which is influenced by the surfactant and water content. The adsorption mechanism of MB is chemisorption dominated by strong electrostatic interaction between CH3 groups of MB and oxygen atoms of Mt surfaces. Besides, physisorption also plays a minor role in MB orientations. It is found that the existence of cationic surfactants can slightly improve the adsorption capacity of MB only at higher water content through enlarging the interlayer space of Mt and reducing mobility of MB. However, there will be a negative impact on the reduction of adsorption sites for dyes especially at low water content. Our results provide a possible application for swelling clay minerals being a promising adsorbent for dyes-surfactants co-existing wastewater treatment.

Adsorption performance and mechanism of methyl orange by layered zinc hydroxide nitrate improved through flame spray pyrolysis method

To adsorb methyl orange (MO) pollutant in wastewater, zinc hydroxide nitrate (ZHN) was synthesized by commonly-used chemical coprecipitation method (CCM) and the flame spray pyrolysis (FSP) method in this work. ZHN can efficiently adsorb MO pollutant. Specially, the adsorption capacity per unit mass of ZHN synthesized by FSP and CCM can reach 732.72 mg/g and 611.76 mg/g. ZHN synthesized by FSP possesses faster initial adsorption rate, larger adsorption capacity per unit mass and better adsorption characteristics for MO solution which may be due to the more voids between the lamellar of ZHN synthesized by FSP and the fact that the grain size, lamina thickness, and specific surface area of ZHN synthesized by FSP are 1/2, 1/6 and 4 times of those of ZHN synthesized by CCM, respectively. The adsorption kinetics of ZHN on MO is conformed to the pseudo-second-order kinetic equations. Moreover, at a low equilibrium concentration of MO, the adsorption behavior of ZHN is closer to the monolayer surface adsorption. When the adsorption on the ZHN surface tends gradually to the saturation with the increase of the equilibrium concentration, it will show the characteristics of interlayer adsorption.

Adsorption of ammonium ions onto the external surface of smectite: Effects of layer charge, concentration, anion and comparison with interlayer adsorption

The residual and release of ammonium in ion-adsorption rare earth tailings and soil are related to the interaction of NH4+ with clay minerals. Molecular dynamics simulations were used to study the adsorption and diffusion of NH4+ on the external surface of smectite. The factors, such as layer charge position (octahedral and tetrahedral substitution) and charge density (0.5, 1.0 and 1.5 e⋅uc−1, where uc denotes unit cell), initial NH4+ concentration (from 0.64 to 1.60 mol⋅L−1), and anion types (Cl−, NO3− and SO42−) were considered. The results of NH4+ adsorption on the external surface were also compared with that in the interlayer. The layer charge of smectite significantly affected the interaction between NH4+ and smectite. With increasing charge density, the major NH4+ surface complexes were changed from outer-sphere to inner-sphere, resulting in increased adsorption density and stability, which was verified by calculated hydration structure and adsorption free energies. The inner-sphere NH4+ complexes were located above the centers of the hexagonal rings and the Si tetrahedras for montmorillonite at a charge density of 1.0 e⋅uc−1, while only located in the vicinity of tetrahedral substitutions for beidellite. NO3− promoted the adsorption of NH4+ as inner-sphere complexes relative to Cl− and SO42−. As the water content in the interlayer increased, the confinement effect weakened and the mobility of NH4+ increased, and the NH4+ adsorption in the interlayer exhibited similar characteristics to that on the external surface at a water content of 10 H2O⋅uc−1.