Layered double hydroxide nanocomposites as promising nanomaterials for removal of inorganic pollutants from aquatic systems: A review

Inorganic pollutants as heavy metal cations, radioactive nuclides and major anionic contaminants are highly toxic, nonbiodegradable and accumulative when released to the aquatic systems and therefore, causing a worldwide environmental pollution problem. Recent research interests have been mainly intensified and devoted to find out regenerable, low-cost and eco-friendly environmental materials with targeted and incorporated characteristics for sequestration and removal of theses inorganic pollutants from wastewater and aquatic systems. Layered double hydroxides (LDHs) are classified as low-cost materials due to their preparation and formation of some simple metal salts as widely available and low-cost constituents. Therefore, LDHs and their composites of clays, polymers, nanometal oxides, and different organic derivatives have attracted pronounced focuses in recent years owing to their wide range of applications. One of the most important applications of LDHs is related to water remediation from various contaminants, including organics, pesticides, heavy metals, and radioactive nuclides. The characteristics of these materials as anionic clays are based on the positive charge layers via interlayer guest anions, in addition to their high specific surface area for excellent adsorption efficiency via ion exchange mechanisms. In this review article, the recent applications of LDHs materials were reviewed and surveyed in adsorptive removal of various toxic heavy metal pollutants. These include monovalent metals such as Ag, divalent metals like Cu, Pb, Hg, Cd, Co, Zn, Ni, and Mn, besides other metal species including removal of As(III, VI), Cr(VI), Mo(III), V(V). In addition, removal of these ions was classified into single, bi-, tri-, and multi-toxic heavy metal cations as well as single, bi-, tri-, and multi-toxic anions. Moreover, radioactive nuclides as pollutants of major concern were also surveyed and covered in this review. Removal processes of these toxic metals have been classified in diverse categories as the well-known synthetic approaches to prepare LDHs and their composites are also discussed.