Adsorption mechanism of humic and fulvic acid onto Mg/Al layered double hydroxides

The adsorption of humic and fulvic acids onto nitrate, chloride and carbonate intercalated layered double hydroxides with Mg/Al ratios ranging from 85/15 to 60/40 was studied by adsorption isotherms at different ionic strengths and the characterization of the preferentially adsorbed humic substance size fractions. The adsorption of humic and fulvic acids onto LDHs occurred by ion exchange with both the intercalated and surface anions of the LDH and ligand exchange reactions with surface groups. The contribution of both mechanisms to the total HA and FA adsorption was estimated. Lower molecular weight humic and fulvic acids were preferentially adsorbed because these fractions can more easily enter the mesoporous LDHs and contain more carboxylic groups, which are known to be involved in ligand exchange reactions with e.g. surface Al-OH groups. Intercalation of the entire HA or FA molecules in-between the LDH sheets is unlikely to occur.     

Adsorption of phosphate by layered double hydroxides in aqueous solutions

Adsorption of phosphate by various calcined layered double hydroxides (LDHs) such as Mg–Al, Zn–Al, Ni–Al, Co–Al, Mg–Fe, Zn–Fe, Ni–Fe and Co–Fe was investigated. LDHs were found to have high anion exchange capacity that enhances their capability to remove anionic contaminants from aqueous system. The nature and content of di- and trivalent cations in LDH have strong influence on the adsorption process. Calcined Mg–Al LDH (CLDH) with Mg–Al molar ratio of 2.0 showed higher adsorption capacity compared to other calcined LDHs as it possessed higher Al³⁺ content. Results indicate that the adsorption isotherm could be fitted to a linearised form of Langmuir and Freundlich equations. The adsorption phenomenon has been well supported by XRD study. The adsorption process was spontaneous and exothermic in nature and followed first order kinetics. Competitive anions were found to have detrimental effect on the percentage of adsorption.     

Adsorption of fluoride ions by Zn–Al layered double hydroxides

Zn–Al layered double hydroxides (LDHs) with different molar ratios Zn/Al (0, 0.17, 0.34, 0.97, 3.47, ∝) were prepared by the co-precipitation of chlorides, characterized and evaluated for their fluoride adsorption at room temperature from aqueous solutions. The fluoride adsorption of the as-synthesized LDHs was influenced by the chemical composition of the LDHs and ZA-11 (Zn/Al = 0.97) had the highest capacity for fluoride adsorption (1.14–4.16 mg/g). The adsorption increased after calcination of the LDH up to 500 °C. The equilibrium data were fitted to the Freundlich, Langmuir, and Temkin equations. The kinetics of fluoride adsorption followed the pseudo-second order model.     

层状复合氢氧化物的结构调控及其吸附重金属离子的研究进展

层状复合氢氧化物（LDHs）是具有特殊层状结构的阴离子黏土,具有化学组成可调、比表面积大及结构记忆效应独特等性质,在废水处理方面备受关注。调控LDHs自身结构,是进一步扩大其应用范围、提高其吸附性能的有效途径。该文介绍了LDHs特殊的层状结构和其自身性质;总结了LDHs最常用的制备方法,即共沉淀法、离子交换法、尿素水解法、煅烧复原法和溶胶-凝胶法等,分别介绍了各种制备方法的原理和特点;综述了LDHs的结构调控对其吸附重金属离子性能的影响,并总结了LDHs对重金属离子的吸附机理;最后,指出了目前LDHs在处理含有重金属离子废水研究中面临的挑战,并对该材料未来的研究方向和发展趋势进行了展望。     

Intercalation of layered double hydroxides by poly(oxyalkylene)-amidocarboxylates: tailoring layered basal spacing

Mg-Al layered double hydroxides (LDHs) were intercalated with various molecular-weight poly(oxypropylene)-bis-amidoacid salts (POP-acid), synthesized from polyoxyalkylene-diamines and maleic anhydride. The intercalation involves an ionic exchange reaction of LDHs at 120 °C and under N₂ atmosphere in an autoclave to afford a series of organoclays with a maximal basal spacing of 92 Å, revealed by X-ray diffraction and transmission electron microscopy analyses. The unusually wide interlayer spacing was ascribed to the self-alignment of the hydrophobic POP backbone in the layer confinement. In contrast, the intercalation of poly(oxyethylene)-bis-amidoacids (POE-acid) afforded a low basal spacing (7.8 Å) due to their oxyethylene-backbone interaction with the layered surface. The resultant POP- and POE-acid intercalated LDHs also exhibited different dispersing properties in toluene or water. Particularly, the organically-modified and space-enlarged hybrids with a proper amount of the embedded POPs possessed an amphiphilic property of lowering the toluene/water interfacial tension.     

Simultaneous incorporation of palladium and zirconium ions in Mg-Al layered double hydroxides by co-precipitation

Simultaneous incorporation of palladium and zirconium ions in Mg-Al layered double hydroxides (LDHs) was attempted by co-precipitation. Mixed oxides were obtained by calcination at 500 °C. XRD patterns of as-synthesized samples showed the formation of well-crystallized LDHs at lower Zr contents. The CO₃²⁻ content of the solids has been determined to have information on the possible incorporation of Zr⁴⁺ into the brucite layer of the hydrotalcites. The values obtained showed that Zr⁴⁺ can incorporate into brucite sheets at Zr content lower than 10 mol%. SEM and TEM images indicated that the resulted LDHs with lower Zr content exhibited plate-like structure. Thermal calcination at 500 °C results in the formation of mixed oxides containing MgO, PdO, ZrO₂ crystallites and a solid solution formed by some of the Zr⁴⁺ cations dissolving along with Al³⁺ to MgO lattice.     

Adsorption and photocatalytic degradation of phenol and 2,4 dichlorophenoxiacetic acid by Mg–Zn–Al layered double hydroxides

Mg–Zn–Al layered double hydroxides (LDHs) with varying amounts of zinc were prepared by the coprecipitation method. Solids were analyzed by XRD and N₂ physisorption, confirming the formation of pure LDH phase; and the production of mixed oxides with high specific surface areas (182–276 m² g⁻¹) after calcination. Band gap energy was also determined, presenting the expected decreasing tendency on increasing zinc amounts. These mixed oxides were tested both for the adsorption of 2,4 dichlorophenoxiacetic acid (2,4-d) and for the photocatalytic degradation of 2,4-d and phenol. Nearly total (97%) degradation of initial 1.45 mmol L⁻¹ of 2,4-d, with 1 g calcined LDH per liter, was accomplished in 9 h, while phenol half-life was as short as 3.5 h, with the catalyst with lowest zinc amount (5 wt.%). Langmuir adsorption isotherms are presented. Solids were also characterized by XRD and FTIR analysis after photocatalytic and adsorption activity, to determine the presence of 2,4-d. The versatility of LDH decomposition products in the elimination of different contaminants by different mechanisms puts them forward as a viable alternative for environmental remediation.     

Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of V---O in-plane bonds around the reduced V centres.     

Co-based catalysts from Co/Fe/Al layered double hydroxides for preparation of carbon nanotubes

Carbon nanotubes have been prepared via catalytic chemical vapor deposition of acetylene on a series of catalysts derived from Co/Fe/Al layered double hydroxides (LDHs). The catalytically active cobalt particles were obtained by calcination of LDHs containing cobalt (II) ions followed by reduction. The obtained products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, high-resolution electron microscopy and Raman spectroscopy. The content of Co in the precursors had a distinct effect on the growth of carbon nanotubes. Increasing Co content enhanced the carbon yield, due to good dispersion of a large number of active Co species. This indicated that the agglomeration of metallic Co particles did not take place even at high Co content. Higher Co content led to the formation of carbon nanotubes with smaller diameters and less structural disorder.     

Adsorption of reactive dyes from aqueous solutions by layered double hydroxides

BACKGROUND: Water‐soluble reactive azo dyes are the most problematic dye house effluents, as they tend to pass through conventional treatment systems unaffected. The release of these compounds into the environment is undesirable and their removal becomes environmentally important. In this work, synthesis, characterization and sorption properties of hydotalcite‐like compounds (Mg/Al and Mg/Fe), calcined and uncalcined, were investigated for the removal of the reactive azo dye Remazol Red 3BS. RESULTS: The calcined compounds present higher surface area than the uncalcined. The optimum pH for sorption was found to be 6. Thermodynamic analysis reveals that the sorption is spontaneous and endothermic. Equilibrium data were fitted by a Langmuir model, and kinetic data by a second‐order model. The calcined Mg/Al compound showed the highest sorption capacity, at 0.125 mmol g^?1. Regeneration of dye loaded derivative is achieved using the surfactant SDS. CONCLUSIONS: The prepared hydotalcite‐like compounds and especially calcined Mg‐Al exhibited significant adsorption capacity, kinetics, and regenerative ability. Its potential applicability as sorbent should be tested in a large‐scale implementation. Copyright © 2011 Society of Chemical Industry     

Self-assembled hierarchical porous layered double hydroxides by solvothermal method and their application for capacitors

In this paper, for the first time, Ni/Al layered double hydroxides (LDHs) with high specific surface area and high porosity were developed via a facile and environmentally friendly solvothermal approach. The obtained Ni/Al LDHs have a diameter of 1-10 μm, which are self-assembled by 15 nm-sized primary particles and 200 nm size secondary particles. The BET surface area of the sample is determined to about 300 m² g⁻¹, which is about three times than that prepared by conventional hydrothermal route. The average pore size is 3.97 nm. Notedly, the specific capacitance of this hierarchical porous Ni/Al LDHs reaches 477 F g⁻¹ at a current density of 2.5 A g⁻¹, which is nearly double that of Ni/Al LDHs synthesized by conventional hydrothermal procedure. The capacitance loss is less than 5% after 400 charge-discharge cycles, suggesting the high potential as electrode materials in electrochemical capacitors. The formation mechanism of self-assembled hierarchical porous Ni/Al LDHs through solvothermal method was also proposed.     

Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

Changes in cation composition and M²⁺/M³⁺ ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations.Heating Mg–Al–CO₃ hydrotalcites with Mg/Al=2, 3 and 3.7, at 600 °C for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations.The presence of Mg²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cu²⁺ salts or of low soluble hydromagnesite increased the M²⁺/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn²⁺, Ni²⁺, Co²⁺ and Cu²⁺ cations in the newly formed hydrotalcite was detected, while Mg²⁺ remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)₃, the M²⁺/Al ratio approximated the minimal possible value of 2.The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value.The results obtained support the conception of the dissolution–crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.     

Synthesis, characterization and release of curcumin-intercalated Mg–Al-layered double hydroxides

A drug-inorganic clay composite involving a pharmaceutically active compound curcumin (Cur) intercalated Mg–Al layered double hydroxides (Cur-LDHs) with Mg/Al = 3 has been assembled by co-precipitation and ion-exchange methods. Powder X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR) analysis indicate a successful intercalation of Cur between the layers with monolayer horizontal (parallel to the layer) and vertical (perpendicular to the layer) orientations of Cur anion. (TG-DTA) analyses shows that the thermal stability of intercalated organic species is largely enhanced due to host–guest interaction involving the hydrogen bond compared to pure form before intercalation. The release studies show that release percentages decrease with increasing pH from 4.0 to 6.5 due to possible change of release mechanism during the release process. The kinetic simulation for the release data indicates that the dissolution mechanism is mainly responsible for the release behavior of Cur-LDHs at pH 4.0, while the ion-exchange one is responsible for that at pH 6.5. It suggests that layered double hydroxides may have application as the basis of a novel tunable drug delivery system.     

Thermal shielding performances of nano-structured intumescent coatings containing organo-modified layered double hydroxides

Cone calorimetry tests performed at 50 kW/m² heat flux have been exploited for assessing the fire resistant properties of nano-structured intumescent coatings containing modified layered double hydroxides (hydrotalcites, LDHs) and deposited on steel plates. The effects of different types of modified hydrotalcites (i.e. magnesium–aluminum lactate hydrotalcite, magnesium–aluminum gluconate hydrotalcite, magnesium–aluminum hydrotalcite modified with a fatty acid, magnesium–aluminum hydrotalcite modified with rosin) on the thermal shielding performances of the intumescent coatings and their intumescent degree have been thoroughly discussed and compared with the pristine unfilled counterparts.More specifically, the coatings containing organo-modified LDHs showed better thermal shielding performances with respect to the reference intumescent coating; on the contrary, the use of unmodified hydrotalcite in the intumescent formulations was found detrimental. The thermal shielding performances of the coatings filled with modified LDHs were found to be strictly related to the intumescent degree developed during the cone calorimetry tests. In addition, it was possible to compare the thermal shielding performances of the nanofilled coatings by evaluating the temperatures achieved after 2000 s exposure to the 50 kW/m² heat flux of the cone: the thermal shielding performance sequence was LDH-GL > LDH-RS > LDH-LA > LDH-FA > LDH).Finally, the intumescent degree of the modified coatings was found to decrease with increasing the hydrotalcite content, hence lowering their thermal shielding performances.     

Towards solving the crystal structure of polytype 3R2 Mg–Al layered double hydroxides

An analysis of the crystal structure of polytype 3R₂ layered double hydroxides has been carried out based on XRPD diffractograms. Measurement of the transmission-XRPD pattern carried out in a spinning and tilting capillary using synchrotron radiation has been shown to diminish the preferred orientation effect of the sample. In order to approach the structure of polytype 3R₂, structural models, based on possible interlayer anion arrangement as revealed by FT-IR, 27-Al solid state NMR and XRPD, were constructed. A structural model is proposed that is consistent with the by XRPD measured interlayer distance, in which tetrahedral aluminate ions are grafted with their apical oxygen ions onto the octahedral sheet. The presence of a measured extra reflection at 2θ value of 6.3, which is not predicted by this structure, can be obtained however by enlargement of the unit cell to a′ = a√3 = 0.5279 nm and c′ = c = 2.1985 nm, which simulates the aluminate anions ordering in the interlayer.     

Layered double hydroxides as containers of inhibitors in organic coatings for corrosion protection of carbon steel

The present work focuses on the use of layered double hydroxides (LDH) as containers for corrosion inhibitors in an epoxy coating. 2-Benzothiazolylthio-succinic acid (BTSA), used as corrosion inhibitor, was intercalated by co-precipitation in magnesium–aluminum layered double hydroxides. The obtained LDH–BTSA was characterized by infrared spectroscopy, X-ray diffraction and scanning electron microscopy. BTSA release from LDH–BTSA in NaCl solutions was investigated by UV–vis spectroscopy. The inhibitive action of LDH–BTSA on carbon steel corrosion was characterized by electrochemical methods and the protective properties of an epoxy coating containing LDH–BTSA were evaluated by electrochemical impedance spectroscopy. It was shown that the BTSA was intercalated in the layered double hydroxide and its loading was about 33%. The BTSA release was dependent on the NaCl concentration in the electrolyte. The polarization curves obtained on the carbon steel sample showed that the LDH–BTSA is an anodic inhibitor. Its efficiency was about 90% at a concentration of 3 g/l. The impedance results showed that the incorporation of LDH–BTSA (3%) in the epoxy matrix improved the corrosion protection of the carbon steel.     

Uptake of Cu, Cd and Pb on Zn–Al layered double hydroxide intercalated with edta

Hydrotalcite-like compound [Zn₂Al(OH)₆]₂edta·nH₂O(ZnAl-edta) was obtained from the precursor [Zn₂Al(OH)₆]NO₃·nH₂O (ZnAl-NO₃), by the anion exchange method, with the aim of uptake Cu²⁺, Cd²⁺ and Pb²⁺ from the aqueous solutions by chelating process between edta and metal cations. The amount of Cu²⁺, Cd²⁺ and Pb²⁺ adsorbed was monitorized by atomic absorption technique at different contact time, pH and metal concentrations. The results indicate the very fast adsorption of the metal cations by ZnAl-edta reaching the equilibrium of the uptake reaction in two hours for Cu and Pb and 24 h for Cd. The shape of the adsorption isotherms suggests specific interaction and high host–guest affinity. At pH 5.5 and initial concentration C_i = 10 mM, the amount adsorbed was C_s = 1117, 375 and 871 μmol/g for Cu²⁺, Cd²⁺ and Pb²⁺, respectively.     

Relaxation behavior of layered double hydroxides filled dangling chain-based polyurethane/polymethyl methacrylate nanocomposites

A series of dangling chain based-polyurethane/poly(methyl methacrylate) (DPU/PMMA) filled with exfoliated layered double hydroxides (LDH) were synthesized by methyl methacrylate in-situ intercalative polymerization. The dangling chains were introduced by using vegetable oils as chain extender. The effect of dangling chain and the contents of LDH on the molecular dynamics of DPU/PMMA was investigated by a combination of dynamic mechanical analysis and broadband dielectric relaxation spectroscopy. Compared with polyurethane/poly(methyl methacrylate) (PU/PMMA) without dangling chain, the glass transition temperature (T_g) of DPU/PMMA shifts to lower temperature and the segmental dynamics becomes faster. A plateau with a high loss factor value above T_g significantly broadens the damping temperature range due to the synergy effect between the dangling chains and LDH layers. In DPU/PMMA/LDH nanocomposites, the α-relaxation associated with the glass transition of the polymer matrix becomes slower with the increase of LDH content, which indicates a restricted molecular mobility in the interfacial regions between polymer and LDH. However, the local relaxations at relatively low temperature remain unaffected by dangling chain or the addition of LDH. When the LDH content increases, Maxwell–Wagner–Sillars (MWS) interfacial polarization process caused by charge accumulation at interfaces becomes faster because of the smaller mean distance d between the exfoliated LDH layers.     

Electrocatalysis of gold nanoparticles/layered double hydroxides nanocomposites toward methanol electro-oxidation in alkaline medium

A nanocomposite based on layered double hydroxides (LDHs) and gold nanoparticles (AuNPs) was prepared via hydrothermal treatment followed by a reduction procedure. The AuNPs were obtained in Mg-Al LDHs, and they maintained good stability. The electrocatalytic activities of AuNPs/LDH-modified glassy carbon electrodes for methanol oxidation in alkaline medium were investigated in detail. Under the same conditions, the modified electrode exhibited higher electrocatalytic activity than both the pure AuNPs-modified electrode and LDH-modified electrode. The role of the AuNPs and LDHs in this composite system was explored by cyclic voltammetry and chronoamperometry, respectively. Further studies demonstrated that the promoting effect of LDHs could be due to its strong adsorption and partly to the discharge of OH⁻ during methanol oxidation. This work indicates that LDHs is expected to be a good supporting material in the development of methanol anode catalysts.     

Effect of surfactant concentration on the stacking modes of organo-silylated layered double hydroxides

The effects of the surfactant concentration on the structure, morphology and thermal property of silylated hydrotalcites have been investigated. By in-situ coprecipitation, the surfaces of layered double hydroxides (LDHs) have been modified by using 3-aminopropyltriethoxysilane (APTS) and anionic surfactant, Na-dodecylsulfate (SDS). Two different stacking modes in the resultant materials were detected by X-ray diffraction (XRD). One has an identical structure of LDHs, in which the SDS and APTS only bond to the outside surfaces and plate edges of LDH. The other is with enlarged interlayer distance, in which SDS and APTS combined with the inside surfaces of LDH. With the increased loading of SDS and APTS, the surface of the modified LDH appeared rough as observed in the transmission electron microscopy (TEM) images. The attenuated total reflection Fourier-transform infrared (ATR FTIR) spectra of the silylated hydrotalcites showed a series of bands attributed to –NH₂ and Si–O–M (M = Mg and Al), proving that APTS has successfully been grafted onto LDH. The thermogravimetric curves (TG) showed that the silane grafted samples have less –OH concentration and less interlayer water, as a result of the –OH consumption during the condensation reaction between Si–OH and –OH on LDH surface. These nanomaterials are of potential applications including clay-based nanocomposites, adsorbents for removal of organic contaminants from water and flame retardant materials.