Influence of a doubly charged cation nature on the formation and properties of mixed oxides MAlO x (M = Mg2+, Zn2+, Ni2+) obtained from the layered hydroxide precursors

Influence of the nature of a doubly charged cation in the layered double hydroxides (LDH) on the conditions of formation and properties of mixed oxide phase MAlO _x (M = Mg²⁺, Zn²⁺ and Ni²⁺), its ability to reconstruct the structure of the original precursor under contact with water has been studied. Hydrotalcite-like compounds and corresponding oxides with different M²⁺: M³⁺ ratio were investigated by XRD, TEM, TG-DTG-DTA, ²⁷Al NMR, N₂ adsorption, and differentiating dissolution. It has been found that the nature of the cation M²⁺ influences the conditions of LDH thermal decomposition, structural and textural characteristics of the formed mixed oxides. The obtained data can be used to synthesize the oxide supports with desired acid-base and adsorption properties.     

Removal of sulfate from wastewater via synthetic Mg–Al layered double hydroxide: An adsorption,kinetics, and thermodynamic study

Sulfate-contaminated water is a major environmental problem that alters the taste of water, disturbs the digestive systems of animals and humans, and erodes both soil and metals. In this study, the layered double hydroxide LDH 4Mg2Al·NO₃ and LDH 8Mg2Al·NO₃ were prepared using a co-precipitation technique, and applied in the adsorption of SO₄^2- from an aqueous solution. The reaction is well described by the Langmuir adsorption model. LDH 4Mg2Al·NO₃ and LDH 8Mg2Al·NO₃ afforded maximum SO₄^2- adsorption values of 135.14 and 92.59 ​mg/g, respectively. The reaction is best explained by a pseudo-second-order mechanism, which suggests that chemisorption is the rate-determining step. The activation energies of LDH 4Mg2Al·NO₃ and LDH 8Mg2Al·NO₃ indicate that the adsorption of SO₄^2- on synthetic LDHs predominantly follows an anion-exchange mechanism, wherein SO₄^2- ions in the aqueous medium replaces intercalated NO₃^- ions in the synthetic LDHs. The thermodynamic parameters (ΔH̊, ΔS̊, and ΔG̊) were also calculated. The reaction was endothermic, and the synthetic LDHs afforded feasible and spontaneous adsorption of SO₄^2-.     

Sorption Removal of Pb（Ⅱ） from Solution by Uncalcined and Calcined MgAl-Layered Double Hydroxides

Layered double hydroxide （LDH） with a Mg/Al molar ratio of 1 ： 1 was synthesized by using a co-precipitation method and its calcined product （CLDH） was obtained by calcination of the MgAl-LDH at 500 ℃. The sorption removal of Pb^2＋ from solution was investigated, finding that both LDH and CLDH show good sorption ability and they could be used as a new type of environmental sorbent for the removal of Pb^2＋ from water. The sorption kinetics and the sorption isotherms of Pb^2＋ on both LDH and CLDH can be described by the pseudo-second order kinetics and Freundlich isotherm, respectively, under the studied conditions. The sorption amounts of Pb^2＋ on LDH and CLDH are independent of pH in a pH range of about 3-10. The presence of NaNO3 may inhibit the sorption of Pb^2＋ on LDH while hardly affect that on CLDH. The sorption mechanism of Pb^2＋ on LDH and CLDH may be attributed to the surface precipitation and the surface complex adsorption. The surface complex adsorption may be further distinguished to the chemical binding adsorption forming the inner-sphere surface complexes and the electrostatic binding adsorption forming the outer-sphere surface complexes. The sorption mechanism of Pb^2＋ on LDH may be attributed to the surface precipitation and the electrostatic binding adsorption, while that on CLDH may be attributed to the surface precipitation and the chemical binding adsorption.     

Studies on Mechanochemical Method to Synthesize LDH Nanoparticles

Magnesium‐ferrum layered double hydroxide (Mg‐Fe‐LDH) and zinic‐aluminum layered double hydroxide (Zn‐Al‐LDH) compounds were prepared through a mechanochemical method. The influence of molar ratio of M²⁺ to M³⁺ (R value) on the property of LDH nanoparticles has been studied and the results showed that R=3:1 is the optimum value for the both samples. Besides pure water, the mixture of water and ethanol with the volume ratio of 3:1 is also used to wash the precipitates and used as suspending agent during the peptization process and our results showed that the addition of ethanol can improve the monodispersity of LDH nanoparticles greatly.     

Adsorption of Metanil Yellow Azoic Dye from Aqueous Solution onto Mg‐Fe‐NO3 Layered Double Hydroxide

We report the preparation and characterization of a layered double hydroxide (LDH) adsorbent for azoic dye, metanil yellow (yellow GX; YGX) removal. The nanoparticles of Mg‐Fe‐LDH‐NO₃ adsorbent were prepared with Mg/Fe molar ratio of 3:1 by a hydrothermal process and coprecipitation method at pH 9.5 and were characterized by X‐ray powder diffraction (XRPD), thermal gravimetric analysis (TGA), elemental analysis, and Fourier transform infrared spectroscopy (FT‐IR). The size and morphology of nanoparticles were examined by scanning electron microscopy (SEM). The XRD patterns indicate that the intercalation of YGX between the LDH layers has not occurred and surface adsorption has happened. In the adsorption experiments, the Gibbs free energy ΔG⁰ values, the enthalpy ΔH⁰, and entropy ΔS⁰ was determined. The isotherms showed that the adsorption of YGX by Mg‐Fe‐LDH‐NO₃ was both consistent with Langmuir and Freundlich equations.     

Adsorption of 4‐Chloro‐2‐methylphenoxy Acetic Acid (MCPA) from Aqueous Solution onto Cu‐Fe‐NO3 Layered Double Hydroxide Nanoparticles

The nanoparticles of Cu‐Fe‐NO₃ layered double hydroxide (LDH) were prepared with Cu/Fe molar ratio of 2:1 by a thermal process and co‐precipitation method at pH 9 and were characterized by X‐ray powder diffraction (XRPD), thermal gravimetric analysis (TGA), atomic adsorption spectroscopy (AAS) and fourier infrared spectroscopy (FT‐IR). The size and morphology of nanoparticles were examined by transmission electron microscopy (TEM). Cu‐Fe‐NO₃‐LDH was studied as a potential adsorbent of the acid herbicide MCPA [(4‐chloro‐2‐methylphenoxy) acetic acid] as function of pH, contact time and temperature. The results showed high and rapid adsorption of MCPA on the LDH. The characterization of the adsorption products by XRD indicates that the intercalation of MCPA between the LDH layers has not occurred and surface adsorption has happened. The adsorption kinetic was tested for pseudo‐first‐order, pseudo‐second‐order, elovich and intra‐particle diffusion kinetic models and rate constants were calculated. Freundlich and Langmuir isotherm models were applied to experimental equilibrium data obtained from the measurements of pesticide adsorption. Langmuir isotherm slightly better fitted to the experimental data than that of Freundlich. In the adsorption experiments, the Gibbs free energy ΔG⁰ values, the enthalpy ΔH⁰, and entropy ΔS⁰ were determined.     

Study of the thermal decomposition of MSO4 (M = Zn(II), Cd(II), Hg(II)) in a sodium nitrate—potassium nitrate melt

MSO₄ (M = Zn²⁺, Cd²⁺, Hg²⁺) dissolves in the molten NaNO₃—KNO₃ eutectic and is decomposed on further heating. The kinetics of decomposition have been studied at different temperatures. The decomposition of CdSO₄ and HgSO₄ in the eutectic melt obey first-order kinetics whereas the decomposition of ZnSO₄ at 420–460°C obeys second-order kinetics. However, at 480°C the decomposition of ZnSO₄ obeys first-order kinetics. The mechanism of decomposition has been given as M²⁺ +SO^2?₄ +Na⁺ +K⁺ +2NO^?₃ ? (Na,K)SO₄ + M²⁺ +2NO^?₃ M²⁺ +NO^?₃ → MO+NO⁺₂ NO^?₃ +NO⁺₂ → NO₂ + $\text{1}\text{2}$O₂ Some of the end products have been analysed by X-ray diffraction.     

Adsorption of urea at the mercury—water interface

The adsorption of urea on mercury from aqueous 1 M KNO₃ has been determined from capacity and electrocapillary measurements. A Langmuir isotherm is obeyed with a saturation molecular area of 24 × 10^?10 m² per molecule. The standard free energy of adsorption is a quadratic function of charge with maximum adsorption occurring at a charge of +8 μC cm^?2. These data are interpreted in terms of the partially oriented urea dipole, the contribution of water dipoles and nitrate ions also being considered.     

NMR Crystallography Reveals Carbonate Induced Al-Ordering in ZnAl Layered Double Hydroxide

Layered double hydroxides (LDHs) serve a score of applications in catalysis, drug delivery, and environmental remediation. Smarter crystallography, combining X-ray diffraction and NMR spectroscopy revealed how interplay between carbonate and pH determines the LDH structure and Al ordering in ZnAl LDH. Carbonate intercalated ZnAl LDHs were synthesized at different pH (pH 8.5, pH 10.0, pH 12.5) with a Zn/Al ratio of 2, without subsequent hydrothermal treatment to avoid extensive recrystallisation. In ideal configuration, all Al cations should be part of the LDH and be coordinated with 6 Zn atoms, but NMR revealed two different Al local environments were present in all samples in a ratio dependent on synthesis pH. NMR-crystallography, integrating NMR spectroscopy and X-ray diffraction, succeeded to identify them as Al residing in the highly ordered crystalline phase, next to Al in disordered material. With increasing synthesis pH, crystallinity increased, and the side phase fraction decreased. Using ¹H−¹³C, ¹³C−²⁷Al HETCOR NMR in combination with ²⁷Al MQMAS, ²⁷Al-DQ-SQ measurements and Rietveld refinement on high-resolution PXRD data, the extreme anion exchange selectivity of these LDHs for CO₃²⁻ over HCO₃⁻ was linked to strict Al and CO₃²⁻ordering in the crystalline LDH. Even upon equilibration of the LDH in pure NaHCO₃ solutions, only CO₃²⁻ was adsorbed by the LDH. This reveals the structure directing role of bivalent cations such as CO₃²⁻ during crystallization of [M²⁺₄M³⁺₂(OH)₂]²⁺[A²⁻]₁⋅yH₂O LDH phases.     

Sorption of Pb(Ⅱ) on Mg-Fe Layered Double Hydroxide

Mg‐Fe layered double hydroxide (LDH) with a Mg/Fe molar ratio of 3:1 was synthesized by using a coprecipitation method and the sorption removal of Pb(II) by the LDH sample from Pb(NO₃)₂ solution was investigated. It was found that Mg‐Fe LDH showed a good sorption ability for Pb(II) from Pb(NO₃)₂ solution, indicating that the use of LDH as a promising inorganic sorbent for the removal of heavy metal ions is possible. The sorption kinetics and the sorption isotherm of Pb(II) on the LDH sample obeyed the pseudo‐second order kinetic model and Aranovich‐Donohue equation, respectively. The sorption mechanism of Pb(II) on the LDH may be attributed to the surface‐induced precipitation and the chemical binding adsorption, and the removal ability arising from the surface‐induced precipitation is much higher than that from the chemical binding adsorption.     

The investigation of the adsorptive behaviour of electroactive species by means of admittance analysis: Part II. Anion-induced adsorption of cadmium(II) from bromide and iodide solutions at the dropping mercury electrode

The adsorption of Cd(II) has been studied by measuring the interfacial admittance spectra at different dc potentials of the dropping mercury electrode (DME) in solutions of: (a) 1 M KBr+0.48 mM Cd(NO₃)₂,(b) 1 M KI+0.105 mM CdI₂ and (c) 0.1 M NaI+0.9 M NaClO₄ +0.116 mM CdI₂ The experimental data were analysed using the procedures described in Part I of this series. The frequency dependence of the admittance corresponds to that of a reversible electrode reaction with reactant adsorption. The potential dependence of the resulting adsorption parameters can be mathematically described by a Langmuir isotherm for adsorption of Cd(II) with the adsorption energy being at least a quadratic function of potential. Detailed checks on the consistency of this model have been applied and were found to be satisfactory.     

Complexation of M3+ Lanthanide Cations by Calix[4]arene-CMPO Ligands: A Molecular Dynamics Study in Methanol Solution and at a Water/Chloroform Interface

Abstract

We report a molecular dynamics study on the 1:1 M³⁺ lanthanide (La³⁺, Eu³⁺ and Yb³⁺) inclusion complexes of an important extractant molecule L: a calix[4]arene-tetraalkyl ether substituted at the wide rim by four NH-C(O)-CH₂-P(O)Ph₂ arms. The M(NO₃)₃ and MCl₃ complexes of L are compared in methanol solution and at a water / chloroform interface. In the different environments the coordination sphere of M³⁺ involves the four phosphoryl oxygens and three to four loosely bound carbonyl oxygens of the CMPO-like arms. Based on free energy simulations, we address the question of ion binding selectivity in pure liquid phases and at the liquid-liquid interface where L and the complexes are found to adsorb. According to the simulations, the enhancement of M³⁺ cation extraction in the presence of the calixarene platform, examined by comparing L to the (CMPO)₄ “ligand” at the interface, is related to the fact that (i) the (CMPO)₄Eu(NO₃)₃ complex is more hydrophilic than the LEu(NO₃) one and (ii) the free CMPO ligands spread at the interface, and are therefore less organized for cation capture than L.     

Fabrication of zirconium(IV) cross-linked alginate/kaolin hybrid beads for nitrate and phosphate retention

The extreme nitrate (NO₃⁻) species in drinking water leads to methemoglobinemia (blue baby syndrome) disease in new born toddlers whereas the excess phosphate (PO₄³⁻) and NO₃⁻ contents lead to the eutrophication (algae growth) problem of water sources. Upto date, the environmental researchers have developing the suitable adsorbent materials for providing NO₃⁻ and PO₄³⁻ free water system. In present study, a low-cost alginate (Alg) assisted kaolin (KN) (AlgKN) composite beads were prepared and utilized for the removal of NO₃⁻ and PO₄³⁻. To improve the sorption capacity (SC) and stability, Zr⁴⁺ ions were coated onto AlgKN to get Zr@AlgKN composite beads which were prepared via., hydrothermal (Hydro) and in situ precipitation (In situ) methods. The hydro assisted Zr@AlgKN composite beads possess an enhanced SC than the in situ assisted adsorbents. In batch scale, the parameters responsible for the adsorption process such as contact time, co-ions, adsorbent dosage, pH, initial ions concentration and temperature were optimized. The adsorbents were characterized by XRD, FTIR, BET, EDAX and SEM analysis. The adsorption experimental data was fitted with isotherms, kinetics and thermodynamic parameters. The regeneration and field applicability study of the Zr@AlgKN composite beads were also investigated.     

Ag/AgBr/Co–Ni–NO3 Layered Double Hydroxide Nanocomposites with Highly Adsorptive and Photocatalytic Properties

A facile anion‐exchange precipitation method was used to synthesize bifunctional Ag/AgBr/Co–Ni–NO₃ layered double hydroxide (LDH) nanocomposites by adding AgNO₃ solution to a suspension of Co–Ni–Br LDH. The Ag/AgBr nanoparticles were highly dispersed on the sheets of Co–Ni–NO₃ LDH. The prepared nanocomposites were used to adsorb and photocatalytically degrade organic pollutants from water. Without light illumination, the nanocomposites quickly adsorbed methyl orange, and the adsorptive capacity, which can reach 230 mg g^?1, is much higher than those of Co–Ni–Br LDH, Ag/AgBr, and activated carbon. The photocatalytic activities of the nanocomposites for the removal of dyes and phenol are higher than those of Co–Ni–Br LDH and Ag/AgBr. The proposed method can be applied to prepare other LDH/silver salt composites. The high absorptive capacity and good photocatalytic activity of such nanostructures could have wide applications in wastewater treatment.     

On Demand One-Pot Mild Preparation of Layered Double Hydroxides and Their Hybrid Forms: Advances through the Epoxide Route

Epoxide ring opening driven alkalinization process was explored with the aim of preparing layered double hydroxide (LDH) phases on demand, at room temperature. Employing iodide as nucleophilic agent, the precipitation reaction can be driven under much lower halide concentrations. This scenario favors the selective intercalation of concomitant bulky oxo anions as nitrate or perchlorate in the LDH products, allowing for the one-pot synthesis of an LDH able to delaminate in formamide. Even large dicarboxylic acids, ⁻O₂C-(CH₂)_n-CO₂⁻, with n up to 8, can be quantitively intercalated within the growing LDH phase, providing a versatile one-pot route for hybrid LDHs as well. Under the mild conditions employed, governed by a continuous pH rise from a starting acid condition, a M^II to M^*III ratio of 2 prevails, independently from the overall cationic composition. However, after moderate hydrothermal aging LDH phases bearing a cationic ratio higher than 2 could result. The solubility of a given chloride-containing M^II₂M^*III LDH can be approximated as a linear combination of the solubility of the pure hydroxylated phases of the constitutive cations, M(OH)₂ and M^*(OH)₃.     

Preparation of Bamboo-Based Hierarchical Porous Carbon Modulated by FeCl3 towards Efficient Copper Adsorption

Using bamboo powder biochar as raw material, high-quality meso/microporous controlled hierarchical porous carbon was prepared—through the catalysis of Fe³⁺ ions loading, in addition to a chemical activation method—and then used to adsorb copper ions in an aqueous solution. The preparation process mainly included two steps: load-alkali leaching and chemical activation. The porosity characteristics (specific surface area and mesopore ratio) were controlled by changing the K₂CO₃ impregnation ratio, activation temperature, and Fe³⁺ ions loading during the activation process. Additionally, three FBPC samples with different pore structures and characteristics were studied for copper adsorption. The results indicate that the adsorption performance of the bamboo powder biochar FBPC material was greatly affected by the meso/micropore ratio. FBPC _2.5-900-2%, impregnated at a K₂CO₃: biochar ratio of 2.5 and a Fe³⁺: biochar mass ratio of 2%, and activated at 900 °C for 2 h in N₂ atmosphere, has a very high specific surface area of 1996 m² g⁻¹ with a 58.1% mesoporous ratio. Moreover, it exhibits an excellent adsorption capacity of 256 mg g⁻¹ and rapid adsorption kinetics for copper ions. The experimental results show that it is feasible to control the hierarchical pore structure of bamboo biochar-derived carbons as a high-performance adsorbent to remove copper ions from water.     

Synthesis and characterization of layered double hydroxides with different cations (Mg,Co, Ni,Al), decomposition and reformation of mixed metal oxides to layered structures

Three layered double hydroxides (LDH) [Mg_1−xAl_x(OH)₂]^x+(A^m−)_x/m]·nH₂O and [M^II _1−xM^III _x (OH)₂]^x+(A^m−)_x/m]·nH₂O (M^II — Mg, Co, Ni; MIII — Al; A — CO₃ ²⁻) were successfully synthesized by the low supersaturation method. The as-synthesized LDH samples were thermally decomposed and the derived mixed metal oxides reformed back to layered structures in water and magnesium nitrate media at different temperatures. All synthesized samples were characterized by X-ray diffraction (XRD) analysis, thermogravimetric (TG) analysis, X-ray fluorescence (XRF) analysis and scanning electron microscopy (SEM). The results of XRD and XRF analyses showed that single-phase layered double hydroxides were formed during synthesis and reformation. It was demonstrated, that a partially substituted by cobalt and nickel LDH samples also show memory effect. The crystallite size of regenerated LDH depends on the regeneration media, temperature and chemical composition. The LDH samples after regeneration consist of large particles with sharp edges along with a large amount of smaller particles     

Novel proximity effects and Ortho interactions in 2,2′-disubstituted diphenylamines on electron impact

Unexpected ejections of CH₃NO₂/[^˙CH₃ + ^˙NO₂], N₂O₄/[^˙NO₂ + ^˙NO₂] and CH₃OCH₃/[^˙CH₃ + ^˙OCH₃] were observed from the molecular ions of 2-methoxy-2′-nitrodiphenylamine, 2,2′-dinitrodiphenylamine and 2,2′-dimethoxydiphenylamine, respectively, under electron impact conditions owing to proximity effects. In other competing fragmentation pathways, novel proximity effects triggered by the ortho interactions leading to the unusual eliminations of [^˙CH₃ + H₂O] from M^+˙ of 2-methoxy-2′-nitrodiphenylamine and HNO₃/[^˙NO₂ + ^˙OH] from M^+˙ of 2,2′-dinitrodiphenylamine were observed. Evidence for the interpretation of the main fragmentation pathways was obtained from the metastable ion spectra and high-resolution mass spectrometry. Confirmation of the structures assigned to the ions was provided by collision-activated dissociation mass-analysed ion kinetic energy spectra.     

Solvation of ligandopentaamminecobalt (III) complexes in dimethyl sulfoxide–water mixtures

The rate constants for the replacement of water from the inner-coordination shell of Co(NH₃)₅OH₂³⁺, I, by dimethyl sulfoxide (DMSO) as DMSO gradually replaced water in the solvation shell of I were found to approach, and finally equal, the water-exchange rate constant of I in aqueous media in accordance with expectation for a dissociative mechanism. Also the rate constants for the replacement of DMSO from the innercoordination shell of Co(NH₃)₅DMSO³⁺, II, by water as water replaced DMSO in the solvation shell of II were found to approach, and approximately equal, the DMSO-exchange rate constant for II in liquid DMSO in accordance with expectation for a dissociative mechanism. The DMSO-exchange rate constant for II in liquid DMSO was determined and found to be equal to (3.6 ± 0.8) × 10^?4 sec^?1 at 45°C. The dissociation quotient, [II] [NO₃^?]/[Co(NH₃)₅NO₃²⁺], was found to be equal to 0.28 ± 0.11 M at 45°C by NMR methods. The pseudo first-order rate constants for anation of II by NO₃^? and the solvation of Co(NH₃)₅NO₃ ²⁺ by DMSO were determined at various temperatures.     

Extraction of heterometallic complexes of ruthenium and nonprecious metals by phosphorylmethylated calix[4]arenes from nitrate solutions

Synergetic extraction of [RuNO(NO₂)₄OH]^2? by calix[4]arene phosphine oxides (L) in the form of Ru/M heterometallic complexes was studied in the presence of nonprecious metals (M²⁺). The main extraction laws were recognized for [M(NO₃)₂L _n ] and [RuNO(NO₂)₄OH])ML _m ], where M²⁺ = Zn²⁺, Cu²⁺, Co²⁺, or Ni²⁺ and n, m = 1 or 2; extraction constants were determined for these metals. The variation row of the extraction constants with varying metal (Zn²⁺ > Cu²⁺ > Co²⁺ > Ni²⁺) coincides with the Irving-Williams row. Two or three PO groups of extractant L and the OH and NO₂ groups of the ruthenium anion are coordinated to the M²⁺ atom in Ru/M complexes. The conditions for generation of the Ru/Zn complex and its complete extraction were optimized as applied to the extraction of fission ruthenium from nitrated nitric acid and imitation solutions.