A Comprehensive Review of Layered Double Hydroxide-Based Carbon Composites as an Environmental Multifunctional Material for Wastewater Treatment

Layered Double Hydroxides for Regulating Phosphate in Water to Achieve Long-Term Nutritional Management

Hunger and undernourishment are increasing global challenges as the world's population continuously grows. Consequently, boosting productivity must be implemented to reach the global population's food demand and avoid deforestation. The current promising agricultural practice without herbicides and pesticides is fertilizer management, particularly that of phosphorus fertilizers. Layered double hydroxides (LDHs) have recently emerged as favorable materials in phosphate removal, with practical application possibilities in nanofertilizers. This review discusses the fundamental aspects of phosphate removal/recycling mechanisms and highlights the current endeavors on the development of phosphate-selective sorbents using LDH-based materials. Specific emphasis is provided on the progress in designing LDHs as the slow release of phosphate fertilizers reveals their relevance in making agro-practices more ecologically sound. Relevant pioneering efforts have been briefly reviewed, along with a discussion of perspectives on the potential of LDHs as green nanomaterials to improve food productivity with low eco-impacts.

Efficient Adsorption Capacity of MgFe-Layered Double Hydroxide Loaded on Pomelo Peel Biochar for Cd (II) from Aqueous Solutions: Adsorption Behaviour and Mechanism

A novel pomelo peel biochar/MgFe-layered double hydroxide composite (PPBC/MgFe-LDH) was synthesised using a facile coprecipitation approach and applied to remove cadmium ions (Cd (II)). The adsorption isotherm demonstrated that the Cd (II) adsorption by the PPBC/MgFe-LDH composite fit the Langmuir model well, and the adsorption behaviour was a monolayer chemisorption. The maximum adsorption capacity of Cd (II) was determined to be 448.961 (±12.3) mg·g^-1 from the Langmuir model, which was close to the actual experimental adsorption capacity 448.302 (±1.41) mg·g^-1. The results also demonstrated that the chemical adsorption controlled the rate of reaction in the Cd (II) adsorption process of PPBC/MgFe-LDH. Piecewise fitting of the intra-particle diffusion model revealed multi-linearity during the adsorption process. Through associative characterization analysis, the adsorption mechanism of Cd (II) of PPBC/MgFe-LDH involved (i) hydroxide formation or carbonate precipitation; (ii) an isomorphic substitution of Fe (III) by Cd (II); (iii) surface complexation of Cd (II) by functional groups (-OH); and (iv) electrostatic attraction. The PPBC/MgFe-LDH composite demonstrated great potential for removing Cd (II) from wastewater, with the advantages of facile synthesis and excellent adsorption capacity.

Composites of Layered Double Hydroxides and ANA-Type Zeolite Synthesized from Hazardous Secondary Aluminum Dross for Cationic Dye Wastewater Treatment

This work first transformed hazardous aluminum waste into low-cost MgAl−layered double hydroxide@ANA zeolite (LDHs@ANA) composite for dye wastewater adsorption, which was meaningful for waste recovery and pollution control. Based on this strategy, the Al(OH)3 extracted from secondary aluminum dross (a hazardous waste in the aluminum industry) was used as an aluminum source to synthesize LDHs@ANA composite, which had more excellent adsorption capacity to methylene blue than MgAl−LDHs and ANA alone. The composite consisted of spherical ANA particles uniformly covered with LDH nanosheets, which effectively avoided a large amount of aggregation between nanosheets and increased specific surface areas and pore volumes. The kinetic results indicated that the adsorption process conformed to the pseudo-second-order kinetic model, and the adsorption site was the main factor affecting the adsorption process. The equilibrium studies showed the adsorption process was exothermic, and the Langmuir model best fitted for the adsorption process, with a maximum adsorption capacity reaching 65.27 mg/g. Meanwhile, the effects of pH, adsorbent concentration, initial methylene blue concentration, and adsorption time on the LDHs@ANA were analyzed. Overall, this work provides a fresh concept for the preparation of low-cost adsorbents from aluminum waste.

An Advanced Approach for MgZnAl-LDH Catalysts Synthesis Used in Claisen-Schmidt Condensation

Using organic-base tetramethylammonium hydroxide (TMAH) is a viable, cheap, and fast option for the synthesis of MgZnAl-LDH-type materials by both co-precipitation and mechano-chemical methods. TMAH presents several advantages, such as the smaller quantity of water required in the washing step compared to the use of inorganic alkalis, the prevention of LDH contamination with alkali cations, and its action as a template molecule in texture tailoring. It also has disadvantages, such as its presence in small quantities in the resulting layered materials. Regardless of the use of organic/inorganic bases and co-precipitation/mechano-chemical methods, zincite stable phase was found in all the synthesized solids. The basicity of catalysts followed the trend: mixed oxides > reconstructed > parent LDH. The memory effect of LDH was supported only by the presence of Mg and Al cations, while Zn remained as a zincite stable phase. The catalytic activities for Claisen–Schmidt condensation of benzaldehyde with cyclohexanone provided values higher than 90% after 2 h, with a total selectivity toward 2,6-dibenzylidenecyclohexanone, while self-condensation of cyclohexanone yielded no more than 7.29% after 5 h. These behaviors depended on catalyst basicity as well as on the planar rigidity of the compound.