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

Abstract

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 LDHs is dominated by the edge sites of LDH layers and strongly correlated with the basal spacing doo3 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 between two neighboring 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, a best crystalline LDH material is obtained generally with a M(II)/M(III) ratio of 3 : 1. Replacement of interlayer carbonate with readily exchangeable nitrate greatly increases the sorption capability of a LDH material for pertechnetate, due to the enhanced adsorption on edge sites and the possible contribution from interlayer anion exchanges. The work reported here will help to establish a general structure-property relationship for the related layered materials.

Hydrotalcite—A potentially significant sorbent of organic matter in calcareous alkaline soils

“…Since variable charged groups of layered minerals are mainly located at the surface, we assume the sorption to take place mostly at the external surface sites of the hydrotalcites, by ligand exchange with surface hydroxyl groups and/or by electrostatic attraction. Our results are in line with observations of Wang and Gao (2006) and Inacio et al (2001) who found a decrease in anion sorption with increasing pH. However, experiments with HT20 at pH 9 did not reveal weaker OC sorption than at lower pH values.…”

“…Therefore we assume the more nitrate the larger the capacity of hydrotalcite to bind anionic compounds via exchange reactions, hence the larger the sorption capacity for aromatic acids. This would be in agreement with results of Wang and Gao (2006) Easier displacement of nitrate compared to carbonate has also been observed for organic anions such as 4-chloro-2-methylphenoxyacetic acid by Inacio et al (2001). However, since in HT100 the structural charge is entirely balanced by interlayer carbonate, which is supposed to be the most favoured interlayer anion of hydrotalcites and thus hardly exchangeable, sorption of organic anions by displacement of carbonate at the entrance of the interlayer space seems rather unlikely.…”

Phosphate adsorption from sewage sludge filtrate using zinc–aluminum layered double hydroxides

“…2). Optimal results for other metal combinations have been reported elsewhere: Mg/Al of 2:1 [19], Mg/Mn of 3:1 [21] and Ni/Al of 3:1 [24]. Phosphate adsorption activity of LDHs compounds results from the positively charged hydroxylated sheet, which is developed by partial substitution of trivalent for divalent cations.…”

Technetium‐99 MAS NMR Spectroscopy of a Cationic Framework Material that Traps TcO4− Ions

“…The TcO 4 À ion is also difficult to incorporate into vitrified waste and easily leaches away. [1] These issues have created a need for preparing new materials that are capable of sequestering the TcO 4 À ion. One of these materials is the compound NDTB-1, [(ThB 5 O 6 (OH) 6 )-(BO(OH) 2 )]·2.5 H 2 O, the extraframework borate ions of which are readily exchangeable with the TcO 4 À ion.…”