Short communication
Adsorption of SeO42− by delaminated Mg-Al layered double hydroxide nanosheets

https://doi.org/10.1016/j.inoche.2020.108266Get rights and content

Highlights

  • The NO3⋅Mg-Al LDH and the nanosheets could adsorb the SeO42− in an aqueous solution.

  • The adsorption capacity was greater for the delaminated NO3⋅Mg-Al LDH nanosheets.

  • This is because the specific surface area of the NO3⋅Mg-Al LDH nanosheets is larger.

  • The nanosheets are layered after the anion exchange on the surface of the host layer.

Abstract

Layered double hydroxides (LDHs) are excellent anion-exchange materials, and are useful in adsorbing hazardous anions. The LDHs form nanosheets upon delamination, which improves their efficiency. In this study, we investigated the SeO42− adsorption properties of the nanosheets obtained by the delamination of NO3⋅Mg-Al LDH in an aqueous solution and compared them with those of the NO3⋅Mg-Al LDH. The SeO42− adsorption by both substances was a “pseudo-second-order rate reaction”. The reaction-rate constant during the early stage was greater for the nanosheets as their surface area was larger than the NO3⋅Mg-Al LDH. The amorphous nanosheets reverted to an ordered LDH structure after the anion exchange. Atomic force microscopy (AFM) imaging and the corresponding height profile analysis of the samples confirmed the nanosheets stacking post SeO42− adsorption. We propose a mechanism of the SeO42− adsorption by the NO3⋅Mg-Al LDH nanosheets. These findings may improve the understanding of the anion exchange by the LDHs, which may lead to the development of high-efficiency materials that extract metals such as Se from wastewater.

Introduction

Layered double hydroxides (LDHs) have an alternating layered structure consisting of a host and a guest layer. The host layer is a positively charged octahedral layer formed by the substitution of some of the divalent metal cations (M2+) of a metal hydroxide (M(OH)2) with trivalent metal cations (M3+). The guest layer contains anions, which compensate for the positive charge, and interlayer water [1], [2]. The Mg-Al layered double hydroxides with Mg2+ (M2+) and Al3+ (M3+) are expressed as [Mg2+1−xAl3+x(OH)2]x+[(An−)x/n mH2O]x−, where An− is an n-valent anion and 0.20 ≤ x ≤ 0.33. The layered compounds usually have a cation-exchange capacity; however, LDHs have an anion-exchange capacity because the host layer is positively charged [3]. The larger the charge density of anions, the more easily they are intercalated between the layers. In addition, the layer gap changes depending on the size of the captured anions [3]. LDHs have been widely studied as adsorbents of hazardous anions in aqueous solutions because of their anion-exchange capacity [4], [5], [6], [7], [8]. An efficient use of the layers in the LDHs is expected to provide them high functionality. This can be achieved by delaminating the LDHs to form nanosheets. However, in LDHs, the host- and the guest layers are strongly attracted to each other due to electrostatic interaction; hence, it is difficult to achieve the delamination. At present, the most common method of delamination is to swell the LDH layers using an organic solvent followed by ultrasonic treatment [9], [10], [11], [12]. However, when these nanosheets are used as an adsorbent, the organic solvent can mix with the aqueous medium. Further, the nanosheets can remain in the delaminated form only when dispersed in a solvent. Thus, if the organic solvent is removed by heating or other operations, the nanosheets may condense and re-laminate at high concentrations. Therefore, when using as an adsorbent, it is important to obtain the nanosheets in an aqueous medium. We have developed a new synthetic method for the delamination of LDHs in an aqueous solution [13]. In this method, the LDH is heated under reflux conditions to break the hydrogen bonds between its layers. Then, the ultrasonic treatment is performed to break the hydrogen bonds that were not broken by the thermal treatment. In this study, we investigated the anion (SeO42−) adsorption properties of the nanosheets obtained by the delamination of NO3⋅Mg-Al LDH in an aqueous solution.

Section snippets

Experimental

Delamination of the NO3⋅Mg-Al LDH was carried out in an aqueous solution according to the method described in the previous study [13]. In a three-necked flask, 10.0 g of NO3⋅Mg-Al LDH (Mg/Al molar ratio of 3.3) was added to 100 mL of N2-purged ion-exchanged water. The three-necked flask was placed in an oil bath and its contents were refluxed at 120 °C for 24 h. Then, the contents of the flask were sonicated at 43 kHz and 40 °C for 5 h. The resulting suspension was centrifuged at 3000 rpm for

Results and discussion

Fig. 1 shows the changes in the adsorbate (SeO42−) concentration in the NO3⋅Mg-Al LDH and the NO3⋅Mg-Al LDH nanosheets over time (ICP-AES analysis). The amount of SeO42− adsorbed by both substances increased rapidly. The adsorbate amount leveled off as the time progressed. The NO3⋅Mg-Al LDH and NO3⋅Mg-Al LDH nanosheets were both able to adsorb SeO42− in the aqueous solution. In the early stage of the reaction, the NO3⋅Mg-Al LDH nanosheets adsorbed SeO42− faster than the NO3⋅Mg-Al LDH. However,

Conclusions

The NO3⋅Mg-Al LDH and the NO3⋅Mg-Al LDH nanosheets could adsorb the SeO42− in an aqueous solution. The adsorption of SeO42− by both substances was in agreement with the “pseudo-second-order rate equation”. The reaction-rate constant, and therefore the SeO42− adsorption capacity, were greater for the delaminated NO3⋅Mg-Al LDH nanosheets than that for the NO3⋅Mg-Al LDH during the early stage of the reaction. This is because the specific surface area of the NO3⋅Mg-Al LDH nanosheets is larger than

CRediT authorship contribution statement

Tomohito Kameda: Conceptualization, Supervision, Writing - original draft, Writing - review & editing. Daichi Ikeda: Investigation. Shogo Kumagai: Resources, Visualization. Yuko Saito: Investigation. Toshiaki Yoshioka: Supervision, Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Cited by (9)

  • Facile synthesis of Ni-based layered double hydroxides with superior photocatalytic performance for tetracycline antibiotic degradation

    2022, Journal of Solid State Chemistry
    Citation Excerpt :

    The uniform distribution of metals in the LDH layers result in a fine dispersion of the active ingredients and intimate contact at the atomic level [19–21]. Moreover, the characteristics of nonporous material design, ease of scale-up, oxo-bridged linkage, and tunability make LDH a challenging adsorbing material to eliminate organic or inorganic pollutants in water [22–24]. Recently, LDHs have been received great attention and widely studied as adsorbents of hazardous anions in aqueous solutions because of their anion-exchange capacity [25–28].

  • Synthesis and microstructural analysis of calcined double hydroxide material and its performance for alkaline-decline resistance in concrete pores and chloride adsorption from simulated concrete pore solution

    2022, Chemical Physics Letters
    Citation Excerpt :

    Double hydroxides (DH) of transition metals are a well-known class of materials that have been evaluated as adsorbents to enhance the adsorption capacity or the cost and are being intensively investigated because of their high anion capacity, reusability, and easy interactions with the adsorbates [6–8]. DH materials have been recently reported as effective and significant adsorbents or determination of different ions [9]; Cr(VI) [10,11], SeO42− [12], fluoride [13], iodide [14], or molecules such as Congo red [15]. Additionally, it played a role as anticorrosive material [16] as these DH materials have two significant roles; (i) providing the intercalated inhibitors and (ii) entrapment of aggressive ions such as chloride ions [17,18].

  • Regeneration mechanism, modification strategy, and environment application of layered double hydroxides: Insights based on memory effect

    2022, Coordination Chemistry Reviews
    Citation Excerpt :

    Usually, delamination happens in the ultrasonic oscillation and thermal treatment in a specific solvent [27,53,57], LDHs will be eventually peeled into monolayer material, which shows the characteristics of colloid solution [53]. Kameda et al. delaminated MgAl-LDH in deionized water to form an LDH nanosheet successfully [55]. Delamination-reconstruction could complete the loading of the active sites.

View all citing articles on Scopus
View full text