Rosette-like Layered Double Hydroxides: Adsorbent Materials for the Removal of Anionic Pollutants from Water

Insight into the synthesis of LDH using the urea method: morphology and intercalated anion control

Layered double hydroxides (LDHs) are a class of layered solids applied in many application fields. The study of synthetic methods able to control the interlayer composition and morphology of LDH is an open issue. The urea method, which exploits the thermal decomposition of urea, is known for yielding highly crystalline LDH in the carbonate form. This form is highly stable and, to replace carbonate ions with more easily exchangeable anions, a second step is required. In this work, we modified the urea method to obtain MgAl and ZnAl LDH in the chloride or nitrate form through a one-step synthesis. The effects of the urea/(Al + M(II)) molar ratio (R), reaction time and metal salt concentrations were deeply investigated. We found that LDH in chloride and nitrate forms can be prepared from solutions of metal salts not exceeding 1 M by adjusting R and maintaining the reaction time at 48 hours. The morphology of these products was found to depend on the R value and on the metal salts used in the synthesis. A high R value and nitrate salts favoured the formation of sand-rose crystals, while chloride salts induced the formation of plate-like crystals. The crystal growth mechanism and the parameters influencing the morphology are discussed with reference to ZnAl LDH by monitoring the synthesis over time.

Synthesis and characterization of new biocatalyst based on LDH functionalized with l-asparagine amino acid for the synthesis of tri-substituted derivatives of 2, 4, 5-(H1)-imidazoles

In this study, a new and recyclable biocatalyst (MgAl CO3-LDH@Asn) was synthesized by immobilizing l-asparagine amino acid (Asn) on the surface of 3-(chloropropyl)-trimethoxysilane modified MgAl CO3-layered double hydroxide (LDH). The physicochemical properties of the samples were identified by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and thermogravimetric analysis (TGA) techniques. The MgAl CO3-LDH@Asn was employed in the multi-component assembly process for the synthesis of tri-substituted derivatives of 2,4,5-(H1)-imidazoles from benzyl, various benzaldehyde derivatives, and ammonium acetate. For optimizing the reaction, the main factors, including the amount of MgAl CO3-LDH@Asn, type of solvent, reaction time, and temperature were evaluated. The optimum conditions of the model reaction were achieved using 20 mg of MgAl CO3-LDH@Asn biocatalyst in ethanol solvent after 20 min at reflux temperature. According to the findings above, the results indicated that high-yield products are achieved within a short time frame. Moreover, the high catalytic activity of the MgAl CO3-LDH@Asn was maintained for four cycles without significantly diminishing its performance.

Ultrasound-Assisted Adsorption of Perchlorate Using Calcined Hydrotalcites and the Thermal Stabilization Effect of Recycled Adsorbents on Poly(vinyl chloride)

Due to their high anion exchange and memory effect, the layered double hydroxides (LHDs) have wide applications for some areas. In this work, an efficient and green recycling route for layered double hydroxide based adsorbents is proposed specifically for application as a poly(vinyl chloride) (PVC) heat stabilizer without requiring secondary calcination. Conventional magnesium-aluminum hydrotalcite was synthesized using the hydrothermal method followed by removal of carbonate anion (CO₃^2-) between LDH layers by calcination. The adsorption of perchlorate anion (ClO₄^-) by the memory effect of calcined LDHs with and without ultrasound assistance was compared. Using ultrasound assistance, the maximum adsorption capacity of the adsorbents (291.89 mg/g) was increased, and the adsorption process was fitted using the kinetic Elovich rate equation (R² = 0.992) and Langmuir adsorption model (R² = 0.996). This material was characterized using XRD, FT-IR, EDS, and TGA which demonstrated that ClO₄^- was intercalated into the hydrotalcite layer successfully. The recycled adsorbents were used to augment a commercial calcium-zinc-based PVC stabilizer package applied in a epoxidized soybean oil plasticized cast sheet which is based on an emulsion type PVC homopolymer resin. Use of perchlorate intercalated LDH augmentation yielded significant improvement to static heat resistance as indicated by the degree of discoloration with a life extension of approximately 60 min. The improved stability was corroborated by evaluation of HCl gas evolved during thermal degradation using conductivity change curves and the Congo red test.

A novel remediation method of cadmium (Cd) contaminated soil: Dynamic equilibrium of Cd2+ rapid release from soil to water and selective adsorption by PP-g-AA fibers-ball at low concentration

The acid-extractable fraction Cd(II) in soil accumulates easily in organisms, migrates and transforms in the ecological environment, which has posed potential health risks to human. This study found that the acid-extractable fraction Cd(II) in soil could be released rapidly into water at very low Cd²⁺ concentration. Carboxylated polypropylene (PP-g-AA) fibers-ball with high selectivity as adsorbent was used in the Cd(II) contaminated soil-water system. It could remove promptly trace Cd²⁺ from water even in the presence of interfering metal ions. Moreover, Cd(II) desorbed from soil to water could be continuously adsorbed by PP-g-AA fibers-ball, which kept the Cd²⁺ concentration always at a low level. This forms a dynamic equilibrium of rapid release- selective adsorption toward the acid-extractable fraction Cd(II) in the soil-water system. Here, the migratory pathway for the acid-extractable fraction Cd(II) to be released from contaminated soil to water and adsorbed simultaneously on the surface of PP-g-AA fibers-ball was established. This work offers a novel protocol that can remove more than 90% of the acid-extractable fraction Cd(II) from contaminated soil within 12 h, thereby contributes better to mitigate the risk of Cd(II) from soil to the food chain without changing the physical and chemical properties of soil.

Layered double hydroxides are still out in the bloom: Syntheses, applications and advantages of three-dimensional flower-like structures

Layered double hydroxides (LDHs) have received great attention for years in numerous fields. Controlled and flexible layer composition, as well as the vast assortment of possible anionic guests, and easy adaptability for multipurpose applications, have been some of the many reasons behind their extraordinary success. However, versatility does not only involve the composition or the dimensions of the crystals but also their morphology. Aside from conventional hexagonal, flat structures, three-dimensional assemblies have been reported with architectures closely resembling those of flowers. The possibility of interconnecting the LDH nanosheets in rosette-like geometries has arisen the interest in finding new ways to control, modulate, and guide the particle growth obtaining hierarchical structures to be adapted to specific targets. This review is focused on describing the different strategies implemented to build flower-like assemblies, and on investigating their applications, looking for specific advantages of the use of a three-dimensional architecture over a bi-dimensional one.

Nitrate removal from aqueous solutions by magnetic cationic hydrogel: Effect of electrostatic adsorption and mechanism

Excessive nitrate (NO₃^-) is among the most problematic surface water and groundwater pollutants. In this study, a type of magnetic cationic hydrogel (MCH) is employed for NO₃^- adsorption and well characterized herein. Its adsorption capacity is considerably pH-dependent and achieves the optimal adsorption (maximum NO₃^--adsorption capacity is 95.88 ± 1.24 mg/g) when the pH level is 5.2-8.8. The fitting result using the homogeneous surface diffusion model indicates that the surface/film diffusion controls the adsorption rate, and NO₃^- approaches the center of MCH particles within 30 min. The diffusion coefficient (D_s) and external mass transfer coefficient (k_F) in the liquid phase are 1.15 × 10^-6 cm²/min and 4.5 × 10^-6 cm/min, respectively. The MCH is employed to treat surface water that contains 10 mg/L of NO₃^-, and it is found that the optimal magnetic separation time is 1.6 min. The high-efficiency mass transfer and magnetic separation of MCH during the adsorption-regeneration process favors its application in surface water treatment. Furthermore, the study of the mechanism involved reveals that both -N⁺(CH₃)₃ groups and NO₃^- are convoluted in adsorption via electrostatic interactions. It is further found that ion exchange between NO₃^- and chlorine occurs.