Removal of heavy metals and dyes by clay-based adsorbents: From natural clays to 1D and 2D nano-composites

Natural and synthetic clays have been used as adsorbents for removal of heavy metals and dyes from aqueous solution for many decades. In the last decade, exfoliating the natural clays to low dimensional nanosheets or nanotubes for preparation of multifunctional clay-based adsorbents has attracted considerable attention. In this review, we have summarized the removal of heavy metals and dyes by clay-based adsorbents, from natural clays to 1D clay nanotubes and 2D clay nanosheets. Firstly, the structural and adsorption performance of various natural clays (montmorillonite/bentonite/smectite, kaolinite, illite, sepiolite, palygorskite/attapulgite, halloysite) are presented. Then the adsorption of dyes and heavy metals onto modified clays including thermally treated clays, acid-washed clays, inorganic pillared clays and organic modified clays are discussed. Next, we have introduced the novel nano-clay based adsorbents with low dimensional morphologies (2D nanosheets, 1D nanotubes, 1D nanorods and clay-supported nanoparticles). The clay adsorbents with exfoliated nanosheets or nanotubes show dramatically improved performance because of the fully exposed reactive sites and very high specific surface area. More importantly, the nanosheets and nanotubes can be readily composited with coupling agents or functional polymers to prepare multifunctional adsorbents. In addition, the adsorption behaviors of clay-based adsorbents along with factors affecting adsorption, adsorption mechanisms and regeneration of adsorbents have been discussed. Inspired by the natural and modified clays, future suggestions in designing novel adsorbents for various applications have also been suggested.