A study of growth mechanism of KDP and ADP crystals by means of quantum chemistry

Impurity adsorption, crystal growth by adsorption of growth unit and step-pinning mechanism of metal ion adsorption were investigated for potassium dihydrogen phosphate (KDP; KH₂PO₄) and ammonium dihydrogen phosphate (ADP; NH₄H₂PO₄) by quantum chemistry. In this study, the ideal crystal morphologies, the growth unit and the crystal surface with and without metal ions were calculated and analyzed by using electrostatic property. It is found that the computational results based on electrostatic potential distribution can account for the observed behaviours on KDP and ADP crystal growth.     

Adsorption of Cr(Ⅵ) polluted water by Fe3O4@SiO2-APTMS nanocomposites prepared in the presence of ultrasonic irradiation for sustainable water resources utilization

The research on developing a purification technology for Cr(Ⅵ) polluted water with high efficiency and the low energy consumption is crucial for achieving several Sustainable Development Goals (SDGs). In order to achieve these goals, Fe₃O₄@SiO₂-APTMS nanocomposites were prepared by Fe₃O₄ nanoparticles modified with silica and 3-aminopropyltrimethoxysilane in the presence of ultrasonic irradiation. The nanocomposites were characterized by TEM, FT-IR, VSM, TGA, BET, XRD, XPS and these analytic results proved that the nanocomposites were successfully prepared. The influential factors of Fe₃O₄@SiO₂-APTMS on Cr(Ⅵ) adsorption have been explored and better experimental conditions have been obtained. The adsorption isotherm conformed to the Freundlich model. Pseudo-second-order kinetic model provided a better correlation for the experimental data compared to other kinetic models. Thermodynamic parameters for adsorption indicated that the adsorption of Cr(Ⅵ) was a spontaneous process. It was speculated that the adsorption mechanism of this adsorbent includes redox, electrostatic adsorption and physical adsorption. In summary, the Fe₃O₄@SiO₂-APTMS nanocomposites were of great significance to human health and the remediation of heavy ion pollution, contributing to achievement of the Sustainable Development Goals (SDGs), including SDG-3, SDG-6.     

Preparation of functional composite grafted particles PDMAEMA/SiO2 and preliminarily study on functionality

Micron-sized silica gel particles were first surface-modified with coupling agent, γ-methacryloylpropyl trimethoxysilane (MPS), and the polymerizable double bonds were introduced onto the surfaces of silica gel particles, forming the modified particles MPS-SiO₂. Subsequently, N,N-dimethylaminoethyl methacrylate (DMAEMA) was graft-polymerized on the surfaces of particles MPS-SiO₂ in the manner of “grafting through”, resulting in the grafted particles PDMAEMA/SiO₂. The grafted particles PDMAEMA/SiO₂ were fully characterized with several means. The graft polymerization process of DMAEMA on particles MPS-SiO₂ was studied in detail, and the optimal reaction conditions were determined. Thereafter, the adsorption properties of the grafted particles PDMAEMA/SiO₂ for chromate anion and Cu²⁺ ion were preliminarily examined respectively. The experimental results indicate that the PDMAEMA grafting degree on PDMAEMA/SiO₂ particles is limited because an enwinding polymer layer as a kinetic barrier on the surfaces of silica gel particles will be formed during the graft polymerization, and blocks the graft polymerization. In order to enhance PDMAEMA grafting degree, reaction time and temperature, and the used amount of initiator as well as the monomer concentration should be effectively controlled. The preliminary adsorption tests show that the grafted particles PDMAEMA/SiO₂ are multi-functional. They possess very strong adsorption ability for CrO₄²⁻ anion by right of strong electrostatic interaction, and have also adsorption action towards heavy metal ion by dint of complexing action.     

Relaxation at clean metal surfaces

An electrostatic model of the energy of the surface layers of a metal is shown to describe in detail complex phenomena of surface relaxation in clean metals. The model accounts for relaxation effects that go many layers deep, that have both parallel and perpendicular components and that show large variations from surface to surface of the same metal. The model adds a new physically plausible assumption to the simple electrostatic model previously proposed by Finnis and Heine, which increases the force binding each ion to its bulk position by an amount fixed empirically for each metal. The equilibrium configuration of surface layers is found by minimizing the energy with respect to rigid translations of ion nets in a fixed electronic background density. The many surface structure parameters thus determined fit low-energy electron diffraction data on six surfaces of bcc Fe and six of fcc Al well in almost all cases.     

Preparation, characterization and dye adsorption properties of γ-Fe2O3/SiO2/chitosan composite

A γ-Fe₂O₃/SiO₂/chitosan composite was prepared by water-in-oil emulsification, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). Effects of various factors, including adsorbent dosage, initial dye concentration, solution pH, and competing anions, on the adsorption of methyl orange from aqueous solutions by the resulting composite were studied by batch adsorption experiments. The adsorption kinetics was found to follow the pseudo-second-order kinetic model, and intraparticle diffusion was related to the adsorption, but not as a sole rate-controlling step. The equilibrium adsorption data were well described by the Freundlich isotherm model. Evaluation of the thermodynamic parameters ΔG°, ΔH°, and ΔS° revealed that the adsorption process was naturally feasible, spontaneous, and exothermic. The composite was proven to be efficient, suitable and promising for the removal of methyl orange from aqueous solutions since it has a relatively higher adsorption capacity than other low-cost adsorbents.     

Surface diffusion upon oxygen isotopic exchange on oxide-supported metal nanoclusters

A newly developed real-time kinetic model is presented. The model was used for the characterization of supported metal catalysts' properties in the oxygen isotopic exchange reaction. This model is based on rate equations and includes several elementary processes: adsorption on the metal nanoclusters, reactions on the metal nanoclusters, desorption from the metal nanoclusters, surface and bulk diffusion on/in the oxide support and direct exchange with the gas phase. On the basis of this model, a computer code was developed to determine the kinetic and thermodynamic parameters of the oxygen isotopic exchange reactions. The characteristics of a Rh/CeO₂ catalyst were examined in the temperature range between 200 and 450 °C.     

Preparation and characterization of hexadecyl functionalized magnetic silica nanoparticles and its application in Rhodamine 6G removal

In this paper, a new adsorbent, hexadecyl functionalized magnetic silica nanoparticles (C₁₆/SiO₂-Fe₃O₄ NPs), was prepared by a facile method. The final product was characterized by X-ray diffractometer, transmission electron microscope, Fourier transform infrared spectrometer and vibration sample magnetometer. The preparation and adsorption conditions of the adsorbent were optimized. The adsorbent prepared maintaining volume ratio of tetraethylorthosilicate to hexadecyltrimethoxysilane at 1:0.5 and their total volume at 1100 μL exhibited high adsorption capacity. The optimum pH value for the adsorption experiments was 11.00. The adsorption behavior of Rhodamine 6G onto C₁₆/SiO₂-Fe₃O₄ NPs obeyed pseudo-second-order kinetic model and Langmuir isotherm. Thermodynamic data indicated that the adsorption process was spontaneous and exothermic. The adsorption capacity of the adsorbent could reach to 35.6 mg g⁻¹, owing to the hydrophobic attraction and the enhanced electrostatic attraction. The saturation magnetization of the magnetic adsorbent was 35 emu g⁻¹, which ensured the magnetic separation after adsorption.     

Adsorption and desorption kinetics of Cu and Au on (0001) graphite

The interaction of Au and Cu atoms with the (0001) plane of graphite was studied by mass spectrometric measurements of desorption flux both in the presence and absence of an incident atomic beam. For these systems the condensation coefficient increases from ～0.05 at θ = 0 to unity at θ = 1; furthermore the rate of thermal desorption has a kinetic order of one-half for both systems at low coverage. These observations are consistent with a kinetic model in which two-dimensional nucleation of mobile adsorbed atoms occurs upon adsorption, while the reverse process, loss of atoms from the edges of disc nuclei, is the rate controlling step for desorption. This model implies that bonding of metal atoms to the basal plane of graphite is weak and nonlocalized, with adsorption occurring only when two-dimensional nucleation permits metal-metal bonding.     

Electrostatic potentials in gas–solid fluidized beds influenced by the injection of charge inducing agents

Accumulation of electrostatic charges in gas–solid fluidized beds can lead to particle adhesions, electrostatic discharges, wall sheeting and even explosion. Therefore, it is of vital importance to reduce the electrostatic level in industrial fluidized beds. Adding chemical static agents was reported as one of the most promising ways to control fluidized bed electrostatic potentials. In this study, the electrostatic potential distributions were measured in a three-dimensional column gas–solid fluidized bed with the LLDPE particles for fluidization and the introduction of chemical static agents. Seven charge inducing agents including TiO₂, Al₂O₃, MgO, H₂O, C₂H₅OH, ZnO, and Fe₂O₃ were employed to investigate their influence on electrostatic potentials distribution respectively. Additionally, Al(Et)₃, which is normally used as the cocatalyst for olefin polymerization, was also studied for comparison. It was found that the adding of negative charge inducing agent such as TiO₂ made the positive electrostatic potentials shift to higher values and the negative electrostatic potentials shift to lower values. The injection of positive charge inducing agent such as Al₂O₃ and MgO caused the positive electrostatic potentials dropping to lower values and the negative electrostatic potentials shifting to the values near zero at low injection amount. Affecting mechanism of charge inducing agents was discussed based on the experimental results and the electronegativity of corresponding compounds. It was found that the electronegativity (χ_i) of the metal ion played an important role in determining the charge polarity on the LLDPE particles and the change in the electrostatic potentials after contact and separation with the charge inducing agent particles.     

Nanofibrous membrane of polyacrylonitrile with efficient adsorption capacity for cadmium ions from aqueous solution: Isotherm and kinetic studies

The nanofibrous membrane of polyacrylonitrile (NMP) was successfully synthesized after NaOH and NaHCO₃ treatment aiming its functionalization using electrospinning for cadmium ion (Cd²⁺) adsorption. Field emission scanning electron microscopy (FE-SEM) revealed that small particles attached to the surface of functionalized PAN nanofibers. Equilibrium was attained after 60 min following a rapid uptake of Cd²⁺ with maximum adsorption capacity and percentage removal at an optimum solution pH of 7.0. The adsorbent dose of 0.3 g and 90 mg L⁻¹ of initial Cd²⁺ concentration yielded the maximum adsorption capacity. The pseudo-second-order kinetic model was the best fitted to the adsorption data, indicating that the chemisorption is the controlling mechanism of adsorption. The physisorption was proposed based on the calculated values of the mean free energy of adsorption from the D–R isotherm (E < 8 kJ mol⁻¹). Furthermore, three-parameter isotherm models indicated the homogeneous and heterogeneous Cd²⁺ adsorption onto NMP adsorbent.     

Influence of Ca2+ or Na+ extraframework cations on the thermal dehydration and related kinetic performance of LSX zeolite

Nitrogen adsorption performances of low-silica type X zeolites (LSX) containing Na⁺ or Ca²⁺ ions were studied and compared with Li⁺ ion, and their structural and thermal properties were investigated using various characterizations (XRD, TG-DTG, BET, XPS, SEM, TEM, and EDX with elemental mapping techniques). The kinetics of their thermal dehydration and decomposition was studied using thermogravimetry at three rates (5, 10, and 15 K/min) of linear increase of temperature under non-isothermal heating. Two model free procedures named, Kissinger, and Flynn–Wall–Ozawa (iso-conversional) and one model fitting method called Coats–Redfern based on single TG curves, as well as 41 mechanism functions were used. The kinetic parameters (apparent activation energy E, pre-exponential factor A and model) of the three phases for each sample obtained from the non-isothermal methods were then compared with the results from iso-conversional methods this showed that they strongly depend on the selection of appropriate mechanism function and the corresponding kinetic model from the perspective of crystal structure used. The results demonstrated that the E value obtained at low temperature was lower than that at high temperature, implying that the dehydration process of physisorbed water belongs to diffusion-based control, while decomposition of bonded water (chemisorbed) belongs to kinetic-based control at high temperature. These comparisons allow us to underline the strong effect of cations in association with water and their distribution in the micropores of LSX on the N₂ adsorption performance.     

Preparation of activated carbon from Enteromorpha prolifera and its use on cationic red X-GRL removal

Enteromorpha prolifera was pyrolyzed to prepare activated carbon using chemical activation by zinc chloride. The effect of activation parameters such as activation temperature, weight ratio (Enteromorpha prolifera to ZnCl₂), and activation time was investigated. The BET results showed that the surface area and pore volume of activated carbons were achieved as high as 1722 m²/g and 1.11 cm³/g, respectively, in the optimal activation conditions. Batch adsorption studies were carried out to study the adsorption properties of cationic red X-GRL onto activated carbon by varying the parameters like initial solution pH, contact time, and temperature. The kinetic studies showed that the adsorption data followed a pseudo second-order model. The isotherm analysis indicated that the adsorption data could be represented by the Langmuir isotherm model. The Langmuir monolayer adsorption capacity of cationic red X-GRL was estimated as 263.16 mg/g at pH 6.0.     

Interfacial kinetic enhancement of metal ion adsorption on binary mixed self-assembled monolayers

The adsorption of metal ions, a type of surface reaction on binary mixed self-assembled monolayers (SAMs) on a gold surface composed of 1,6-hexanedithiol (HDT) with 11-mercaptoundecanoic acid (MUA), was monitored by in situ surface plasmon resonance (SPR) measurements. The differential SPR reflectance (ΔR) enables the kinetics of adsorption of Pt²⁺ on the mixed SAMs to be investigated. Unlike single HDT SAM, kinetic analyses of the mixed SAMs showed that the rate of adsorption of Pt²⁺ was enhanced and that it was highly dependent on the fraction of MUA present. These SPR measurements suggest that the adsorption rate of metal ions can be readily manipulated simply by using mixed SAMs.     

Carbon monoxide and carbon dioxide interaction with tantalum

The adsorption of carbon monoxide and carbon dioxide on tantalum and the dissolution of these gases in the adsorbent at T ? 300 K have been studied. The flash-filament method (FFM) in a monopole mass-spectrometer and a field emission microscopy was used in the same apparatus. Carbon monoxide and carbon dioxide dissociate on the tantalum surface, carbon monoxide being desorbed in both cases during the flash. The desorption curves of CO reveal three different binding states: two of them (α and $\text{}\text{?}$gb₁) for the adsorbed particles whereas the high temperature desorption state relates to the adsorbate dissolved in the metal, For the $\text{}\text{?}$gb₁ state of CO the activation energy, the pre-exponential factor and the kinetic order in the kinetic equation of desorption have been estimated. They turned out to be E = 110 kcal/mol, C = 3 × 10¹²sec^?1, and ν = 1. The activation energy of diffusion for CO in tantalum and the energy of outgassing for the metal were found to be 9.4 and 49 kcal/mole, respectively.     

FA(Ga+,In+,Tl+) tunable laser activity and interaction of halogen atoms (F,Cl,Br,I,At) at the (001) surface of KCl crystal: ab initio calculations

An attempt has been made to examine F_A(Ga⁺,In⁺,Tl⁺) tunable laser activity and adsorptivity of halogen atoms (F,Cl,Br,I,At) at the (0 0 1) surface of KCl crystal using an embedded cluster model, CIS and density functional theory calculations with effective core potentials. The ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field at the host surface. The nearest neighbor ions to the defect site were then allowed to relax to equilibrium. Based on the calculated strength of electron–phonon coupling and Stokes-shifted optical transition bands, The F_A(Tl⁺) center was found to be the most laser active in agreement with the experimental observation that the optical emissions of F_A(In⁺) and F_A(Ga⁺) centers were strongly quenched. The disappearance of the anisotropy and np splitting observed in the absorption of F_A(Ga⁺,In⁺,Tl⁺) centers were monotonically increasing functions of the size of the impurity cation. The F_A(Ga⁺,In⁺,Tl⁺) defect formation energies followed the order F_A(Ga⁺)>F_A(In⁺)>F_A(Tl⁺). The Glasner–Tompkins empirical relationship between the principal optical absorption of F centers in solids and the fundamental absorption of the host crystal was generalized to include the positive ion species. As far as the adsorptivity of the halogen atoms is concerned, the F and F_A(In⁺,Tl⁺) centers were found to change the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred from the defect-free surface. The calculated adsorption energies were explainable in terms of the electron affinity, the effective nuclear charge and the electrostatic potentials at the surface. The spin pairing mechanism played the dominant role in the course of adsorbate–substrate interactions and the KCl defect-free surface can be made semiconducting by F or F_A(In⁺,Tl⁺) surface imperfections.     

Flow microcalorimetry: Experimental development and application to adsorption of heavy metal cations on silica

Sorption of metal ions at the oxide mineral-water interfaces is a complex process involving many various contributions that can be explained using thermodynamics. The aim of this study is to obtain experimental thermodynamic data on adsorption of two heavy metal ions (Cd(II) and Pb(II)) on macroporous silica. Thermal signals of adsorption are studied by flow microcalorimetry which has been preferred because physico-chemicals conditions (pH, equilibrium concentration,…) can be controlled (the routine configuration was optimized in order to get a very stable pressure baseline and avoid important fluctuations in the determination of heat). Mechanisms driving the adsorption have been explained. The calculation of the effective charge of ions determined from the speciation diagrams and of the surface charges shows that the interactions between the two metals and the silica surface are mainly electrostatic. The differential enthalpies of adsorption Δ_adsH have been experimentally measured. The heat of cadmium adsorption is low, endothermic and quantitatively equivalent to that of desorption. In the case of lead, the adsorption is athermal. Free energies and entropic effects related to cation adsorption have then been deduced according to the Gibbs’law. The entropy is positive during the adsorption process and at this temperature (298 K) is quite equivalent to free energy. This entropy is due to modification of hydration shell of the ions during their insertion into the interfacial region.     

Highly efficient simultaneous ultrasonic-assisted adsorption of Pb(II), Cd(II), Ni(II) and Cu (II) ions from aqueous solutions by graphene oxide modified with 2,2′-dipyridylamine: Central composite design optimization

In present work, a graphene oxide chemically modified with 2,2′-dipyridylamine (GO-DPA), was synthesized by simple, fast and low-cost process for the simultaneous adsorption of four toxic heavy metals, Pb(II), Cd(II), Ni(II) and Cu(II), from aqueous solutions. The synthesized adsorbent was characterized by FT-IR, XRD, XPS, SEM and AFM measurements. The effects of variables such as pH solution, initial ion concentrations, adsorbent dosage and sonicating time were investigated on adsorption efficiency by rotatable central composite design. The optimum conditions, specified as 8 mg of adsorbent, 20 mg L⁻¹ of each ion at pH 5 and short time of 4 min led to the achievement of a high adsorption capacities. Ultrasonic power had important role in shortening the adsorption time of ions by enhancing the dispersion of adsorbent in solution. The adsorption kinetic studies and equilibrium isotherms for evaluating the mechanism of adsorption process showed a good fit to the pseudo-second order and Langmuir model, respectively. The maximum adsorption capacities (Q_m) of this adsorbent were 369.749, 257.201, 180.893 and 358.824 mg g⁻¹ for lead, cadmium, nickel and copper ions, respectively. The removal performance of adsorbent on the real wastewater samples also showed the feasibility of adsorbent for applying in industrial purposes.     

Theory of ion extraction from plasma by an external electric field in systems of laser isotope separation

A refined model of processes taking place in electrostatic extractors is proposed. The model is based on the analysis of the present-day state of theoretical studies in this field and takes into account both the Langmuir and Bohm mechanisms of ion transport, calculations of initial profile of ionic current to the cathode, and the integrated contribution of ionic current to the anode. The Bohm mechanism is shown to make the dominant contribution to ion extraction for typical values of initial ionic densityn _i ⁰ ≥10¹⁰ cm^?3. Under these conditions, this mechanism not only causes a considerable (by more than an order of magnitude) decrease in plasma relaxation time τ _R in comparison with the values determined by the usual Langmuir mechanism, but also substantially modifies the main structural dependences of the relaxation time, which are found to be now in good agreement with the experimental power dependences. The new results obtained in the work favor the view that electrostatic (nonmagnetic) ion extraction systems provide a rather high efficiency at an increased (n _i ⁰ ～10¹¹ cm^?3) plasma density and, correspondingly, at a high power density.     

Ab-initio electronic structure studies of mobility paths in fast-ion conductors—I: Results for MI4−3 clusters

We report results of minimum-basis Pseudopotential Hartree-Fock studies of MI₄^?3 clusters (M = Na⁺, K⁺, Ag⁺, and Cu⁺), and of HgI₄^?2. The calculations are designed to characterize local-site effects on mobility paths in solid state electrolytes. We observe qualitatively correct behavior, with Ag⁺ predicted to be the most mobile ion. Quadrupolar polarizability of the metal ion, which is produced by s-d mixing, lowers the energy of trigonal transition state, thus accounting for the observation that quadrupole polarizable species are ideal mobile ions in close-packed halide frameworks. Mulliken populations show that there is considerable local covalency, so that electrostatic potential studies must be done very carefully. Expansion of the I₄ tetrahedron lowers the barrier energy.     

Isothermal adsorption kinetics on heterogeneous surfaces

Adsorption kinetics on energetically heterogeneous surfaces under isothermal conditions is analyzed using the uniform energy distribution model. Considering the quasi-equilibrium of surface diffusion between the adsorption sites with different energy, the kinetic equations dΘ/dt=(k_ap−A_dK_diff)(1−Θ) for first-order adsorption and dΘ/dt=k_ap(1−Θ)²−A_dK_diffΘ(1−Θ) for dissociative adsorption are obtained, where K_diff is a coefficient describing the surface diffusion equilibrium, which depends on the coverage and the energy distribution. Under isochoric conditions with p decreasing due to adsorption, surface diffusion accelerates the rate towards equilibrium significantly, as observed in static calorimetric adsorption experiments. An approximate solution in Lagergren form is derived for this condition.