Magnetic layered double hydroxide-based composites as sustainable adsorbent materials for water treatment applications: Progress, challenges, and outlook

Layered double hydroxides (LDHs) have shown exciting applications in water treatment because of their unique physicochemical properties, which include high surface areas, tunable chemical composition, large interlayer spaces, exchangeable content in interlayer galleries, and ease of modification with other materials. Interestingly, their surface, as well as the intercalated materials within the layers, play a role in the adsorption of the contaminants. The surface area of LDH materials can be further enhanced by calcination. The calcined LDHs can reattain their structural features upon hydration through the "memory effect" and may uptake anionic species within their interlayer galleries. Besides, LDH layers are positively charged within the aqueous media and can interact with specific contaminants through electrostatic interactions. LDHs can be synthesized using various methods, allowing the incorporation of other materials within the layers or forming composites that can selectively capture target pollutants. They have been combined with magnetic nanoparticles to improve their separation after adsorption and enhance adsorptive features in many cases. LDHs are relatively greener materials because they are mostly composed of inorganic salts. Magnetic LDHbased composites have been widely employed for the purification of water contaminated with heavy metals, dyes, anions, organics, pharmaceuticals, and oil. Such materials have shown interesting applications for removing contam-inants from real matrices. Moreover, they can be easily regenerated and used for several adsorption-desorption cycles. Magnetic LDHs can be regarded as greener and sustainable because of several green aspects in their synthesis and re-usability. We have critically reviewed their synthesis, applications, factors affecting their adsorption performance, and related mechanisms in this review. In the end, some challenges and perspectives are also discussed.