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Arsenate adsorption by Mg/Al–NO3 layered double hydroxides with varying the Mg/Al ratio

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Abstract.
Arsenate is one of the main contaminants threatening water supplies all over the world. Layered double
hydroxides (LDHs) have large surface areas and high anion exchange capacities, so they may be used to develop
rapid and cost-effective methods for scavenging As from contaminated water. In order to enhance arsenate
adsorption of layered double hydroxides, it is essential to have a fundamental understanding of the interactions between arsenate and LDHs. In this study, the effects of layer charge density and interlayer nitrate orientation of Mg/Al–NO3 LDHs on the preferential adsorption of arsenate were investigated. Mg/Al–NO3 LDHs with a Mg:Al ratio of 2:1, 3:1 and 4:1 were synthesized, using a constant-pH co-precipitation method. The adsorption maxima of As on 2:1, 3:1 and 4:1 LDHs were 1.56, 1.08 and 0.36 mmol g−1, respectively, following the order of their layer charge densities. In addition, nitrate orientation in the LDHs plays a significant role in determining arsenate adsorption. The low arsenate adsorption of 4:1 LDH relative to its anion exchange capacity resulted from the low degree of access arsenate has to the interlayer space, so that arsenate adsorption largely occurred on the external surface of the material. The accessibility of the interlayer of 4:1 LDH to arsenate was restricted by the small basal spacing as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheets. Conversely, interlayer nitrate with an orientation perpendicular to the hydroxide sheets in 2:1 and 3:1 LDHs can be exchanged for arsenate more readily. Consequently, these two LDHs exhibit greater arsenate adsorption capacity, because arsenate can be adsorbed on both their external and interlayer surfaces. The arsenate adsorption of the LDHs alsowas highly dependent upon the type and concentration of competing anions. The negative effect of the competing anions on arsenate adsorption by various LDHs generally followed the order H2PO4 − >HCO3 − >SO42−>F−>Cl−. Due to the difference in adsorptionmechanism, the arsenate adsorption of 4:1 LDH is subject to the negative impact of competing anions to a greater extent than those of 3:1 and 2:1 LDH. These results demonstrated that the preferential adsorption of arsenate by LDHs is dependent upon the nitrate orientation in these materials, as a consequence of changing layer charge density. The selective retention of arsenate for wastewater treatment can be maximized by controlling the orientation of interlayer nitrate in Mg/Al–NO3 LDHs.

Keywords: Arsenate, Adsorption, Layered double hydroxide, Layer charge density

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Informasi Detail

Judul Seri

Applied Clay Science

No. Panggil

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Penerbit

London : Elsevier B.V.., 2009

Deskripsi Fisik

Applied Clay Science 43 (2009) 79–85

Bahasa

English

ISBN/ISSN

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Klasifikasi

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Tipe Isi

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Tipe Media

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Tipe Pembawa

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Edisi

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Subjek

KIMIA

Info Detail Spesifik

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Pernyataan Tanggungjawab

agus

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