Sorption characteristics and mechanisms of oxyanions and oxyhalides having different molecular properties on Mg/Al layered double hydroxide nanoparticles

MgAl-NO3 LDH: Adsorption Isotherms and Multivariate Optimization for Cr(VI) Removal

Within the framework of the various strategies studied for the abatement of polluting agents in water, both from anthropogenic and natural origins, adsorption processes are among the most widespread techniques. In this context, Layered Double Hydroxides (LDHs) play a fundamental role. In this study, a Mg–Al LDH (nitrate intercalated, Mg/Al = 2) was prepared to be used as an anion exchanger for Cr(VI)-removal purposes from water. The LDH was synthesized through a coprecipitation reaction, followed by an aging process under heating. The compound was characterized by means of inductively coupled plasma–atomic emission spectroscopy (ICP-AES), X-ray powder diffraction (XRPD), field-emission scanning electron microscopy (FE-SEM) and Fourier-transform infrared spectroscopy (FT-IR). Regarding LDH adsorption capacity, with respect to Cr(VI), the adsorption isotherms and reaction kinetic were studied, and the adsorption process was well described by the Langmuir model. A central composite design was used for the multivariate optimization of the working parameters. The maximum adsorption capacity was estimated to be 30 mg/g.

Adsorptive removal of heavy metal anions from water by layered double hydroxide: A review

High-valence heavy metals with high ecotoxicity are generally found in water in the form of anions, and this increases heavy metal pollution intensity and treatment difficulty. Recent studies have pointed to the potential efficiency of layered double hydroxides (LDHs) to meet this challenge. In this review, we retrospectively research the development of LDHs using a Java application called CiteSpace. We describe the unique layer structure, highly adjustable chemical properties, and diverse synthesis methods of LDHs, all of which decide the effective adsorption of heavy metal anions by LDHs. Subsequently, we focus on discussing the adsorption mechanism of LDHs on heavy metal anions, as well as the current state of research and future directions for microscopic interaction mechanisms. For practical applications, it is critical to improve the adsorption selectivity and stability. We then recommend solutions to improve the adsorption selectivity and stability after identifying the influencing mechanism. Finally, we provide our perspectives on the future development of LDHs adsorption of heavy metal anions.

Synthesis of acidified magnetic sludge-biochar and its role in ammonium nitrogen removal: Perception on effect and mechanism

An acidified magnetic sludge-biochar (MSB) was prepared to enhance ammonium nitrogen (AN) removal efficiency in eutrophic water, and MSB was obtained by secondary pyrolysis of sludge biochar powder. A series of MSB were prepared under 300, 400, 500, 600 °C and different valence states of iron ions by impregnation pyrolysis, which is based on the deposition of unstable iron minerals on biochar matrix. Physicochemical properties of pristine biochar and MSB were revealed through characterization analysis, suggesting that MSB prepared by ferric chloride at 400 °C presented the largest adsorption capacity, and the acid-modification enhanced the ammonium adsorption capacity by 10.7%. Electrostatic attraction and ion-exchange processes were identified as the main adsorption mechanisms of MSB on AN. As the most dominant mechanism, ion exchange of AN with functional groups containing -OH and CO on the surface of MSB resulted in the relative content of -OH (61.3%) and CO (11.5%) bonds reduced to 34.2% and 7.0% respectively. The novel magnetic sludge-biochar with acid-modification possessed enhanced electron transfer capacity, revealing a removal pathway of ammonium by nitrification. The findings above demonstrated that MSB is a promising adsorbent for ammonium removal and can be applied to the natural nitrogen-rich water regulation.

Selective adsorption of anions on hydrotalcite-like compounds derived from drinking water treatment residuals

Most drinking water treatment residuals (DWTRs) with rich metal resources are landfilled directly without treatment, which results in wasted Al/Fe resources. This work proposes a new method of preparing Mg-Al-Fe Hydrotalcite-like compounds (MAF-HTCLs) by recycling DWTRs as the raw material to selectively adsorb anions in the waste water. In this study, MAF-HTCLs were prepared by the coprecipitation method with recycled DWTRs. The characterizations and adsorption of MAF-HTCLs were studied for the selective adsorption of P, Cr, F, and Br. The adsorption capacity was increasing as the value of pH decreased. For kinetic adsorption, the pseudo-second-order model fit better, and two isotherm models (the Langmuir and Freundlich models) described the isotherm results well. According to the Langmuir model, the maximum adsorption capacities of P, Cr, F, and Br were 55.2, 34.9, 16.84, and 13.9 mg/g, respectively. Based on the results of characterizations before and after adsorption, adsorption mechanisms of Cr, F, and Br were proposed, including physicochemical adsorption, surface complexation, and ion exchange, in which ion exchange was dominant. Finally, we determined that the selective adsorption mechanisms of P on MAF-HTCLs included strong ion exchange and surface chemical precipitation by analyzing the results of X-ray photoelectron spectroscopy.

Layered Double Hydroxides for Sustainable Agriculture and Environment: An Overview

Agricultural practices in modern society have a detrimental impact on the health of the ecosystem, environment, and consumers. The significantly high usage rate of chemicals causes serious harm, and the sector demands the development of innovative materials that can foster improved food production and lessen ecological impacts. The majority of layered double hydroxides (LDH) are synthetic. At the same time, some of them occur in the form of natural minerals (hydrotalcite), which have recently emerged as favorable materials and provided advanced and ingenious frontiers in various fields of agriculture through practical application possibilities that can replace conventional agricultural systems. LDH can exchange anions intercalated between the layers in the interlayer structure, and there is evidence that atmospheric carbon dioxide and moisture can completely break down LDH over time. Due to certain unique properties such as tunable structure, specific intercalation chemistry, pH-dependent stability, as well as retention of the guest molecules within interlayers and their subsequent controlled release, LDHs are increasingly investigated as materials to enhance yield, quality of crops, and soil in recent times. This review aims to present the current research progress in the design and development of LDH-based materials as nanoscale agrochemicals to illustrate its relevance in making agro-practices more sustainable and efficient. Specific emphasis is given to the functionality of these materials as effective materials for the slow release of fertilizers and plant growth factors as well as adsorption of toxic agrochemical residues and contaminants. Relevant research efforts have been briefly reviewed, and the potential of LDH as new generation green materials to provide solutions to agricultural problems for improving food productivity and security has been summarized.

Adsorption performance and mechanism of Ca-Al-LDHs prepared by oyster shell and pop can for phosphate from aqueous solutions

The anion exchange properties of layered composite metal hydroxide (LDHs) make it a significant anion purification material in aquatic environment. Ca-Al-LDHs as a calcium-based adsorbent can significantly improve the removal rate of phosphate. In this manuscript, waste oyster shell was used as the source of calcium and waste cans as the source of aluminum, and compared with the LDHs prepared by chemical reagents. The optimal molar ratio of for both the types of materials prepared from waste material as well as reagents was found to be 4:1 for best adsorption performance, with the waste group and reagent group materials showing similar adsorption capacities of 127.50 mg g^-1 and 126.67 mg g^-1, respectively. The effects of adsorbent dosage, initial solution pH and presence of coexisting ions on the adsorption capacity were investigated. Langmuir adsorption isotherm, quasi-second-order kinetic equation and mass transfer kinetics was found to describe the phosphorus adsorption process of CAs-4 and CA-4 in better way. The maximum adsorption capacities of Langmuir equation for CAs-4 and CA-4 adsorption process were 126.41 mg g^-1 and 127.43 mg g^-1, respectively, for CAs-4 and CA-4. The characterization results of FTIR, XRD and ICP showed that the main adsorption mechanisms were intercalation anion exchange, electrostatic attraction and dissolution precipitation. This work not only provides a method to prepare Ca-Al-LDHs for efficient utilization of solid waste, but also provides theoretical significance for controlling water eutrophication and realize waste recycling.

Materials interacting with inorganic selenium from the perspective of electrochemical sensing

Inorganic selenium, the most common form of harmful selenium in the environment, can be determined using electrochemical sensors, which are compact, fast, reliable and easy-to-operate devices. Despite progress in this area, there is still significant room for developing high-performance selenium electrochemical sensors. To achieve this, one should take into account (i) the electrochemical process that selenium undergoes on the electrode; (ii) the valence state of selenium species in the sample and (iii) modification of the sensor surface by a material with high affinity to selenium. The goal of this review is to provide a knowledge base for these issues. After the Introduction section, mechanisms and principles of the electrochemical reduction of selenium are introduced, followed by a section introducing the modification of electrodes with materials interacting with selenium and a section dedicated to speciation methods, including the reduction of non-detectable Se(VI) to detectable Se(IV). In the following sections, the main types of materials (metallic, polymers, hybrid (nano)materials…) interacting with inorganic selenium (mostly absorbents) are reviewed to show the diversity of properties that may be endowed to sensors if the materials were to be used for the modification of electrodes. These features for the main material categories are outlined in the conclusion section, where it is stated that the engineered polymers may be the most promising modifiers.

Synthesis of Layered Double Hydroxides Intercalated With Drugs for Controlled Release: Successful Intercalation of Ibuprofen and Failed Intercalation of Paracetamol

This work examines the effect of drug structure and ionization degree on the formation and properties of biocompatible layered double hydroxides (LDH) intercalated with ibuprofen and paracetamol. Ibuprofen (pKa = 5.3) is in its anionic form, whereas paracetamol (pKa 9.4) is only partially ionized at the synthesis pH (9.0), and thus intercalation is expected to be different in the two cases. Chemical analyses, X-ray diffraction, electron microscopy, infrared spectroscopy and thermal analyses were applied to characterize the materials. Dissolution kinetics and drug release kinetics were also investigated, in an ample range of pH (3.0-9.0) in NaCl solutions, and in physiological buffers (1.2, 4.5 and 6.8). All characterization techniques showed that an efficient intercalation of ibuprofen took place, resulting in a material with 30% of its weight corresponding to the drug. On the contrary, all techniques revealed a very poor intercalation of paracetamol (1.2%). The dissolution kinetics of LDHs was highly pH-dependent, being higher as pH decreased. The drug release kinetics, conversely, increased as pH increased. In physiological buffers the release rate depended not only on the pH but also on the type of buffer. This last behavior is useful to control the release in different parts of the digestive system.

Facile synthesis of MgAl layered double hydroxides by a co-precipitation method for efficient nitrate removal from water: kinetics and mechanisms

A series of MgAl-LDH as highly efficient adsorbents for removing low concentrations of NO₃⁻ were synthesized. The mechanism of NO₃⁻ removal has been comprehensively discussed in terms of its characterization, adsorption kinetics and thermodynamics.

Adsorption of phosphate from aqueous solution by vegetable biochar/layered double oxides: Fast removal and mechanistic studies

Two biochars, from Chinese cabbage (Cc, Brassica rapa pekinensis) and rape (Ra, Brassia campestris L.), were used to prepare biochar/Mg-Al layered double oxides (LDOs) as adsorbents for phosphate removal from aqueous solution. The biochar/LDOs were horizontally alternated lamellar particles and had abundant groups of oxides and biochars. The phosphate removal percentage remained above 92% at a pH range of 2-10, and above 95% during the first 5 min for 50 mg/L phosphate by 0.05 g biochar/LDOs. The adsorption kinetics and isotherms data were well fitted by the pseudo-second-order kinetic equation, as well as by the Freundlich and Langmuir models. Based on FTIR, XPS, and zeta potential analysis, the interaction mechanisms were defined as "memory effect", electrostatic attraction, surface complexation, and anion exchange. The results indicate that vegetable biochar/LDOs can be considered a novel and efficient sorbent for phosphate removal from water or wastewater.

Competitive adsorption and desorption of arsenate, vanadate, and molybdate onto the low-cost adsorbent materials alum water treatment sludge and bauxite

When low-cost adsorbents are being used to remove contaminant ions (e.g. arsenate, vanadate, and molybdate) from wastewater, competitive adsorption/desorption are central processes determining their removal efficiency. Competitive adsorption of As, V, and Mo was investigated using equimolar oxyanion concentrations in single, binary, and tertiary combinations in adsorption isotherm and pH envelope studies while desorption of previously adsorbed oxyanions was examined in solutions containing single and binary oxyanion combinations. The low-cost adsorbent materials used were alum water treatment sludge (amorphous hydroxy-Al) and bauxite ore (crystalline Al oxides). Adsorption isotherm and pH envelope studies showed that Mo had only a small effect in decreasing adsorption of As and V but V and As had substantial and similar effects in reducing adsorption of the other. As had a greater effect than V in reducing adsorption of Mo and it was concluded that the affinity of oxyanions for the surfaces of water treatment sludge and bauxite followed the order As > V >> Mo. In 0.3 M NaCl electrolyte, desorption of previously adsorbed oxyanions amounted to 0.3-3.4% for V and As, and 11-20% for Mo. As had approximately four times greater effect than Mo in increasing desorption of V while V had about three times the effect of Mo in increasing desorption of As. Thus, the order of oxyanions in inducing desorption of the other oxyanions (i.e. As on V and As) was the same as that for adsorption selectivity: As > V >> Mo. Water treatment sludge was a more effective adsorbent than bauxite because it had a greater adsorption capacity for all three anions and, in addition, they were held more strongly so desorption in the background electrolyte was proportionately less. It was concluded that at similar molar concentrations, arsenate would tend to reduce adsorption of vanadate as well as displace vanadate already held on adsorbent surfaces while both anions will compete effectively with molybdate. The limiting factor for simultaneous removal of As, V, and Mo from multielement solutions by adsorption will therefore be the removal of Mo.

Sealing of anodized magnesium alloy AZ31 with MgAl layered double hydroxides layers

In this work, anodized magnesium alloy AZ31 with and without boiling water sealing was pre-prepared, and then MgAl-layered double hydroxide (LDH) films were fabricated on it through hydrothermal chemical conversion of the pre-prepared anodic layer. The morphology, structure, and composition of the films were characterized by XRD, SEM, EDS, FT-IR, XPS and GDOES. It was found that the porosity of the films was reduced after in situ fabrication of the LDHs. The effects of boiling water sealing treatment on the anodized substrate were also discussed. Moreover, the polarization curve, EIS, and immersion tests showed that LDHs fabricated on the anodized substrate with boiling water sealing treatment exhibited a significant long period of protection for the substrate.

In this work anodized magnesium alloy AZ31 with and without boiling water sealing was pre-prepared, and then MgAl-layered double hydroxide (LDH) films were fabricated on it through hydrothermal chemical conversion of the pre-prepared anodic layer.

Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-Contaminated Soils

It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from chromium ore processing residue (COPR). We modified various parameters of EPA 3060A toward understanding the transformation of COPR minerals in the alkaline extraction and improving extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si were the major elements dissolved from NIST 2701, and their concentrations in solution were correlated with Cr(VI). The extraction fluid leached additional Al and Si from the method-prescribed borosilicate glass vessels which appeared to suppress the release of Cr(VI). Use of polytetrafluoroethylene vessels and intensive grinding of NIST 2701 increased the amount of Cr(VI) extracted. These modifications, combined with an increased extraction fluid to sample ratio of ≥900 mL g^-1 and 48-h extraction time resulted in a maximum release of 1274 ± 7 mg kg^-1 Cr(VI). This is greater than the NIST 2701 certified value of 551 ± 35 mg kg^-1 but less than 3050 mg kg^-1 Cr(VI) previously estimated by X-ray absorption near edge structure spectroscopy. Some of the increased Cr(VI) may have resulted from oxidation of Cr(III) released from brownmillerite which rapidly transformed during the extractions. Layered-double hydroxides remained stable during extractions and represent a potential residence for unextracted Cr(VI).

Sorption-desorption of selenite and selenate on Mg-Al layered double hydroxide in competition with nitrate, sulfate and phosphate

Selenate and selenite are considered emerging contaminants and pose a risk to living organisms. Since selenium anion species are at low concentration in aquatic environments, materials for its retention are required to enable monitoring. Herein, hydrotalcite was calcined and characterised to investigate sorption and desorption of selenite and selenate in competition with nitrate, sulfate and phosphate. Sorption experiments were carried out in batch system and desorption by sequential dilution. Selenite and selenate concentration remaining after N desorption steps was determined by mass balance. The isotherms were adjusted to the dual-mode Langmuir-Freundlich model (R² > 0.99). Maximum sorption capacity ranged from 494 to 563 meq kg^-1 for selenite and from 609 to 659 meq kg^-1 for selenate. Sulfate and phosphate ions showed greater competitive effect on the sorption of selenate and selenite, respectively. Low mobilization factors and high sorption efficiency (MF<3%; SE ≈ 100%) indicated that calcined hydrotalcite has the wanted characteristics for retention of relevant selenium anion species in aqueous media.

Enhanced adsorption of bromate from aqueous solutions on ordered mesoporous Mg-Al layered double hydroxides (LDHs)

An ordered mesoporous Mg-Al layered double hydroxide (meso-LDH350) with a fairly high Brunauer-Emmett-Teller (BET) surface area (126m²g^-1) has been facilely synthesized and then evaluated for the adsorptive removal of bromate from aqueous solutions. Adsorbents were characterized by a variety of techniques (e.g., XRD, FTIR, SEM, TG-DSC, N₂ physisorption, XPS, etc.). The adsorption studies indicated that the presence of background electrolytes and competitive anions can obviously repress the uptake of bromate on LDHs. The adsorption isotherms agree well with the Langmuir model, giving a maximum adsorption capacity of 59.34mgg^-1 (pH 7.5, 10°C) for meso-LDH350, which is much higher than other LDH-type adsorbents reported in literature. The adsorption kinetic data can be well fitted with the pseudo-second-order rate model. Based on the macroscopic and microscopic studies, bromate adsorption on meso-LDH350 was associated with two mechanisms: the reconstruction of the layered structures of meso-LDH350 and the anion-exchange between bromate and the intercalated anions.

Enhanced sulfur removal by a tuned composite structure of Cu, Zn, Fe, and Al elements

Several combinations of high sulfur affinity elements of Cu, Zn, Fe, and Al were used to prepare sorbent materials that remove hydrogen sulfide (H₂S) from air contaminated streams. The combination of these four elements in composite crystallinity structure resembling hydrotalcite-like and aurichalcite-like compounds showed excellent H₂S uptake. Further tuning of the relative ratio among these elements resulted in outstanding H₂S uptake. XRD revealed that the final sorbent material was featured by crystallinity structure that had two adjacent lower reflection angles. The experimental test showed H₂S uptake was around 39% of the sorbent material weight when the concentration of H₂S in the outlet was less than 0.5% of its concentration in the inlet. The sorbent material showed high sulfur removal efficiency at ambient temperature without prior calcination.

Biochar pyrolyzed from MgAl-layered double hydroxides pre-coated ramie biomass (Boehmeria nivea (L.) Gaud.): Characterization and application for crystal violet removal

A novel biochar/MgAl-layered double hydroxides composite (CB-LDH) was prepared for the removal of crystal violet from aqueous solution by pyrolyzing MgAl-LDH pre-coated ramie stem (Boehmeria nivea (L.) Gaud.). Pyrolysis played dual role for both converting biomass into biochar and calcining MgAl-LDH during the pyrolysis process. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and zeta potential analysis were used to characterize the CB-LDH. The results of characterization suggested that the calcined LDH was successfully synthesized and coated on biochar. The resulted CB-LDH had higher total pore volume and more functional groups than the pristine biochar. Adsorption experimental data fitted well with the pseudo-second order kinetics model and the Freundlich isotherm model. The rate-controlled step was controlled by film-diffusion initially and then followed by intra-particle diffusion. Thermodynamic analysis showed that the adsorption of crystal violet was a spontaneous and endothermic process. The higher pH and temperature of the solution enhanced the adsorption performance. CB-LDH could also have excellent ability for the removal of crystal violet from the actual industrial wastewater and groundwater with high ionic strength. LDH adsorption, electrostatic attraction, pore-filling, π-π interaction and hydrogen bond might be the main mechanisms for crystal violet adsorption on CB-LDH. The results of this study indicated that CB-LDH is a sustainable and green adsorbent with high performance for crystal violet contaminated wastewater treatment and groundwater remediation.

Enhancing adsorptive removal of sulfate by metal layered double hydroxide functionalized Quartz-Albitophire iron ore waste: preparation, characterization and properties

In this study, the seemingly worthless Quartz-Albitophire iron ore mineral waste has been modified with metal double hydroxides (Mg–Al, Ni–Fe, Ni–Al and Mg–Fe) and utilized as a sulfate adsorbent which is characterized by XRD, XRF, FTIR and SEM analysis.

Production of biochars from Ca impregnated ramie biomass (Boehmeria nivea (L.) Gaud.) and their phosphate removal potential

This work explored the efficiency and mechanisms of phosphate (P) removal by Ca-impregnated biochar prepared from CaCl₂-pretreated ramie stem (Ca-RSB) and ramie bark (Ca-RBB).

Synthesis of aminopropyltriethoxysilane grafted/tripolyphosphate intercalated ZnAl LDHs and their performance in the flame retardancy and smoke suppression of polyurethane elastomer

Synthesis procedure of APTS grafted/P₃O₁₀⁵⁻ intercalated LDHs.

Equilibrium and kinetics studies on As(V) and Sb(V) removal by Fe2+ -doped Mg-Al layered double hydroxides

Mg-Al layered double hydroxides (Mg-Al LDHs) doped with Fe(2+) adsorbed As(V) [Formula: see text] and Sb(V) [Formula: see text] from an aqueous solution through anion exchange with Cl(-) intercalated in the LDH interlayer. Fe(2+)-doped Mg-Al LDH exhibited superior As(V) removal compared with Mg-Al LDH. The oxidation of Fe(2+) doped in the Mg-Al LDH host layer to Fe(3+) increased the positive layer charge of the LDH, thus increasing the anion-uptake capacity owing to stronger electrostatic attractive force between the positively charged layer and the anion. However, Fe(2+)-doped Mg-Al LDH was not superior to Mg-Al LDH in terms of Sb(V) removal. This was attributed to the preferential intercalation of OH(-) over [Formula: see text] . The As(V) and Sb(V) removal by LDH followed Langmuir-type adsorption, which proceeded via a pseudo-first-order reaction. The equilibrium and kinetics studies confirm that the adsorption of As(V) and Sb(V) by Fe(2+)-doped Mg-Al LDH was the result of chemical adsorption, involving the anion exchange of [Formula: see text] and [Formula: see text] with the intercalated Cl(-).

Metal composition of layered double hydroxides (LDHs) regulating ClO(-)4 adsorption to calcined LDHs via the memory effect and hydrogen bonding

A series of calcined carbonate layered double hydroxides (CLDHs) with various metal compositions and different M(2+)/M(3+) ratios were prepared as adsorbents for perchlorate. Adsorption isotherms fit Langmuir model well, and the adsorption amount followed the order of MgAl-CLDHs ≥ MgFe-CLDHs >> ZnAl-CLDHs. The isotherms of MgAl-CLDHs and MgFe-CLDHs displayed a two-step shape at low and high concentration ranges and increased with an increase in the M(2+)/M(3+) ratio from 2 to 4. The two-step isotherm was not observed for ZnAl-CLDHs, and the adsorption was minimally affected by the M(2+)/M(3+) ratio. The LDHs, CLDHs and the reconstructed samples were characterized by X-ray diffraction, SEM, FT-IR and Raman spectra to delineate the analysis of perchlorate adsorption mechanisms. The perchlorate adsorption of MgAl-CLDHs and MgFe-CLDHs was dominated by the structural memory effect and the hydrogen bonds between the free hydroxyl groups on the reconstructed-LDHs and the oxygen atoms of the perchlorates. For ZnAl-CLDHs, the perchlorate adsorption was controlled by the structural memory effect only, as the hydroxyl groups on the hydroxide layers preferred to form strong hydrogen bonds with carbonate over perchlorate, which locked the intercalated perchlorate into a more confined nano-interlayer. Several distinct binding mechanisms of perchlorate by CLDHs with unique M(2+) ions were proposed.

Effect of calcination temperature on Mg-Al bimetallic oxides as sorbents for the removal of F(-) in aqueous solutions

Bimetallic oxides were synthesized from hydrotalcite using increasing calcination temperatures (873, 1073, 1273 K). These bimetallic oxides were fully characterized and the sorption density of F(-) was investigated. X-ray diffraction patterns for the produced bimetallic oxides showed that MgO was the primary phase within the range of investigated calcination temperatures, but MgO crystallinity increased with calcination temperature and an additional MgAl2O4 phase was formed. In the process of F(-) sorption, the bimetallic oxides were primarily transformed into hydrotalcite with intercalation of F(-). The Higher calcination temperature increased the MgAl2O4 phase, which did not contribute to the immobilization of F(-). These findings show that optimizing the calcination temperature can be used to maximize the sorption density of this material for F(-) removal.

Selectivity of layered double hydroxides and their derivative mixed metal oxides as sorbents of hydrogen sulfide

In the context of finding high efficient sorbent materials for removing hydrogen sulfide (H2S) from air stream, a screening study was performed to find the best combination of metals for the synthesis of layered double hydroxides (LDHs) and their derivative mixed metal oxides. Based on selectivity of 998 natural mineral species of sulfur-containing compounds, Cu(2+), Ni(2+) and Zn(2+) were selected as divalent metals, and Fe(3+), Al(3+) and Cr(3+) as trivalent metals to synthesis the LDHs sorbents. 10 LDHs materials and their calcined mixed metal oxides, Ni(0.66)Al(0.34), Cu(0.35)Ni(0.32)Al(0.33), Zn(0.66)Al(0.34), Cu(0.36)Zn(0.32)Al(0.32), Ni(0.64)Fe(0.36), Cu(0.35)Ni(0.31)Fe(0.34), Ni(0.66)Cr(0.34), Cu(0.35)Ni(0.31)Cr(0.34), Zn(0.66)Cr(0.34), Cu(0.33)Zn(0.32)Cr(0.35) were synthesized, characterized chemically and physically, and then tested using breakthrough test to determine their sulfur uptake. Ni(0.64)Fe(0.36) mixed metal oxides was found to have the best uptake of hydrogen sulfide (136 mg H₂S/g). Regeneration of spent Ni(0.64)Fe(0.36) mixed metal oxides was studied using two different mixture solutions, NaCl/NaOH and acetate-buffer/NaCl/NaOH. The latter mixture successfully desorbed the sulfur from the Ni0.64Fe0.36 sorbent for 2 cycles of regeneration/sorption.

Fe(II)-Al(III) layered double hydroxides prepared by ultrasound-assisted co-precipitation method for the reduction of bromate

Bromate is recognized as an oxyhalide disinfection byproduct in drinking water. In this paper, Fe(II)-Al(III) layered double hydroxides (Fe-Al LDHs) prepared by the ultrasound-assisted co-precipitation method were used for the reduction of bromate in solution. The Fe-Al LDHs particles were characterized by X-ray diffractometer, scanning electron microscopy and thermogravimetry-differential scanning calorimetry. It was found that ultrasound irradiation assistance promoted the formation of the hydrotalcite-like phase and then improved the removal efficiency of bromate. In addition, the effects of solid-to-solution ratio, contact time, initial bromate concentration, initial pH, coexisting anions on the bromate removal were investigated. The results showed the bromate with an initial concentration of 1.56 μmol/L could be completely removed from solution by Fe-Al LDHs within 120 min. When the initial bromate concentration was 7.81 μmol/L, the Fe-Al LDHs with irradiation time of 30 min exhibited the optimum removal efficiency and the bromate removal capacity (qe) was 6.80 μmol/g. In addition, the appearance of sulfate and production of bromide were observed simultaneously in this process, which suggested that ion-exchange between sulfate and bromate, and the reduction of bromate to bromide by Fe(2+) were the main mechanisms responsible for the bromate removal by Fe-Al LDHs.

Layered-double-hydroxide-modified electrodes: electroanalytical applications

Two-dimensional inorganic solids, such as layered double hydroxides (LDHs), also defined as anionic clays, have open structures and unique anion-exchange properties which make them very appropriate materials for the immobilization of anions and biomolecules that often bear an overall negative charge. This review aims to describe the important aspects and new developments of electrochemical sensors and biosensors based on LDHs, evidencing the research from our own laboratory and other groups. It is intended to provide an overview of the various types of chemically modified electrodes that have been developed with these 2D layered materials, along with the significant advances made over the last several years. In particular, we report the main methods used for the deposition of LDH films on different substrates, the conductive properties of these materials, the possibility to use them in the development of membranes for potentiometric anion analysis, the early analytical applications of chemically modified electrodes based on the ability of LDHs to preconcentrate redox-active anions and finally the most recent applications exploiting their electrocatalytic properties. Another promising application field of LDHs, when they are employed as host structures for enzymes, is biosensing, which is described considering glucose as an example.

Complexation at the edges of hydrotalcite: the cases of arsenate and chromate

Sorption of CrO(4)(2-) and HAsO(4)(2-) by hydrotalcite, in its chloride form, was studied as a function of anion concentration. In both cases, the shape of the isotherms is langmuirian. The maximum uptake of CrO(4)(2-) equals the ion-exchange capacity of the solid, whereas sorption of HAsO(4)(2-) saturates at a higher value. Chloride ions inhibit the uptake of both anions, the amount of sorbed CrO(4)(2-) declining rapidly to zero. The uptake of HAsO(4)(2-), however, attains a constant value at high chloride concentrations. The excess of arsenate uptake follows, at constant pH, a langmuirian dependence with equilibrium concentration and decreases with increasing pH, depicting a marked change in slope at pH≈pQ(a3). CrO(4)(2-) and HAsO(4)(2-) have notable, albeit different, effects on the electrophoretic behavior of hydrotalcite; the positive particle charge is screened almost completely by CrO(4)(2-), whereas sorption of HAsO(4)(2-) produces charge reversal. These results reflect the formation of inner-sphere arsenate surface complexes at the edges of hydrotalcite particles. The underlying rationale is discussed in terms of the crystal structure of hydrotalcite surfaces.

Layered double hydroxides: a novel nano-sorbent for solid-phase extraction

The nickel-aluminum layered double hydroxide (Ni-Al LDH) was synthesized by a simple co-precipitation method with controlled pH and followed by hydrothermal treatment. The obtained nano-structured inorganic material was employed, for the first time, as a new solid-phase extraction (SPE) sorbent for the extraction and pre-concentration of trace levels of fluoride ions from aqueous solutions. An indirect method was used for monitoring of extracted fluoride ions. The method is based on the quenching effect of extracted fluoride ions upon the fluorescence intensity of Al-oxine complex via the forming of AlF(6)(3-), which was determined spectrofluorometrically at λ(em)=510 nm with excitation at λ(ex)=404 nm. The effect of several parameters such as type of interlayer anion in Ni-Al LDH structure, pH, sample flow rate, elution conditions, amount of nano-sorbent, sample volume and co-existing ions on the extraction efficiency of the analyte were investigated. The results showed that fluoride ions could be retained on the Ni-Al (NO(3)(-)) LDH at pH 6.0 and stripped by 1.2 mL of 3.0 mol L(-1) NaOH. In the optimum experimental conditions, the limit of detection (3s) and enrichment factor were 9.0 ng mL(-1) and 50, respectively. The optimized method was successfully applied to the determination of fluoride concentration in various water samples. The results obtained from the proposed method were successfully compared with those provided by standard SPADNS method.