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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]

This study undertook the testing of Cu2+ and Ni2+ adsorption using synthesized oxalic acid-crosslinked chitosan (CTS)-modified montmorillonite (MMT), as well as a kinetic analysis and equilibrium analysis. We believe that our study makes a significant contribution to the literature because we showed that the interlayer distance of untreated MMT was increased from 1.23 to 2.05–2.21 nm, and thus verified the intercalation of CTS into the MMT interlayer. The maximum adsorption of Cu2+, as calculated using the Langmuir equation, was significantly higher for oxalic acid-crosslinked CTS-modified MMT, relative to untreated MMT or the non-crosslinked CTS-modified-MMT. The oxalic acid-crosslinked CTS-modified MMT could also take up more Cu2+ than Ni2+ from a solution containing a mixture of Cu2+-Ni2+. The selective adsorption of oxalic acid-crosslinked CTS-modified-MMT (i.e. Ni2+ < Cu2+) is governed by the selective adsorption of CTS and the oxalic acid. This reveals the effect of the complex formation with CTS and oxalic acid. [ABSTRACT FROM AUTHOR]

Owing to the low atmospheric concentration of CO 2 (400 ppm), direct air capture technologies require adsorbents that offer high CO 2 adsorption capacities at low CO 2 concentrations. In this study, we investigated the potential of CO 3 ·Mg–Al layered double hydroxides (LDHs; Mg/Al molar ratio = 2, 3, and 4) as CO 2 adsorbents. The CO 2 adsorption and desorption behavior was studied at various molar ratios, adsorption/desorption temperatures, and linear speeds. CO 2 adsorption by CO 3 ·Mg–Al LDH followed the Langmuir model, indicating that monolayer adsorption occurs. The maximum CO 2 adsorption quantity calculated using the Langmuir equation was 0.794 mmol/g for CO 3 ·Mg–Al LDH with a Mg/Al molar ratio of 3. In dry CO 2 adsorption experiments, a maximum CO 2 adsorption quantity of 0.319 mmol/g was recorded with a Mg/Al molar ratio of 3 and temperature of 60 °C. In dry CO 2 desorption experiments, the CO 2 concentration and desorption quantity reached maximum values at 300 °C and a linear speed of 0.250 m/min. Notably, the maximum CO 2 concentration was 6232 ppm, showing a 15.6-fold increase in comparison with the initial concentration of 400 ppm. Overall, our results demonstrate that CO 3 ·Mg–Al LDH has high potential as a CO 2 adsorbent. The study findings have significant implications for the development of new CO 2 adsorbents for the direct air capture and storage of CO 2. • We have developed new CO 2 adsorbents for the direct air capture and storage of CO 2. • Our results demonstrate that CO 3 ·Mg–Al LDH has high potential as a CO 2 adsorbent. • CO 2 adsorption followed the Langmuir model, indicating that monolayer adsorption occurs. [ABSTRACT FROM AUTHOR]

The removal of toxic anions from wastewater by layered double oxides (LDOs) is highly efficient and studies related to the synthesis of LDOs have been actively attempted. This paper reports the synthesis of a triatomic LDO consisting of Mg, Fe, and Al by a novel sol-gel synthesis process using organic chains (previously developed by our group). This adsorbent exhibited an excellent adsorption performance for the removal of arsenate anions with a maximum adsorption capacity calculated by the Langmuir equation of 304.6 mg/g and an absorbance density of 1.33 mg/m2. The Mg–Fe–Al LDO synthesized via the sol-gel method adsorbed arsenate anions better than an Mg–Al LDO prepared by the same method. Furthermore, a highly effective process for the desorption of arsenate from the adsorbent was investigated. The results revealed that the maximum arsenate anion desorption was achieved via an anion exchange with an NaHCO 3 solution. Namely, we have succeeded in synthesizing a triatomic LDO of Mg, Fe, and Al by the sol-gel method that we established, and the adsorbent has excellent arsenate anion adsorption performance. [Display omitted] • The synthesis of a triatomic LDO consisting Mg, Fe and Al by this sol-gel process. • 1.7 times increase in specific surface area compared to Mg–Al LDO (coprecipitation). • A maximum adsorption capacity and an absorbance density of Mg–Fe–Al LDO (sol-gel) are the highest compared to both Mg–Al LDO (coprecipitation) and Mg–Al LDO (sol-gel). • A highly effective process for desorption of arsenate anion has been established. [ABSTRACT FROM AUTHOR]

Layered double oxides (LDO) are effective in removing toxic anions from aqueous solutions; however, related synthesis studies have not been thoroughly presented. In this study, we successfully established a new method for the sol-gel synthesis of LDO. This new synthesis method, in which organic chains are added in the sol-gel synthesis process, is simple and fast and improves the adsorption capacity. The specific surface area of the LDO synthesized by the sol-gel method was approximately 2.6 times larger than that of the LDO synthesized by the coprecipitation method. In addition, LDO synthesized by adding organic chains in the sol-gel synthesis process showed further improvement in specific surface area. A specific surface area value approximately 2.7 times higher than that of LDO synthesized by the coprecipitation method was obtained. The maximum adsorption amount calculated from the Langmuir equation for the arsenic adsorption on the LDO synthesized by the sol-gel method was 282.6 mg/g. It was confirmed that the adsorption capacity of LDO increased with an increase in the specific surface area, and LDO with an increased adsorption capacity were successfully synthesized using this method. [Display omitted] • Increase in specific surface area by this sol-gel process. • 2.7 times increase in specific surface area compared to Mg-Al LDO (coprecipitation). • Removal of arsenate anion from water by Mg-Al LDO (sol-gel). • Approximately 2 times increase in adsorption amount compared to Mg-Al LDO (coprecipitation). [ABSTRACT FROM AUTHOR]

The adsorption of Cu2+ and Ni2+ on synthesized tripolyphosphate-crosslinked-chitosan-modified montmorillonite (TPP-crosslinked-CTS-modified MMT) was examined by kinetic and equilibrium studies. The adsorption followed a pseudo-second order kinetics. Based on the activation energies, chemisorption due to electron transfer between TPP-crosslinked-CTS-modified MMT and the Cu2+ and Ni2+ cations was concluded to be the rate-determining step. Equilibrium analysis for the adsorption of each cation shows a good fit to the Langmuir equation. The adsorption of metal cations onto TPP-crosslinked-CTS-modified MMT proceeds because of the cation adsorption capacity of MMT, complexation with CTS, and complexation with the crosslinking agent (TPP). The selectivity toward Cu2+ was high because of the selective adsorption of this cation by CTS. Comparison of maximum adsorption amount of heavy metal ions by both adsorbents. Image 1 • Montmorillonite modified by crosslinked tripolyphosphate was prepared. • Kinetics and equilibrium of the adsorption of Cu2+ and Ni2 ions have been analyzed. • We were able to achieve selective adsorption of Cu2+ over Ni2+. [ABSTRACT FROM AUTHOR]

The adsorption of Cu2+ and Ni2+ by chitosan-modified montmorillonite (CTS-modified MMT) was investigated. Kinetic analyses suggested that the adsorption of Cu2+ and Ni2+ by MMT, CTS, and CTS-modified MMT can be modeled using a pseudo-second-order kinetic equation. The adsorption isotherms clearly indicate that the adsorption follows the Langmuir equation. The adsorption of metal cations by CTS-modified MMT is thought to be attributable to the cation adsorption capacity of MMT and the ability of CTS to form complexes with metal ions. Desorption and re-adsorption experiments were also conducted to determine the possibility of reusing the synthesized adsorbent. Using a 100-mmol/L HNO 3 solution, a desorption rate of approximately 90% was achieved for the CTS-modified MMT. The CTS-modified MMT obtained after desorption could achieve the same level of adsorption as fresh the CTS-modified MMT, indicating that this adsorbent can be reused. Image 1 • Cu2+ and Ni2+ adsorption by CTS-modified MMT was investigated. • Adsorption follows a pseudo-second-order kinetic equation. • Adsorption isotherms show that adsorption follows the Langmuir equation. • Adsorption is attributable to MMT cation adsorption and CTS interaction with ions. • Reused CTS-modified MMT achieves a desorption rate of approximately 90%. [ABSTRACT FROM AUTHOR]