# A Review of the Synthesis and Applications of Polymer-Nanoclay Composites
## 3 Application
### Wastewater Treatment  
The contamination of water due to the presence of an extensive variety of toxic substances, such as heavy metals, aromatic molecules, and dyes, has become a critical problem due to potential health concerns [209]. Adsorption is a widely used technique for the removal of pollutants from water; a number of novel adsorbents have been developed in recent years [210]. Polymer/nanoclay composites are able to deliver a high adsorption capacity and an excellent life cycle for water treatment/remediation due to their easy processability, effective cation exchange, large surface area, and their relatively low cost and toxicity [211]. The adsorption performance (capacity, selectivity and reusability) is substantially affected by exchangeable cations, the nanoclay structure in the matrix, the technique employed in their modification, and the existence of water molecules between the clay layers [212; 213]. Various advanced polymer/nanoclay composites have been developed and used in emerging adsorbents [211]. In addition, a number of current studies demonstrate the remediation potential and effectiveness of various synthesized polymer/nanoclay composites as promising adsorbents. [PERSON] et al. [214] fabricated poly(vinylimidazole) (PVI)/sepiolite nanoclay composites using an in-situ polymerization method. The synthesized composite was shown to be an effective adsorbent for Hg(II) in wastewater. PVI/sepiolite composites exhibited a dense, smooth surface and a uniform dispersion of nanoclay in the polymer matrix, which delivered a higher adsorption capacity compared to sepiolite alone. The optimum pH value for Hg(II) adsorption was observed to occur at a pH of six, whereas its adsorption capacity values increased with the temperature. [PERSON] et al. [215] employed PVI/bentonite nanoclay composites in wastewater treatment to remove anionic dye remazol black B (RB) from water. The maximum adsorption (230 mg/g) was achieved at a pH of two. [PERSON] et al. [216] synthesized PANI/bentonite nanoclay composites by plasma-induced polymerization to remove radioactive uranium(VI) ions from aqueous solutions. Temperature, pH and ionic strength had significant effects on adsorption of uranium(VI) on PANI/bentonite surfaces. The adsorption capability of the PANI/bentonite was significantly enhanced within the presence of humic acid (pH! 6.5). [PERSON] et al. [217] reported the development of thiourea-formaldehyde/bentonite nanoclay composites for Pb(II), Mn(VII) and Cr(VI) adsorption in aqueous solution. The optimum pH was approximately four for Pb(II) and Mn(VII), two for Cr(VI). The maximum adsorption values were 13.38, 14.81 and 4.20 mg/g for Pb(II), Mn(VII) and Cr(VI), respectively. Regeneration experiments indicated removal efficiencies over 90% after four successive adsorption/desorption cycles [217]. Chitosan incorporated with nanoclay adsorbents has emerged as a new class of pollutant removal technique due to its relatively low cost, natural biodegradability and nontoxicity. [PERSON] et al. [218] focused on developing an inexpensive and environmentally-friendly adsorbent for the removal of Cr(VI) from wastewater. A chitosan/bentonite nanoclay composite was synthesized with a 5% nanoclay loading content. An adsorption test was carried out on the wastewater from a tannery in the city of Fez in Morocco; the composites delivered a maximum adsorption capacity of 223 mg/g of Cr(VI) at a pH of five. Several polymer-nanoclay composites are able to remove various pollutants from aqueous solutions and have been shown to be effective in water treatment processes. Safety concerns remain regarding the potential for nanoparticles to be released into the environment and cause harm via chemical transformation, biological transformation, physical transformation or aggregation, and interactions between particles and natural organic matter or biomolecules [219; 220; 221].