Comment on "Factors influencing the removal of fluoride from aqueous solution by calcined Mg-Al-CO3 layered double hydroxides"

Lv et al. [L. Lv, J. He, M. Wei, D.G. Evans, X. Duan, Factors influencing the removal of fluoride from aqueous solution by calcined Mg-Al-CO3 layered double hydroxides, J. Hazard. Mater. B 133 (2006) 119-128] previously investigated the fluoride removal using calcined Mg-Al-CO3 layered double hydroxides (CLDH) as the sorbents. The present comment further discusses the mechanism of fluoride adsorption onto CLDH.     

Bromide ion removal from contaminated water by calcined and uncalcined MgAl-CO3 layered double hydroxides

A fundamental investigation on the uptake of bromide ion from contaminated water by calcined and uncalcined MgAl-CO3 layered double hydroxides (LDHs) were conducted in batch mode. The uptake capacity of calcined LDHs (CLDH) is higher than that of uncalcined LDHs, due to their different mechanisms which are confirmed by powder X-ray diffraction, FT-IR spectroscopy and TG-MS measurements. The former mechanism is based on the reconstruction of CLDH to Br-LDHs, whilst the latter is related to the surface adsorption. It has been found that the LDHs calcined at 500 degrees C with Mg/Al molar ratio of 4 represents the highest capacity to remove bromide ion from aqueous solution. The equilibrium isotherms of uptake of bromide by CLDH were well fitted by the Langmuir equation, and thermodynamic parameters such as Delta G0, Delta H0 and Delta S0 were calculated from Langmuir constants. The negative value of Delta H0 confirms the exothermic nature of adsorption. Three kinetics models were used to fit the kinetics experimental data, and it was found that the pseudo-second order kinetics model could be used to describe the uptake process appropriately. The value of Ea was calculated to be 79.9 kJ/mol, which suggests that the process of uptake bromide is controlled by the reaction rate of bromide with the CLDH rather than diffusion.     

Removal of indigo carmine dye from water to Mg-Al-CO(3)-calcined layered double hydroxides

Layered double hydroxides (LDHs) calcined, denoted as CLDHs, have been shown to recover their original layered structure in the presence of appropriate anions. In the light of this so-called "memory effect", the removal of indigo carmine (IC), an anionic dye, from aqueous solution by calcined Mg-Al-CO(3) LDHs was investigated in batch mode. We looked at the influence of pH values, dye-adsorbent contact time, initial dye concentration and various temperatures of heating of LDHs on the decolorization rate of IC. The adsorption isotherms, described by Freundlich model are L-type. The characterization of the solids CLDHs, both fresh and after removal of IC, by X-ray diffraction and infrared spectroscopy shows that the IC adsorption on CLDHs is enhanced by reconstruction of a matrix hydrotalcite intercaled by the dye, and the intercalation of the organic ion was clearly evidenced by the net increase in the basal spacing from 0.76 nm for [Mg-Al-CO(3)] to 2.13 nm for the organic derivative.     

Removal of chloride ion from aqueous solution by ZnAl-NO(3) layered double hydroxides as anion-exchanger

The layered double hydroxides (LDHs) containing nitrate as the interlayer anion has a high anion-exchange capacity in the presence of appropriate anions. In the light of this, ZnAl-NO(3) LDHs have been employed to remove chloride ion from aqueous solution in a batch mode. The influences of conditions for chloride ion uptake, including dosage of LDHs, pH of aqueous solution, and temperature on anion-exchange have been investigated, respectively. The thermodynamic parameters including Gibbs free energy (DeltaG(0)), standard enthalpy change (DeltaH(0)), and standard entropy change (DeltaS(0)) for the process were calculated using the Langmuir constants. It was found from kinetics test that the pseudo-second order kinetics model could be used to well describe the uptake process. An E(a) value of 10.27 kJ/mol provides evidence the anion-exchange process. The explanation of anion-exchange phenomenon has also been supported by X-ray diffraction and FT-IR spectra.     

酒石酸柱撑水滑石吸附水中的Ni~(2+)的特性研究

采用离子交换法制得酒石酸插层MgAl水滑石(MgAl-TA LDHs),并进行了XRD、IR、SEM、比表面积及孔径分析表征。考察了MgAl-TA LDHs吸附剂对溶液中Ni 2+的吸附能力,探讨了水滑石吸附剂投加量、Ni 2+溶液浓度、pH值以及吸附温度对Ni 2+吸附率的影响,并进行了动力学和热力学特征的研究。结果表明,适宜的水滑石投加量为2g/L,pH值以中性为宜,对于Ni 2+浓度不超过100 mg/L的溶液室温下Ni 2+吸附率在60%以上,而高温有利于提高Ni 2+吸附率。吸附过程符合二级动力学模型,吸附等温曲线可用R-P模型来描述。吸附过程能够自发进行,为吸热过程,属于化学吸附。     

Chloride adsorption by calcined layered double hydroxides in hardened Portland cement paste

This study investigated the feasibility of using calcined layered double hydroxides (CLDHs) to prevent chloride-induced deterioration in reinforced concrete. CLDHs not only adsorbed chloride ions in aqueous solution with a memory effect but also had a much higher binding capacity than the original layered double hydroxides (LDHs) in the cement matrix. We investigated this adsorption in hardened cement paste in batch cultures to determine adsorption isotherms. The measured and theoretical binding capacities (153 mg g⁻¹ and 257 mg g⁻¹, respectively) of the CLDHs were comparable to the theoretical capacity of Friedel's salt (2 mol mol⁻¹ or 121 mg g⁻¹), which belongs to the LDH family among cementitious phases. We simulated chloride adsorption by CLDHs through the cement matrix using the Fickian model and compared the simulation result to the X-ray fluorescence (XRF) chlorine map. Based on our results, it is proposed that the adsorption process is governed by the chloride transport through the cement matrix; this process differs from that in an aqueous solution. X-ray diffraction (XRD) analysis showed that the CLDH rebuilds the layered structure in a cementitious environment, thereby demonstrating the feasibility of applying CLDHs to the cement and concrete industries.     

Sorption study of an acid dye from an aqueous solution on modified Mg-Al layered double hydroxides

In this paper, experimental methods and results are reported on the removal of the dye Green Bezanyl-F2B (an acid dye) from MgAlCO(3) (HT) and from intercalated anionic surfactant, "sodium dodecylsulfate (SDS)", into the Mg-Al layered double hydroxides by the calcination-rehydration reaction using Mg-Al oxide precursors calcined at 773 K. Dodecylsulfate hydrotalcite was prepared by the calcination-rehydration method. The surfactant intercalation in the interlayer space of hydrotalcite was investigated by XRD and FT-IR spectroscopy where the resulting materials were found to be similar to those reported in the literature and were used to remove an acid dye from an aqueous solution. Equilibrium time and rate-determining step of the dye Green Bezanyl-F2B sorption were determined. Two simplified kinetic models were tested to investigate the sorption. The adsorption capacity data were also fitted to Langmuir and Freundlich equation as well. The sorption data fitted to the Langmuir model gave good values of the determination coefficient.     

Sorption of an anionic dye by uncalcined and calcined layered double hydroxides: a case study

Layered double hydroxides (LDHs) with a Mg/Al molar ratio of 2:1 were synthesized by using a co-precipitation method and their calcined products (CLDHs) were obtained by heating Mg/Al-LDHs at 500 °C. Sorption of a weak acid dye, Brilliant Blue R (BBR), by LDHs and CLDHs indicated equilibrium time required for BBR sorption by CLDHs was less than 12 h, regardless of initial concentration of BBR, whereas BBR sorption by LDHs was longer than 20 h. Sorption capacity of CLDHs was much larger than that of LDHs. Therefore, CLDHs could be used to remove anionic dyes of relatively high concentrations while LDHs may only be used to remove anionic dyes of low concentrations. Isotherms for BBR sorption by CLDHs and LDHs were well described using the Freundlich and Langmuir equations, respectively. When the initial pH of BBR solutions was lower than 8.0, the final pH of the solution after sorption was enhanced and stabilized at 10.6–10.8. The effect of initial pH (<8.0) on BBR removal was negligible, which would be environmentally important for precipitation/co-precipitation of co-existing metal cations. The effects of both Cl⁻ and SO₄²⁻ on BBR sorption by CLDHs were minimal; but the presence of CO₃²⁻ markedly reduced BBR removal. Thermal regeneration for re-use of LDHs and CLDHs after BBR sorption was feasible only within the first two cycles, after which the regenerated materials suffered from a large loss in their sorption capacities.     

Synthesis of layered double hydroxides from eggshells

Ca-Al and Ca-Fe layered double hydroxides (LDHs) were successfully synthesized from chicken eggshells by an ultrasonic wave assistant method. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) techniques. XRD and TEM analyses showed that the 4Ca-Al LDHs were of high purity but other samples were not. The present study provides a simple, efficient and environmental friendly method to obtain LDHs from biowaste eggshells, in which additional alkaline solution is not required for synthesis. Moreover, eggshells provide all the requisite bivalent metal ions, which are needed to form layered double hydroxides.     

Sorption of radioiodine from aqueous solutions on layered double magnesium aluminum hydroxides at 300 K

Sorption of ¹³¹I^? and ¹³¹IO ₃ ^? from aqueous solutions at 300 K on solid layered double magnesium aluminum hydroxides (Mg-Al-LDH) in the carbonate (Mg-Al-LDH-CO₃) and nitrate (Mg-Al-LDH-NO₃) forms was studied. The distribution coefficients of ¹³¹I^? and ¹³¹IO ₃ ^? between solid Mg-Al-LDH-CO₃ and an aqueous solution after contact of these phases for 60 min are 38 and 34 ml g^?1, respectively. The distribution coefficients of ¹³¹I^? and ¹³¹IO ₃ ^? between solid Mg-Al-LDH-NO₃ and an aqueous solution, measured under the similar conditions, are about 5 ml g^?1. After contact of the solid and liquid phases for 14 h, the distribution coefficients of ¹³¹I^? and ¹³¹IO ₃ ^? between an aqueous phase and solid layered double magnesium aluminum oxides (Mg-Al-LDO) prepared by thermolysis of Mg-Al-LDH are 7999 and 7068 ml g^?1, respectively.     

Sorption of cesium, strontium, and yttrium radionuclides from the aqueous phase on layered double hydroxides

Sorption of ¹³⁷Cs, ^85,90Sr, and ⁹⁰Y from aqueous solutions on the solid phase of layered double hydroxides (LDHs) and layered double oxides (LDOs) of various compositions was studied. On the solid phase of Mg-Al and Cu-Al LDHs and Mg-Al LDO, the Sr and Cs radionuclides are very weakly sorbed from aqueous solutions containing 10^?5 M of the corresponding element (Cs⁺ or Sr²⁺). Introduction of EDTA ions into Mg-Al LDH increases the distribution coefficients K _d of Sr by a factor of more than 40. After 96-h contact of the solid and liquid phases, K _d of radioactive Sr in sorption from aqueous solution on Mg-Al LDH and Mg-Al LDH-EDTA is 2.4 and 100 ml g^?1, respectively. The Sr and Y radionuclides are efficiently sorbed from aqueous solutions containing 10^?5 M Sr²⁺ and Y³⁺ on the Mg-Nd LDH solid phase. After 5-min contact of the solid and liquid phases, K _d of Sr exceeds 10⁵ ml g^?1. For Y, the distribution coefficients equal to 700–800 ml g^?1 are attained after 30-min contact of the solid and liquid phases. Aging of the Mg-Nd LDH precipitate does not affect its sorption properties toward Sr and Y radionuclides. With an increase in the Sr(NO₃)₂ concentration in the solution from 10^?5 to 10^?1 M, the distribution coefficients K _dz of Sr drastically decrease (virtually to zero) and those of Y change insignificantly.     

Evaluation of removal efficiency of fluoride from aqueous solution using quick lime

Several methods are available to remove fluoride from the aqueous environment but they are not feasible in all places and conditions due to various reasons. In the present work, F(-) has been removed by using activated and ordinary quick lime. The removal of fluoride was 80.6% and the final concentration was 9.7 mg/L at optimum condition from the synthetic solution having initial fluoride concentration of 50 mg/L. Adsorption kinetic study revealed that the adsorption process followed first order kinetics. And the removal process followed Langmuir adsorption isotherm. The SEM micrographs and XRD studies revealed that the removal of fluoride was mainly due to chemi-sorption and precipitation. However, they can be suitably used to remove fluoride from industrial effluent where the concentration is high. But this method cannot be employed to treat water for domestic purpose, since it cannot bring fluoride within permissible limit and also increases the pH of treated water.     

Surface control of layered double hydroxides by in-situ initiating&terminating polymerization

Appropriate surface modification or functionalization is prerequisite for the application of inorganic nanoparticles.And surface control between organic and inorganic interface plays an important role in constructing organic-inorganic composites.In-situ polymerization has been extensively studied to improve the compatibility and dispersibility of inorganic nanoparticles,but the polymerized nanoparticles tend to concatenate and form large composites,restricting further applications.Herein,uniform and dense polyacrylic acid(PAA)membranes have been grafted on layered double hydroxide(LDH)nanosheets via an in-situ initiating and terminating radical graft polymerization method.With initiating and terminating on the same particle,the size,morphology and density of grafted PAA onto the surface of LDHs can be controlled by adjusting the ratio of initiated sites to terminated sites,the amount of redox initiator or monomer.As a result,with only 17.33%organic grafting ratio,PAA@LDHs with largely improved compatibility can be monodispersed in polyethylene(PE)and polyvinyl chloride(PVC)matrices,which is determined by a fluorescence microscope technique.     

Hydrated cement: a promising adsorbent for the removal of fluoride from aqueous solution

The present study was carried out to investigate the potential of cement hydrated at various time intervals for the removal of excess F- from aqueous solution by using batch adsorption studies. The influence of different adsorption parameters, viz. effect of adsorbent dose, initial concentration, pH, interfering ions and contact time were studied for their optimization. It was observed that the adsorbent exhibited reasonably significant F- removal over a wide range of pH. The presence of carbonate and bicarbonate ions in aqueous solution were found to affect the F- removal indicating that these anions compete with the sorption of F- on adsorbent. The equilibrium adsorption data were fitted well for both the Freundlich and Langmuir isotherms and the adsorption capacities were calculated. Comparative studies for F- removal in simulated and field water show relatively higher F- removal in simulated water. XRD and SEM patterns of the hydrated cement were recorded to get better insight into the mechanism of adsorption process. From the experimental results, it may be concluded that HC was an efficient and economical adsorbent for F- removal.     

Sorption of 60Co from aqueous solutions on layered double hydroxides of Mg, Al, and Nd

Sorption of ⁶⁰Co from aqueous solutions of various compositions on layered double hydroxides (LDHs) of Mg, Al, and Nd in carbonate and hydroxide forms and on layered double oxides (LDOs) of Mg and Al was studied. ⁶⁰Co is poorly sorbed from aqueous nitrate solutions onto LDH-Mg-Al-OH. The ⁶⁰Co distribution coefficient K _d does not exceed 50 ml g^?1 at a phase contact time of 15 min and V/m = 50 ml g^?1. At the same time, ⁶⁰Co is efficiently sorbed from 10^?3?C10^?5 M aqueous Co(NO₃)₂ solutions onto LDH-Mg-Al and LDH-Mg-Nd with CO ₃ ^2? in the interlayer space. At a phase contact time of 15 min and V/m = 50 ml g^?1, K _d exceeds 2 × 10⁴ ml g^?1 for LDH-Mg-Nd and does not exceed 5 × 10³ ml g^?1 for LDH-Mg-Al. The ⁶⁰Co desorption from LDH-Mg-Nd-CO₃ into 0.05?C0.2 M solutions of Na₂CO₃, NaNO₃, (NH₄)₂C₂O₄, and Na₂H₂EDTA and into distilled water was studied. Na₂H₂EDTA is the most efficient desorbing agent. After 15-min contact of LDHMg(⁶⁰Co)-Nd-CO₃ with 0.1 and 0.05 M Na₂H₂EDTA solutions, the degree of desorption of ⁶⁰Co is ??100 and ??99%, respectively.     

Oxidative leaching of chromium from layered double hydroxides: Mechanistic studies

The layered double hydroxide (LDH) of Zn with Cr on treatment with a hypochlorite solution releases chromate ions as a result of oxidative leaching by a dissolution-reprecipitation mechanism. The residue is found to be ε-Zn(OH)₂. The LDH of Mg with Cr on the other hand is resistant to oxidative leaching. In contrast, a X-ray amorphous gel of the coprecipitated hydroxides of Mg and Cr yields chromate ions. These results suggest that the oxidation potential of Cr(III) in LDHs is determined by the nature of the divalent ion and the crystallinity of the phase while being unaffected by the nature of the intercalated anions.     

Catalytic characterization of bi-functional catalysts derived from Pd-Mg-Al layered double hydroxides

Hydrotalcite like precursors containing Pd^II-Mg^II-Al^III with varying molar ratios, (Pd + Mg)/Al ≈ 3 and Mg/Pd ≈ 750 to 35, were prepared by coprecipitation of metal nitrates at constant pH. Characterization of samples as synthesized and their calcined products by elemental analyses, powder XRD, TG-DTA, FT-IR spectroscopy, TPR and N₂ physisorption indicated a well crystalline hydrotalcite like structure with incorporation of Pd²⁺ in the brucite layers. Thermal decomposition of hydrotalcite precursors at intermediate temperatures led to amorphous mixed oxides, Pd/MgAl(O), which on reduction yielded bi-functional catalyst, Pd°/MgAl(O). The resultant catalysts with acid, base and hydrogenating sites, were highly active and selective for one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and hydrogen. The results showed an optimal balance between acid-base and metallic sites were required to increase the selectivity of MIBK and stability of the catalysts.     

Chromium removal from aqueous solution by the ferrite process

This research summarises the results of the study on the removal of chromium by applying the ferrite process to the solutions obtained from two different Cr(VI) reduction processes utilising sodium sulphite and ferrous sulphate as reducing agents. For both solutions containing trivalent chromium ions, the optimum treatment conditions were determined. The generated sludges were characterised by XRD analysis and physical tests. In addition, to explore the dissolution properties of the sludges obtained, they were contacted with the solutions of sulphuric, citric, tartaric, oxalic and ascorbic acids and EDTA. Also, the sludge samples were subjected to standard toxicity characterisation leaching procedure (TCLP) test of USEPA in order to determine the pollution potential. An efficient Cr(III) removal (about 100%) in the solution from the Cr(VI) reduction process utilising sodium sulphite as reducing agent was achieved when the solution was treated at pH 9 and 50 degrees C for 60 min in the presence of Fe2+/Cr3+ weight ratio of 16. For the other Cr(III) solution prepared from Cr(VI) reduction by ferrous sulphate, a Fe2+/Cr3+ weight ratio of 17.9 at the same conditions was found to produce complete removal of Cr(III). It was determined that the spynel chromium-iron compounds obtained in the process were in the form of chromite (Cr2FeO4). Dissolution experiments and TCLP tests show that the concentrations of the chromium dissolved from both sludges were below the limit given as 5 mg l(-1) by USEPA. The results showed that Cr(III) removal through ferrite process provides the advantages that the sludges generated are non-voluminous, easily separable and environmentally stable.     

Surfactant-assisted fatty acid intercalation of layered double hydroxides

Surfactant-mediated intercalation of aliphatic carboxylic acids into a commercial layered double hydroxide (LDH) with approximate composition ] was explored. The reaction was conducted at elevated temperatures with the LDH powder as a suspension in a stearic acid oil-in-water emulsion. The acidic fatty acid, e.g., stearic acid, reacts with the basic carbonate anions, CO₂ is released and the fatty acid is intercalated as a bilayer. High-concentration anionic or nonionic surfactants, i.e., sodium dodecylsulfate or Tween 60, aid the intercalation process by emulsifying the molten acid and dispersing the hydrotalcite particles. X-ray diffraction, thermal analysis, and infrared spectroscopy confirmed that a bilayer-intercalated hydrotalcite was formed and that the surfactant is not co-intercalated. The method is convenient, economical, and environmentally friendly: It employs the readily available carbonate form as starting reagent; water is used as medium rather than organic solvents; low reaction temperatures suffice, i.e., calcinations of the clay are superfluous and there is no need for working under a CO₂-free atmosphere.