Compositional and structural control on anion sorption capability of layered double hydroxides (LDHs)

• Published in: Journal of colloid and interface science 2006-09, Vol.301 (1), p.19-26
• Main Authors: Wang, Yifeng, Gao, Huizhen
• Format: Article
• Language: English
• Subjects: Cage effect; Chemistry; Edge site adsorption; Exact sciences and technology; General and physical chemistry; Layered double hydroxide; Oxyanion; Pertechnetane anion; Structure–property relationship; Surface adsorption; Surface physical chemistry; Waste disposal
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2006.04.061

Layered double hydroxides (LDHs) have shown great promise as anion getters. In this paper, we demonstrate that the sorption capability of a LDH for a specific oxyanion can be greatly increased by appropriately manipulating material composition and structure. We have synthesized a large set of LDH materials with various combinations of metal cations, interlayer anions, and molar ratios of divalent cation M(II) to trivalent cation M(III). The synthesized materials have then been tested systematically for their sorption capabilities for pertechnetate (TcO − 4). It is discovered that for a given interlayer anion (either CO 2− 3 or NO − 3) the Ni–Al LDH with a Ni/Al ratio of 3:1 exhibits the highest sorption capability among all the materials tested. The sorption of TcO − 4 on M(II)–M(III)–CO 3 LDHs may be dominated by the edge sites of LDH layers and correlated with the basal spacing d 003 of the materials, which increases with the decreasing radii of both divalent and trivalent cations. The sorption reaches its maximum when the layer spacing is just large enough for a pertechnetate anion to fit into a cage space among three adjacent octahedra of metal hydroxides at the edge. Furthermore, the sorption is found to increase with the crystallinity of the materials. For a given combination of metal cations and an interlayer anion, the best crystalline LDH material is obtained generally with a M(II)/M(III) ratio of 3:1. Synthesis with readily exchangeable nitrate as an interlayer anion greatly increases the sorption capability of a LDH material for pertechnetate. The work reported here will help to establish a general structure–property relationship for the related layered materials. Pertechnetate sorption on layered double hydroxides increases with basal spacing d 003 . Further expansion of d 003 by replacing carbonate with sulfate results in LDHs with no sorption capabilities, suggesting a possible cage effect on pertechnetate sorption.