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Recently, layered double hydroxides (LDHs) have been extensively researched for their anion exchange capabilities and exhibit a layered structure with a host layer of octahedrally coordinated metal cations and a guest layer (interlayer) of anions. In this study, we synthesized a CM-β-CD•Zn–Al LDH by anion exchange method to study the anionization of cyclodextrin (CD) to promote its incorporation into the LDH interlayers and its stability therein. Ni2+ adsorption by the CMCD•Zn–Al LDH by the anion exchange method was compared with the results co-precipitation method. The X-ray diffraction spectra of NO3•Zn–Al LDH and CM-β-CD•Zn–Al LDH synthesized by the anion exchange method exhibit that the product peaks were attributable to Z n 6 A l 2 O H 16 C O 3 4 H 2 O and possessed an LDH structure. Furthermore, all CM-β-CD•Zn–Al LDHs exhibited the presence of some CM-β-CD. The CM-β-CD•Zn–Al LDH was hypothesized to be composed of two types of layers: a CM-β-CD layer in which two molecules of CM-β-CD7-, with cavity axes perpendicular to the LDH host layer, are vertically overlaid, and a NO3-type Zn–Al LDH layer used as a precursor. CM-β-CD•Zn–Al LDH adsorbed Ni2+ in an aqueous solution. To determine whether Ni2+ adsorption by the CM-β-CD•Zn–Al LDH is driven chemically or physically, we fitted the data to the Freundlich and Langmuir adsorption models; the results showed that Ni2+ adsorption by the CM-β-CD•Zn–Al LDH is consistent with the Langmuir equation and could be considered a chemical adsorption. [ABSTRACT FROM AUTHOR]

Nitrate-intercalated Mg Al layered double hydroxide (LDH) was successfully delaminated in water by a facile and effective method upon reflux at 120 °C for 24 h followed by sonication at 40 °C for 5 h. This process is environmentally friendly since water is the only solvent used. The delaminated nanosheets were characterized by microscopic, spectroscopic, and particle size analyses. The delamination process successfully produced octahedron-shaped single-layer nanosheets 50–150 nm in size. X-ray photoelectron spectroscopy (XPS) data confirmed that the surface elements and their chemical status are consistent with the basic layer of Mg Al LDH. The delaminated nanosheets displayed higher adsorption capacity for removing heavy metals from waste water than the original powdered LDH. After treating the waste water, a sharp and intense peak in the X-ray powder diffraction (XRD) pattern of the precipitate confirms the restacking of the LDH nanosheets. [ABSTRACT FROM AUTHOR]

A selective catalytic reduction process is generally used to treat NOx; however, this approach results in the leakage of ammonia and requires expensive catalysts. In light of these issues, the specific objective of this study was to develop a new treatment method for a NOx gas stream with a Mg−Al oxide slurry. In this study, we examined the ability of a Mg−Al oxide slurry to treat NO and NO2 gas streams. The Mg−Al oxide slurry only partially removed the NO and NO2. The ability of Mg−Al oxide to remove NO is based on the adsorption of NO molecules onto the surface of the Mg−Al oxide rather than the formation of an Mg−Al layered double hydroxide (Mg−Al LDH) intercalated with NO– 2. NO2 removal was greater than NO removal because of the higher solubility of NO2. For NO2 removal, the Mg−Al oxide adsorbed NO– 2 and NO– 3 onto the surface and formed Mg−Al LDH intercalated with NO– 2 and NO– 3. Based on these results, we conclude that the Mg−Al oxide slurry is an effective treatment for NOx. [ABSTRACT FROM AUTHOR]

• Simultaneous removal of HCl, SO 2 , and NO from the mixed gas was performed using MnO 2 /Mg–Al LDH. • Regenerated MnO 2 /Mg–Al LDH could remove HCl, SO 2 , and NO gases. • We efficiently removed the gases from the mixed gas by applying the two-step treatment process. Combustion methods are popularly used to treat acidic exhaust gases to reduce the emission of pollutants. However, exhaust gas treatment can be difficult and expensive, rendering it essential to develop a new treatment process for acidic exhaust gases. In the present study, we investigated the removal of NO from the HCl–SO 2 –NO mixed gas using MnO 2 /Mg–Al layered double hydroxide (MnO 2 /Mg–Al LDH). We analyzed the influence of CO 2 gas on the NO removal. Additionally, we regenerated MnO 2 /Mg–Al LDH after the acid gas adsorption using the wet method and examined the possibility of its cyclical use. MnO 2 /Mg–Al LDH could simultaneously remove HCl, SO 2 , and NO from the mixed gas. Because HCl and SO 2 influenced the NO removal rate, we proposed a two-step treatment process with MnO 2 /Mg–Al LDH and CO 3 •Mg–Al LDH to efficiently remove HCl, SO 2 , and NO from the HCl–SO 2 –NO mixed gas. The NO removal rate decreased with an increase in the CO 2 concentration, confirming the influence of CO 2. Additionally, the results confirmed the possible cyclic use of MnO 2 /Mg–Al LDH after removing acid gases. However, the Mn elution and Mn(IV) concentration decreased the removal rates of SO 2 and NO, respectively. Thus, it is necessary to develop the regeneration method further. [ABSTRACT FROM AUTHOR]

[Display omitted] • The adsorption follows a pseudo-second-order kinetic equation. • The Δ G values for adsorption systems ranged from − 20 to 0 kJ/mol. • It indicates that adsorption could be classified as physical adsorption. • NP was adsorbed on OS•Cu-Al LDH by hydrophobic interaction. • AP was adsorbed on SCD•Cu-Al LDH by CH/π interactions. Our previous study revealed that Cu-Al layered double hydroxides (LDHs) intercalated with 1-octanesulfonate and sulfonated β-cyclodextrin showed good adsorption selectivity toward m -nitrophenol (NP) and m -aminophenol (AP), respectively. We also found that the adsorption behaviors of these LDHs fit the Freundlich equation. Herein, we performed a kinetic analysis to elucidate the adsorption mechanism of the LDHs for phenols in greater detail. The adsorption results were fitted to a pseudo-first-order kinetic equation, a pseudo-second-order kinetic equation, and an intraparticle diffusion model. The correlation coefficients of the pseudo-second-order kinetic equation for each adsorption process were fairly high, and the measured and calculated equilibrium adsorption amounts were relatively close. Thus, the adsorption of NP and AP by OS•Cu-Al LDH and SCD•Cu-Al LDH, respectively, fits the pseudo-second-order kinetic equation. Because the ΔG values for each system ranged from − 20 to 0 kJ/mol, the adsorption processes in these systems may be classified as physical adsorption. Specifically, NP was adsorbed on OS•Cu-Al LDH by hydrophobic interactions, whereas AP was adsorbed on SCD•Cu-Al LDH by CH/π interactions. Our findings provide new insights into the adsorption of phenols onto modified Cu-Al LDHs and could serve as a foundation for future studies on the adsorption properties of LDHs intercalated with anions as guest layers. [ABSTRACT FROM AUTHOR]

[Display omitted] • MnO 2 /Mg–Al LDH is able to remove both HCl and SO 2 gases in addition to NO. • The HCl removal proceeds mainly through the reaction of HCl with the CO 3 2− and OH− in the LDH layers. • The SO 2 removal takes place through the adsorption of SO 2 onto the LDH and MnO 2 surfaces. • The NO removal occurs through the oxidation of NO to NO 2 by MnO 2. • It follows by the reaction of NO 2 with the CO 3 2− and OH− in the LDH layers. The gas exhaust generated by waste incinerators contains toxic acid gases such as HCl, SO 2 , and NO x , which require appropriate treatment following their release. Although several methods exist for this purpose, they have various drawbacks. Thus, toward the search for a new removal method, the ability of Mg–Al layered double hydroxide (Mg–Al LDH) to eliminate HCl, SO 2 , and NO gases from simulated exhaust gas was examined. In addition, the impact of the amount of LDH employed was investigated in terms of the toxic gas removal efficiencies and process temperature. Furthermore, the mechanism of removing HCl, SO 2 , and NO by MnO 2 /Mg–Al LDH was considered. More specifically, HCl removal proceeded mainly by the reaction of HCl with the CO 3 2− and OH− species present between the Mg-Al LDH layers, while that of SO 2 took place through the adsorption of SO 2 onto the LDH and MnO 2 surfaces. Moreover, NO was removed through the oxidation of NO to NO 2 by MnO 2 , followed by the reaction of NO 2 with the CO 3 2− and OH− species present between the Mg–Al LDH layers. These results are expected to help pave the way for the use of Mg–Al LDH to remove toxic acid gases from the gas exhausts of waste incinerators, thereby addressing the various issues related to the use of current technologies. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) nanomaterials have drawn widespread attention because of their unique physical characteristics compared to three-dimensional materials. Delamination and nanosheet formation of layered composites are necessary for 2D nanomaterials. Layered double hydroxides (LDH) exhibit anion exchange properties that can be maximized using different methods such as organic modifications and crystal growth control; delamination is one such method. Delamination of LDH render them in the form of nanosheets, which could not be expressed as interlayers, allowing them to exhibit physical properties completely different from those of bulk LDH. Herein, we propose a novel synthesis method to delaminate LDH in an aqueous solvent. In this method, heat reflux is used to break interlayer hydrogen bonds in the LDH, and the remaining hydrogen bonds are broken using ultrasonic treatment. Finally, we dispersed the material in the solvent to obtain LDH nanosheets. We compared the delamination of Mg Al LDH intercalated with lactate (lactate•Mg–Al LDH) and Mg Al LDH intercalated with NO 3 − (NO 3 ·Mg–Al LDH) in an aqueous solvent. Furthermore, we used the nanosheets obtained from NO 3 ·Mg–Al LDH and described their Ni2+ uptake characteristics. Our results show that without using organic solvents, we could delaminate lactate•Mg–Al LDH and NO 3 ·Mg–Al LDH by applying the proposed method. The lactate•Mg–Al LDH had approximately ten Mg Al LDH host layers, whereas the corresponding nanosheets had three Mg Al LDH host layers. NO 3 ·Mg–Al LDH had 14–15 Mg–Al LDH host layers, whereas the corresponding nanosheets had 2–3 Mg–Al LDH host layers. Furthermore, NO 3 ·Mg–Al LDH and its nanosheets could adsorb Ni2+ in an aqueous solvent. Thus, Ni2+ uptake capabilities were enhanced by delaminating NO 3 ·Mg–Al LDH to fabricate nanosheets. • A method for delaminating layered double hydroxides (LDHs) is proposed. • Heat reflux is used to remove hydrogen bonds in LDHs. • Remaining hydrogen bonds are removed by ultrasonic treatment. • We delaminated lactate-intercalated Mg Al LDH and NO3-intercalated Mg Al LDH. • Ni2+ uptake capabilities increased by delaminating NO3-intercalated Mg Al LDH. [ABSTRACT FROM AUTHOR]

This study investigates the influence of CO 2 gas on the treatment of single acidic gases (HCl, SO 2 , and NO 2) using Mg Al layered double hydroxide intercalated with CO 3 2− (CO 3 ·Mg-Al LDH) as the adsorbent. The gas-removal kinetics is analyzed, and the reaction mechanisms are discussed. The coexisting CO 2 gas decreased the removal rate of all three acidic gases. This is likely due to the exchange reaction between intercalated CO 3 2− and gas-phase CO 2 , which inhibits the exchange between intercalated CO 3 2− and HCl, SO 2 , or NO 2. The HCl removal by CO 3 ·Mg-Al LDH follows pseudo first order kinetics, suggesting that the main removal mechanism is chemisorption (chemical reaction between CO 3 ·Mg-Al LDH and HCl). Meanwhile, the removal of SO 2 and NO 2 by CO 3 ·Mg-Al LDH also follows pseudo first order kinetics. However, the corresponding adsorption mechanisms are classified as physisorption onto the Mg Al LDH. • 10% coexisting CO 2 reduced the acidic gases absorption ratio by CO 3 ·Mg-Al LDH. • This is attributed to the exchange reaction between CO 2 gas and pre-intercalated CO 3 2−. • All the kinetic data were well fitted to pseudo-first order models. [ABSTRACT FROM AUTHOR]

Mg − Al layered double hydroxide intercalated with CO 3 2– (CO 3 ·Mg − Al LDH) was successfully used to remove SO 2 from a gas mixture at 140, 170, and 200 °C. The removal of SO 2 with CO 3 ·Mg − Al LDH occurred via surface adsorption of SO 2 and anion exchange between CO 3 2– in the interlayer of Mg − Al LDH and SO 2 from the gas mixture. The removal of SO 2 followed the order of Ca(OH) 2 > CO 3 ·Mg − Al LDH (Mg/Al = 2) > CO 3 ·Mg − Al LDH (Mg/Al = 4). The removal of SO 2 also followed the order of CO 3 ·Mg − Al LDH > Mg(OH) 2 > SO 4 ·Mg − Al LDH. CO 3 ·Mg − Al LDH was regenerated from SO 4 ·Mg-Al LDH using a Na 2 CO 3 solution through anion exchange between SO 4 2− and CO 3 2−, leading to its recyclability for removal of SO 2 from a gas mixture three times, demonstrating the feasibility of recycling the material. Unlabelled Image • This study deals with the development of CO 3 ·Mg − Al LDH for the removal of SO 2. • The SO 2 removal increased with increasing temperature and SO 2 concentration. • The removal proceeds under mass transfer control rather than surface chemical reaction control. [ABSTRACT FROM AUTHOR]

The removal of toxic nitric oxide (NO) from exhaust gas is challenging and expensive. MnO 2 /Mg-Al LDH, a composite of manganese dioxide and Mg-Al layered double hydroxide, was synthesized via the rehydration method for potential application in NO removal. MnO 2 /Mg-Al LDH was found to be complexed with MnO 2 , pillared between the LDH layers and intercalated with CO 3 2− and OH−. We investigated the performance of a MnO 2 /Mg-Al LDH treatment in removing NO from exhaust gas. The exhaust gas was circulated in a reaction tube filled with MnO 2 /Mg-Al LDH, CO 3 ･Mg-Al LDH, or a MnO 2 + CO 3 ･Mg-Al LDH mixture, and the NO removal efficiency of the MnO 2 /Mg-Al LDH composite was compared with that of the other compounds. When NO gas (150 ppm, linear velocity 1.0 m/min) was distributed at 170 °C, treatment with the MnO 2 ･Mg-Al LDH composite and CO 3 ･Mg-Al LDH yielded removal rates of 83.9% and 0%, respectively. The significantly higher NO removal rate of MnO 2 /Mg-Al LDH was ascribed to the transformation of NO to NO 2 via oxidation over MnO 2. Further, the NO removal rate after 90 min through the application of a simple mixture of MnO 2 and CO 3 ･Mg-Al LDH (Mn: 4.2 wt%) was only 1.4%, suggesting that a synergistic effect was generated by the MnO 2 /Mg-Al LDH composite. Though NO removal by MnO 2 /Mg-Al LDH was reduced at temperatures of 140 °C and higher, the results suggest that the synthesized compound can be much more effective than those commonly used for NO removal from exhaust gas. [Display omitted] • One-step treatment for simpler and more effective NO removal is proposed. • MnO 2 /Mg-Al LDH is synthesized via rehydration. • MnO 2 or Mn 2 O 3 is pillared to the host layer and CO 3 2−–OH intercalation occurs. • MnO 2 /Mg-Al LDH shows the highest NO removal rates. • A synergistic effect was generated by the MnO 2 /Mg-Al LDH composite. [ABSTRACT FROM AUTHOR]