C. Barriga

Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity

Hydrotalcite-like compounds (HT) and their calcined product (HT500) remove diverse anionic pollutants from waters by two mechanisms: anionic exchange for HT and reconstruction for HT500, and both can be affected by structural characteristics of these sorbents such as degree of layer substitution, interlayer anion and crystallinity. This work deals with assessing the effect of crystallinity and interlayer anion of the sorbent on the adsorption of two anionic organic contaminants in water: 2,4,6-trinitrophenol (TNP) and dodecylbenzylsulfonate (DBS). [Mg3Al(OH)8]2CO3·nH2O (HTCO3) and Mg3Al(OH)8Cl·nH2O (HTCl) were prepared by the coprecipitation method and subjected to hydrothermal treatment in air at 80°C and 130°C for 24 h. The adsorption of TNP and DBS was followed by adsorption isotherms, quantitatively described by application of the Langmuir equation. The adsorption of TNP and DBS on HT by anionic exchange is dramatically affected by the interlayer anion. On the calculated anion exchange capacity (AEC) basis, HTCO3 adsorbed 23% of TNP and 0% of DBS, whereas HTCl adsorbed up to 100% of both anionic contaminants. The specific affinity of carbonate anion for HT layer strongly complicates its displacement. These exchange reactions were scarcely modified by the crystallinity, more important in the case of carbonate-samples. Inversely, the adsorption of TNP and DBS by reconstruction on HT500 was affected by the crystallinity in all cases; however, HT500 derived from HTCO3 samples adsorbed better than those derived from HTCl samples. The adsorption of TNP and DBS on HT500 (from the untreated samples) was about 50% in both sorbents, but TNP adsorption increased up to 77% on HTCO3500 and only up to 52% on HTCl500 (from 130°C treated samples), and DBS adsorption increased up to 100% on HTCO3500 and 70% on HTCl500. A crystallinity increase of HT seems to render probably a better ordered HT500, favouring the memory effect.

Hydrotalcites as sorbent for 2,4,6-trinitrophenol: influence of the layer composition and interlayer anion

This work evaluates the effects of different variables such as MII/MIII ratio (layer charge), interlayer anion (CO32− or Cl−) and nature of the trivalent cation (Al3+ or Fe3+) in the sorption of 2,4,6-trinitrophenol (TNP) by layered double hydroxides (LDH) [MII1 − xMIIIx(OH)2]An−x/n·mH2O (MII = Mg2+, MIII = Al3+ or Fe3+, A = CO32− or Cl−, x = 0.33 − 0.20) and its calcined products MII1 − xMIIIxO1 + x/2. TNP adsorption by anionic exchange is strongly affected by exchangeable interlayer anion, being dramatically favoured in the case of MgAlCl. TNP adsorption by anionic exchange on the MgAlCO3 system is very low and it is not affected by the Mg/Al ratio while it is in MgAlCl. In the case of the MgFeCl system, the higher polarizing character of Fe3+ makes the exchange reaction less favorable. TNP adsorption by reconstruction of the calcined products decreases in the order MgAlCO3500 > MgAlCl500 > MgFeCl500, and it is favored by the layer charge decrease.

Microwave-assisted homogeneous precipitation of hydrotalcites by urea hydrolysis.

The use of urea as a precipitating agent in the synthesis of Ni-Al and Zn-Al layered double hydroxides having a hydrotalcite-like structure via a microwave-hydrothermal method is reported. For comparison purposes, the samples were also prepared by a conventional hydrothermal method. Ni-Al compounds with the hydrotalcite-like structure were obtained in shorter periods of time by the microwave method than by the conventional method, whereas when zinc cations were involved, no successful synthesis was achieved regardless of the method used. In order to find the best synthesis conditions for the Ni-Al solids, samples were submitted to microwave-hydrothermal treatment at different temperatures for increasing periods of time, and the structural, thermal, and textural properties of the synthesized materials were evaluated. All of the solids were fully characterized by chemical elemental analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, and transmission and scanning electron microscopy as well as by N 2 adsorption/desorption at -196 degrees C for assessment of specific surface area and porosity. The PXRD patterns showed that the layered structure appeared after merely 10 min when the synthesis was carried out at 125 degrees C; however, the FT-IR spectra showed the presence of some cyanate groups that were formed during urea hydrolysis and were quite difficult to remove completely. When the conventional hydrothermal treatment was used, longer periods of time were required in order to develop the hydrotalcite-like structure, but increasing the aging time improved the crystallinity of the compounds and yielded large particles.

Layered double hydroxides as adsorbents and carriers of the herbicide (4-chloro-2-methylphenoxy)acetic acid (MCPA): systems Mg-Al, Mg-Fe and Mg-Al-Fe.

Hydrotalcite-like compounds [Mg(3)Al(OH)(8)]Cl x 4H(2)O; [Mg(3)Fe(OH)(8)]Cl x 4H(2)O; [Mg(3)Al(0.5)Fe(0.5)(OH)(8)]Cl x 4H(2)O (LDHs) and calcined product of [Mg(3)Al(OH)(8)]Cl x 4H(2)O, Mg(3)AlO(4.5) (HT500), were studied as potential adsorbents of the herbicide MCPA [(4-chloro-2-methylphenoxy)acetic acid] as a function of pH, contact time and pesticide concentration, and also as support for the slow release of this pesticide, with the aim to reduce the hazardous effects that it can pose to the environment. The information obtained in the adsorption study was used for the preparation of LDH-MCPA complexes. The results showed high and rapid adsorption of MCPA on the adsorbents as well as that MCPA formulations based on LDHs and HT500 as pesticide supports displayed controlled release properties and reduced herbicide leaching in soil columns compared to a standard commercial MCPA formulation. Thereby, we conclude that the LDHs employed in this study can be used not only as adsorbents to remove MCPA from aqueous solutions, but also as supports for the slow release of this highly mobile herbicide, thus controlling its immediate availability and leaching.

Layered Ni(II)-Zn(II) hydroxyacetates. Anion exchange and thermal decomposition of the hydroxysalts obtained

Layered Ni,Zn hydroxyacetates have been prepared by hydrothermal methods and have been used for anion exchange of the original acetate anions by chloride, bromide, carbonate, nitrate, sulfate, and phosphate. Exchange was complete in all cases. The solids have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analysis (thermogravimetric and differential), FT-IR spectroscopy and electron microscopy; XAS spectra were also recorded in some cases. Thermal decomposition in air leads to removal of interlayer water at 150–200 °C. Mixed NiO–ZnO oxides are formed at higher temperatures. Chloride, sulfate, and phosphate salts are also formed at intermediate calcination temperature, and phosphate remains even after calcination at 1000 °C.

Intercalation of iron hexacyano complexes in Zn,Al-hydrotalcite

Abstract Zn,Al-hydrotalcites interlayered with hexacyanoferrate(II) and hexacyanoferrate(III) ions have been synthesized by different methods. These include rehydration of the product obtained by calcination of the carbonate form at 500 °C, anionic exchange of nitrate- and terephtalate-containing hydrotalcites, and direct synthesis. Both hexacyanoferrate(II) and (III) species are present in the interlayer space, but in different ratios, suggesting that a redox process takes place in this interlayer.

Preparation and thermal stability of manganese-containing hydrotalcite, [Mg0.75MnII0.04MnIII0.21(OH)2](CO3)0.11·nH2O

A magnesium–manganese hydroxycarbonate with the hydrotalcite structure has been synthesized by coprecipitation from aqueous solutions of Mg2+ and Mn2+. According to temperature-programmed reduction data, 84% of the manganese is oxidized to Mn3+ during synthesis. Fourier-transform infrared spectroscopy, powder X-ray diffraction and transmission electron microscopy data support the hydrotalcite structure. Thermal decomposition in air at 390 °C leads to an amorphous material with a simultaneous increase of the specific surface area. Further calcination at higher temperatures (670 and 1000 °C) leads to crystallization of MgO and Mg2MnO4 spinel and to sintering. Reduction of the spinel takes place in two separate steps, probably through formation of intermediate Mn3O4 and finally MnO.

Organo/layered double hydroxide nanohybrids used to remove non ionic pesticides

The preparation and characterization of organo/layered double hydroxide nanohybrids with dodecylsulfate and sebacate as interlayer anion were studied in detail. The aim of the modification of the layered double hydroxides (LDHs) was to change the hydrophilic character of the interlayer to hydrophobic to improve the ability of the nanohybrids to adsorb non-ionic pesticides such as alachlor and metolachlor from water. Adsorption tests were conducted on organo/LDHs using variable pH values, contact times and initial pesticide concentrations (adsorption isotherms) in order to identify the optimum conditions for the intended purpose. Adsorbents and adsorption products were characterized several physicochemical techniques. The adsorption test showed that a noticeable increase of the adsorption of the non-ionic herbicides was produced. Based on the results, the organo/LDHs could be good adsorbents to remove alachlor and metolachlor from water. Different organo/LDHs complexes were prepared by a mechanical mixture and by adsorption. The results show that HTSEB-based complex displays controlled release properties that reduce metolachlor leaching in soil columns compared to a technical product and the other formulations. The release was dependent on the nature of the adsorbent used to prepare the complexes. Thus, it can be concluded that organo/LDHs might act as suitable supports for the design of pesticide slow release formulations with the aim of reducing the adverse effects derived from rapid transport losses of the chemical once applied to soils.

Lithium ferrite formation by precipitation from Fe(III) solutions

Abstract The formation of a new lithium ferrite modification structurally similar to γ-Fe 2 O 3 and LiFe 5 O 8 by direct precipitation from Fe(III) solutions is described. It is used as intermediate in the preparation of other lithium ferrites and contains no Fe(II) as revealed by Mossbauer spectroscopy and chemical analysis. Thermogravimetric results show a complex water loss at 80–500°C yielding a solid solution of γ-Fe 2 O 3 and LiFe 5 O 8 that decomposed to LiFe 5 O 8 and α-Fe 2 O 3 at 850°C. Hydrothermal treatment of the initial gel at 160°C leads to the progressive formation of α, β, and γ-LiFeO 2 .

Precursor dependence of the nature and structure of non-stoichiometric magnesium aluminium vanadates

Hydrotalcite-like compounds containing (a) MgII, AlIII and VIII in the layers and carbonate in the interlayers, or (b) MgII and AlIII in the layers and decavanadate in the interlayers, have been synthesized and characterized using powder X-ray diffraction, thermal analysis, FTIR spectroscopy. temperature-programmed reduction, and specific surface-area determination. The crystalline phases formed after calcination in air at 700 °C have been identified using these same experimental techniques. It has been found that the nature of the precursor determines the nature of the Mg–V–O phases formed; thus, when starting from (a), Mg3(VO4)2 containing isolated [VO4] units is formed, while when starting from (b), MgV2O6, containing pairs of edge-sharing [VO6] octahedra, is formed preferentially. Such a reactivity appears to depend on the nature of the support, and not on the stoichiometry (i.e., Mg/V atomic ratio) of the starting materials.