Outer-sphere Adsorption Research Articles

Aluminum adsorption using different models of hydroxyapatite via Molecular Dynamic simulations

The activities of the anodizing industry, mining, and the inadequate use of coagulants and flocculants for wastewater treatment are some of the causes of an alarming increase in aluminum levels in water bodies. Adsorbents, such as bone char, are used for the removal of metal ions from water due to their easy operation, regeneration, and low cost. Bone char is constituted mainly by hydroxyapatite and in order to understand the mechanism of aluminum adsorption onto hydroxyapatite an adequate analysis of the effect of the surface exposed to the aqueous solution is necessary. Different facets of hydroxyapatite, 001 and 010, were studied, which have been protonated to represent the acidic conditions of the wastewater from the anodizing industry. The results of Molecular Dynamic simulations show that the protonation of the 001 facet leads to an increase in the adsorption of aluminum. The coarse surface of the 010 facet at acidic pH conditions cause that the water molecules adsorb on the sub-layers of the surface. The adsorption of aluminum occurs preferentially on the phosphates of hydroxyapatite and is due to the formation of outer-sphere adsorption complexes.

Direct observations of nanoscale brushite dissolution by the concentration-dependent adsorption of phosphate or phytate

With the development of agricultural intensification, phosphorus (P) accumulation in croplands and sediments has resulted in the increasingly widespread interaction between inorganic and organic P species, which has been, previously, underestimated or even ignored. We quantified the nanoscale dissolution kinetics of sparingly soluble brushite (CaHPO4·2H2O, DCPD) over a broad range of phosphate and/or phytate concentrations by using in situ atomic force microscopy (AFM). Compared to water, we found that low concentrations of phosphate (1–1000 µM) or phytate (1–100 µM) inhibited brushite dissolution by slowing single step retraction. However, with increasing phosphate or phytate concentrations to 10 mM, there was a reverse effect of dissolution promotion at brushite-water interfaces. In situ observations of the coupled dissolution-reprecipitation showed that phosphate precipitated more readily than phytate on brushite surfaces, with the formation of amorphous calcium phosphate (ACP). For a fundamental understanding, zeta potential and in situ Raman spectroscopy (RS) revealed that the concentration-dependent dissolution is attributed to the reverse of outer-sphere to inner-sphere adsorption with increasing phosphate or phytate concentrations. In addition, the mineralization of phytate with outer-sphere adsorption by phytase was higher than that with inner-spere adsorption, and the presence of phytate delayed ACP phase transformation to hydroxylapatite (HAP). These in situ observations and analyses may fill the knowledge gaps of interaction between inorganic and organic P species in P-rich terrestrial and aquatic environments, thereby implicating their biogeochemical cycling and the associated availability.

Adsorption of hydrated Fe(OH)<sup>2+</sup> on the kaolinite surface: A density functional theory study

The present study employed density functional theory (DFT) to analyze the adsorption configuration and mechanism of Fe(OH)<sup>2+</sup> on the kaolinite (001) surface. The findings demonstrated that Fe(OH)<sub>2</sub>(H<sub>2</sub>O)<sup>4+</sup> is the main type in which hydrated Fe(OH)<sup>2+</sup> can be found in aqueous solution. On the surface of kaolinite, Fe(OH)<sub>2</sub>(H<sub>2</sub>O)<sup>4+</sup> will be adsorbed. There are two forms of adsorption: outer-sphere and inner-sphere coordination (monodentate/bidentate) adsorption. Fe(OH)<sub>2</sub>(H<sub>2</sub>O)<sup>4+</sup> has a moderate propensity to adsorb on the alumina octahedral sheet of kaolinite when the outer-sphere coordination adsorption takes place. In cases of inner-sphere coordination adsorption, Fe exhibits a tendency to form monodentate adsorption compounds in conjunction with Ou atoms. Additionally, it prefers to create bidentate adsorption compounds through coordination with both Ot and Ou atoms. The adsorption mechanism analysis results show that the ionic property of Fe atom decreases after outer-sphere coordination adsorption. After inner-sphere coordination adsorption, some electrons of Fe atom are transferred to the surface O atom. The presence of electrons between the Fe and O atoms enhances the formation of bonds, hence enhancing the covalent nature of the Fe-O bond. Theoretical FT-IR (Fourier transform infrared spectroscopy) calculations show that the formation of Fe-O chemical bonds. Because of the lower adsorption energy and more chemical bonds, hydrate Fe(OH)<sup>2+</sup> is more likely to be bidentate adsorbed on the kaolinite surface.

Theoretical investigation of outer-sphere adsorption of hydrated yttrium ion on the basal surfaces of kaolinite: A density functional study

Ion-adsorption deposits (IADs) are important types of REE deposits. Previous studies have found that, in IAD, REE3+ adsorb on the surface of clay minerals in the form of multi-coordinated hydrated complexes, [REE(H2O)8/9]3+, forming exchangeable outer-sphere adsorption complex. Actually, there are two types of basal surfaces in clay minerals, the (001) surface and the (00−1) surface, and it is not clear whether there are differences in the adsorptions on these two types. Additionally, the electron transfer mechanism in the adsorption structures cannot be revealed. To address these issues, we employ first-principles calculations based on density functional study to simulate the outer-sphere adsorption structures of representative Y3+ ion on the two types of basal surface of kaolinite. The study reveals that both the (001) surface and the (00–1) surface can serve as adsorption surfaces, but the (00–1) surface forms more stable adsorption structures with adsorbate-[Y(H2O)8]3+ due to the absence of hydroxyl group repulsion effects. The adsorption process involves electron transfer from the kaolinite basal surface to the adsorbate, forming hydrogen bonds. The primary orbital interaction in this process is between the O-2p orbitals of the adsorbate and the surface O-2p orbitals. This work contributes to a quantum-level understanding of the nature of ion adsorption in ion-adsorption deposit.

Competitive adsorption between sodium citrate and naphthenic acids on alumina surfaces: Experimental and computational study

The adsorption of naphthenic acids (NAs) on clay surfaces induces an increase in surface hydrophobicity, which is detrimental to crude oil recovery and froth quality. Sodium citrate (Na3Cit), with more functional groups than NAs, is expected to compete with NAs for adsorption on mineral surfaces. The adsorption of NAs on alumina surfaces was quantified in the presence and absence of Na3Cit at pH 8 ± 0.1. An increase in hydrophobicity of the alumina surface was observed after the adsorption of NAs without Na3Cit. However, adding Na3Cit significantly reduced NAs adsorption and protected the surface wettability. The viscoelasticity and electrokinetic properties of adsorbed materials indicate Na3Cit was preferentially covered on alumina surfaces and repelled NAs by its negative charges. The competition between Na3Cit and NAs for adsorption sites was explained using ab initio Car-Parrinello molecular dynamics (CPMD) simulation and periodic plane-wave density functional theory (DFT) to calculate inner-sphere adsorption and outer-sphere adsorption, respectively. Favourable adsorption of sodium citrate than NAs representatives on aluminum (hydr)oxide cluster and the basal and edge planes of gibbsite were observed from the perspective of free energy barrier and adsorption energy, respectively.

Understanding the sources of mercury release from coal: A combined experimental and molecular simulation study

Understanding the occurrence modes of mercury in coal is important as its release poses long-term adverse effects on the environment and human health during coal production and utilization. However, the matter still remains a subject of controversy due to differing results from direct and indirect analyses, which suggest various possible modes of occurrence for mercury in coal. Additionally, the experimental measurement of Hg concentration presents challenges, further contributing to the complexity of the issue. A comprehensive investigation of experiments and molecular simulations is conducted herein. Electron probe microanalysis and elemental mapping analysis show that elemental Hg is concentrated in framboidal pyrites while absent in organic matter. To understand the occurrence modes of mercury in inorganic and organic materials at the atomic level, molecular simulations are performed for Hg2+ adsorption and retention in MMT, pyrite, and kerogen slit nanopores. It is found that the inorganic MMT and pyrite surfaces have a greater adsorption capacity than the organic kerogen surface (pyrite > MMT > kerogen). The outer-sphere adsorption is mainly observed with at least one monolayer of water molecules exiting between the ion and mineral surfaces. MMT has the highest retention for Hg2+ transport as the self-diffusion coefficient is the smallest among the three slit pores (MMT < pyrite < kerogen). The high adsorption and retention originate from the strong Hg2+-mineral interaction. These results suggest that mercury in coal is most likely associated with inorganic minerals instead of organic matter.

Evolution of the electrical double layer with electrolyte concentration probed by second harmonic scattering.

Investigating the electrical double layer (EDL) structure has been a long-standing challenge and has seen the emergence of several sophisticated techniques able to probe selectively the few molecular layers of a solid/water interface. While a qualitative estimation of the thickness of the EDL can be obtained using simple theoretical models, following experimentally its evolution is not straightforward and can be even more complicated in nano- or microscale systems, particularly when changing the ionic concentration by several orders of magnitude. Here, we bring insight into the structure of the EDL of SiO2 nanoparticle suspensions and its evolution with increasing ionic concentration using angle-resolved second harmonic scattering (AR-SHS). Below millimolar salt concentrations, we can successively characterize inner-sphere adsorption, diffuse layer formation, and outer-sphere adsorption. Moreover, we show for the first time that, by appropriately selecting the nanoparticle size, it is possible to retrieve information also in the millimolar range. There, we observe a decrease in the magnitude of the surface potential corresponding to a compression in the EDL thickness, which agrees with the results of several other electroanalytical and optical techniques. Molecular dynamics simulations suggest that the EDL compression mainly results from the diffuse layer compression rather than outer-sphere ions (Stern plane) moving closer to the surface.

Nanogap-Resolved Adsorption-Coupled Electron Transfer by Scanning Electrochemical Microscopy: Implications for Electrocatalysis.

Here, we demonstrate for the first time that the mechanism of adsorption-coupled electron-transfer (ACET) reactions can be identified experimentally. The electron transfer (ET) and specific adsorption of redox-active molecules are coupled in many electrode reactions with practical importance and fundamental interest. ACET reactions are often represented by a concerted mechanism. In reductive adsorption, an oxidant is simultaneously reduced and adsorbed as a reductant on the electrode surface through the ACET step. Alternatively, the non-concerted mechanism mediates outer-sphere reduction and adsorption separately when the reductant adsorption is reversible. In electrocatalysis, reversibly adsorbed reductants are ubiquitous and crucial intermediates. Moreover, electrocatalysis is complicated by the mixed mechanism based on simultaneous ACET and outer-sphere ET steps. In this work, we reveal the non-concerted mechanism for ferrocene derivatives adsorbed at highly oriented pyrolytic graphite as simple models. We enable the transient voltammetric mode of nanoscale scanning electrochemical microscopy (SECM) to kinetically control the adsorption step, which is required for the discrimination of non-concerted, concerted, and mixed mechanisms. Experimental voltammograms are compared with each mechanism by employing finite element simulation. The non-concerted mechanism is supported to indicate that the ACET step is intrinsically slower than its outer-sphere counterpart by at least four orders of magnitude. This finding implies that an ACET step is facilitated thermodynamically but may not be necessarily accelerated or catalyzed by the adsorption of the reductant. SECM-based transient voltammetry will become a powerful tool to resolve and understand electrocatalytic ACET reactions at the elementary level.

Effect of layer charge density and cation concentration on sorption behaviors of heavy metal ions in the interlayer and nanopore of montmorillonite: A molecular dynamics simulation

Soil and ground water pollution by heavy metal is an identified global environment concern. Montmorillonite is widely adopted as an adsorbent used in retardation of heavy metals due to its excellent adsorption capacity, conveniency, and stability. However, the adsorption mechanism of heavy metal ions in the interlayer and nanopore of montmorillonite needs to be further studied. The classical grand canonical monte carlo (GCMC) and molecular dynamics (MD) simulations were performed to quantify the structural and dynamic properties of heavy metal ions in the interlayer and nanopore of montmorillonite (MMT). Two layer charge densities of MMT, two cation concentrations, and three typical heavy metal ions (Pb2+, Ba2+, and Cs+) were considered. With the increase of layer charge density, outer-sphere adsorption (OSA) was transformed into inner-sphere adsorption (ISA) for three heavy metal ions. The adsorption quantity increased with the increase of layer charge density and cation concentration. The calculated diffusion coefficients of three heavy metal caions in montmorillonite was bulk solution > nanopore > interlayer. The results and adsorption mechanism are helpful for understanding the natural processes of heavy metal ions in environment and developing more efficient remediation materials.

Interaction mechanism of uranium(VI) with chitosan hydrogel: Insights from the perspective of adsorbent and adsorbate

Chitosan-based hydrogels have been used as adsorption materials and offered remarkable prospects, due to their abundant, biodegradable, inexpensive features and rich amino and hydroxyl groups. In this work, the structure of chitosan (CS) hydrogel and its adsorption performance toward uranyl species (UO22+ and UO2(CO3)34−) were comprehensively investigated on the basis of various characterizations and theoretical simulation. Systematic experiments show that CS hydrogel bears excellent adsorption performance for low-concentration UO2(CO3)34− solution in column adsorption experiment and the removal efficiency is >99 %. The outer-sphere complexation adsorption has been postulated for the case of UO2(CO3)34−, where intermolecular hydrogen bonds dominate in the adsorption. Differently, the inner-sphere one is attributed for capturing UO22+ species, which is coordinated by CN, -NH2 and -OH groups of CS hydrogel. Since the 3D network structure of CS hydrogel is prone to couple with water, it is difficult for U(VI) to enter inside the hydrogels, which reduces the adsorption performance. However, this phenomenon has been effectively weakening by using column adsorption experiment, which accomplished the exchange between water and U(VI). These detailed investigation toward adsorption mechanism might open a doorway to gain deeper insights on the design and application of efficient adsorption materials for uranium extraction.

Competitive adsorption of nitrate, phosphate, and sulfate on amine-modified wheat straw: In-situ infrared spectroscopic and density functional theory study

Amine-modified wheat straw (AMWS) has already been reported as a promising adsorbent for nitrate (NO3) removal due to its cost-effectiveness and high efficiency. However, the NO3 removal mechanism has not been well understood, especially in the presence of co-existing ions. Here, the effect of co-existing anions on NO3 removal by AMWS was investigated and the underlying mechanisms were revealed using a combination of in-situ infrared (IR) spectroscopy and computational modeling. The in-situ IR results indicated that NO3, sulfate (SO4), and phosphate (PO4) are all adsorbed as outer-sphere complexes on AMWS. The two-dimensional-correlation spectroscopy analysis implied the adsorption sequence of SO4 > PO4 > NO3. The adsorption energies obtained from density functional theory calculation range from −0.24 to 0.51 eV (−23.2 to 49.2 kJ/mol), confirming that these anions adsorb on AMWS as outer-sphere complexes. For the first time, this study provides direct spectroscopic evidence of the outer-sphere adsorption of NO3 on AMWS, as well as identifies the adsorption sequence, confirmed by computational modeling. The competitive mechanism of NO3, SO4, and PO4 revealed in this study is helpful to understand and predict the applications of AMWS.

Molecular-level understanding of metal ion retention in clay-rich materials

Clay minerals retain or adsorb metal ions in the Earth’s critical zone. Rocks, sediments and soils rich in clay minerals can concentrate rare earth elements (REEs) in ion adsorption-type deposits (IADs) and are similarly effective at metallic contaminant remediation. However, the molecular-scale chemical and physical mechanisms of metal ion retention remain only partly understood. In this Review, we describe the nature, location and energy requirements of metal retention at clay mineral surfaces. Retention originates mainly from electrostatic interactions during cation exchange at low pH and chemical bonding in surface complexation and precipitation at neutral and high pH. Surface complexation can induce surface redox reactions and precipitation mechanisms including neoformation of clay mineral layered structures. In IADs, outer-sphere adsorption is the major retention mechanism of REE ions. By contrast, the use of clay minerals in pollution control relies on various mechanisms that can coexist, including cation exchange, surface complexation and nucleation growth. To more effectively leverage clay mineral–metal interactions in resource recovery and contaminant remediation, complex mechanisms such as surface precipitation and redox reactions must be better understood; for instance, by utilizing advances in quantum mechanical calculations, close combination between synchrotron and simulation techniques, and upscaling of molecular-level information in macroscopic thermokinetic predictive models. Clay minerals can retain metal ions, concentrate rare earth elements and be exploited for industrial waste disposal. This Review discusses the molecular-level mechanisms of metal ion retention in clay minerals and their importance for environmental and industrial applications.

Infiltration of salt solutions through illite particles: Effect of nanochannel size and cation type

Water infiltration through soil particles is of great importance in determining the availability of elements and soil texture. However, process of water infiltrate into the capillary nanochannels and influence of nanochannel size and cation type as well as the underlying mechanism are still lacking, especially at the molecular level. Here, molecular dynamics simulation was used to investigate the infiltration of NaCl and CaCl2 solutions through 1.5, 3, 5, and 10 nm illite channels. At the initial stage, obvious regression of the advanced frontier of salt solution was observed. Water molecules and cations easily accumulated at the entrance of nanochannels, which was clearer in small-sized nanochannels. The infiltration velocity increased with increasing nanochannel size, though time for water saturating the illite nanochannel showed an opposite trend. Both Na+ and Ca2+ exhibited obvious inner-sphere adsorption during the infiltration, but Ca2+ showed more outer-sphere adsorption compared to Na+. Diffusion coefficients of both water and cations were higher in the small- than large-sized nanochannel, which was related to the unsaturated condition, capillary force, and interplay between illite-water interaction and surface tension. By using MD method, this study is the first time, to our knowledge, to investigate the unsaturated capillary infiltration of water in clay soil nanochannels, which is hard to achieve using traditional experimental tools. Results showed that, besides the generally considered interaction between soil particles, cation-clay interactions coupled with nanochannel size were of great importance in affecting the unsaturated infiltration of water and cations. This study provides a new clue in understanding the availability of soil water and stability of soil aggregates.

Removal of Sb(V) from wastewater via siliceous ferrihydrite: Interactions among ferrihydrite, coprecipitated Si, and adsorbed Sb(V)

Although ferrihydrite (Fh) exhibits good Sb(V) adsorption behavior, the instability of its amorphous structure limits its engineering applications. In this study, siliceous ferrihydrite (SiFh) was prepared via coprecipitation to resolve these limitations. X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and SiFh aging tests revealed that the growth of Fh particles covered with Fe–O–Si links was inhibited while maintaining their amorphous structure. Meanwhile, the XRD patterns indicated that SiFh maintained excellent stability after five adsorption–desorption cycles. During the aging process, the added Si decreased the electrostatic interaction between SiFh and Sb(V), which weakened the affinity between Sb(V) and Fh; however, most of the Sb(V) still entered the Fe lattice after seven days of aging, which was favorable for Sb(V) recovery during reutilization. Furthermore, Sb(V) adsorbed from the simulated textile wastewater onto SiFh had the highest adsorption energy (Eads), which meant its unstable inner-sphere complexation on the surface of SiFh. Meanwhile, the presence of SO42−, NO3−, Ca2+, and Mg2+ contributed to Sb(V) outer-sphere adsorption. Both of these factors were conducive to Sb(V) desorption. Hence, SiFh is a promising adsorbent owing to its facile preparation process, stability, and optimal regeneration properties.

Second Harmonic Scattering Reveals Ion-Specific Effects at the SiO2 and TiO2 Nanoparticle/Aqueous Interface

Ion-specific effects play a crucial role in controlling the stability of colloidal systems and regulating interfacial processes. Although mechanistic pictures have been developed to explain the electrostatic structure of solid/water colloidal interfaces, ion-specific effects remain poorly understood. Here we quantify the average interfacial water orientation and the electrostatic surface potential around 100 nm SiO2 and TiO2 colloidal particles in the presence of NaCl, RbCl, and CaCl2 using polarimetric angle-resolved second harmonic scattering. We show that these two parameters can be used to establish the ion adsorption mechanism in a low ionic strength regime (<1 mM added salt). The relative differences between salts as a function of the ionic strength demonstrate cation- and surface-specific preferences for inner- vs outer-sphere adsorption. Compared to monovalent Rb+ and Na+, Ca2+ is found to be preferentially adsorbed as outer-sphere on SiO2 surfaces, while a dominant inner-sphere adsorption is observed for Ca2+ on TiO2. Molecular dynamics simulations performed on crystalline SiO2 and TiO2 surfaces support the experimental conclusions. This work contributes to the understanding of the electrostatic environment around colloidal nanoparticles on a molecular level by providing insight into ion-specific effects with micromolar sensitivity.

Mechanisms of selenium removal by partially oxidized magnetite nanoparticles for wastewater remediation

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions selenite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we characterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption capacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form 2C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions 2E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes.

Molecular Dynamics Simulation of Ion Adsorption and Ligand Exchange on an Orthoclase Surface.

Orthoclase (K-feldspar) is one of the natural inorganic materials, which shows remarkable potential toward removing heavy metal ions from aqueous solutions. Understanding the interactions of the orthoclase and metal ions is important in the treatment of saline wastewater. In this paper, molecular dynamics simulations were used to prove the adsorption of different ions onto orthoclase. The adsorption isotherms show that orthoclase has remarkable efficiency in the removal of cations at low ion concentrations. Aluminol groups are the preferential adsorption sites of cations due to higher negative charges. The adsorption types and adsorption sites are influenced by the valence, radius, and hydration stability of ions. Monovalent cations can be adsorbed in the cavities, whereas divalent cations cannot. The hydrated cation may form an outer-sphere complex or an inner-sphere complex in association with the loss of hydration water. Na+, K+, and Ca2+ ions mainly undergo inner-sphere adsorption and Mg2+ ions prefer outer-sphere adsorption. On the basis of simulation results, the mechanism of ion removal in the presence of orthoclase is demonstrated at a molecular level.

A modeling approach for unveiling adsorption of toxic ions on iron oxide nanocrystals

The era of advanced computer simulations in materials science enables a great potential to design in silico computational experiments for (nano-)material performance. The adsorption efficiency of nanoparticles in various environments can be unveiled by atomistic models and computer simulations. Arsenic (As) is one of the important globally distributed contaminants with a hazardous impact on human health and environment, and it can strongly bind with iron nanocrystals (e.g., hematite (Fe2O3)) depending on their shape and size. Here, we developed a novel Kinetic Monte Carlo (KMC) model capable of exploring and delineating shape-efficiency dependence for Fe2O3 nanocrystals in contact with arsenate-contaminated water. This newly designed model demonstrated the performance of nanocrystals for removal of toxic (As) ions on their surface. The current model opens new avenues for designing further advanced KMC models for nanoparticles-toxic ions interactions, under varying environmentally relevant situations, e.g., groundwater, wetlands, and water treatment systems. In addition to bidentate adsorption complexes, implemented in the model presented, monodentate and outer-sphere adsorption complexes should be incorporated into the KMC model. Detailed environmental controls can be addressed by implementation of pH and background ions.

Adsorption of lead and antimony in the presence and absence of EDTA by a new vermiculite product with potential recyclability.

A new two-step modification method has been proposed where 1.8% HCl and 3.1% HNO3 were applied to modify the interlayer of vermiculite (VMT). This product was given 90 °C of heat in 30% H2SO4 solution that was used for Pb (II) and Sb (III) adsorption. The EDTA presence on the individual adsorption was assessed. X-ray diffraction revealed that the VMT inter-stratified reflection through acid intercalation within the interlayer decreased the parallel gaps between the atoms, witnessing on the outer-sphere adsorption. The driving force was found electrostatic, which fits well with pseudo-second-order kinetics and Langmuir isotherm. The Pb (II) and Sb (III) uptake followed descending order adsorption with increasing concentration of chelating EDTA. Three consecutive desorption cycles revealed that the prepared adsorbent was suitable that may be regarded as a good candidate for complex wastewaters.

2D-layered Mg(OH)2 material adsorbing cellobiose via interfacial chemical coupling and its applications in handling toxic Cd2+ and UO22+ ions

The interfacial chemistry of nanocomposite materials is of overarching importance in the separation and purification science; moreover, its understanding helps to guide synthesis, clarify structure-property relationship and unearth novel applications. However, the composites feature rather complicated local structures and hydrogen bonds are often involved in the interface and the vicinity of active sites. In this regard, density functional theory first-principle calculations associated with experimental study have synergistically examined two-dimensional (2D) magnesium hydroxide material with different layers and their adsorption toward cellobiose. Hydrogen bonds are found responsible for the interfacial coupling, which make it vital to cover the dispersion correction in the calculation. The average adsorption energy ranges from −0.29 to −0.35 eV, falling well within the range of reported hydrogen-bonding strength. On the basis of calculated structural/interfacial properties and experimental findings, the 2D Mg(OH)2 in terms of three-layer model was unraveled to substitute toxic Cd2+ ion and sorb radioactive UO22+ that is coordinated by water and hydroxyl groups. These reactions are thermodynamically feasible. The ion-exchanging mechanism was proposed for cadmium removal and the outer-sphere adsorption one for uranium extraction.