Short communicationAdsorption of SeO42− by delaminated Mg-Al layered double hydroxide nanosheets
Graphical abstract
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.
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