Layered Double Hydroxides Phase Research Articles

Zn-Al layered double hydroxide / polyurethane as a novel nanocomposite for cefixime antibiotic adsorption

Wastewater contaminated with antibiotics requires innovative solutions to remove such pollutants efficiently. In this work, Zn-Al layered double hydroxide (LDH) adsorbent supported on polyurethane (PU) (Zn-Al LDH/PU) was synthesized using a simple co-precipitation method. The synthesized material was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), zeta potential, and hydrodynamic size. This adsorbent was investigated for the removal of cefixime (CFX), which is a model third-generation cephalosporin antibiotic. The LDH phase showed a typical hexagonal layered morphology with layer sizes larger than 200 nm in diameter. CFX adsorption on PU, Zn-Al LDH and Zn-Al LDH/PU showed a maximum adsorption capacity of 94.43, 57.17, and 115.69 mg/g, respectively. The Redlich-Peterson and Baudu isotherm models were found to be the best fit models for CFX adsorption of Zn-Al LDH and Zn-Al/PU, respectively. The equilibrium time was found to be 175 and 250 min for CFX adsorption of Zn-Al LDH and Zn-Al/PU, respectively. CFX adsorption kinetics on Zn-Al LDH/PU best fitted the pseudo-second order model indicating chemisorption and diffusion limited adsorption process. Numerous types of bonding between functional groups of CFX and both Zn-Al LDH and PU could explain the adsorption mechanism. The selectivity study of CFX adsorption on Zn-Al LDH/PU using Sulfamethoxazole and Ciprofloxacin with different concentrations was investigated and discussed. The recyclability of this Zn-Al LDH/PU was also studied. Also, the effect of temperature was investigated at (23, 35, 45, 55 °C) and the thermodynamic parameters (∆H°, ∆S° and ∆G°) were calculated showing exothermic and spontaneous adsorption process.The methyl thiazolyl tetrazolium (MTT) assay indicated that the biocompatibility of Zn-Al LDH nanostructures was enhanced following modification with PU, demonstrating regulated and minimal cytotoxic effects towards normal liver cell line (WRL-68). The Zn-Al LDH/PU can be considered as a promising low-cost adsorbent for CFX antibiotic from wastewater streams.

Growth mechanism and corrosion resistance of layered double hydroxide film on magnesium alloy without external addition of magnesium and aluminum salts

This study first reports in-situ growth of layered double hydroxide (LDH) coatings on magnesium alloy surface without external adding magnesium (Mg2+) and aluminum (Al3+) salts with the aid of a chelating agent at ambient pressure and relatively low temperature (≤ 95 °C). The morphology, composition, and structure of the deposits on the magnesium alloy surface were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), etc. Combined with open-circuit potential (OCP)-time curves, the mechanism of growth and formation process of the LDH film was proposed. The process was divided into four stages, i.e., dissolution of the Mg substrate, formation of Mg(OH)2, nucleation of LDH by transformation of Mg(OH)2 phase, and growth of LDH phase. The corrosion resistance of the LDH coating before and after treatment with vanadate and tungstate was evaluated through electrochemical impedance spectroscopy (EIS), Tafel curves, and immersion measurements. Compared to bare magnesium alloy and pristine and vanadate-treated LDH, the LDH coating modified with tungstate exhibited the most effective corrosion protection for magnesium alloy AZ31, without obvious signs of corrosion after exposure to a 3.5 wt% NaCl solution for seven days.

Molecular and Cationic Engineering of Fe-Based Layered Double Hydroxides for Water Oxidation

Layered double hydroxides (LDHs) are a class of anionic clays with a layered structure, that can be exfoliated giving place in 2D materials. Recently, these materials are attracting increasing attention from the point of view of their electrochemical performance towards energy storage and conversion applications. [1] In the field of electrocatalysts for the oxygen evolution reaction, Fe-based LDHs stand out as one of the best oxygen evolution reactions (OER) electrocatalysts in alkaline conditions. In the attempt to improve the electrochemical performance of Fe-LDHs-based electrodes, the effect of the microstructure, exfoliation, metal composition, crystallinity, doping, defect engineering and so on has been reported to increase the knowledge of electrochemical behavior and obtained better performances.[2] Beyond these well-studied parameters, interlayer space, Fe-clustering and covalent functionalization have been not explored, being the last two synthetic challenges within the field of LDHs.On the one hand, to explore the influence of interlayer space in the behaviour of Fe–LDHs as an oxygen evolution reaction (OER) electrocatalyst, we develop a family of NiFe-LDHs with the same metal composition, morphology, and dimensions and different alkyl sulfate to reach extreme ranges of basal spacing (from 8 to 32 Å). Increasing the basal space of the LDH results in a higher electrochemical surface area and a reduction of the resistance related to the chemisorption of oxygen leading to better kinetic behaviour. Indeed, the Tafel slope for the NiFe–LDHs with the highest basal space studied is similar to that obtained for benchmark exfoliated NiFe nanosheets and shows better stability as a consequence of the tridimensional robustness of the hybrid material. However, an excessive increment of the interlayer space compromises the onset potential.[3]On the other hand, to shed light on the influence of Fe-clustering on this topic, firstly, we have selected MgFe-based LDHs phases as model systems to decipher whether the Fe-clustering exert an effect on the OER performance. By optimization of a hydrothermal method based on triethanolamine different Fe-clustering degree has been achieved, and thanks to the diamagnetic behaviour of Mg, a simple magnetic characterisation allows us to identify the Fe-clustering degree. Nevertheless, both samples behave identically in terms of OER performance when glassy carbon electrodes are used. Surprisingly, when the samples are tested on the most employed electrode, nickel foam, striking differences arise. Specifically, the sample exhibiting the lower Fe-clustering behaves as a better electrocatalyst with a reduction of the overpotential values of more than 50 mV to reach 100 mA/cm2, as a consequence of a favoured surface transformation of MgFe-LDHs phases into more reactive oxyhydroxide NiFe-based ones during the electrochemical tests.[4]Last but not least, the covalent functionalization of LDHs has been explored to enhance their processability and stability as an electrocatalyst, for this, we develop for the first time the covalent functionalization of Fe-LDHs beyond silane-based molecules due to the reversible character of this bond in alkaline media. By a modified hydrothermal method from Kuroda et al.,[5] the covalent functionalization of NiFe-LDHs with tris(hydroxymethyl) aminomethane (Tris) was performed. The NiFe@Tris LDH were fully characterized using X-ray diffraction, infrared spectroscopy, thermogravimetric analysis coupled with gas chromatography-mass spectrometry, magnetic measurement, XAS in order to identify the structural differences with a bare-LDH. At the same time, the stability of the covalent functionalization in basic media was proved. Interestingly, the Tris molecule anchored to LDHs increases the electrocatalytic stability in absence of binder material. Thus, after 40 hours of the OER, the overpotential increase is 37% lower in the NiFe@Tris than in the NiFe-Cl@bare. To check the origin of this overpotential decrease, ICP analysis of electrolytes pre-treated with acid after the stability test was performed. A reduction of 27% and 17% of Ni and Fe respectively is observed in the electrolyte for functionalized samples. Consequently, it is demonstrated that, regardless of the decomposition process (detachment or dissolution), the NiFe-LDH functionalized with the tripodal ligand Tris presents greater stability.[6]Overall, we demonstrated as molecular and cation engineering of LDHs can be used to enhance the electrochemical activity and increase the stability for their use as electrocatalysts for the oxygen evolution reaction.

Investigating the interactions between hydrotalcite and U(IV) nanoparticulates

In the UK, the decommissioning of legacy spent fuel storage facilities at the Sellafield nuclear facility requires the retrieval of radioactive sludge resulting from Magnox fuel corrosion. However, sludge retrievals may enhance uranium mobility including via sorption of radionuclide nanoparticles onto colloidal phases such as hydrotalcite (Mg4Al2(OH)16(CO3).4H2O). Hydrotalcite is a Mg-Al layered double hydroxide (LDH) which is a corrosion product of Magnox fuel cladding. Currently, there are a paucity of studies examining interactions between actinide nanoparticles and LDH phases such as hydrotalcite. Here, a multi-technique approach was used to investigate the interactions between colloidal hydrotalcite and three different forms of nanoparticulate U(IV): nanoparticulate uraninite (UO2); nanoparticulate UO2 reacted with silica (UO2-Si); and U(IV)-Si-coprecipitate under anoxic, neutral-to-alkaline conditions. Ultrafiltration and zeta potential analyses indicated that for UO2 and UO2-Si nanoparticulate phases, sorption to colloidal hydrotalcite was limited due to rapidly settling UO2 and UO2-Si aggregates (>450 nm). By contrast, ultrafiltration and zeta potential analyses confirmed the U(IV)-Si-coprecipitate nanoparticle phase showed significantly higher sorption to colloidal hydrotalcite. This was due to the increased colloidal stability of intrinsic U(IV)-silicate nanoparticles which in turn promoted increased sorption to hydrotalcite. TEM imaging showed some evidence for smaller UO2 and UO2-Si aggregates (<20 nm) sorbed to colloidal hydrotalcite. Similar behaviour was observed in TEM images of authentic pond effluent samples from Sellafield, providing confidence that the model laboratory experiments provided a bridge to the highly radioactive spent nuclear fuel pond interactions. This study highlights the potential for U(IV) nanoparticles to form a new type of colloid-colloid interaction with hydrotalcite, especially when silica is present. This further informs predictions of U(IV) (and An(IV)) behaviour in the legacy pond and silo environments, as well as in environmental scenarios where LDH mineral phases and silica are present (e.g. in geological disposal of radioactive waste).

NHC-heterocycle carbene gold catalyst intercalated in Layered Double Hydroxides as reusable hybrid catalyst in acidic medium

NHC-heterocyclic carbene metal complexes are efficient catalysts used for the oxidation, amination, cycloaddition or hydration of alkynes. Immobilization of such complex is a strategy to avoid its decomposition/deactivation and allow its recycling. One limitation of the immobilization of carbene metal complexes is the acidic pH required to perform the catalysis of hydration of alkynes. The support used must be resistant in such conditions. Layered Double Hydroxides (LDH) is a widely used material as catalyst support due to their tunable chemical composition and flexible open structure. However, the LDH phases usually found as catalyst support in the literature are chemically instable under severe conditions. In particular, when operating in extreme acidic pH. This study describes the development of a pH-resistant support by screening of LDH with various composition and formulation in order to obtain a stable material at pH down to 3.0, conditions required for a model reaction, the hydration of propargyl alcohol. Then, the intercalation of a NHC-heterocyclic carbene gold anionic complex (NHC-Au) was optimized and fully characterized. The newly heterogeneous catalysts were evaluated for the targeted hydration catalysis reaction. The selected support, a LiAl2 phase obtained by urea method and dried at 80 °C, allowed to immobilized up to 75% of the gold complex. The intercalated catalyst was finally used for the hydration of several alkynes with yields between 84% and 100% and was recycled up to 12 cycles without any activity loss.

Controllable Doping of Mn into Ni0.075-xMnxAl0.025(OH)2(CO3)0.0125·yH2O for Efficient Adsorption of Fluoride Ions.

Here, the structural, optical, and adsorptive behaviors of Ni0.075-xMnxAl0.025(OH)2(CO3)0.0125·yH2O (Ni-Mn/Al) layered double hydroxides (LDHs) are investigated to capture fluoride from aqueous media. The 2D mesoporous plate-like Ni-Mn/Al LDHs are successfully prepared via a co-precipitation method. The molar ratio of divalent to trivalent cations is maintained at 3:1 and the pH at 10. The X-ray diffraction (XRD) results confirm that the samples consist of pure LDH phases with a basal spacing of 7.66 to 7.72Å, corresponding to the (003) planes at 2θ of 11.47o and the average crystallite sizes of 4.13 to 8.67nm. The plate-like Mn-doped Ni-Al LDH consists of many superimposed nanosheets with a size of 9.99nm. Energy-dispersive X-ray and X-ray photoelectron spectroscopies confirm the incorporation of Mn2+ into the Ni-Al LDH. UV-vis diffuse reflectance spectroscopy results indicate that incorporating Mn2+ into LDH enhances its interaction with light. The experimental data from the batch fluoride adsorption studies are subjected to kinetic models such as pseudo-first order and pseudo-second order. The kinetics of fluoride retention on Ni-Mn/Al LDH obey the pseudo-second-order model. The Temkin equation well describes the equilibrium adsorption of fluoride. The results from the thermodynamic studies also indicate that fluoride adsorption is exothermic and spontaneous.

Operando spectroscopies unveil interfacial FeOOH induced highly reactive β-Ni(Fe)OOH for efficient oxygen evolution

Interfacial engineering is an effective strategy for the design of active oxygen evolution electrode. Yet the intrinsic mechanism of heterogeneous interfaces remains largely unclear for the lack of direct evidence of real active phases. Herein, operando ultraviolet-visible (UV–vis) and Raman spectroscopies were used to probe the surface reconstruction behaviors of FeOOH@NiFe layered double hydroxides (LDH) hybrid catalysts, which revealed that FeOOH@NiFe LDH was converted into highly active FeOOH@ β-Ni(Fe)OOH phases during the OER process. The FeOOH-NiFe LDH interface can promote interfacial charge transfer, delay the Ni oxidation and induce the NiFe LDH phases to highly reactive β-Ni(Fe)OOH. The FeOOH@ β-Ni(Fe)OOH exhibited excellent OER performance (252 mV at 100 mA cm−2) and a 1.6 times increase (at overpotential of 300 mV) in turnover frequency (TOF) with respect to the active phase γ-Ni(Fe)OOH of NiFe LDH. This work clarifies that ration interface engineering can regulate the generation of highly active phases in the OER process.

Influence of aluminum sulfate on properties and hydration of magnesium potassium phosphate cements

Aluminum sulfate shows a positive effect on preventing efflorescence of magnesium potassium phosphate (MKP) cements. Its influence on the MKP cement properties and hydration is explored in this study. Compared to a reference cement (Mg/PO4 molar ratio = 4, w/c ratio = 0.25), aluminum sulfate retards setting, increases compressive strength, and has little influence on the short-term volume stability. Aluminum sulfate slows down the hydration kinetics, decreases the pH values, accelerates the depletion of KH2PO4, and thus mitigates the formation of efflorescence. It also modifies the hydrate assemblage. In addition to K-struvite, aluminum sulfate leads to the formations of arcanite and aluminum-containing hydrates such as amorphous aluminum phosphate, possibly phosphate containing LDH (layered double hydroxide) and minyulite-like phases. The formation of amorphous MgHPO4 and minor amounts of bobierrite and brucite as observed in the reference paste, are suppressed in the presence of aluminum.

Enhanced photoactivity for the methyl orange on heterostructure LaCO3OH/Ni–Fe layered double hydroxides prepared from complexing agent-assisted homogeneous precipitation techniques

LaCO3OH/Ni–Fe-LDH was synthesized as a new type of layered double hydroxides (LDHs) by complexing agent-assisted homogeneous precipitation techniques. According multiple characterizations, LaCO3OH/Ni–Fe-LDH has LDH phase with platelet-like particles and mesoporous properties with 141 m2 g−1 of specific surface area. In addition, both lanthanum and iron are present in the composites, resulting in a significant decrease in the band gap to 2.04 eV, which is lower than that of either Ni–Fe-LDH (2.43 eV) or Ni–Al-LDH (4.86 eV). Compared with Ni–Fe-LDH and Ni–Al-LDH, LaCO3OH/Ni–Fe-LDH significantly improved photocatalytic performance and effectively decomposed methyl orange (MO) under visible light exposure. The increased photocatalytic efficiency from LaCO3OH/Ni–Fe-LDH is mainly ascribed to the lower band gap and higher adsorption performance. In addition, LaCO3OH/Ni–Fe-LDH has an interface region produced by the recombination of semiconductors with LaCO3OH and Ni–Fe-LDH structures, which can effectively improve the separation of photocarriers and expand the energy range of photoexcitation. Moreover, a plausible mechanism for photocatalytic decomposition of MO was presented.

Aqueous solubility of Zn incorporated into Mg-Al-layered double hydroxides

Hydrotalcite minerals are layered double hydroxides (LDH) which play an important role in immobilizing hazardous compounds to decontaminate industrial wastewaters. The stability of an LDH is mostly evaluated in terms of its low solubility in water. However, the solubility of divalent trace metals immobilized by Mg-Al-type LDHs is not well known. Hydrotalcites containing Zn in solid solution, (Mg+Zn)3-Al-LDH, were synthesized by alkaline co-precipitation. A series of eleven LDH phases with Zn mole fractions XZn = Zn/(Mg+Zn) of 0–1 were characterized by powder X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, thermogravimetry (TGA), scanning and transmission electron microscopy (SEM/TEM), Brunauer-Emmett-Teller (BET) surface area analysis, and inductively coupled plasma mass (ICP-MS) spectrometry. The XRD analysis provided sharp characteristic spacings for d003 and d006 which occurred for all samples, confirming a layered LDH structure. Cell parameters (a, c) obeyed Vegard’s law and confirmed the formation of a regular solid-solution series without a mixing gap. An aqueous equilibrium time was determined by kinetic dissolution experiments. Steady-state solubility occurred after 120 days, but the experiments continued up to 240 days. The XRD and SEM/TEM analyses indicated no phase changes during the long-term dissolution experiments; neither were phase impurities detected after 240 days. The solubility products of the Mg- and Zn-bearing endmember compositions were calculated from experimentally determined total cation and anion concentrations using the Visual Minteq code for considering element speciation and ion pairing. The solubility product decreased as the Zn mole fraction increased, suggesting that the Zn-bearing LDH phases were more stable than the pure Mg3-Al-LDHs. Solid-solution aqueous-solution thermodynamic equilibrium modeling using the Lippmann “total solubility product” approach and applying Lippmann diagrams with logarithmic x-axes revealed a log-linear decrease in aqueous Zn solubility. The results are promising for remediation of metal-bearing liquid wastes because the metals that co-precipitated with the LDH were more strongly retained and, therefore, less soluble than the hydroxides or carbonates of the trace metal.

Preparation of well-structured hybrid material through ion exchange of chloride by 2,4,5-trichlorophenoxyacetic herbicide in a layered double hydroxide

The [Zn2–Al–Cl] layered double hydroxide (LDH) was used to successfully exchange chloride ions by 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), as a model of the carboxylate herbicide family. The total exchange was obtained after 6 h of stirring with a 2,4,5-T/Cl molar ratio of 10 and a pH of 7. The characterization by various techniques (XRD, FTIR, TG-DTG and SEM) indicates that this exchange led to the production of a new stable hybrid LDH phase intercalated by the herbicide. The structural analysis of the material, based on the interlayer distance of 1.855 nm, indicates that the 2,4,5-T anion takes a vertical arrangement in a double layer. The cohesion of the structure is ensured by hydrogen bonds between the carboxylate group of the herbicide and the hydroxyl groups of the LDH sheets. The exchange causes a modification of the type of symmetry from a 3R stacking mode for [Zn2–Al–Cl] (space group R3‾m) to a 2H for [Zn2–Al-2,4,5-T] (space group P63/mmc). This result confirms the already proven use of LDH materials as efficient anion exchangers for the trapping and elimination of this type of pollutant from wastewater.

Ce-Containing MgAl-Layered Double Hydroxide-Graphene Oxide Hybrid Materials as Multifunctional Catalysts for Organic Transformations.

The combination of layered double hydroxides (LDH) with graphene oxide (GO) enables the formation of nanohybrids with improved properties. This work focuses on the structural and catalytic properties of Ce-containing MgAl LDH-GO composites bearing different concentrations of GO in the range of 5–25 wt.%. The synthesis of the composites was performed by co-precipitating the LDH phase in the presence of GO, while their characterization was performed using XRF, XRD, DRIFT, Raman, SEM, nitrogen adsorption-desorption, and acidity-basicity measurements. The LDH-GO composites, showing redox, basic, and acid catalytic functions, were tested in two different types of organic transformations: (i) Knoevenagel condensation and (ii) one-pot cascade oxidation-Knoevenagel condensation. (i) The cinnamic acid was synthesized by the Knoevenagel condensation of benzaldehyde with diethylmalonate. The composites showed catalytic performances in strong contrast to neat LDH or GO, suggesting a synergistic interaction between the two components. During Knoevenagel condensation, the catalytic activity increased with the GO content in the hybrids up to 15 wt.% and decreased afterwards. (ii) 2-Benzoyl-3-phenylacrylonitrile was synthesized by the aerobic oxidation of benzyl alcohol followed by the Knoevenagel condensation with benzoyl acetonitrile using three different non-polar solvents, i.e., toluene, benzene, and mesitylene. The conversion of benzyl alcohol was higher for the hybrid materials compared to the individual components but decreased with the increase of the graphene oxide concentration.

Operando leaching of pre-incorporated Al and mechanism in transition-metal hybrids on carbon substrates for enhanced charge storage

Operando leaching of pre-incorporated Al and mechanism in transition-metal hybrids on carbon substrates for enhanced charge storage

One-pot synthesis of Mg[sbnd]Al layered double hydroxide (LDH) using MgO and metakaolin (MK) as precursors

Layered double hydroxides (LDH) are a class of basic inorganic layered compounds, which are widely used in the fields of adsorption and catalysis owing to their unique structure and properties. In previous studies, costly soluble Al salts or Al(OH)3 were used as the Al sources for the synthesis of MgAl LDH. The present work proposes a facile and one-pot method to synthesize MgAl LDH in a water bath with MgO and MK as precursors, which are both obtained from abundant natural resources. The effects of liquid/solid ratio (30:1 or 60:1), synthesis temperature (55–90 °C) and alkali concentration (3 or 6 mol/L) were investigated on the composition and characteristics of the synthetic products. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Thermogravimetric analysis (TGA), Scanning electron microscope (SEM) and Nuclear magnetic resonance (NMR). Findings showed that a highly alkaline solution (6 M NaOH) promoted the dissolution of MK, resulting in the release of trivalent Al for the formation of LDH. Increasing temperature also led to faster dissolution of MK and released more Al and Si, which resulted in the formation of both LDH and zeolite. More importantly, it was found that Mg2+ released during the hydrolysis of MgO combined with Al-containing substances directly to precipitate LDH, whereas its hydration product, i.e., brucite, remained stable in the alkaline condition and released little Mg2+ to participate in the formation of LDH. Zeolites may form depending on the Si/Al ratio of the solution which is governed by the various synthesis parameters. The findings not only shed lights on the reaction mechanism between the MgO and MK and the role of key synthetic conditions in the formation of LDH and zeolitic phases, but also demonstrated the feasibility of using widely-available, low-cost natural minerals to produce commercial adsorbents/catalysts.

Insights into the Structure and the Electrochemical Reactivity of Cobalt-Manganese Layered Double Hydroxides: Application to H2O2 Sensing

A series of CoRMn-LDH (R = 1, 2, 3, 4, 5) was synthesized by the coprecipitation method. The formation of layered double hydroxide (LDH) phases is confirmed by powder X-ray diffraction (XRD) and infrared spectroscopy (FTIR). The energy-dispersive X-ray analysis (EDX) shows the total coprecipitation of Co and Mn. The X-ray photoelectron spectroscopy (XPS) result gives evidence of the joint presence of Co(II), Co(III), and Mn(III) in the LDH structures in proportion depending on the initial R value. The Rietveld refinement of the structure using the XRD data reveals that the sample with a Co/Mn ratio of 3:1 displays a pure LDH phase containing mainly Co(II) and Mn(III) with a M(II)/M(III) of nearly 2.1, in agreement with the X-ray absorption spectroscopy (XAS) results at the Co and Mn K-edges, strong hydrogen-bonding network involving CO32–/OH– charge-compensating anions in the interlayers. When compared with the other CoRMn-LDHs, the Co3Mn-LDH displays the best redox properties in an alkaline medium (0.1 M NaOH) and in a neutral pH (Tris buffer). An in-depth study of the Co3Mn-LDH obtained after electrochemical oxidation was also performed. The evolution of Co and Mn oxidation states under electrochemical oxidation was evidenced by XPS. Finally, the performance of the Co3Mn-LDH-modified electrode was determined for the electrocatalytic detection of hydrogen peroxide.

Use of Raman spectroscopy to assess nitrate uptake by calcined LDH phases

Layered double hydroxides (LDHs) are natural or synthetic compounds with a layered structure that become mixed oxides of the metals that contain upon calcination at 450–700 °C. The resulting oxides have shown to be excellent solid sorbents for nitrate ions in aqueous solutions. The structure of the original LDH is recovered by rehydration as a result of the so-called “memory effect”. This process, which follows a Lagergren´s pseudo-first order kinetic model, is monitored via the NO stretching vibration band for nitrate ion at 1051 cm–1 in the Raman spectrum. The nitrate uptake is mainly governed by the temperature and the metal ratio in the initial LDH. At higher Mg/Al ratio, the nitrate adsorption rate was lower. Unlike, it was increased with the adsorption temperature with non-linear response, suggesting an endothermic process. The obtained results present Raman spectroscopy a reliable choice for monitoring nitrate uptake, overcoming to many existing alternatives in terms of accuracy and expeditiousness.

Characterization of blast furnace slag-blended Portland cement for immobilization of Co

The leaching behavior of Co2+ from blast furnace slag-blended cement is investigated. Semi-dynamic leaching test results show that the performance of the cement with regard to the retention of Co2+ is increased by replacing Portland cement with slag (~70 wt%). The changes in the formation and structure of calcium-silicate-hydrate gels in the blended cement as a function of the slag content are insignificant after 28 days of curing, while slag in the binder leads to the formation of more layered double-hydroxide (LDH) structured phases, including AFm and CoAl LDH phases. The enhanced Co uptake by the LDH phases together with the refined pore structure of the slag-blended cement significantly improves the performance for retention of Co2+ in the cement matrix.

Comprehensive study of the formation of stable colloids of Cu[sbnd]Al layered double hydroxide assisted by double hydrophilic block copolymers

The formation of stable aqueous colloidal suspensions of CuAl layered double hydroxide (LDH) assisted by double-hydrophilic block copolymers was investigated. It was of utmost importance since the direct preparation LDH colloids of controlled size is still a major challenge; moreover, regarding the CuAl system, the mechanism of formation of the LDH phase is not well understood yet. CuAl LDH nanoparticles were obtained by complexing Cu2+ and Al3+ cation mixture (Cu2+/Al3+ = 2) with an asymmetric poly(acrylamide)-b-poly(acrylic acid) block copolymer, followed by metal cation hydroxylation. The mechanism of formation of the CuAl LDH was studied and compared to that of MgAl LDH. ICP-MS analysis of the hybrid polyion complex (HPIC) micelles obtained at increasing complexation ratio, i. e. the ratio of the molar concentration of complexing acrylate group of DHBC to M2+ and Al3+ cations (R = AA/(M2+ + Al3+)), showed higher selectivity of DHBC toward Al3+ and, among M2+ cations, for Cu2+ than for Mg2+. Hydroxylation of the HPIC micelles led to macroscopic precipitates at R values below flocculation thresholds, i. e. R1 = 0.43 for DHBC/Cu-Al and 0.13 for DHBC/Mg-Al suspensions, while stable suspensions were formed above R1. The hydrodynamic diameter (Dh) of the colloids determined by DLS decreased from 160 to 50 nm and from 350 to 50 nm in the DHBC/Cu-Al and DHBC/Mg-Al colloids, respectively, when R increased from R1 to 1. XRD and TEM analyses of the dried colloidal suspensions (R > R1) revealed the presence of CuAl LDH and MgAl LDH phases and elemental analyses confirmed that M(II)/Al = 2. Particle sizes were lower in CuAl LDH than in MgAl LDH. Titration curves corresponding to progressive hydroxylation of DHBC/Cu(or Mg)-Al solutions at fixed complexation ratio R allowed establishing that formation of CuAl and MgAl LDH obeyed to a similar sequential mechanism in two steps but involving different aluminum hydroxide precursors according to the different pH values of precipitation. CuAl LDH was obtained by combination of aluminum poly(hydr)oxide species with dissolved copper-based species at pH ~ 5.5, while MgAl LDH resulted from combination of Al(OH)3 and dissolved Mg2+ at pH ~ 8.

Layered Double Hydroxides with Intercalated Permanganate and Peroxydisulphate Anions for Oxidative Removal of Chlorinated Organic Solvents Contaminated Water

The contamination by chlorinated organic solvents is a worldwide problem as they can deeply penetrate aquifers, accumulating in the sub-surface as lenses of highly hazardous pollutants. In recent years, so called in situ oxidation processes have been developed to remediate chlorinated organic solvents from groundwater and soil by injecting solutions of oxidising agents such as permanganate or peroxydisulphate. We here present modified layered double hydroxides (LDHs) with intercalated oxidising agents that might serve as new reactants for these remediation strategies. LDHs might serve as support and stabiliser materials for selected oxidising agents during injection, as the uncontrolled reaction and consumption might be inhibited, and guarantee that the selected oxidants persist in the subsurface after injection. In this study, LDHs with hydrotalcite- and hydrocalumite-like structures intercalated with permanganate and peroxydisulphate anions were synthesised and their efficiency was tested in batch experiments using trichloroethene or 1,1,2-trichloroethane as the target contaminants. All samples were characterised using powder X-ray diffraction, thermal analysis coupled with mass spectrometry to directly analyse evolving gases, and Fourier-transform infrared spectroscopy. Additionally, particle size distribution measurements were carried out on the synthesised materials. Results of the batch experiments confirmed the hypothesis that oxidising agents keep their properties after intercalation. Permanganate intercalated LDHs proved to be most efficient at degrading trichloroethene while peroxydisulphate intercalated Ca,Al-LDHs were the most promising studied reactants degrading 1,1,2-trichloroethane. The detection of dichloroethene as well as the transformation of the studied reactants into new LDH phases confirmed the successful degradation of the target contaminant by oxidation processes generated from the intercalated oxidising agent.

The characteristics and formation mechanism of the dark rim in alkali-activated slag

This paper presents comprehensive investigation on the microstructure and micromechanical properties of the hydration products in alkali-activated slag by adopting the coupled techniques of nanoindentation, optical microscopy, backscattering electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS), aiming to provide insight into the formation mechanism of the dark rim in AAS. It is found that the dark rim is primarily comprised of a mixture of Mg-Al layered double hydroxide (LDH) and C-A-S-H gel, and the existence of this LDH phase contributes to the relatively high hardness and elastic modulus of the dark rim. The dark rim is in-situ formed in the outer layer of GGBFS. The dissolution and diffusion of Ca and Si in the outer layer of GGBFS results in a lower average atomic number in the dark rim region, contributing to its relatively lower grey level. Based on the experimental results, the formation mechanism of the dark rim is discussed in this investigation.