Calcined Layered Double Hydroxides Research Articles

Integrated study of antiretroviral drug adsorption onto calcined layered double hydroxide clay: experimental and computational analysis

This study focused on the efficacy of a calcined layered double hydroxide (CLDH) clay in adsorbing two antiretroviral drugs (ARVDs), namely efavirenz (EFV) and nevirapine (NVP), from wastewater. The clay was synthesized using the co-precipitation method, followed by subsequent calcination in a muffle furnace at 500 °C for 4 h. The neat and calcined clay samples were subjected to various characterization techniques to elucidate their physical and chemical properties. Response surface modelling (RSM) was used to evaluate the interactions between the solution’s initial pH, adsorbent loading, reaction temperature, and initial pollutant concentration. Additionally, the adsorption kinetics, thermodynamics, and reusability of the adsorbent were evaluated. The results demonstrated that NVP exhibited a faster adsorption rate than EFV, with both reaching equilibrium within 20–24 h. The pseudo-second order (PSO) model provided a good fit for the kinetics data. Thermodynamics analysis revealed that the adsorption process was spontaneous and exothermic, predominantly governed by physisorption interactions. The adsorption isotherms followed the Freundlich model, and the maximum adsorption capacities for EFV and NVP were established to be 2.73 mg/g and 2.93 mg/g, respectively. Evaluation of the adsorption mechanism through computational analysis demonstrated that both NVP and EFV formed stable complexes with CLDH, with NVP exhibiting a higher affinity. The associated adsorption energies were established to be −731.78 kcal/mol for NVP and −512.6 kcal/mol for EFV. Visualized non-covalent interaction (NCI) graphs indicated that hydrogen bonding played a significant role in ARVDs-CLDH interactions, further emphasizing physisorption as the dominant adsorption mechanism.

Removal of direct blue FFRL by layered double hydroxides: Thermodynamics and kinetics

In this paper, MgAl layered double hydroxides (LDH) with molar ratio of nMg:nAl = 2:1 was successfully synthesized by conventional co-precipitation method to remove direct blue FFRL (FFRL). In order to study the influence of calcined temperature to FFRL removal, a series of calcined layered double hydroxides (CLDH) samples was obtained by calcination at 200, 450, 600, 800 °C for 6 h, respectively. BET, XRD and SEM were used to analyze the structure of MgAl-CLDH calcined at different temperature. The MgAl-CLDH calcined at 450 °C exhibits the most considerable removal efficiency, which is corresponding to the results of BET. The equilibrium isotherm of FFRL removal by MgAl-CLDH can be better described by Langmuir model. The process of FFRL removal by MgAl-CLDH is spontaneous and exothermic with an increasing in molecular order. Removal of FFRL by MgAl-CLDH also conforms to the pseudo-first-order model with activating energy of 129.6 kJ/mol, which proves that the removal of FFRL by MgAl-CLDH is controlled by a chemical reaction between FFRL and MgAl-CLDH.

Mechanisms of CLDH seeding on hydration kinetics of slag-based geopolymer: Towards aluminosilicate cement phase engineering

Geopolymer and related chemically activated materials are more sustainable aluminosilicate cementitious materials that have less natural materials consumption and CO2 emissions. An intrinsic challenge comes from the lack of sufficient crystalline phases in the binder, which leads to the suffering of volume instability and other issues for this type of cements. This study proposes phase engineering concept, for the first time, and investigates the stargate of using active crystal seeds. Calcined layered double hydroxides (CLDH) was used as a type of seed to understand the impact on hydration kinetics and microstructure of slag-based geopolymer. Through various characterization techniques, it is revealed that during the hydration process of slag, CLDH adsorbs OH− ions and other anions from the solution, leading to a rapid recrystallization and formation of hydrotalcite (Ht) phase. The generated Ht phase serves as nucleation sites, accelerating the formation of C-A-S-H amorphous gel and more Ht phase. Ht nanocrystals consistently exists in the binder, at least in the studied period, together with C-A-S-H nanocrystals and more ordered gel phases. Low modulus of silicate activator is found to enhance the nucleation effect of Ht. This research provides valuable insights into the interactions and mechanisms governing the hydration kinetics and phase evolution in geopolymer, and proves the effectiveness of phase engineering via seeding.

Study on the anti-corrosion and barrier ability of modified sulfoaluminate cement mortar cutoff wall against sulfate

Calcined layered double hydroxide (CLDH) was used to modify sulfoaluminate cement (SAC) mortar to develop a new sulfate-resistant barrier material. Through investigations into mechanical properties, permeability, microstructure, and migration characteristics, the results demonstrated that the addition of CLDH improved the mechanical properties and pore structure of SAC mortar and inhibited the diffusion of SO42-. Based on its anti-corrosion and barrier ability, the optimal CLDH dosage in SAC mortar was determined to be 2 % of the cement mass. Service life calculations and Visual MODFLOW simulations demonstrated that the SAC-CLDH cutoff wall has significant durability and barrier effect on sulfate. Compared to the site without the SAC-CLDH cutoff wall, the effective dispersion distance and area of sulfate decreased by 88.79 % and 93.43 % during 4000 days. This finding provides a new reference for the containment and management of high-concentration sulfate-contaminated sites.

Improving U(VI) retention efficiency and cycling stability of GCN-supported calcined-LDH composite: Mechanism insight and real water system applications

The synthesis and characterization of graphitic carbon nitride (GCN) and its composites with calcined layered double hydroxide (CLDH) were examined in this investigation. The goal was to assess these composites' maximum adsorption capacity (qmax) for U(VI) ions in wastewater. Several different characterization methodologies were utilized to examine the fabricated substances. These methods encompass X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The GCN-CLDH composite displayed enhanced adsorption ability towards U(VI) ions due to its high surface functionality. Langmuir adsorption isotherm analysis showed that more than 99% of U(VI) ions were adsorbed, with a qmax of 196.69 mg/g. The kinetics data exhibited a good fit for a pseudo-second-order (PSO) model. Adsorption mechanisms involving precipitation and surface complexation via Lewis's acid-base interactions were proposed. The application of the GCN-CLDH composite in groundwater demonstrated adsorption below the maximum permissible limit established by USEPA, indicating improved cycling stability. These observations underscore the capacity of the GCN-CLDH composite's proficiency in adsorbing U(VI) aqueous solutions containing radioactive metals.

Structured LDH/Bentonite Composites for Chromium Removal and Recovery from Aqueous Solutions.

This study focuses on chromium removal through adsorption and ion exchange using structured calcined layered double hydroxide (LDH) (MgAl)-bentonite composites. Firstly, the powders were structured into granulates to study the effect on Cr sorption kinetics to circumvent the limitations of working with powders in real-life applications. Secondly, the regeneration of the structured composites was optimized to enable multi-cycling operation, which is the key for their applicability beyond laboratory scale. Firstly, the LDH/bentonite ratio was optimized to obtain the best performance for the removal of Cr3+ and Cr6+ species. In powder form, the calcined adsorbent containing 80 wt% LDH and 20 wt% bentonite performed best with an adsorption capacity of 48 and 40 mg/g for Cr3+ and Cr6+, respectively. The desorption was optimized by studying the effect of the NaCl concentration and pH, with a 2 M NaCl solution without pH modification being optimal. The kinetic data of the adsorption and desorption steps were modelled, revealing a pseudo-second order model for both. This was also demonstrated using XRD and Raman measurements after the Cr3+ and Cr6+ adsorption tests, indicating successful uptake and revealing the adsorption mechanism. Finally, five consecutive adsorption-desorption cycles were performed, each showing nearly 100% adsorption and desorption.

Removal of single dye and dye mixture from aqueous solution with alginate-coated calcined layered double hydroxide and illite clay composite beads

ABSTRACT Alginate-coated calcined layered double hydroxide and illite clay (Alg@CLDH@ Illite) composite beads were fabricated and tested for single and mixture dye adsorption from aqueous solution. Adsorption characteristics of the as-fabricated beads were assessed by using Methylene Blue (MB) and Congo Red (CR) as adsorbates. As-prepared samples were characterised by using X-ray diffraction (XRD), Thermogravimetry/Differential Scanning Calorimetry (TG–DSC), Fourier transform infrared spectroscopy (FTIR), and point of zero charge (pHpzc). The characterisations demonstrated that the incorporation of CLDH powder/clay mixture can not only enhance thermal stability but also can improve the adsorption performance. The adsorption kinetics of dyes onto composite beads followed the linear form of pseudo-second-order model and their equilibrium data were fitted well to the Freundlich model. The results showed that the Alg@CLDH@Illite composite is able to adsorb anionic and cationic dyes from a binary dye mixture with high sorption capacities in short time 50 min.

Effect of Layered Double Hydroxides on the Deterioration Process of Cement Paste under Sulfate Attack

This study investigated the effect of layered double hydroxides (LDHs) on the deterioration process of cement paste in the sulfate environment. Cement pastes with the addition of original and calcined LDHs at 2.5 wt.% and 5.0 wt.% of cement were exposed to Na2SO4 solution for 360 days. The macroscopic performance of the cement paste was assessed based on mass variation, porosity, compressive strength, and content of sulfate ions. Furthermore, the microhardness, microstructures, and composition of the degraded pastes were examined using Vickers hardness (HV), mercury intrusion porosimetry (MIP), scanning electron microscope (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The results indicate that cement paste incorporated with LDHs can mitigate the corrosion caused by sulfate effectively, especially in the case of calcined LDHs (C-LDHs), which primarily increase the adsorption of sulfate. Compared with the control specimen, the 180 d compressive strength loss ratio of specimens with 2.5 wt.% and 5.0 wt.% of C-LDHs decreased by 63.66% and 80.51%, respectively. Moreover, LDHs can reduce the amount of ettringite crystals, densify the microstructure, and refine the pore structure to mitigate the cement paste's sulfate corrosion significantly. Compared with the control specimen, the 180 d harmful pore volume fraction of specimens laced with 2.5 wt.% and 5.0 wt.% C-LDHs decreased by 43.77% and 54.51%, respectively. In terms of the content of C-LDHs, an optimal content of C-LDHs could ensure the dominant effect of adsorption, while excessive C-LDHs could refine pores. In addition, Vickers hardness has an excellent correlation with compressive strength, which could precisely predict the compressive strength. Moreover, by combining the Vickers hardness distribution and content distribution of sulfate ions, the cross-section of the paste could be classified into four regions to evaluate the deterioration process accurately: the degraded zone, the strengthened zone, the invaded zone, and the intact zone.

Green synthesis of Ag ultra-fine nano-catalyst supported on layered double oxide and chitosan: Accelerated reduction of 4-nitrophenol to 4-aminophenol

In this research, the green synthesis of a silver nanoparticle (Ag NPs) based nano-catalyst for reducing 4-nitrophenol (4-NP) pollutants were explained. Hence, instead of using chemical reductant, natural extract of green tea was utilized as reducing agent to prepare Ag NPs. The obtained Ag NPs had uniform size and shape. The small size of Ag NPs helps exited electron to be stabilized and extends lifespan during a reduction reaction. Then, layered double hydroxides (LDHs) was prepared, and after calcination was added to Ag NPs. The calcination of LDH leads to a significant increase in the number of catalytic active sites, thereby enhancing catalyst function. Besides, chitosan (CS) was chosen as a natural polymer matrix. Ag-LDH hybrids can remain immobilized and well-dispersed on CS matrix. The CS matrix also facilitates the separation process. More importantly, it enhances reusability of the nano-catalyst noticeably and accordingly plays an important role in the synthesis of the reusable high-performance nano-catalyst. Finally, the prepared CS/Ag-LDH NC film reduced 4-NP to 4–aminophenol in 5 min with a conversion percentage of 99.01% ± 0.13. The effect of various parameters like pH, catalyst amount, NaBH4 concentration, and 4-NP concentration was evaluated. Based on the obtained findings, reaction kinetics is pseudo-first-order with an apparent rate constant of 1.65 × 10−2 (s−1), and the activation energy was calculated to be 13.93 kJ mol−1. Presence of CS enhanced reusability of the nano-catalyst noticeably so that, performance of the nano-catalyst remained unchanged after five reuse cycles.

Synthesis and physicochemical characterization of new calcined layered double hydroxide Mg Zn Co Al-CO3; classical modeling and statistical physics of nitrate adsorption

This research focuses on the study of a new material, a layered double hydroxide, which was synthesized using a co-precipitation method at pH constant and, then, tested for its retention of nitrate anions. The LDH was prepared from four metal cations Mg-Zn-Co-Al to produce the quadratic material mentioned, Mg Zn Co Al-CO3, which was calcined to obtain metal oxides. Different techniques of characterization were considered, namely FTIR, X-ray diffraction, BET, SEM and EDX. Adsorption experiments for nitrate were carried out to test the effect of contact time, adsorbent mass, pH values, mass effect, and initial surfactant concentrations. When equilibrium was reached after 300 min (R2 = 0.998), the Pseudo-second order model was found to fit accurately the kinetic data. The intraparticle diffusion model recorded a rate constant of between 0.277 and 0.757 (g/mg. min−0.5) and the Sips adsorption isotherm best fitted the experimental data obtained. Maximum adsorption capacity for nitrate was found to be 55.97 mg g−1 which was in accordance with the experimental results. Thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic. The values of ΔH˚ and ΔS˚ of the adsorption process were 14.81 and 59.04 kJ mol−1, respectively. The low value of ΔH˚ (<40 kJ mol−1) indicated that the adsorption process was associated with physical forces. Finally, for a deeper insight into the nitrate adsorption mechanism, the statistical physics model was considered to quantify the number of adsorbed nitrate molecules per site, the anchorage number, the receptor sites density, the adsorbed quantity at saturation, the concentration at half saturation and the molar adsorption energy. A statistical physics model, with two binding sites, proved to be the best option to characterize and interpret the removal of nitrate on the tested LDH.

Molecular scale assessment of defluoridation of coal-mining wastewater by calcined Mg/Al layered double hydroxide using 19F solid-state NMR, XPS, and HRTEM.

Calcination is an effective way to improve the F- adsorption capacity of layered double hydroxide (LDH) materials, however, a molecular scale understanding of the enhanced defluoridation capability of calcined LDHs (CLDH) is lacking. This study investigated the mechanisms of F- adsorption by CLDH using 19F solid-state NMR, X-ray photoelectron spectroscopy (XPS), and high-resolution TEM. Under calcination process, LDH underwent three periods: surface dehydration below 200°C, structural dehydroxylation at 200-400°C, and release of interlayer carbonate groups above 400°C. Additionally, XPS and XRD characterization showed that CLDH could not recover to the original structural symmetry even after rehydration and reconstitution. The F- affinity was greatly enhanced for the calcined LDH, especially at high pH. At pH 10, the adsorption capacity could reach 22.0mgF-/g for CLDH (500°C calcined), about 6 times larger than that of LDH. The XRD analyses revealed that the F-adsorbed CLDH had a poorer crystalline degree as the calcination temperature increased, consistent with the TEM observation of abundant defects and Mg/Al oxides on the CLDH sheets. 19F solid-state NMR spectra of the CLDH after F- adsorption showed that the formation of surface Al-F is the predominant F- adsorption mode at pH 7, whereas the Mg-F local coordination mode is the pronounced F- adsorption mechanism under alkaline conditions (pH 10). The present study provided a comprehensive understanding of CLDH in F- adsorption and suggested that calcination is a promising treatment for promoting the efficacy of polluted anion scavenging.

Flocculation of polystyrene nanoplastics in water using Mg/Al layered double hydroxides via heteroaggregation

Nanoplastics (NPs) nano-scale effects and have strong mobility, posing potential risks to ecosystems and human health. Therefore, it is of great significance to study the interactions of minerals and NPs in water to understand the migration, transformation, and fate of NPs in the environment. This study investigated the flocculation of NPs on layered double hydroxides (LDHs) in water via heteroaggregation under various conditions such as different Mg/Al ratios of LDHs, calcination temperatures, coexisting ions, solution pH, and contact time. We found that the overall reaction achieved high efficiency that almost surpassed 70% and varied with solution pH. The highest flocculation performance (85.87%) was achieved at a Mg:Al ratio of 3:1, followed by 4:1 and 2:1. Moreover, the highest removal rate (80.85%) of NPs on cLDHs was achieved at calcination temperature of 300 °C. The heteroaggregation mechanism, bonding type, and interface reaction of the NPs–LDHs system were analyzed using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, spectroscopy, and microscopy. At short separation distance within 5 nm, the total interaction energy was governed by electrostatic repulsion. However, at longer distance, the interparticle forces became dominated by van der Waals and electrostatic attractions, leading to attachment of two types of particles. The NPs could be removed by attachment onto the surface of LDHs without forming other functional groups. However, the calcined LDHs and those prepared by hydrothermal reactions showed different attachment morphologies under scanning electron microscopy, which revealed the mechanisms of surface attachment and encased precipitation. These results provided new insights and theoretical support for controlling NPs pollution in surface water, which could reduce the risks of NPs to ecosystems and human health.

Analysis of Highly Efficient Adsorption of Au(S2O3)2 3- by Calcined Cu/Fe Layered Double Hydroxides.

One of the main problems affecting the application of gold thiosulfate leaching is the difficulty in recovering gold from the leaching solution. Our previous studies revealed that when using calcined Mg/Al layered double hydroxides (LDHs), the single adsorption capacity of Au(S2O3)23– can reach 7.76 mg/g. In order to further examine the recovery of Au(S2O3)23– by various types of LDHs, the divalent metal and trivalent metal ions in LDHs were altered. Also, thiosulfate anions are introduced between the layers of LDHs to reduce the competitive adsorption of thiosulfate ions in the solution. Results show that the calcined LDHs (CLDH) prepared with Cu/Fe-LDHs are the best at adsorbing Au(S2O3)23–. Compared with Mg/Al-CLDH, the single adsorption capacity reaches 48.6 mg/g. The pseudo-second-order kinetic model is more suitable for describing the adsorption of Au(S2O3)23– by Cu/Fe-CLDH. The adsorption isotherm fitting experiment indicates that the adsorption by Cu/Fe-CLDH of Au(S2O3)23– conforms to the Langmuir model. During the process of Cu/Fe-CLDH adsorbing Au(S2O3)23–, CLDH restored part of the layered structure, and Au(S2O3)23– was inserted between the layers as a counteranion. Furthermore, the reduction of Au(S2O3)23– by low-valent iron compounds in the adsorbent promoted the process of Cu/Fe-CLDH adsorbing Au(S2O3)23–.

Hydration and microstructure of calcined hydrotalcite activated high-volume fly ash cementitious composite

This work aims to propose a novel method to treat the defects of delayed hydration and low early-age strength of cementitious composites blended with high-volume fly ash (HVFA). The influence of calcined layered double hydroxide (CLDH) on mechanical properties, hydration process and microstructure of HVFA cementitious composites have been explored. Assessed properties of HVFA cementious composites incorporated various contents of CLDH included compressive strength, flowability, pore size distribution and the alkalinity of pore solution. The hydration process and microstructure were detected by XRD, TGA, Barrett-Joyner-Halenda Analysis (BJH), and SEM-EDS. This work revealed the promising potential of using CLDH as the hardening accelerator for HVFA composites. Results indicated that 0.5–2 wt% CLDH addition can play an important role in early-age strength increment and the HVFA mixture with 0.5 % CLDH addition performed better than other contents. In addition, microstructural analyses demonstrated an acceleration in the hydration of HVFA mortars after CLDH added, thereby higher contents of hydrated products can be observed. Meanwhile, less than 1 % CLDH addition is shown to change the morphology and composition of C-(A)-S-H to be slenderer and with higher Ca/Si ratio. Increasing the pore alkalinity by hydrolysis process and the seeding effect are the main enhancement mechanisms of CLDH.

Remediation of chromium-contaminated soil using calcined layered double hydroxides containing different divalent metals: Temperatures and mechanism

MgAl and CaAl were calcined at different temperatures and applied to the immobilization of Cr(VI) in the soil. The maximum adsorption capacity of Mg-500 °C and Ca-900 °C was 13.89 mg/g and 33.78 mg/g, respectively. TCLP and phytotoxicity experiments showed that Mg-500 °C and Ca-900 °C could immobilize Cr(VI) stably in the soil, prevent the biological transfer of Cr(VI) from soil to plants. XRD, SEM, FTIR, and other characterizations were utilized to analyze the sorption mechanism. The adsorption mechansim of MgAl and Mg-300 °C are mainly interlayer anion exchange and surface adsorption. But for the Mg-500 °C, memory effect is predominant. Since the main structure of Mg-900 °C is mainly spinel. After long-term adsorption, only a small part returns to the original layered structure of LDHs. So its adsorption performance is poor. Immobilization of Cr(VI) by CaAl is mainly through interlayer anion exchange and surface adsorption. After calcination, the main structure of Ca-300 °C is Ca4Al2O3(OH)6·(NO2)2, which can regeneration into LDHs during adsorption. But part of it dissolved and combined with CO32– to form CaCO3 leading to a poor performance. Main form of Ca-500 °C is CaO and Ca(OH)2, which dissolves to produce Ca2+ during adsorption and mainly produces CaCO3 after reaction, so the adsorption of Cr (VI) was also poor. Differently from others, two stages were included in the immobilization of by Ca-900 °C. In the first, Cr(VI) were adsorbed on the alumina gel resulted from the hydration of calcined products. In this stage, Cr(VI) can be attracted as an interlayer anion into the new forming LDHs in the following stage and then immobilized into the interlayer of LDHs.

Chloride binding by calcined layered double hydroxides and alumina-rich cementitious materials in mortar mixed with seawater and sea sand

Seawater and sea sand can alleviate the shortage of resources and reduce the transportation distance of raw materials at the construction of offshore islands, whereas the high content of chloride ions in concrete mixed with seawater and sea sand induces reinforcement corrosion, which is the biggest obstacle to its wide application. This study aims to explore the way to reduce the free chloride ions content and improve the chloride binding capacity of mortar mixed with seawater and sea sand by calcined layered double hydroxide (CLDH) and alumina-rich cementitious materials (metakaolin (MK), fly ash (FA), and calcium aluminate cement (CAC)). The results show that both CLDH, MK, FA, and CAC can effectively improve the chloride binding capacity of mortar by 65%, 65%, 35%, and 200%, when the optimal dosages are 7.5%, 30%, 30%, and 100%, respectively. Moreover, molecular dynamics simulation shows that the radius correlation function (RDF) value of Cl-Ca ionic pairs is cut down at the condition that has CLDH, which reveals that the adsorption of chloride ions by CLDH occurs in the mortar is confirmed.

Mixed Metal Oxide by Calcination of Layered Double Hydroxide: Parameters Affecting Specific Surface Area.

Mixed metal oxide (MMO) is one of the widely utilized ceramic materials in various industries. In order to obtain high performance, the specific surface area of MMO should be controlled. Calcination of layered double hydroxide (LDH) is a versatile way to prepare MMO with homogeneous metal distribution and well-developed porosity. Although researchers found that the specific surface area of LDH-originated MMO was relatively high, it had not been systematically investigated how the surface area is controlled under a certain parameter. In this review, we summarized LDH-originated MMO with various starting composition, calcination temperature, and pore developing agent in terms of specific surface area and porosity. Briefly, it was represented that MMOs with Mg-Al components generally had higher specific surface area than Mg-Fe or Zn-Al components. Calcination temperature in the range 300–600 °C resulted in the high specific surface area, while upper or lower temperature reduced the values. Pore developing agent did not result in dramatic increase in MMO; however, the pore size distribution became narrower in the presence of pore developing agents.

Effective utilization of calcined MgAl-layered double hydroxides for adsorption of gold (I) thiosulfate complexes

Layered double hydroxides (LDHs) have been widely used to remove anions from water owing to their ability to exchange interlayer anions. In this study, LDHs synthesized by coprecipitation method were used as raw materials to produce calcined layered double hydroxides (CLDH), in order to investigate the adsorption of gold (I) complexes from a thiosulfate solution. The effects of solution pH, adsorption temperature, initial gold concentration, and thiosulfate concentration on the adsorption of Au(S2O3)23- by the CLDH were systematically studied. The results showed that the adsorption of Au(S2O3)23- by the CLDH is a very fast endothermic process, and that the adsorption equilibrium can be reached in 2 h. More than 85% of Au(S2O3)23- could be recovered over a wide gold concentration range. Increasing the initial gold concentration resulted in faster and more efficient CLDH adsorption of Au(S2O3)23-, the cumulative adsorption capacity of activated carbon for gold was 11.8 mg/g. The adsorption followed pseudo-second-order kinetics and was well described by the Freundlich model. The adsorption mechanism involves the CLDH adsorbing Au(S2O3)23- species in an anionic solution based on the memory effect, which restores its original layered structure. The Au(S2O3)23- loaded CLDH (CLDH-Au) could be recycled by elution with CO32-. The present results reveal that the recovery of gold ions from a thiosulfate leaching solution using CLDH has good application prospects in gold hydrometallurgy.

China coal-to-chems margins pressured as crude plunges

The structure, porosity, and adsorption properties of mixed metal oxides (MMO) were investigated according to the physical property of the starting materials. Layered double hydroxides (LDH), the pristine MMO, were prepared to have the same composition of Mg2Al(OH)6(CO3)0.5, but with different particle size. The prepared LDH were calcined at 400 °C to obtain MMO named CMO, HMO, and UMO corresponding to the pristine MMO synthesized by coprecipitation, hydrothermal treatment, and urea hydrolysis method, respectively. The particle sizes of CMO, HMO, and UMO were comparable to those of the pristine MMO in the order UMO > HMO > CMO; on the other hand, the crystallite sizes were almost the same as each other, regardless of the pristine MMO. According to the high-resolution transmission electron microscopy and X-ray diffraction, we could suggest that a single particle of MMO was composed of smaller domains of metal oxide crystallite. Consequently, small MMO particle tended to have interparticulate macropore, while large MMO particle had more possibility of containing intraparticulate mesopores, which was cross-confirmed by the N2 adsorption–desorption isotherms. Toluene and water vapor adsorption tests on three different MMO showed that the intraparticulate mesopores were advantageous to adsorb a variety of adsorbate. It was therefore concluded that MMO obtained by calcination of LDH would have high adsorption property when the pristine LDH has large particle dimension, thanks to the large portions of intraparticulate mesopores.

Removal of arsenic in groundwater using Slag based calcined layered double hydroxides (CLDHs) with dual functions of adsorption and photo-catalysis

We employed Ti-bearing blast furnace slag (Ti-BFS) as raw material to prepare calcined TiO2-loaded layered double hydroxides (TiO2-CLDHs) through acidolysis-alkali precipitation coupled with pyrolysis strategy. During the pyrolysis process, the involvement of TiO2 and LDHs made the TiO2-CLDHs composites have superior photocatalytic ability and abundant porosities, resulting in highly-efficient removal of arsenic (As) from groundwater. Meanwhile, OH played crucial role in the As(III) oxidation process through the results of Electron-spin resonance spectroscopy (ESR) and X-ray photoelectron spectroscopy (XPS) measurements. In addition, the co-existing of humic acid in groundwater was found to compete for OH and adsorption sites of TiO2-CLDHs, leading to a decrease in As removal. This study not only provided a novel strategy for Ti-BFS disposal, but also evaluated the application of TiO2-CLDHs in the treatment of As contaminants in groundwater environment.