Ca Al Layered Double Hydroxides Research Articles

Development of a low carbon binder system based on metakaolin: Focus on corrosion inhibitor-intercalated layered double hydroxides

In this work, MgO-CaO activated blends were produced by using metakaolin (MK). Roles of sodium nitrite (NaNO2) corrosion inhibitor on phase assemblance, micro-structure, and chloride binding of layered double hydroxides (LDH) in the systems were systematically studied. The results showed that the compressive strength in the middle and late stages were increased for 32.78% and 73.74% with the addition of CaO, and pore structure of the MgO-CaO-MK blends was optimized. After the addition of 5% NaNO2, NO2− anions were preferably intercalated into the layers of self-formed MgAl LDH and CaAl LDH. The chloride binding capacity of the samples was stronger than that of cement paste, which was attributed to the generation of more LDH by accelerating hydration of the system.

Layered Double Hydroxides Reduce the Availability of Cadmium and Lead in Soil and the Content of Cadmium and Lead in Grain

ABSTRACT Layered double hydroxides (LDHs) have excellent immobilization effects on heavy metals (HMs) in water and soil. For the first time in Hunan province, the immobilization effect of Ca-Al LDH (SR-02-02) and Mg-Al LDH (SR-02-03) for the remediation of Cd and Pb was examined in contaminated soils. With treatments of SR-02-02 at 3000 kg/ha(L2–3), the immobilization effect of available Cd and Pb was 54.74% and 20.85%; with treatments of SR-02-03 at 3000 kg/ha(L3–3), the immobilization effect of available Cd and Pb was 68.02% and 26.70%, which showed that Cd, and Pb were gradually solidified into stable oxidizable and residual state. Less HMs were absorbed by rice as a result of the immobilization effect of SR-02-02 and SR-02-03, and the concentration of Cd and Pb in the root, stem, husk, and grain of rice were also significantly reduced. In particular, under the treatment of L2–3, the content of Cd and Pb in grain decreased by 69.13% and 72.67%, respectively; under the treatment of L3–3, the content of Cd and Pb in grain decreased by 80.76% and 100%, respectively. This was because LDHs materials reacted with HMs to form Cd-OH, Pb-OH, CdAl-LDH, and Ca/Mg-O-Pb etc. which reduced the availability of HMs. In conclusion, LDHs material is a promising material for the synergistic remediation of Cd and Pb contaminated soil.

Sebacate intercalated Ca[sbnd]Al layered double hydroxide pigments for corrosion protection of low carbon steel: Anion exchange and electrochemical properties

Sebacate (SB) molecules were incorporated into CaAl layered double hydroxides (CaAL-LDH) using a hydrothermal method. The resulting products underwent characterization through X-ray diffraction, infrared spectroscopy, and scanning electron microscopy. To investigate the release kinetics and thermodynamics of the corrosion inhibitor, total organic carbon analysis (TOC) was employed. A comparison was made with CaAl-LDH prepared without corrosion inhibitors, in which nitrates were the primary anions within the layered clay structure. An assessment of anion exchange between nitrates and chlorides was conducted using total nitrogen analysis (TN). The SB anion comprised approximately 49.2% of the total dried powder synthesized, with a maximum release efficiency of around 86.7% fitting a Langmuir model. For the evaluation of the corrosion inhibition effect on steel surfaces, electrochemical impedance spectroscopy, potentiodynamic polarization, and scanning vibrating electrode technique were employed. The corrosion inhibition impact was ascribed to the liberation of SB anions from the CaAl-LDH pigments, accompanied by alkalinization stemming from partial particle dissolution. Additionally, the incorporation of organic molecules notably enhanced the stability of pigments in aqueous solutions compared to similar ones hosting nitrates. These findings suggested that CaAl-LDH pigments have potential as nanocontainers for organic inhibitors, showing promising prospects in the field of corrosion research.

Effect of Graphene Oxide-Modified CaAl-Layered Double Hydroxides on the Carbon Dioxide Permeation Properties of Fluoroelastomers.

This work aimed to investigate the CO2 gas barrier and mechanical properties of fluorine rubber nanocomposites filled with Ca/Al layered hydroxide (graphene oxide [GO]/LDH-Ca2Al) modified by GO. GO/LDH-Ca2Al nanocomposite fillers were prepared by depositing Ca/Al layered hydroxide (LDH-Ca2Al) into the surface of alkalized GO (Al-GO). The prepared GO/LDH-Ca2Al nanocomposite fillers and complexes were characterized by Fourier infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) for structural and micromorphological characterization. The results showed that GO/LDH-Ca2Al was successfully prepared with strong interactions between Al-GO and LDH, and the compatibility of GO/LDH-Ca2Al nanocomposite fillers with the polymer was significantly improved compared with that of LDH-Ca2Al. Consequently, both the fracture strength (σb) and strain (εb) of GO/LDH-Ca2Al nanocomplexes remarkably increased, and they exhibited excellent mechanical properties. Differential scanning calorimetry and thermogravimetric analysis were used to characterize the thermal stability of GO/LDH-Ca2Al nanocomposite fillers, and GO/LDH-Ca2Al nanocomposite fillers have better thermal stability than LDH-Ca2Al. The reaction products (S-LDH-Ca2Al and S-GO-Ca2Al) of LDH-Ca2Al and GO/LDH-Ca2Al with CO2 were characterized using XRD and TGA, respectively, and the results show that LDH-Ca2Al reacts readily and chemically with CO2, resulting in a lower diffusion coefficient of CO2 in the LDH-Ca2Al nanocomplexes than that of the GO/LDH-Ca2Al nanocomplexes and leading to the destruction of the laminar structure of LDH-Ca2Al, while GO/LDH-Ca2Al has better CO2 resistance stability. GO/LDH-Ca2Al nanocomplexes exhibited a reduced content of hydroxyl groups with pro-CO2 nature exposed on the surface of LDH-Ca2Al, improving the interfacial interaction between the nanofillers and the rubber matrix and enhancing the dispersion of GO/LDH-Ca2Al in the polymers. Moreover, CO2 in the soluble GO/LDH-Ca2Al nanocomposites was significantly reduced, while the diffusion properties demonstrated weak temperature dependence on solubility. The mechanism of the CO2 gas barrier of polymers filled with GO/LDH-Ca2Al was proposed on the basis of the Arrhenius equation.

Effects of nitrite ion intercalated CaAl and MgAl layered double hydroxides on the properties of concrete mortar

Nitrite ion intercalated Ca-Al and Mg-Al layered double hydroxides (LDHs) had been successfully synthesized by two modified methods in order to improve intercalation rate of nitrite ion and the effects of the obtained LDHs on the mechanical properties, anticorrosion properties of concrete mortar were discussed. The results revealed that two kinds of LDHs exhibited the improved crystallinity and uniform morphology, and Ca-Al LDH synthesized by hydrothermal method (H-C-LDH) exhibited better properties. The intercalated LDHs had different degrees of improvement in the mechanical properties, chloride penetration resistance and permeability resistance of cement mortars without apparent decline in the fluidity, especially for 2% H-C-LDH sample. Electrochemical tests also showed that the intercalated LDH led to a more positive potential and larger charge transfer resistance, and H-C-LDH sample expressed the best corrosion resistance. The exchanged ions and X-ray diffraction (XRD) results confirmed that synthetic LDHs could reduce the corrosion risk of carbon steel by providing physical barrier, chloride ion absorption and inhibitor release.

In-situ formation of layered double hydroxides in MgO–NaAlO2-activated GGBS / MSWI BA: Impact of Mg2+ on reaction mechanism and leaching behavior

Leaching problems of municipal solid waste incineration bottom-ash (MSWI BA) have been extensively reported in the literature. The outstanding heavy-metal-ion binding capacity of layered double hydroxides (LDHs) can potentially address this problem. This study aimed to investigate the in-situ formation of Mg–Al LDHs in MgO–NaAlO2-activated ground granulated blast-furnace slag (GGBS) and BA blends, and the evolution of pH, ions, reaction products, microstructure, strength, and leaching behavior were characterized. The results indicate that the formation of Mg–Al LDHs is promoted (up to 69.2%) using a higher MgO/NaAlO2 molar ratio, which improves the heavy-metal-ion binding capacity. Simultaneously, the compressive strength increases up to 78.4% with higher MgO/NaAlO2 molar ratios compared to NaAlO2-activated (NA) sample. Al(OH)4– reacts with extra Mg2+ ions rather than Ca2+ ions to preferentially form Mg–Al LDHs over Ca–Al LDHs. Meanwhile, the extra Mg2+ ions snatch the Al(OH)4– from Si(OH)4, increasing the Mg–Al LDH content and reducing the zeolite content. Moreover, the in-situ-formed LDHs absorbed more SO42– than Cl–.

Ternary Ca-Mg-Al layered double-hydroxides for synergistic remediation of As, Cd, and Pb from both contaminated soil and groundwater: Characteristics, effectiveness, and immobilization mechanisms.

Layered double hydroxides (LDH) are the cost-effective and high-efficiency materials for remediation of potentially toxic elements (PTEs) in contaminated soil and groundwater. Herein, the effectiveness and mechanisms of a ternary Ca-Mg-Al LDH (CMAL) for the synergistic remediation of As, Cd, and Pb were investigated in contaminated soils and simulative groundwaters for the first time. The immobilization efficiencies of As, Cd, and Pb in both black soil (BS) and red soil (RS) amended by CMAL at 5wt% were all >75%. CMAL amendment transferred more mobile As, Cd, and Pb fractions in soils to immobile species than did Ca-Al LDH and Mg-Al LDH treatments. Furthermore, using a pump-and-treat technology, 82-98% of these 3 PTEs from contaminated groundwater were successfully immobilized in both CMAL treated BS and RS top-soils. Meanwhile, leaching of Ca, Mg, and Al from CMAL was minimal indicating the material was stable. The excellent immobilization performance of CMAL for these PTEs was attributed to the coating of soil microparticles by CMAL nanosheets that allowed complexation of Ca-O-As/Cd or Mg-O-As/Cd/Pb formation, co-precipitation of Ca/Fe-As and Cd(OH)2, and formation of Ca-bridged ternary complex (FeO-Ca-As/Cd). The adverse effect of oppositive pH/Eh-dependence between As and Cd/Pb was overshadowed by these mechanisms and thus allowed As immobilization. Immobilization of As, Cd, and Pb by CMAL amendment was more favorable for RS soil due to its lower reduction potential and more participation of metal-(hydr)oxides for complexation. Overall, the ternary-LDH is a promising synergistic remediation strategy for multi-PTEs contaminated soil and groundwater.

Effect of layered double hydroxides Ca-Al LDHs and polycarboxylate ethers on the hardening of Portland limestone cement

The use of layered double hydroxides (LDHs) is a new direction in chemistry of cement. The influence of calcium/aluminum double hydroxides (Ca-Al LDHs) and polycarboxylate ethers (PCE) on the strength development of Portland limestone cement has been investigated. It has been shown that Ca-Al LDHs form in cement slurry with the introduction of alkaline aluminate accelerator Na[Al(OH)4]. It was determined that this contributes to a significant increase in the early strength of cement stone. However, this reduces the setting time of hardening, and there is a decrease in strength with age. At an early stage of structuring in cement paste with the addition of Na[Al(OH)4], metastable high-water hydrate phases of Ca-Al LDHs of type C4AH19 ([Ca2Al(OH)6]2(ОН)2·12H2O) are formed, which, due to the conversion phenomenon, transform to cubic C3AH6. Such processes are accompanied by an increase in overall porosity and decreases in the strength of cement stone. Stabilization of the Ca-Al LDH structure is achieved by introducing anions [NO3]- into the interlayer space with the formation of Ca-Al-NO3 LDHs through the use of sol-gel technology. X-ray phase analysis, IR spectroscopy, and electron microscopy proved the fact of synthesis of AFm-phases of type [Ca2Al(OH)6]2(NO3)2.6H2О. When modifying such hydrates with polycarboxylate ethers, a nanocomposite Ca-Al-NO3 LDHs-РСЕ is formed. It was found that when the Ca-Al-NO3 LDHs-РСЕ nanocomposite was introduced to СЕМ ІІ/A-LL 42,5 R, the strength over the period of 8...24 hours increases by 2.0–1.5 times, and, after 2 and 28 days, 58 and 144 % of the standard strength is achieved (Rс28=62.5 MPa). The obtained nanomodified Portland limestone cement refers to extra rapid-hardening and high-strength cement, which makes it possible to solve the task of carrying out work on the rapid restoration of reinforced concrete structures' elements

Preparation of organic dye-modified calcium-aluminum layered double hydroxide powders and their cytotoxicity

Incorporation of anions of indigo carmine dye and dodecyl sulphate to CaAl-layered double hydroxides (LDHs) has been carried out by the coprecipitation method. The effect of Ca2+/Al3+ ions ratio on the color and photostability of the composites was studied. The FE-SEM images suggested that the CaAl/Indigo and CaAl/SDS/Indigo composites formed plate-like structure with loose stacking. XRD results showed that the CaAl-LDH were mainly composed of CaAl-hydrotalcites structure. The FT-IR spectra revealed the interaction between the sulfate groups and the LDH surface, and the functional groups of each counterpart were present. The UV-Vis absorption spectra of the CaAl/Indigo and CaAl/SDS/Indigo composites exhibited intense blue color. Furthermore, the CaAl/Indigo and CaAl/SDS/Indigo composites exhibited high photo-stability under UV irradiation. Cytotoxicity was then assessed using the methyl tetrazolium test (MTT) and the cell survival rate was determined. Cytotoxicity tests showed that most CaAl/Indigo and CaAl/SDS/Indigo composites are biocompatible to the GF cells except the SDS modified CaAl-hydrotalcites structure.

Novel Ca-SLS-LDH nanocomposites obtained via lignosulfonate modification for corrosion protection of steel bars in simulated concrete pore solution

A novel nanocomposite (Ca-SLS-LDH) of Ca-Al-layered double hydroxides (Ca-LDH) modified with sodium lignosulfonate (SLS) was synthesized using hydrothermal synthesis method. Mechanistically, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) analysis revealed that SLS modified the nanocomposite by adsorbing onto the surface of Ca-LDH. The specific surface area of Ca-SLS-LDH was larger and the average particle size was smaller than that of Ca-LDH. This enabled the Ca-SLS-LDH to bind chlorides more effectively and is hence suitable for application in reinforced concrete. Ca-SLS-LDH binds to chlorides through ion exchange and surface adsorption, which is similar to the binding mechanism of Ca-LDH to chlorides.Ca-SLS-LDH efficiently controlled the corrosion of carbon steel in simulated concrete pore (SCP) solution and had a stronger inhibitory effect compared to unmodified LDHs at equal concentrations. Ca-SLS-LDH improved the resistance to corrosion by binding more chlorides in SCP solution. In addition, it may be due to the anti-corrosion effect of SLS on steel bars, thereby delaying the initiation of corrosion.

Lactate adsorption by layered double hydroxides in aqueous solution and cell culture medium

When cells proliferate in a culture medium, metabolites such as lactic acid gradually accumulate and eventually hinder cell growth. Herein, we considered various layered double hydroxides (LDH) and layered double oxides (LDO) for the selective removal of lactate in aqueous solution and a culture medium by anion exchange. NO3•Mg-Al LDH showed high lactate adsorption, low adsorption of glucose (a nutrient in culture medium), appropriate pH stability, and low cytotoxicity within a limited contact time. In contrast, NO3•Ni-Al LDH, NO3•Ca-Al LDH, Mg-Al LDO, and Cl•Mg-Al LDH had a lower performance than NO3•Mg-Al LDH. To further improve the performance, the anion of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES, a common pH buffer in culture media) was tested as the intercalated anion in Mg-Al LDH. HEPES•Mg-Al LDH (Mg/Al = 2) was found to be the most promising adsorbent for culture medium regeneration because its adsorption behavior was similar to that of NO3•Mg-Al LDH, while it had a much lower cytotoxicity. The traditional methods for regenerating a culture medium by removing accumulated lactate to increase the cell density in large-scale cell culture and lower the costs, including electrolysis, cell modification, and modification of the culture medium’s composition, are complicated to operate and expensive. The adsorption method using HEPES•Mg-Al LDH presented in this study, instead, can be easily operated and has low costs. Thus, it is expected to be applied in actual cases for regenerating culture media.

Autogenous formation and smart behaviors of nitrite- and nitrate-intercalated layered double hydroxides (LDHs) in Portland cement-metakaolin-dolomite blends

In this work, the influence of sodium nitrite (NaNO2) and sodium nitrate (NaNO3) corrosion inhibitors on the composition, structure, and chloride binding behaviors of layered double hydroxides (LDHs) formed in ternary ordinary Portland cement-metakaolin-dolomite (OPC-MK-DM) systems is studied. The results show that the nitrite and nitrate anions are preferably intercalated in the CaAl LDHs (AFm phases), but not in the MgAl LDHs (hydrotalcite-type phases) due to its limited formation in these ternary cementitious systems cured at ambient temperature. The autogenously formed nitrate- and nitrite-AFm phases are decomposed upon chloride exposure accompanied by Friedel's salts formation, potentially releasing corrosion inhibitive ions to the pore solution in a progressive manner. The NaNO2 and NaNO3 incorporation in ternary OPC-MK-DM binders marginally lowers chloride binding capacity but reduces its penetration resistance mainly due to pore coarsening. Nevertheless, a strong linear correlation can be established between the water-soluble and total chloride contents in ternary OPC-MK-DM systems, regardless of OPC replacement level and corrosion inhibitor incorporation.

Novel fluoride rechargeable dental composites containing MgAl and CaAl layered double hydroxide (LDH)

ObjectiveThis study aims to incorporate 2:1 MgAl and 2:1 CaAl layered double hydroxides (LDHs) in experimental dental-composites to render them fluoride rechargeable. The effect of LDH on fluoride absorption and release, and their physico-mechanical properties are investigated. Methods2:1 CaAl and 2:1 MgAl LDH-composite discs prepared with 0, 10 and 30wt% LDH were charged with fluoride (48h) and transferred to deionized water (DW)/artificial saliva (AS). Fluoride release/re-release was measured every 24h (ion-selective electrodes) with DW/AS replaced daily, and samples re-charged (5min) with fluoride every 2 days. Five absorption-release cycles were conducted over 10 days. CaAl and MgAl LDH rod-shaped specimens (dry and hydrated; 0, 10 and 30wt%) were studied for flexural strength and modulus. CaAl and MgAl LDH-composite discs (0, 10, 30 and 45wt% LDH) were prepared to study water uptake (over 7 weeks), water desorption (3 weeks), diffusion coefficients, solubility and cation release (ICP-OES). ResultsCaAl LDH and MgAl LDH-composites significantly increased the amount of fluoride released in both media (P<0.05). In AS, the mean release after every recharge was greater for MgAl LDH-composites compared to CaAl LDH-composites (P<0.05). After every recharge, the fluoride release was greater than the previous release cycle (P<0.05) for all LDH-composites. Physico-mechanical properties of the LDH-composites demonstrated similar values to those reported in literature. The solubility and cation release showed a linear increase with LDH loading. SignificanceLDH-composites repeatedly absorbed/released fluoride and maintained desired physico-mechanical properties. A sustained low-level fluoride release with LDH-composites could lead to a potential breakthrough in preventing early stage carious-lesions.

Optical properties of the europium (II) and (III) ions doped metal oxides obtained from sintering layered double hydroxides, and their fine structures

We examined the optical properties of sintered layered double hydroxides (LDH) as phosphors. Eu3+-doped MgAl LDH and CaAl LDH were synthesized via co-precipitation methods. We sintered the obtained LDH at temperatures ranging from 300 to 1000 °C. The sintered MgAl LDH showed the same X-ray diffraction (XRD) pattern as rock salt in the temperature range between 300 and 800 °C and partly transitioned to spinel at 1000 °C. The sintered LDH showed red luminescence with a main peak at 613 nm. This dominant red luminescence is assigned to the 5D0-7F2 electric dipole transition of Eu3+, which is allowed for structures without inversion symmetry. The dominant intensity of the electric dipole transition is inconsistent with the highly symmetric rock salt structure. We concluded that this phenomenon is triggered by the reduction of symmetry around the Eu3+ ion due to the introduction of the Al3+ ion and the cation vacancy in the MgO rock salt host and/or the strain induced by the introduction of these dopants. Our conclusion is based on the X-ray absorption near edge structure (XANES) data and the extended X-ray absorption fine structure (EXAFS) analysis of the sintered NiTi LDH with the same rock salt structure as the sintered MgAl LDH.Eu2+-doped MgAl and CaAl binary metal oxides, synthesized by sintering Eu3+-doped MgAl and CaAl LDH in quartz tubes in the presence of Ti powder, showed broad peaks at the green (540 nm) and blue (440 nm) light regions, respectively. These results have demonstrated that red-green-blue (RGB) luminescence can be obtained using a uni-type host structure.

Removal of selenium from caustic solution by adsorption with Ca[sbnd]Al layered double hydroxides

Removal of selenium from caustic solution by adsorption with CaAl Layered Double Hydroxides (CaAl LDHs) was proposed. CaAl LDHs with a hexagonal platelet structure were successfully synthetized by co-precipitation. 94% of Se(VI) was removed at 30 °C for 30 min with NaOH concentration of 1 mol·L−1 and adsorbent dosage of 40 g·L−1. Temperature played an important role in Se(VI) removal. At low temperature (30 °C–70 °C), CaAl LDHs underwent an anion exchange with Se(VI) to form Ca4Al2O6SeO4(H2O)10, with the d-spacing expanding from 7.95 Å to 10.15 Å. When increasing temperature above 80 °C, Ca4Al2O6SeO4(H2O)10 began to decomposed to Ca3Al2O6(H2O)6 and Ca(OH)2, resulting in SeO42− into the liquor. Kinetic data were well fitted to pseudo-second-order model, indicating Se(VI) removal process was controlled by the chemisorption. The adsorption isotherms were in good agreement with Langmuir isotherm model, suggesting Se(VI) removal was a monolayer adsorption. The maximum adsorption capacity was calculated to be 138.76 mg·g−1 at 30 °C. Desorption and regeneration results indicated that CaAl LDHs can be reused at least three times without significantly reducing its absorption capacity for Se(VI).

Influence of Nitrate Corrosion Inhibitors on Phase Stability of Alkali-Activated Slag against Chloride Binding and Natural Carbonation

Nitrate and nitrite-based corrosion inhibitors have been studied for decades for mitigating corrosion of steel embedded in ordinary portland cement (OPC) concrete; however, few studies are available regarding the effect of nitrate or nitrite on the performance of alkali-activated binders. This work, for the first time, investigates the influence of three nitrate-containing activators (i.e., sodium nitrate, magnesium nitrate, and aluminum nitrate, each mixed with sodium hydroxide solution) on the strength development, pore structure, phase assemblage, and phase stability of alkali-activated slag (AAS) binders upon exposure to chloride and natural carbonation. The results show that the addition of nitrate, regardless of its form, does not substantially alter the type of main reacted phases, i.e., calcium-aluminosilicate-hydrate (C-A-S-H) and Mg-Al layered double hydroxides (LDHs), in AAS; however, the addition of aluminum nitrate improves its compressive strength, boosts the formation of Ca-Al LDHs (AFm-type phase), and likely inserts nitrate anions in the interlayer of formed Ca-Al or Mg-Al LDHs. The findings suggest that the nitrate-intercalated Ca-Al LDHs and/or Mg-Al LDHs, if formed to some extent, can potentially release nitrate ions during chloride binding and carbonation, working as a smart corrosion inhibitor.

Reactivity and Heavy Metal Removal Capacity of Calcium Alginate Beads Loaded with Ca–Al Layered Double Hydroxides

Layered double hydroxides (LDHs) present multiple applications due to their versatility and reactivity. Thus, Ca–Al LDHs with Friedel’s salt structure (HC) have been proposed as heavy metal scavengers due to their buffering capacity at basic pHs. Nevertheless, the control of the reactivity of LDHs such as HC is necessary to optimize their applications. Here, the reactivity of an HC prepared by a coprecipitation method was modified by its inclusion in calcium alginate (CaAlg) beads prepared by ionic gelation. The obtained beads (CaAlg/HC) showed good dispersion of the HC particles in the alginate matrix and were used to test the acid base reactivity and heavy metal uptake capacity compared with pure CaAlg beads and HC powder separately. The pH buffering capacity of CaAlg beads was enriched by the inclusion of HC that, in turn, was modulated in its reactivity. Thus, the HC dissolution times changed from mere seconds for the powder to tens of minutes when enclosed in the beads in a kinetic profile determined by the diffusive step. On the other hand, Cu2+ uptake capacity of CaAlg/HC beads combined the Cu(OH)2 precipitation capacity of HC with the complexation capacity of alginate, reaching good affinity and capacity for the obtained beads. Nevertheless, the precipitation of the hydroxide was produced outside the bead, which would induce the addition of an additional separation step to produce an acceptable Cu2+ elimination.

Novel route to synthesize CaAl- and MgAl-layered double hydroxides with highly regular morphology

A facile, sol–gel method was developed to obtain Al-containing layered double hydroxides (LDHs) with Ca or Mg divalent cations. Ca- and Al-alkoxide were added to aqueous NaNO3 solution, and, after rapid gelation, a solid product, phase-pure, highly crystalline CaAl–LDH was formed with exceptionally regular morphology without impurities for the first time with sol–gel method. The lack of salt in the solution used for the hydrolysis resulted in a non-desirable by-product (katoite). For the sol–gel synthesis of MgAl–LDH, it was found that its formation was less sensitive to the presence of nitrate ions, and pure water or aqueous NaOH worked equally well. The product was also a highly crystalline material with thin, plate-like morphology as revealed by X-ray diffractometry and scanning electron microscopy. IR measurements indicate the lack of organic substance in the interlamellar space for both materials.

Super adsorption capability of rhombic dodecahedral Ca-Al layered double oxides for Congo red removal

In this work, a novel rhombic dodecahedral structure of Ca-Al-layered double hydroxides (Ca-Al-LDHs) was synthesized by a two-phase solvothermal method. Subsequently, Ca-Al layered double oxides (Ca-Al-LDOs) were obtained by calcinating Ca-Al-LDHs at 600 °C which maintained the rhombic dodecahedral structures well. The morphologies of as-obtained samples were characterized by both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The phase structure and elemental composition were analyzed by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), respectively. Fourier transformed infrared (FT-IR) spectra were performed to confirm the existence of LDH structures. The Ca-Al-LDOs exhibited high adsorption rates and superb adsorption capacities for removal of Congo red (CR) from aqueous solution. The maximum adsorption capacities of Ca-Al-LDOs towards CR reached to 1536.1 mg g−1, which is extremely higher than most of hydrotalcite-like materials. The adsorption process of Ca-Al-LDHs and Ca-Al-LDOs were described by the Langmuir isotherm model and pseudo-second-order model well. The adsorption mechanisms mainly dominated by electrostatic adsorption, ion exchange process, hydrogen bonding interactions and surface complexation. This general strategy afforded a wide possibility to prepare a novel and advanced LDOs adsorption material for water treatment.

Selenite Uptake by Ca–Al LDH: A Description of Intercalated Anion Coordination Geometries

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO32-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO32--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO3, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl2 shows much stronger affinity and larger distribution ratio (Rd ∼ 17800 L kg-1) toward SeO32- than AFm-SO4 (Rd ∼ 705 L kg-1). At stoichiometric SeO32- loading for anion exchange, the newly formed AFm-SeO3 phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO32- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl-, SO42-, MoO42-, and SeO32-). Contrary to AFm-SeO3 with inner-sphere SeO32- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO32- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.