The layered double hydroxides (LDHs) of Mg with Al readily scavenge dissolved chromate ions under a wide range of conditions. While the chromate uptake is quantitative in the LDH containing 33 mol% Al, the uptake is only 58% of the stoichiometric value in the LDH containing 25 mol% Al. This indicates that the lower symmetry of the NO 3 À ions in the LDH with 33 mol% Al facilitates the intercalation of chromate ions even under conditions of equilibration with excess dissolved nitrate ions. The chromate uptake obeys the Langmuir adsorption isotherm suggesting that the entire interlayer region of the LDH behaves like a surface. This surface is structural rather than morphological as the chromate uptake correlates negatively with the BET surface area of the LDHs.

The LDH of Ni with Fe, having the formula Ni(1-x)Fe(x)(OH)2(A(n-))(x/n)yH2O (A = NO3-, Cl-; x = 0.25, 0.33), scavenges CrO4(2-) ions from solution throughout the concentration range examined (0.00625-0.25 N). The CrO4(2-) uptake capacity is independent of the anion in the starting LDH but is higher when x = 0.25 (3.60 meq g(-1)) as compared to x = 0.33 (2.40 meq g(-1)). These values are higher than those observed for control compounds beta-Ni(OH)2 (1.86 meq g(-1)) and FeO(OH) (1.26 meq g(-1)), which do not have any interlayer chemistry, showing that chromate uptake takes place by its incorporation in the interlayer region by a stoichiometric anion-exchange reaction, rather than by adsorption. Nevertheless, the interaction between the LDH and the chromate ions is weak. The weak interaction is due to the mismatch between the symmetry of the chromate ions and the symmetry of the interlayer site, which introduces turbostratic disorder in the chromate-intercalated LDHs. The chromate ions can be completely leached out by soaking the LDH in a sodium carbonate solution.

The NO3 −- and Cl−-containing layered double hydroxides (LDHs) of Mg with Al interact weakly with anions in solution under equilibrated conditions and yield a type V isotherm. The uptake of anions under nonequilibrated conditions varies monotonically with the layer charge when the outgoing anion is NO3 −, whereas it goes through a maximum when the outgoing anion is Cl−. These observations suggest that Coulombic interactions play a dominant role in Cl−-LDHs, whereas weak interactions governing the mode of intercalation of NO3 − ions influence the exchange reactions of NO3 −-LDHs. These studies have significant implications for the remediation of insidious anions by LDHs.

The arsenate-intercalated layered double hydroxide (LDH) of Mg and Al is synthesized by coprecipitation. The higher thermodynamic stability and the consequent lower solubility of the unitary arsenates preclude the formation of arsenate-intercalated LDHs of other metals directly from solution. However other M/AlAsO 4 (M = Co, Ni, Zn) LDHs could be prepared by anion exchange, showing that arsenate intercalation proceeds topotactically. The intercalation of various species of As(V) into the interlayer of LDHs and the subsequent arsenate carrying capacity are dependent upon the pH of the solution. Upon thermal decomposition, the intercalated arsenate ion undergoes reductive deintercalation to give a mixture of As(III) and As(V) oxides. The product oxides revert back to the LDH upon soaking in water on account of the compositional and morphological metastability of the former. This is in contrast with the phosphateintercalated LDHs, in which the reversibility is suppressed, consequent to the formation of stable metal phosphates.

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Partial cation exchange of Mg 2+ ions with Fe 3+ ions employing solid Mg(OH) 2 as precursor yielded an ordered layered double hydroxide of Mg and Fe in the presence of carbonate anions. Structure refinement revealed that the compound adopts the polytype structure 3R 1 (space group R3m, a = 3.108 Å, c = 23.08 Å) and does not show any signs of cation order. It crystallizes with a unique cation ratio of [Mg]/ [Fe] = 4. At this ratio, the compound shows a single sharp absorption in its electronic spectrum at 280 nm. Attempts to prepare the LDH with a higher Fe content resulted in the IntroductionThe layered double hydroxide (LDH) of Mg and Fe has potential applications in catalysis, [1] water purification, [2,3] safe storage of vitamins, [4] anion exchange, [5] as well as cation sorption.[6] The oxide residue obtained by the thermal decomposition of the Mg/Fe LDH is a bifunctional catalyst with both acidic and basic surface sites.[7] The Fe 0 /Fe 2+ / Fe 3+ composition of the catalyst can be finely tuned by controlled reduction, whereby catalysts with high olefin selectivity can be generated for use in the Fischer-Tropsch reaction. Given the magnetic properties of the oxides of Fe III , the Mg/Fe LDH is also used as a precursor for the synthesis of single-phase magnetic ferrites [8] and magnetic nanocomposites.[9] Furthermore, the Mg/Fe LDH is less toxic than many combinations of di-and trivalent cations and is proposed as a model medium for drug delivery. [10] The Mg/Fe LDH is obtained by the isomorphous substitution of a fraction x of Mg 2+ ions by Fe 3+ ions in Mg-(OH) 2 to yield positively charged layers with the composi-x+ . [11] Anions are incorporated into the interlayer. Many pairs of di-and trivalent cations can be combined with a variety of anions [12][13][14] to obtain a large and diverse family of layered double hydroxides. The Mg/ Fe LDH in many ways stands apart from other LDHs.(1) There is limited literature on the anion-exchange and -uptake properties of the pristine and calcined Mg/Fe LDH. Evidence for the anion-exchange of Mg/Fe LDHs is phase separation of excess Fe into X-ray amorphous binary compounds, the existence of which can be discerned only by the appearance of absorptions at λ Ͼ 350 nm, a characteristic of oxide-hydroxides of Fe 3+ . The nitrate-containing compound also forms with a similarly low Fe content. At this composition, the compound does not exhibit any anion-exchange properties as the nitrate ions intercalated in layered hydroxides of low layer charge are not labile. This explains the paucity of information on anion-exchange reactions of layered double hydroxides of Mg and Fe.at best equivocal and there are some definitive reports, [4] including this work, of the absence of anion-exchange reactions in Fe-containing LDHs.(2) In general, the a parameter of LDHs decreases with increasing trivalent metal content.[ (3) If the composition is fixed at x = 0.25, as suggested by some authors, [17] or at x = 0.2, as suggested by others, [5,18] does the Mg/Fe LDH crystallize in a cation-ord...

The layered double hydroxides are obtained by the stacking of metal hydroxide layers one above another. The stacking sequence is determined by the molecular symmetry of the intercalated anion. Anions select for those stacking sequences which provide interlayer sites having a local symmetry compatible with their own molecular symmetry. Oxoanions, XO3 n– (X = S, I), are unique in that their molecular symmetry, C 3v , is compatible with two different polytypes of rhombohedral symmetry. The SO3 2– ion selects for the more ubiquitous 3R1 polytype, whereas the IO3 – ion selects for the much rarer 3R2 polytype. Structure refinement shows that neither anion departs significantly from the structure of the free species on intercalation. The higher charge-to-size ratio of the SO3 2– ion compared to the IO3 – ion and its consequent stronger basicity is responsible for the nucleation of the 3R1 polytype, wherein the trigonal prismatic interlayer sites facilitate the superior hydrogen bonding capacity of the SO3 2– ion. The poorer basicity of the IO3 – ion and its consequent poorer proclivity to form hydrogen bonds select for the 3R2 polytype wherein the IO3 – ion is lodged in interlayer sites of octahedral symmetry.

a b s t r a c tThe iodide-containing layered double hydroxides (LDHs) of Mg and Zn with Al crystallize by the inclusion of extensive positional disorder of I À ions in the interlayer region. I À ion given its poor charge to size ratio can neither screen effectively the positive charge nor participate in H-bonding with the metal hydroxide layers. Thereby the I À ions are not stabilized in sites close to the seat of positive charge of the metal hydroxide layers (6c), nor in sites that facilitate H-bonding (3b or 18h). On the other hand, OH À from water can do both and effectively displaces I À from the interlayer.