Defluoridation by Bare Nanoadsorbents, Nanocomposites, and Nanoadsorbent Loaded Mixed Matrix Membranes

In groundwater, excess fluoride concentration (>1.5 mg/L) is a primary concern for various countries. Nanoadsorbents have been proven to possess higher adsorption capacity than conventional adsorbents. This review aims to provide insights into recent advancements in the adsorption of fluoride using various nanoadsorbents. Nanoparticles have very high specific surface area but are usually found unsuitable for real field applications as they tend to agglomerate. They are also difficult to recover after use and pose a significant threat to the environment through leaching. These limitations have given rise to the trend of the development of nanocomposites for defluoridation. Nanocomposites often involve a polymeric matrix that serves as a medium for their homogenous dispersion. This reduces agglomeration and leaching into the surroundings without altering the original activities of the nanoadsorbent. This review classifies the fluoride nanoadsorbents into three categories: bare nanoparticles, nanocomposites with nanoparticles inside the porous matrix, and nanocomposites with nanoparticles coated with or anchored on the matrix surface. Critical analyses of the importance and shortcomings of these classes have been presented. In general, bare nanocomposites show the best fluoride adsorption performance compared to other classes of nanoadsorbents.